Long-range pseudorapidity dihadron correlations in d+Au collisions at sNN=200 GeV

Abstract

Dihadron angular correlations in d+Au collisions at sNN=200\sqrt{s_{\rm NN}}=200
GeV are reported as a function of the measured zero-degree calorimeter neutral
energy and the forward charged hadron multiplicity in the Au-beam direction. A
finite correlated yield is observed at large relative pseudorapidity
(Δη\Delta\eta) on the near side (i.e. relative azimuth Δϕ∼0\Delta\phi\sim0). This
correlated yield as a function of Δη\Delta\eta appears to scale with the
dominant, primarily jet-related, away-side (Δϕ∼π\Delta\phi\sim\pi) yield. The
Fourier coefficients of the Δϕ\Delta\phi correlation, Vn=⟨cos⁡nΔϕ⟩V_{n}=\langle\cos
n\Delta\phi\rangle, have a strong Δη\Delta\eta dependence. In addition, it is
found that V1V_{1} is approximately inversely proportional to the mid-rapidity
event multiplicity, while V2V_{2} is independent of it with similar magnitude
in the forward (d-going) and backward (Au-going) directions.

Full text

Long-range pseudorapidity dihadron correlations in d+Au collisions at √sNN = 200 GeV
L. Adamczyk,1J. K. Adkins,21 G. Agakishiev,19 M. M. Aggarwal,32 Z. Ahammed,49 I. Alekseev,17 J. Alford,20
A. Aparin,19 D. Arkhipkin,3E. C. Aschenauer,3G. S. Averichev,19 A. Banerjee,49 R. Bellwied,45 A. Bhasin,18
A. K. Bhati,32 P. Bhattarai,44 J. Bielcik,11 J. Bielcikova,12 L. C. Bland,3I. G. Bordyuzhin,17 J. Bouchet,20
A. V. Brandin,28 I. Bunzarov,19 T. P. Burton,3J. Butterworth,38 H. Caines,53 M. Calder’on de la Barca S’anchez,5
J. M. campbell,30 D. Cebra,5M. C. Cervantes,43 I. Chakaberia,3P. Chaloupka,11 Z. Chang,43 S. Chattopadhyay,49
J. H. Chen,41 X. Chen,23 J. Cheng,46 M. Cherney,10 W. Christie,3M. J. M. Codrington,44 G. Contin,24
H. J. Crawford,4S. Das,14 L. C. De Silva,10 R. R. Debbe,3T. G. Dedovich,19 J. Deng,40 A. A. Derevschikov,34
B. di Ruzza,3L. Didenko,3C. Dilks,33 X. Dong,24 J. L. Drachenberg,48 J. E. Draper,5C. M. Du,23
L. E. Dunkelberger,6J. C. Dunlop,3L. G. Efimov,19 J. Engelage,4G. Eppley,38 R. Esha,6O. Evdokimov,9
O. Eyser,3R. Fatemi,21 S. Fazio,3P. Federic,12 J. Fedorisin,19 Feng,8P. Filip,19 Y. Fisyak,3C. E. Flores,5
L. Fulek,1C. A. Gagliardi,43 D. Garand,35 F. Geurts,38 A. Gibson,48 M. Girard,50 L. Greiner,24 D. Grosnick,48
D. S. Gunarathne,42 Y. Guo,39 S. Gupta,18 A. Gupta,18 W. Guryn,3A. Hamad,20 A. Hamed,43 R. Haque,29
J. W. Harris,53 L. He,35 S. Heppelmann,33 A. Hirsch,35 G. W. Hoffmann,44 D. J. Hofman,9S. Horvat,53
H. Z. Huang,6X. Huang,46 B. Huang,9P. Huck,8T. J. Humanic,30 G. Igo,6W. W. Jacobs,16 H. Jang,22 K. Jiang,39
E. G. Judd,4S. Kabana,20 D. Kalinkin,17 K. Kang,46 K. Kauder,9H. W. Ke,3D. Keane,20 A. Kechechyan,19
Z. H. Khan,9D. P. Kikola,50 I. Kisel,13 A. Kisiel,50 D. D. Koetke,48 T. Kollegger,13 L. K. Kosarzewski,50
L. Kotchenda,28 A. F. Kraishan,42 P. Kravtsov,28 K. Krueger,2I. Kulakov,13 L. Kumar,32 R. A. Kycia,31
M. A. C. Lamont,3J. M. Landgraf,3K. D. Landry,6J. Lauret,3A. Lebedev,3R. Lednicky,19 J. H. Lee,3
X. Li,42 X. Li,3W. Li,41 Z. M. Li,8Y. Li,46 C. Li,39 M. A. Lisa,30 F. Liu,8T. Ljubicic,3W. J. Llope,51
M. Lomnitz,20 R. S. Longacre,3X. Luo,8L. Ma,41 R. Ma,3G. L. Ma,41 Y. G. Ma,41 N. Magdy,52 R. Majka,53
A. Manion,24 S. Margetis,20 C. Markert,44 H. Masui,24 H. S. Matis,24 D. McDonald,45 K. Meehan,5N. G. Minaev,34
S. Mioduszewski,43 B. Mohanty,29 M. M. Mondal,43 D. A. Morozov,34 M. K. Mustafa,24 B. K. Nandi,15
Md. Nasim,6T. K. Nayak,49 G. Nigmatkulov,28 L. V. Nogach,34 S. Y. Noh,22 J. Novak,27 S. B. Nurushev,34
G. Odyniec,24 A. Ogawa,3K. Oh,36 V. Okorokov,28 D. L. Olvitt Jr.,42 B. S. Page,16 Y. X. Pan,6Y. Pandit,9
Y. Panebratsev,19 T. Pawlak,50 B. Pawlik,31 H. Pei,8C. Perkins,4A. Peterson,30 P. Pile,3M. Planinic,54 J. Pluta,50
N. Poljak,54 K. Poniatowska,50 J. Porter,24 M. Posik,42 A. M. Poskanzer,24 N. K. Pruthi,32 J. Putschke,51
H. Qiu,24 A. Quintero,20 S. Ramachandran,21 R. Raniwala,37 S. Raniwala,37 R. L. Ray,44 H. G. Ritter,24
J. B. Roberts,38 O. V. Rogachevskiy,19 J. L. Romero,5A. Roy,49 L. Ruan,3J. Rusnak,12 O. Rusnakova,11
N. R. Sahoo,43 P. K. Sahu,14 I. Sakrejda,24 S. Salur,24 A. Sandacz,50 J. Sandweiss,53 A. Sarkar,15 J. Schambach,44
