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Recent Results from BaBar

Workshop on RECENT DEVELOPMENTS IN HIGH ENERGY PHYSICS AND COSMOLOGY Athens, 17-20 April 2003. All results are preliminary unless otherwise stated. Recent Results from BaBar. On the behalf of the BaBar collaboration.  HELLENIC SOCIETY FOR THE STUDY OF HIGH ENERGY PHYSICS. Adrian Bevan.

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Recent Results from BaBar

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  1. Workshop onRECENT DEVELOPMENTS INHIGH ENERGY PHYSICSAND COSMOLOGY Athens, 17-20 April 2003 All results are preliminary unless otherwise stated Recent Results from BaBar On the behalf of the BaBar collaboration  HELLENIC SOCIETY FOR THE STUDY OF HIGH ENERGY PHYSICS Adrian Bevan

  2. Talk Outline • Theoretical Motivation • The PEP-II accelerator & BaBar Detector • Experimental issues • Vertex reconstruction • Flavour tagging • Particle Identification • background suppression techniques • Results • sin2, sin2, direct CP Violation searches, rare decays • Summary and Outlook Athens, '03

  3. Physics Background – CP Violation • CP Violation is one of Sakharov’s conditions for obtaining a matter • antimatter asymmetry in the universe. • The observed CP Violation explained by the Standard Model of Particle • Physics is 9 orders of magnitude too small to explain our universe • There must be some kind of new physics to discover in the flavour sector • Standard Model mechanism for CP Violation in weak interactions is given by a single • complex phase in the CKM matrix. A. D. Sakharov (1967) JTEP Letters 5, 24 large uncertainty in magnitude and phase →related to a phase in the unitarity triangle CP Violating phase present Athens, '03

  4. Experimental constraints before the B-factory era (CP Violation measurements prior to 1999) • Observed effects have been entirely in the kaon system • This defined our knowledge of CP Violation in the SM • 1964, Christensen et al. discovered CP Violation in K2 decay • Following 37 years: measured several CP Violation effects in • Semi-leptonic kaon decay,  ~10-3 • Non-leptonic kaon decay, +-, 00: • CP Violation in mixing:  ~10-3 • CP Violation in decay: ’/  ~10-3 (KTeV/NA48) • Observed a CP Violating asymmetry in KL→+-e+e- (13% effect) • (observed a T odd asymmetry in this decay as well) Athens, '03

  5. CP-LEAR • measured T violation • obtained an upper limit on CPT Violation from • There are still important measurements to do with kaons • over-constraining the CKM matrix is the aim • With the advent of the B factories, we were able to test • this description for the first time! Athens, '03

  6. (,) (0,0) (1,0) The CKM Matrix in terms of Bd/u decay Unitarity of the CKM matrix gives 6 triangles in the complex plane; 9 relations in all – few triangles have all sides with the same order in  – kaon system is not one of these – interesting one for the Bd/u system is “The Unitarity Triangle”: mixing Athens, '03

  7. mass eigenstates=CP eigenstates if no CP Violation in mixing (q/p=1) Neutral Meson Phenomenology • For neutral K, B, D … mesons • strong eigenstates are not CP eigenstates • mass eigenstates are an admixture of different strong eigenstates • particle  antiparticle (mixing), f = 2 CP even CP odd if q/p1 have CP Violation in mixing; e.g. K=2.3x10-3 (mixing is very suppressed in the case of D mesons) Athens, '03

  8. direct CPV Direct CP Violation (f = 1) • need interference between diagrams with different strong (i) and weak phases (i): • Direct CPV only seen in ; ' ~ few 10-6 • theory predicts large asymmetries in B+/0(few to ~80%) Athens, '03

  9. Analyse time evolution of B0B0system (assume DG=0) direct CP violation→ C ≠ 0 Indirect CP violation → S ≠ 0 Observing CP violation at the U(4S) • Three observable interference effects: • CP violation in mixing (|q/p| ≠ 1) • (direct) CP violation in decay (|A/A| ≠ 1) • CP violation in interference of mixing and decay (Iml≠ 0) Athens, '03

