November 6, 3 3: WSPC/INSRUCION FILE PIC Jian International Journal of Modern Physics: Conference Series c World Scientific Publishing Company DIBOSON PRODUCION A LHC AND EVARON JIAN WANG on behalf of the ALAS, CMS, CDF, DØ collaborations Universite Libre de Bruxelles Brussels, 5, Belgium Jian.Wang@cern.ch Received Day Month Year Revised Day Month Year his is a report at the conference Physics In Collisions 3. he experimental results on physics of diboson production are reviewed. he measurements use pp collisions at the LHC with center-of-mass energy s = 7 and 8 ev, and p p collisions at the evatron with s =.96 ev. hese include measurements of Wγ,, WW, W and production. he results are compared with Standard Model predictions, and are interpreted in terms of constraints on charged and neutral anomalous triple gauge couplings. Keywords: PACS numbers:. Introduction he study of diboson production provides an important test of the Standard Model (SM) of particle physics at ev energy scale. Especially, it is sensitive to the selfinteractions among vector bosons via triple gauge couplings. Any significant deviation of the production cross section or kinematic distributions from the SM predictions gives an indication of new physics. In addition, non-resonant diboson production measurements are important to a precise estimation of irreducible backgrounds for the Higgs study. he experimental results reviewed here, use p p collision at the evatron at a center-of-mass energy s =.96 ev, with an integrated luminosity up to about fb, and pp collision at s = 7 and 8 ev at the LHC, with integrated luminosities up to about 5 fb and fb respectively. Emphasis is placed on the latest results released in the past year.. Cross section Measurement.. Wγ and he Wγ and productions have been studied in W γ lνγ and llγ decay channels at s = 7 ev., In the Wγ measurement, events are selected
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian J. Wang by requiring an isolated electron or muon, and missing transverse energy, E miss, from the undetected neutrino, in addition to an isolated photon. he dominant background comes from W+jets, where a jet is misidentified as a photon. In the measurement, events are selected by requiring a same flavor, opposite sign electron or muon pair with an invariant mass close to the boson mass, in addition to an isolated photon. he photon is required to be separated from the lepton to suppress the contribution from final state radiation photons. CMS has measured the cross sections for γ E >5 GeV and m ll > 5 GeV. he measured Wγ cross section times W lν branching ratio is 37.±.8 (stat.)± 4. (syst.) ±.8 (lumi.)pb. he measured cross section times ll branching ratio is 5.33 ±.8 (stat.) ±.5 (syst.) ±. (lumi.)pb. he results are consistent with SM predictions. he differential cross sections measured by ALAS, comparing with theoretical predictions, are shown in Figure. In general, the NLO parton-level MC, MCFM, 3,4 agrees with the exclusive (Njet = ) production cross section measurements, while LO MC (ALPGEN 5 or SHERPA 6 ) with multiple parton emission reproduce the γ E spectrum. he ννγ decay channel has also been measured.,7 he backgrounds are jets misidentified as photons, and instrumental sources such as beam-gas interactions. Very tight photon E and E miss cuts are applied to suppress such backgrounds. Detector timing is also also used to reduce instrumental backgrounds. Both ALAS and CMS results are in agreement with SM predictions. ] [fb GeV d(pp l) de heory heory - ALAS L dt = 4.6 fb s=7ev (Inclusive) SHERPA ALPGEN MCFM (Inclusive). (Inclusive).5 (Inclusive) (Exclusive) SHERPA ALPGEN MCFM (Exclusive). (Exclusive).5 (Exclusive) 5 3 4 6 E [GeV] ] [fb GeV ) - l + d(pp l de heory heory - -3 ALAS L dt = 4.6 fb + - s=7ev pp l l (Inclusive) SHERPA MCFM (Inclusive). (Inclusive) (Exclusive) SHERPA MCFM (Exclusive). (Exclusive) 5 3 4 6 E [GeV] Fig.. ALAS measured γ E differential cross sections of the lνγ process (left) and the llγ process (right), in the inclusive and exclusive (Njet = ) extended fiducial regions, at s = 7 ev.. WW he WW production cross section has been measured in the W W lνl ν final state. 