R. P. Scharenberg,35 A. M. Schmah,24 W. B. Schmidke,3N. Schmitz,26 J. Seger,10 P. Seyboth,26 N. Shah,6
E. Shahaliev,19 P. V. Shanmuganathan,20 M. Shao,39 M. K. Sharma,18 B. Sharma,32 W. Q. Shen,41 S. S. Shi,24
Q. Y. Shou,41 E. P. Sichtermann,24 R. Sikora,1M. Simko,12 M. J. Skoby,16 N. Smirnov,53 D. Smirnov,3
D. Solanki,37 L. Song,45 P. Sorensen,3H. M. Spinka,2B. Srivastava,35 T. D. S. Stanislaus,48 R. Stock,13
M. Strikhanov,28 B. Stringfellow,35 M. Sumbera,12 B. J. Summa,33 Y. Sun,39 Z. Sun,23 X. M. Sun,8X. Sun,24
B. Surrow,42 D. N. Svirida,17 M. A. Szelezniak,24 J. Takahashi,7A. H. Tang,3Z. Tang,39 T. Tarnowsky,27
A. N. Tawfik,52 J. H. Thomas,24 A. R. Timmins,45 D. Tlusty,12 M. Tokarev,19 S. Trentalange,6R. E. Tribble,43
P. Tribedy,49 S. K. Tripathy,14 B. A. Trzeciak,11 O. D. Tsai,6T. Ullrich,3D. G. Underwood,2I. Upsal,30
G. Van Buren,3G. van Nieuwenhuizen,25 M. Vandenbroucke,42 R. Varma,15 A. N. Vasiliev,34 R. Vertesi,12
F. Videbaek,3Y. P. Viyogi,49 S. Vokal,19 S. A. Voloshin,51 A. Vossen,16 Y. Wang,8F. Wang,35 H. Wang,3
J. S. Wang,23 G. Wang,6Y. Wang,46 J. C. Webb,3G. Webb,3L. Wen,6G. D. Westfall,27 H. Wieman,24
S. W. Wissink,16 R. Witt,47 Y. F. Wu,8Z. Xiao,46 W. Xie,35 K. Xin,38 Z. Xu,3Q. H. Xu,40 N. Xu,24 H. Xu,23
Y. F. Xu,41 Y. Yang,8C. Yang,39 S. Yang,39 Q. Yang,39 Y. Yang,23 Z. Ye,9P. Yepes,38 L. Yi,35 K. Yip,3
I. -K. Yoo,36 N. Yu,8H. Zbroszczyk,50 W. Zha,39 J. B. Zhang,8X. P. Zhang,46 S. Zhang,41 J. Zhang,23 Z. Zhang,41
Y. Zhang,39 J. L. Zhang,40 F. Zhao,6J. Zhao,8C. Zhong,41 L. Zhou,39 X. Zhu,46 Y. Zoulkarneeva,19 and M. Zyzak13
(STAR Collaboration)
1AGH University of Science and Technology, Cracow 30-059, Poland
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Brookhaven National Laboratory, Upton, New York 11973, USA
4University of California, Berkeley, California 94720, USA
5University of California, Davis, California 95616, USA
arXiv:1502.07652v1 [nucl-ex] 26 Feb 2015

2
6University of California, Los Angeles, California 90095, USA
7Universidade Estadual de Campinas, Sao Paulo 13131, Brazil
8Central China Normal University (HZNU), Wuhan 430079, China
9University of Illinois at Chicago, Chicago, Illinois 60607, USA
10Creighton University, Omaha, Nebraska 68178, USA
11Czech Technical University in Prague, FNSPE, Prague, 115 19, Czech Republic
12Nuclear Physics Institute AS CR, 250 68 ˇ
Reˇz/Prague, Czech Republic
13Frankfurt Institute for Advanced Studies FIAS, Frankfurt 60438, Germany
14Institute of Physics, Bhubaneswar 751005, India
15Indian Institute of Technology, Mumbai 400076, India
16Indiana University, Bloomington, Indiana 47408, USA
17Alikhanov Institute for Theoretical and Experimental Physics, Moscow 117218, Russia
18University of Jammu, Jammu 180001, India
19Joint Institute for Nuclear Research, Dubna, 141 980, Russia
20Kent State University, Kent, Ohio 44242, USA
21University of Kentucky, Lexington, Kentucky, 40506-0055, USA
22Korea Institute of Science and Technology Information, Daejeon 305-701, Korea
23Institute of Modern Physics, Lanzhou 730000, China
24Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
25Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA
26Max-Planck-Institut fur Physik, Munich 80805, Germany
27Michigan State University, East Lansing, Michigan 48824, USA
28Moscow Engineering Physics Institute, Moscow 115409, Russia
29National Institute of Science Education and Research, Bhubaneswar 751005, India
30Ohio State University, Columbus, Ohio 43210, USA
31Institute of Nuclear Physics PAN, Cracow 31-342, Poland
32Panjab University, Chandigarh 160014, India
33Pennsylvania State University, University Park, Pennsylvania 16802, USA
34Institute of High Energy Physics, Protvino 142281, Russia
35Purdue University, West Lafayette, Indiana 47907, USA
36Pusan National University, Pusan 609735, Republic of Korea
37University of Rajasthan, Jaipur 302004, India
38Rice University, Houston, Texas 77251, USA
39University of Science and Technology of China, Hefei 230026, China
40Shandong University, Jinan, Shandong 250100, China
41Shanghai Institute of Applied Physics, Shanghai 201800, China
42Temple University, Philadelphia, Pennsylvania 19122, USA
43Texas A&M University, College Station, Texas 77843, USA
44University of Texas, Austin, Texas 78712, USA
