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Study of the B → J/ψ decays

Study of the B → J/ψ decays. Giuseppe E Bruno Università di Bari and INFN - Italy. Outline motivations strategy status of the analysis outlook. Motivations. The study of the channel B  J/ y +X is an important issue since

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Study of the B → J/ψ decays

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  1. Study of the B → J/ψ decays Giuseppe E Bruno Università di Bari and INFN - Italy Outline motivations strategy status of the analysis outlook III Convegno Nazionale sulla Fisica di ALICE

  2. Motivations The study of the channel B  J/y +X is an important issue since • a large fraction (≈30% ) of total J/y come from beauty hadron decays • it provides another measurement of the sbb cross-section besides (competitive with) the single electron channel (B e +X) e+e- G.E. Bruno

  3. Pros & cons of BJ/y w.r.t Be+X • Being the J/y much heavier than the electron, it resembles more an “exclusive” channel • It can allow a determination of the dsbb/dpt cross-section down to pt≈0 • Invariant mass analysis • Branching ratios: B e+n+X 10.8% B J/y + X 1.16±0.10% J/y e+e- 5.94±0.06% BJ/ye+e- 0.069±0.007% G.E. Bruno

  4. Experience at colliders: CDF√sNN=1960 GeV • CDF measures J/y in the dimuon channel using SVX II (five layers) to separate primaries from secondaries (i.e. from B decays) • the channel e+e- has a worsen mass resolution than the mm due to bremsstrahlung • Performance of CDF • CDF resolution on primary vertex is 30m (runbyrun) • CDF s(pt)/pt2=0.0017 GeV/c-1 (COT) G.E. Bruno

  5. Experience at colliders: CDF√sNN=1960 GeV • Since the B hadrons with as little as zero momenta produce J/y with momenta as large as 2 GeV/c, the measured cross section for pt(J/y)>1.25 GeV/c is sensitive to the complete pt(B) spectrum Ref: D. Acosta et al Phys. Rev. D 71, 032001 (2005) G.E. Bruno

  6. Strategy • Analysis of total J/Y is quite simple • Combinatorics between positives and negatives • PID selection of e+e- pairs: TRD + TPC • Invariant mass analysis • background evaluation • This analysis aims at separating secondary J/Y from prompt J/Y • it relies on the spatial resolution of the ITS (and of the SPD in particular) G.E. Bruno

  7. Impact parameter resolution • d0 resolution Silicon Pixel Detector • 2 layers, R=4 and 7 cm, ~107 channels pt > 1 GeV/c < 60 mm (rf) ~12m asymptotic BEAM (Z) pixel size 50425 m d0 resolution G.E. Bruno

  8. Particle IDentification scheme • Two possibilities implemented in AliBtoJPSitoEleAnalysis • TString fPID=“TRDTPCparam” (only for Sim) • TString fPID=“ESDCombinedPID” (default) • The ESD pid of PDC06 is not “reliable”  in the present analysis a parameterisation of the pid performance has been used (for TPC and TRD) • Each MC particle is assigned a weight, depending on the specie: • e+ and e- 0.81 • p+ and p-  peff(p) (≈10-4 10-2 see next slides) • All the others  0. G.E. Bruno

  9. TPC-TRD parameterisation of the probability to identify a p as an electron Particle IDentification scheme G.E. Bruno

  10. MC samples used for the analysis • Sample with secondary J/Y • equivalent to 9.9·109 min. bias pp events • Sample with prompt J/Y • equivalent to 14.9·109 min. bias pp events • Sample of m.b. events to study the background • 2.8*106min. bias pp events, i.e. all available (pre-staged) data at the Cern Analisys Facility dedicated production on the Bari farm G.E. Bruno

  11. Kinematical distributions:prompt J/yversus secondary J/y • rapidity and transverse momentum (all distribution normalized to 1 m.b. event) Primary J/y y Secondary J/y but pT dependent G.E. Bruno

  12. Kinematical distributions:prompt J/yversus secondary J/y • Invariant mass The shape is due to: • detector resolution • bremsstrahlung process on the path through the detecotor material • “internal” bremsstrahlung process J/ye+e-g are not considered (effect would be of the order of 5%) Primary J/y Secondary J/y G.E. Bruno

  13. Kinematical distributions:prompt J/yversus secondary J/y • Impact parameters d0 Primary J/y Secondary J/y e+ e- G.E. Bruno

