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Review Article
Ultrahigh Energy Neutrinos at the Pierre Auger Observatory
P. Abreu,1M. Aglietta,2M. Ahlers,3E. J. Ahn,4I. F. M. Albuquerque,5D. Allard,6
I. Allekotte,7J. Allen,8P. Allison,9A. Almela,10,11 J. Alvarez Castillo,12 J. Alvarez-Muñiz,13
R. Alves Batista,14 M. Ambrosio,15 A. Aminaei,16 L. Anchordoqui,17 S. Andringa,1
T. AntiIiT,18 C. Aramo,15 E. Arganda,19,20 F. Arqueros,20 H. Asorey,7P. Assis,1J. Aublin,21
M. Ave,22 M. Avenier,23 G. Avila,24 T. Bäcker,25 A. M. Badescu,26 M. Balzer,27 K. B. Barber,28
A. F. Barbosa,29 R. Bardenet,30 S. L. C. Barroso,31 B. Baughman,9,32 J. Bäuml,33 J. J. Beatty,9
B. R. Becker,34 K. H. Becker,35 A. Bellétoile,36 J. A. Bellido,28 S. BenZvi,3C. Berat,23
X. Bertou,7P. L. Biermann,37 P. Billoir,21 F. Blanco,20 M. Blanco,21,38 C. Bleve,35
H. Blümer,22,33 M. BoháIová,39 D. Boncioli,40 C. Bonifazi,21,41 R. Bonino,2N. Borodai,42
J. Brack,43 I. Brancus,44 P. Brogueira,1W. C. Brown,45 R. Bruijn,46,47 P. Buchholz,25
A. Bueno,48 R. E. Burton,49 K. S. Caballero-Mora,50 B. Caccianiga,51 L. Caramete,37
R. Caruso,52 A. Castellina,2O. Catalano,53 G. Cataldi,54 L. Cazon,1R. Cester,55 J. Chauvin,23
S. H. Cheng,50 A. Chiavassa,2J. A. Chinellato,14 J. Chirinos Diaz,56 J. Chudoba,39 M. Cilmo,15
R. W. Clay,28 M. R. Coluccia,54 R. Conceição,1F. Contreras,57 H. Cook,46 M. J. Cooper,28
J. Coppens,16,58 A. Cordier,30 S. Coutu,50 C. E. Covault,49 A. Creusot,6A. Criss,50 J. Cronin,59
A. Curutiu,37 S. Dagoret-Campagne,30 R. Dallier,36 B. Daniel,14 S. Dasso,60,61 K. Daumiller,33
B. R. Dawson,28 R. M. de Almeida,62 M. De Domenico,52 C. De Donato,12 S. J. de Jong,16,58
G. De La Vega,63 W. J. M. de Mello Junior,14 J. R. T. de Mello Neto,41 I. De Mitri,54
V. de Souza,64 K. D. de Vries,65 L. del Peral,38 M. del Río,40,57 O. Deligny,66 H. Dembinski,22
N. Dhital,56 C. Di Giulio,40,67 M. L. Díaz Castro,29 P. N. Diep,68 F. Diogo,1C. Dobrigkeit,14
W. Docters,65 J. C. D’Olivo,12 P. N. Dong,66,68 A. Dorofeev,43 J. C. dos Anjos,29 M. T. Dova,19
D. D’Urso,15 I. Dutan,37 J. Ebr,39 R. Engel,33 M. Erdmann,69 C. O. Escobar,4,14 J. Espadanal,1
A. Etchegoyen,10,11 P. Facal San Luis,59 H. Falcke,16,70 G. Farrar,8A. C. Fauth,14 N. Fazzini,4
A. P. Ferguson,49 B. Fick,56 A. Filevich,11 A. FilipIiI,71,72 S. Fliescher,69 C. E. Fracchiolla,43
E. D. Fraenkel,65 O. Fratu,26 U. Fröhlich,25 B. Fuchs,22 R. Gaior,21 R. F. Gamarra,11
S. Gambetta,73 B. García,63 S. T. Garcia Roca,13 D. Garcia-Gamez,30 D. Garcia-Pinto,20
A. Gascon Bravo,48 H. Gemmeke,27 P. L. Ghia,21 M. Giller,74 J. Gitto,63 H. Glass,4
M. S. Gold,34 G. Golup,7F. Gomez Albarracin,19 M. Gómez Berisso,7P. F. Gómez Vitale,24
P. Gonçalves,1J. G. Gonzalez,33 B. Gookin,43 A. Gorgi,2P. Gouffon,5E. Grashorn,9
S. Grebe,16,58 N. Griffith,9M. Grigat,69 A. F. Grillo,75 Y. Guardincerri,61 F. Guarino,15
G. P. Guedes,76 P. Hansen,19 D. Harari,7T. A. Harrison,28 J. L. Harton,43 A. Haungs,33
T. Hebbeker,69 D. Heck,33 A. E. Herve,28 C. Hojvat,4N. Hollon,59 V. C. Holmes,28
P. Homola,42 J. R. Hörandel,16 P. Horvath,77 M. Hrabovský,39,77 D. Huber,22 T. Huege,33
A. Insolia,52 F. Ionita,59 A. Italiano,52 C. Jarne,19 S. Jiraskova,16 M. Josebachuili,11
K. Kadija,18 K. H. Kampert,35 P. Karhan,78 P. Kasper,4I. Katkov,22 B. Kégl,30 B. Keilhauer,33
A. Keivani,79 J. L. Kelley,16 E. Kemp,14 R. M. Kieckhafer,56 H. O. Klages,33 M. Kleifges,27
J. Kleinfeller,33,57 J. Knapp,46 D.-H. Koang,23 K. Kotera,59 N. Krohm,35 O. Krömer,27
D. Kruppke-Hansen,35 F. Kuehn,4D. Kuempel,25,69 J. K. Kulbartz,80 N. Kunka,27
G. La Rosa,53 C. Lachaud,6D. LaHurd,49 L. Latronico,2R. Lauer,34 P. Lautridou,36
Advances in High Energy Physics
S. Le Coz,23 M. S. A. B. Leão,81 D. Lebrun,23 P. Lebrun,4M. A. Leigui de Oliveira,81
A. Letessier-Selvon,21 I. Lhenry-Yvon,66 K. Link,22 R. López,82 A. Lopez Agüera,13
K. Louedec,23,30 J. Lozano Bahilo,48 L. Lu,46 A. Lucero,11 M. Ludwig,22 H. Lyberis,41,66
M. C. Maccarone,53 C. Macolino,21 S. Maldera,2D. Mandat,39 P. Mantsch,4A. G. Mariazzi,19
J. Marin,2,57 V. Marin,36 I. C. Maris,21 H. R. Marquez Falcon,83 G. Marsella,84 D. Martello,54
L. Martin,36 H. Martinez,85 O. Martínez Bravo,82 H. J. Mathes,33 J. Matthews,79,86
J. A. J. Matthews,34 G. Matthiae,40 D. Maurel,33 D. Maurizio,55 P. O. Mazur,4
G. Medina-Tanco,12 M. Melissas,22 D. Melo,11 E. Menichetti,55 A. Menshikov,27 P. Mertsch,87
C. Meurer,69 S. MiTanoviT,18 M. I. Micheletti,88 I. A. Minaya,20 L. Miramonti,51
L. Molina-Bueno,48 S. Mollerach,7M. Monasor,59 D. Monnier Ragaigne,30 F. Montanet,23
B. Morales,12 C. Morello,2E. Moreno,82 J. C. Moreno,19 M. Mostafá,43 C. A. Moura,81
M. A. Muller,14 G. Müller,69 M. Münchmeyer,21 R. Mussa,55 G. Navarra,2J. L. Navarro,48
S. Navas,48 P. Necesal,39 L. Nellen,12 A. Nelles,16,58 J. Neuser,35 P. T. Nhung,68 M. Niechciol,25
L. Niemietz,35 N. Nierstenhoefer,35 D. Nitz,56 D. Nosek,78 L. NoDka,39 J. Oehlschläger,33
A. Olinto,59 M. Ortiz,20 N. Pacheco,38 D. Pakk Selmi-Dei,14 M. Palatka,39 J. Pallotta,89
N. Palmieri,22 G. Parente,13 E. Parizot,6A. Parra,13 S. Pastor,90 T. Paul,91 M. Pech,39
J. PWkala,42 R. Pelayo,13,82 I. M. Pepe,92 L. Perrone,84 R. Pesce,73 E. Petermann,93
S. Petrera,67 A. Petrolini,73 Y. Petrov,43 C. Pfendner,3R. Piegaia,61 T. Pierog,33
P. Pieroni,61 M. Pimenta,1V. Pirronello,52 M. Platino,11 M. Plum,69 V. H. Ponce,7
M. Pontz,25 A. Porcelli,33 P. Privitera,59 M. Prouza,39 E. J. Quel,89 S. Querchfeld,35
J. Rautenberg,35 O. Ravel,36 D. Ravignani,11 B. Revenu,36 J. Ridky,39 S. Riggi,13
M. Risse,25 P. Ristori,89 H. Rivera,51 V. Rizi,67 J. Roberts,8W. Rodrigues de Carvalho,13
G. Rodriguez,13 I. Rodriguez Cabo,13 J. Rodriguez Martino,57 J. Rodriguez Rojo,57
M. D. Rodríguez-Frías,38 G. Ros,38 J. Rosado,20 T. Rossler,77 M. Roth,33
B. Rouillé-d’Orfeuil,59 E. Roulet,7A. C. Rovero,60 C. Rühle,27 A. Saftoiu,44
F. Salamida,66 H. Salazar,82 F. Salesa Greus,43 G. Salina,40 F. Sánchez,11 C. E. Santo,1
E. Santos,1E. M. Santos,41 F. Sarazin,94 B. Sarkar,35 S. Sarkar,87 R. Sato,57 N. Scharf,69
V. Scherini,51 H. Schieler,33 P. Schiffer,69,80 A. Schmidt,27 O. Scholten,65
H. Schoorlemmer,16,58J. Schovancova,39 P. Schovánek,39 F. Schröder,33
S. Schulte,69 D. Schuster,94 S. J. Sciutto,19 M. Scuderi,52 A. Segreto,53 M. Settimo,25
A. Shadkam,79 R. C. Shellard,29 I. Sidelnik,11 G. Sigl,80 O. Sima,95 A. UmiaBkowski,74
R. Šmída,33 G. R. Snow,93 P. Sommers,50 J. Sorokin,28 H. Spinka,4,96 R. Squartini,57
Y. N. Srivastava,91 S. Stanic,72 J. Stapleton,9J. Stasielak,42 M. Stephan,69 A. Stutz,23
F. Suarez,11 T. Suomijärvi,66 A. D. Supanitsky,60 T. Šuša,18 M. S. Sutherland,79 J. Swain,91
Z. Szadkowski,74 M. Szuba,33 A. Tapia,11 M. Tartare,23 O. TaGcSu,35 R. Tcaciuc,25
N. T. Thao,68 D. Thomas,43 J. Tiffenberg,61 C. Timmermans,16,58 W. Tkaczyk,74
C. J. Todero Peixoto,64 G. Toma,44 L. Tomankova,39 B. Tomé,1A. Tonachini,55
P. Travnicek,39 D. B. Tridapalli,5G. Tristram,6E. Trovato,52 M. Tueros,13 R. Ulrich,33
M. Unger,33 M. Urban,30 J. F. Valdés Galicia,12 I. Valiño,13 L. Valore,15 A. M. van den Berg,65
E. Varela,82 B. Vargas Cárdenas,12 J. R. Vázquez,20 R. A. Vázquez,13 D. VeberiI,71,72
V. Verzi,40 J. Vicha,39 M. Videla,63 L. Villaseñor,83 H. Wahlberg,19 P. Wahrlich,28
