Outline Brief theoretical overview Search strategies Results Summary

1. Search for charged Higgs boson at CMS Outline • Brief theoretical overview • Search strategies • Results • Summary Aruna Kumar Nayak LIP, Lisbon DHEP Seminar, TIFR, Mumbai.

2. The Standard Model • Standard Model is the most successful theory of particle physics • Most of the particles of SM have been discovered and their • properties have been measured precisely • However, the Higgs boson (the fundamental ingredient of the • model) is not yet discovered • And there are also quite a few shortcomings. • Naturalness or fine-tunning problem The radiative correction to the Higgs mass squared diverges quadratically ( ~L2, L is cut of scale). In order to keep the Higgs mass in the range of electroweak symmetry breaking scale, an unnatural fine adjustment of parameters is needed. • Lack of Gauge Coupling Unification Electromagnetic, weak and Strong interactions do not converge at certain energy scale • Dark matter problem • SM particles only account for a small fraction of the • matter observed in the universe strong force

3. Supersymmetry • SUSY postulates a new symmetry between bosons and fermions For each SM fermionic spin-½ field there is bosonic spin-0 partner with exactly the same gauge quantum numbers. Similarly, each SM spin-1 field has a SUSY spin-½ partner. • Loops of particles and their supersymmetric partners have the ability to cancel the quadratic divergences in the Higgs field self-couplings hence solving the Naturalness problem. • SUSY foresees unification of couplings at • large energy scales ~ 1015 GeV • Provides candidates for Dark Matter (LSP)

4. MSSM Higgs Sector The MSSM Higgs Sector • 2 Higgs doublets, 5 physical bosons • 3 neutral ( H and h with CP even; A with CP odd ) • 2 charged (H±) The observation of a charged Higgs boson will be an unambigous signature of new physics • Controlled by two parameters at tree level • tan (ratio between vacuum expectation values of the 2 Higgs fields) • mA (mass of neutral CP-odd Higgs bosons) • Other parameters are important for radiative corrections • Here, we discuss only the search for a light charged Higgs boson in the decay of top quark.

5. Production and decay at tree level depends on MA and tanb = v1/v2(ratio of the v.e.vs) Light charged Higgs (MH+ < Mtop) : Heavy charged Higgs (MH+ > Mtop) : Search assumption : mH+ < mt,t→H+b, H+→tn, BR(H+→tn) = 1 (high tanb) Three search channels included 1) Hadronic tau decay, hadronic W decay (t + jets) : 2) Hadronic tau decay, leptonic W decay (m + t) : 3) Leptonic tau decay, leptonic W decay (e + m) : H+ in Top quark decay CMS-PAS-HIG-11-008 D0 Note 5715-CONF

6. The Large Hadron Collider • Superconducting magnets (NbTi) opperated at superfluid helium temp.(1.9K) and low vacuum (10-13 atm) • 26.7 km pp (/heavy ion) collider located at swiss-french borders (CERN) close to Geneva reaching √s =7 TeV ( designed for 14 TeV ) • Operating at instantaneous luminosity : L ~ 2 x10 33 cm-2s-1 • Delivered integrated luminosity by 8th Aug : ~ 2.1 fb-1 • Expected to go beyond 5 fb-1 in 2011 • Analysis presented today with 2011 data • Int. luminosity ~ 1 fb-1

7. The Compact Muon Solenoid Detector

8. Hadronic Tau Identification • Tau decays • To light leptons (e,μ) and 2 neutrinos • with BR ~35% • To hadrons(τh) and one neutrino • with BR ~65 % • Dominantly via π+/Κ+ ,ρ+ → π+π0and • a1 → π+π-π+(π+π0π0) • Particle‐Flow based Algorithm : Hadron + Strips (HPS) algorithm • Decay mode Finding : Require there is a valid π,ρ,a1combination (mass constraints on combination of particle candidates) • 3 possible isolation working points : Loose, medium, Tight depending on the cuts on particles in isolation cone • Electron rejection : Low electron compatibility for leading charged hadron • Muon rejection : Low muon compatibility for leading charged hadron

9. Tau Identification Performance Commissioned in data : Fake rates for jets in di-jet events, W + jets and muon enriched sample Fake rates for electron/muons with tag & probe method using Z→ll sample Efficiency measured (1) using Ζ → tt → μ + th with tag and probe, (2) Simultaneous fit for tau id efficiency and Z→tt cross section and (3) comparison of measured Z→tt cross section to measured Z→ee,mm cross section at CMS CMS-PAS-TAU-11-001

10. b-Jet Tagging Track Counting b-Tagging Algorithm: b-tag discriminator = 3-dimentional impact parameter significance = IP/sIP Two variants : High Efficiency : Two tracks with IP3D significance above a given threshold Discriminator : IP3D significance of 2nd track High Purity : Three tracks with IP3D significance above a given threshold Discriminator : IP3D significance of 3rd track

