DOI 10.1393/ncc/i2015-11855-0
Colloquia: LaThuile14
Diboson and electroweak physics at ATLAS
A. Veston behalf of the ATLAS Collaboration
Technische Universit¨at - Dresden, Germany
ricevuto il 31 Luglio 2014
Summary. — LHC data at centre-of-mass energies of 7 and 8 TeV are used to
perform measurements of electroweak diboson and Zjj production with the ATLAS detector. These measurements comprise combinations of W , Z and isolated photons, and use leptonic as well as semi-leptonic decay channels. No significant deviations from the Standard Model have been observed and the measurements are used to place constraints on anomalous gauge boson couplings. In addition, the prospects for vector boson scattering and triboson production at the LHC at a centre-of-mass energy of 14 TeV are discussed.
PACS 14.70.-e – Gauge bosons.
PACS 29.20.db – Storage rings and colliders.
1. – Introduction
Measurements of electroweak gauge boson pair production processes provide excellent tests of the electroweak sector of the Standard Model (SM). Any deviation from SM gauge constraints can cause a significant enhancement in the production cross section at high diboson invariant mass due to anomalous triple gauge boson couplings (aTGC). In the SM, triple gauge couplings (TGC) are predicted at tree level only with charged bosons, while neutral TGC are forbidden. Furthermore, diboson measurements are an evident prerequisite for the measurement of vector boson scattering, where dibosons plus at least two jets are produced.
This note presents measurements of the diboson and electroweak Zjj production cross sections in proton-proton collisions at centre-of-mass energies of√s = 7 TeV with
an integrated luminosity of L = 4.6 fb−1 and at√s = 8 TeV with integrated luminosities
of L = 13 fb−1 and 20 fb−1, respectively, with the ATLAS [1] experiment. The cross sections of the studied processes are calculated at NLO in QCD using MCFM [2] with PDF set CT10 [3]. Furthermore, examples of limits on aTGC from the W W and the electroweak Zjj production measurement are given. In addition, the prospects for vector boson scattering and triboson production at the LHC at a centre-of-mass energy of
√
s = 14 TeV are discussed.
c
CERN on behalf of the ATLAS Collaboration
Four-Lepton Mass [GeV] 100 200 300 400 500 600 700 800 900 Events / 20 GeV 10 20 30 40 50 60 70 80 90 Data ZZ Background Total Uncertainty Preliminary ATLAS -1 = 20 fb dt L = 8 TeV s llll ZZ [TeV] s 0 2 4 6 8 10 12 14 [pb] ZZ total 1 10 <116 GeV) ll ) (66<m p ZZ (p <116 GeV) ll ZZ (pp) (66<m =8 TeV) s LHC Data 2012 ( =7 TeV) s LHC Data 2011 ( =1.96 TeV) s Tevatron ( -1 <116 GeV) L=20 fb ll llll (66<m ATLAS ZZ -1 <120 GeV) L=5.3 fb ll llll (60<m CMS ZZ -1 <116 GeV) L=4.6 fb ll ) (66<m ll(ll/ ATLAS ZZ -1 <120 GeV) L=5.0 fb ll llll (60<m CMS ZZ -1 <120 GeV) L=8.6 fb ll ) (60<m ll(ll/ D0 ZZ -1 ) (on-shell) L=6.0 fb ll(ll/ CDF ZZ Preliminary ATLAS NLO QCD (MCFM, CT10.0)
Fig. 1. – Left: The invariant mass of the four-lepton system for the ZZ selection. Right: Com-parison of experimental measurements and theoretical predictions of the total ZZ production cross section as a function of the centre-of-mass energy√s. The solid red line shows the
theoret-ical prediction for the ZZ production cross section in pp collisions, calculated using the natural width of the Z boson in the mass range 66 to 116 GeV.
2. – ZZ production
The analyses of Z boson pair production is performed in the 4l and 2l2ν final states [4, 5]. The ZZ → llll signature comprises four isolated leptons and the invariant mass of each reconstructed opposite-sign same-flavor lepton pair forming a Z is required to be within 66 GeV < mll< 116 GeV. For the ZZ→ llνν channel the signature is two opposite-sign same-flavor high pTisolated leptons within 76 GeV < mll< 106 GeV. Fig-ure 1 (left) shows the invariant mass of the four-lepton system for the ZZ→ llll selection. The total measured ZZ production cross sections are σtot = 6.7± 0.7(stat)+0.4−0.3(syst)±
0.3(lumi) pb for √s = 7 TeV and σtot = 7.1+0.5−0.4(stat)± 0.3(syst) ± 0.2(lumi) pb for
√
s = 8 TeV, and the theory predictions are σth
tot = 5.89+0.22−0.18pb and σtotth = 7.2+0.3−0.2pb
for √s = 7 TeV and 8 TeV, respectively. Figure 1 (right) illustrates the comparison of
experimental measurements and theoretical predictions of the total ZZ production cross section as a function of the centre-of-mass energy.
