Search for W' -> tb resonances with left- and right-handed couplings to fermions D0 Collaboration

Abstract

We present a search for the production of a heavy gauge boson, W', that decays to third-generation quarks, by the D0 Collaboration in ppbar collisions at sqrt(s)= 1.96 TeV. We set 95% confidence level upper limits on the production cross section times branching fraction. For the first time, we set limits for arbitrary combinations of left- and right-handed couplings of the W' boson to fermions. For couplings with the same strength as the standard model W boson, we set the following limits for M(W') > m(nu_R): M(W')>863 GeV for purely left-handed couplings, M(W')>885 GeV for purely right-handed couplings, and M(W')>916 GeV if both left- and right-handed couplings are present. The limit for right-handed couplings improves for M(W') < m(nu_R) to M(W')>890 GeV.

Full text

Physics Letters B 699 (2011) 145–150
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Physics Letters B
www.elsevier.com/locate/physletb
Search for W→tb resonances with left- and right-handed couplings to fermions
D0 Collaboration
V.M. Abazov ai, B. Abbott bt,B.S.Acharya
ac,M.Adams
av,T.Adams
at,G.D.Alexeev
ai,G.Alkhazov
ac,
A. Altonbh,1, G. Alverson bg,G.A.Alves
b,L.S.Ancu
ah,M.Aoki
au,M.Arov
be,A.Askew
at,B.Åsman
an,
O. Atramentov bl, C. Avila h,J.BackusMayes
ca,F.Badaud
m,L.Bagby
au,B.Baldin
au,D.V.Bandurin
at,
S. Banerjee ac,E.Barberis
bg, P. Baringer bc,J.Barreto
c,J.F.Bartlett
au, U. Bassler r,V.Bazterra
av,S.Beale
f,
A. Beanbc, M. Begalli c, M. Begel br, C. Belanger-Champagne an, L. Bellantoni au,S.B.Beri
aa, G. Bernardi q,
R. Bernhard v,I.Bertram
ao,M.Besançon
r, R. Beuselinck ap, V.A. Bezzubov al,P.C.Bhat
au, V. Bhatnagar aa,
G. Blazey aw, S. Blessing at, K. Bloom bk, A. Boehnlein au, D. Boline bq,T.A.Bolton
bd, E.E. Boos ak,
G. Borissov ao, T. Bose bf,A.Brandt
bw,O.Brandt
w,R.Brock
bi, G. Brooijmans bo, A. Bross au,D.Brown
q,
J. Brown q,X.B.Bu
au, M. Buehler bz, V. Buescher x, V. Bunichev ak,S.Burdin
ao,2, T.H. Burnett ca,
C.P. Buszello an,B.Calpas
o, E. Camacho-Pérez af, M.A. Carrasco-Lizarraga bc, B.C.K. Casey au,
H. Castilla-Valdez af, S. Chakrabarti bq,D.Chakraborty
aw,K.M.Chan
ba, A. Chandra by,G.Chen
bc,
S. Chevalier-Théry r,D.K.Cho
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M. Diesburg au, A. Dominguez bk , T. Dorland ca,A.Dubey
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A. Evdokimov br, V.N. Evdokimov al, G. Facini bg,T.Ferbel
bp,F.Fiedler
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H.E. Fisk au,M.Fortner
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br, A. Garcia-Bellido bp,V.Gavrilov
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W. Geist s,W.Geng
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A. Goussiou ca, P.D. Grannis bq,S.Greder
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bq, G. Gutierrez au,
P. Gutier rez bt , A. Haas bo,3, S. Hagopian at,J.Haley
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J. Hays ap,T.Head
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I. Heredia-De La Cruz af, K. Herner bh , M.D. Hildrethba, R. Hirosky bz,T.Hoang
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ap, K. Johns ar,
M. Johnson au, D. Johnston bk, A. Jonckheere au, P. Jonsson ap, J. Joshi aa,A.Juste
au,4, K. Kaadze bd,
E. Kajfasz o,D.Karmanov
ak,P.A.Kasper
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A. Khanovbu, A. Kharchilava bn, Y.N. Kharzheevai,D.Khatidze
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au, J. Lellouch q,L.Li
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au, J. Linnemann bi, V.V. Lipaev al, R. Lipton au,Y.Liu
g,Z.Liu