8,9, Events are selected by requiring two opposite charged isolated leptons, electron or muon, accompanied by significant E miss. he +jets background in the ee and µµ channels is suppressed by cut off large E miss and a veto. o minimise the contribution from top-quark background, events containing jets are rejected (jet
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian Diboson Production at LHC and evatron 3 veto). his leads to a significant theoretical uncertainty in the jet veto efficiency. he WW production cross sections measured by ALAS and CMS at s = 7 ev are 5.9 ±. (stat.) ± 3.9 (syst.) ±. (lumi.)pb and 5.4 ±. (stat.) ± 4.5 (syst.) ±. (lumi.)pb respectively. he ALAS measurement of differential cross section as a fuction of the leading lepton p is shown in Figure. CMS has also performed a first measurement of WW production at s = 8 ev. he cross sections is determined to be 69.9 ±.8 (stat.) ± 5.6 (syst.) ± 3. (lumi.)pb. hese measured cross sections are slightly higher than but still compatible with the SM predictions. Systematic uncertainties dominate the total uncertainty. ] [GeV fid /dp WW.3.5 ALAS ( s=7 ev) L dt = 4.6 fb Monte Carlo (MC@NLO) Stat. Uncertainty Full Uncertainty d WW / fid..5..5 /MC.5.5 5 4 6 8 4 6 8 35 Leading lepton p [GeV] Fig.. ALAS measured normalized differential WW fiducial cross section as a function of the leading lepton p compared to the SM prediciton..3. W he W production cross sections have been measured in the W lνl decay channel.,,3 his final state has very low background after requiring exactly three isolated leptons (electron or muon), a pair of which is same-flavor and has an invariant mass close to the mass of boson, in addition to significant E miss from W decay. In ALAS measurement at s = 7 ev, totally 37 candidates are observed with a background expectation of 68 events. In the measurement at s = 8 ev, 94 candidate events are observed in total, with a background expectation of 7 events. he W cross sections are measured to be 9. +.4.3 (stat.)±.9 (syst.)±.4 (lumi.)pb at s = 7 ev, and.3 +.8.7 (stat.)+.. (syst.)+.7.6 (lumi.)pb at s = 8 ev, for the boson mass in the range of 66 to 6 GeV. CMS has measured the W production cross section for the boson mass between 7 to GeV. he cross sections are determined to be.76 ±.3 (stat.) ±.3 (syst.) ±.46 (lumi.)pb at s = 7 ev, and 4.6 ±.76 (stat.) ±.3 (syst.) ±.8 (lumi.)pb at s = 8 ev. Since the LHC is a pp collider, the W + and
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian 4 J. Wang W cross sections are not equal. he ratios of production cross sections for W + and W have also been measured. hey are.94 ±.5 (stat.) ±.4 (syst.) and.8±. (stat.)±.3 (syst.) at s = 7 ev and 8 ev respectively, in agreement with SM predictions. he total production cross section of WV (WW+W) has also been studied in the W V lνqq final state at s = 7 ev. 4,5 he resolution of reconstructed dijet mass is about GeV, which cannot distinguish W from here. his channel has higher branching ratio with respect to the fully leptonic decay mode, at the cost of larger W/+jets background. Events are selected by requiring an isolated electron or muon, E miss and exactly two high-p jets. Signal is extracted by fitting di-jet mass distribution. ALAS measures a cross section of 7 ± 9 (stat.) ± 5 (syst.) ± 3 (MC stat.)pb and CMS measures a cross section of 68.9±8.7 (stat.)