Hot Topics at D0

1

• Hot Topics at D0
• David Buchholz (Northwestern Univ)
• On behalf of the DØ Collaboration

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D0 Results

• D0 Talks at this conference-
• b physics
• B mixing and lifetimes- Piedra
• Bs decays and B leptonic decays- Van Kooten
• t physics
• Top Physics Clement
• Many new b and t physics results
• Watch for these in other talks
• Hot Topics at D0
• Bs and Bd mixing
• CP violation result in B decays
• Flavor Changing Neutral Current results (D?
  pµµ- )

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Excellent Tevatron Performance

• Data sample corresponding to over 1 fb-1 of the
  integrated luminosity used for the Bs mixing
  analysis
• Results are being reported on this 1 fb-1 sample

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D0 Detector
B Physics Program based on excellent performance
of 1) muon system, ?lt2.0, pT gt 3,4,5 GeV 2)
silicon microstrip tracker 3)Good single and
dimuon triggers
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Unitarity Triangle- B Mixing Contribution
Measurement of ?ms contributes to better
determination of Vtd
Dmd (lots of QCD)xVtd
Dms (lots of QCD)xVts
Dms/Dmd (much less QCD)xVts/Vtd
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Mixing and Oscillations
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Analysis Outline
Opposite Side
Reconstructed Side
X
µ
B
µ(e)
LT
p -
D-S
f
K-
?
K

• Select Bs candidate
• Concentrate on the most clean decay mode
  Bs??µDs(?fp)
• For each Bs candidate
• BS flavor at decay time from muon sign at the
  reconstructed side
• Transverse length LT and its error
• Transverse momentum PT(Bs) (use PT(Dsµ))
• B-hadron flavor at the opposite side (indicates
  BS flavor at production time)

8
Proper Decay Length

• Proper Decay Length is determined from the
  Visible Proper Decay Length
• K-factor takes into account the escaping
  neutrino and other missing particles
• From MC for each decay mode- different excited
  states

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Resolution Calibration Using Data

• Use J/??µµ sample
• Fit pull distribution for J/? Proper Decay
  Length with 2 Gaussians
• Resolution Scale Factor is 1.0 for 72 of the
  events and 1.8 for the rest
• Confirmed by Impact Parameter tuning procedure
  in MC

DØ Run II Preliminary
resolution
resolution plus decays
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Efficiency Dependence on VPDL

• From MC
• Cross-checked and tuned using data
• Note that efficiency at VPDL0 is not 0

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Opposite Side Tagging
p
m,e
K,p
K K
n
Ds
B
m,e
Pa
Bs
Secondary Vertex
Primary Vertex
Tagging side
Reconstructed B-meson

• Muon (electron) jet charge defined as
• Sum is taken over all tracks in the cone ?Rlt0.5
  around muon
• SV Charge
• Event charge
• all tracks with ?R lt 1.5 w.r.t. reconstructed B
  direction are used

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Calibration of Dilution Using Bd?Dµ?X
Increasing dilution
Increasing dilution
?mHFAG 0.507 0.004 ps-1
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Final Signal Selection

• Use likelihood ratio method
• Set of discriminating variables xi constructed
  for each event
• Helicity Angle (Ds,K1)
• ?Ds Isolation
• pT(K1K2)
• m(?Ds)
• ?2 of Ds Vertex Fit
• m(K1K2 or K1?)
• Construct likelihood ratio for each variable
• bgrd from m(Ds) sidebands
• signal from bgrd-sub peak

• Combine into single variable
• Use for final selection

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µ?? sample _at_ D0 (1 fb-1)
Opposite-side flavor tagging
µD 7,422281
µDs 26,710560
µD 1,51996
Tagging efficiency 20
µDs 5,601102
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Cross-check Using Bd?XµD(???)
Amplitude Scan
DØ Run II Preliminary

• The Amplitude Scan reveals the Bd oscillations
• at correct place ? no lifetime bias
• with correct amplitude ? correct dilution
  calibration

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Event-by-Event Fit

• Minimize the Likelihood function
• Probability Density Functions (PDF) for each
  source
• Proper Decay Length
• Dilution
• Proper Decay Length Error
• Mass
• Signal Selection Variable

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Vertex Resolution
DØ Run II Preliminary
e.g. Period of oscillations _at_ 19ps-1
Limit of sensitivity to mixing at 22ps-1

• Determined by vertex fitting procedure

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Results of the Lifetime Fit
Most important region

• Different background models are used for
  cross-check and systematic errors
• Trigger biases have been studied
• Different efficiency models
• Central values for ctBs 404 - 416 µm
• Statistical error 10 µm
• HFAG value ctBs 438 12 µm

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Amplitude Scan For Bs

• Deviation of the amplitude at 19 ps-1
• 2.5s from 0
• 1.6s from 1

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Log Likelihood Scan
In agreement with the amplitude scan

• Resolution
• K-factor variation
• BR (Bs??DsX)
• VPDL model
• BR (Bs?DsDs)

Systematic
Have no sensitivity above 22 ps-1
17 lt Dms lt 21 ps-1 _at_ 90 CL assuming Gaussian
errors Most probable value of Dms 19 ps-1
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World Average
With current D0 result
HFAG Preliminary Correlated systematics not yet
included
_at_19ps-1 1.5s ? 2.3s
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Ensemble Tests