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ah, A. Melnitchouk bj, D. Menezes aw,P.G.Mercadante
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M. Merkin ak,A.Meyer
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e,
0370-2693/$ – see front matter ©2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physletb.2011.03.066

146 D0 Collaboration / Physics Letters B 699 (2011) 145–150
T. Nunnemann y,G.Obrant
am,J.Orduna
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ap,J.Osta
ba, G.J. Otero y Garzón
a,M.Owen
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M. Padilla as, M. Pangilinan bv, N. Parashar az, V. Parihar bv,S.K.Park
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N. Parua ay,A.Patwa
br, B. Penning au,M.Perfilov
ak,K.Peters
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aq, G. Petrillo bp,P.Pétroff
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R. Piegaia a,J.Piper
bi, M.-A. Pleier br, P.L.M. Podesta-Lermaaf,6,V.M.Podstavkov
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P. Polozov aj,A.V.Popov
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ay, S. Protopopescu br, J. Qian bh ,A.Quadt
w,B.Quinn
bj,
M.S. Rangel b, K. Ranjan ab,P.N.Ratoff
ao,I.Razumov
al,P.Renkel
bx, M. Rijssenbeek bq, I. Ripp-Baudot s,
F. Rizatdinova bu, M. Rominsky au,C.Royon
r,P.Rubinov
au,R.Ruchti
ba,G.Safronov
aj,G.Sajot
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A. Sánchez-Hernández af,M.P.Sanders
y, B. Sanghi au,A.S.Santos
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au,L.Sawyer
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T. Scanlon ap,R.D.Schamberger
bq,Y.Scheglov
am, H. Schellman ax ,T.Schliephake
z, S. Schlobohm
ca,
C. Schwanenberger aq, R. Schwienhorst bi, J. Sekaric bc, H. Severini bt, E. Shabalina w,V.Shary
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j, T. Sinthuprasith bv,V.Sirotenko
au,P.Skubic
bt,P.Slattery
bp,
D. Smirnov ba, K.J. Smith bn,G.R.Snow
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bs,S.Snyder
br, S. Söldner-Rembold aq, L. Sonnenschein u,
A. Sopczak ao, M. Sosebee bw,K.Soustruznik
i,B.Spurlock
bw,J.Stark
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M. Strauss bt,D.Strom
av, L. Stutte au , L. Suter aq,P.Svoisky
bt, M. Takahashi aq, A. Tanasijczuka,W.Taylor
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M. Titov r, V.V. Tokmenin ai,Y.-T.Tsai
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bq, B. Tuchming r,C.Tully
bm,P.M.Tuts
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L. Uvarov am,S.Uvarov
am,S.Uzunyan
aw,R.VanKooten
ay, W.M. van Leeuwenag, N. Varelas av,
E.W. Varnes ar, I.A. Vasilyev al, P. Verdier t, L.S. Vertogradov ai, M. Verzocchi au, M. Vesterinen aq,
D. Vilanova r, P. Vint ap,P.Vokac
j,H.D.Wahl
at, M.H.L.S. Wang bp,J.Warchol
ba,G.Watts
ca, M. Wayneba,
M. Weber au,7, L. Welty-Rieger ax,A.White
bw,D.Wicke
z, M.R.J. Williams ao, G.W. Wilson bc,
S.J. Wimpenny as,M.Wobisch
be, D.R. Wood bg,T.R.Wyatt
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au,C.Xu
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W.-C. Yang aq,T.Yasuda
au, Y.A. Yatsunenko ai,Z.Ye
au,H.Yin
au,K.Yip
br,S.W.Youn
au,J.Yu
bw,
S. Zelitch bz,T.Zhao
ca,B.Zhou
bh,J.Zhu
bh,M.Zielinski
bp,D.Zieminska
ay, L. Zivkovic bv
aUniversidad de Buenos Aires, Buenos Aires, Argentina
bLAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil
cUniversidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
dUniversidade Federal do ABC, Santo André, Brazil
eInstituto de Física Teórica, Universidade Estadual Paulista, São Paulo, Brazil
fSimon Fraser University, Vancouver, British Columbia, and York University, Toronto, Ontario, Canada
gUniversity of Science and Technology of China, Hefei, People’s Republic of China