  10. s,d Searching for Direct CP Violation & probing New Physics with Bu/d • Large ACP requires amplitudes of similar order • b→u: suppressed tree: i.e. charmless decays • large predicted ACP • b→s,d: penguins: radiative decays good for constraining BSM • small predicted ACP • Understand penguins • Access to ,  and  • Sensitive to New Physics effects via loops • minimal SUGRA: B→Xs, K+, K0+ … • R-parity Violating SUSY: KS … • SUSY searches – K* Athens, '03

  11. Experimental Issues • interesting modes have small branching fractions • large light quark continuum background qq • quarks below threshold are u,d,s & c • other B background • Need good K/ separation • Need to boost to do time dependent CP measurements • require good vertex resolution for CP measurements • need to ‘tag’ the flavour of the B (as a particle/anti particle) • understand charge bias • detector: trigger, tracking; reconstruction • event selection, particle ID, analysis • physics: differences in (anti)particle interaction in matter (e.g. K) Athens, '03

  12. The Accelerator & Experiment • On the whole - very similar designs with many common features for both • Belle and BaBar: • e+e- colliders • run on the U(4S) resonance – produce ~100% BB pairs • run 40 MeV below resonance for qq background studies • To measure CP violation in the B system, need to measure vertex • difference of the evolving BB system (Dt) silicon device • need asymmetric beam energies (PDG 2002) Athens, '03

  13. Most results use: 81 fb-1 on-resonance. 88 million BB events (as for ICHEP02). The PEP-II e+e- collider E(e-) = 9.0 GeV E(e+) = 3.1 GeV  0.56 Design Achieved Luminosity (cm-2 s-1) 3 x 1033 5 x 1033 Int. Lum / day (pb-1) 135 341 Int. Lum / month (fb-1) 3.3 7.2 Athens, '03

  14. Electromagnetic Calorimeter (EMC) Detector for Internally reflected Cherenkov radiation (DIRC) 1.5 T Solenoid Drift chamber (DCH) Instrumented Flux Return (IFR) Silicon Vertex Detector (SVT) The BABAR experiment SVT: 5 layers double-sided Si. Crucial for measuring t. DCH: 40 layers in 10 super- layers, axial and stereo. DIRC: Array of precisely machined quartz bars. Excellent Kaon identification. EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID, 0 and  reco. IFR: Layers of RPCs within iron. Muon and neutral hadron (KL) Athens, '03

  15. Quartz bar Active Detector Surface Particle Cherenkov light Particle Identification (PID) Detection of Internally Reflected Cherenkov Light (DIRC) • Measure angle of Cherenkov light • Transmitted by internal reflection • Detected by~10,000 PMTs • excellent K/ separation Athens, '03

  16. Measuring CP Violation from time dependence 1. Start with data sample of BB pairs 2. Reconstruct one B in a CP eigenstate decay mode 3. “Tag” the other B to make the matter/antimatter distinction 4. Determine the time between the two B decay vertices, t 5. Plot t distribution and do CP fit for S and C. Athens, '03

  17. signal B background Event Selection Techniques Use beam energy to constrain mass & energy difference E  ~ 15-80 MeV; larger with neutrals MES ~ 3 MeV qq background Athens, '03

  18. u,d,s,c background Signal Arbitrary Units Fisher Discriminant Event Selection Techniques B event u,d,s,c B events are spherical u,d,s,c is jets • shape variables • flavour-tagging (e, , K, slow  from other B) • Maximum Likelihood fits or cut based analysis • off-resonance & E sidebands are used to parameterise light quark • background Athens, '03

