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

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

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microvertex detectors u production of many species of b hadrons u high boost of b hadrons n … but many results being superseded by pp colliders and b factories

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

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

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

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

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

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

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

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

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)

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

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

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

ISMD2003 Andrea Sciabà INFN-CNAF Model independent results n Mean values for x B: u OPAL:0.7193±0.0016(stat.) (syst.) u DELPHI:0.7153±0.0007(stat.) (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

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 

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

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!

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

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!

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

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

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

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

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

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

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

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…