hUniversidad de los Andes, Bogotá, Colombia
iCharles University, Faculty of Mathematics and Physics, Center for Particle Physics, Prague, Czech Republic
jCzech Technical University in Prague, Prague, Czech Republic
kCenter for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
lUniversidad San Francisco de Quito, Quito, Ecuador
mLPC, Université Blaise Pascal, CNRS/IN2P3, Clermont, France
nLPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France
oCPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France
pLAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France
qLPNHE, Universités Paris VI and VII, CNRS/IN2P3, Paris, France
rCEA, Irfu, SPP, Saclay, France
sIPHC, Université de Strasbourg, CNRS/IN2P3, Strasbourg, France
tIPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, and Université de Lyon, Lyon, France
uIII. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany
vPhysikalisches Institut, Universität Freiburg, Freiburg, Germany
wII. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
xInstitut für Physik, Universität Mainz, Mainz, Germany
yLudwig-Maximilians-Universität München, München, Germany
zFachbereich Physik, Bergische Universität Wuppertal, Wuppertal, Germany
aa Panjab University, Chandigarh, India
ab Delhi University, Delhi, India
ac Tata Institute of Fundamental Research, Mumbai, India
ad University College Dublin, Dublin, Ireland
ae Korea Detector Laboratory, Korea University, Seoul, Republic of Korea
af CINVESTAV, Mexico City, Mexico
ag FOM – Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
ah Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
ai Joint Institute for Nuclear Research, Dubna, Russia
aj Institute for Theoretical and Experimental Physics, Moscow, Russia
ak Moscow State University, Moscow, Russia
al Institute for High Energy Physics, Protvino, Russia
am Petersburg Nuclear Physics Institute, St. Petersburg, Russia
an Stockholm University, Stockholm and Uppsala University, Uppsala, Sweden
ao Lancaster University, Lancaster LA1 4YB, United Kingdom
ap Imperial College London, London SW7 2AZ, United Kingdom
aq The University of Manchester, Manchester M13 9PL, United Kingdom
ar University of Arizona, Tucson, AZ 85721, USA
as University of California Riverside, Riverside, CA 92521, USA

D0 Collaboration / Physics Letters B 699 (2011) 145–150 147
at Florida State University, Tallahassee, FL 32306, USA
au Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
av University of Illinois at Chicago, Chicago, IL 60607, USA
aw Northern Illinois University, DeKalb, IL 60115, USA
ax Northwestern University, Evanston, IL 60208, USA
ay Indiana University, Bloomington, IN 47405, USA
az Purdue University Calumet, Hammond, IN 46323, USA
ba University of Notre Dame, Notre Dame, IN 46556, USA
bb Iowa State University, Ames, IA 50011, USA
bc University of Kansas, Lawrence, KS 66045, USA
bd Kansas State University, Manhattan, KS 66506, USA
be Louisiana Tech University, Ruston, LA 71272, USA
bf Boston University, Boston, MA 02215, USA
bg Northeastern University, Boston, MA 02115, USA
bh University of Michigan, Ann Arbor, MI 48109, USA
bi Michigan State University, East Lansing, MI 48824, USA
bj University of Mississippi, University, MS 38677, USA
bk University of Nebraska, Lincoln, NE 68588, USA
bl Rutgers University, Piscataway, NJ 08855, USA
bm Princeton University, Princeton, NJ 08544, USA
bn State University of New York, Buffalo, NY 14260, USA
bo Columbia University, New York, NY 10027, USA
bp University of Rochester, Rochester, NY 14627, USA
bq State University of New York, Stony Brook, NY 11794, USA
br Brookhaven National Laboratory, Upton, NY 11973, USA
bs Langston University, Langston, OK 73050, USA