3. – W Z production
The analysis of W Z production is performed with 3lν final states [6,7]. The signature of these topologies is two opposite-sign same-flavor isolated leptons forming the Z boson within a tight invariant mass window around the Z pole of|mZ−mPDG| < 10 GeV which
suppresses the background significantly. The residual third isolated lepton and the Emiss T
are forming the W boson. In fig. 2 (left) the transverse mass mT of the W boson formed by the third lepton and Emiss
T is shown. The total W Z production cross section
measurements are σtot = 19.0+1.4−1.3(stat)± 0.9(syst) ± 0.4(lumi) pb for
√
s = 7 TeV and σtot = 20.3+0.8−0.7(stat)+1.2−1.1(syst)+0.7−0.6(lumi) pb for
√
s = 8 TeV. The theory predictions are σtottheory= 17.6+1.1−1.0pb and σtheorytot = 20.3± 0.8 pb for√s = 7 TeV and 8 TeV, respectively.
In fig. 2 (right) experimental measurements and theoretical predictions of the total W Z production cross section as a function of the centre-of-mass energy √s are compared.
The total cross section is calculated using the natural width of the Z boson in the mass range 66 to 116 GeV.
) [GeV] miss T (l,E T M 20 40 60 80 100 120 140 160 Events / 5 GeV 0 20 40 60 80 100 120 data WZ ZZ W/Z+ W+jet Z+jet Top ATLAS Preliminary -1 L dt = 13 fb = 8 TeV, s [TeV] s 0 2 4 6 8 10 12 14 [pb] totalWZ 1 10 <116 GeV) ll )(66<m p WZ (p <116 GeV) ll WZ (pp)(66<m =8 TeV) s LHC Data 2012 ( =7 TeV) s LHC Data 2011 ( =1.96 TeV) s Tevatron ( -1 <116 GeV) L=13 fb ll ll (66<m l ATLAS WZ -1 <116 GeV) L=4.6 fb ll ll (66<m l ATLAS WZ -1 <120 GeV) L=8.6 fb ll ll (60<m l D0 WZ -1 ll L=7.1 fb l CDF WZ Preliminary ATLAS NLO QCD (MCFM, CT10)
Fig. 2. – Left: The W transverse mass for the W Z selection. The rightmost bin includes the overflow. Right: Comparison of experimental measurements and theoretical predictions of the total W Z production cross section as a function of the centre-of-mass energy √s. The solid
red line shows the theoretical prediction for the W Z production cross section in pp collisions, calculated using the natural width of the Z boson in the mass range 66 to 116 GeV.
4. – W W production
The W W → lνlν signal is measured in final states with two opposite-sign isolated leptons and large missing transverse energy [8]. Candidate W W events are required to have no jets reconstructed in the final state in order to reduce the background from t¯t production. The Z + jets processes with mis-measured jets are suppressed by a cut on Emiss
T . The measured total W W production cross section amounts to
σtot = 51.9± 2.0(stat) ± 3.9(syst) ± 2.0(lumi) pb compared to a theory prediction of
σtottheory = 44.7+2.1−1.9pb, being slightly larger than the SM prediction. Figure 3 shows the normalized differential W W fiducial cross section as a function of the leading lepton pT
compared to the SM prediction.
[GeV] T Leading lepton p 40 60 80 100 120 140 160 180 Data/MC 0.5 1 1.5 25 40 60 80 100 120 140 350 ] -1 [GeV T /dp WW fid d WW fid 1/ 0.005 0.01 0.015 0.02 0.025 0.03 ATLAS =7 TeV) s Data 2011 ( -1 L dt = 4.6 fb
Monte Carlo (MC@NLO) Data
Stat. Uncertainty Full Uncertainty
Fig. 3. – The normalized unfolded differential W W fiducial cross section as a function of the leading lepton pTcompared to the SM prediction.