O. Wainberg,10,11 D. Walz,69 A. A. Watson,46 M. Weber,27 K. Weidenhaupt,69 A. Weindl,33
F. Werner,33 S. Westerhoff,3B. J. Whelan,28 A. Widom,91 G. Wieczorek,74 L. Wiencke,94
B. Wilczyñska,42 H. Wilczyñski,42 M. Will,33 C. Williams,59 T. Winchen,69 M. Wommer,33
B. Wundheiler,11 T. Yamamoto,59,97 T. Yapici,56 P. Younk,25,98 G. Yuan,79 A. Yushkov,13
B. Zamorano Garcia,48 E. Zas,13 D. Zavrtanik,71,72 M. Zavrtanik,71,72 I. Zaw,8,99 A. Zepeda,85
Y. Zhu,27 M. Zimbres Silva,14,35 and M. Ziolkowski25
Advances in High Energy Physics
1LIP and Instituto Superior T´
ecnico, Technical University of Lisbon, Lisboa, Portugal
2Istituto di Fisica dello Spazio Interplanetario (INAF), Universit`
a di Torino and Sezione INFN, Torino, Italy
3University of Wisconsin, Madison, WI, USA
4Fermilab, Batavia, IL, USA
5Universidade de S˜
ao Paulo, Instituto de F´
ısica, S˜
ao Paulo, SP, Brazil
6Laboratoire AstroParticule et Cosmologie (APC), Universit´
e Paris 7, CNRS-IN2P3, Paris, France
7Centro At´
omico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), San Carlos de Bariloche, Argentina
8New York University, New York, NY, USA
9Ohio State University, Columbus, OH, USA
10Universidad Tecnol ´
ogica Nacional-Facultad Regional Buenos Aires, Buenos Aires, Argentina
11 Instituto de Tecnolog´
ıas en Detecci´
on y Astropart´
ıculas (CNEA, CONICET, UNSAM), Buenos Aires, Argentina
12Universidad Nacional Autonoma de Mexico, Mexico, DF, Mexico
13Universidad de Santiago de Compostela, Santiago de Compostela, Spain
14Universidade Estadual de Campinas, IFGW, Campinas, SP, Brazil
15Universit`
a di Napoli “Federico II” and Sezione INFN, Napoli, Italy
16IMAPP, Radboud University Nijmegen, Nijmegen, e Netherlands
17University of Wisconsin, Milwaukee, WI, USA
18Rudjer Boˇ
skovi´
c Institute, 10000 Zagreb, Croatia
19IFLP, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
20
Universidad Complutense de Madrid, Madrid, Spain
21LaboratoiredePhysiqueNucl
´
eaire et de Hautes Energies (LPNHE), Universit´
es Paris 6 et Paris 7, CNRS-IN2P3, Paris, France
22
Karlsruhe Institute of Technology-Campus S¨
ud-Institut f ¨
ur Experimentelle Kernphysik (IEKP), Karlsruhe, Germany
23
LaboratoiredePhysiqueSubatomiqueetdeCosmologie(LPSC),Universit
´
e Joseph Fourier, INPG, CNRS-IN2P3, Grenoble, France
24
Observatorio Pierre Auger and Comisi´
on Nacional de Energ´
ıa At´
omica, Malarg¨
ue, Argentina
25
Universit¨
at Siegen, Siegen, Germany
26
University Politehnica of Bucharest, Bucharest, Romania
27
Karlsruher Institut f¨
ur Technologie-Campus Nord-Institut f¨
ur Prozessdatenverarbeitung und Elektronik, Karlsruhe, Germany
28
University of Adelaide, Adelaide, SA, Australia
29
Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, RJ, Brazil
30
Laboratoiredel’Acc
´
el´
erateur Lin´
eaire (LAL), Universit´
e Paris 11, CNRS-IN2P3, Orsay, France
31Universidade Estadual do Sudoeste da Bahia, Vitoria da Conquista, BA, Brazil
32University of Maryland, College Park, MD, USA
33Karlsruhe Institute of Technology-Campus North-Institut f¨
ur Kernphysik, Karlsruhe, Germany
34
University of New Mexico, Albuquerque, NM, USA
35Bergische Universit¨
at Wu pper ta l, Wupp e r tal , G e rmany
36
SUBATECH, ´
Ecole des Mines de Nantes, CNRS-IN2P3, Universit´
edeNantes,Nantes,France
37Max-Planck-Institut f¨
ur Radioastronomie, Bonn, Germany
38
Universidad de Alcal ´
a, Alcal´
a de Henares, Madrid, Spain
39
Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic
40
Universit`
a di Roma II “Tor Vergata” and Sezione INFN, Roma, Italy
41Instituto de F´
ısica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil
42
Institute of Nuclear Physics PAN, Krakow, Poland
43
Colorado State University, Fort Collins, CO, USA
44
Horia Hulubei National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, Romania
45
Colorado State University, Pueblo, CO, USA
46
School of Physics and Astronomy, University of Leeds, Leeds, UK
47
Universit´
e de Lausanne, Lausanne, Switzerland
48
Universidad de Granada & C.A.F.P.E., Granada, Spain
49
Case Western Reserve University, Cleveland, OH, USA
50
Pennsylvania State University, University Park, PA, USA
51Universit`
a di Milano and Sezione INFN, Milan, Italy
52Universit `
a di Catania and Sezione INFN, Catania, Italy
53Istituto di Astrosica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy
54
Dipartimento di Fisica dell’Universit`
a del Salento and Sezione INFN, Lecce, Italy
55Universit `
a di Torino and Sezione INFN, Torino, Italy
56
Michigan Technological University, Houghton, MI, USA
57
Observatorio Pierre Auger, Malarg¨
ue, Argentina
58
Nikhef, Science Park, Amsterdam, e Netherlands
59
Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
Advances in High Energy Physics
60
Instituto de Astronom´
ıa y F´
ısica del Espacio (CONICET-UBA), Buenos Aires, Argentina
61Departamento de F´
ısica, FCEyN, Universidad de Buenos Aires y CONICET, Buenos Aires, Argentina
62
Universidade Federal Fluminense, EEIMVR, Volta Redonda, RJ, Brazil
63
Faculty Mendoza (CONICET/CNEA), National Technological University, Mendoza, Argentina
64
Instituto de F´
ısica, Universidade de S˜
ao Paulo, S˜
ao Carlos, SP, Brazil
65
Kernfysisch Versneller Instituut, University of Groningen, Groningen, e Netherlands
66
Institut de Physique Nucl´
eaire d’Orsay (IPNO), Universit´
e Paris 11, CNRS-IN2P3, Orsay, France
67
Universit`
adell’AquilaandINFN,L’Aquila,Italy
68
Institute for Nuclear Science and Technology (INST), Hanoi, Vietnam
69
III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany
70
ASTRON, Dwingeloo, e Netherlands
71J. Stefan Institute, Ljubljana, Slovenia
72Laboratory for Astroparticle Physics, University of Nova Gorica, Nova Gorica, Slovenia
73Dipartimento di Fisica dell’Universit`
a and INFN, Genova, Italy
74University of Ł ´
od´
z, Ł´
od´
z, Poland
75INFN, Laboratori Nazionali del Gran Sasso, Assergi, L’Aquila, Italy
76Universidade Estadual de Feira de Santana, Feira de Santana, Brazil
77
RCPTM, Palacky University, Olomouc, Czech Republic
78InstituteofParticleandNuclearPhysics,FacultyofMathematics and Physics, Charles University, Prague, Czech Republic
79
Louisiana State University, Baton Rouge, LA, USA
80
Universit¨
at Hamburg, Hamburg, Germany
81Universidade Federal do ABC, Santo Andr´
e, SP, Brazil
82
Benem´
erita Universidad Aut´
onomadePuebla,Puebla,Mexico
83Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico
84
Dipartimento di Ingegneria dell’Innovazione dell’Universit`
a del Salento and Sezione INFN, Lecce, Italy
85
Centro de Investigaci´
on y de Estudios Avanzados del IPN (CINVESTAV), M´
exico, DF, Mexico
86
Southern University, Baton Rouge, LA, USA
87
Rudolf Peierls Centre for eoretical Physics, University of Oxford, Oxford, UK
88
Instituto de F´
ısica Rosario (IFIR), CONICET/U.N.R. and Facultad de Ciencias Bioqu´
ımicas y Farmac´
euticas U.N.R.,
Rosario, Argentina
89
Centro de Investigaciones en L´
aseres y Aplicaciones, CITEDEF and CONICET, San Carlos de Bariloche, Argentina
90
Instituto de F´
ısica Corpuscular, CSIC-Universitat de Val`
encia, Valencia, Spain
91Northeastern University, Boston, MA, USA
92
Universidade Federal da Bahia, Salvador, BA, Brazil
93University of Nebraska, Lincoln, NE, USA
94