45University of Houston, Houston, Texas 77204, USA
46Tsinghua University, Beijing 100084, China
47United States Naval Academy, Annapolis, Maryland, 21402, USA
48Valparaiso University, Valparaiso, Indiana 46383, USA
49Variable Energy Cyclotron Centre, Kolkata 700064, India
50Warsaw University of Technology, Warsaw 00-661, Poland
51Wayne State University, Detroit, Michigan 48201, USA
52World Laboratory for Cosmology and Particle Physics (WLCAPP), Cairo 11571, Egypt
53Yale University, New Haven, Connecticut 06520, USA
54University of Zagreb, Zagreb, HR-10002, Croatia
Dihadron angular correlations in d+Au collisions at √sNN = 200 GeV are reported as a function
of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity
in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (∆η)
on the near side (i.e. relative azimuth ∆φ∼0). This correlated yield as a function of ∆ηappears to
scale with the dominant, primarily jet-related, away-side (∆φ∼π) yield. The Fourier coefficients of
the ∆φcorrelation, Vn=hcos n∆φi, have a strong ∆ηdependence. In addition, it is found that V1is
approximately inversely proportional to the mid-rapidity event multiplicity, while V2is independent
of it with similar magnitude in the forward (d-going) and backward (Au-going) directions.
PACS numbers: 25.75.-q, 25.75.Dw
Relativistic heavy-ion collisions are used to study
quantum chromodynamics (QCD) at high energy den-
sities at the Relativistic Heavy Ion Collider (RHIC) and
the Large Hadron Collider (LHC) [1–5]. Final-state par-

3
ticle emission in such collisions is anisotropic, quantita-
tively consistent with hydrodynamic flow resulting from
the initial-state overlap geometry [6, 7]. Two-particle
correlations are widely used to measure anisotropic flow
and jet-like correlations [8]. A near-side long-range cor-
relation (at small relative azimuth ∆φand large rela-
tive pseudorapidity ∆η), called the “ridge,” has been ob-
served after elliptic flow subtraction in central heavy-ion
collisions at RHIC and the LHC [9–14]. It is attributed
primarily to triangular flow, resulting from a hydrody-
namic response to initial geometry fluctuations [15, 16].
As reference, p+p,p+A and d+Au collisions are of-
ten used to compare with heavy-ion collisions. Hydro-
dynamics is not expected to describe these small-system
collisions. However, a large ∆ηridge has been observed
in high-multiplicity p+p[17] and p+Pb [18–21] colli-
sions at the LHC after a uniform background subtrac-
tion. The similarity to the heavy-ion ridge is suggestive
of a hydrodynamic description of its origin, in conflict
with early expectations. Indeed, hydrodynamic calcula-
tions with event-by-event fluctuations can describe the
observed ridge and attribute it to elliptic flow [22, 23].
Other physics mechanisms are also possible, such as the
color glass condensate where the two-gluon density is en-
hanced at small ∆φover a wide range of ∆η[24–26], or
quantum initial anisotropy [27].
Furthermore, a back-to-back ridge is revealed by sub-
tracting dihadron correlations in low-multiplicity p+Pb
from those in high-multiplicity collisions at the LHC [19–
21]. A similar double ridge is observed in d+Au collisions
at RHIC by PHENIX within 0.48 <|∆η|<0.70 us-
ing the same subtraction technique [28]. A recent STAR
analysis has challenged the assumption of this subtrac-
tion procedure that jet-like correlations are equal in high-
and low-multiplicity events [29]. It was shown that the
double ridge at these small-to-moderate ∆ηhas a sig-
nificant contribution from residual jet-like correlations
despite performing event selections via forward multi-
plicities [29]. A recent PHENIX study of large ∆ηcor-
relations, without relying on the subtraction technique,
suggests a long-range correlation consistent with hydro-
dynamic anisotropic flow [30]. In order to further un-
derstand the underlying physics mechanism, here in this
Letter, we present our results on long-range (large ∆η)
correlations in d+Au collisions at √sNN = 200 GeV as a
function of ∆ηand the event multiplicity. The large ac-
ceptance of the STAR detector is particularly well suited
for such an analysis over a wider range in ∆η.