±9.7 (syst.)±.5 (lumi.)pb. Both measurements agree with the SM predictions..4. he production cross sections have been measured using the high purity fourlepton ( ll ) decay channel. 6,7,8,9. Even though the branching ratio to four-lepton final state is small, this process is really clean, with negligible amount of background. Events are selected by requiring two pairs of electrons or muons, with opposite-charge and same-flavour. he invariant mass of each pair is compatible with the boson mass. he challenge is the four-lepton analysis is the optimization for lepton efficiencies, especially for low p leptons. Some leptons might fall outside the acceptance of the detector while some others may fail the criteria used to select a lepton. With four chances to miss a lepton, even small inefficiencies will add up. In ALAS measurement at s = 7 ev, lν decay mode has also been measured, by applying a tight cut on a E miss -related variable in order to suppress the dominant +jets background. ll and lν results are then combined, assuming SM branching ratios. he production cross section is determined to be 6.7 ±.7 (stat.) +.4.3 (syst.) ±.3 (lumi.)pb at s = 7 ev, where both bosons in the mass range of 66 to 6 GeV. In the measurement at s = 8 ev, only fourlepton channel is used. otally 35 candidate events are observed with a background expectation of.4. he SM expectation for the number of signal event is 9.5. he production cross section at s = 8 ev is measured to be 7. +.5.4 (stat.) ±.3 (syst.) ±. (lumi.)pb. he CMS ll measurements include lτ decay mode as well. he measured cross sections are 6.4 +.86.8 (stat.)+.4.3 (syst.) ±.4 (lumi.)pb at s = 7 ev and 7.7 +.5.5 (stat.)+.5.4 (syst.) ±.4 (theo.) ±.3 (lumi.)pb at s = 8 ev, for both bosons produced in the mass region of 6 to GeV. Differential cross sections are also measured (as shown in Figure 3) and well described by the theoretical predictions. CDF and DØ have measured cross section in p p collisions at s =.96 ev. CDF has studied production through ll and lν final states, using a dataset
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian Diboson Production at LHC and evatron 5 ) (/GeV) d σ fid /d(lead. lep. p /σ fid /MC.35.3.5..5..5 CMS Preliminary s = 8 ev, L = 9.6 fb Unfolded otal Error POWHEG+gg 4 6 8 4 Lead. lep. p (GeV).5.5 ) (/GeV) p d σ fid /d( /σ fid /MC.45.4.35.3.5..5..5 CMS Preliminary s = 8 ev, L = 9.6 fb Unfolded otal Error POWHEG+gg 5 5 5 p (GeV).5.5 Fig. 3. Differential cross section normalized to the fiducial cross section in the CMS ll measurement at s = 8 ev. he differential cross sections in bins of p are presented for the leading lepton (left) and the higher-p (right). corresponding to 9.7fb integrated luminosity. he combined measured cross section is.4.4 +. (stat.)+.5.8 (syst.). DØ has measured the four-lepton channel, and combined it with a previous study of lν channel, resulting a cross section of.3 +.9.5 (stat.) ±. (syst.) ±.4 (lumi.). A summary of WW, W and production cross section measurements are listed and compared with the relevant theoretical predictions in able. he theoretical predictions are computed using MCFM to QCD NLO. Please note the theoretical predictions are different for same process measured by different experiments, because the phase spaces are not exactly same defined. 