Colorado School of Mines, Golden, CO, USA
95
Physics Department, University of Bucharest, Bucharest, Romania
96
Argonne National Laboratory, Argonne, IL, USA
97
Konan University, Kobe, Japan
98
Los Alamos National Laboratory, Los Alamos, NM, USA
99
NYU Abu Dhabi, Abu Dhabi, UAE
Correspondence should be addressed to e Pierre Auger Collaboration; auger spokesperson@fnal.gov
Received February ; Accepted June
Academic Editor: Kara Homan
Copyright © 2013 P. Abreu et al. is is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
e observation of ultrahigh energy neutrinos (UHEs) has become a priority in experimental astroparticle physics. UHEscan
be detected with a variety of techniques. In particular, neutrinos can interact in the atmosphere (downward-going )orinthe
Earth crust (Earth-skimming ), producing air showers that can be observed with arrays of detectors at the ground. With the
surface detector array of the Pierre Auger Observatory we can detect these types of cascades. e distinguishing signature for
neutrino events is the presence of very inclined showers produced close to the ground (i.e., aer having traversed a large amount
of atmosphere). In this work we review the procedure and criteria established to search for UHEs in the data collected with the
ground array of the Pierre Auger Observatory. is includes Earth-skimming as well as downward-going neutrinos. No neutrino
candidates have been found, which allows us to place competitive limits to the diuse ux of UHEs in the EeV range and above.
Advances in High Energy Physics
1. Introduction
e observation of ultrahigh energy cosmic rays (UHECR)
of energy – EeV (18–20 eV) has stimulated much
experimental as well as theoretical activity in the eld of
Astroparticle Physics [,]. Although many mysteries remain
to be solved, such as the origin of the UHECRs, their
production mechanism and composition, we know that it
is very dicult to produce these energetic particles without
associated uxes of ultrahigh energy neutrinos (UHEs) [].
In the so-called “bottom-up” models, protons and nuclei
are accelerated in astrophysical shocks, where pions are
believed to be produced by cosmic ray interactions with mat-
ter or radiation at the source []. In the so-called “top-down”
scenarios, protons and neutrons are produced from quark
and gluon fragmentation, a mechanism which is known to
produce much more pions than nucleons []. Furthermore,
protons and nuclei also produce pions in their unavoidable
interactions responsible for the Greisen-Zatsepin-Kuzmin
(GZK) cuto [–]. e ux of UHECRs above ∼×19 eV
is known to be largely suppressed with respect to that at lower
energies, a feature seen in the UHECR spectrum [–]thatis
compatible with the interaction of UHECRs with the cosmic
microwave background (CMB) radiation. If the primaries
are protons, the interaction responsible for the GZK eect is
photopion production, and the decays of the charged pions
produce UHE neutrinos. However, their uxes are uncertain
[], and if the primaries are heavy nuclei, the UHEyield
wouldbestronglysuppressed[].
e observation of UHE neutrinos could provide impor-
tant hints to the origin of UHECRs [,]. Unlike cosmic
rays, neutrinos point directly to the source where they were
produced, without being deected by galactic and extragalac-
tic magnetic elds. Unlike photons they travel undisturbed
from the sources carrying a footprint of the production
model.
High energy neutrinos can be detected with a variety of
techniques [,]. In particular, they can be observed with
arrays of detectors at ground level that are currently being
used to measure extensive showers produced by cosmic rays
[]. e main challenge in this technique lies in separating
showers initiated by neutrinos from those induced by regular
cosmic rays. It was suggested in the s that this could be
done at high zenith angles []becausetheatmosphereslant
depth provides a quite large target for neutrino interactions.
e idea is that neutrinos, having very small cross-sections,
can interact at any point along their trajectories, while
protons, nuclei, or photons interact shortly aer entering the
atmosphere.esignatureforneutrinoeventsisthusinclined
showers that interact deep in the atmosphere.
Inclined showers were rst observed in the s by
several groups [–]. With the surface detector array (SD)
ofthePierreAugerObservatory[] we can detect inclined
showers and identify neutrinos with energies typically above
. EeV. ere are two ways of performing this task.
() Neutrinos of all avours can collide with nuclei in
the atmosphere and induce an extensive air shower
close to the ground [,]. In this so-called
“downward-going” neutrino channel, both charged
current (CC) and neutral-current (NC) interactions
contribute to the neutrino event rate.
() Neutrinos of tau avour (𝜏)areexpectedtobemost
sensitively observed through the detection of showers
induced by the decay products of an emerging
lepton, aer the propagation and interaction of an
upward-going 𝜏inside the Earth [,]. is
“Earth-skimming” channel benets from the long
range of the lepton (∼km for the shower energies
relevant in this analysis) which sets the scale of the
eective volume. Only charged-current interactions
of 𝜏are relevant in this case.
In both the Earth-skimming and downward-going chan-
nelstheshowerscanbeidentiedandseparatedfromcosmic
ray induced showers with the SD of the Pierre Auger Obser-
vatory if the zenith angle is large enough, typically larger than
∼∘–∘. A number of properties of the shower front, mostly
stemming from the time distribution of the shower particles,
can be used to distinguish neutrino-induced showers. As
shown in Section , even though the criteria to identify
neutrinosinbothchannelsbeingbasedonsimilarideasand
variables, two dierent analyses were designed. e main
reason for that concerns background reduction. e Earth-
skimming neutrino search is restricted to a very narrow
angular range where the background of nucleonic showers is
expected to be very small. On the other hand, in the broader
angular range of the downward-going neutrino search the
background contamination is expected to be larger, and the
selection criteria need to be more restrictive. is calls for
specic algorithms and methods, capable of optimizing the
separation of neutrino-induced showers from nucleonic ones
as will be explained later in the paper.
In this work we review the procedure to search for UHEs
with the SD of the Auger Observatory, for both the Earth-
skimming and downward-going channels. In Section we
give a brief overview of the SD of the Pierre Auger Obser-
vatory. In Section we concentrate on the general strategy to
search for UHEs. Section is devoted to describe the simu-
lations of neutrino-induced showers cr ucial to establish selec-
tion criteria and to compute the exposure to UHEswhichis
reported in Section .InSectionwe give a detailed descrip-
tion of the neutrino selection criteria. When these criteria are
applied blindly to the data collected at the SD no candidates
arefound.eresultinglimitstothediuseuxofUHEsare
presented in Section .Finally,inSectionwe summarize the
paper and give some prospects for future observations.
