1 / 22

Measurement of diboson production with CMS in early LHC data: the example of WZ production

Measurement of diboson production with CMS in early LHC data: the example of WZ production. Vuko Brigljevi ć Ru đ er Bo š kovi ć Institute, Zagreb on behalf of the CMS Collaboration. Physics @ LHC Split, 29 September – 4 October 2008. Motivation.

hammer
Download Presentation

Measurement of diboson production with CMS in early LHC data: the example of WZ production

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Measurement of diboson production with CMS in early LHC data:the example of WZ production Vuko Brigljević Ruđer Bošković Institute, Zagreb on behalf of the CMS Collaboration Physics @ LHC Split, 29 September – 4 October 2008

  2. Motivation • Prediction of the non-abelian SM gauge structure: Couplings between gauge bosons • Measuring the coupling between the gauge bosons tests a central part of the SM • Deviations could hint to new physics • Complementary to direct search for new physics Manifestation of gauge boson couplings at the LHC: production of final states with boson pairs (W,Z,g) V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  3. V1 V3 V2 Gauge boson couplings Triple gauge couplings (W,Z,g) • Charged couplings WWZ and WWg Allowed in the SM • Neutral couplings ZZZ, ZZg Forbidden in the SM V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  4. g λ λ κ g g g g κ λ ~ κ κ κ g g λ V V V Z γ Z Z = κ -1 = -1 ~ Δ Δ Δ Δ Δ V V V Z γ Z γ V 1 4 5 1 1 1 1 WWZ : WWγ : ( = 1 : EM gauge invariance) Triple Gauge boson couplings Most general description of the TGC vertex by a Lorentz invariant effective Lagrangian V=Z,γ K. Hagiwara et al. PRD 41, 2113 V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  5. q V1 q q V2 V1 V0 V1 q’ q q q q’ V2 V2 Diboson production at the LHC Production Processes at the LHC • Leading order Feynman diagrams: • Only s-channel has three boson vertex • Anomalous couplings tend to manifest in: • Cross section enhancement • Enhancement at high pT of V1,2. • Production angle. V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  6. WZ Cross section: s=51.5 pb (MCFM, NLO) s-channel dominated, sensitive to TGC WW Cross section: s=117 pb (MCFM NLO) s-channel dominated, sensitive to TGC ZZ* Cross section: s= 18 pb (MCFM, NLO, m(Z*) = 91 +/- 45 GeV ) t-channel dominated only at tree level Wg Cross section: s = 140 pb (Baur NLO, pt(g)>10 GeV) s-channel, sensitive to TGC Zg Cross section: s = 74 pb (Baur NLO, pt(g)>10 GeV) Large cross sections Expect tens to hundreds of events in first fb-1 Part of the SM rediscovery we can do with luminosity from ~50-100 pb-1 CMS working on all these channels Today present prospects for WZ measurement with CMS Diboson processes at the LHC (@14 TeV) V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  7. Sensitivity to anomalous couplings Signatures of anomalous couplings enhancement of cross section enhancement at high PT e.g. WW cross section σ(fb) λ Δκ Atlas-Phys-Pub-2006-011 V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  8. Measurement of pp → WZ →3lin early LHC data Considering 4 channels: “3e” : Z →ee, W →en “2e1m” : Z →ee, W →mn “2m1e” : Z →mm, W →en “3m” : Z →mm, W →mn • Background for searches with 3 leptons (+ MET) • Background for searches with WZ in final state (W’, Techni-Rho) V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  9. WZ production at the LHC Production • s-channel dominates • NLO cross section (MCFM) • sNLO (pp → W+Z) = 31.9 pb • sNLO (pp → W-Z) = 19.6 pb • Apply pt(Z)-dependent k-factor to Pythia • Accounts for dependence of k-factor on PT(Z): affects Z reconstruction efficiency k-factor vs Pt(Z) V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  10. Background samples Sample Generator Z + jets Alpgen W + jets Alpgen ttbar + jets Alpgen Zg Pythia Zbb CompHEP ZZ Pythia • Constant k-factors applied to all backgrounds • All samples fully simulated with conditions (calibration, alignment) corresponding to 100 pb-1 V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  11. Electrons Good match in position and energy between calorimeter and tracker Narrow shower in ECAL Tracker isolation Additional requirement for W →en: calorimetric (ECAL + HCAL) isolation Muons Combine central tracker and muon chamber information Require calorimetric and tracker isolation Require impact parameter significance consistent with primary vertex WZ Preselection:Trigger and lepton ID 1.Trigger requirements Single electron trigger for Z →ee channels Single muon trigger for Z →mm channels Minimize trigger bias on W lepton 97-100% efficient for selected events 2. 