Sinad M' Farrington

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Rare B Decays at CDF

• Sinéad M. Farrington
• University of Liverpool
• for the CDF Collaboration
• ICHEP
• 28th July 2006

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Outline

• Will show two analyses from CDF
• Bd,s ? mm-
• very rare decay (10-9 in Standard Model)
• strong probe of new physics scenarios
• Briefly also show
• Bs ? Ds Ds
• not so rare (10-3)
• interesting CP properties

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Bd,s ? mm
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B ? mm in the Standard Model

• In Standard Model FCNC decay B ? mm heavily
  suppressed
• Standard Model predicts

A. Buras Phys. Lett. B 566,115

• Bd ? mm further suppressed by CKM coupling
  (Vtd/Vts)2
• Both are below sensitivity of Tevatron
  experiments
• SUSY scenarios (MSSM,RPV,mSUGRA) boost the BR by
  up to 100x

Observe no events ? set limits on new
physics Observe events ? clear evidence for
new physics
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The Challenge
pp collider trigger on dimuons
search region
Mass resolution separate Bd, Bs
s(Mmm)24MeV

• Large combinatorial background
• Key elements are
• select discriminating variables
• determine efficiencies
• estimate background

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Methodology

• Vertex muon pairs in Bd/Bs mass windows
• Unbiased optimisation, signal region blind
• Aim to measure BR or set limit
• Reconstruct normalisation
• mode (B?J/y K)
• Construct likelihood discriminant to select B
  signal and suppress dimuon background
• Measure remaining background
• Measure the acceptance and efficiency ratios

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B ? mm Selection
Discriminating variables

• Proper decay length (l)
• Pointing (Da) f B f vtx
• Isolation (Iso)

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Likelihood Ratio Discriminant

• Likelihood discriminant more powerful than cuts
  alone
• i index over all discriminating variables
• Psig/bkg(xi) probability for event to be signal
  or background for a given measured xi
• Obtain probably density functions of variables
  using
• background Data sidebands
• signal Pythia Monte Carlo sample

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Optimisation

• Likelihood ratio discriminant

• Optimise likelihood
• for best 90 C.L. limit
• Bayesian approach
• include statistical and systematic
• errors
• Optimal cut Likelihood ratio gt0.99

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Unblinded Results
central-central central-extension
1 event found in Bs search window (expected
background 0.88?0.30)
2 events found in Bd search window (expected
background 1.86?0.34)
(In central-central Muon sample only)
BR(Bs?mm) lt 1.010-7 _at_ 95 CL
lt 8.010-8 _at_ 90 CL BR(Bd?mm) lt 3.010-8 _at_
95 CL lt 2.310-8 _at_ 90 CL
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(These are currently world best limits)
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Bs ? DsDs
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Bs ? Ds Ds

• Results of Bs mixing analysis at CDF have yielded
  measurement of Dms (see talk at this conference
  by Stefano Giagu)
• DG/G gives complementary insight into CKM matrix
• 1/G(CP even) lt1/G(CP odd) since the fast
  transition b ?ccs is mostly CP even
• Biggest contribution to lifetime difference comes
  from
• Bs ? Ds Ds (pure CP even)
• Can constrain DG/G by measuring branching
  fractions

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Relative BR Measurement

• BR (Bs ? Ds Ds ) measured relative to B0 ?Ds D-

3 Ds modes Reconstructed Total yield395
Control mode
3 Ds modes Reconstructed Total yield23
Bs ? Ds Ds
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Summary

• Made BR measurement of CP mode Bs? Ds Ds
• Bd,s ?mm- are a powerful probe of new physics
• Could give first hint of new physics at the
  Tevatron
• These are world best limits
• Impacting new physics scenarios phase space

Constrained MSSM
SO(10)
hep-ph/0507233
Phys. Lett B624, 47, 2005
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Back-up
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Searching for New Physics

• Two ways to search for new physics
• direct searches seek e.g. Supersymmetric
  particles
• indirect searches test for deviations from
  Standard Model predictions e.g. branching ratios
• In the absence of evidence for new physics
• set limits on model parameters

l
Z
c02
q
l-
BR(B? mm)lt1x10-7
q
c01
q
c
l
W
n
Trileptons
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Expected Background

• Extrapolate from data sidebands to obtain
  expected events
• Scale by the expected rejection from the
  likelihood ratio cut
• Also include contributions from charmless B
  decays
• B?hh (hK/p) (use measured fake rates)

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Limits on BR(Bd,s ? mm)

• BR(Bs?mm) lt 1.010-7 _at_ 90 CL
• lt 8.010-8 _at_ 95 CL
• BR(Bd?mm) lt 3.010-8 _at_ 90 CL
• lt 2.310-8 _at_ 95 CL
• These are currently world best limits
• The future
• Need to reoptimise after 1fb-1 for
• best results
• Assume linear background
• scaling

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B ? mm in New Physics Models

• SUSY could enhance BR by orders of magnitude
• MSSM BR(B ? mm) ? tan6b
• may be 100x Standard Model

• R-parity violating SUSY tree level diagram via
  sneutrino
• observe decay for low tan b
• mSUGRA B ? mm search complements direct SUSY
  searches
• Low tan b ? observation of trilepton events
• High tan b ? observation of B ? mm
• Or something else!