2. The Pierre Auger Observatory
e Pierre Auger Observatory [] is a hybrid UHECR
detector combining an array of particle detectors at ground
level, and uorescence telescopes housed in four buildings,
for redundancy and calibration. It is located near the town of
Malarg ¨
ue, in the province of Mendoza in Argentina. In this
paperwefocusonthesurfacedetectorarray[,]whichis
briey described in the following.
Advances in High Energy Physics
(a) (b)
F : (a) One of the ∼ water Cherenkov stations that constitute the surface detector array of the Pierre Auger Observatory (forefront),
and one of the four uorescence buildings housing six of the uorescence telescopes (background). (b) Layout of the SD array with ∼
water Cherenkov stations (depicted as dots), spread over a surface of ∼ km2(blue area), with a distance between stations of . km. e
four uorescence buildings at the edges of the observatory are also indicated.
2.1. e Surface Detector Array. e surface detector array
[] consists of water Cherenkov detectors in the form of
cylinders of . m diameter and . m height, each containing
tonnes of puried water. Charged particles entering the
stationemitCherenkovlightwhichisreectedatthewalls
byadiusiveTyvekliner,andcollectedbythree-inch
photomultiplier tubes (PMT) at the top surface and in
optical contact with the water. e PMT signals are sampled
by ash analog to digital converters (FADC) with a time
resolution of ns. Each station is regularly monitored and
calibrated in units of vertical equivalent muons (VEM)
corresponding to the signal produced by a muon traversing
the tank vertically through its center []. In Figure we
show a picture of one of the water Cherenkov stations. e
stations are autonomous, with all their components (PMTs,
local processor, GPS receiver, and radio system) powered by
batteries coupled to solar panels. Once installed, the local
stations work continuously without external intervention.
e SD was completed in . ere are ∼ water
stations arranged in a triangular grid with . km spacing
between them, spanning an almost at surface of ∼ km2,
at an approximate altitude of m above sea level, or
equivalently an atmospheric depth ground = 880 gcm
−2.e
layout of the SD array is sketched in the right panel of Figure .
2.2. Surface Detector Trigger. e stations transmit infor-
mation by conventional radio links to the Central Data
Acquisition System (CDAS) located in Malarg¨
ue. ere are
two types of trigger conditions. A local trigger at the level
of an individual station (second order or T trigger), and
a global trigger (third order or T trigger). e T trigger
condition is the logical OR of two conditions: either a given
threshold signal (. VEM) is passed in at least one time bin
of the FADC trace—the so-called “reshold trigger”— or a
somewhat lower threshold (. VEM) is passed in at least
bins within a s time window (i.e., bins)—the so-called
“Time-over-reshold (ToT) trigger.” e ToT condition was
designed to trigger on signals broad in time, characteristic of
the early stages of the development of an extensive air shower,
and is crucial for neutrino identication as explained below.
e data acquisition system receives the local T triggers
and builds a global T trigger requiring a relatively compact
conguration of at least three local stations compatible in
time,eachsatisfyingtheToTtrigger,orfourtriggeredstations
withanytypeofTtrigger[]. With the completed array,
the global T trigger rate is about two events per minute, one
third being actual shower events at energies above 3×1017 eV.
3. Generalities of UHE Neutrino Search
With the SD of the Pierre Auger Observatory we can detect
and identify UHE neutrinos in the EeV range and above
[–]. e main challenge from the experimental point of
view is to identify neutrino-induced showers in the large
background of showers initiated by nucleonic cosmic rays.
e concept for identication is relatively simple. While pro-
tons, heavier nuclei and even photons interact shortly aer
entering the atmosphere, neutrinos can generate showers ini-
tiated deeply into the atmosphere. When considering vertical
showers, even the ones initiated by protons or heavy nuclei
have a considerable amount of electromagnetic component at
the ground (“young” shower front). However, when looking
at high zenith angles (>75
∘) the atmosphere is thick enough
(thicker than about three vertical atmospheres) so that the
cosmic rays interacting high in the atmosphere have shower
fronts dominated by muons at ground (“old” shower front). A
neutrino with >75
∘interacting deep will present a young
shower front and, consequently, can be distinguished.
AttheSDlevel,youngshowersinducesignalsspread
in time over hundreds of nano-seconds in a fraction of the
stations triggered by the shower, while old showers induce
narrow signals spreading over typically tens of nano-seconds
in practically all the stations of the event. With the ns
time resolution of the FADC of the water Cherenkov stations,
Advances in High Energy Physics
0 1000 2000 3000
5
4
3
2
1
0
Signal (VEM)
Time (ns)
Energy of shower ∼5 EeV
Distance to shower axis ∼1 km
Zenith angle ∼22∘(“young shower”)
(a)
0 1000 2000 3000
5
4
3
2
1
0
6
Signal (VEM)
Time (ns)
Energy of shower ∼5 EeV
Distance to shower axis ∼1 km
Zenith angle ∼80∘(“old shower”)
(b)
F : FADC traces of stations at km from the shower core for two real showers of EeV. (a) shower arriving in the early stages of
development (“young” shower). (b) “old” extensive air shower ( ∼ 80∘).
the distinction between traces induced by young and old
shower fronts can be easily accomplished. In Figure we show
an example of those two types of traces.
With this simple idea, we can search for two types of
neutrino-induced showers at the surface detector array of the
Pierre Auger Observatory, as follows.
() Earth-skimming showers induced by tau neutrinos
(𝜏) that travel in the upward direction with respect
to the vertical to ground. 𝜏can skim the Earth’s
crust and interact relatively close to the surface
inducing a tau lepton which escapes the Earth and
decays in ight in the atmosphere, close to the SD.
Typically, only Earth-skimming 𝜏-induced showers
with zenith angles 90∘<<95
∘may be identied.
() Showers initiated by any neutrino avour moving
down at large angles with respect to the vertical
at ground that interact in the atmosphere close to
the surface detector array. We include here showers
induced by 𝜏interacting in the mountains sur-
roundingthePierreAugerObservatory.Although
this latter process is exactly equivalent to the “Earth-
skimming”mechanism,itisincludedinthisclass
because such showers are also going downwards.
In the following we will refer to all these types of
showers as “downward-going” -induced showers. In
this paper we restrict ourselves to downward-going -
induced showers with zenith angles 75∘≤≤90
∘.
In Figure we show a pictorial representation of the
dierent types of inclined showers that can be detected.
4. Simulation of Neutrino Showers
Monte Carlo simulations of neutrino-induced showers are
crucial to establishing identication criteria and computing
the acceptance of the SD to UHEs. e whole simulation
chain is divided into three stages.
() High energy processes:
(a) the -nucleon interaction in the atmosphere for
downward-going neutrinos is simulated with
F : Pictorial representation of the dierent types of inclined
showers that can be detected at the surface detector array of the
Pierre Auger Observatory. (1) An inclined shower induced by a
proton interacting high in the atmosphere whose electromagnetic
component is absorbed and only the muons reach the detector.
Inclined showers presenting signicant electromagnetic component
at the detector level: (2) a deep downward-going -induced shower;
(3) an Earth-skimming 𝜏interacting in the Earth crust and
producing an upward-going lepton that decays in ight and
induces a shower in the atmosphere; and (4)a𝜏interacting in the
mountains, producing a downward-going lepton that decays close
to the detector and initiates a shower.
HERWIG []. e output of HERWIG includes
the types, energies, and momenta of the sec-
ondary particles produced for both charged
(CC) and neutral current (NC) neutrino inter-
actions (see Figure for a pictorial summary of
all the channels considered in this work);
(b) in the case of 𝜏CC interactions, the lepton
propagationintheEarthand/orintheatmo-
sphere is simulated with a dedicated, fast, and
exiblecodewhichallowsustoeasilystudy
the inuence on the outgoing lepton ux of
dierent 𝜏interaction cross sections, energy
loss models, and so forth. e simulation of the
decay of the (when necessary) is performed
with the TAUOLA package [].
() Shower development in the atmosphere: e AIRES
Monte Carlo code []isusedtopropagatethe
particles produced in a high energy interaction,
or in the decay of a lepton. e types, energies,
momenta,andtimesoftheparticlesreachingtheSD
level are obtained.
Advances in High Energy Physics
Hadronic
jet
High energy
electron High energy
tau
Neutral current
Charged current
Hadronic
jet Hadronic
jet Hadronic
jet
μνx
𝜈𝑒𝜈𝜇𝜈𝜏𝜈𝑥
F : Sketch of the dierent types of showers induced by UHE neutrinos. All the channels depicted contribute to the neutrino event rate
due to downward-going induced showers.