50 100 150 200 250 Entries / 5 GeV 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Data Multijet +single-t t t W/Z + HF W/Z + jets WW/WZ -1 L dt = 4.7 fb = 7 TeV s ATLAS Preliminary + 2 jets ) l (e, W
Dijet Mass [GeV]
50 100 150 200 250 (Data-MC)/M C -0.1 0.0 0.1 ] 2 Dijet Mass [GeV/c
50 100 150 200 250 Data - MC -200 0 200 400 600 800 1000 + 2 jets ) l(e, W -1 L dt = 4.7 fb = 7 TeV s ATLAS Preliminary
Fig. 4. – Left: dijet invariant-mass distribution of reconstructed W/Z → jj candidates. The rightmost bin includes the overflow. The lower panel displays the fractional difference between the data and the MC expectation. The yellow band displays the systematic uncertainty due to the JES uncertainty only. Right: Background-subtracted dijet invariant-mass distribution of reconstructed W/Z→ jj candidates. The errors bars represents statistical uncertainty of data and MC.
5. – W W and W Z production in the single-lepton final state
W W and W Z production where one W or the Z boson decays into jets are also
established [10] for √s = 7 TeV. Candidate events have one high pT isolated lepton,
ETmiss > 30 GeV and exactly two jets with pT > 25/30 GeV. Figure 4 shows the dijet
invariant-mass distributions of reconstructed W/Z→ jj candidates and the background-subtracted dijet invariant-mass distribution of reconstructed W/Z→ jj candidates. The
W/Z resonance has been observed with 3.3 σ, and the measured cross section is σtot =
72± 9(stat) ± 15(syst) ± 13(MC stat) pb, compared to the theory prediction of σtheorytot =
63.4± 2.6 pb.
6. – W γ and Zγ production
The candidate events for production of W and Z bosons in association with photons are selected from the production processes pp → lνγ + X, pp → llγ + X and pp →
ννγ +X [9]. The experimental signature comprises a high-pTisolated photon and missing
transverse energy for the lνγ and ννγ final states or two oppositely charged same-flavor leptons for the llγ final state. In order to suppress contributions from final-state radiation, the spacial separation between the lepton and the photon is required to be ΔR(l, γ) > 0.7. Cross section measurements are performed in the inclusive and exclusive final state, depending on the number of jets produced. In fig. 5 the measured ETγ differential cross section for pp→ W γ → lνγ is shown for both the inclusive and exclusive measurements. The resulting measured and expected inclusive and exclusive cross sections for W γ and Zγ production processes are compatible within uncertainties.
] -1 [fb GeV T dE ) l (pp d -2 10 -1 10 1 10 2 10 Data 2011 (Inclusive) 1.0 (Inclusive) SHERPA 1.5 (Inclusive) ALPGEN MCFM (Inclusive) Data 2011 (Exclusive) 1.0 (Exclusive) SHERPA 1.5 (Exclusive) ALPGEN MCFM (Exclusive) =7TeV s -1 L dt = 4.6 fb
ATLAS Theory Data 0 1 2 [GeV] T E Theory Data 0 1 2 15 20 30 40 60 100 1000Fig. 5. – Measured ET differential cross sections of the pp → lνγ process, using combined
electron and muon measurements in the inclusive (Njet≥ 0) and exclusive (Njet= 0) extended
fiducial regions. [pb] Zjj -1 10 1 10 ATLAS -1 L dt = 20.3 fb = 8 TeV s Data 2012 Powheg (Zjj) + Sherpa (VZ) baseline T
high p search control high mass
theory data 0.8 0.9 1 1.1 / 250 GeV obs N 1 10 2 10 3 10 4 10 Data (2012) Background Background + EW Zjj ATLAS -1 L dt = 20.3 fb = 8 TeV s search region 0.5 1 1.5 [GeV] jj m 500 1000 1500 2000 2500 3000 3500 0 0.5 1 Data BKG + EW Data BKG constrained unconstrained
Fig. 6. – Left: Fiducial cross section measurements for inclusive Zjj production in the l+l−
de-cay channel, compared to the POWHEG prediction for strong and electroweak Zjj production and the small contribution from diboson initiated Zjj production predicted by SHERPA [12]. Right: The dijet invariant-mass distribution in the search region. The signal and (constrained) background templates are scaled to match the number of events obtained in the fit. The lowest panel shows the ratio of constrained and unconstrained background templates to the data.
