Diboson production with CMS Vuko Brigljevic Rudjer Boskovic

Diboson production with CMS Vuko Brigljevic Rudjer Boskovic Institute, Zagreb on behalf of the CMS Collaboration Physics at LHC Cracow, July 3 -8 2006 1

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 at LHC, Cracow, July 2006

Gauge boson couplings V 1 V 3 V 2 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 at LHC, Cracow, July 2006

Diboson production at the LHC q Production Processes at the LHC V 1 V 0 – Leading order Feynman diagrams: – Only s-channel has three boson vertex – Anomalous couplings tend to manifest in: q V 2 q • Cross section enhancement • Enhancement at high p. T of V 1, 2. • Production angle. V 1 q’ q We present here: Study of discovery potential for WZ and ZZ production V. Brigljevic Dibosons with CMS V 2 q’ V 1 q Physics at LHC, Cracow, July 2006

WZ and ZZ production at the LHC s channel t channel ZZ WZ • s-channel dominates • Sensitive to TGC • Only t-channel at tree level • s-channel suppressed by O(10 -4) no ZZZ TGC Constitute backgrounds (partly irreducible) for some new physics searches V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

ZZ signal definition Need to consider g* contribution Our signal V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

ZZ signal definition Zg* • Define on-shell Z as: ZZ 70 Ge. V < MZ < 110 Ge. V g*g* no g* • Distinguish 3 states: ZZ (72%), Zg* (26%), g*g* (2%) NLO cross section from MCFM: • We consider only ZZ events s. LO (pp → ZZ → 4 e) = 13. 7 fb Cut on 4 lepton pt values: s. NLO(pp → ZZ → 4 e) = 18. 7 fb pt > 30, 20, 15, 10 Ge. V/c Extract overall k-factor V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Z/g* → 4 e lepton spectra highest pt e 2 nd highest pt e with g* without g* Cut on leptons pt : 3 rd highest pt e lowest pt e p 1 t > 30 Ge. V/c p 2 t > 20 Ge. V/c p 3 t > 15 Ge. V/c p 4 t > 10 Ge. V/c Keep only ZZ! V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ signal definition 4 lepton invariant mass for process pp → WZ → eemnm (MCFM NLO) g* contribution included • Restrict to on-shell Z boson: | M(l+l-) - MZPDG | < 20 Ge. V • Implicitly forces W on shell NLO cross section from MCFM s. NLO (pp → W-Z → 3 l, l=e, m, t) = 0. 63 pb s. NLO (pp → W+Z → 3 l, l=e, m, t) = 1. 03 pb V. Brigljevic Dibosons with CMS Apply overall k-factor to Pythia Physics at LHC, Cracow, July 2006

Studied final states 1. pp → ZZ → 4 e 2. pp → WZ → 3 l (l=e, m) Effort on Signal efficiency 4 different event categories: 1. 2. 3. 4. “ 3 e”: “ 2 e 1 m”: “ 2 m 1 e”: “ 3 m”: W →en, Z → ee W →mn, Z → ee W → en, Z →mm W → mn, Z →mm Effort on Background rejection Backgrounds Events with 3 or more (true or fake) leptons and/or real Z: ttbar, Z+jets (especially Zbb), … V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Signal and Background samples Cross sections times BR (NLO) Signals ZZ(4 e) 18. 7 fb WZ(3 l, l=e, m, t) 1. 6 pb Main backgrounds tt(2 l) 62. 3 pb Z(ee)bb 60. 3 pb Z(mm)bb 60. 3 pb tt(4 e) 194 fb ZZ(2 e 2 m) 32. 3 fb k-factor 1. 37 1. 92 1. 6 2. 4 1. 6 1. 35 (bg. For WZ) tt(2 l) generated with Top. Re. X, Zbb with Comp. HEP, all others with Pythia V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Full detector simulation and reconstruction WZ event: W → en, Z →mm • Signal and backgrounds passed through full detector simulation • Used reconstructed objects: – Tracks – ECAL clusters – Electrons • ECAL clusters + tracks – Muons • Muon system + tracker – Jets • For jet veto (WZ) V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