bt University of Oklahoma, Norman, OK 73019, USA
bu Oklahoma State University, Stillwater, OK 74078, USA
bv Brown University, Providence, RI 02912, USA
bw University of Texas, Arlington, TX 76019, USA
bx Southern Methodist University, Dallas, TX 75275, USA
by Rice University, Houston, TX 77005, USA
bz University of Virginia, Charlottesville, VA 22901, USA
ca University of Washington, Seattle, WA 98195, USA
article info abstract
Article history:
Received 19 February 2011
Received in revised form 27 March 2011
Accepted 28 March 2011
Available online 31 March 2011
Editor: H. Weerts
Keywords:
Wprime
Resonances
Top qu ark
Bottom quark
Left–right couplings
We present a search for the production of a heavy gauge boson, W, that decays to third-generation
quarks, by D0 Collaboration in p¯
pcollisions at √s=1.96 TeV. We set 95% confidence level upper limits
on the production cross section times branching fraction. For the first time, we set limits for arbitrary
combinations of left- and right-handed couplings of the Wboson to fermions. For couplings with the
same strength as for the standard model Wboson, we set the following limits, assuming that there are
right-handed neutrinos νRfor all three generations with M(W)>m(νR):M(W)>863 GeV for purely
left-handed couplings, M(W)>885 GeV for purely right-handed couplings, and M(W)>916 GeV if
both left- and right-handed couplings are present. The limit for right-handed couplings improves for
M(W)<m(νR)to M(W)>890 GeV.
©2011 Elsevier B.V. All rights reserved.
In the standard model (SM), the charged weak current is medi-
ated by Wbosons. Many models of physics beyond the standard
model predict the existence of additional charged bosons, generally
called W, that are more massive than the Wboson of the stan-
dard model. The chiral structure of the couplings of the Wboson
to fermions depends on the details of the model.
In models of universal extra dimensions [1],theWis the low-
est Kaluza–Klein excitation of the Wboson. It therefore has the
same couplings to fermions as the Wboson and couples exclu-
sively to left-handed fermions. Left–right symmetric models [2]
1Visitor from Augustana College, Sioux Falls, SD, USA.
2Visitor from The University of Liverpool, Liverpool, UK.
3Visitor from SLAC, Menlo Park, CA, USA.
4Visitor from ICREA/IFAE, Barcelona, Spain.
5Visitor from Centro de Investigacion en Computacion – IPN, Mexico City, Mexico.
6Visitor from ECFM, Universidad Autonoma de Sinaloa, Culiacán, Mexico.
7Visitor from Universität Bern, Bern, Switzerland.
add an additional group SU(2)Rto the SM. Thus there are two
bosons, WLand WR, associated with the SM SU(2)Lgroup and
the additional SU(2)Rgroup that would mix to form the physical
states. One of these physical states is identified with the Wbo-
son. Since the Wboson is known experimentally to couple only
to left-handed fermions, the other state, W, would have to al-
most exclusively couple to right-handed fermions. If there exists a
light right-handed neutrino νR,thedecayW→νR(where is
a charged lepton) is allowed. Otherwise, the Wboson can only
decay to quarks, making it harder to observe.
Independent of specific models, the most general, lowest-
dimension Lagrangian for the interaction of a Wboson with
fermion fields fis given by [3]
L=Vijgw
2√2¯
fiγμaR
ij1+γ5+aL
ij1−γ5Wμfj+h.c.,(1)
where aL/R
ij are the left/right-handed couplings of the Wboson
to the fermion doublet fiand fjand gwis the weak coupling

148 D0 Collaboration / Physics Letters B 699 (2011) 145–150
constant of the SM. If the fermions are quarks, the Vij are the
elements of the CKM matrix for quark flavors i,j. If the fermions
are leptons, the Vij are the elements of a 3 ×3 identity matrix.
