Diboson Physics at the Tevatron

Abstract

Tevatron diboson measurements are reviewed, and new or recent results
reported for Wgamma, Zgamma, and ZZ production in the leptonic decay modes, and
for WW/WZ production in the lepton plus jets decay mode. The most stringent
limits on anomalous triple gauge couplings are reported for each final state.

Full text

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Diboson Physics at the Tevatron
Aidan Robson1,afor the CDF and D0 Collaborations
1SUPA, School of Physics and Astronomy, University of Glasgow, G12 8QQ, Scotland
Abstract. Tevatron diboson measurements are reviewed, and new or recent results reported for Wγ, Zγ and
ZZ production in the leptonic decay modes, and for WW/WZ production in the lepton plus jets decay mode. The
most stringent limits on anomalous triple gauge couplings are reported for each final state.
1 Introduction
Following the ending of the Tevatron collider program, we
can review the significant progress that has been made in
the diboson sector over the ten years of Run 2. At the start
of Run 2, WW was the only massive diboson state to have
been measured, with only a handful of events. In the in-
tervening years, the WZ and ZZ processes have been ob-
served (in 2007 and 2008 respectively), and the new avail-
ability of theoretical tools such as mcfm [1] and mc@nlo
[2] has allowed the standard model to be tested in the di-
boson sector. Measuring diboson production addresses the
basicphysicsinterestofobservingfundamentalelectroweak
processes.Measuringincreasinglysmallcross-sectionsisa
stepping-stone to new physics; and as diboson production
is a major background to Higgs searches, it is important
to understand it. Furthermore, measuring diboson produc-
tion allows access to triple gauge couplings, which could
provide indications of new physics.
2 W/Z + photon
2.1 Wγ
D0 has a new result in Wγ production from September
this year, using 4.2fb−1of integrated luminosity. Events
are selected with an electron or muon, a photon, and miss-
ing transverse energy (?ET). This analysis uses a neural
network for photon identification to improve sensitivity to
WWγ coupling. Backgrounds are at the 20 − 25% level,
overwhelmingly W+jets, and are estimated from data. An
importantpropertyofthestandardmodelpredictionatlead-
ing order is that interference between the s- and t-channel
amplitudes produces a zero in the total Wγ yield at a spe-
cificangleθ∗betweentheWbosonandtheincomingquark
in the Wγ rest frame. Although it is difficult to measure
the angle directly, this so-called ‘radiation amplitude zero’
is also visible in the charge-signed photon-lepton rapidity
difference as a dip at around -1/3. Figure 1 shows the dip,
ae-mail: aidan.robson@glasgow.ac.uk
compared with the signal prediction from the Baur-Berger
dedicated event generator [3] interfaced to pythia [4] for
showering. The measured cross-section for the kinematic
region ET(γ) > 15GeV and ∆R(?γ) > 0.7 is
σ(p¯ p → Wγ + Z → ?γ + X) = (7.6±0.4(stat)±0.6(sys))pb,
ingoodagreementwiththestandardmodelprediction7.6±
0.2pb. If there were anomalous triple gauge couplings, the
photon ETspectrum would be modified and more high-ET
photons observed. The photon ETspectrum may therefore
be used to derive limits on anomalous WWγ couplings.
A binned likelihood fit to data is used, and the 1-d lim-
its 95% CL limits obtained are −0.4 < ∆κγ < 0.4 and
−0.08 < λγ< 0.07 for a new physics scale Λ = 2TeV.
2.2 Zγ
D0hasanewresultforthisconferenceinZγ,using6.2fb−1
ofintegratedluminosity.Again,aneuralnetworktechnique
that uses five variables from tracking, calorimetry, and the
preshowerdetectorsprovidesrobustdifferentiationbetween
photons and jets. Background is at the 5 − 10% level and
is dominated by Z+jets. Around 1000 events are observed
in each of the final states Z → e+e−+γ and Z → µ+µ−+γ.
The Zγ system has the property that initial state photon
radiation (ISR) may be selected preferentially over final
state photon radiation by requiring the three-body invariant
mass M(??γ)tobeabovetheZbosonmass.With M(??γ) >
110GeV/c2, around 300 events are observed in each of
the final states. The differential cross-section dσ/dpT(γ)
is measured, using matrix inversion to unfold the exper-
imental distribution, and is shown in Figure 2 both for
all M(??γ), and for the ISR-dominated sample M(??γ) >
110GeV/c2. Prior to this analysis, these differential distri-
butions had not been shown. The data are compared with
the NLO prediction from mcfm, and are seen to be consis-
tent. Total cross-sections are also quoted: for the kinematic
region |η(γ)| < 1, ET(γ) > 10GeV, ∆R(?γ) > 0.7 and
M(??γ) > 60GeV/c2the result is σ(p¯ p → Zγ → ??γ) =
(1.09 ± 0.04(stat) ± 0.07(sys))pb, to be compared with the
standard model prediction 1.10±0.03pb; and for M(??γ) >
110GeV/c2the result is σ(p¯ p → Zγ → ??γ) = (0.29 ±
arXiv:1201.4771v1 [hep-ex] 23 Jan 2012

