1. Results on B Lifetimes from DØ
Breese Quinn
University of Mississippi
for the DØ Collaboration
Introduction
Semileptonic lifetime measurements
in J/ mode
Explain plots more
More physics, less technical, esp after slide 25,26
Watch jargon

2. B Lifetime Physics
All B hadrons predicted to have equal lifetimes by simple quark spectator model
Heavy Quark Effective field Theory, on the other hand, predicts a lifetime hierarchy
τ(B+) ≥ τ(B0) ≈ τ(Bs) > τ(Λb) >> τ(Bc)
QCD expansions in orders of 1/mb
Measurements of lifetime ratios eliminate much systematic uncertainty
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

3. DØ Detector Tracking 2T Solenoid Nearly hermetic muon system
Coverage to =3
New Layer 0 detector for improved vertexing and b-tagging
2T Solenoid
Nearly hermetic muon system
High efficiency muon triggers
Toroids provide muon pT at Level 1
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

4. Layer 0 Silicon Installed April 2006 48 modules at r = 1.7 cm
Greatly improved resolution
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

5. Tevatron B’s Tevatron has delivered in excess of 2 fb-1
1.68 fb-1 recorded at DØ
B physics at the Tevatron
Plus: all B hadrons produced there with large cross sections
Minus: very large backgrounds produced as well
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

6. Semileptonic B Decays High statistics
Reconstruct primary and secondary vertices for B hadron
Lose some momentum via ν
Select events in triggers which are unbiased with regard to lifetime
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

7. Semileptonic B Decays
Must calculate proper decay length, , from the transverse decay length, Lxy.
But due to the missing neutrino, we do not measure pT(b), we measure pT(c), for a visible proper decay length.
Where K is a correction factor for the momentum difference obtained from Monte Carlo. z
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

8. b: Event Selection
Select events satisfying only triggers which do not bias lifetime distribution
pT(μ) > 2.0 Gev, at least 2 track segments in muon chambers
Reconstructed K0S:
pT(K0S) > 0.7 GeV
480.0 < M(π π) < MeV – mass constrained to PDG KS mass
pT(p) > 1.0 GeV, at least 2 hits in SMT
Λc vertex χ2 < 9 and Λb vertex χ2 < 9
3.4 < M(Λc) < 5.4 GeV
dT(PVΛc) – dT(PVΛb) >-3σ
dT(Λb Λc) < 3.3σ
Isolation: Λb fraction of total momentum in cone [ (()2+()2)1/2 < 0.5 ] around Λc system > 0.5
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

9. b: Likelihood Ratio BG Rejection
To reduce background further, construct a probability distribution function, y, by combining likelihood ratios of several discriminating variables, xi
Measure fi from B0dD-+X(DKS) and B0sD-S+X(DSKSK) data
Similar kinematics to b signal
fis= fi(signal region)- fi(sidebands)
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

10. b: Likelihood Ratio BG Rejection
Variables used:
pT (K0S)
pT of proton
pT (Λ+c )
M( Λ+c)
Isolation of Λ0b candidate
Cut at ln(y) < 0.1
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

11. b: Final Sample
V > 0.02 cm
Fit is a Gaussian signal plus fourth order polynomial background
Red histogram shows MC estimated Bd and Bs reflected events
n(b) = 4437 ± 329 events
M(b) = ± 1.7 MeV
Width = 20.6 ± 1.7 MeV
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

12. b: Lifetime Fit
There are several decay modes that will result in a cνX final state
Each one has a different K factor distribution
The individual distributions are determined from properly weighted Monte Carlo
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

13. b: Lifetime Fit M prob. Dist. for events in the signal peak
Measure V and its error for each event and split sample into V bins and fit the mass distribution of each bin to a Gaussian signal and 4th order polynomial to determine bin by bin yields and errors, ni and I
Minimize:
NTOT: total number of signal events, free parameter
F(M): M probability distribution for events in the signal peak
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

