Chris Barnes, Imperial CollegeWIN 2005 B mixing at DØ B mixing at DØ WIN 2005 Delphi, Greece Chris Barnes, Imperial College.
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WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
B mixing at DØWIN 2005
Delphi, Greece
Chris Barnes, Imperial College
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Outline
• Mixing phenomenology• Tevatron • DØ detector• Flavour tagging
• Bd cross-checks
• Bs mixing and limit
• Prospects• Summary
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Mixing
• Transition of neutral mesons between particle and antiparticles
• Mass eigenstates superpostion of flavour eigenstates
• An initial B0 will oscillate in time between B0 and B0
)]cos(1[2
1)( 0 mteBp t
LH mmm
)]cos(1[2
1)( 0 mteBp t
*tbV
*tbVtdV
b sd ,tdV
0B
bsd ,
W W 0B
tcu ,,
tcu ,,
*tbVtdV
b
tcu ,,
sd ,tdV*tbV
0B
bsd ,
tcu ,,
W
W
0B
00 BqBpBL 00 BqBpBH
_
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Motivation
• Why study mixing?
• In standard parameterisation
unitarity approximated by
• Mixing constrains least known side of unitarity triangle
• Experimental limit
• SM predicts (incl exp)
2
2
||
||
td
ts
d
s
V
V
m
m
Some theoretical
uncertainties cancel
cbi
tdi
ub VeVeV
tdV12.226.15 psms
15.14 psms
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Experimental issues
• Need to tag initial flavour of B
Tagging power εD2 with efficiency ε and dilution
• Need excellent propertime resolution σt – decay length and p resolution
• The average significance of a measurement is given by
• Experimentally useful
to define asymmetry)()(
)()()(
tNtN
tNtNtA
oscoscno
oscoscno
2
)(22
2
tsm
eBS
SDN
D = 2*purity – 1 tagwrongtagcorrect
tagwrongtagcorrect
NN
NND
mtDtA cos
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Setting limitsAmplitude method
• Fit to data - free parameter
• Obtain as a function of
• Measurement of gives
and otherwise
• At 95% CL
sensitivity
excluded
• Combine measurements from experiments
)]cos(1[2
1tmAep s
t
sm
A
sm 1A0A
A
1645.1 A
1645.1 AA 15.14 psms
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
460pb-1
200pb-1
Tevatron
• Only place in world to study Bs mesons
• Tevatron performing well
– Inst L reached ~1.2×1032 cm/s2 (1.5 – 3 ×1032 cm/s2 design)
Total integrated L = 4-8 fb-1 in Tevatron Run II programme
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
DØ Detector
• Excellent coverage
– Muon |η| < 2
– Tracking |η| < 3
• Robust muon triggers
• All B physics events require muons
Muon System
Tracking System
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Samples
0D)2010(D
200 pb-1
20K D* + μ
92KD0 + μ
0* DD KD 0
)2010(0 DBd
0DB
85% Bd, 15% B+ 85% B+, 15% Bd
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Samples
13.3K Ds
5K D±
13.3K Ds
5K D±
XDB ss 0460 pb-1 sD KK
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
B reconstruction
XDB ss 0
- Hadronic environment – large backgrounds
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Flavour Tagging • Need to determine b flavour
at production
• Taggers developed• Soft Lepton (SLT): ε = (5.0 ± 0.2) % • Jet Charge (JetQ)• Same side pion (SST)• Combined JetQ + SST: ε = (68.3 ± 0.9) %
• Opposite side μ + SV tagging
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Bd mixing
)2010(0 DBd 0DB
Δmd = 0.456 ± 0.034 (stat) ± 0.025 (sys) ps-1 World average = 0.502 ± 0.007 ps-1
SLT D0 = 0.448 ± 0.051
JetQ+SST D+ = 0.279 ± 0.012
JetQ+SST D0 = 0.149 ± 0.015
difference in dilutions
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Flavour Tagging
• Opposite side combined μ+SV tagging• Require muon with angular separation cosΔφ < 0.8
• Use discriminating variables xi - combine using likelihood ratio
– Charge of jet containing the muon
