b lifetimes Simone Donati INFN & University of Pisa 8th International Symposium on Heavy Flavour Physics University of Southampton 25 - 29 July 1999

Outline Introduction - Theoretical Framework - Crucial detector elements B+, B0 lifetime and t(B+ )/t(B0 ) - Dl and D0l samples - B vertex charge - Exclusive reconstruction B0s lifetime - DS l correlation - DS h samples Lb lifetime - Lc lepton combination Summary and Conclusions

Theoretical Framework q, l Baseline Spectator Model: the light quark acts as a spectator  All b - hadrons have the same lifetime. G = (GF2 mb5/192p3) · | Vcb | 2 · F • W q, n b c • Vcb B D q q But, for charm hadrons: t (D-) ~ 2.5 t (D0) ~ 2.5 t (Ds-) ~ 5.0 t (Lc-) q, l p+ b c w D- b B0 W W B+ b c B+ D0 p+ u q,n d u u u W exchange B+ annihilation D0 b c w p+ Expected lifetime difference ~ 5 - 10 % between B0 and B+ (t (B+)  t (B0) ) due to nonspectator effects. u u Destructive interference between external / internal W emission

Theoretical Framework Effects scale as 1/m2q - Important effects for c hadrons: t (D+) / t (D0) = 2.55  0.04 - Up to 10 % differences expected for b hadrons (from HQE) t (Lb)  t (B0d)  t (B0s)  t (B+) Theoretical predictions still unstable Measurements with a precision of few % needed

B+, B0 meson lifetimes and t(B+) / t(B0) Dl and D0l samples: CDF, ALEPH, DELPHI, OPAL B vertex charge: DELPHI, L3, OPAL, SLD Exclusive reconstruction: ALEPH (BDp), CDF (BJ/y K) Inclusive reconstruction of Dl combinations (B0 only): DELPHI, ALEPH, L3

Crucial for B physics Silicon Microstrip Detector s d = ( 13 + 40 / pt ) mm 2D vertex error ~ 60 mm Central Tracking Chamber B field = 1.4 T, Radius = 1.4 m (dpt/pt)2 = (0.0066)2 +(0.0009 pt)2 J/y Ks0 mass resolution ~10 MeV/c2 Lepton (e / m) Detection: fundamental for B - triggers - Inclusive lepton trigger b  lncX or b  cX, c  lnY (  pt ( B ) ~ 20 GeV/c ) - Dilepton ( em , mm ) trigger b  J/y X, y  m+ m- (  pt ( B ) ~ 10 GeV/c ) b  m- X , b  e+ Y

CDF Experimental Technique (A) lep All lifetime results are based on the Decay Length measurement n Lxy B   lep-D direction (VB - VP) ·pt (D-lep)  VP VB D Lxy = |pt (D-lep)|  Fully reconstructed decays ct (B) = Lxy m(B) / pt(B) proper D.L. Partially reconstructed decays l = Lxy m(B)/pt(D-lep) pseudo-proper D.L. Correction factor pt(D-lep) / pt(B) from MC, introduced statistically in the fit. pt(D-S -l+) / pt(B0S)

CDF Experimental Technique (B) # events signal sample # events bck. sample Lifetime and background shape are determined from a simultaneous fit of Signal and Background samples. Signal Signal prob. distr. nS nB L =  [ f sig  F isig + (1- f sig )  F ibck ]   F jbck i j Signal fraction under the “D mass” peak Bck. prob. distr. Convolution F isig = [Decay exponential  Gaussian resolution ]  [ pt(Dl) / pt(B) Smearing ] Background F ibck = Gaussian resolution + [Positive exp. + Negative exp.]  Gaussian resolution Only for part. rec. decays Zero -lifetime bck HF contribution

CDF: B+ and B0 lifetime from B  D()lnX decays D candidates searched for close to the trigger lepton. a) D0  K+p- ( D0 not from D -) b) D -  D0 p-, D0  K+p- c) D - D0 p-, D0  K+p- p+ p- d) D - D0 p-, D0  K+p- p0 Crosstalk from D  resonance B0  D  -l + X, D  -  D0 X B+  D  0l + X, D 0  D - X decomposed using MonteCarlo (main source of systematic error) t(B-) = (1.637  0.058+0.045) ps -0.043 t(B0) = (1.474  0.039+0.052) ps t(B-) / t(B0) = 1.110  0.056+0.033 -0.051 -0.030 l (B-) l (B0)