In this Letter, we report on a search for a Wboson that decays
to a top quark and an anti-bottom quark (or charge conjugates) [3–
5] using 2.3 fb−1of integrated luminosity from proton–antiproton
collisions at √s=1.96 TeV, accumulated by the D0 experiment [6]
at the Fermilab Tevatron Collider between 2002 and 2007. This is
the same dataset used in the observation of single top quark pro-
duction by D0 Collaboration [7].Weusethenotationtb to repre-
sent the sum of the t¯
band the ¯
tb final states. The tb decay channel
is sensitive to Wbosons with left- and right-handed couplings
regardless of the existence of a light right-handed neutrino. For
left-handed couplings, the process p¯
p→W+X→tb +Xinter-
feres with SM single top quark production p¯
p→W∗+X→tb+X.
We set limits on the production cross section for such Wbosons
and translate them into limits on left- and right-handed gauge
couplings. We have previously excluded Wbosons with masses of
M(W)<731 GeV for purely left-handed couplings and M(W)<
739 (768)GeV for the case of purely right-handed couplings with
(without) a light right-handed neutrino [8]. The CDF Collaboration
has published similar limits [9], neglecting the interference with
SM single top quark production. Depending on the Wmass and
couplings, the interference term may contribute as much as (16–
33)% of the total rate [3]. Relative to our earlier publication, we
use more than twice the integrated luminosity and improve the
sensitivity of the analysis to a Wboson signal by using boosted
decision trees (BDT). In Ref. [7] it was demonstrated that the sin-
gle top quark signal, which has an overall signal-to-background
ratio of a few percent, is clearly identifiable by the BDT tech-
nique. For the first time, we perform a general analysis of the
left- and right-handed gauge couplings aL
ij,aR
ij of the Wboson,
0<aL,R
ij <1. For comparison with the earlier work, we also quote
results for three special cases: (1) purely left-handed couplings,
aL
ij =a,aR
ij =0∀i,j; (2) purely right-handed couplings, aL
ij =0,
aR
ij =a∀i,j; and (3) an equal mixture of left- and right-handed
couplings, aL
ij =aR
ij =a∀i,j. Finally, we set limits on the mass
of such Wbosons under the assumption of a “SM-like” coupling
strength, i.e., a=1.
We search for events in which a Wboson is produced and
subsequently decays into tb. The top quark decays to Wb and the
Wbosonisrequiredtodecaytoeνor μν.Wboson decays into τ-
leptons or quarks suffer from an overwhelming background. Thus
our final state contains an electron or muon, missing transverse
momentum from the undetected neutrino, and two jets from the
fragmentation of the two b-quarks. Additional jets may arise from
initial- or final-state radiation. We acquire these events using a
logical OR of many trigger conditions that require a combination
of jets, electrons and muons.
Object selections and definitions are identical to Ref. [7].Events
are selected with exactly one isolated electron with pT>15 GeV
for two-jet events or pT>20 GeV for three- and four-jet events,
and |η|<1.1 or one isolated muon with pT>15 GeV and |η|<
2.0, where ηis the pseudorapidity. The leading jet must have
pT>25 GeV and there must be a total of two, three, or four jets
with pT>15 GeV and |η|<3.4. Jets are defined using a cone
algorithm [10] with a cone radius R=(φ)2+( y)2=0.5,
where yis the rapidity. Missing pTis required to be greater than
20 (25) GeV for two-jet (three- or four-jet) events. To enhance the
signal content of the selected data sample, one or two of the jets
are required to be identified as originating from the fragmentation
of a bquark [11]. We call these jets b-tagged. In order to minimize
the effect of interference with SM single top quark production, we
require the invariant mass, √ˆ
s, of the reconstructed charged lep-
ton, the jets, and missing pTto be greater than 400 GeV.
