1. b physics at LEP Andrea Sciabà INFN-CNAF XXXIII International Symposium on Multiparticle Dynamics September 5-11, 2003 Krakow

2. ISMD2003 Andrea Sciabà INFN-CNAF b physics n Goals: u CKM matrix F B oscillations (|V td |, |V ts |) F B semileptonic BRs (|V cb |, |V ub |) F b lifetimes F CP violation u masses u Electroweak F sin 2 θ W (asymmetries) F R b (sensitive to new physics, m top ) u QCD processes F fragmentation F two-photon events, gluon splitting, etc. u New physics (CP violation, rare decays, etc.) u … B 0 oscillations V ud V ub * A B C V td V tb * V cd V cb * b  cℓν b  uℓν

3. ISMD2003 Andrea Sciabà INFN-CNAF b physics at LEP n LEP data sample features and advantages u ~ 3.6×10 6 Z 0  bb events u very clean environment u high heavy quark tagging power due to silicon microvertex detectors u production of many species of b hadrons u high boost of b hadrons n … but many results being superseded by pp colliders and b factories

4. ISMD2003 Andrea Sciabà INFN-CNAF b physics at LEP n LEP data still contributes significantly to u b semileptonic BRs u B s oscillations u Asymmetries, R b (no more measurements until Z factories) u B s, Λ b, excited b hadron masses u Fragmentation u heavy quark production in 2-photon collisions u gluon splitting into heavy quarks n Only a few topics will be covered here

5. ISMD2003 Andrea Sciabà INFN-CNAF B s oscillationsB s oscillations: Basic concepts n Main motivation:  m d  |V td | gives a large error  m d /  m s  |V td | with a 3 times better theoretical error! B s,d 0 b b s,d W-W-W-W- W+W+W+W+ V tb VtqVtqVtqVtq Vtq*Vtq*Vtq*Vtq* Vtb*Vtb*Vtb*Vtb* t t s,d  15% theor. error  |V cb | 2 SU(3) ~ 5% error n Oscillation probability: fit A for any assumed value ω of Δm s ω << Δm s  A  0 ω ≃ Δm s  A  1 ω excluded at 95%CL if A+1.65σ A <1 A

6. ISMD2003 Andrea Sciabà INFN-CNAF Strategy n Flavour tagging at production u jet charges, vertex charges, kaons, leptons, etc. u typical mistag   25% n Flavour tagging at decay u lepton or D charge u identify final state (b , b  c , etc.) n Distinguish B s from other hadron species n Measure B s decay length, momentum u  t = (m/p)  L  (  p /p) t n Maximum likelihood fit to amplitude A at given  m s Production flavour tagging Decay flavour tagging

7. ISMD2003 Andrea Sciabà INFN-CNAF Oscillation analyses n Exclusive analyses u Fully reconstructed B s (A, D) n Semi-exclusive analyses u D s + - pairs (A, D, O) u D s + h - pairs (D) u Φ - pairs (D) n Inclusive analyses u semileptonic decays (A, D, O) u inclusive vertices (D) Very low statistics High purity Very high p.t. resolution! Low statistics Fair purity Good p.t. resolution High statistics Low purity Low p.t. resolution

8. ISMD2003 Andrea Sciabà INFN-CNAF Fully reconstructed B s (ALEPH) total  and  0 reconstruction  efficiency and purity increase    180  m  p /p   0.5 – 3 % Well reconstructed Missing  ’s or  ’s  t  0.08 ps Excellent proper time resolution! Add a photon

9. ISMD2003 Andrea Sciabà INFN-CNAF D s + - pairs (DELPHI) (updated) Semileptonic modes Hadronic modes n Event-by event: u proper time resolution u B s purity Sensitivity:  m s = 8.6 ps -1  m s > 4.9 ps -1 at 95% CL

10. ISMD2003 Andrea Sciabà INFN-CNAF Inclusive analysis (ALEPH) n Topological reconstruction of the D vertex n Extensive event-by-event treatment of: u proper time resolutions and corrections u sample composition u flavour tags u  huge sensitivity enhancement! Lepton with high p  Topologically rec. “D” track “B” track Photon Primary vertex BsBs “D”  Sensitivity:  m s = 13.6 ps -1  m s > 11.9 ps -1 at 95% CL