The data were taken during the d+Au run in 2003 by
the STAR experiment [31, 32]. The details of the STAR
detector can be found in Ref. [33]. Minimum-bias d+Au
events were triggered by coincidence of signals from the
Zero Degree Calorimeters (ZDC) [34] and the Beam-
Beam Counters (BBC) [33]. Particle tracks were recon-
structed in the Time Projection Chamber (TPC) [35] and
the forward TPC (FTPC) [36]. The primary vertex was
determined from reconstructed tracks. In this analysis,
events were required to have a primary vertex position
|zvtx|<50 cm from the TPC center along the beam axis.
TPC(FTPC) tracks were required to have at least 25(5)
out of the maximum possible 45(10) hits and a distance
of closest approach to the primary vertex within 3 cm.
Three measurements were used to select d+Au events:
neutral energy by the ZDC and charged particle multi-
plicity within −3.8< η < −2.8 by the FTPC [31, 32],
both in the Au-beam direction, and charged particle
multiplicity within |η|<1 by TPC. Weak but posi-
tive correlations were observed between these measure-
ments; the same event fraction defined by these measures
corresponded to significantly different d+Au event sam-
ples [29]. In this work we study 0-20% high-activity and
40-100% low-activity collisions according to each mea-
sure.
The pairs of particles used in dihadron correlations are
customarily called the trigger and the associated particle.
Two sets of dihadron correlations are analyzed: TPC-
TPC correlations where both the trigger and associated
particles are from the TPC (|η|<1), and TPC-FTPC
correlations where the trigger particle is from the TPC
but the associated particle is from either the FTPC-Au
(−3.8< η < −2.8) or FTPC-d(2.8< η < 3.8). The
pTranges of the trigger and associated particles are both
1< pT<3 GeV/c. The associated particle yields are
normalized per trigger particle. The yields are corrected
for the TPC and FTPC associated particle tracking ef-
ficiencies of 85% ±5% (syst.) and 70% ±5% (syst.), re-
spectively, which do not depend on the event activity in
d+Au collisions [31, 32].
The detector non-uniformity in ∆φis corrected by the
event-mixing technique, where a trigger particle from one
event is paired with associated particles from another
event. The mixed events are required to be within 1 cm in
zvtx, with the same multiplicity (by FTPC-Au or TPC)
or similar energy (by ZDC-Au). The mixed-event corre-
lations are normalized to 100% at ∆η= 0 for TPC, and
at ±3.3 for FTPC-dand FTPC-Au associated particles,
respectively.
Two analysis approaches are taken. One is to analyze
the correlated yields after subtracting a uniform combi-
natorial background. The background normalization is
estimated by the Zero-Yield-At-Minimum (ZYAM) as-
sumption [9, 37]. ZYAM is taken as the lowest yield
averaged over a ∆φwindow of π/8 radian width, after
the correlated yield distribution is folded into the range
of 0 <∆φ<π. The ZYAM systematic uncertainty is
estimated by the yields averaged over windows of half
and three half the width. We also fit the ∆φcorrela-
tions by two Gaussians (with centroids fixed at 0 and π)
plus a pedestal. The fitted pedestal is consistent with
ZYAM within the statistical and systematic errors be-
cause the near- and away-side peaks are well separated
in d+Au collisions. The systematic uncertainties on the

4
correlated yields are taken as the quadratic sum of the
ZYAM and tracking efficiency systematic uncertainties.
The other approach is to analyze the Fourier coefficients
of the ∆φcorrelation functions, Vn=hcos n∆φi. No
background subtraction is required. Systematic uncer-
tainties on the Fourier coefficients are estimated, by vary-
ing analysis cuts, to be less than 10% for V1and V2, and
smaller than the statistical errors for V3.
Figure 1 shows the ZYAM-subtracted correlated yields
as a function of ∆φin ZDC-Au low- and high-activity
d+Au collisions. The TPC-TPC correlation at large ∆η
is shown in panel (a), whereas the TPC-FTPC correla-
tions are shown in panels (b) and (c) for Au- and d-going
directions, respectively. The ZYAM statistical error is
included as part of the systematic uncertainty drawn in
Fig. 1 because it is common to all ∆φbins. No difference
is observed in TPC-TPC correlations between positive
and negative ∆η, so they are combined in Fig 1(a). The
away-side correlated yields are found to be larger in high-
than low-activity d+Au collisions for TPC and FTPC-Au
correlations. The opposite behavior is observed for the
FTPC-dcorrelations, Fig. 1(c).
On the near side, the correlated yields are consistent
with zero in the low-activity events and, in FTPC-d, in
the high-activity events as well. (Note that the yield
value cannot be negative because of the ZYAM assump-
tion.) In contrast, in TPC and FTPC-Au, finite corre-
lated yields are observed in high-activity events. A sim-
ilar result was observed by PHENIX [30]. In Fig. 1, the
event activity is determined by ZDC-Au. For event ac-
tivity determined by FTPC-Au or TPC multiplicity, the
data are qualitatively similar. In Table I, the correlated
yields integrated over the near side (|∆φ|< π/3) and
the away side (|∆φ−π|< π/3), normalized by the in-
tegration range, are tabulated together with the ZYAM
magnitude for low- and high-activity events determined
by the various measures.
