# Petrakou Eleni

## Transcription

Petrakou Eleni

Anomalous Trilinear Gauge Couplings in the Z(Z(->ll) γ channel at the CMS experiment Eleni Petrakou (NCSR Demokritos) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology Thessaloniki, 25-28 March 2010 Trilinear Gauge Couplings (1/4) SelfSelf-interactions of gauge bosons enter the SM because of the nonnon-Abelian character of the electroweak sector. +/-: WWZ, WWW, WWγ Trilinear couplings involving the W+/WWγ Their existence was an important prediction of the SM. However, interactions involving only electrically neutral vector bosons are prohibited at Born level. (At one-loop level, fermionic loops contribute much less than the current detector sensitivity at LHC energies.) Simultaneous Zγ Zγ production would require such couplings, -- therefore, its detection would be a signal of new physics. Trilinear Gauge Couplings (2/4) In the specific case of the Zγ Zγ channel with leptonic Z decay, decay, the the only detectable SM contributions are: γ: initial-state radiation γ: final-state radiation And they act as backgrounds to the present study. Trilinear Gauge Couplings (3/4) It is possible to study TGCs in a modelmodel-independent way: Starting from all possible terms in the lagrangian and keeping only very basic symmetries (EM gauge invariance, Lorentz invariance of on-shell photon, assuming negligible lepton masses), we arrive at an effective Lagrangian. Lagrangian. The result for the Zγ ZγV vertex includes 4 general anomalous coupling parameters, hiZ: CP violating Z (q1) V (P) CP Conserving γ (q2) Trilinear Gauge Couplings (4/4) U.Baur et al. used helicity amplitudes summation to calculate the contribution of the Zγ ZγV diagrams to the Zγ Zγ crosscross-section. The main signatures of anomalous TGCs would be the enhancement of the total production cross-section, and the photon Pt distribution towards higher values; there are also expected differences in angular distribution etc. In addition, they provided software for the calculation at leading and nextnext-toto-leading logarithmic order, and QCD corrs. In this study we use an implementation of the "Baur generator" in the framework of the CMS experiment. Baur & Berger, PhRevD 47 (93) Baur et al, PhRevD 57 (98) Zgamma Baur generator The signal samples (both SM and beyond) were generated with the "Baur Zgamma" generator, generator, hadronized with Pythia, and passed through full CMS simulation. Hard scattering Baur Zgamma Hadronization, showers, FSR Detector simulation PYTHIA GEANT 4 Reconstruction of event CMSSW framework Of the four parameters in the ZγV vertex, the CPCP-conserving h3,4Z were set to the values h3Z: {±5 { 5‧10-2, 0}, h4Z: {±1 { 1‧10-3, 0} 0}, giving eight nonnon-SM combinations. The SM sample was produced by setting all hiZ = 0. Baur Production: • centre of mass E = 10TeV both Z->ee and µµ decays • cut-off scale Λ = 2TeV lepton Pt > 3GeV, |η| < 2.7 • pdf: CTEQ Set 5L photon Pt > 50GeV, |η| < 3 • only ZZγ vertex enabled ~9,500 events per sample • leading-order calculations https://twiki.cern.ch/twiki/bin/view/CMS/BaurZgamInterface The CMS experiment Event selection (1/7 (1/7) : Samples Signal: "Zγ "Zγ Baur" SM production σ @10TeV 0.12 pb Background ("official"): Z+Jets Muon QCD EMEM-enriched QCD σ @10TeV 3.7 nb 0.5 mb 0.1 mb Baur Production: • centre of mass E = 10TeV • cut-off scale Λ = 2TeV • pdf: CTEQ Set 5L • only ZZγ vertex enabled • leading-order