b lifetimes
description
Transcript of b lifetimes
b lifetimes
Simone Donati INFN & University of Pisa
8th International Symposium on Heavy Flavour PhysicsUniversity of Southampton 25 - 29 July 1999
Outline
• Introduction - Theoretical Framework - Crucial detector elements • B+, B0 lifetime and (B+ )/(B0 ) - Dl and D0l samples - B vertex charge - Exclusive reconstruction • B0
s lifetime - DS l correlation - DS h samples - Exclusive reconstruction • b lifetime - c lepton combination • Summary and Conclusions
Theoretical Framework
Baseline Spectator Model: the light quark actsas a spectator All b - hadrons have the same lifetime. = (GF
2 mb5/1923) · | Vcb | 2 ·
B D•
•W
q q
b cVcb
q, lq,
But, for charm hadrons: (D-) ~ 2.5 (D0) ~ 2.5 (Ds-) ~ 5.0 (c
-)
Expected lifetime difference ~ 5 - 10 %
between B0 and B+ (B+) (B0) due to nonspectator effects.
B0 W D-
+
W exchange
b c
d u
B+ Wb
u
q, l
q,
B+ annihilation
B+
w +
D0b c
u u
D0
+
u u
wb c
Destructive interference betweenexternal / internal W emission
Theoretical Framework • Effects scale as 1/m2
q
- Important effects for c hadrons: (D+) / (D0) = 2.55 0.04 - Up to 10 % differences expected for b hadrons (from HQE)
b (B0d) (B0
s) (B+)
• Theoretical predictions still unstable• Measurements with a precision of few % needed
B+, B0 meson lifetimes and (B+) / (B0)
• Dl and D0l samples: CDF, ALEPH, DELPHI, OPAL
• B vertex charge: DELPHI, L3, OPAL, SLD
• Exclusive reconstruction: ALEPH (BD), CDF (BJ/K)
• Inclusive reconstruction of Dl combinations (B0 only): DELPHI, ALEPH, L3
Crucial for B physics
· Silicon Microstrip Detectord = ( 13 + 40 / pt ) m 2D vertex error ~ 60 m
· Central Tracking Chamber B field = 1.4 T, Radius = 1.4 m (pt/pt)2 = (0.0066)2 +(0.0009 pt)2
J/Ks0 mass resolution ~10 MeV/c2
• Lepton (e / ) Detection: fundamental for B - triggers - Inclusive lepton trigger b lcX or b cX, c lY ( pt ( B ) ~ 20 GeV/c )
- Dilepton ( e, ) trigger
b J/X, ( pt ( B ) ~ 10 GeV/c )
b X , b e+ Y
CDF Experimental Technique (A)
All lifetime results are based on the Decay Length measurement
Lxy =(VB - VP) ·pt (D-lep)
|pt (D-lep)|
• Fully reconstructed decays ct (B) = Lxy m(B) / pt(B) proper D.L.
• Partially reconstructed decays = 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(B0
S)
D
B
lep
VBVP
lep-D directionLxy
CDF Experimental Technique (B)
Lifetime and background shape are determined from a simultaneous fit of Signal and Background samples.
Signal
L = [ f sig F isig + (1- f sig ) F i
bck ] F jbck
i j
nS nB
# events signal sample
Signal fraction under the “D mass” peak
Signal prob. distr.
Bck. prob. distr.
# events bck. sample
F isig = [Decay exponential Gaussian resolution ] [ pt(Dl) / pt(B) Smearing ]
BackgroundF i
bck = Gaussian resolution + [Positive exp. + Negative exp.] Gaussian resolution
Only for part. rec. decays
Zero -lifetime bck HF contribution
Convolution
CDF: B+ and B0 lifetime from B D()lX decays
• D candidates searched for close to the trigger lepton. a) D0 K+( D0 not from D -)
b) D - D0 , D0 K+
c) D - D0 , D0 K+
d) D - D0 , D0 K+
•Crosstalk from D resonance
B0 D -l + X, D D0 X
B+ D 0l + X, D 0 D - Xdecomposed using MonteCarlo (main source of systematic error)
(B-) (B0)
(B-) = (1.637 0.058+0.045) ps-0.043
(B0) = (1.474 0.039+0.052) ps
(B-) / (B0) = 1.110 0.056+0.033
-0.051
-0.030
ALEPH: B+ and B0 lifetime from B D()lX decays (A)
• Technique similar to CDF - D + D0 sample D0 K
D0 K
D0 K
D0 K0S
- D0 sample (D0 not from D +) D0 K
D0 K
D0 K0S
ALEPH: B+ and B0 lifetime from B D()lX decays (B)
(B0) = ( 1.524 0.053+0.035 ) ps (B+) = ( 1.646 0.056+0.036 ) ps (B+) / (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
M-tag vertex charge Qvtx = wi qiZ0
e+
e
b
b
T-tag : identify bb events
M-tag : performlifetime measurement
( ~ 10,000 reconstructed vertices)
Probability that the track exits from secondary vertex
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.