For trigger particles in our pTrange of 1 < pT<
3 GeV/c, the away-side correlation in d+Au collisions is
expected to be dominated by jet-like correlations [38]. In-
specting the near-side correlation amplitude at large ∆η,
any possible non-jet, e.g. anisotropic flow, contributions
on the away side should be order of magnitude smaller.
Perhaps the observed away-side dependence on ZDC-Au
event activity arises from a correlation between jet pro-
duction and the forward beam remnants. Or, the under-
lying physics may be more complex; for example the op-
posite away-side trends in the Au- and d-going directions
may arise from different underlying parton distributions
in high- and low-activity collisions. The finite correlated
yield on the near side is, on the other hand, rather sur-
prising because jet-like contributions should be minimal
at these large ∆ηdistances. Hijing simulation [38] of
d+Au collisions indicates that jet correlations within our
pTrange after ZYAM background subtraction is consis-
tent with zero at |∆η|>1.5.
To study the ∆ηdependence of the correlated yields
in the TPC and FTPC, the correlation data are divided
into multiple ∆ηbins. In Fig. 2(a), the near- and away-
side correlated yields are shown as a function of ∆η. To
avoid auto-correlations, we have used ZDC-Au for event
selections for both the TPC and FTPC correlation data.
Unlike in Fig. 1, the ZYAM statistical errors are depen-
dent of ∆ηand are therefore included in the statistical
error bars of the data points. The away-side correla-
tion shape, noticeably concaved for TPC, is presumably
determined by the underlying parton-parton scattering
kinematics. On the near side, finite correlated yields are
observed at large ∆ηon the Au-going side in all bins,
while the yields are consistent with zero on the d-going
side. As aforementioned, similar results have been previ-
ously observed in heavy-ion [9–14], p+p[17], and p+Pb
collisions [18–21]. There, the trigger and associated par-
ticles were taken from the same ηregion. As a result, the
correlated yields were approximately uniform in ∆η[39],
and were dubbed the “ridge.” In the three groups of cor-
relation data in Fig. 2(a), the trigger particles come from
the TPC, but the associated particles come from different
ηregions. Significant differences in pair kinematics result
in the steps at ∆η=±2 even though their ∆ηgaps are
similar. Despite this, for simplicity, we refer to the large
∆ηcorrelated yields in our data also as the “ridge.”
In order to elucidate the formation mechanism of the
ridge, we study in Fig. 2(b) the ratio of the near- to
away-side correlated yields. Because the ZYAM value is
common for the near and away side, its statistical error
is included as part of the systematic uncertainty; this
part of the systematic uncertainty is uncorrelated be-
tween ∆ηbins. While the large peak at ∆η∼0 is due
to the near-side jet, the ratio at ∆η < −1 is rather in-
sensitive to ∆η, whether the correlations are from TPC
or FTPC-Au. A linear fit (dashed-line in Fig. 2(b)) to
those data points at ∆η < −1 yields a slope parameter
of −0.023 ±0.019+0.020
−0.010 with χ2/ndf = 2.6/3, indicating
that the ratio is consistent with a constant within one
standard deviation. The rather constant ratio is remark-
able, given the nearly order of magnitude difference in
the away-side jet-like correlated yields across ∆η=−2
due to the vastly different pair kinematics. Since the
away-side correlated yields are dominated by jets [38],
the finite, ∆η-independent ratio at ∆η < −1 may sug-
gest a connection between the near-side ridge and jet
production, even though any possible jet contribution to
the near-side ridge at |∆η|>1 should be minimal. On
the other hand, the near-side ridge does not seem to scale
with the ZYAM value, which represents the underlying
background. A linear fit to the ratio of the near-side cor-
related yield over ZYAM in the same ∆η < −1 region
gives a slope parameter of 6.5±1.6+3.7
−2.1×10−3, signifi-
cantly deviating from zero.
The correlated yields discussed above are subject to the
ZYAM background subtraction. Another way to quantify

5
φ∆dη∆N/d
2
) d
trig
(1/N
0 1 2 3
0
20
40
60
-3
10×
ZDC 0-20%
ZDC 40-100%
|<1.8η∆(a) TPC-TPC, 1.2<|
<3 GeV/c
T
1<p
φ∆
0 1 2 3
0
5
10
-3
10×
ZDC 0-20%
ZDC 40-100%
<-2η∆(b) TPC-FTPC-Au, -4.5<
<3 GeV/c
T
1<p
φ∆
0 1 2 3
0
2
4
6-3
10×
ZDC 0-20%
ZDC 40-100%
<4.5η∆(c) TPC-FTPC-d, 2<
<3 GeV/c
T
1<p
φ∆
FIG. 1: Correlated dihadron yield, per radian per unit of pseudorapidity, as a function of ∆φin three ranges of ∆ηin d+Au
collisions. Shown are both low and high ZDC-Au activity data. Both the trigger and associated particles have 1 < pT<3 GeV/c.
The arrows indicate ZYAM normalization positions. The error bars are statistical and histograms indicate the systematic
uncertainties.