3. Limits On Anomalous riple Gauge Couplings he SM describes exactly how vector bosons couple with each other, and diboson productions are sensitive to these couplings. Even if new physics is at very high energy scale, beyond the each of current colliders (which means direct pair production of new particle is impossible ), it could still have indirect effect on triple gauge couplings through virtual corrections. Anomalous triple gauge couplings (agc) could be modeled by adding terms to the SM Lagrangian, using a set of parameters, listed in able. All these parameters are equal to zero in the SM. his is a common approach to parameterize low energy effects from high energy scale new physics, which allows for experimental results to be interpreted as model independent constrains on agc. he presence of agc would increase diboson production cross sections, in particular at high mass and high p regions. Experimental measurements made by the ALAS, CMS, CDF and DØ collaborations have found no excess over the SM predictions, leading to limits on charged and neutral agc, as shown in Figure 4
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian 6 J. Wang and Figure 5 respectively, together with previous LEP results. For charged agc, LEP results remain competitive while the sensitivity from LHC is approaching. For neutral agc, LHC results dominate. Feb 3 κ γ λ γ ALAS Limits CMS Limits D Limit LEP Limit Wγ -.4 -.46 4.6 fb Wγ -.38 -.9 5. fb WW -. -. 4.9 fb WV -. -.4 5. fb D Combination -.58 -.55 8.6 fb LEP Combination -.99 -.66.7 fb Wγ -.65 -.6 4.6 fb Wγ -.5 -.37 5. fb WW -.48 -.48 4.9 fb WV -.38 -.3 5. fb D Combination -.36 -.44 8.6 fb LEP Combination -.59 -.7.7 fb -.5.5.5 agc Limits @95% C.L. Feb 3 ALAS Limits CMS Limits D Limit LEP Limit WW -.43 -.43 4.6 fb κ WV -.43 -.33 5. fb LEP Combination -.74 -.5.7 fb WW -.6 -.59 4.6 fb λ WW -.48 -.48 4.9 fb W -.46 -.47 4.6 fb WV -.38 -.3 5. fb D Combination -.36 -.44 8.6 fb LEP Combination -.59 -.7.7 fb WW -.39 -.5 4.6 fb g WW -.95 -.95 4.9 fb W -.57 -.93 4.6 fb D Combination -.34 -.84 8.6 fb LEP Combination -.54 -..7 fb -.5.5.5 agc Limits @95% C.L. Fig. 4. Limits at 95% C.L. on WWγ (top) and WW (bottom) agc. 4. First Studies On Quartic Gauge Couplings CMS has studied the exclusive two-photon production of WW using 5.5 fb of data at s = 7 ev. Events are selected by requiring a µ ± e vertex with no associated charged tracks, and p (µ ± e ) > 3 GeV.wo events are observed in the data, compared to a SM expectation of. ±.5 signal events with.84 ±.3 background. he significance of the signal is.σ, with a 95% CL upper limit on the SM cross section of 8.4 fb. DØ has studied the exclusive two-photon production of WW in events with an electron, a positron and E miss. No excess above the background expectation has been found.
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian Diboson Production at LHC and evatron 7 July 3 ALAS Limits CMS Limits γ f4 f4 γ f5 f5 -.5 -.5 4.6 fb -.4 -.4 9.6 fb -.3 -.3 -.4 -.4 4.6 fb 9.6 fb -.6 -.5 4.6 fb -.5 -.5 -.3 -.3 -.5 -.5 9.6 fb 4.6 fb 9.6 fb -.5.5.5 x agc Limits @95% C.L. Feb 3 ALAS Limits CMS Limits CDF Limit γ h3 h3 γ h4 x h4 x -.5 -.6 4.6 fb -.3 -.3 5. fb -. -. 5. fb -.3 -.4 4.6 fb -.3 -.3 5. fb -. -. 5. fb -.9 -.9 4.6 fb -. -. 5. fb -.9 -.9 4.6 fb -. -. 5. fb -.5.5.5 x agc Limits @95% C.L. Fig. 5. Limits at 95% C.L. on, (top) and, (bottom) agc. A study of the WVγ, three vector boson production has also been perform by CMS, using 9.3 fb data from pp collisions at s = 8 ev. 3 he analysis selects events containing a W boson decaying to leptons, a second V (W or ) boson decaying to two jets, and a photon. he number of observed events in data is 3, while the estimated background yield is 34.5. his is consistent with the SM predictions, and corresponds to an upper limit of 4 fb at 95% CL for WVγ with photon p > GeV. he results of the above analyses are studied for deviations from the SM, and used to constrain anomalous quartic gauge boson couplings. 5. Summary Latest results of diboson production measurements by ALAS, CMS, CDF and DØ experiments are reviewed. he measured cross sections are typically consistent with SM predictions. he results are used to constrain new physics, by setting limits on