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Data
Nominal MC
Matrix Element Likelihood Ratio (LRWW)
Events / 0.04
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CDF Run II Preliminary
-1
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Fig. 1. (upper) The charge-signed photon-lepton rapidity differ-
ence for Wγ candidates from D0, showing the radiation ampli-
tude zero as a dip at around -1/3; and (lower) the matrix element
likelihood discriminant for WW candidates from CDF.
0.02(stat) ± 0.01(sys))pb, to be compared with the stan-
dard model prediction 0.29 ± 0.01pb.
ThemoststringentanomalouscouplinglimitsinZγ are
from CDF, in another 2011 result [5]. Here, Z → ?+?−+ γ
eventsareselectedwith M(??γ) > 100GeV/c2,andZ → νν
is also included through events having ?ET> 50GeV. For
the Z → νν + γ selection, events with tracks having pT >
10GeVorjetshaving ET> 15GeVarerejected,andcalorime-
ter timing information is used to reject cosmic-ray tracks.
The photon ET spectrum shows no evidence for anoma-
lous couplings and is used to set limits; at 95% CL they
are −0.020 < hZ
−0.022 < hγ
Λ = 1.5TeV.
D0 has also used the Zγ signature to look for reso-
nances and has set limits on generic scalar or vector res-
onances, such as might occur in technicolour models, at
the level of 1pb [6].
3< 0.021, −0.0009 < hZ
3< 0.020, and −0.0008 < hγ
4< 0.0009,
4< 0.0008, for
3 Massive Dibosons
3.1 WW
The WW final state is intimately connected with Higgs
searches, and CDF’s WW measurement was done in paral-
Fig. 2. The differential cross-section dσ/dpT(γ) for Zγ events
from D0, for (upper) all values of M(??γ), and (lower) the ISR-
enhanced dataset M(??γ) > 110GeV/c2.
lel with the search for H → WW, using the same tools [7].
Events having two oppositely-charged leptons and ?ETare
selected. Around 12% of the acceptance comes from τ lep-
tons decaying to electrons or muons. Control samples such
as same-sign dileptons check background modelling. The
analysis uses a matrix element probability approach, where
transfer functions derived from simulation are applied to
the measured four-vectors, which are inputs to matrix el-
ements that allow the computation of probabilities that an
event comes from signal or one of several background pro-
cesses. These probabilities are put together in a likelihood
ratio, and the cross-section is extracted from a fit as shown
in Figure 1. With 3.6fb−1of integrated luminosity, CDF
measures σ(p¯ p → WW) = (12.1 ± 0.9(stat)+1.6
A small excess in the high tail of the lepton pTspectrum
makes anomalous triple gauge coupling limits less strin-
gent than expected. The probability that the observed dis-
−1.4(sys))pb.