TABLE I: Near- (|∆φ|< π/3) and away-side (|∆φ−π|< π/3) correlated yields and ZYAM background magnitude, per radian
per unit of pseudorapidity, at large ∆ηin low- and high-activity d+Au collisions. Positive(negative) ηcorresponds to d(Au)-
going direction. Both the trigger and associated particles have 1 < pT<3 GeV/c. All numbers have been multiplied by 104.
Errors are statistical except the second error of each ZYAM value which is systematic and applies also to the corresponding
near- and away-side yields. An additional 5% efficiency uncertainty applies.
Event Event 1.2<|∆η|<1.8 Event −4.5<∆η < −2 2 <∆η < 4.5
activity selection ZYAM near away selection ZYAM near away ZYAM near away
40-100% ZDC 1896±7+1
−13 10±4 346±5 ZDC 978±2+1
−22±1 55±1 361±1+1
−21±1 38±1
0-20% 3043±11+15
−26 53±7 456±7 1776±4+2
−110±2 70±2 438±2+1
−21±1 31±1
40-100% FTPC 1324±7+2
−67±4 347±5 TPC 636±2+1
−26±1 59±1 309±2+1
−13±1 45±1
0-20% 3468±10+7
−543±6 429±7 1899±3+2
−515±2 75±2 445±1+1
−32±1 27±1
the ridge is via Fourier coefficients of the azimuthal cor-
relation functions without background subtraction. Fig-
ure 3 shows the second harmonic Fourier coefficient (V2)
as a function of ∆ηfor both high and low ZDC-Au energy
collisions. The V2values are approximately the same in
high- and low-activity collisions at large ∆η. Both de-
crease with increasing |∆η|from the small ∆η, jet dom-
inated, region to the large ∆η, ridge, region by nearly
one order of magnitude. The ∆ηbehavior of V2, a mea-
sure of modulation relative to the average, is qualitatively
consistent with the ∆η-dependent ratio of the near-side
correlated yield over ZYAM. One motivation to analyze
correlation data using Fourier coefficients is their inde-
pendence of a ZYAM subtraction procedure. One way for
V2to develop is through final-state interactions which, if
prevalent enough, may be described in terms of hydro-
dynamic flow. If V2is strictly of a hydrodynamic elliptic
flow origin, the data would imply a decreasing collective
effect at backward/forward rapidities that is somehow
independent of the activity level of the events.
To gain further insights, the multiplicity dependencies
of the first, second and third Fourier coefficients V1,V2
and V3are shown in Fig. 4. Three ∆ηranges are pre-
sented for FTPC-Au, TPC, and FTPC-dcorrelations, re-
spectively. Results by both the ZDC-Au and FTPC-Au
event selections are shown, plotted as a function of the
corresponding measured charged particle pseudorapidity
density at mid-rapidity dNch/dη. The absolute value of
the V1parameter in each ∆ηrange varies approximately
as (dNch/dη)−1(see the superimposed curves). This is
consistent with jet contributions and/or global statistical
momentum conservation. On the other hand, the V2pa-
rameter in each ∆ηrange is approximately independent
of dNch/dη over the entire measured range (the dashed
lines are to guide the eye). Similar behavior of V2is
also observed in p+Pb collisions at the LHC [13, 40, 41].
Figure 4 shows that the V3values are small and mostly
consistent with zero, except for TPC-TPC correlation at
the lowest multiplicity.
In d+Au collisions, dihadron correlations are domi-
nated by jets, even at large ∆η, where the away-side
jet contributes [38]. The behavior of V1suggests that
the jet contribution to Vnis diluted by the multiplicity.
The similar V2values and ∆ηdependencies in different
multiplicity collisions are, therefore, rather surprising.
In order to accommodate a hydrodynamic contribution,
there must be a coincidental compensation of the reduced
jet contribution with increasing multiplicity, over the en-

6
-4 -2 0 2 4
-3
10
-2
10
-1
10
1
Near-side Away-side
<3 GeV/c
T
(a) ZDC 0-20%, 1<p
φ∆dη∆N/d
2
) d
trig
(1/N
-4 -2 0 2 4
-1
10
1
ZDC 0-20%
<3 GeV/c
T
(b) 1<p
Near/Away yield ratio
η∆
FIG. 2: The ∆ηdependence of (a) the near- (|∆φ|< π/3)
and away-side (|∆φ−π|< π/3) correlated yields, and (b)
the ratio of the near- to away-side correlated yields in d+Au
collisions. Positive(negative) ηcorresponds to d(Au)-going
direction. Only high ZDC-Au activity data are shown. The
error bars are statistical and histograms indicate the system-
atic uncertainties (for ∆η > 2 in (b) the lower bound falls
outside the plot). The dashed curve in (b) is a linear fit to
the ∆η < −1 data points.
-4 -2 0 2 4
-2
10
-1
10 ZDC 40-100%
ZDC 0-20%
<3 GeV/c
T
1<p
η∆
2
V
FIG. 3: The ∆ηdependence of the second harmonic Fourier
coefficient, V2, in low and high ZDC-Au activity d+Au colli-
sions. The error bars are statistical. Systematic uncertainties
are 10% and are shown by the histograms, for clarity, only for
the high-activity data.
tire measured multiplicity range, by an emerging, non-jet
contribution, such as elliptic flow.
