Bd and Bs mixing at D

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Bd and Bs mixing at DØ

• BEAUTY 2005, Assisi, Perugia
• Tania Moulik,
• University of Kansas
• (For the DØ Collaboration)

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Outline

• Introduction
• Data taking, Triggers
• Bd mixing measurement
• Reconstruction
• Flavor Tagging
• Bs mixing measurement
• Decay modes
• Amplitude scan
• Sensitivity studies.

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Introduction

• Oscillations in the B0-B0 system first observed
  by ARGUS in 1987 (18 years since then!)
• Signalled a large top mass
• CP violation in B decays.
• Many measurements on Bd mixing LEP, SLD, CDF,
  DØ, Belle, Babar
• Tevatron has unique opportunity to measure Bs
  mixing in 2 complementary analyses. Dms (This
  talk) and DGs/Gs (Alberto Sanchezs talk)

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Bd and Bs Mixing

• B mixing implies a B0 transition to B0 (second
  order weak interactions)
• Light and heavy B meson mass eigenstates differ
  from flavor eigenstates

Vtb
Vtb
t
Vtb
Vtb
b
b
W
b
b
W
W
t
t
d(s)
d(s)
d(s)
Vtd(s)
Vtd
t
d(s)
Vtd(s)
Vtd
W
Time evolution follows the Schrodinger equation
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Bd, Bs mixing

• The case with DG?0, mixed and unmixed decay
  probabilities become,
• Extract mixing parameter Dm from asymmetry

Time evolution of B0 and B0 states,
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Ultimate Goal Constraing the CKM matrix -
complex phase in the CKM matrix (CP violation).
Wolfenstein parametrisation - expansion in l
0.22
complex
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Dmd, Dms, and CKM
x2 (x 1.24 ?0.04?0.06)
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Data Taking
1.8 TeV
1.96 TeV
460 pb-1
200 pb-1
Data taking e 80 - 88
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B physics triggers
m
n
p

• Single muon ? Muon track match at Level 1 and a
  muon at L2
• Unbiased single muon triggers(PTgt3,4,5) Bd and
  Bs mixing, prescaled or turned off depending on
  luminosity.
• Impact parameter biased triggers Hadronic
  decays (Not being used yet).

Muon system ? Layers of drift chambers and
scintillators. Trigger (B mixing) ? Single
inclusive muon trigger with h lt 2.0, PT gt
3,4,5 GeV, Dimuon trigger (second muon for
tagging)
Toroid
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Bd mixing Motivation and Main ingredients
m-

• High statistics sample
• m- determines charge of the B on reconstruction
  side.
• Flavor tagging determine charge of opposite B
• Sample composition Other B decays contribute to
  the same final state.
• Validate flavor tagging to be used for main
  analysis, Bs mixing.

W
D0? K p, D ? D0 ps
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D0 and B selection
92 K total in 200 pb -1

• D0 selection
• 2 tracks, opposite charge,
• Ptgt0.7, hlt2.0
• Displaced vertex
• d0/s(d0) of tracks gt 2
• Lxy/s(lxy) gt 4,
• cos(a)gt 0.9
• Good vertex fit.
• D0m (B) selection
• If lxyB/s(lxyB) gt 4, then cos(aB ) gt 0.95
• lxyB lt lxyD within 3s
• Does not form a D with a soft pion.

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Bd mixing measurement

• Fit the c2 in VPDL bins to extract Dm and purity
  of tag

Measured
Expected
PT(B)
Visible Proper Decay Length
Charge of B determined from m charge on
reconstructed side Opposite B charge from Flavor
Tag
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Expected Asymmetry

• Distribution of oscillated and non-oscillated
  events are function of K-factor,h, Dm
• B not fully reconstructed. Measure
• average k-factor K Pt(mD0)/Pt(B)

K-factor
Reconstruction efficiency of jth channel
VPDL resolution
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Bd mixing Decay length resolution
793 K readout channels
F-Disks
Barrel
H-disks

