Studies of the decay B  c K at the BaBar experiment

1. Studies of the decay B c K at the BaBar experiment Nick Barlow University of Manchester December 2003

2. The BaBar Collaboration • The BaBar collaboration consists of • Nearly 600 physicists • From 10 countries • 75 Institutions • The canalyses presented here were mainly performed by: • Me • Witold Kozanecki (Saclay) • Stefania Ricciardi (RHUL) • Frank Jackson (Manchester) • Gautier Hamel de Monchenault (Saclay)

3. Outline • The PEP-II B-factory, and BaBar detector • CP violation in the quark sector of the S.M. • CKM matrix and Unitarity Triangle • 3 Types of CP violation in B decays • Measuring the CKM parameter sin2 using neutral B decays to charmonium + K. (Specifically Bc KS) • Reconstructing B candidates • B flavour tagging • Time-independent and time-dependent fits

4. Motivation for the BaBar experiment • CP violation is interesting because • It is one of the least tested areas of the Standard Model • Has some relevance to the matter/antimatter asymmetry in the Universe • Before BaBar and Belle, CP violation had only been observed in the kaon system • Hadronic uncertainties make comparison with Standard Model parameters difficult • High luminosity of PEP-II facilitates numerous other precision measurements in B physics • (and tau physics…)

5. The PEP-II B-factory e+e-(4S)  BBbar 9 GeV electron beam 3.1 GeV positron beam Boost (4S) in lab frame: 0.55

6. PEP-II performance • Design peak luminosity = 3.0*1033cm-2s-1 • Record peak luminosity = 6.8* 1033cm-2s-1 • 24 hr record integrated = 484.3 pb-1 • 7 day record integrated = 2.49 fb-1 • Records improving all the time, largely thanks to trickle injection… • Results in this talk use 81fb-1 (Runs 1 and 2).

7. Trickle injection • Inject charge continuously into LER, while BaBar is taking data • Will increase integrated luminosity by at least 15% • If backgrounds are under control…

8. The BaBar detector Instrumented Flux Return Detector of Internally Reflected Cherenkov light e+ (3.1GeV) e- (9GeV) Electro-Magnetic Calorimeter Silicon Vertex Tracker Drift CHamber 1.5T solenoid

9. Tracking and Vertex finding • SVT consists of 5 layers of double-sided silicon strip detectors • Single hit resolution ~80 microns. • DCH consists of 40 layers (10 superlayers) of wires, arranged in axial and stereo layers in helium/isobutane gas mixture • Measures momentum and position of charged particles • (pT)/PT=0.13%PT0.45%

10. Particle Identification • Charged hadrons (, K) identified using dE/dx in the DCH and SVT, and Cherenkov angle in the DIRC • K- separation>3.4 for P<3.5GeV/c • Leptons identified using the IFR () and EMC (electrons)

11. Neutral particles • EMC consists of 6480 CsI(Tl) crystals, arranged in a barrel+endcap configuration. • Measures energy of electrons and photons over a wide energy range.

12. Im (,)    1 Re CKM matrix and Unitarity Triangle • Quark mixing is described by the CKM matrix Wolfenstein parametrization Unitarity condition: Complex phase CP violation Can be represented as a triangle on complex plane:

13. Im (,)    1 Re Unitarity triangle cont. • Angles are given by:

14. Errors are dominated by theoretical uncertainties! Existing constraints on Unitarity Triangle Constraints come from - CP violation in K0 mesons - Vub and Vcb measurement - Bd and Bs mixing http://ckmfitter.in2p3.fr

15. mixing decay CP 3 Types of CP violation in B decays • CP violation in decay (direct CP violation) • Amplitude for a decay is not same as amplitude for CP conjugate decay • CP violation in mixing (indirect CP violation) • Mass eigenstates cannot be chosen to be CP eigenstates • CP violation in interference between decays with and without mixing • Time-dependent asymmetry in decays of and to common CP eigenstate.