7. – Electroweak Zjj production
The three main electroweak Zjj production mechanisms are vector boson fusion (VBF), Z-boson bremsstrahlung, and non-resonant l+l−jj production and have been
same-Z -0.1 0 0.1 Z -0.1 0 0.1 ATLAS LEP Scenario 95% C.L. = 7 TeV s -1 L = 4.6 fb Z -0.2 -0.1 0 0.1 0.2 1 Z g -0.2 -0.1 0 0.1 0.2 0.3 ATLAS LEP Scenario 95% C.L. = 7 TeV s -1 L = 4.6 fb Z -0.1 0 0.1 1 Z g -0.1 0 0.1 ATLAS LEP Scenario 95% C.L. = 7 TeV s -1 L = 4.6 fb
Fig. 7. – Two-dimensional 95% C.L. contour limit on λZvs. ΔκZ(left), ΔgZ1 vs. ΔκZ(middle),
and Δg1Z vs. λγ (right) for the LEP scenario for Λ =∞. Except for the two parameters under
study, all other anomalous couplings are set to zero.
flavor lepton pair within 81 GeV < mll < 101 GeV and at least two high-pT jets. For
the inclusive Zjj production a measurement of fiducial cross sections in five phase space regions with different sensitivity to the electroweak component have been performed, and are shown in fig. 6 (left). In the search regions the two highest-pT jets (“tagging
jets”) are required to fulfill mjj > 250 GeV and 1 TeV, respectively. Additionally, a jet veto on additional jets in the rapidity interval between the two tagging jets is applied. The electroweak Zjj component is extracted using a two-component template fit to the
mjj spectrum. The background-only hypothesis could be rejected with a significance above the 5 σ level. Figure 6 (right) shows the dijet invariant-mass distribution in the search region. The measured fiducial cross sections in the search region mjj > 250 GeV is σEW= 54.7± 4.6(stat)+9.8−10.4(syst)± 1.5(lumi) fb, compared to the theory prediction of
σtheory = 46.1± 0.2(stat)+0.3−0.2(scale)± 0.8(PDF) ± 0.5(model) fb.
8. – Exclusion limits on anomalous triple gauge couplings
The effect of physics beyond the Standard Model is modeled by an effective Lagrangian with anomalous triple gauge coupling (aTGC) parameters which are zero in the Standard Model [13, 14]. The five charged aTGC parameters are ΔgZ
1, ΔκZ, Δκγ, λZ, and λγ. The eight neutral aTGCs parameters are hZ3, hZ4, hγ3, h
γ
4, f4Z, f5Z, f4γ, f
γ
5. In order to
avoid unitarity violation, form factors F(s) = (1 + ˆs/Λ2)−n are used, where Λ is the
scale of the form factor, ˆs is the effective centre-of-mass energy, and n is an arbitrary
exponent. Figure 7 shows exemplary one- and two-dimensional confidence intervals at the 95% confidence level for the charged aTGC parameters derived from the W W production measurement [8] within the LEP parametrization, which defines ΔκZ = gZ1 − Δκγtan θw and λZ = λγ, and therewith reduces the number of independent parameters to three. No dipole form factor unitarization is used here. First ever aTGC limits at a hadron collider from VBF Z production, extracted from the search region mjj > 1 TeV of the electroweak Zjj production measurement, with and without form factor unitarization are also derived [11].
[TeV] jj leading m 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Entries 20 40 60 80 100 120 140 160 180 200 220 240 SM VV Diboson Non-VV (g = 1.75) 1.0 TeV Res SM VV + -1 L dt = 3000 fb
(Simulation) Preliminary ATLAS [TeV] 4l m 0.2 0.3 0.4 0.5 0.6 1 Entries 10 20 30 40 50 SM VV Diboson Non-VV (g = 1.75) 1.0 TeV Res SM VV + -1 L dt = 3000 fb
(Simulation) Preliminary ATLASFig. 8. – Left: Leading jet-jet invariant-mass (mjj) distribution in the pp→ ZZ + 2j → llll + 2j
channel. Right: Reconstructed 4-lepton mass (m4l) spectrum for the pp→ ZZ + 2j → llll + 2j
channel after requiring mjj> 1 TeV.