• Using data
• Simulate ?ms8 by
• randomizing the sign
• of flavor tagging
• Probability to observe
• ?log(L)gt1.9
• (as deep as ours)
• in the range 16 lt ?ms lt 22 ps-1 is 3.8
• 5 using lower edge of syst. uncertainties band
• Region below 16 ps-1 is experimentally excluded
• No sensitivity above 22 ps-1
• Using MC
• Probability to observe ?log(L)gt1.9 for the true
  ?ms19 ps-1 in the range 17 lt ?ms lt 21 ps-1 is 15

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Impact on the Unitarity Triangle
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Impact on the Unitarity Triangle
25

CP Violation in Mixing
Three phenomenological signatures
First in K
CPV in Mixing eK KL pp ? eB ll - l-l-
?
Mixing vs Decay h KL p0nn ? b/f1 J/y
Ks ?
CPV in Decay e//e KL p0p0 , pp- ? ACP Kp-,
K-p ?
Last in B
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Analysis Strategy
Goal is to measure eB with part per mil precision
by looking for asymmetry in like-sign dimuons
Toroid
How? Using independent spectrometers and
reversing the polarities
Solenoid
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Raw Results
After removing detector based asymmetries
Now need the fraction of m from B
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Sample Composition
signal
b m b b m
b m b c m
Backgrounds with false asymmetries Reaction
K-N?Yp (Y?,S...) has no KN analog
Backgrounds that dilute the asymmetry
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Sample Composition
signal
b m b b m
b m b c m
Backgrounds with false asymmetries Reaction
K-N?Yp (Y?,S...) has no KN analog
Backgrounds that dilute the asymmetry
Cross-check
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Results
Raw Asymmetry
Asymmetry from Mixing
CP Violation Parameter
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Results
(0.41.40.9)
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Flavor Changing Neutral Currents
Strict limits from b s and s d Possible to
have effects in up sector and not down sector
D p m m-
10-4 10-6 10-8 10-10
RPV in the up sector and not the down
sector Burdman et al. hep-ph/0112235
(1/G)(dG/dm2) (GeV-2)
c u m m-
Little Higgs models with new up sector vector
quark Fajfer et al. hep-ph/0511048
0.5 1.0 1.5
m( m m-) (GeV)
factors of gt1000 over SM not ruled out.
Similar in magnitude to Bs mm but orthogonal
parameter space
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c u m m- Analysis Strategy
DØ Preliminary
1 fb-1

• First find Ds f p
• BF(Ds f p) (3.6 /- 0.9)
• BF(f mm-) (2.85 /- 0.19) x 10-4
• Product 1 x 10-5
• Then search for excess in the continuum region
• D p m m- with m(mm) ! f
• Start with low mass dimuon candidate, add track
  to form D candidate
• Reduce background with lifetime and topological
  cuts

55k w
51k f
h(?), r
Dimuon invariant mass
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Results for D f p p m m
DØ Preliminary
Optimized cuts
1 fb-1
n(Ds) 65 11
n(D) 26 9
m(pmm) GeV/c2 for candidates with 0.96 lt m(mm) lt
1.06 GeV/c2
DØ BF( D f p p mm-) (1.75 /- 0.70 /-
0.50) x 10-6
CLEO-c (2.7 3.6 -1.8 /- 0.2) x 10-6
Expected 1.77 x 10-6
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Results for D pmm-
DØ Preliminary
DØ Preliminary
1 fb-1
1 fb-1
n(D) 6
n(Ds) 3
n(bkg) 20.9 /- 3.4
n(SB) 4.6 /- 0.7
n(SB) 5.3 /- 1.5
0.96 lt m(mm) lt 1.06 GeV/c2
0.2 lt m(mm) lt 0.96 GeV/c2 1.06 lt m(mm) lt 1.76
GeV/c2
Good agreement with expectations
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Results for D pmm-

• Branching fraction limit
• D p mm-
• DØ lt 4.7 x 10-6 (at 90)
• CLEO-c(pee-) 7.4x 10-6 at 90
• FOCUS 8.8 x 10-6 at 90
• New limit on second generation RPV couplings

DØ Preliminary
1 fb-1
n 17
n(bkg) 20.9 /- 3.4
0.2 lt m(mm) lt 0.96 GeV/c2 1.06 lt m(mm) lt 1.76
GeV/c2
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Check of SM with Lifetimes and BR

• In SM ( with CP conserved) eigenstates are CP
  specific- check decay portion of matrix
• Measure of decay rates and widths
• in Bs0-Bs0 system
• Gs measures lifetime average
• 1/?Gs measures off diagonal decay matrix element

• Measure
• Flavor specific, e.g.
• Branching ratios
• Lifetimes Differences

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1 Sigma Contours on ?Gs and 1/Gs
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Conclusions

• Tevatrons first fb-1 opening up several new
  opportunities in flavor physics
• Today, 3 worlds best results
• Rare FCNC charm decays
• CP Violation in B mixing
• First double sided bound on Dms
• More improvements this summer-
• extend analysis and new hardware layer 0

19 years after the observation of Bd mixing and
12 years after the first lower limit on Dms, we
now have first direct double sided bound on the
Bs oscillation frequency.
17 lt Dms lt 21 ps-1 _at_ 90 CL
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Backup Slides
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Initial State Tagging

• Get best estimate for reconstructed B meson to
  contain b(b) at origin
• Definitions
• Efficiency
• Dilution
• Tagging power
• use to compare the performance of taggers

42
Dilution

• Determine dilution on event-by-event basis from
  Bd?Dµ?X events
• Use opposite side events
• Plot dpr ( predicted) versus D measured