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