) [GeV] -l + log10 M(l 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 Entries/Bin 0 5 10 15 20 25 30 35 40
45 ATLAS Simulation Preliminary
-1 L dt = 3000 fb
= 14 TeV s -4 = 0.4 TeV 4 / T8 f SM J Z ZJJ ] -4 ) [TeV -l + l (Z 4 / T8 f -1 -0.5 0 0.5 1 ] Significance [ 0 1 2 3 4 5 6 7 8 9 10 -1 3000 fb -1 300 fb ATLAS Simulation PreliminaryFig. 9. – Left: Reconstructed mass spectrum of the l+l−γγ production process. Right: Signal
significance as a function of fT 8/Λ4.
9. – Prospects for vector boson scattering and triboson production at√
s = 14 TeV
Vector boson scattering (VBS) is the key process to experimentally probe the nature of the mechanism of electroweak symmetry breaking. In VBS processes additional res-onances of physics beyond the SM might show up. The signal investigated here is an anomalous VBS ZZ tensor singlet resonance f0 with a resonance mass of m = 1 TeV,
coupling g = 1.75, and width Γ = 50 GeV, and has been chosen is a benchmark [15, 16]. With an integrated luminosity of 3000 fb−1 this resonance could be observed with a sig-nificance of about 5.5 σ. The studied signature is two opposite-sign, same-flavor lepton pairs, and at least two jets with mjj > 1 TeV. Figure 8 shows for simulated events in the pp→ ZZ + 2j → llll + 2j channel the mjj distribution (left) and the reconstructed 4-lepton mass spectrum (right).
Triboson production gives access to quartic gauge couplings, and the example dis-cussed here is the Zγγ production [17]. Candidate events have two opposite-sign,
same-flavor high pT leptons within the Z mass window of |mZ − mll| < 10 GeV, and two high-pTwell-separated photons. The expected sensitivity to anomalous Zγγ production
at√s = 14 TeV in 5 σ discovery values for the anomalous quartic gauge coupling
param-eters are fT 8/Λ4= 0.4 TeV−4 and fT 9/Λ4= 0.7 TeV−4 with an integrated luminosity of 3000 fb−1. Figure 9 shows the reconstructed mass spectrum of the l+l−γγ production
(left) and the signal significance as a function of fT 8/Λ4.
10. – Summary and conclusion
Measurements of electroweak diboson production with the ATLAS experiment con-tinue to improve. Total and fiducial production cross section measurements of ZZ, W Z,
W W , and W/Zγ diboson production as well as electroweak Zjj production have been
performed at a centre-of-mass energies of 7 TeV and/or 8 TeV. No significant deviations from the SM have been observed. Unfolded differential cross sections are available for all fully leptonic results at 7 TeV. The measured cross sections are consistent with the SM predictions, and limits on aTGC have been derived. Moreover, prospects for vec-tor boson scattering and triboson production processes, which provide access to quartic gauge couplings, have been studied at a centre-of-mass energy of 14 TeV. An increase of significances for anomalous resonances or couplings with higher luminosity is expected.
REFERENCES
[1] ATLAS Collaboration, JINST, 3 (2008) S08003. [2] Campbell J. et al., arXiv:1007.3492.
[3] Nadolsky P. M. et al., Phys. Rev. D, 78 (2008) 013004. [4] ATLAS Collaboration, JHEP, 03 (2013) 128.
[5] ATLAS Collaboration, ATLAS-CONF-2012-090, https://cds.cern.ch/record/1460409.
[6] ATLAS Collaboration, Eur. Phys. J. C, 72 (2012) 2173. [7] ATLAS Collaboration, ATLAS-CONF-2013-021,
https://cdsweb.cern.ch/record/1525557.
[8] ATLAS Collaboration, Phys. Rev. D, 87 (2013) 112001. [9] ATLAS Collaboration, Phys. Rev. D, 87 (2013) 112003. [10] ATLAS Collaboration, ATLAS-CONF-2012-157,
https://cdsweb.cern.ch/record/1493586. [11] ATLAS Collaboration, JHEP, 04 (2014) 031. [12] Gleisberg T. et al., JHEP, 0902 (2009) 007. [13] Hagiwara K. et al., Nucl. Phys. B, 282 (1987) 253. [14] Ellison J. et al., Ann. Rev. Nucl. Part. Sci., 48 (1998) 33. [15] ATLAS Collaboration, ATL-PHYS-PUB-2012-005,
http://cds.cern.ch/record/1496527. [16] Alboteanu A. et al., arXiv:0806.4145.
[17] ATLAS Collaboration, ATL-PHYS-PUB-2013-006, http://cds.cern.ch/record/1558703.