We simulate the complete chain of Wdecays, taking into
account finite widths and spin correlations between the produc-
tion of resonance states and their decay using the singletop
Monte Carlo package [12] based on the singletop event gener-
ator [13]. We set the top quark mass to 172.5 GeV; the factor-
ization scale is set to M(W)and we use the CTEQ6M parton
distribution functions [14].Thesingletop generator reproduces the
next-to-leading-order (NLO) kinematic distributions, and the inter-
ference term between the Wboson signal and the SM Wboson.
The width of the Wboson varies between 20 and 30 GeV for
Wmasses between 600 and 900 GeV [3,5]. It is 25% smaller with-
out a light right-handed neutrino. The width does not have a sig-
nificant effect on our search because it is smaller than the detector
resolution. Therefore, the only relevant effect of a massive neutrino
is the larger branching fraction for W→tb. Unless mentioned
otherwise we will assume in the following that there is a right-
handed neutrino with M(W)>m(νR). In this case the limits for
right-handed couplings will be weaker than for M(W)<m(νR)
because of the smaller branching fraction for W→tb.Inthe
absence of interference between Wand Wbosons and in the
presence of a light right-handed neutrino, there is no difference
between Wbosons with left- and right-handed couplings for our
search.ToprobeforWbosons with masses above the currently
published limits, we simulate Wbosons at nine mass values from
600 to 1000 GeV.
Background yields are estimated using both Monte Carlo sam-
ples and data. The procedure is identical to that used in Ref. [7].
The multijet background arises from events in which a jet mim-
ics the signature of an isolated charged lepton. It is modeled us-
ing control data samples with non-isolated leptons. The dominant
background for our search originates from W+jets production.
Smaller sources of backgrounds come from t¯
tpair, t-channel sin-
gle top quark (tqb), diboson, and Z+jets events. The diboson
(WW,WZ ,ZZ) backgrounds are modeled using pythia [15].The
other background processes are modeled using the alpgen [16]
event generator and subsequently hadronized using pythia.All
processes except the W+jets and multijets background are nor-
malized to their expected cross sections [17,18]. The relative frac-
tion of W+heavy flavor events (Wb¯
band Wc¯
c) is adjusted by
scaling the cross sections calculated by alpgen to the NLO cross
section by a factor of 1.47 [19]. An additional correction factor of
0.95 ±0.13, derived from data samples with different number of
b-tagged jets, is applied. Similarly, the Wc+light parton cross sec-
tions obtained from alpgen are scaled by a factor of 1.38 to include
NLO effects [19].TheW+jets (including Wb¯
b,Wc¯
c, and Wc) and
multijets yields are obtained by normalizing to the data sample
after subtracting all other backgrounds and before selecting events
with b-tagged jets. At this point in the selection, the data are com-
pletely dominated by background and any contamination from a
Wboson signal is negligible.
The sensitivity of the search is maximized by dividing the data
into 24 independent channels based on lepton flavor (e,μ), jet
multiplicity (2, 3, 4), number of b-tagged jets (1, 2), and two data
collection periods, to take into account different signal acceptances
and signal-to-background ratios [7]. After applying all selections,
we find the event yields for data and backgrounds as shown in
Table 1. The requirement √ˆ
s>400 GeV accepts most of the W
contribution but eliminates most of the Wboson contribution. As
the mass of the Wboson increases, its contribution to the tb fi-
nal state decreases relative to that of the Wboson, and therefore
the overall efficiency for selecting tb final states decreases from
3.5% to 0.5% for Wboson masses between 600 and 1000 GeV.

D0 Collaboration / Physics Letters B 699 (2011) 145–150 149
The expected Wsignal yields, for two representative masses, cor-
responding to the different scenarios for the couplings (aL,aR)are
listed in Table 1.
