Study of B s         Supreet Pal Singh Prof. Kajari Mazumdar Prof. J.B.Singh 1India LHC weekely meeting (6th July 2009)

The branching ratio is predicted to be 1.6 x in Standard Model. source : C. Q. Geng and C. C. Liu, J. Phys. G 29, 1103 (2003). The latest update on the branching ratio is an upper limit from D0 experiment: B( Bs   ) < 3.2 x  (pp  bb) ~ 440  b at 10 TeV. Prob(b  Bs) = 10% So,for 100 pb -1 luminosity we expect about 4400 events of the type Bs   to be produced at 10 TeV pp collision. A fraction of these will be within the detector acceptance. But this number will further reduce due to non-zero trigger thresholds on final state particles, ie, muons. 2India LHC weekely meeting (6th July 2009) Introduction to Bs  

K+K+ K-K-  .. PV SV B0sB0s B s mass = GeV 3India LHC weekely meeting (6th July 2009) General strategy B s decays  Secondary vertex properties utilized during offline analysis in this study. (displaced vertex trigger may be implemented after initial phase) Combine 2 muons  2 energetic tracks from  (        BR ~ 50 %) Since CMS does not have particle identification assign kaon mass to hard tracks other than muons and then try to form the  resonance. Most of the time the other b-quark in the event is expected to produce jet in the opposite hemisphere. B-tagging is not used in the present study though. Trigger events with 2 muons only.

India LHC weekely meeting (6th July 2009)4 CMS software, data samples etc. for this work The analysis has been done using PAT in CMSSW_2_2_6 via CRAB. Signal events have been generated privately through full reconstruction chain in CMSSW_2_2_3 on the line of official data production. (Tier2 facility at Uni. Wisconsin has been used mainly.) Pythia for generic QCD, (MSEL =1, no thresholds on hard scattered partons)  bb pair produced due to gluon fusion, flavour excitation and gluon splitting processes. EvtGenLHC generator has been used to generate the decay mode with standard format for user_decay_file : “GeneratorInterface/EvtGenInterface/data/Bs_phimumu_KK.dec” Requirement at gen. level: Presence of Bs meson 2 muons in the event with P T > 2.5 GeV and |  | < 2.5  Generator efficiency = 1.73 x No. of signal events analyzed = 8665  Integrated Luminosity = 1.0 fb-1

More on Signal and Background events No. of signal events analyzed = 8665  Integrated Luminosity = 1.0 fb-1  Average momentum of Bs ~ 10 GeV  The daughter particles in 3-body decay are quite soft!  Not a convenient situation for analysis! India LHC weekely meeting (6th July 2009)5 P T vs.  plot for Bs mesons Main source of background  QCD Background samples analyzed : InclusivePPmuX/Summer08_IDEAL_V9_v4/GEN-SIM-RECO For this dataset: MSEL=1 At least one  in the event with P T > 2.5 GeV and |  | < 2.5 Integrated Luminosity = 0.04 pb-1  Large scale factor needed to extrapolate results to 10 pb-1.

Kinematic variables from signal sample at generator level India LHC weekely meeting (6th July 2009)6 Distributions are normalized to unity  K  K  

P T and  of the leading reconstructed muons India LHC weekely meeting (6th July 2009)7 Leading Muon 2 nd Leading muon Events are normalized to unity Events lost in the central region

Analysis Criteria and Candidate Selection Two opposite charged leading pair of muons are selected. Trigger condition not applied, but same cuts at offline on recoed muons : P T  > 3.0 GeV & |   |< 2.4 All track pairs of opposite charges with P T tk > 0.4 GeV and lying in the hemisphere opposite to the leading jet (|  tk |. |  jet |< 0) are chosen to form (K+K- system ) invariant mass by assigning kaon mass. There are many combinations possible, (for background events)! The pair closest to  mass (1.005 < M  <1.035 GeV) are taken as candidate kaons from Bs decay. Events are studied in sideband region of the signal mass window (5.245 GeV – GeV) to avoid biases due to peaking backgrounds. B-Physics Meeting 2nd June 20098

Invariant Mass Distributions India LHC weekely meeting (6th July 2009)9 Huge dominance of QCD over signal formation of Bs in signal sample formation of 