Whether or not a finite correlated yield appears on
the near side depends on the interplay between V1and
V2(higher order terms are negligible). Although the V2
parameters are similar, the significantly more negative
V1in low- versus high-multiplicity events eliminates the
near-side V2peak in ∆φ. The same applies also to the
TPC-FTPC correlation comparison between the Au- and
d-going directions. The V2values are rather similar for
FTPC-d(forward rapidity) and FTPC-Au (backward ra-
pidity) correlations, but the more negative V1for d-going
direction eliminates the near-side V2peak. If the relevant
physics in d+Au collisions is governed by hydrodynam-
ics, then it may not carry significance whether or not
there exists a finite near-side long-range correlated yield,
which would be a simple manifestation of the relative V1
and V2strengths.
Our V2data are qualitatively consistent with that from
PHENIX [30]. While PHENIX focused on the pTdepen-
dence, we study the Fourier coefficients as a function of
∆ηafforded by the large STAR acceptance, as well as the
event multiplicity. Hydrodynamic effects, if they exist in
d+Au collisions, should naively differ over the measured
multiplicity range and between Au- and d-going direc-
tions. However, the V2parameters are approximately
constant over multiplicity, and quantitatively similar be-
tween the Au- and d-going directions. On the other
hand, the correlation comparisons between low- and high-
activity data reveal different trends for the Au- and d-
going directions. The high- and low-activity difference in
the FTPC-Au correlation in Fig. 1(b) may resemble ellip-
tic flow, but that in the FTPC-dcorrelation in Fig. 1(c)
is far from an elliptic flow shape. In combination, these
data suggest that the finite values of Vncannot be ex-
clusively explained by hydrodynamic anisotropic flow in
d+Au collisions at RHIC.
In summary, dihadron angular correlations are re-
ported for d+Au collisions at √sNN = 200 GeV as a
function of the event activity from the STAR experi-
ment. The event activity is classified by the measured
zero-degree neutral energy in ZDC, the charged hadron
multiplicity in FTPC, both in the Au-going direction,
or the multiplicity in TPC. In a recent paper we have
shown that the short-range jet-like correlated yield in-
creases with the event activity [29]. In this paper we
focus on long-range correlations at large |∆η|, where jet-
like contributions are minimal on the near side, although
the away side is still dominated by jet production. Two
approaches are taken, one to extract the correlated yields
above a uniform background estimated by the ZYAM
method, and the other to calculate the Fourier coeffi-
cients, Vn=hcos n∆φi, of the dihadron ∆φcorrelations.
The following points are observed: (i) The away-side cor-
related yields are larger in high- than in low-activity col-
lisions in the TPC and FTPC-Au, but lower in FTPC-

7
5 10 15 20
-0.06
-0.04
-0.02
0(a)
1
Fourier coefficient V
5 10 15 20
0.01
0.02
(b)
2
Fourier coefficient V
5 10 15 20
-0.01
-0.005
0(c)
η/d
ch
Uncorrected mid-rapidity dN
3
Fourier coefficient V
|<1.8)η∆TPC (1.2<| <-2)η∆FTPC-Au (-4.5<
<4.5)η∆FTPC-d (2<
Open: ZDC event activity
Filled: FTPC event activity
FIG. 4: Fourier coefficients (a) V1, (b) V2, and (c) V3versus
the measured mid-rapidity charged particle dNch/dη. Event
activity selections by both ZDC-Au and FTPC-Au are shown.
Trigger particles are from TPC, and associated particles from
TPC (triangles), FTPC-Au (circles), and FTPC-d(squares),
respectively. Systematic uncertainties are estimated to be
10% on V1and V2, and smaller than statistical errors for
V3. Errors shown are the quadratic sum of statistical and
systematic errors. The dashed curves are to guide the eye.
d; (ii) Finite near-side correlated yields are observed at
large ∆ηabove the estimated ZYAM background in high-
activity collisions in both the TPC and FTPC-Au (re-
ferred to as the “ridge”); (iii) The ridge yield appears
to scale with the away-side correlated yield at the cor-
responding ∆η < −1, which is dominated by the away-
side jet; (iv) The V2coefficient decreases with increasing
|∆η|, but remains finite at both forward and backward
rapidities (|∆η| ≈ 3) with similar magnitude; (v) The
V1coefficient is approximately inversely proportional to
the event multiplicity, but the V2appears to be indepen-
dent of it. While hydrodynamic elliptic flow is not ex-
cluded with a coincidental compensation of jet dilution
by increasing flow contribution with multiplicity and an
unexpected equality of elliptic flow between forward and
backward rapidities, the data suggest that there exists a
long-range pair-wise correlation in d+Au collisions that
is correlated with dijet production.
We thank the RHIC Operations Group and RCF at
BNL, the NERSC Center at LBNL and the Open Sci-
ence Grid consortium for providing resources and sup-
port. This work was supported in part by the Offices
of NP and HEP within the U.S. DOE Office of Sci-
ence, the U.S. NSF, the Sloan Foundation, the DFG clus-
ter of excellence ‘Origin and Structure of the Universe’
of Germany, CNRS/IN2P3, STFC and EPSRC of the
United Kingdom, FAPESP CNPq of Brazil, Ministry of
Ed. and Sci. of the Russian Federation, NNSFC, CAS,
MoST, and MoE of China, GA and MSMT of the Czech
Republic, FOM and NWO of the Netherlands, DAE,
DST, and CSIR of India, Polish Ministry of Sci. and
Higher Ed., Korea Research Foundation, Ministry of Sci.,
Ed. and Sports of the Rep. Of Croatia, Russian Ministry
of Sci. and Tech, and RosAtom of Russia.