3 leptons (e or m) with pt>15 GeV, |h|<2.5 (2.4) for e (m) V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  12. WZ Candidate selection • Z selection: • Look for e+e- or m+m- pair with Mass in [50-120] GeV • Keep large mass window for background estimation • Veto if more than one Z candidate • W reconstruction: • Associate 3rd lepton to W-decay, • require pt(lW)>20 GeV • Use neutrino presence (MET): MT(W) > 50 GeV V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  13. W Selection: using the neutrino • Exploit MET to discriminate WZ from Z+jets: MT(W) > 50 GeV 2e1m 3e CMS Preliminary 300 pb-1 CMS Preliminary 300 pb-1 3m 2m1e CMS Preliminary 300 pb-1 CMS Preliminary 300 pb-1 V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  14. Event yields after all cuts Expected number of events for 300 pb-1 • Dominant background: Z+jets (including bbll) • More background in W →enchannels: jets much more likely to fake electrons than muons V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  15. Z mass distribution W →en W →mn 3e 2e1m Z →ee CMS Preliminary 300 pb-1 2m1e 3m Z →mm V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  16. Signal extraction strategy Question: how do we extract WZ signal (cross section) from accepted events? 3 categories of background • Non-genuine Z background: ttbar+jets, W+jets • Sideband fit • For low luminosity (no events in sideband): subtract from MC • Genuine Z Physics background (irreducible): ZZ, Zg • Estimate from MC • Genuine Z instrumental background (fake leptons): Z+jets • Data-driven background estimation (“Matrix method”) All channels, 300 pb-1 CMS Preliminary 300 pb-1 3 2 1 V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  17. Data-Driven background estimation Procedure known as “Matrix method”, e.g. used in D0: Define 2 samples • “tight”: final selection • “loose”: relaxed requirement on W lepton Nloose = Nl + Njet Ntight = etight Nl + pfake Njet Nl : # isolated true leptons Njet : # fake or non-isolated leptons etight • Efficiency for true isolated lepton to pass from loose to tight • Determine from data with Tag & Probe with Z →ee / mm pfake • Efficiency for fake or non-isolated lepton to pass from loose to tight • Determine from data control sample: W+jets, QCD, bbbar V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  18. Data-driven method: pfake • Loose sample definition: • Electrons: relax calorimetric isolation • Muons: relax isolation and impact parameter significance requirement • Pfake determination for electrons: • with W+jets: • Standard W →mn selection, trigger on muon • Select loose electron with same charge as m (reject ttbar bg) • Count number passing tight requirement • With multi-jet events: • Start with jet trigger • Select loose electron separated from triggering jet • Count number of electrons passing tight requirement Methods 1 and 2 can cross-check each other! (in agreement on MC) • Pfake determination for muons: as for electrons, can also use bb-bar sample V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  19. Matrix method results • Apply matrix method to our MC sample as if it were data • Pfake and etight determined from independent samples • Numbers and errors correspond to what is expected with 300 pb-1 • Method gives correct signal estimation • Powerful: can be applied to any distribution bin by bin • If statistics available, can correct as a function of pt,h V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  20. Systematic uncertainties Systematic uncertainties estimated for a scenario of 300 pb-1 of integrated luminosity V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  21. Observation potential • Estimate expected signal significance with toy MC: • Vary expected number of events within systematics • Dice signal and background events and compute significance for each try and estimate 68% and 95% C.L. regions • Can achieve 5s observation with less than ~350 pb-1 at 95% C.L. CMS Preliminary V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

  22. Summary • Diboson production test a central area of the electroweak theory, and their measurement at the LHC is an important part of the Standard Model rediscovery • Observation of all diboson processes expected with luminosity smaller than 1 fb-1 • Diboson production is sensitive to new physics and is important background for many searches • Prospects for WZ observation with CMS: • 5s observation with less than 350 pb-1 • Validated data-driven background estimation V. Brigljevic Dibosons with CMS Physics@LHC 2008, Split

More Related