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

2. 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

3. 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 ~ %
between B0 and B+ (t (B+)  t (B0) )
due to nonspectator effects. u u
Destructive interference between
external / internal W emission

4. 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

5. 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

6. Crucial for B physics
Silicon Microstrip Detector
s d = ( / 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 +( 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

7. 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)

8. 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

9. 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  ) ps
-0.043
t(B0) = (1.474  ) ps
t(B-) / t(B0) = 
-0.051
-0.030
l (B-)
l (B0)

10. 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 +)

11. ALEPH: B+ and B0 lifetime from B  D()lnX decays (B)
t (B0) = (  ) ps
t (B+) = (  ) ps
t (B+) / t (B0) =   0.018
-0.032
-0.034

12. 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

13. 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) = (   ) ps
t (B+) = (   ) ps
t (B+) / t (B0) =   0.041
Excess decay length

14. 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) = (   ) ps
t (B+) = (   ) ps
t (B+) / t (B0) =  0.024
Decay Length (cm)
-0.024

15. 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

16. Summary of B+ meson lifetime

17. Summary of B0d meson lifetime

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

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

20. 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

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

22. 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

23. DELPHI: BS lifetime from DS h decays (B)
t(B0S) = ( (stat)+0.07(syst)) ps
-0.15
-0.08
Sources of systematic error
- Sample composition
- Background fraction
- Back. Parameterization
- BS purity
- 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
DGBs / GBs  C.L.
-0.016
-0.050
-0.012
-0.015
-0.08
DS sidebands

24. 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) = (  0.05) ps
-0.19
B0S proper decay length

25. 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  21 ev
- D-S  K 0 K-, K 0  K+ p 20 ev
- D-S  K0S K-, K0S  p+ p  8 ev
- D-S  f m- n, f  K+K  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  C.L.
Same data after
switching K- to p-
K0S K- Mass MC MC

26. Summary of B0S meson lifetime

27. 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) = 0.03 ps
-0.16
e only
m only

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

29. 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) =  0.03 ps
-0.12

30. 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

31. Summary of Lb lifetime

32. Summary of b lifetime

33. 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