– pTrel weighted muon charge
– SV charge • Look for SV in event (muon not included)
6.06.0 )(/)( iTi
iTi
iSV ppqQ
iT
iiT
p
qpJetQ
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Flavour Tagging
• Likelihood ratio
• For each variable xi, x > 0 tags initial B flavour as b quark
• Construct tagging variable d
• Where y is a product of ratios
of probability density fns to give
wrong and right tags
• PDFs determined from reconstructed Bd →D*-μ+ X data small decay lengths - minimal oscillations
pdfs of variable xi for wrong and right tags
|d| related to dilution for an event
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Dilution cross-check• Opposite side tagging dilution should be same for all B species• Cross-check performed on Bd Bu
• Dilutions consistent• Use value of D = 0.468 ± 0.015 in Bs mixing fits
D = 0.448 ± 0.021 D = 0.468 ± 0.015
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Strategy
• One decay mode Bs → Ds μ X, Ds → φ π, φ → KK
• Opposite side μ+SV tagging
• Binned asymmetry in Visible Proper Decay Length
• Minimise observed and expected values of asymmetry
• Inputs to fitting procedure
– Sample composition MC
– K factors (corrections for non-reconstructed particles) MC
– Efficiencies MC
– Dilution obtained from Bd mixing
– VPDL resolution MC/DATA
BDxy
DxyxyM M
P
PLx
s
s
2)(
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Fit parameters
Decay Sample composition
Bs→Dsμν 20.6%
Bs→D*sμν 57.2%
Bs→D*0sμν 1.4%
Bs→D*1sμν 2.9%
Bs→DsDsX 11.3%
B0→DsDX 3.2%
B-→DsDX 3.4%
Sample composition Efficiency correction due to impact parameter cuts on tracks from Ds
cc estimated at ~ 3.5 ± 2.5 % VPDL (cm)
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
K-factors
• Correction factor to pT accounting for
non-reconstructed particles
Decay <k>
Bs→Dsμν 0.878
Bs→D*sμν 0.857
Bs→D*0sμν 0.829
Bs→D*1sμν 0.817
Bs→DsDsX 0.738
B0→DsDX 0.681
B-→DsDX 0.687
)(
)(
sT
sT
Bp
DpK
VPDLKc
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
VPDL resolution
• VPDL resolution parametrised with three Gaussians
• Track impact parameter errors tuned on data
• Correction dependent on number of vertex hits, location of hits, track polar angle and momentum
• Tuning results in one scale factor applied to all three Gaussians
SF = 1.095
Corrected resolution
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Systematics
• Uncertainty on input fit parameters contribute to systematic error
• Uncertainties in variables included– Dilution– Mass fits– Resolution– Sample composition– K-factor– ΔΓ– Efficiencies
A
sysA
AAA
)1(
Each error added in quadrature
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Result
ms > 5.0 ps-1 at 95% CL No obvious oscillations
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Projections• Include hadronic Bs sample
– Better proper decay time resolution in hadronics (no )
– Hadronic events at DØ triggered by opposite side muon
Provides high efficiency tag (~1)
• Detector addition summer 2005: “Layer 0” Silicon detector
~25% gain in σt
• Proposal to increase rate to tape from 50 to 100 Hz later in 2005– Dedicated B-physics bandwidth– Lower trigger thresholds~ factor 3 gain in statistics
• Expect to cover full SM range
WIN 2005 B mixing at DØ
Chris Barnes, Imperial College
Summary
• Developed all tools to perform mixing measurement
• Flavour tagging demonstrated on Bd → D* μ X
Δmd = 0.456 ± 0.034 (stat) ± 0.025 (sys) ps-1
• Using 460 pb-1 peformed first Bs mixing analysis at DØ
• Decay mode Bs → Ds μ X
Δms > 5.0 ps-1 at 95% CL
Future• Layer-0 and increased L3 bandwidth• Improve tagging - electrons, more variables, possibly SST?• Additional semileptonic modes • Hadronic mode
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