() Surface detector array simulation: is is performed
with the O line soware []. Firstly, particles reach-
ingasurfacedetectorstationareinjectedintothesta-
tion, and with the aid of GEANT []theamountof
Cherenkov light produced in water is calculated. en
the FADC traces of the PMT signals are obtained,
and the total signal due to the particles entering the
station, as well as several quantities characterizing
the FADC trace which will be relevant for neutrino
identication are computed (see below). Also both the
local trigger condition (T—either threshold or ToT),
and the global trigger condition (T) are applied to
the simulated events in the same way as for collected
data.
e phase space of the simulations—namely, neutrino
energy, zenith angle of incidence, interaction depth in the
atmosphere for downward-going neutrinos, and altitude of
the decay in the case of Earth-skimming 𝜏—spans a
suciently wide range of numerical values as to guarantee
that at the edges of the phase space none of the simulated
showers fullls the global trigger conditions. is is taken as
aclearindicationthatacompletesampleofshowershasbeen
produced without introducing any bias and therefore that the
Monte Carlo sample correctly represents the characteristic
of showers that could trigger the SD of the Pierre Auger
Observatory. For the Earth-skimming channel, showers were
simulated at zenith angles between .∘and .∘and
at an altitude of the decay point above the Pierre Auger
Observatory up to m. In the case of downward-going
neutrinos, simulations were performed at zenith angles in the
range ∘–∘.
5. Identifying Neutrino-Induced Showers
As stated above, the selection of potential neutrino-induced
showers (neutrino candidates) is based on two steps.
() Firstly,weselectamongthedatacollectedattheSDof
the Pierre Auger Observatory those events that arrive
in inclined directions with respect to the vertical.
() Secondly, we select among the inclined events those
with FADC traces that are spread in time, indicative of
the presence of an inclined shower in the early stage of
development, a clear signature of a deeply interacting
neutrino triggering the SD.
Although the two steps above are the same for all the
neutrino-induced showers searched for at the Pierre Auger
Observatory, due to the dierent nature of Earth-skimming
and downward-going neutrino-induced showers, the criteria
and selection cuts that are applied to data are slightly
dierent.
5.1. Selection of Inclined Events. Firstofall,eventsoccurring
during periods of data acquisition instabilities []are
excluded.
For the remaining events the FADC traces of the triggered
stations are rst “cleaned” to remove accidental signals
induced (mainly) by atmospheric muons arriving closely
before or aer the shower front—produced in showers dier-
ent than the triggering one and which are below the energy
threshold of the Pierre Auger Observatory. e trace-cleaning
procedure is detailed in []. Aer that, the start times of
thesignalsinallstationsincludedintheglobaltriggerare
requestedtobecompatiblewithaplaneshowerfrontmoving
at roughly the speed of light. is compatibility is realized
through upper bounds on both, the largest residual and the
mean quadratic residual from the planar t. If the condition
is not fullled, ts are attempted removing one station; for
this operation, the stations are sorted by increasing quality
(based on the integrated amplitude and the duration of the
signal), and the procedure is stopped as soon as a satisfactory
solution is found. If none is found, trials are made removing
two stations, and so on. e event is accepted if at least three
(four) stations in the Earth-skimming (downward-going)
case belong to the conguration.
e second step in both channels is the selection of
inclined showers. From the pattern (footprint) of stations at
ground (see Figure ) we can extract a length along the
arrivaldirectionoftheevent(i.e.,themainaxisoftheevent)
and a width perpendicular to it characterizing the shape
of the footprint (see [] for complete details). e ratio /
depends on zenith angle. Vertical events have / ∼ 1 and
this ratio increases gradually as the zenith angle increases.
Very inclined events typically have elongated patterns on
the ground along the direction of arrival, and hence large
values of /.Acutin/ is therefore a good selector
of inclined events. e exact value of this cut is dierent
Advances in High Energy Physics
Main axis
𝐿
𝑊Δ𝑡𝑖𝑗
𝑑𝑖𝑗
F : Schematic view of the footprint of a shower triggering the
surface detector array of the Pierre Auger Observatory. e shower
triggers the array from the le to the right of the gure, along the
“main axis.” e circles represent the position of the stations, with
their sizes being proportional to the collected signal in the PMTs.
See text for more details.
for downward-going and Earth-skimming events and was
determined through Monte Carlo simulations of -induced
showers performed at dierent zenith angles. For downward-
going events with >75
∘the requirement is / > 3, while
for Earth-skimming it is more restrictive / > 5 since only
quasihorizontal showers with largely elongated footprints
can trigger the array. e axis of Earth-skimming showers
travelling in the upward direction does not intersect ground,
contrary to the downward-going showers case. For this
reason, we exploit the properties of the footprint generated
by the shower particles that deviate laterally from the shower
axis and trigger the water Cherenkov stations. (see [,Figure
]).
Another indication of inclined events is given by the
apparent speed of the trigger from a station to a station
, averaged over all pairs (,) of stations in the event. is
observable denoted as is obtained in a straightforward
manner from the distance between the stations aer projec-
tion along the “main axis” of the footprint at ground (𝑖𝑗)
as depicted in Figure , and from the dierence in trigger
times of the stations (𝑖𝑗). Vertical showers have apparent
average speeds exceeding the speed of light since all triggers
occuratroughlythesametime,whileinveryinclinedevents
is concentrated around the speed of light. Moreover its
root-mean-square (RMS()) is small. For downward-going
(Earth-skimming) events is required to be below .
mns
−1 ( ∈ [0.29,0.31]mns
−1)andRMS()/ <
0.08 (RMS() < 0.08mns
−1). e values of these selection
requirements are based on comparisons between data and
Monte Carlo simulations. Also, and only for downward-going
events, a further quality cut is applied consisting on a simple
reconstruction of the zenith angle rec and the requirement
that rec >75
∘(see [] for full details).
In the top of Table the cuts applied to the observables
used to select inclined events are summarized.
5.2. Selection of Young Showers. Once inclined showers are
selected, the next step is to identify young showers among the
data collected at the SD of the Pierre Auger Observatory.
To optimize the numerical values of the cuts and tune
the algorithms needed to separate neutrino-induced showers
from the much larger background of hadronic showers, we
divided the whole data sample into two parts (excluding
periods of array instability). A fraction of the data (train-
ing period) is dedicated to dene the selection algorithm.
ese data are assumed to be overwhelmingly constituted of
background showers. e applied procedure is conservative
because the presence of neutrinos would result in a more
severe denition of the selection criteria. e remaining
fraction is not used until the selection procedure is estab-
lished, and then it is “unblinded” to search for neutrino
candidates. In Table we indicate the periods used for
training and “blind” search. e blind search period for the
Earth-skimming (downward-going) analysis corresponds to
an equivalent of ∼. yr (∼ yr) of a full surface detector array
consisting of stations working continuously without
interruptions.
It is worth remarking that data instead of Monte Carlo
simulationsofhadronicshowersareusedtooptimizethe
identication cuts. e rst reason for this is that, the com-
position of the primary UHECR ux—a necessary input in
the simulations—is not accurately known. Also, the detector
simulation may not account for all possible detector defects
and/or uctuations that may induce events that constitute a
background to UHE neutrinos, while they are accounted for
in collected data, including those which are not well known,
or even not yet diagnosed.
is is the general strategy followed in the search for
Earth-skimming 𝜏and downward-going -induced show-
ers. However, the two searches dier in several aspects that
wedetailinthefollowingsections.
5.2.1. Earth-Skimming Analysis. In the Earth-skimming anal-
ysis we identify young showers by placing a cut on the
fraction of stations in the event that fulll two conditions:
(1) the station passes the ToT local trigger condition and
(2) the ratio of the integrated signal over the peak height—
the so-called Area-over-Peak (AoP), a variable that carries
information on the time spread of the signal—is greater than
.. By convention, both the “area” and the “peak” values are
normalized to in signals induced by isolated muons.
e aim of both conditions is to identify broad signals in
time such as those induced by showers developing close to
the array. In particular, with the second condition we reject
background signals induced by inclined hadronic showers,
in which the muons and their electromagnetic products are
concentrated within a short time interval, exhibiting AoP
values close to the one measured in signals of isolated muons.
In order to reject inclined hadronic events, at least %
ofthetriggeredstationsintheeventarerequiredtofulllthe
two conditions above (Table ). e selection conditions were
optimized using data collected during the training period
indicated in Table . It is important to remark that this is the
same selection procedure and training period as in previous
publications [,], which is applied in this work to a larger
data set. e nal choice of the actual values of the neutrino
selection cuts was done by requiring zero background events
in the training data sample. When the Earth-skimming cuts
in Table are applied blindly to the data collected during the
search period, no events survived.
Advances in High Energy Physics
T : Observables and numerical values of cuts applied to select inclined and young showers for Earth-skimming and downward-going
neutrinos. See text for explanation.
Earth-skimming Downward-going
Number of Stations ≥ Number of Stations ≥
—rec >∘
/ > 5 / > 3
Inclined showers 0.29mns
−1 < < 0.31mns
−1 < 0.313mns
−1
RMS() < 0.08mns
−1 RMS()/ < 0.08
Yo u n g s h o we r s At least 60% of stations with ToT
trigger and AoP >.
