Exclusive γγ WW and Anomalous Quartic Gauge Couplings at CMS Universite catholique de Louvain Email: jjhollar@cern.ch Recent CMS results sensitive to quartic couplings of gauge bosons are discussed. he signatures investigated are exclusive and quasiexclusive γγ WW scaering, and triboson production in the WWγ and WZγ channels. In the γγ WW analysis two candidate events pass the selection criteria, consistent with the Standard Model expectation. No deviations from the Standard Model are observed in the high p tails of either measurement, and the resulting limits on anomalous quartic gauge couplings are the most stringent to date. PoS(EPSHEP )8 he European Physical Society Conference on High Energy Physics EPSHEP 8 July Stockholm, Sweden Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons AributionNonCommercialShareAlike Licence. hp://pos.sissa.it/
. Introduction he existence of quartic couplings of the gauge bosons, WWγγ, WWZZ, WWWW, and WWZγ, in conjunction with triple gauge couplings, is a fundamental prediction of the electroweak sector of the Standard Model (SM). While deviations from the SM triple gauge couplings have been extensively searched for at the Large Hadron Collider (LHC) and earlier experiments, the quartic couplings are less constrained experimentally. he high energies and large integrated luminosity available at the LHC greatly improve the sensitivity to beyond SM effects in these interactions, typically parameterized in terms of anomalous quartic gauge couplings (AQGC s). At the LHC, AQGC s can be investigated through two complimentary approaches: gauge boson scaering, such as γγ WW, and production of triboson final states, such as WWγ and W Zγ. First results from both approaches have been obtained at the Compact Muon Solenoid (CMS) experiment in pp collisions at s = 7 ev and s = 8 ev.. γγ WW analysis Exclusive or quasiexclusive production is characterized by the detection of two W bosons, with no additional activity from the underlying event. his signature may arise when both incident protons remain intact and scaer undetected along the beamline, or, in the case of quasiexclusive production, when one or both protons dissociate into a lowmass system that goes undetected. he CMS analysis [] uses.fb of data collected at s =7 ev, with a mean of 9 interactions ( pileup ) per bunch crossing. In order to reduce the sensitivity to pileup, the exclusive selection is based only on charged tracks associated to a dilepton vertex. Only final states in which the WW pair decays to an electron, muon, and neutrinos are considered when searching for a signal. In the case of sameflavor lepton final states, the contribution from DrellYan and direct γγ l + l production is more than an order of magnitude larger. herefore the sameflavor µ + µ events are instead used as a control sample to study the backgrounds, the contribution of proton dissociation in highmass γγ interactions, and the effect of pileup on the selection. Events /. GeV 8 CMS, s = 7 ev, L =. fb m(µµ) > GeV with Z region removed DrellYan τ τ DrellYan µ µ (doubledissociation) (singledissociation) (elastic) Events /. GeV CMS, s = 7 ev, L =. fb m(µµ) > GeV with Z region removed LPAIR γγ µ µ Inclusive WW DrellYan τ + τ DrellYan µ + µ (doubledissociation) (singledissociation) (elastic) PoS(EPSHEP )8..... p (µµ) [GeV] 7 8 9 p (µµ) [GeV] Figure : Distribution of p (µ + µ ) for dimuon events with zero extra tracks, for selections enriched in elastic (left) and proton dissociation (right) γγ µ + µ events. In a kinematic region enhanced in elastic pp pµ + µ p events, good agreement is observed between data and the prediction from the LPAIR [, ] Monte Carlo generator (Fig. left). In
a region enhanced in proton dissociation events, a deficit is observed compared to LPAIR, which does not include rescaering corrections. he ratio of the observed γγ µ + µ to the prediction is therefore used to rescale all γγ cross sections, including the γγ W + W signal. In the µe channel the Standard Model signal region is defined by requiring a µe vertex with zero additional associated tracks, and p (µe) > GeV. he expected background is.8 ±. events, while the expected SM signal is. ±. events. In the data two events are observed surviving all selection criteria. he properties of the selected events are consistent with the expectation from the sum of backgrounds and Standard Model γγ W + W signal (Fig. ). he results are converted into a cross section times branching fraction of σ(pp p ( ) W + W p ( ) p ( ) µ ± e p ( ) ) =. +.. fb, with an upper limit of. fb at 9% confidence level. Events/ GeV 8 7 Elastic γγ γγ W W CMS, s = 7 ev, L =. fb (SM) DrellYan τ τ Inelastic γγ 8 m(eµ) [GeV] Events/. 8 7 Elastic γγ γγ W W CMS, s = 7 ev, L =. fb (SM) DrellYan τ τ Inelastic γγ.......7.8.9 φ(eµ)/π Events/ GeV 8 7 Elastic γγ γγ W W CMS, s = 7 ev, L =. fb (SM) DrellYan τ τ Inelastic γγ E [GeV] Figure : Dilepton invariant mass (left) and acoplanarity (center), and missing transverse energy (right), for µe events with zero associated tracks and p (µe) > GeV. he solid histograms indicate the SM backgrounds, while the open histograms indicate the expected SM signal, stacked on top of the backgrounds. he p of the µe pair is used as the observable to search for evidence of anomalous couplings, by counting events in the region p (µe) > GeV, where the SM background expectation is. events, dominated by γγ W + W. No events with p (µe) > GeV are observed (Fig. ), and a model independent upper limit on the partial cross section for p (µ,e) > GeV, (η(µ,e)) <., and p (µ ± e ) > GeV is set at.9 fb at 9% CL. Events/ GeV Elastic γγ γγ W W (SM) CMS, s = 7 ev, L =. fb DrellYan τ τ Inelastic γγ a W a W C γγ W W ( = *, =, Λ cutoff =GeV) Λ Λ a a W W C γγ W W ( = *, = 8*, Λ cutoff =GeV) Λ Λ PoS(EPSHEP )8 p (eµ) [GeV] Figure : p (µe) for events with zero extra tracks. he solid histograms indicate the SM backgrounds, while the open histograms indicate the expected γγ W + W signal for the SM (solid line) and two example AQGC scenarios (doed and dotdashed lines).