At this stage, the expected Wboson signal would constitute
only a small fraction of the selected data sample. To improve dis-
crimination, we employ a multivariate discriminant based on BDTs
that separates the Wboson signal from background and thus
enhances the probability to observe Wboson production. We
compute the discriminant independently in each of the 24 chan-
nels. The input variables used to train the BDT discriminants take
into account the kinematic properties and angular correlations of
the reconstructed objects and the topology of the event. A BDT
is trained for each Wboson mass value using the Monte Carlo
sample generated with aL=aR=1. The final discriminant for all
24 channels combined is shown in Fig. 1(a). To represent the dis-
criminant distribution expected from an arbitrary combination of
couplings, we combine samples of Wdecays generated with left-
handed, right-handed and mixed couplings and SM s-channel tb
production based on theoretical expectations from Ref. [3].Back-
ground events preferentially populate the low discriminant region
whereas signal events would be clustered towards high discrimi-
nant values. We observe good agreement between the background
prediction and data in all channels. The data show no deviation
from background-only expectations and we set 95% C.L. upper lim-
its on the cross section for the process W→tb →νbb using
Bayesian statistics [20]. A Poisson distribution for the observed
counts and a flat non-negative prior probability for the signal cross
section are assumed. Systematic uncertainties are taken into ac-
count with Gaussian priors.
The systematic uncertainties that affect the signal and back-
ground models are described in Ref. [21]. The largest sources of
Tabl e 1
Event yields after selection for Wsignal corresponding to the different scenarios
for the couplings (aL,aR), data and SM background with systematic uncertainties.
Process Events
Signals M(W)
850 GeV 900 GeV
SM +W(aL,aR)=(1,0)9.97.0
SM +W(aL,aR)=(0,1)23.618.9
SM +W(aL,aR)=(1,1)21.518.1
Backgrounds
tqb 26.4±2.5
t¯
t424.7±58.4
W+light partons 125.3±10.6
W+heavy flavor 154.2±13.1
Z+jets 26.0±3.2
Dibosons 13.0±1.6
Multijets 60.5±10.8
Total background 830 ±62
Data 831
systematic uncertainties are the jet energy scale calibration and
the modeling of b-tagging performance. Smaller uncertainties arise
from the finite size of the MC samples, the corrections of the flavor
composition of W+jets events, and from the normalization of the
background. The total uncertainty in the background yield varies
between 8% and 10% for the different channels. In determining the
effect of the uncertainties from jet energy scale calibration, b-tag
modeling, and W+jets modeling, we take into account changes in
the shape of the discriminant distributions in addition to normal-
ization effects.
The cross section for single top quark production in the pres-
ence of a Wboson for any set of coupling values can be written
in terms of the cross sections σLfor purely left-handed couplings
(aL,aR)=(1,0),σRfor purely right-handed couplings (aL,aR)=
(0,1),σLR for mixed couplings (aL,aR)=(1,1), and σSM for SM
couplings (aL,aR)=(0,0).Itisgivenby:
σ=σSM +aL
udaL
tb(σL−σR)
+aL
udaL
tb2+aR
udaR
tb2(σR−σSM )
+1
2aL
udaR
tb2+aR
udaL
tb2(σLR −σL−σR+σSM ). (2)
The predicted cross section for SM single top quark production
through s-channel Wboson exchange is σSM =1.12±0.05 pb [18].
The predicted cross sections σL,σR, and σLR as a function of
Wboson mass, taking into account the interference effects [3],
are listed in Table 2. We assume that the couplings to first gen-
eration quarks, aud, which are important for the production of the
Wboson, and the couplings to third generation quarks, atb, which
are important for the decay of the Wboson, are equal. For given
values of aLand aR, the distributions are obtained by combining
the four samples according to Eq. (2).
We vary both aLand aRbetween 0 and 1 in steps of 0.1, for
each Wboson mass value. We consider each of these 121 combi-
nations of aL,aR, and M(W)as a model. For each of these mod-
els, the ratio of the s-channel and the Wcross sections is fixed.
We then vary the total cross section to determine the expected
and observed 95% C.L. upper limits. For the three special cases
(aL,aR)=(1,0),(0,1), and (1,1), the measurements are listed
along with the theoretical cross sections in Table 2. For small W
masses, the cross section limits drop below the cross section for
SM single top quark production in the s-channel, because the data
do not accommodate the large Wcomponent that would have to
be present for these models. The cross section limit reflects the
maximum total cross section that the data can accommodate for
the ratio of s-channel and Wproduction in the particular model.