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XXIInd Hadron Collider Physics Symposium
tribution is drawn from the standard model is not too small
(7%), so the small excess is ascribed to a statistical fluctu-
ation. The best limits come from D0’s 1fb−1analysis [8],
and at 95% CL are −0.54 < ∆κγ < 0.83, −0.14 < λγ =
λZ < 0.18, and −0.14 < ∆gZ
scale Λ = 2TeV.
1< 0.30 for a new physics
3.2 WZ
The WZ final state is little-studied as it is charged, and
therefore produced only at hadron colliders. CDF’s recent
analysis in the ???ν final state using 6fb−1of integrated
luminosity incorporates improvements in lepton selection
and shows very good resolution, as demonstrated by the
W boson transverse mass in Figure 3. The measured cross-
sectionisnormalisedtothemeasuredZbosoncross-section
to remove some systematic uncertainties, in particular the
luminosityuncertainty.Thisisthenexchangedforasmaller
theoretical uncertainty when multiplying by a calculation
of the Z boson production cross-section in order to re-
cover the WZ cross-section: σ(p¯ p → WZ)/σ(p¯ p → Z) =
(5.5±0.9)×10−4and σ(p¯ p → WZ) = (4.1±0.7)pb. Triple
gauge couplings were not studied in this analysis and the
best anomalous coupling limits are set by D0’s 4.1fb−1
analysis, using the Z boson pTdistribution shown in Fig-
ure 3: −0.400 < ∆κZ< 0.675, −0.077 < λZ< 0.093, and
−0.056 < ∆gZ
[9].
1< 0.154 for a new physics scale Λ = 2TeV
3.3 ZZ
CDF has a new measurement of the ZZ production cross-
section in the four-lepton final state using 6fb−1of in-
tegrated luminosity [11]: σ(p¯ p → ZZ) = (2.3+0.9
0.2(sys))pb, to be compared with the NLO standard model
prediction 1.4±0.1pb. A clustering of events at high mass,
shown in Figure 4, caused excitement. However, analysis
of the other ZZ final states ZZ → ?+?−νν and ZZ → ?+?−jj
showed them to be more sensitive to a resonance of mass
around 327GeV/c2decaying to ZZ, and the data in those
channelsareinagreementwithstandardmodelpredictions.
The four-lepton events therefore appear to arise from stan-
dard model sources.
D0 also has a recent measurement of ZZ → ?+?−?+?−,
with increased muon acceptance compared to previous re-
sults [12]. The measured cross-section is σ(p¯ p → ZZ) =
(1.26+0.47
between the planes of the lepton pairs, computed in the
ZZ rest-frame, is sensitive to the production mechanism of
the Z pair: for example, a Z pair arising from the decay
of a Higgs boson would result in a different angular dis-
tribution. This distribution is tested for the first time and
is shown in Figure 4; it is seen to be consistent with the
standard model expectation.
The only anomalous triple coupling limits from ZZ are
fromanearlierD0result[13]:−0.28 < fZ
fγ
95% CL for Λ = 1.2TeV.
−0.8(stat) ±
−0.37(stat) ± 0.14(sys))pb. The distribution in angle
40< 0.28,−0.26 <
50< 0.28 at
40< 0.26, −0.31 < fZ
50< 0.29, and −0.30 < fγ
)
2
(W) (GeV/c
T
m
0020 40 60 80 100 120 140160 180 20020 40 60 80 100 120 140160 180 200
2
Events / 10 GeV/c
00
22
44
66
88
1010
1212
data
WZ
→
Z
Z
+ jets
µµ
ee + jets
→
µ
→γ
Z
ee
→γ
Z
γ
γ
µ
ZZ
-1
L dt = 6 fb
∫
CDF Run II Preliminary
Hist/wz_124:zzx MT(W)
(GeV) (GeV)
TT
Z pZ p
002020404060608080100100120120140140
Events/30 GeV
00
55
1010
15 15
2020
2525
Data
Background
SM WZ + Background
!"
= -0.1,
#
!"
= -0.1,
#
= 0.2
= -0.2
Events/30 GeV
Fig. 3. (upper) The W boson transverse mass mT in WZ can-
didates recorded by CDF; and (lower) the Z boson pT in WZ
candidates recorded by D0.
Finally, CDF has measured ZZ → ?+?−νν in 5.9fb−1
of integrated luminosity using techniques from the Higgs
search, and in that channel found
σ(p¯ p → ZZ) = (1.45+0.45
−0.42(stat)+0.41
−0.30(sys))pb.
4 Diboson final states with jets
Given their similarity to key Higgs boson signatures, there
have been ongoing efforts to observe diboson production
in final states with jets.
Two CDF analyses observed WW and WZ production
inthe?νjjfinalstatein2010.Thisfinalstateisverysimilar
to that expected from WH associated production. W+jets
is the overwhelming background. In the first analysis, the
background contribution from QCD was fitted from data
using the ?ETdistribution, where for the analysis selection,
QCD enters at low values, and electroweak processes have
high values. The signal was extracted from a χ2fit to the
dijet mass distribution as shown in Figure 5, giving an ex-
tracted cross-section σ(WW + WZ) = (18.1 ± 3.3(stat) ±
2.5(sys))pb with 5.2σ significance [14]. The second anal-
ysis used a matrix element technique, for which the final