  14. Kinematical distributions:prompt J/yversus secondary J/y • product of impact parameters d0xd0 Primary J/y Secondary J/y G.E. Bruno

  15. Kinematical distributions:prompt J/yversus secondary J/y • “pseudo proper decay time” x Primary J/y Secondary J/y G.E. Bruno

  16. e+e- tot eh h+h- Background: different components • The dominant component is e+e- • The available statistics (≈3M m.b.events) is not enough to study it • The event mixing will be tried • With the aim of developing the tools for analysis, I will consider in the following the bkgnd. as equal to h+h- normalized to the total background G.E. Bruno

  17. using a cosq* cut only J/y signal versus normalized h+h- background tot J/y (prompt+secondary) bkg for N=109 events: in the seected mass window Events/10 MeV G.E. Bruno

  18. Performance plot w.r.t. CDF tot tot J/y J/y from B bkg ALICE CDF pT>0 G.E. Bruno

  19. How to measure the fraction of secondary J/y Events/10 MeV • In order to extract the fraction fB of J/y from b-hadron decays, one should fit simultaneously • the invariant mass spectrum • one distribution which can discriminate prompt from detached J/y (e.g. x or d0e+·d0e-) • the tails of the M(e+e-) distribution measures the percentage of background G.E. Bruno

  20. How to measure the fraction of secondary J/y • It has been implemented the approach used in CDF [Acosta et al. PRD 71 032001 (2005)], i.e., simulataneous mass and lifetime fit using the log-likelihood function: N is the number of events in |mee-MJ/y|<D, D=200 MeV G.E. Bruno

  21. How to measure the fraction of secondary J/y fraction of secondary J/y MonteCarlo templates of the x distribution cMC(x,pt) resolution function convoluted with G.E. Bruno

  22. How to measure the fraction of secondary J/y the x MC distributions (as obtained by Pythia) interpreted (after interpolation) as the template for cMC G.E. Bruno

  23. How to measure the fraction of secondary J/y resolution function • to be determined (from MC) looking at prompt J/Y • in this first exercise, parametrized as sum of 3 Gaussian centred at 0 (5 param) G.E. Bruno

  24. Parameterisation of • In CDF, the di-muon mass shape is taken as the sum of two gaussian distributions • not adequate for the di-electron channel • the best parameterisation looks to be that of Hera_B (used in p-A for both e+e- and m+m-), also adopted by PHENIX • ref: A. Spiridonov hep-ex/0510076 • the first attempt to introduce this parameterisation in the likelihood fit has failed • for the time being, I’ve used an ad hoc parameterisation: the sum of 2 Landau with same mpv, one of them square-rooted and reflected about mpv (4 param.) G.E. Bruno

  25. How to measure the fraction of secondary J/y G.E. Bruno

  26. How to measure the fraction of secondary J/y • The mass background is modeled using a linear distribution • function normalized to unity over the mass range [meemin, meemax] • Mslope is the only fit parameter G.E. Bruno

  27. How to measure the fraction of secondary J/y a c++ program has been implemented to perform the fit, based on the Root module Minuit2 it requires Root + Minuit2 multidimensional fit (13 parameters) performed in about 13 hours for a statistics equivalent to 10*109 min bias event optimisation have to be implemented G.E. Bruno

  28. How to measure the fraction of secondary J/y tot tot J/y J/y from B bkg Result of the likelihood fit: fB=0.375±0.020 (imput was 0.401) G.E. Bruno

  29. Outlook realistic description of the background to be tried with event mixing improvements on the method to extract the fraction of secondary J/y mandatory parameterisation of the signal mass distribution and x resolution R(x) to be improved improvements in the code algorithm robustness of the minimization to be investiagated try other variables to separate J/y from B, e.g. d0xd0 KF vertexing package performance in Pb-Pb under study, C. DiGiglio G.E. Bruno

  30. Extra G.E. Bruno

  31. J/y signal versus normalized h+h- background tot J/y bkg for N=109 events: PWGR3 Sept 3rd 2007 G. E. Bruno 31 G.E. Bruno

  32. J/y signal versus normalized h+h- background tot J/y bkg Events/10 MeV for N=109 events: a |cosq*|>0.8 cut reduces drastically the bkgnd PWGR3 Sept 3rd 2007 G. E. Bruno 32 G.E. Bruno

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