  19. b c s K- cleaner signal larger mistag prob B Flavour Tagging • Tagging algorithm with physics-based neural networks • Inputs include leptons, kaons, slow- (from D*), and high-momentum tracks • Outputs combined and categorized by mistag probability (w) • 5 mutually exclusive categories: • Lepton – isolated high-momentum leptons • Kaon I – high quality kaons or correlated K+ and slow-- • Kaon II – lower quality kaons, or slow- • Inclusive – unidentified leptons, poor-quality kaons, high-momentum tracks • Untagged – no flavor information is used Athens, '03 Q = (1-2w)2 = (28.1  0.7)%

  20. Tagging: example of Charmless B Decays 81/fb B→ h+h- sample split by tagging category • Tagging efficiency is very different for signal and background • Strong bkg suppression in categories with the lowest mistag prob (Lepton/Kaon) • plots shown are for h+h-, a rare decay with significant backgrounds. 150 100 Athens, '03

  21. Exclusive Brec reconstruction • z resolution dominated by tag side (other B) • Average z resolution ~180m • Average z ~260 m BREC Vertex BREC daughters Interaction Point Beam spot Example in B →  BTAG Vertex z BTAG direction e+e- → qq TAG tracks, V0s B →  t (ps) Vertex Reconstruction Resolution function parameters obtained from data for both signal and background • Signal from sample of fully reconstructed B decays to flavor eigenstates: D*(, , a1) • Background from data sideband sample without the boost would have z~20m … e.g. CLEO Athens, '03

  22. Measuring  c g c b c b s c c W+ W+ s d d d d Tree Penguin VcdVcb are almost real - only phase is from mixing Decay K0 mixing B0 mixing Golden Mode for CP Violation in B decay(theoretically clean) • Dominant amplitudes for b  ccs decay: • Both amplitudes have the same weak phase: • For B0-> J/Y Ks we obtain: Athens, '03

  23. Vtb*Vtd b Vcb*Vcd • CP = -1 • B  J/ Ks0, Ks0p+p-, p0p0 • B (2S) Ks0 • B c1 Ks0 • B c Ks0 • B  J/ K*0, K*0  Ks0 J/KL signal J/X background other background • CP = +1 • B  J/ KL0 E=Eb-Ebeam (GeV) sin 2b mostly CP even Athens, '03

  24. Ks modes KL modes 2641 tagged events (78% purity; 66% tagged ) tagging efficiency Q=(28.1  0.7)% Still statistics limited… hep-ex/ 0207042 (PRL) sin2b = 0.741  0.067  0.034 || = 0.948  0.051  0.030 Latest result on 88M BB events Athens, '03

  25. pure penguin • sensitive to new physics • measured  in this mode should • agree with • In SM: 84 million BB pairs (supersedes ICHEP ‘02) C = 0 Goodness-of-fit: 23% need more statistics to see if the SM holds out B0Ks ~60 events • c.f. world average: sin2 = 0.73 ± 0.06 • >2  difference. • Belle measures -0.73±0.64±0.18 Athens, '03

  26. “reference” sin2b pure penguin mostly penguin? colour- & CKM-suppressed tree competing penguin CKM-suppressed treesmall penguin pollution Also have results from D*D*, ’Ks, J/0 but interpretation of the measurement of S and C in terms of the weak phase  is plagued by uncertainty … Athens, '03

  27. Measuring a Interesting modes to measure a need to perform an Isospin analysis of branching ratios of B0 and B0 to 2p final states to determine shift in a due to the presence of penguin diagrams doing a ‘quasi 2 body analysis’ – will eventually have to analyse the whole Dalitz plot. need a neutral B meson decay with several contributing processes that interfere in order to be able to measure the phase In analogy to these modes one needs to analyse the time dependence of this decay and extract a from an isospin analysis of each of the three partial waves (L=0,1,2) in the final state (full angular analysis is required) The shift in a from penguin diagrams is expected to less than that in pp Work is now in preparation for analysis of modes  the time evolution of p+p- gives the shifted value of a Athens, '03

  28. Tree (T) Level: With Penguins (P): decay mixing • penguins are significant: P/T~0.2 • Need to measure the weak phase  • can use isospin relations to extract shift CP Violation in B0 → p+p- Athens, '03