ZZ Event selection • Find 4 electron candidates 2 entries per event – Ordered by Pt, must satisfy: Pt > 30, 20, 15, 10 Ge. V/c – |h|<2. 5 • Form Z candidate: – Combine e+e- pair – 50 Ge. V < M(e+e-) < 120 Ge. V – Order Z’s according to nearness to MZ Contribution from Zg* • Z 1 and Z 2 • Form ZZ candidate – Combine two non-overlapping Z’s V. Brigljevic Dibosons with CMS • Z 1 biased towards true Z mass and higher pt lepton • Consider Z 1 and Z 2 on equal footing Physics at LHC, Cracow, July 2006

ZZ best candidate selection • Best Z association: – possible ambiguity in lepton assignment – Choose ZZ pairing with masses closest to PDG Z mass Correct pairing for ~100% of all on shell ZZ events • Best ZZ candidate – Due to fake electrons more than one ZZ candidate can be formed (2. 5 % events) – Retain pair with highest pt non common lepton Correct choice for 98. 3 % of events V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ Event Selection / Background rejection Strategy • Find 3 isolated electrons or muons with no lifetime pt>10 Ge. V/c and |h|<2. 5 m impact parameter significance • Form Z candidate m isolation WZ in tracker – Combine same flavour. CUT opposite charge pair Sum(Pt) in cone around m – | M(l+l-) - MZPDG | < 20 Ge. V/c 2 – Veto event if more than one Z candidate WZ CUT • Find third lepton (from W) ttbar Z(ee)bb – Pt > 20 Ge. V/c – If more than one candidate, select the one with the Z(ee)bb highest pt Z(mm)bb • No jet with Etraw > 25 Ge. V – Do not consider jets close to the 3 leptons V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

ZZ→ 4 e expected signal & background M(e+e-) after all cuts (2 entries per event) Expected signal and background yields for 1 and 10 fb-1 L = 10 fb-1 Nearly background free! V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ → 3 l expected signal & background L = 1 fb-1 M(l+l-) after all cuts 4 channels combined (3 e, 2 e 1 m, 2 m 1 e, 3 m) Presence of peaking backgrounds: • Zbb • ZZ (irreducible) Expected signal and background yields for 1 fb-1 High significance in the first fb-1 ! V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Systematic uncertainties WZ ZZ Systematic uncertainties on cross section Systematic uncertainties on significance V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ and ZZ Discovery Potential ZZ→ 4 e signal significance WZ → 3 l signal significance Ideal simulation miscalibration Systematic effects 0 5 s discovery at: • ZZ : ~1 fb-1 • WZ : ~150 pb-1 V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Summary • We have investigated the discovery potential for the SM reactions pp → ZZ → 4 e and pp → WZ → 3 l (l=e, m) • Signal and background samples were processed through the full simulation, reconstruction analysis chain of the CMS detector • For 1 fb-1 we expect: – 7. 1 signal and 0. 4 background events for ZZ → 4 e – 97 signal and 23 background events for WZ → 3 l Expect to establish those signals already in the first run • Possibility to probe anomalous gauge couplings already with a few fb-1 V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Reference Study of Di-Boson Production with the CMS Detector at LHC, CERN-CMS Note 2006/108 V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

Backup slides 22

WZ event V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ & ZZ k-factors V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ yields by category V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

MET: why not use it? No clear Signal / Background separation V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

WZ Event Selection • Find 3 isolated electrons or muons pt>10 Ge. V/c and |h|<2. 5 • Form Z candidate – Combine same flavour opposite charge pair – | M(l+l-) - MZPDG | < 20 Ge. V/c 2 – Veto event if more than one Z candidate • Find third lepton (from W) – Pt > 20 Ge. V/c – If more than one candidate, select the one with the highest pt • No jet with Etraw > 25 Ge. V – Do not consider jets close to the 3 leptons V. Brigljevic Dibosons with CMS Physics at LHC, Cracow, July 2006

m isolation in tracker Sum(Pt) in cone around m CUT m impact parameter significance WZ CUT Z(ee)bb V. Brigljevic WZ Z(ee)bb Dibosons with CMS ttbar Z(mm)bb Physics at LHC, Cracow, July 2006