. WV γ analysis riboson production in the CMS analysis [] is probed via WV γ events, in which the W decays to a µν or eν final state, and V denotes either a second W or a Z boson decaying to two jets. he analysis uses 9.fb of data collected at s =8 ev. he main background after all selections arises from Wγ+jets, which is estimated from data by fiing to the m j j mass distribution outside the W/Z signal region (7 < m j j < GeV). Further details of the event selection and backgrounds can be found in Refs. [, ]. Events/ GeV Ratio /MC CMS preliminary Muon MC Uncertainty L dt = 9. fb W SM + a / Λ = ev s = 8 ev jets γ bkg Zγ+Jets top Wγ+jets WVγ Photon p (GeV) Events/ GeV Ratio /MC CMS preliminary Electron MC Uncertainty L dt = 9. fb W SM + a / Λ = ev s = 8 ev jets γ bkg multijet Zγ+Jets top Wγ+jets WVγ Photon p (GeV) Figure : Photon p for selected events in the muon (left) and electron (right) channels. he solid histograms indicate the SM signal and backgrounds, while the open histogram shows an example of an AQGC signal. In the muon channel 8 events pass the final selection criteria, while in the electron channel the number is 9. hese values are consistent with the expectations of 9.9 ±.9(stat.) ±.8(syst.) ±.(lumi) and 7. ±.8(stat.) ± 9.(syst.) ±.7(lumi) in the muon and electron channels, respectively. An upper limit on the combined cross section is set at fb at 9% CL, approximately. times larger than the SM prediction. he photon p is used as the observable to search for evidence of anomalous couplings (Fig. ). Fits to binned templates of the p (γ) distribution are used to compute the limits, with the rightmost bin treated as an overflow. No excess above the SM prediction is observed at high p (γ). PoS(EPSHEP )8. AQGC Results he results are interpreted in terms of anomalous quartic gauge couplings. In the γγ W + W analysis the dimension formalism is used [], while in the WV γ analysis limits are given in both the dimension and dimension8 [7] approaches. he limits from γγ W + W are obtained both with and without a dipole form factor having Λ cutoff = GeV. In the case of the WV γ analysis, no form factor is used. he one and twodimensional limits obtained from the γγ W + W analysis with Λ cutoff = GeV are shown in Fig. at left. he resulting limits are approximately x more stringent
than those obtained at LEP [8, 9,,,, ], and x more stringent than those obtained from γγ W + W searches at the evatron [] with Λ cutoff = GeV. ] [GeV a W C /Λ.. CMS, s = 7 ev, L =. fb July LEP L limits D limits CMS WWγ limits CMS γ γ WW limits Anomalous WWγ γ Quartic Coupling limits @9% C.L. Channel Limits L a W /Λ ev WW γ [, ].fb. ev γ γ WW [, ] 9.7fb.9 ev WW γ [, ] 9.fb 8. ev γ γ WW [, ].fb 7. ev s.. Standard Model CMS 9% confidence region (Λ cutoff = GeV) CMS D limit, 9% confidence region (Λ cutoff = GeV).. W a /Λ [GeV ] a W C /Λ ev f, /Λ ev WW γ [ 8, ].fb. ev γ γ WW [, ] 9.7fb.9 ev WW γ [, ] 9.fb 8. ev γ γ WW [, ].fb 7. ev WW γ [, ] 9.fb 8. ev Figure : Left: twodimensional limits on dimension anomalous WWγγ quartic couplings from the CMS γγ W + W analysis, with a form factor of Λ cutoff = GeV. Right: Compilation of onedimensional limits on dimension and dimension8 anomalous WWγγ quartic couplings, with no form factors. he limits obtained by CMS in the WV γ and γγ W + W analyses with no form factors are shown in Fig. at right, compared to the limits from previous measurements at LEP and the evatron. With no form factors, the limits on the dimension anomalous WWγγ couplings are up to two orders of magnitude more stringent than previous constraints obtained at the evatron with no form factors. he WV γ analysis further derives the first experimental constraints on the CPconserving anomalous WWZγ coupling parameters κ W /Λ and κ W C /Λ, finding at 9% CL:. Summary < κ W /Λ < ev, 8 < κ W C /Λ < 7eV. PoS(EPSHEP )8 he investigation of quartic gauge boson couplings at the LHC has begun, with the first results obtained by CMS in protonproton collisions at center of mass energies of 7 and 8 ev. In a search for γγ W + W production two candidate events are selected, compared to a Standard Model expectation of.8 ±. background with. ±. signal. In a search for triboson production WV γ, an upper limit on the cross section is set at. times the Standard Model prediction. No excess is observed in the high p (µe) or p (γ) tails of either analysis, and limits are set on anomalous quartic gauge couplings. he limits on the anomalous WW γγ couplings are up to times more restrictive than those obtained at LEP or the evatron, while the first limits are set on CPconserving anomalous WW Zγ couplings.
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