We can now assume values for any two of the three parameters,
aL,aR,andM(W), and interpolate the cross section limit in the
third parameter value. The value of the third parameter at which
the cross section limit equals the theory cross section [3] repre-
Tabl e 2
NLO production cross section times branching fraction, σ(p¯
p→W/W→tb), and expected and observed 95% C.L. upper limits for different Wboson masses, assuming
M(W)>m(νR),inpb.
M(W)(GeV)(aL,aR)=(0,1)(aL,aR)=(1,0)(aL,aR)=(1,1)
σRobs exp σLobs exp σLR obs exp
600 3.22 0.12 0.31 2.16 0.09 0.22 4.13 0.09 0.21
650 2.37 0.16 0.37 1.43 0.10 0.23 2.62 0.10 0.26
700 1.86 0.32 0.46 1.03 0.18 0.26 1.74 0.15 0.23
750 1.56 0.60 0.64 0.80 0.37 0.36 1.24 0.27 0.28
800 1.38 0.64 0.92 0.68 0.39 0.51 0.95 0.24 0.34
850 1.28 0.85 1.44 0.61 0.48 0.78 0.78 0.28 0.46
900 1.21 1.39 2.06 0.58 0.93 1.23 0.69 0.56 0.57
950 1.18 2.23 2.81 0.57 1.50 2.05 0.64 0.90 1.17
1000 1.15 2.39 3.22 0.57 2.23 3.06 0.62 1.24 1.80

150 D0 Collaboration / Physics Letters B 699 (2011) 145–150
Fig. 1. (a) Distribution of the discriminant for data (points) compared to the background model summed over all analysis channels (filled histograms) and expected Wboson
signal with mixed couplings (aL,aR)=(1,1)(open histogram), (b) Contour plots of 95% C.L. lower limits on M(W)in the (aL,aR) plane, and (c) 95% C.L. upper limits on
the coupling aLin the (aR,M(W))plane.HereM(W)>m(νR).
Fig. 2. Expected and observed upper limits and theory prediction for the production cross section (the shaded band indicates the uncertainty [5])forWbosons with
(a) left-handed couplings (aL,aR)=(1,0), (b) right-handed couplings (aL,aR)=(0,1)and, (c) mixed couplings (aL,aR)=(1,1)as a function of the Wboson mass. Here
M(W)>m(νR).
sents the limit on the third parameter. Fig. 1(b) shows contours
of lower limits for the Wboson mass in the (aL,aR) plane, and
Fig. 1(c) shows contours of upper limits for the coupling aLin
the (aR,M(W)) plane for M(W)>m(νR).Fig. 2 shows the cross
section limits for the three special cases. As the Wboson mass
increases, the selection efficiency decreases and the upper limits
on the cross section increase.
In conclusion, we have carried out a search for a massive
charged gauge boson, W, that decays to tb. We considered a
model-independent approach in which the Wboson may couple
to fermions with any combination of left- and right-handed cou-
plings. We do not observe any deviations from SM expectations
and set upper limits on the cross section for the production of
Wbosons. We compare upper limits to theory cross sections to
extract the following limits for M(W)>m(νR):M(W)>863 GeV
for purely left-handed couplings, M(W)>885 GeV for purely
right-handed couplings, and M(W)>916 GeV if both left- and
right-handed couplings are present. The limit for right-handed
couplings improves for M(W)<m(νR)to M(W)>890 GeV.
These mass limits improve previously published results by more
than 100 GeV.
Acknowledgements
We thank the staffs at Fermilab and collaborating institutions,
and acknowledge support from the DOE and NSF (USA); CEA
and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CNPq,
FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE and DST (India);
Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Ko-
rea); CONICET and UBACyT (Argentina); FOM (The Netherlands);
STFC and The Royal Society (United Kingdom); MSMT and GACR
(Czech Republic); CRC Program and NSERC (Canada); BMBF and
DFG (Germany); SFI (Ireland); The Swedish Research Council (Swe-
den); and CAS and CNSF (China).
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