  29. The Isospin Analysis: Need to measure decay of B and B to final states in order to determine the shift Can bound the shift on a using BR(B→  0) and upper limit on B0→00 Already have Measuref (f’) for B(B) from Need to measure Grossman Quinn bound Athens, '03

  30. large qq background • /K separation pp qq + Kp Kp e+e→qq The first side of the Isospin triangle: B+→p+p- Fit projection in sample of pp-selected events Athens, '03 fit , K simultaneously PRL 2002 89 281802 

  31. Fit region e+e- → qq e+e- → qq The Base of the Isospin Triangle: B+→p+p0 • large qq background • /K separation • Potential background from r+p- • Minimize with tight cut on DE Athens, '03 Simultaneously fit for pp0/Kp0 hep-ex/0303028, submitted to PRL

  32. p0p0 rp0 e+e- → qq The Missing Sides : B0→p0p0 • Small signal; BR  few 10-6 • 0 background • Background suppression • Event shape and flavour tagging to reduce qq • Cut on M(p+p0) and DE(p+p0p0) to reduce rp0 background, then fix in the fit Significance including systematic errors = 2.5s Athens, '03 hep-ex/0303028, submitted to PRL

  33. Need ~20 the data to do a better isospin analysis than using a bound in B→ • Use Grossman-Quinn Bound: • assumes isospin • most conservative of several bounds in literature • account for correlated systematic • uncertainties common to both analyses Bounding penguin pollution: Athens, '03

  34. Not a CP eigenstate direct CPV related to a • K is self tagging • CK, SK, SK =0, CK =-1 • large expected: • ACP() & ACP(K) signal SXF+BBg Athens, '03

  35. 2.1 B0→+- ~88106 B pairs continuum B-background total fit Dilution from background Athens, '03

  36. Direct CP Violation Searches • Looked for direct CP Violation • in many modes • so far no evidence • most precise measurement ~4.6% • charge asymmetry in •  is 2.5  effect •  is 2.1  effect • look for updates using more data Athens, '03

  37. Rare Decays: • very rare process • BR ~10-7 • penguin + FCNC process • consistent with SM • sensitive to new physics Athens, '03

  38. (Cabibbo suppressed) (Cabibbo favoured) signal region s,d B00 B++ Rt E (GeV) B0 MES (GeV) Phys Rev Lett 88 101805 (2002) • radiative penguins • new physics probes • constrain |Vtd/Vts|(Rt) 20.7fb-1 @90% C.L. 78fb-1 Athens, '03

  39. hep-ex/0304021 Observation of a Narrow Meson Decaying to D+s0 at a Mass of 2.32 GeV/c2 0 D+s Athens, '03

  40. Other BaBar physics I’ve not covered • BaBar has also produced results on • B lifetime and mixing; B, m • semi-leptonic B decays • D mixing • fB+B-/fB0B0 • to name but a few • in addition to B; have very large  and D samples to study • produced in excess of 35 publications • several more preprints have been submitted for publication http://www-public.slac.stanford.edu/babar/BaBarPublications.htm Athens, '03

  41. Summary • Since the start up of BaBar and Belle we have learnt? • SM description of CP Violation passed its first real test • Observed CP Violation in the B System • CP Violation in B system consistent with that in kaons • Still not enough CP Violation to explain matter-antimatter • asymmetry in the universe • Expect ~500 fb-1 by 2006 (5x current data set) • With this we will: • start to test SM by doing alternate measurements of sin2 • start to measure  with greater precision in several modes • continue the search for direct CP Violation • work towards over constraining the unitarity triangle • can we break it? • search for and constrain new physics in the flavour sector Athens, '03

  42. Dominated by results • from B physics! • main constraint: sin2 • fitting using Belle • result and indirect • constraints ~1.50 • measuring  is next • test of SM Constraints on (,) CKM Fitter Group: A. Höcker, H. Lacker, S. Laplace, F. Le Diberder, Eur. Phys. Jour. C21 (2001) 225, [hep-ph/0104062] Athens, '03

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