11. Muon + Hadronic tau decay -jet  H+  g/q t  g b b t l g/q W- μ 1.09 fb-1 of data used • Main backgrounds : , W+jets • Event Selection : • Isolated single muon trigger (pT > 17 GeV/c) • All physics objects are reconstructed using Particle Flow method with charged hadrons originating from pileup vertices removed. • One isolated muon pT >20 GeV/c • At least 2 jets pT>30 GeV/c (AK5, particle flow), |h| < 2.4 • ET miss (particle flow) > 40 GeV • One tau pT >20 GeV/c • Opposite-Sign between muon and tau • At least one b-jet , Track Counting High Efficiency Loose working point (At least two tracks with 3D impact parameter significance > 1.7)

12. Muon Selection • Preselection : 1 muon + 3 jets • (2 jets pT> 30 GeV/c + 1 jet pT> 20 GeV/c) • ID : Global muon prompt tight • min tracker hits > 10, • min muon hits > 1 • c2/ndof < 10 • Global & tracker muon • pT >20 GeV/c , || < 2.1 • |d0| < 200 m • R (muon, jet) > 0.3 • PF isolation cone 0.3 , rel.iso. < 0.2 • Loose muon (electron) veto • pT >10 GeV/c (15 GeV/c)

13. Jet Selection • Preselection : 1 muon + 3 jets • (2 jets pT > 30 GeV/c + 1 jet pT > 20 GeV/c) • AK5 PFlow Jets • pT > 30 GeV/c, || < 2.4 • R (muon, jet) > 0.3 • L1 FastJet + L2 (relative) +L3 (absolute) ,L2L3 Residual (data)

14. MET and b-tagging MET distribution for events with one isolated muon and at least three jets B-tag jet multiplicity for events with one isolated muon+at least 3 jets +MET

15. Tau Selections • Preselection : 1 muon + 3 jets + MET + ≥ 1 btag • pT > 20 GeV/c, || < 2.4 • Hadron Plus Strips loose isolation • medium electron rejection, loose muon rejection Not used in the selection

16. Muon + Tau event yields Event yields at each selection step An excess of event yields expected in the presence of charged Higgs boson

17. Data driven -fake background Fake Rates as function of jet pt • Main background from “fake” tau jets W+≥1jet • Data driven background estimation : • - Select jets in events with : • 1 lepton + MET + 3 jets • + ≥ 1 b-tagged jet • - Apply to every jet a • “jettau probability (pt,eta,jet Width)” QCD

18. Jets in l+ETmiss+3Jets events • Jets that can fake taus in events with : • 1 isolated muon pT>20 GeV/c • At least 2 jets with pT>30 GeV/c • ( ≥ 1 b-tagged jet ) • + 1 jet with pT>20 GeV • (If there is only one b-tagged jet, it is not counted. However if there are more than one b-tag jet in an event, all of them are counted) • MET > 40 GeV • Dominated by W+jets and tt~ -> l+jets events

19. Uncertainties on -fake background -fake background fake rate applied in bins pT, eta, jet width MVA method is used to apply tau fake probability to jets. k Nearest Neighbour (kNN) is trained with jets passing/failing the tau discriminators Contribution from real tau jets estimated from MC closure test within uncertainty The fake-tau background is estimated at “tau selection step”. In order to get the estimated fake-tau background at “OS selection step” an efficiency 0.66 +/- 0.04 (estimated from MC) is applied.

20. Uncertainties on -fake background Systematics • Uncertainty on tau ID efficiency : 7% • JES,JER and MET uncertainty evaluated as function of jet pT and h (includes unc. due to pileup, jet flavor b-jet, unclustered jet energy) Uncertainty on ttbar cross section : 20% • B-tagging/mis-tagging uncertainty : Data/MC scale factor for b-tagging efficiency and mis-tag rates used (CMS-PAS-BTV-11-001) to rescale MC and the corresponding uncertainty is propagated. • muon trigger & ID & Isolation: 1%

21. Muon + Tau event yields Summary of event yields after final selection Background measured from data Data agrees well with the SM expectations within the uncertainty NO excess observed

22. Fully Hadronic final state -jet  H+ g/q  t  g b b t q g/q W- q 1.08 fb-1 of data used • Main backgrounds: QCD multi-jet, ttbar, W+jets • General selection strategy is to suppress • QCD multi-jet background below ttbar and • other backgrounds • Event Selection : • Trigger: Single tau + ETmiss trigger • HLT_IsoPFTau35_Trk20_MET45 (340 pb-1), HLT_IsoPFTau35_Trk20_MET60 (740 pb-1) • HLT MET > 60 GeV required offline to have uniform thresholds • Require one  jet pT > 40 GeV/c, pT(leading particle)>20 GeV/c, |h| < 2.4 • Take as -jet candidates the jets matching to the trigger  jets • HPS, tight isolation, againstElectronMedium, againstMuonTight, one prong • ETmiss (particle flow) > 70 GeV • At least 3 jets (AntiKt 0.5, particle flow), pT > 30 GeV/c, || < 2.4 • At least one b-tagged jet, Track Counting High Efficiency Loose working point (At least two tracks with 3D impact parameter significance > 1.7)

23. Tau and ETmiss Selection Distributions after id, excluding tau isolation

24. Jet selection Number of selected jets (pT > 30 GeV/c, |eta| < 2.4) after id, lepton veto and MET cut, excluding tau isolation. Number of selected b-jets after id, lepton veto, MET cut and jet number requirement, excluding tau isolation. B-tagging scale factors are applied.