calculations both Z->ee and µµ decays lepton Pt > 3GeV, |η| < 2.7 photon Pt > 50GeV, |η| < 3 ~9,500 events per sample C.Karafasoulis, A.Kyriakis, A.Markou, E.P. NCSR Demokritos, Athens P.Adzic, M. Djordjevic Vinca Institute, Belgrade Triggers: electron candidates with Pt>14 GeV/c 1(2) muon candidates with Pt>9(3) GeV/c ** √s=10TeV ** L = 200pb-1 ** 1. SM analysis: Electrons selection Muons selection + brem. correction Photon selection 2. Anomalous TGCs analysis Event selection (1/6) : Electrons Electron selection • Pt > 20 GeV/c, |η| < 2.5 excl. gaps • |Δηin|< 0.0040 / 0.0066 (barrel/endcaps) • |Δφin|< 0.025 / 0.020 (barrel/endcaps) • H / E < 0.01 • σiηiη< 0.0099 / 0.028 (barrel/endcaps) • |Mee – MZ| < 10 GeV/c2 • Isolation: (Tracker+Ecal+Hcal)/Pt < 0.15 • Exactly one pair of e+,e- Efficiency 95% Rejection 93% Event selection (2/6) : Muons I Muon selection • Pt > 10GeV/c, |η| < 2.5 excl. gaps • silicon detector track fit, χ2/n.d.f. < 2 • silicon hits > 10, d0 < 0.2 mm • |Mμμ – MZ| < 10 GeV/c2, after correcting Mμμ with brem (if any) • Isolation: (Tracker+Hcal)/Pt < 0.2 • Exactly one pair of μ+,μ- Efficiency 99% Rejection 80% Event selection (3/6) : Muons II Bremsstrahlung selection • Pt > 10 GeV • Distance from closest muon, dR<0.5 • H / E < 0.2 • No pixel hits γ jet (97% brem., 3% "contamination") Dimuon invariant mass for signal before (red) and after (blue) brem correction.. Dimuon invariant mass for Z+Jets background before (red) and after (blue) brem correction.. Event selection (4/6) : Photons Photon identification • Not identified as brem. (for Z->μμ) • Pt > 60 GeV, |η| < 2.5 excl. gaps • H / E < 0.2 • No pixel hits • Isolation: (Tracker+Hcal)/Pt < 0.1 • Exactly one photon passing all criteria Efficiency 96% Rejection 96% Event selection (5/6) : Event yields for electrons Event selection (6/6) : Event yields for muons Anomalous TGCs (1/4) The "discovery variable of choice" is photon's Pt: Z->ee Z->µµ Anomalous TGCs (2/4) The vertex amplitude is a linear function of the couplings, so the number of predicted events can be expressed as a function of the couplings' values {h3 h4}. More specifically, it is an elliptical paraboloidal function: Ni(h3, h4) = NiSM + Ai·h3 + Bi·h4 + Ci·h32 + Di·h42 + Ei·h3·h4 Given only some specific combinations of h3, h4, this permits the prediction of the number of events for a larger range of coupling values. To predict the coefficients we fit over the number of events for the eight pairs of h3, h4 in each Pt bin: Muller et al, CMS NOTE 2000/017 Anomalous TGCs (3/4 (3/4) Using this function for predicting the number of events –not only for the fixed pairs h3, h4– we can employ the extended log-likelihood method, for calculating the likelihood that the data from anomalous TGC's are consistent with SM and find the sensitivity of CMS to anomalous values. Z->ee Z->µµ Anomalous TGCs (4/4) CMS sensitivity on h3 and h4 values @ 95% CL: L = 0.2 fb-1 L = 1.0 fb-1 Limits from previous experiments: Tevatron: Λ=1.2TeV NLO arXiv:0810.3766v1 [hep-ex] (08) h3Ζ h4Ζ LEP II -0.20 0.07 -0.05 0.12 D0 (1.1fb-1) -0.083 0.082 -0.0053 0.0054 CDF(1.1fb-1e, 2.0fb-1µ) -0.083 0.083 -0.0047 0.0047 this study (0.2fb-1) -0.034 0.034 -0.00066 0.00069 Current work – To do Repeat the analysis at 7 TeV Data-driven background estimation ("fake-rate method") Next-to-leading order: reweighting Next-to-leading order: included in new production Systematics' analysis.