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian 8 J. Wang anomalous triple gauge couplings. First studies on quartic gauge couplings have started as well. References. G. Aad et al. [ALAS Collaboration], Phys. Rev. D 87, 3 (3) [arxiv:3.83. S. Chatrchyan et al. [CMS Collaboration], arxiv:38.683 [hep-ex]. 3. J. M. Campbell and R. K. Ellis, Nucl. Phys. Proc. Suppl. 5-6, () [arxiv:7.349 [hep-ph]]. 4. J. M. Campbell, R. K. Ellis and C. Williams, JHEP 7, 8 () [arxiv:5. [hep-ph]]. 5. M. L. Mangano, M. Moretti, F. Piccinini, R. Pittau and A. D. Polosa, JHEP 37, (3) [hep-ph/693]. 6.. Gleisberg, S..Hoeche, F. Krauss, M. Schonherr, S. Schumann, F. Siegert and J. Winter, JHEP 9, 7 (9) [arxiv:8.46 [hep-ph]]. 7. S. Chatrchyan et al. [CMS Collaboration], JHEP 3, 64 (3) [arxiv:39.7 8. G. Aad et al. [ALAS Collaboration], Phys. Rev. D 87, (3) [arxiv:.979 9. S. Chatrchyan et al. [CMS Collaboration], arxiv:36.6 [hep-ex].. S. Chatrchyan et al. [CMS Collaboration], Phys. Lett. B 7, 9 (3) [arxiv:3.4698. G. Aad et al. [ALAS Collaboration], Eur. Phys. J. C 7, 73 () [arxiv:8.39. [ALAS Collaboration], ALAS-CONF-3-. 3. CMS Collaboration [CMS Collaboration], CMS-PAS-SMP-6. 4. [ALAS Collaboration], ALAS-CONF-57. 5. S. Chatrchyan et al. [CMS Collaboration], Eur. Phys. J. C 73, 83 (3) [arxiv:.7544 6. G. Aad et al. [ALAS Collaboration], JHEP 33, 8 (3) [arxiv:.696 [hepex]]. 7. [ALAS Collaboration], ALAS-CONF-3-. 8. S. Chatrchyan et al. [CMS Collaboration], JHEP 3, 63 (3) [arxiv:.489 9. CMS Collaboration [CMS Collaboration], CMS-PAS-SMP3-5.. V. M. Abazov et al. [D Collaboration], Phys. Rev. D 88, 38 (3) [arxiv:34.54. S. Chatrchyan et al. [CMS Collaboration], JHEP 37, 6 (3) [arxiv:35.5596. V. M. Abazov et al. [D Collaboration], Phys. Rev. D 88, 5 (3) [arxiv:35.58 3. CMS Collaboration [CMS Collaboration], CMS-PAS-SMP3-9.
November 6, 3 3: WSPC/INSRUCION FILE PIC Jian Diboson Production at LHC and evatron 9 able. Summary of diboson production cross section measurements. Experiment s Integrated luminosity Measured cross section heoretical prediction ( ev) (fb ) (pb) (pb) WW ALAS 7 4.6 5.9 ±. (stat.) ± 3.9 (syst.) ±. (lumi.) 44.7 +..9 CMS 7 4.9 5.4 ±. (stat.) ± 4.5 (syst.) ±. (lumi.) 47. ±. CMS 8 3.5 69.9 ±.8 (stat.) ± 5.6 (syst.) ± 3. (lumi.) 57.3 +.3.6 W ALAS 7 4.6 9. +.4.3 (stat.) ±.9 (syst.) ±.4 (lumi.) 7.6+.. ALAS 8 3.3 +.8.7 (stat.)+.. (syst.)+.7.6 (lumi.).3 ±.8 CMS 7 4.9.76 ±.3 (stat.) ±.3 (syst.) ±.46 (lumi.) 7.8 +.7.5 CMS 8 9.6 4.6 ±.76 (stat.) ±.3 (syst.) ±.8 (lumi.).9 +.7.88 WV (V= W or ) ALAS 7 4.7 7 ± 9 (stat.) ± 5 (syst.) ± 3 (MC stat.) 63.4 ±.6 CMS 7 5 68.9 ± 8.7 (stat.) ± 9.7 (syst.) ±.5 (lumi.) 65.6 ±. ALAS 7 4.6 6.7 ±.7 (stat.) +.4.3 (syst.) ±.3 (lumi.) 5.89+..8 ALAS 8 7. +.5.4 (stat.) ±.3 (syst.) ±. (lumi.) 7.+.3. CMS 7 5. 6.4 +.86.8 (stat.)+.4.3 (syst.) ±.4 (lumi.) 6.3 ±.4 CMS 8 9.6 7.7 +.5.5 (stat.)+.5.4 (syst.) ±.4 (theo.) ±.3 (lumi.) 7.7 ±.6 CDF.96(p p) 9.7.4 +..4 (stat.)+.5.8 (syst.).4 ±. DØ.96(p p) 9.8.3 +.9.5 (stat.) ±. (syst.) ±.4 (lumi.).43 ±. able. Parameterization of agc. Coupling Parameters Channels WWγ κ γ, λ γ WW, Wγ WW g, κ, λ WW,W h 3, h 4 γ h γ 3, hγ 4 f4, f 5 f γ 4, f γ 5