Fisher discriminant Fbased on
Area-over-Peak (AoP)
T : Training and blind search periods for the search for Earth-skimming and downward-going neutrino candidates. In the rd row we
indicate the equivalent period of time of a full surface detector array. In the th row we give the number of candidates found in the search
period aer unblindly applying the cuts selecting inclined and young showers (see Table ). In the th row we give the numerical value of
the 90% C.L. limit to the normalization of a diuse ux of UHE neutrinos assumed to behave with energy as / = −2.Systematic
uncertainties are included in the value of the limit (see Section . for details). In the last row we indicate the energy range where the limits
apply, typically the energy interval where % of the events are expected.
Earth-skimming Downward-going
Training period Nov – Dec Jan – Oct
Blind search period Jan – May Nov – May
Equivalent full auger blind search period . yr . yr
candidates
Diuse limit % C.L. (GeVcm−2 s−1 sr−1 ) < 3.2× 10−8 < 1.7× 10−7
Energy range (EeV) ∼.–. ∼.–.
5.2.2. Downward-Going Analysis. In the search for
downward-going events, the discrimination power is
optimized with the aid of a multi-variate technique known
as the Fisher discriminant method []. e method consists
on constructing a linear combination of observables denoted
as Fwhich optimizes the separation between two samples
of events, in our case background hadronic inclined showers
occuring during the downward-going training period (see
Table ), and Monte Carlo simulated -induced showers.
e method requires as input a set of variables which can
discriminate between the two samples. For that purpose we
use variables depending on the Area-over-Peak (AoP)—as
dened above—of the FADC traces. In the rst few stations
hit by a deep inclined shower, the typical AoP values range
between and (Figure (a)).
Aer training the Fisher method, a good discrimination is
found when the following ten variables are used to construct
the linear Fisher discriminant variable F: the AoP of the four
stations that trigger rst (early stations) in each event, their
squares, the product of the four AoPs, and a global parameter
that measures the asymmetry between the average AoP of the
early stations and those triggering last (late stations) of the
event.
e product of the AoP of the earliest four stations
in the event aims at minimizing the relative weight of an
accidentally large AoP produced, for instance, by a single
muon which does not belong to the shower front arriving
at a station before or aer the shower itself. is variable is
also a very good discriminator as shown in Figure (b).We
have also checked in Monte Carlo simulations that neutrino-
induced events typically have an asymmetry parameter larger
than proton or nucleus-induced showers.
As the shower front is broader at larger distance from the
core for both young and old showers, the discrimination is
better when splitting the samples according to the number
of selected stations . A Fisher discriminant polynomial was
obtained separately for 4≤≤6,7≤≤11,and≥
12. An excellent separation is achieved for events in each of
thethreesubsamples.eindividualAoPsoftherstfour
tanks have the largest weights in the Fisher polynomials. In
Figure we show as an example the distribution of Fin the
subsample with the smallest number of selected stations (the
distributions corresponding to the three subsamples can be
found in [,Figure]).
Once the Fisher discriminant Fis dened, the next step
is to dene a numerical value of F, denoted as Fcut,that
separates neutrino candidates from regular hadronic show-
ers. One of the advantages of the Fisher discriminant method
is that it allows us to estimate the expected rate of background
events and, hence, to tune the value of Fcut so that the
background is kept at a very low value. is is important
given the fact that the expected rate of detected neutrino
events will be small. Data in the training period indicated in
Table were exploited to produce a reasonable prediction of
the background (see [] for full details). In practice, we x
Fcut so that the estimated number of background events is 1
in yr of data taking by a full Auger SD. With this cut, and
for our search sample we have an estimated background of
Advances in High Energy Physics
MC 𝜈simulations
Training data
Log10 (AoP1)
1
10−1
10−2
10−3
10−4
−0.2 0 0.2 0.4 0.6 0.8 1 1.2
(a)
0 0.5 1 1.5 2 2.5 3
Log10 (AoP1×AoP2×AoP3×AoP4)
MC 𝜈simulations
Training data
1
10−1
10−2
10−3
10−4
(b)
F:DistributionsoftheArea-over-Peak(AoP,seetext)oftheearlieststation(a)andoftheproductoftheAoPoftherstfourstations
in the event (b). In each panel we show the distribution of the corresponding variable in background events (i.e., data events in the training
sample as indicated in Table ), and in simulated electron neutrino-charged current events. ese are two of the ten variables depending on
the AoP used in constructing the multivariate Fisher discriminant linear polynomial to optimize the separation between background and
neutrino-induced showers. See text for more details on the remaining eight variables.
.eventsforeachmultiplicityclassthatadduptoatotalof
. events with a statistical uncertainty of %. It is important
to remark that this estimate relies on the a priori hypothesis
that the background has an exponential distribution in F.
Given the fact that we do not have a solid estimation of
the actual background, a conservative approach was taken
assuming the background is zero, in other words, the esti-
mated . background events were not used to improve our
upper limit on the ux [](seeSection. ).
As exemplied in Figure for the low multiplicity events,
the identication cuts reject only ∼% of the simulated
neutrino events, and those are mainly neutrinos interacting
far from the ground that, being similar to nucleonic-induced
showers, are not expected to be identied.
Finally, when the downward-going cuts in Table are
appliedtothedatacollectedduringthesearchperiod,no
neutrino candidates appeared (see Table ).
6. Exposure to UHE Neutrinos
6.1. Neutrino Identication Eciencies. With the criteria to
select neutrino-induced showers indicated in Table ,we
obtain a relatively large identication eciency both for
Earth-skimming 𝜏and downward-going -induced show-
ers. e eciency has been computed with Monte Carlo
simulations as the fraction of simulated events identied as
neutrinos.
In the case of Earth-skimming 𝜏induced showers, and a
full Auger SD working without interruption, the eciencies
depend only on the energy of the emerging leptons (𝜏)and
onthealtitudeofthe“centeroftheshower”(𝑐)aboveground
(averaged over the decay channels). is is conveniently
dened as the altitude of the shower axis at a distance
of km away from the decay point along the shower
axis. Showers induced by leptons with the same energy
but with dierent zenith angles—the range in being very
Fisher discriminant value
Fisher distribution-low mult. (4≤𝑁≤6)
Training data MC 𝜈simulations
Events
−4 −2 024
103
102
10
1
10−1
1 yr
20 yrs
100 yrs
F : Distribution of the value of the Fisher polynomial (F,
see text for details) for events with number of selected stations
4≤≤6. Data in the training period (see Table ) describe
the nucleonic background,while Monte Carlo simulated downward-
going neutrinos correspond to the signal. e vertical lines indicate
Fcut needed to expect event in the labeled periods of time (full SD
array).
narrow—have approximately the same eciency as long as
the corresponding altitudes of their shower maxima 𝑐are the
same. e maximum eciency that can be reached is 82.6%,
the 17.4% remaining corresponds to the channel in which the
decays into a which is unlikely to produce a detectable
shower close to ground. In Figure we show the trigger and
identication eciencies as a function of 𝑐for dierent
energies. As expected, the eciency increases with 𝜏and
dropsasthedecays at increasing altitude from ground.
In the case of downward-going neutrinos the identica-
tion eciency depends on neutrino avour, type of inter-
action (CC or NC), neutrino energy (𝜈), zenith angle (),
and distance () measured from ground along the shower
Advances in High Energy Physics
100
90
80
70
60
50
40
30
20
10
0 0.5 1 1.5 2 2.5 3 3.5
Trigger eciency (%)
Shower height at 10 km from decay (km)
(a)
100
90
80
70
60
50
40
30
20
10
0 0.5 1 1.5 2 2.5 3 3.5
Trigger eciency (%)
Shower height at 10 km from decay (km)
(b)
100
90
80
70
60
50
40
30
20
10
0 0.5 1 1.5 2 2.5 3 3.5
Trigger eciency (%)
Shower height at 10 km from decay (km)
(c)
100
90
80
70
60
50
40
30
20
10
0 0.5 1 1.5 2 2.5 3 3.5
Trigger eciency (%)
Shower height at 10 km from decay (km)
(d)
F : T trigger (open dots) and identication (closed dots, cuts as in Table ) eciency in the Earth-skimming analysis, as a function
of the height above ground of the shower at km from the decay point 𝑐. e eciency is shown for Monte Carlo showers induced by s
with energy (clockwise from (a)) ., , and EeV. e eciencies are calculated in a full SD array (see text for details).
axis at which the neutrino is forced to interact in the
simulations. An example of the eciency that can be achieved
in a full SD array is shown in Figure .eeciency
is dierent from zero between a minimal depth close to
ground (a minimal amount of matter needed for the -
induced shower to reach a sucient lateral expansion), and
a maximal one (such that the electromagnetic component is
almost extinguished at ground level and hence the neutrino
cannot be identied). e eciency as well as the slice of
atmosphere where it is dierent from zero, typically increase
with neutrino energy, and depend on the neutrino avour
and interaction. As an extreme example, high energy 𝜏
interacting in the atmosphere through the CC channel can be
identied regardless the interaction depth in the atmosphere,
as long as the energetic produced in the interaction decays
andproducesashowerclosetoground.