ALEPH: B+ and B0 lifetime from B  D()lnX decays (A) Technique similar to CDF - D +  D0 p+ sample D0  K- p+ D0  K- p+p-p+ D0  K- p+p0 D0  K0S p+p- - D0 sample (D0 not from D +)

ALEPH: B+ and B0 lifetime from B  D()lnX decays (B) t (B0) = ( 1.524  0.053+0.035 ) ps t (B+) = ( 1.646  0.056+0.036 ) ps t (B+) / t (B0) = 1.080  0.062  0.018 -0.032 -0.034

OPAL: B+ and B0 lifetime from Vertex Charge (A) Inclusive approach b events selected requiring displaced vertices and high momentum leptons Hemisphere tag T-tag : Jet-charge, vertex charge and lepton charge used to tag b flavour M-tag: used to determine decay length and perform lifetime fit e+ T-tag : identify bb events ( ~ 10,000 reconstructed vertices) b M-tag vertex charge Qvtx =  wi qi Z0 b Probability that the track exits from secondary vertex e- M-tag : perform lifetime measurement

OPAL: B+ and B0 lifetime from Vertex Charge (B) Excess decay length method To reduce the bias from the M-tag the excess to the minimum decay length which results in a resolvable secondary vertex is used. t (B0) = ( 1.523  0.057  0.053 ) ps t (B+) = ( 1.643  0.037  0.035 ) ps t (B+) / t (B0) = 1.079  0.064  0.041 Excess decay length

SLD: B+ and B0 lifetime from Topological Vertexing Inclusive 3D vertex reconstruction performed exploiting the excellent performance of the vertex detector Decay length  1 mm Vertex mass  2 GeV/c2 to eliminate charm and light flavor background Vertex charge obtained adding the charge of the corresponding tracks Charge purity enhanced using - Beam polarization - Opposite hemisphere jet charge Reconstructed Vertex Charge t (B0) = ( 1.585  0.021  0.043 ) ps t (B+) = ( 1.623  0.020  0.034 ) ps t (B+) / t (B0) = 1.037+0.025  0.024 Decay Length (cm) -0.024

CDF: B+ and B0 lifetime from exclusive B  J/y K decays 82436 fully reconstructed B+ - B+  J/y K+ - B+  J/y K+ - B+  y (2S) K+ - B+  y (2S) K+ 43627 fully reconstructed B0 - B0  J/y K0S - B0  J/y K 0 - B0  y (2S) K0S - B0  y (2S) K 0 M (m+ m- ) M (m+ m- p + p - ) l(B +) peak region l (B 0) peak region t(B+) = (1.680.070.02) ps t(B0) = (1.580.090.02) ps t(B+) / t(B0)=1.060.070.02 l(B +) sideband region l (B 0) sideband region

Summary of B+ meson lifetime

Summary of B0d meson lifetime

Summary of t (B+) / t (B0)

B0S meson lifetime DS l correlation: ALEPH, DELPHI, OPAL, CDF DS - hadron decays: ALEPH, DELPHI J/y f exclusive decay: CDF

DELPHI: BS lifetime from DS l correlation (A) 3.6 million Z0 hadronic decays B0S  D-S l+nX - D-S  f p+, K0K-, K0SK-, f p+p-p+, f p+p0, K0K - - D-S  f e-n, f m-n - D-S  f h-X (partially reconstructed) background sources - B  D(* ) D(* )+X (D l -nX) (reduced by high pt lepton and mass) - Reflections from B+  K- p+p+ (if one p is misidentified as K) f l h candidates