16. CP violation in neutral B decays • Consider neutral B decays into a CP eigenstate fCP that is accessible to both B0 and B0: • Can define measurable quantity :

17. CP violation in neutral B decays • Time dependent asymmetry given by: • If || = 1 (no direct CP violation), this reduces to:

18. c s c b b J/Y , c c s c K0 B0 d d d d BCharmonium K0 • For BCharmonium K0, both ‘tree’ and dominant ‘penguin’ diagrams have same weak phase, so no direct CP violation, || = 1 = chK sin2

19. BCharmonium K0 • BJ/KS is the ‘Golden channel’ at BaBar • The J/ decays to leptons – clean reconstruction with good efficiency • The c has a much larger intrinsic width than the J/, and only decays hadronically • Need to work harder to reduce background • Less than half the total width is accounted for by known decay channels • Although J/ and c have opposite intrinsic CP, the J/ is a vector, so (-1)L term in CP of 2-body state means that BJ/KSand BcKS decays have the same CP eigenvalue (-1)

20. Tag flavour of B-meson using e.g. charge of a primary lepton Btag (4S) e- e+ Fully reconstructed B decay (CP eigenstate) BCP t  z/( c) e.g. BJ/Ks, B  cKs Experimental method • BBbar pair evolve as coherent P-wave state. • Always exactly 1 B0 and 1 B0 • Boosted wrt lab frame: can measure distance z between B decay vertices.

21. Time-dependent asymmetry • In principle, just measure time difference t for events where Btag is tagged as a B0 or a B0. • In practice, tagging and t determination are not perfect – need to modify Probability Density Functions (PDFs) with mistag fractions (w) and resolution functions. B0 tags B0 tags

22. Time dependent PDF P(t) exp(–|t|/B) ( 1 ± (1-2w)CPsin2sin(mt) ) R(t) CP eigenvalue Resolution function dilution • Can extract sin2b from time-dependent fit to t distributions of tagged events, BUT • Need to know mistag fraction and resolution function • Need to add other PDFs for background events, which in general will have different time dependence

23. Neutral B-meson mixing • Neutral B mesons can mix • Btag and Bflav can be opposite flavour (no mixing) or same flavour (mixing) Tag flavour of B-meson using e.g. charge of a primary lepton Btag (4S) e- e+ Fully reconstructed B decay (flavour eigenstate) Bflav t  z/( c) e.g. BD-+

24. B-mixing (2) • PDF for B-mixing is given by: unmixed Real probability of B-mixing mixed Resolution function Dilution = 1-2w, where w is fraction of wrongly tagged events

25. B flavour tagging • Fully reconstructing Btag not practical – would be too inefficient. • Instead, use Neural Net tagging algorithms based on different physics processes • Primary lepton from W in bc transition • Charged kaons from bcs transition • Charge of slow pions from D* decays • Charge of highest pT track • Divide into 4 hierarchical, mutually exclusive ‘Tagging Categories’, • Lepton, Kaon1, Kaon2, Other • Each tagging category has its own mistag fraction, t resolution function, measured on real data using B mixing sample.

26. Tagging performance • Important parameter is effective tagging efficiency Q=eff(1-2w)2 • This is what goes into the error on sin2b

27. Measuring t • Resolution on t is dominated by determination of z decay position of Btag • Use constraints from knowledge of the beamspot position, and the momentum of Brec t= z /( c) + small correction due to small momentum of Bs in Y(4S) frame

28. t resolution • Different PDF used for each tagging category • All use triple Gaussian (core, tail, outliers), and event-by-event errors. • t resolution usually better for lepton category • 8 free parameters: • Relative fractions of tail and outliers • Scale factor for width of core Gaussian • Bias for tail Gaussian • Bias factors for core Gaussians in each tagging category

29. Data samples • BCP sample – reconstructed B0 c KS events • Used for measuring sin2b • Bflav sample – events where 1 B is reconstructed into a flavour eigenstate e.g. D*0- • Used for measuring mistag fractions and t resolution • Bsig+ sample - reconstructed B+ c K+ events • Used for cross-checks, higher statistics than neutral mode. • Numerous Monte Carlo samples • Used for cross-checks, and tuning the fit – can vary generated value of sin2b.