25. Fully Hadronic Event Yields Event yields at each selection step An excess of event yields expected in the presence of charged Higgs boson Signal selection efficiency at each selection step

26. Background measurements • QCD multi-jet background, measured from data • Method based on factorisation of ETmiss + b-tagging from other selections • Apply same selections like signal analysis but in different order. • Factorize e(ETmiss + b-tagging ) at a selection level where QCD contribution dominates. e (ETmiss + b-tagging ) estimated in bins of t pT. • EWK and tt~ background (genuine taus within pT, η acceptance), measured from data • Based on tau embedding method • Select events with only one isolated lepton (pT > 40 Gev/c, |h| < 2.1) and 3 jets (pT > 30 GeV/c, |h| < 2.4). Replace the muon by a fully simulated and reconstructed tau with same momentum. • EWK and tt~ fake tau background (e/μ/jets mis-identified as taus, or genuine taus outside pT, η acceptance) • Expected to be small, estimated from simulation

27. Systematics

28. Fully Hadronic Event Yields Summary of event yields after final selection The major backgrounds are measured from data Data agrees well with the SM expectations within the uncertainty NO excess observed

29. Uncertainties on -fake background em final state  e/μ H+ l g/q t g   b b l t g/q W- μ/e 0.98 fb-1 of data used • Tau decays leptonically • Main background: • Event selection: • e-m trigger • one e (pT>20 GeV/c ), one  (pT>20 GeV/c ) • At least 2 jets (pT>30 GeV/c) Deficit of total events expected in the presence of charged Higgs boson, because e/m from t decay become soft

30. Uncertainties on -fake background Systematics Low systematic uncertainty compared to other two channels b-tagging is not used

31. Uncertainties on -fake background Expected events vs BR mth channel em channel x = BR(tbH+) Ntt (in presence of H+) = NWH 2(1-x)x + NHH x2 + NttSM (1-x)2

32. Statistical Interpretation A frequentist approach (CLs) is being used to obtain upper limit on BR(t-> bH+) Define a likelihood ratio Q : Run 105 toy expts to generate -2lnQ distributions for “background only” and “background + signal” hypothesis Adjust “r” to get value of CLs = 0.05 “r” is signal strength modifier

33. Uncertainties on -fake background Upper limit on BR (t → H+b) muon+tau fully hadronic electron+muon 95 % CL upper limit on BR(t→H+b) using CLs method. The signal is modelled as the excess (or deficit) of events yields in presence of H+ Nexcess (deficit)= NttSUSY – NttSM = NWH 2(1-x)x + NHH x2 + NttSM ((1-x)2 – 1) , x = BR(t→H+b)

34. Uncertainties on -fake background Results from Combination of three channels combination of the fully hadronic, muon + tau and electron + muon channels New World limit exclusion region in MSSM (mhmax ) MH+-tan parameter space Tevatron limit : 0.15 – 0.2

35. Summary • The charged Higgs boson in the decay of Top quark is searched assuming BR(H+→tn) = 1 • Three search channels have been included : tau + jets, muon + tau and electron + muon • Major backgrounds have been measured from data • No significant excess/deficit of events have been observed • Upper limit on BR(t→H+b) = 0.04 – 0.05 (new world limit) depending on H+ mass

36. Thank You

37. Previous limits on BR(tbH+) CMS BR Limit ( single channel) D0 BR Limit (multi-channel) HIG-11-002

38. QCD multi-jet background Sample purity 60-90%

39. EWK and tt~ tau background (genuine taus) data-driven • Control sample selection • One muon, pT > 40 GeV/c, |η|<2.1 • Isolation by requiring no HPSTight-quality PFCandidates in 0.1 < ΔR < 0.4 • Veto of isolated electrons and other muons, pT > 15 GeV/c • At least 3 PF jets, pT > 30 GeV/c, |η| < 2.4 • Tau embedding at PF level • Simulate and reconstruct tau with same momentum as muon • Normalisation • Tau trigger efficiency • MET trigger efficiency with ”vector sum caloMET” > 60 GeV • Muon trigger and ID efficiency with Tag and Probe • Result: 71 ± 5 (stat) ± 15 (syst)MC expectation: 78 ± 7 (stat) Beforeisolation Afterisolation

40. EWK and tt~ bkg measurement