11. ISMD2003 Andrea Sciabà INFN-CNAF Oscillation results n Almost final LEP results on B s oscillations n Lower limit on  m s : 14.4 ps -1 n Now, we must wait for Tevatron to measure  m s

12. ISMD2003 Andrea Sciabà INFN-CNAF b quark fragmentation n The fragmentation function models the non-perturbative confinement of the quark into colourless hadrons n different choices of the variable: not accessible experimentally ISR, FSR, hard gluon emission effects not unfolded n Model-dependent analyses (fit to a given fragmentation model) n Model-independent analyses (x B spectrum reconstruction)

13. ISMD2003 Andrea Sciabà INFN-CNAF Inclusive analyses n Event selection based on: u b-tagging u secondary vertex reconstruction n Direct x B reconstruction (OPAL) u neural networks to distinguish particles from b decay and from hadronization  σ E ≃ 5 GeV n x B, z reconstructed using neural networks (DELPHI), input variables correlated to: u kinematics (track rapidity, multiplicity, etc.) u jet/total energy u b-tagging preliminary

14. ISMD2003 Andrea Sciabà INFN-CNAF Exclusive analyses (ALEPH) n B  D (*) ℓν decays n Much lower statistics u ~ 3400 events n E ν reconstructed from missing energy n σ E ~ 8% x B spectra not efficiency-corrected

15. ISMD2003 Andrea Sciabà INFN-CNAF Model dependent results n Favoured models: u Lund / Lund-Bowler u Kartvelishvili n Disfavoured models: u Peterson et al. u Collins

16. ISMD2003 Andrea Sciabà INFN-CNAF Model independent results -0.0033 -0.0052 n Mean values for x B: u OPAL:0.7193±0.0016(stat.) +0.0038 (syst.) u DELPHI:0.7153±0.0007(stat.) +0.0049 (syst.) u ALEPH:0.716±0.006(stat.)±0.006(syst.) n All measurements consistent n bias w.r.t. old measurements due to the assumption of the Peterson model

17. ISMD2003 Andrea Sciabà INFN-CNAF A b FB measurements (I) n Forward-backward asymmetry at the Z pole: n Related to sin 2 θ W n A b FB most sensitive to sin 2 θ W n Need to measure: u cosθ (from thrust axis) u quark charge u quark flavour n QCD corrections to be calculated u gluon radiation u thrust axis approximation u most important systematic error e-e- e+e+ f f 

18. ISMD2003 Andrea Sciabà INFN-CNAF A b FB measurements (II) n Use a lepton tag u lepton charge correlated to the quark charge u need to distinguish u correct for the dilution by B mixing n Use the jet/vertex/kaon charge u b-tagging to enhance purity u purity from data u quark charge estimator Q u charge separation δ from data F effect of B mixing included

19. ISMD2003 Andrea Sciabà INFN-CNAF A b FB with lepton tag (DELPHI) n b-tagging (η EVT ) u Track I.P., sec.vertex mass and energy, track rapidities u Flavour composition vs. η EVT from data with standard double counting method n Lepton p and p  n Charge tagging u Lepton charge, opposite jet charge as cross check  Uses Q × Q opp (-/+: right/wrong correlation) Probability for a lepton of coming from b , b  c , c , bckgnd function of (p, p  ), (η EVT, Q × Q opp ) n 2-dim fit of the obs asymmetry to A b FB, A c FB over bins of (cos  T, P b  -P b  c , P c  ) New!