30. Reconstruction of B c K candidates • Use decay channels c KSK+- and c K+K-0 • Most BaBar (and Belle, CLEO) B analyses use the variables mES and E, which are (nearly) uncorrelated. • E is the measured – expected energy of the reconstructed B candidate • mES is the beam-energy substituted B-mass (assumes E=0) 5.2 5.3 mES (GeV

31. Fighting background • First run a ‘skim’ – loose cuts on topological variables, charged track multiplicity etc. to reduce dataset to a manageable size • Apply cuts on reconstructed masses of KS, 0 etc. • All cuts optimized to maximise S/sqrt(S+B) • Estimated from signal MC and E sidebands of on-resonance data • Use a ‘Fisher Discriminant’ to fight background from u,d,s,c quark events • Linear combination of ‘event shape’ variables • Light quark events tend to have jet-like structure in CM frame, while BBbar events tend to be more spherical, as B mesons are produced almost at rest in CM frame

32. K+ K- 0 K+ C B+ B+ Types of background • Combinatorial background, from light quark events or BBbar events where Brec is reconstructed using tracks from both Bs • Can be studied using off-resonance data, MC, and E sidebands of on-resonance data • Modeled using a threshold function (ARGUS function) in mES • Peaking background from BBbar events • In our case, studies on high-statistics BBbar MC indicate that this is dominated by decays to the same final state as our signal, but without an intermediate c resonance: • Neither of these backgrounds is expected to have any structure in mX – the mass of the reconstructed c candidate.  K*+

33. Time independent fit • Extract signal and background yields using 2D unbinned maximum likelihood fit to mES and mX c signal: Breit-Wigner  Gaussian2 in mX Gaussian1 in mES J/ signal: Gaussian2 in mX Gaussian1 in mES Peaking background: Gaussian1 in mESlinear in mX Combinatorial background: Argus shaped in mES, linear in mX

34. Results of time independent fit • KsKPi neutral channel:

35. The sin2 fit • The value of sin2 is obtained from a simultaneous fit to the Bflav and BCP samples • Takes into account potential small correlations between sin2b and resolution functions etc., and uses statistical power of BCP sample. • Altogether there are 34 free parameters • Mainly due to resolution functions for each tagging category for signal and background, and the composition and time-dependence of the backgrounds in both the BCP and Bflav samples.

36. Results on MC B0c KSsignal, generated with sin2b=-0.7 sin2b=-0.744±0.074 B+c K+signal, no CP violation expected sin2b=-0.062±0.059

37. Results on Bsig+ sample • No CP violation expected: sin2b=0.11±0.16

38. Results on BCP sample • i.e. the actual result! sin2b=0.51±0.33

39. More cross-checks • Can use ‘Toy MC’ experiments, where events are generated to according to a PDF, to check validity of result: • Compare maximised likelihood found in fit to data with distribution of values found in 500 Toy MC experiments • Compare statistical error in fit to data with those found in 500 Toy MC experiments

40. Systematic errors

41. Comparison with other decay modes hCP =-1 hCP =+1 sin2b = 0.755  0.074 sin2b = 0.723  0.158

42. Constraints on the Unitarity triangle BcKS only All BaBar results http://ckmfitter.in2p3.fr

43. Conclusions • BaBar is a good place to do B physics • Very high luminosity of PEP-II • Clean environment of e+e- collisions • BaBar and Belle have made precision measurements of CP violation in B mesons • Values of sin2b agree well with S.M. expectations • It is (just about) possible to measure sin2b using the decay channel BcKS • Helped reduce the statistical error by 0.01 for the last publication!! • Studies on MC indicate it will be advantageous to include this channel in the main sin2b measurement for the forseeable future

45. Backup slides

46. What is an c ? • c is the ground state of the charmonium system • Mass = (2979.7  1.5) MeV/c2 (PDG 2002) • Width is very poorly known: • BaBar has just made a new measurement using c from 2 photon production: Use J/psi from ISR to measure detector resolution This is the value we use in our analyses