Study of Bs

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 10-6 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 10-6

• (ppbb) ~ 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 -

. .PV

SVB0

s

Bs mass = 5.369 GeV

3India LHC weekely meeting (6th July 2009)

General strategy

Bs 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.

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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 PT > 2.5 GeV and || < 2.5 Generator efficiency = 1.73 x 10 -4

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-1Average momentum of Bs ~ 10 GeVThe daughter particles in 3-body decay are quite soft!Not a convenient situation for analysis!

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PT vs. plot for Bs mesons

Main source of background QCDBackground samples analyzed : InclusivePPmuX/Summer08_IDEAL_V9_v4/GEN-SIM-RECO For this dataset: MSEL=1 At least one in the event with PT > 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

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Distributions are normalized to unity

K

K

PT and of the leading reconstructed muons

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Leading Muon2nd 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 : PT

> 3.0 GeV & ||< 2.4

• All track pairs of opposite charges with PTtk > 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 – 5.491 GeV) to avoid biases due to peaking backgrounds.

B-Physics Meeting 2nd June 2009 8

Invariant Mass Distributions

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Huge dominanceof QCD over signal

formation of Bs in signal sampleformation 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 χ2 are required to satisfy quality criteria.

• The PT of the reconstructed shd. not be too soft Pt(Bs ) > 11 GeV

• muons and should be close by ΔR between and < 0.4• Around Bs candidate do not expect too much extra activityisolation parameter using tracker information:

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Preliminary Cut

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1.4 GeV < M < 4.4 GeV (excl. J/ and Ψ(2S) resonances)

Selection Criteria Bs QCD bkg

Efficiency Efficiency

No. of Events analyzed 8665 4663731

Trigger (PT > 3.0 GeV & ||< 2.4) 0.489 0.006

Candidate Selection for Bs0 0.684 0.731

Events within Sidebands 0.259 0.047

Preliminary Cut 0.66 0.534

QCD events: Invariant mass distribution of ( k k) system in the relevant mass range

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Signal mass window: 5.245 to 5.491 GeVIntegral fraction: Nev in signal window/Nev in full range = 0.132

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.

PT distribution of Bs candidate & ΔR distribution between and system

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PT of the Bs candidate > 11 GeV ΔR < 0.4

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Decay Length () < 0.1

I > 0.78

χ2 < 2.0

Displaced Vertex

Tracker Isolation

lxy < 0.1

Un-correlation between PT(Bs), I, ΔR and displaced vertex parameters

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These are uncorrelated to one another the cuts can be applied Factorially.

Total efficiency is theProduct of individualEfficiencies.

The relative efficiency at each step of event selection for Signal and QCD background

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Selection Criteria Bs QCD bkg

Efficiency Efficiency

No. of Events after prelim. cuts 513 487

PTBs > 11 GeV (ЄA ) 0.665 0.227

ΔR< 0.4 (ЄB ) 0.371 0.121

I > 0.78 (ЄC ) 0.306 0.158

lxy > 0.1 & χ2 < 2.0 (ЄD) 0.271 0.006

We got various efficiencies,Number of signal and background events expected to survive in the signal mass window (5.245-5.391 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: Nsig = Ns . ЄA . ЄB . ЄC . ЄD

where

Ns = the number of signal events surviving in the signal mass region (5.245-5.492 GeV) after preliminary cuts = 391

Nsig = 7.95 for 1 fb-1

Total cumulative signal efficiency, εs = 7.95/8665 9.2 x 10-4 .

For 100 pb-1 number of signal events expected to survive: ns = 0.795

• Total number of background events expected after applying all the selections for 100 pb-1 luminosity :

nb = 4.75India LHC weekely meeting (6th July 2009) 17

Calculations for 100 pb-1 luminosity• The upper limit on branching fraction of the process Bs BUL = NUL(90%) / (εs . gen . NBs) εs = signal efficiency = 9.2 x 10-4

εgen = Kinematic acceptance that the produced Bs meson decays into dimuon and according to generator level cuts = 0.13

NBs = No. of Bs mesons expected to be produced for 100 pb-1 luminosity. NUL (90%) = upper limit on mean of Poisson variable n at 90% confidence level.

• NUL is obtained using TRolke method in root package taking into account the number of background events (nb) and total number of observed events (ntot) using relation ntot = nb + ns .

NUL = 4.94

Now, NBs = bb . L . fs = 8.8 x 1011

where bb = b production cross section = 440 b L = integrated luminosity = 100 pb-1

fs = 0.1 (percentage of b-bbar to form Bs meson) 18India LHC weekely meeting (6th July 2009)

Conclusion

• Our analysis derived a signal selection efficiency of 9.2 x 10-4

• 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(Bs 6.7 x 10-6 .

• 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.

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