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)
2
(GeV/c
ZZ
M
100150200250 300350 400 450500
2
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Fig. 4.
ZZ → ?+?−?+?−candidates, and (lower) angular separation of
the lepton pair planes, measured in the ZZ rest frame, for D0
ZZ → ?+?−?+?−candidates.
(upper) The reconstructed four-lepton mass in CDF
event probability discriminant is shown in Figure 5. Here,
theextractedcross-sectionwasσ(WW + WZ) = 16.5+3.3
with 5.4σ significance [14].
New for this conference is D0’s latest update in the
?νjjfinalstate,using4.3fb−1ofintegratedluminosity,which
makes significant advances. A random forest multivariate
discriminant is used to separate signal from background,
andsinceZbosonscandecaytob-quarkpairsbutWbosons
cannot, b-tagging is employed both to improve the signifi-
cance of the observation, and to separate the WW and WZ
components. Both the random forest discriminant output,
and the dijet invariant mass for the no b-tag data sample,
are shown in Figure 6. A cross-section σ(WW + WZ) =
19.6+3.1
of the separated WW and WZ cross-sections are given in
Figure 6.
Further results of diboson analyses with decays to b-
quark pairs are given elsewhere in these proceedings [17].
An earlier version of this D0 analysis using 1fb−1of
integrated luminosity set limits on anomalous triple cou-
−3.0pb,
−3.0pb is measured, with 8σ significance, and contours
2222
GeV/c GeV/c GeV/c GeV/c
jjjj jjjj
MMMM
50505050 100100100 100 150150150 150 200200 200200
2
Events/4 GeV/c
0000
500500 500500
1000 10001000 1000
/ndf = 25.61/37
2
!
Muon Data
W+jets
QCD
Z+jets
Top
WW+WZ
2
Events/4 GeV/c
-1
= 4.30 fbL dt
"
CDF Run II Preliminary
2
Events/4 GeV/c
2
Events/4 GeV/c
# /
$
-1
0
1
Event Probability DiscriminantEvent Probability DiscriminantEvent Probability Discriminant
000000000.2 0.20.20.20.2 0.20.20.2 0.4 0.40.40.40.40.4 0.40.4 0.60.60.6 0.60.60.60.6 0.6 0.80.8 0.80.80.80.8 0.80.811111111
Events / 0.05
10 1010 1010101010
22222222
10 10 1010 101010 10
33333333
101010 10 101010 10
44444444
1010 10101010 1010
Events / 0.05Events / 0.05
WW+WZ
W+jets
Non-W
Z+jets
Top
Data
-1
CDF Run II Preliminary, L=4.6 fb
0.70.70.75 0.750.8 0.80.850.850.90.90.95 0.9511
00
50 50
100 100
150 150
200 200
Fig. 5. (upper) The dijet invariant mass spectrum in the WW/WZ
muon+jets channel from CDF, and (lower) the event probability
discriminant in the matrix-element probability approach.
plings: −0.44 < ∆κγ < 0.675, −0.10 < λZ = λγ < 0.11,
and −0.12 < ∆gZ
Finally,anearlyanalysisfromCDFinZZ → ?+?−jjus-
ing 1.9fb−1of integrated luminosity set anomalous cou-
pling limits: at 95% CL, −0.12 < fZ
fγ
Λ = 1.2TeV.
1< 0.20 at 95% CL for Λ = 2TeV [16].
4< 0.12, −0.10 <
5< 0.11 for
4< 0.10, −0.13 < fZ
5< 0.12, and −0.11 < fγ
5 Outlook
A rich programme of Tevatron diboson physics has made
huge advances over the ten years of Run 2, testing the
standard model, probing for new physics, and underpin-
ning electroweak symmetry-breaking searches. D0 com-
bined anomalous coupling limits with 1fb−1of integrated
luminosity,resultinginmorestringentlimits.Someofthose
have now been superseded, and there is work on a new
combination.Bothexperimentshaveafinaldatasetofaround
10fb−1,soaswellasbeingcombined,theseanalysesshould
be updated once more for legacy measurements.

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XXIInd Hadron Collider Physics Symposium
Fig. 6.
nal state: (upper) random forest multivariate discriminant output;
(centre) background-subtracted dijet mass; (lower) contours of
WW and WZ production cross-section.
Results from D0’s WW/WZ analysis in the ?νjj fi-
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