• 6 Barrels 4 layers, Double sided
• except for layers 1 and 3 of the outermost
  barrels.
• Interspersed F disks double sided
• H disks single sided, 24 wedges per h-disk

22.1-1.7 mm 50.5-3.1 mm
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Flavor Tagging
----We have explored --- Jet charge, Soft muon,
Same side tagging
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Flavor Tagging Definitions
Same side Tagging

• Flavor tagging merit (eD2)

B meson
PBPtrk
Jet Charge
Ptrel
Candidate track Ptrk
Tagger e () D eD2()
SLT 5.0?0.2 44.8?5.1 1.0?0.1
jetQSST 68.3?0.9 14.9?1.5 1.5?0.5
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Fitted asymmetry

• Simultaneous fit to D and D0 sample to extract
  Dm.

Muon Tagger
B?D0 m X
JetQSST
Dm 0.456?0.034?0.025 ps-1 (D) Bd
contribution 83 (D0) B contribution 76
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Bs Mixing

• Very similar analysis to the Bd mixing. Decay
  mode used Bs ? Ds m n, Ds?fp, f?KK-

67543
133329?277
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ForBs mixing observation
S signal events eD2 tagging power
S/B signal/background st proper time
resolution
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Significant Improvements to flavor tagging

• Add Secondary Vertex Charge
• Tagging

Likelihood ratios for variables discriminating
between b and b ? ptrel.q, svcharge,jetcharge
Normalized Tagging variable d ? Monotonic
function between (-1,1) dgt0 (b) , dlt0(b)
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Proof of principle Dmd measurement
Dmd 0.558-0.048(stat.) ps-1 eD2 1.0 (OST
Tagging only)
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Bs mixing limit Amplitude fit method
Ideal non-osc/osc number of events as a function
of VPDL (x) ? Fit for Amplitude A
Fit for A as a function of Dms ( in steps of 1
ps-1)
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Impact parameter resolution and tuning.
Variable scale factor for VPDL resolution (s.s)
Overall scale factor 1.10

• Tune track errors for data and MC taking taking
  into account dependence on
• Track momentum
• Track angles
• Silicon detector hit configuration
• Silicon cluster width

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Sample composition
Decay Sample composition
Bs?Dsµ? 20.6
Bs?Dsµ? 57.2
Bs?D0sµ? 1.4
Bs?D1sµ? 2.9
Bs?DsDsX 11.3
B0?DsDX 3.2
B-?DsDX 3.4
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Bs mixing Limit (460 pb-1)

• Expect A1, for freq trueDms,
• A0 for freq ? trueDms
• 95 C.L
• smallest value of Dms at which, A1.645sA 1
• Sensitivity
• Dms value with 1.645sA 1

Dms gt 5.0 ps-1 Sensitivity 4.6 ps-1
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To be Added

• Reconstructed candidates 4933 ?376

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World averages for Dmd world Bs mixing limit
Dms gt 14.5 ps-1 Sensitivity 18.2 ps-1
0.502?0.006 ps-1
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Future Perspectives
With additional Channels and Improvements In
flavor tagging Dms upto 11 ps-1 Within reach
at 3.5 fb-1
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Future perspectives
With increase in bandwidth Dms upto 18 ps-1
Addition of hadronic decay Modes and Layer0 50
Hz Bandwidth Dms upto 26 ps-1
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Conclusions and Summary

• Bd mixing measurement
• We can contribute to the Bd mixing measurement in
  a small way and an important cross check of the
  measurement in a hadron collider environment. It
  is mainly important as a validation tool for our
  flavor tagging.
• Bs mixing measurement
• More statistics
• Improvements in flavor tagging, introduction of
  electron tagging (Almost ready!), Unbinned
  likelihood fit
• More decay modes,
• Bs?D s e n X
• Addition of Ds ? KK
• Addition of hadronic decay modes
• Addition of Layer0 ? improvements in impact
  parameter resolution
• We are excited. Lots more to come.

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• BACKUP

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Inner Tracker
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D0 Silicon RunIIa disks
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Silicon Alignment