20. ISMD2003 Andrea Sciabà INFN-CNAF A b FB with lepton tag (OPAL) n Neural networks to distinguish b  (NET b ) and c  (NET c ) u Lepton p and p , lepton jet energy, (∑p  ) jet u Jet decay length significances, lepton i.p. significance n Likelihood fit to the number of single-lepton and double-lepton events u Bins of cos , NET b and NET c u Background fraction fitted from data u average mixing parameter  fitted from data using same-sign dilepton events u A b FB and A c FB fitted

21. ISMD2003 Andrea Sciabà INFN-CNAF A b FB with incl. tag (DELPHI) n high purity b-tagging (b-tag) u lifetime and sec vertex info, rapidities u b-tag selection efficiency from data counting events with 0,1,2 tagged hemi  sample composition (b, c, uds) n charge tagging (flav hem ) u jet charge, sec. vertex charge u flavour tag combining P(same Q|B) for all tracks u prob. of charge mistag from data (unlike/like-sign double tagged events) u for charm events, mistag from data using charge of reconstructed D * (new!) n fit to no. of single and double tagged events in bins of (cosθ, b-tag) n Main systematics from jet charge hem. correlations New!

22. ISMD2003 Andrea Sciabà INFN-CNAF A b FB with incl. tag (OPAL) n b events tagged with u NN (secondary vertex info) u leptons (p, p , isolation) u flavour composition from data (singly and doubly tagged hemispheres)  P f n charge tagging estimator Q from u jet charge (always) u sec. vertex charge (if any) u kaon charge (from b  c  s) (if any) all charge estimators available

23. ISMD2003 Andrea Sciabà INFN-CNAF A b FB summary n A b FB measurements all LEP almost finalized n well compatible (also with SLD) n sin 2  lept eff : 2.9σ discrepancy with A LR (SLD) u need to wait for linear colliders SLD A b LEP A b FB LEP+SLD A

24. ISMD2003 Andrea Sciabà INFN-CNAF Open b production in  events n Test of perturbative QCD u at LO, only direct and single resolved terms (of the same order) u more robust calculations because of the large quark mass u 2-3 orders of magnitude suppression w.r.t open charm because of larger mass, smaller charge u photons mostly quasi-real  anti-tagged events n Analysis strategy u select anti-tagged photon-photon events u reconstruct lepton candidates u reconstruct jets u measure lepton p  w.r.t. jet axis e - e + e - b b e + e - b b direct term single resolved term

25. ISMD2003 Andrea Sciabà INFN-CNAF  bbX in L3 n 627 pb -1, 189 GeV <  s < 209 GeV n muons AND electrons  large statistics n Fit to p  distribution: u p  spectra from MC (PYTHIA for   ) u N b, N c, N uds left free n Consistent e and  results n Also measurement of u consistent with previous L3 results muons electrons 435 events 3850 events

26. ISMD2003 Andrea Sciabà INFN-CNAF  bbX in DELPHI (new) n 463 pb -1, 189 GeV <  s < 209 GeV n only muons n Fit to p  distribution: u p  spectra from MC (PYTHIA for   ) u N c fixed to LEP average for  (e + e -  cc) u N uds measured selecting hadrons instead of muons u N b left free n Study of K-lepton correlations (first time!) u b  K - - Xc  K - + X u increased purity of c (K ±  ) and b (K ± ± ) events u measured cross sections: 651 events

27. ISMD2003 Andrea Sciabà INFN-CNAF  bbX in OPAL n 371 pb -1, 189 GeV <  s < 202 GeV n only muons n Fit to p  distribution: u p  spectra from MC (PYTHIA for   ) u N c fixed to OPAL measurement for  (e + e -  cc) with D * mesons u N uds measured selecting hadrons instead of muons u N b left free n x T =2p T /W vis distribution shows the consistency of the 1:1 ratio of direct to single-resolved contributions 444 events

28. ISMD2003 Andrea Sciabà INFN-CNAF  bb(cc) summary n Consistent results among LEP experiments n Good agreement with Drees et al. model for charm data n EXCESS for beauty data: 4  discrepancy n No satisfactory explanation at present

29. ISMD2003 Andrea Sciabà INFN-CNAF Conclusions n LEP HF physics program almost finalized u B s oscillations (  m s > 14.4 ps -1 at 95% CL) u heavy quark asymmetries (difficult interpretation) u Heavy quark fragmentation function well measured and consistent (both in average and shape) u Large discrepancy in  bbX rate (but not in  ccX rate) n Impossible to show many other results…