6.2. Exposure. Ideally,forthecalculationoftheexposure
of the SD of the Auger Observatory to ultrahigh energy
neutrinos, the simulated neutrino showers should be ran-
domly distributed over the actual congurations of the array,
applying to the shower at ground the trigger and neutrino
identication conditions to obtain the active (eective) area
of the array at every second, and as a function of the parame-
ters of the neutrino-induced showers (neutrino energ y, zenith
angle, 𝑐, etc.). A sum over time and integration in solid
anglewouldthenyieldtheexposure (E) to UHE neutrinos
Advances in High Energy Physics
Eciency
Ground
Trigger e ciency
Selection before Fisher
Top of the atmosphere
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
00 500 1000 1500 2000 2500 3000
Trigger and identication eciency for 𝜈𝑒CC channel: 85∘—1 EeV
Slant depth (g cm−2)
𝜈selection (aer Fisher)
F : Fraction of electron neutrinos of energy EeV and =85
∘
triggering the array (solid grey line) and passing the downward-
going analysis cuts in the second column of Table (solid black line)
as a function of the slant depth of the interaction above the ground.
e dashed line represents the fraction of events passing all cuts
except for the cut on the Fisher discriminant F(see Section .).
e eciencies are calculated in a full SD array (see text for details).
in both the Earth-skimming and downward-going neutrino
analyses. During the search periods considered for both
Earth-skimming and downward-going neutrino searches, the
surface detector array of the Pierre Auger Observatory was
growing continuously. Since the number of working stations
and their status are monitored every second, we know with
very good accuracy the SD conguration at any instant as well
as its evolution with time.
In practice, to avoid having to cope with an unaordable
number of congurations, dierent strategies were devised
to calculate in an accurate and less time-consuming manner
the eective area of the SD array to Earth-skimming and
downward-going -induced showers.
For downward-going neutrinos, the calculation of the
exposure involves folding the SD array aperture with the
interaction probability, the identication eciency, and
integrating in time. Changes in the conguration of the array
introduce a dependence of the eciency on the position of
thecoreoftheshower
= (,) in the surface covered by
thearrayandontime.
Assuming a : : avour ratio (as expected due to the
eects of neutrino oscillations during propagation from the
sources), the total exposure can be written as []:
EDG 𝜈
=2
𝑖𝑖𝜈 sin cos 𝑖eff ,,𝜈,,
()
Earth-skimming
(3.5 yr of full auger )
Down-going
(2 yr of full auger )
Tot a l
Exposure
CC 𝑒
CC 𝜇
CC 𝜏
NC 𝑥
CC 𝜏mountains
Exposure (cm2s sr)
𝜈energy (eV)
1017
1016
1015
1014
1013
1017 1018 1019 1020
F : Exposure of the surface detector array of the Pierre
Auger Observatory on the data search periods to Earth-skimming
-induced showers (equivalent to . yr of full Auger) and to
downward-going -induced showers (equivalent to yr of full
Auger).
where the sum runs over the three neutrino avours and
the CC and NC interactions, with 𝑖the corresponding -
nucleon interaction cross-section []andthe nucleon
mass. e integral is performed over the zenith angle ,the
interaction depth of the neutrino (in units of g cm−2), and
the blind search period. 𝑖eff is the eective area of the SD
array given by:
𝑖eff 𝜈,,,=𝑖
,,,𝜈,, ()
where the integral is performed over the core positions
of
the showers.
For the Earth-skimming neutrinos the calculation of the
exposure is described in [].
e exposures obtained for the search periods indi-
cated in Table are plotted in Figure , where for the
downward-going neutrino-induced showers, we also plot the
contribution of the dierent channels (Figure )tothetotal
exposure. Among them we have included the possibility that
downward-going 𝜏sinteract with the mountains surround-
ing the Observatory which provide a dense target for neutrino
interactions.
e exposure to Earth-skimming neutrinos is higher
than that to downward-going neutrinos by a factor between
∼and∼ depending on the neutrino energy, partially
due to the longer search period in the Earth-skimming
analysis ∼. yr of full Auger, compared to ∼. yr in the
case of the downward-going analysis. When normalized to
the same search time, the Earth-skimming channel is still
a factor ∼.–moresensitivewhenintegratedoverthe
whole energy range, mainly due to the larger density of the
Advances in High Energy Physics
T : Main sources of systematic uncertainty and their impact on the Earth-skimming [] and downward-going [] exposures.
Source of uncertainty Earth-skimming Downward-going
Monte Carlo simulation of shower +%, −% +%, −%
-nucleon cross-section +%, −% +%, −%
energy losses +%, −% +%, −%
Topography +%, % —
Earth’s crust where 𝜏interactions can occur, compared to
the atmosphere. e larger number of neutrino avours and
interaction channels that can be identied in the downward-
going analysis, as well as the broader angular range (75∘<
<90
∘compared to 90∘<<95
∘), partly compensate the
dierence.
6.3. Systematic Uncertainties. Several sources of systematic
uncertainty have been carefully considered. Some of them are
directly related to the Monte Carlo simulation of the showers,
that is, generator of the neutrino interaction either in the
Earth or in the atmosphere, parton distribution function, air
shower development, and hadronic model. Others have to
do with the limitations on the theoretical models estimating,
for instance, the interaction cross-section or the energy
loss at high energies. Some of these sources play a dominant
role on the Earth-skimming analysis, while others do on the
downward-going neutrino one.
In both analyses the procedure to incorporate the sys-
tematic uncertainties is the same. Dierent combinations
of the various sources of systematic uncertainty render
dierent values of the exposure, and the nal uncertainty is
incorporated in the value of the limit itself through a semi-
Bayesian extension [] of the Feldman-Cousins approach
[]. In Table we summarize the dominant sources of
systematic uncertainty and their impact on the exposure.
In the Earth-skimming analysis the model of energy loss
for the is the dominant source of uncertainty, since it
determines the energy of the emerging saerpropagationin
the Earth; the impact of this on the downward-going neutrino
analysis is much smaller since energy losses are only
relevant for 𝜏interacting in the mountains, a channel that
is estimated to contribute only ∼% to the total exposure.
e uncertainty on the shower simulation, that stems mainly
from the dierent shower propagation codes and hadronic
interaction models that can be used to model the high energy
collisions in the shower, contributes signicantly in both
cases. e presence of mountains around the Observatory—
which would increase the target for neutrino interactions
in both cases—is explicitly simulated and accounted for
when obtaining the exposure of the SD to downward-going
neutrino-induced showers, and as a consequence does not
contribute directly to the systematic uncertainties. However,
it is not accounted for in the Earth-skimming limit shown
in Table . Instead, we take the topography around the
observatory as a source of systematic uncertainty and we
estimated that accounting for it would have increased the
event rate by ∼% (Table ).
7. Results
We have searched for neutrino candidates over the search
data periods and no events fullling either the Earth-
skimming or the downward-going selection cuts were found.
isallowsustoputlimitstotheUHEdiuseneutrinoux.
7.1. Limits to the Diuse Flux of UHE Neutrinos. Under the
assumption that the UHE neutrino ux Φ() behaves with
neutrino energy as:
Φ()=
=
−2 GeV−1cm−2s−1sr−1,()
the integrated limit on the value of is:
= up
∫𝐸max
𝐸min −2E(),()
where E() is the exposure. e actual value of the upper
limit on the signal events (up ) depends on the number
of observed and expected background events. We recall
here that, according to [], up = 2.44 at % C.L. for
zero candidates and no expected background events. When
systematic uncertainties are included (Section .)thevalue
of up changes.
e nal limits are reported in Table where we give the
normalization obtained in the search periods (indicated in
thesametable)fortheEarth-skimminganddownward-going
searches.
In Figure we show the Earth-skimming and downward-
going integrated neutrino ux which indicate the level of a
diuse neutrino ux assumed to behave with energy as −2,
needed to detect up events with a Poisson probability of ∼
% given the exposure accumulated during the . years for
Earth-skimming neutrinos (. years for downward-going)
of equivalent time of a full SD.