(B0) = ( 1.523 0.057 0.053 ) ps (B+) = ( 1.643 0.037 0.035 ) ps (B+) / (B0) = 1.079 0.064 0.041
Excess decay length
• 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
SLD: B+ and B0 lifetime from Topological Vertexing
(B0) = ( 1.585 0.021 0.043 ) ps (B+) = ( 1.623 0.020 0.034 ) ps (B+) / (B0) = 1.037+0.025 0.024
-0.024
Decay Length (cm)
Reconstructed Vertex Charge
CDF: B+ and B0 lifetime from exclusive B J/ K decays
82436 fully reconstructed B+
- B+ J/ K+
- B+ J/ K+
- B+ (2S) K+
- B+ (2S) K+
43627 fully reconstructed B0
- B0 J/ K0S
- B0 J/ K 0
- B0 (2S) K0S
- B0 (2S) K 0
(B+) = (1.680.070.02) ps
(B0) = (1.580.090.02) ps
(B+) / (B0)=1.060.070.02
( +) peak region (B 0) peak region
( +) sideband region (B 0) sideband region
M ( ) M ( )
Summary of B+ meson lifetime
Summary of B0d meson lifetime
Summary of (B+) / (B0)
B0S meson lifetime
• DS l correlation: ALEPH, DELPHI, OPAL, CDF
• DS - hadron decays: ALEPH, DELPHI
• J/ exclusive decay: CDF
• 3.6 million Z0 hadronic decays B0
S DS l+X
- DS , K0K, K0
SK, , 0, K0K
- DS e,
- DS hX (partially reconstructed)
• background sources- B D() D()X (D l X(reduced by high pt lepton and mass)- Reflections from B+ K (if one is misidentified as K)
l h candidates
DELPHI: BS lifetime from DS l correlation (A)
DELPHI: BS lifetime from DS l correlation (B)
l h sample
(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 - (B+) (1.65 0.04 ps) 0.0010
- (B0d) (1.56 0.04 ps) 0.0012
- t resolution 0.008 - t acceptance 0.010 - Simulated evts. Statisitcs 0.020 Total 0.03
Bs / Bs 0.46 @95 C.L.
0.13
0.0130
0.0110
0.0020
0.0010
0.0130
• 3.5 million Z0 hadronic decays - B0
S DS or D
S a+1
- DS 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 - 2 of the DS vertex - Opposite hemisphere B tag
DELPHI: BS lifetime from DS h decays (A)
DELPHI: BS lifetime from DS h decays (B)
(B0S) = ( 1.49+0.16(stat)+0.07(syst)) ps0.15 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- (B+) (1.65 0.04 ps) 0.021- (B0
d) (1.56 0.04 ps) 0.019- Analysis bias corr. 0.040 Total +0.07
Bs / Bs 0.58 @95 C.L.
0.016
0.050
0.012
0.015
0.08
DS sidebands
CDF: BS lifetime from BS J/ decay
Use 58 12 BS J/ events
Simultaneous fit of the mass andproper decay length distribution
-0.19(B0S) = (1.34+0.23 0.05) ps
M ( ) M (J/ )
B0S proper decay length
• D-S candidates are searched for close
to the trigger lepton
- D-S , K+K 220 21 ev
- D-S K 0 K, K 0 K+ 12520
ev
- D-S K0
S K, K0
S 33 8
ev
- D-S , K+K 205 38
ev
• Problem: BS signal faked by Bd decays
D-d K 0 and D-
d K0S
can
fake D-S signal if is assumed as K
• Solution: simultaneous fit of the K 0
K
K 0 mass distributions to estimate both components (crosscheck from D
d and DS lifetime difference)
(B0S) = (1.36 0.09 0.05)
psWorld’s best measurement from a single experiment
Bs / Bs 0.83 @95 C.L.
CDF: BS lifetime from BS D-S l+ decays
K0S
KMass
Same data afterswitching K to
MC MC
Summary of B0S meson lifetime
• Bc meson observed through the decay
Bc J/ l X (l = e or )
M(Bc)=6.400.390.13 GeV/c2
(Bc) = 0.46+0.18 0.03 ps
CDF: Bc discovery and lifetime from Bc J/ l X decays
-0.16
e only
only
b lifetime
• c lepton combination : ALEPH, DELPHI, OPAL, CDF
• l+l method : ALEPH, OPAL
• 193 Fully reconstructed c l
- +c p K +
- +c p K0
- +c + +
- +c +
• t b = LM / p - 3D decay length ~ 180 m- p from
c l and energy
(b) = 1.18+0.13 0.03 ps
ALEPH: b lifetime from b cl decays
-0.12
• 197 25 fully reconstructed cl
- +c p K +
- Energy loss in the CTC used for statistical particle identification- Physical background from B c DsX b cDsX reduced using kinematics - Combinatorial background fraction estimated from wrong sign cl pairs- pt correction for missing from MC- Background lifetime shape estimated from +
c p K + sidebands
(b) = 1.32 0.15 0.07 ps
CDF: b lifetime from b cl decays
Summary of b lifetime
Summary of b lifetime
Conclusion
• Current status of b hadrons lifetime measurements (CDF, LEP and SLD) has been reviewed• World average for (B+) / (B0) is starting to put in evidence a significant difference from unity• bstill much lower than prediction• More data and more work needed from single experiments