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

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