Another way of presenting the results is to display the
upper limit in dierential form. In this procedure we assume
that the diuse neutrino ux behaves as −2 within energy
bins of . width on a decimal logarithmic scale, and is
given by 2.44/(0.5log(10) ⋅ ⋅ E()), assuming again no
background. e dierential limit obtained in this way is
showninFigure for the Earth-skimming and downward-
going cases. We achieve most (∼%) of the sensitivity in
the energy range ∼.– EeV (∼.– EeV) for Earth-
skimming (downward-going) neutrinos. In Figure we also
show several predictions of dierent theoretical models of
Advances in High Energy Physics
Auger down-going (2 yr )
Auger earth-skim. (3.5 yr)
IceCube-40 (333.5 days )
Anita-II (28.5 days )
Cosmogenic models
p, Fermi-LAT
p, evol-FRII
Fe, uniform
𝑘=𝐸
2
𝜈Φ(𝐸𝜈) (GeV cm−2 s−1 sr−1)
1017 1018 1019 1020 1021
10−5
10−6
10−7
10−8
10−9
10−10
10−11
Single flavour neutrino limits (90% CL)
𝐸𝜈(eV)
𝜈limits
F : ick lines represent dierential and integrated upper
limits (at 90% C.L.) to the diuse ux of UHE neutrinos (single
avour assuming equipartition) from the Pierre Auger Observatory
for downward-going (equivalent search period = yr of full
Auger) and Earth-skimming 𝜏(equivalent search period = . yr
of full Auger). Limits from other experiments are also plotted [–
]. All limits have been scaled to single avour. e IceCube
dierential limit is scaled by a factor 1/2 due to the dierent
binning in energy with respect to the Auger dierential limits. in
lines: Expected uxes for three theoretical models of cosmogenic
neutrinos (scaled to single avour when necessary). “p, Fermi-LAT”
[] corresponds to the best t to UHECR spectrum incorporating
the Fermi-LAT bound assuming that the transition from Galactic to
extragalactic CRs takes place at 1019 eV. “p, evol-FRII” [] assumes
the FRII strong source evolution with a pure proton composition,
dip transition model and maximum energy of UHECRs at the
sources 𝑝,max =10
21.5 eV. “Fe, uniform” [] represents an extreme
model assuming an iron rich composition, low 𝑝,max, uniform
evolution of the UHECR sources.
cosmogenic neutrino production [,]. Predictions for
cosmogenic neutrino uxes depend on several unknown
parameters including the evolution with redshi of the
sources and the injected UHECR composition. Given the
uncertainties in these parameters, and in particular the pos-
sible presence of heavy primaries in the UHECR spectrum
[], we have plotted a range of models to illustrate the wide
range of predictions available [].
7.2. Event Rate Predictions. In Table we give the expected
number of events from a diuse ux of cosmogenic neutrinos
(produced in the interaction of cosmic ray protons with
background radiation elds) [], from a model of neutrino
production through the bottom-up mechanism in Active
Galactic Nuclei (AGN) [], and from a theoretical model
[] in which neutrinos are the product of the decay of super-
heavy relic particles of the early stages of the Universe. Opti-
mistic theoretical ux predictions for cosmogenic neutrinos
are within reach of our present sensitivity and some models of
neutrinos produced in accelerating sources are already being
constrained. Exotic models are severely disfavored. Note that
all such top-down models are also tightly constrained by the
limits of the Pierre Auger Obser vatory on the photon fraction
in UHECR [].
8. Summary and Prospects
Inthispaperwehavereviewedthesearchesforastrophysical
sources of ultrahigh energy neutrinos at the Pierre Auger
Observatory [–].
e neutrino detection technique is based on the obser-
vation of extensive air showers induced by downward-
going neutrinos of all avours as they interact with the
atmosphere, and by upward-going 𝜏’s t h r o u gh t h e E art h -
skimming mechanism. ese -induced showers display
characteristic features that allow us their identication in
the overwhelming background of regular UHE hadronic
showers. At ground level, high zenith angle neutrino events
wouldhaveasignicantelectromagneticcomponentleading
to a broad time structure of detected signals in the surface
detector array, in contrast to nucleonic-induced showers.
We have shown that, using Monte Carlo simulations
and training data samples, identication criteria for UHE
neutrinos can be dened and used to perform a blind search
on the remaining data sample. e analysis of the collected
data at the Pierre Auger Observatory until May reveals
no candidate events for either downward-going or Earth-
skimming neutrinos. Based on this negative result, stringent
limits have been placed on the diuse ux of UHE neutrinos.
Even though the Auger Observatory was designed to measure
properties of UHECRs, the limits reported in Table provide
at present one of the most sensitive bounds on neutrinos at
EeV energies, which is the most relevant energy to explore
the predicted uxes of cosmogenic neutrinos.
ere are several lines of work in progress inside the
Auger Collaboration related to the neutrino search which
will be the subject of future reports. Some of the eorts
concentrate on the combination of the downward-going and
Earth-skimming channels into a single analysis. is will
simplify the search procedure and will obviously translate
into an improvement of the diuse neutrino limit. e
extension of the downward-going neutrino search to lower
zenith angles (<75
∘) is also very promising. Exploring
the sky down to ∼60
∘implies a sizeable increase on
theexposureandhenceonthelimitincasenocandidates
arefound.emaindrawbackofdecreasingis that
the atmosphere slant depth reduces and nucleonic-induced
showers look “younger” when arriving at ground, making
their separation from -induced showers more challenging.
On the other hand, the sensitivity to neutrino detection could
also be extended to lower energies by reducing the separation
between SD stations. Monte Carlo studies indicate that using
a conguration of stations similar to the currently existing
“inll” array (∼ stations spaced by m) would lead to
a signicant increase of the neutrino detection probability at
lowerenergies(below.EeV)withrespecttothestandard
Advances in High Energy Physics
T : Number of expected events for several theoretical models of UHE neutrino production, given the exposure of the surface detector
array of the Pierre Auger Observatory to earth-skimming and downward-going neutrinos (Table ).
Model and reference Earth-skimming Downward-going
Cosmogenic (Fermi) []∼. ∼.
AGN []∼. ∼.
Exotic (SH relics) []∼. ∼.
SD array. Nevertheless, due to the small size of the current
inll array, the exposure does not appear to be competitive.
Finally, it is worth mentioning that the sensitivity of the
Pierre Auger Observatory to the detection of UHEsfrom
potential astrophysical point-like sources is being evaluated.
e absence of candidates in the searches for diuse neutrino
uxes described in this report allows us to place limits on the
neutrino uxes coming from sources in the eld of view of
the SD of the Auger Observatory. Preliminary results indicate
that with the SD we are sensitive to a large fraction of the sky
spanning ∼∘in declination [].
Acknowledgments
e successful installation, commissioning, and operation of
thePierreAugerObservatorywouldnothavebeenpossible
without the strong commitment and eort from the technical
and administrative sta in Malarg¨
ue.eauthorsarevery
grateful to the following agencies and organizations for nan-
cial support: Comisi´
on Nacional de Energ´
ıa At´
omica, Fun-
daci´
on Antorchas, Gobierno De La Provincia de Mendoza,
Municipalidad de Malarg¨
ue, NDM Holdings and Valle Las
Le˜
nas, in gratitude for their continuing cooperation over land
access, Argentina; the Australian Research Council; Conselho
Nacional de Desenvolvimento Cient´
ıco e Tecnol´
ogico
(CNPq), Financiadora de Estudos e Projetos (FINEP),
Fundac¸˜
ao de Amparo `
aPesquisadoEstadodeRiodeJaneiro
(FAPERJ), Fundac¸˜
ao de Amparo `
aPesquisadoEstadodeS
˜
ao
Paulo (FAPESP), Minist´
erio de Ciˆ
encia e Tecnologia (MCT),
Brazil; AVCR AVZ and AVZ, GAAV
KJB, MSMT-CR LA, LG, MEB,
MSM, LA, and TACR TA, Czech
Republic; Centre de Calcul INP/CNRS, Centre National de
la Recherche Scientique (CNRS), Conseil R´
egional Ile-de-
France, D´
epartement Physique Nucl´
eaire et Corpusculaire
(PNC-INP/CNRS), D´
epartement Sciences de l’Univers
(SDU-INSU/CNRS), France; Bundesministerium f¨
ur Bil-
dung und Forschung (BMBF), Deutsche Forschungsgemein-
scha (DFG), Finanzministerium Baden-W ¨
urttemberg,
Helmholtz-Gemeinscha Deutscher Forschungszentren
(HGF), Ministerium f¨
ur Wissenscha und Forschung,
Nordrhein-Westfalen, Ministerium f¨
ur Wissenscha,
Forschung und Kunst, Baden-W¨
urttemberg, Germany;
Istituto Nazionale di Fisica Nucleare (INFN), Ministero
dell’Istruzione, dell’Universit`
a e della Ricerca (MIUR), Italy;
Consejo Nacional de Ciencia y Tecnolog´
ıa (CONACYT),
Mexico; Ministerie van Onderwijs, Cultuur en Wetenschap,
Nederlandse Organisatie voor Wetenschappelijk Onderzoek
(NWO), Stichting voor Fundamenteel Onderzoek der
Materie(FOM),eNetherlands;MinistryofScience
and Higher Education, Grants no. N N and
N N , Poland; Fundac¸˜
ao para a Ciˆ
encia e a
Tecnologia, Portugal; Ministry for Higher Education,
Science,andTechnology,SlovenianResearchAgency,
Slovenia; Comunidad de Madrid, Consejer´
ıa de Educaci´
on
delaComunidaddeCastillaLaMancha,FEDERfunds,
Ministerio de Ciencia e Innovaci´
on and Consolider-
Ingenio (CPAN), Xunta de Galicia, Spain; Science
and Technology Facilities Council, UK; Department of
Energy, Contract nos. DE-AC-CH and DE-
FR-ER, National Science Foundation, Grant no.
, e Grainger Foundation, USA; NAFOSTED,
Vietnam; ALFA-EC/HELEN and UNESCO.
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