[1] I. Arsene et al. (BRAHMS Collaboration), Nucl.Phys.
A757, 1 (2005), nucl-ex/0410020.
[2] B. Back et al. (PHOBOS Collaboration), Nucl.Phys.
A757, 28 (2005), nucl-ex/0410022.
[3] J. Adams et al. (STAR Collaboration), Nucl.Phys. A757,
102 (2005), nucl-ex/0501009.
[4] K. Adcox et al. (PHENIX Collaboration), Nucl.Phys.
A757, 184 (2005), nucl-ex/0410003.
[5] B. Muller, J. Schukraft, and B. Wyslouch,
Ann.Rev.Nucl.Part.Sci. 62, 361 (2012), 1202.3233.
[6] J.-Y. Ollitrault, Phys.Rev. D46, 229 (1992).
[7] U. Heinz and R. Snellings, Ann.Rev.Nucl.Part.Sci. 63,
123 (2013), 1301.2826.
[8] F. Wang, Prog. Part. Nucl. Phys. 74, 35 (2014),
1311.4444.
[9] J. Adams et al. (STAR Collaboration), Phys.Rev.Lett.
95, 152301 (2005), nucl-ex/0501016.
[10] B. Abelev et al. (STAR Collaboration), Phys.Rev. C80,
064912 (2009), 0909.0191.
[11] B. Alver et al. (PHOBOS Collaboration), Phys.Rev.Lett.
104, 062301 (2010), 0903.2811.
[12] B. Abelev et al. (STAR Collaboration), Phys.Rev.Lett.
105, 022301 (2010), 0912.3977.
[13] S. Chatrchyan et al. (CMS Collaboration), Phys.Lett.
B724, 213 (2013), 1305.0609.
[14] K. Aamodt et al. (ALICE Collaboration), Phys.Rev.Lett.
107, 032301 (2011), 1105.3865.
[15] B. Alver and G. Roland, Phys.Rev. C81, 054905 (2010),
erratum-ibid. C82, 039903 (2010), 1003.0194.
[16] L. Adamczyk et al. (STAR Collaboration), Phys. Rev.
C88, 014904 (2013), 1301.2187.
[17] V. Khachatryan et al. (CMS Collaboration), JHEP 09,
091 (2010), 1009.4122.
[18] S. Chatrchyan et al. (CMS Collaboration), Phys.Lett.
B718, 795 (2013), 1210.5482.
[19] B. Abelev et al. (ALICE Collaboration), Phys.Lett.
B719, 29 (2013), 1212.2001.
[20] G. Aad et al. (ATLAS Collaboration), Phys.Rev.Lett.
110, 182302 (2013), 1212.5198.
[21] B. B. Abelev et al. (ALICE Collaboration), Phys.Lett.
B726, 164 (2013), 1307.3237.

8
[22] P. Bozek, Eur.Phys.J. C71, 1530 (2011), 1010.0405.
[23] P. Bozek and W. Broniowski, Phys.Lett. B718, 1557
(2013), 1211.0845.
[24] A. Dumitru et al., Phys.Lett. B697, 21 (2011),
1009.5295.
[25] K. Dusling and R. Venugopalan, Phys.Rev. D87, 054014
(2013), 1211.3701.
[26] K. Dusling and R. Venugopalan, Phys.Rev. D87, 094034
(2013), 1302.7018.
[27] D. Molnar, F. Wang, and C. H. Greene (2014), 1404.4119.
[28] A. Adare et al. (PHENIX Collaboration), Phys.Rev.Lett.
111, 212301 (2013), 1303.1794.
[29] L. Adamczyk et al. (STAR Collaboration) (2014), ac-
cepted by Phys.Lett.B, 1412.8437.
[30] A. Adare et al. (PHENIX Collaboration) (2014),
1404.7461.
[31] J. Adams et al. (STAR Collaboration), Phys.Rev.Lett.
91, 072304 (2003), nucl-ex/0306024.
[32] B. Abelev et al. (STAR Collaboration), Phys.Rev. C79,
034909 (2009), 0808.2041.
[33] K. Ackermann et al. (STAR Collaboration),
Nucl.Instrum.Meth. A499, 624 (2003).
[34] C. Adler et al., Nucl.Instrum.Meth. A499, 433 (2003).
[35] M. Anderson et al., Nucl.Instrum.Meth. A499, 659
(2003), nucl-ex/0301015.
[36] K. Ackermann et al., Nucl.Instrum.Meth. A499, 713
(2003), nucl-ex/0211014.
[37] N. Ajitanand et al., Phys.Rev. C72, 011902 (2005), nucl-
ex/0501025.
[38] M. Gyulassy and X.-N. Wang, Comput.Phys.Commun.
83, 307 (1994), nucl-th/9502021.
[39] L. Xu, C.-H. Chen, and F. Wang, Phys.Rev. C88, 064907
(2013), 1304.8120.
[40] G. Aad et al. (ATLAS Collaboration), Phys.Lett. B725,
60 (2013), 1303.2084.
[41] B. B. Abelev et al. (ALICE Collaboration), Phys.Rev.
C90, 054901 (2014), 1406.2474.