DELPHI: BS lifetime from DS l correlation (B) t(B0S) = ( 1.42+0.14(stat)  0.03 (syst)) ps Sources of systematic error - Background fraction +0.0090 - Cascade decays -0.0100 - Background from B -0.0020 - XDs discrim. var. +0.008 - pt discrim. Var. 0.004 - t(B+) (1.65  0.04 ps) -0.0010 - t(B0d) (1.56  0.04 ps) -0.0012 - t resolution 0.008 - t acceptance 0.010 - Simulated evts. Statisitcs 0.020 Total 0.03 DGBs / GBs  0.46 @95 C.L. -0.13 -0.0130 +0.0110 +0.0020 +0.0010 +0.0130 f l h sample

DELPHI: BS lifetime from DS h decays (A) 3.5 million Z0 hadronic decays - B0S  D-S p+ or D-S a+1 - D-S  f p- or K 0 K+ Larger statistics than DS l, but lower purity (hadron ambiguity) B vertex found constraining the DS h from a common vertex BS Purity increased using - DS mass and momentum - |cos(y)| - c2 of the DS vertex - Opposite hemisphere B tag

DELPHI: BS lifetime from DS h decays (B) t(B0S) = ( 1.49+0.16(stat)+0.07(syst)) ps -0.15 -0.08 Sources of systematic error - Sample composition +0.013 - Background fraction +0.046 - Back. Parameterization +0.017 - BS purity +0.005 - t resolution  0.019 - t(B+) (1.65  0.04 ps)  0.021 - t(B0d) (1.56  0.04 ps)  0.019 - Analysis bias corr.  0.040 Total +0.07 DGBs / GBs  0.58 @95 C.L. -0.016 -0.050 -0.012 -0.015 -0.08 DS sidebands

CDF: BS lifetime from BS  J/y f decay Use 58  12 BS  J/yf events Simultaneous fit of the mass and proper decay length distribution M (m+ m- ) M (J/y f ) t(B0S) = (1.34+0.23  0.05) ps -0.19 B0S proper decay length

CDF: BS lifetime from BS  D-S l+n decays D-S candidates are searched for close to the trigger lepton - D-S  f p-, f  K+K- 220  21 ev - D-S  K 0 K-, K 0  K+ p- 12520 ev - D-S  K0S K-, K0S  p+ p- 33  8 ev - D-S  f m- n, f  K+K- 205  38 ev Problem: BS signal faked by Bd decays D-d  K 0 p- and D-d  K0S p- can fake D-S signal if p is assumed as K Solution: simultaneous fit of the K 0 K- K 0 p- mass distributions to estimate both components (crosscheck from D-d and D-S lifetime difference) t (B0S) = (1.36  0.09  0.05) ps World’s best measurement from a single experiment DGBs / GBs  0.83 @95 C.L. Same data after switching K- to p- K0S K- Mass MC MC

Summary of B0S meson lifetime

CDF: Bc discovery and lifetime from Bc J/y l X decays Bc meson observed through the decay Bc  J/y l X (l = e or m) M(Bc)=6.400.390.13 GeV/c2 t (Bc) = 0.46+0.18 0.03 ps -0.16 e only m only

Lb lifetime Lc lepton combination : ALEPH, DELPHI, OPAL, CDF L l+l- method : ALEPH, OPAL

ALEPH: Lb lifetime from Lb  L+cl- n decays 193 Fully reconstructed L+c l- - L+c  p K- p+ - L+c  p K0 - L+c  L p+ p+ p- - L+c  L p+ t (Lb) = LM / p - 3D decay length (s ~ 180 mm) - p from L+c l- and n energy t(Lb) = 1.18+0.13  0.03 ps -0.12

CDF: Lb lifetime from Lb  L+cl- n decays 197  25 fully reconstructed L+cl- - L+c  p K- p+ - Energy loss in the CTC used for statistical particle identification - Physical background from B  Lc DsX Lb  LcDsX reduced using kinematics - Combinatorial background fraction estimated from wrong sign Lcl pairs - pt correction for missing n from MC - Background lifetime shape estimated from L+c  p K- p+ sidebands t(Lb) = 1.32  0.15  0.07 ps

Summary of Lb lifetime

Summary of b lifetime

Conclusion Current status of b hadrons lifetime measurements (CDF, LEP and SLD) has been reviewed World average for t (B+) / t (B0) is starting to put in evidence a significant difference from unity t (Lb) still much lower than prediction More data and more work needed from single experiments