14. b: Lifetime Fit Free Parameters NTOT b Fcc
M prob. Dist. for events in the signal peak
fcc: fraction of prompt c + c hadron in signal peak
Gcc: double Gaussian VPDL dist. from MC
M prob. Dist. for signal
H(K): total K factor distribution
Resolution function
p(): error prob. dist. for signal
s: scale factor for mis-estimation of errors
Free Parameters
NTOT
b
Fcc
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

15. b: Lifetime Fit DØ RunII Preliminary
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

16. b: Systematic Uncertainties
Mass fitting variations
Linear BG
Variation of binning
Scale factor varied by ±20%
K factor
Unknown BR varied over a wide range
Dependence on pT() cut
Uncertainties in B hadron generation and decay
cc BG
Widths and fractions of two Gaussians varied by their uncertainties
Many consistency checks also performed
Fit method tested on full and toy signal MC
Variety of data splits
Source
Systematic Uncertainty
in c(m)
Detector alignment
±5.4
Mass fitting method
±20
K factor determination
±12
cc background
±2.6
Scale factor
±10
Total
±26
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

17. b: S.L. Decay Lifetime Result
DØ RunII Preliminary
PDG:
CDF (prelim.):
1.3 fb-1
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

18. Previous S.L. Results: (B+)/ (B0)
As a function of VPDL, measure the ratio of the number of events of
Dominated by B+ decays
To the number of
Dominated by B0 decays
Binned likelihood fit gives the lifetime ratio
October, 2004
0.44 fb-1
World’s best measurement until Belle’s 100’s of fb-1
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

19. Previous S.L. Results: B0S
Unbinned maximum log-likelihood fit to the VPDL distribution
April, 2006
0.4 fb-1
WORLD’s BEST!
PDG:
DØ RunII Preliminary
DØ RunII Preliminary
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

20. b: J/  Mode Select b events with dimuon trigger
+- reconstruct to a J/
2 addl. tracks reconstruct to , with p assigned to higher pT track
Reconstruct b via  +- fit to common vertex with +- constrained to J/ mass
Similar process to select B0d J/KS events for ratio measurement
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

21. bJ/ : Lifetime/Mass Fit
Information on mass and proper decay length are used simultaneously in an unbinned likelihood fit to determine lifetime.
Ns, Nb: number of signal, bg
SM: signal mass dist. – single gaussian
BM[sh)lg)]: prompt(non-prompt) bg mass dist. – flat (2nd O polyn.)
S: signal PDL dist.: exp. convoluted with gauss. Resolution function, G(j,j)
B[sh)lg)]: prompt(non-prompt) bg PDL dist. – G(j,j) (+ and – exp. For combinatoric bg plus exp. for long-lived bg)
SE, BE:  ( error) dist. for signal (bg) – gauss. + 1 exp. (guass. + 2 exp.)
f0: prompt bg fraction
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

22. bJ/ : Systematic Uncertainties
Source
b(m)
B0(m)
Ratio
Detector alignment
5.4
Models for lifetime and mass distributions
0.7
13.0
0.026
Long-lived background components
1.0
0.4
0.001
Contamination
13.9 -
0.031
Total
15.0
14.1
0.041
Lifetime and mass dist. models: vary functional forms
Long-lived bg: replace single exp. with separate low and high mass exp. To investigate effect of any difference
Contamination: 6.5% of B0 pass b cuts, add model distributions to fit
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

23. bJ/ : Fit Results 1.2 fb-1 DØ RunII Preliminary
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

24. b: Comparison of Results
Both DØ results are consistent with the world average and theoretical prediction
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06

25. Conclusions
DØ has produced several very nice B hadron lifetime measurements in RunII
Two recent (b) results with more than 1 fb-1 of data are consistent with world average and with NLO prediction.
Good (B+)/ (B0) measurement with only 0.4 fb-1 in good agreement with world average and theory
World’s best (B0S) measurement (in PRL shortly!) in good agreement with world averages
B. Quinn University of Mississippi
CKM2006, Nagoya, Japan 12/14/06