A. Dedes et al, hep-ph/0207026
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Expected Background

• Tested background prediction in several control
  regions and find good agreement

OS- opposite sign muon, negative lifetime SS
same sign muon, positive lifetime SS- same
sign muon, negative lifetime FM fake muon,
positive lifetime
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Likelihood p.d.f.s

• Input p.d.f.s
• Isolation
• Pointing angle
• Ct significance

New plots
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CDF

• six dedicated rare B triggers
• using all muon chambers to h?1.1
• Tracking capability leads to good mass resolution
• Use two types of muon pairs
• central-central
• central-extension

Central Muon Extension (0.6lt h lt 1.0)
Central Muon Chambers (h lt 0.6)
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Bd,s ? mm K/K/f

• B Rare Decays
• B ? mm K
• B0 ? mm K
• Bs ? mm f
• Lb ? mm L
• FCNC b ? sg
• Penguin or box processes in the Standard Model
• Rare processes Latest Belle measurement

hep-ex/0109026, hep-ex/0308042, hep-ex/0503044
observed at Babar, Belle
m
m
m
m
x10-7
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Motivations

• 1) Would be first observations in Bs and Lb
  channels
• 2) Tests of Standard Model
• branching ratios
• kinematic distributions (with enough statistics)
• Effective field theory for b ? s (Operator
  Product Expansion)
• Rare decay channels are sensitive to Wilson
  coefficients which are calculable for many models
  (several new physics scenarios e.g. SUSY,
  technicolor)
• Decay amplitude C9
• Dilepton mass distribution C7, C9
• Forward-backward asymmetry C10

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Samples (CDF)

• Dedicated rare B triggers
• in total six Level 3 paths
• Two muons other cuts
• using all chambers to h?1.1
• Use two types of dimuons
• CMU-CMU
• CMU-CMX
• Additional cuts in some
• triggers
• Spt(m)gt5 GeV
• Lxygt100mm
• mass(m m)lt6 GeV
• mass(m m)gt2.7 GeV

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Signal and Side-band Regions

• Use events from same triggers for
• B and Bs(d) ?mm reconstruction.
• Search region
• - 5.169 lt Mmm lt 5.469 GeV
• - Signal region not used in
• optimization procedure

s(Mmm)24MeV
Monte Carlo
Search region

• Sideband regions
• - 500MeV on either side of search region
• - For background estimate and analysis
• optimization.

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MC Samples

• Pythia MC
• Tune A
• default cdfSim tcl
• realistic silicon and beamline
• pT(B) from Mary Bishai
• pT(b)gt3 GeV y(b)lt1.5
• Bs?mm-
• (signal efficiencies)
• B?JK?mm-K
• (nrmlztn efncy and xchks)
• B?Jp?mm-p
• (nrmlztn correction)

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SO(10) Unification Model
R. Dermisek et al., hep-ph/0304101

• tan(b)50 constrained by unification
• of Yukawa coupling
• All previously allowed regions (white)
• are excluded by this new measurement
• Unification valid for small M1/2
• (500GeV)
• New Br(Bs?mm) limit strongly
• disfavors this solution for
• mA 500 GeV

??h2gt0.13
mhlt111GeV
m?lt104GeV
Excluded by this new result
Red regions are excluded by either theory or
experiments Green region is the WMAP preferred
region Blue dashed line is the Br(Bs?mm)
contour Light blue region excluded by old Bs?mm
analysis
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Method Likelihood Variable Choice
Prob(l) probability of Bs?mm yields lgtlobs (ie.
the integral of the cumulative
distribution) Prob(l) exp(-l/438 mm)

• yields flat distribution
• reduces sensitivity to
• MC modeling inaccuracies
• (e.g. L00, SVX-z)

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Step 4 Compute Acceptance and Efficiencies

• Most efficiencies are determined directly from
  data using inclusive
• J/y?mm events. The rest are taken from Pythia
  MC.
• a(B/Bs) 0.297 /- 0.008 (CMU-CMU)
• 0.191 /- 0.006 (CMU-CMX)
• eLH(Bs) ranges from 70 for LHgt0.9 to
• 40 for
  LHgt0.99
• etrig(B/Bs) 0.9997 /- 0.0016 (CMU-CMU)
• 0.9986 /- 0.0014
  (CMU-CMX)

Red From MC Green From Data Blue
combination of MC and Data

• ereco-mm(B/Bs) 1.00 /- 0.03 (CMU-CMU/X)
• evtx(B/Bs) 0.986 /- 0.013 (CMU-CMU/X)
• ereco-K(B) 0.938 /- 0.016 (CMU-CMU/X)