Analysis Strategy From invariant mass distribution of dimuons, peaking backgrounds of J/  and Ψ(2S) are vetoed out. The events for the background are then studied in the sideband region of signal mass region (5.369 ± 2.5  ) so as to reduce bias and double counting of peaking backgrounds. The four tracks (  ±,K ± ) are expected from one mother Bs  kinematically fitted to a common vertex using Kalman Vertex Fitter technique.  Corresponding decay length of the secondary vertex from the primary vertex in the XY-plane along with χ 2 are required to satisfy quality criteria. The P T of the reconstructed shd. not be too soft  Pt(B s ) > 11 GeV muons and  should be close by  ΔR between  and  < 0.4 Around Bs candidate do not expect too much extra activity  isolation parameter using tracker information: India LHC weekely meeting (6th July 2009)10

Preliminary Cut India LHC weekely meeting (6th July 2009) GeV < M  < 4.4 GeV (excl. J/  and Ψ(2S) resonances) Selection Criteria B s   QCD bkg Efficiency No. of Events analyzed Trigger (P T  > 3.0 GeV & |   |< 2.4) Candidate Selection for B s Events within Sidebands Preliminary Cut

QCD events: Invariant mass distribution of (  k k) system in the relevant mass range India LHC weekely meeting (6th July 2009)12 Signal mass window: to GeV Integral fraction: Nev in signal window/Nev in full range = We proceed to estimate the number of background events within signal mass window by first estimating from events within the whole mass range (excluding the Bs mass region), the efficiencies for subsequent selections.  Side band estimation.

P T distribution of B s candidate & ΔR distribution between  and  system India LHC weekely meeting (6th July 2009)13 P T of the B s candidate > 11 GeVΔR  < 0.4

India LHC weekely meeting (6th July 2009)14 Decay Length (  ) < 0.1 I > 0.78 χ 2 < 2.0 Displaced Vertex Tracker Isolation l xy < 0.1

Un-correlation between P T (B s ), I, ΔR  and displaced vertex parameters India LHC weekely meeting (6th July 2009)15 These are uncorrelated to one another  the cuts can be applied Factorially. Total efficiency is the Product of individual Efficiencies.

The relative efficiency at each step of event selection for Signal and QCD background India LHC weekely meeting (6th July 2009)16 Selection Criteria B s   QCD bkg Efficiency No. of Events after prelim. cuts P T Bs > 11 GeV (Є A ) ΔR   < 0.4 (Є B ) I > 0.78 (Є C ) l xy > 0.1 & χ 2 < 2.0 (Є D ) We got various efficiencies, Number of signal and background events expected to survive in the signal mass window ( GeV) after all the cuts for a given luminosity can be calculated now.

Number of Signal and Background events for 100 pb-1 No. of signal events left after surviving all the cuts: N sig = N s. Є A. Є B. Є C. Є D where N s = the number of signal events surviving in the signal mass region ( GeV) after preliminary cuts = 391 N sig = 7.95 for 1 fb-1 Total cumulative signal efficiency, ε s = 7.95/8665  9.2 x For 100 pb-1 number of signal events expected to survive: n s = Total number of background events expected after applying all the selections for 100 pb-1 luminosity : n b = 4.75 India LHC weekely meeting (6th July 2009)17

Calculations for 100 pb-1 luminosity The upper limit on branching fraction of the process B s   B UL = N UL (90%) / (ε s.  gen. N Bs ) ε s = signal efficiency = 9.2 x ε gen = Kinematic acceptance that the produced B s meson decays into dimuon and  according to generator level cuts = 0.13 N Bs = No. of B s mesons expected to be produced for 100 pb-1 luminosity. N UL (90%) = upper limit on mean of Poisson variable n at 90% confidence level. N UL is obtained using TRolke method in root package taking into account the number of background events (n b ) and total number of observed events (n tot ) using relation n tot = n b + n s. N UL = 4.94 Now, N Bs =  bb. L. f s = 8.8 x where  bb = b production cross section = 440  b L = integrated luminosity = 100 pb -1 f s = 0.1 (percentage of b-bbar to form B s meson) 18India LHC weekely meeting (6th July 2009)

Conclusion Our analysis derived a signal selection efficiency of 9.2 x Since we considered data of accumulated lumi only 100 pb-1 and the SM expectation of Br is about 10 -6, again it is clear that we cannot observe a significant number of signal events with early data in CMS. The upper limit on the branching fraction : B(B s   6.7 x However the analysis method is established to search for this rare decay and with more data one expects to get better result. Also at higher energy signal selection will be more efficient. 19India LHC weekely meeting (6th July 2009)