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Search for FCNC Decays Bsd

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Title: Search for FCNC Decays Bsd


1
Search for FCNC Decays Bs(d) ? µµ-
  • Motivation
  • Tevatron and CDF
  • Analysis Method
  • Results
  • Conclusion

BEACH 04
J. Piedra
1
2
The Standard Model
  • Successes of the Standard Model
  • Simple comprehensive theory
  • Explains the hundreds of common particles atoms,
    protons, neutrons, electrons
  • Explains the interactions between them
  • Basic building blocs
  • 6 quarks up, down...
  • 6 leptons electrons...
  • Force carrier particles photon...
  • All common matter particles are composites of the
    quarks leptons which interact by the exchange
    of force carrier particles

2
M. Herndon
CMU Seminar
3
Beyond the Standard Model
  • Why look for physics beyond the Standard Model
  • Gravity not a part of the SM
  • What is the very high energy behaviour?
  • At the beginning of the universe?
  • Grand unification of forces?
  • Where is the Antimatter?
  • Why is the observed universe mostly matter?
  • Dark Matter?
  • Astronomical observations of gravitational
    effects indicate that there is more matter than
    we see

3
M. Herndon
CMU Seminar
4
Searches For New Physics
  • How do you search for new physics at a collider?
  • Direct searches for production of new particles
  • Particle-antipartical annihilation
  • Example the top quark
  • Indirect searches for evidence of new particles
  • Within a complex decay new particles can occur
    virtually
  • Tevatron is at the energy frontier and
  • a data volume frontier 1 billion B and Charm
    events on tape
  • So much data that we can look for some very
    unusual decays
  • Where to look
  • Many weak decays of B and charm hadrons are very
    low probability
  • Look for contributions from other low probability
    processes Non Standard Model

4
M. Herndon
CMU Seminar
5
Bs ? µµ Beyond the SM
  • Look at decays that are suppressed in the
  • Standard Model Bs(d) ? µµ-
  • Flavor changing neutral currents(FCNC) to leptons
  • No tree level decay in SM
  • Loop level transitions suppressed
  • CKM , GIM and helicity(ml/mb) suppressed
  • SM BF(Bs(d) ? µµ-) 3.5x10-9(1.0x10-10)
  • G. Buchalla, A. Buras, Nucl. Phys. B398,285
  • New physics possibilities
  • Loop MSSM mSugra, Higgs Doublet
  • 3 orders of magnitude enhancement
  • Rate ?tan6ß/(MA)4
  • Babu and Kolda, Phys. Rev. Lett. 84, 228
  • Tree R-Parity violating SUSY

5
M. Herndon
CMU Seminar
6
CDF and the Tevatron
-
  • 1.96TeV pp collider
  • Performance substantially improving each year
  • Record peak luminosity in 2006 1.8x1032sec-1cm-2
  • CDF Integrated Luminosity
  • 1fb-1 with good run requirements
  • All critical systems operating including silicon
  • Analysis presented here uses 780pb-1
  • Doubled data in 2005, predicted to double again
    in 2006

6
CMU Seminar
7
CDF Detector
  • Silicon Tracker
  • ?lt2, 90cm long, rL00 1.3 - 1.6cm
  • Drift Chamber(COT)
  • 96 layers between 44 and 132cm
  • Muon coverage
  • ?lt1.5
  • Triggered to ?lt1.0
  • Outer chambers high purity muons

7
CMU Seminar
8
Rare B Decay Physics Triggers
  • Large production rates
  • s(pp ? bX, y lt 1.0, pT(B) gt 6.0GeV/c) 30µb,
    10µb
  • All heavy b states produced
  • B0, B, Bs, Bc, ?b, ?b
  • Backgrounds gt 3 orders of magnitude higher
  • Inelastic cross section 100 mb
  • Challenge is to pick one B decay from gt103 other
    QCD events
  • Di-muon trigger
  • pT(µ) gt 1.5 GeV/c, ? lt 1.0
  • B yields 2x Run I (lowered pT threshold,
    increased acceptance)
  • Di-muon triggers for rare decay physics
  • Bs(d) ? µµ-, B ? µµ-K, B0 ? µµ-K0, Bs ?
    µµ-f, ?b? µµ-?
  • Trigger on di-muon masses from near 0GeV/c2 to
    the above Bs mass
  • Reduce rate by requiring outer muon chamber hits
    pT(µ) gt 3.0 GeV/c or
    ?pTµ gt 5.0GeV/c

-
8
M. Herndon
9
Bs ? µµ Experimental Challenge
  • Primary problem is large background at hadron
    colliders
  • Analysis and trigger cuts must effectively reduce
    the large background around mBs
    5.37GeV/c2 to find a possible handful of events
  • Key elements of the analysis are determining the
    efficiency and rejection of the discriminating
    variables and estimating the background level

9
M. Herndon
CMU Seminar
10
Analysis Method
  • Performing this measurement requires that we
  • Demonstrate an understanding of the background
  • Optimize the cuts to reduce background
  • Accurately estimate a and e for triggers,
    reconstruction and selection cuts
  • SM predicts 0 events, this is essentially a
    search
  • Emphasis on accurately predicting the Nbg and
    performing an unbiased optimization
  • Blind ourselves to the signal region
  • Estimate background from sidebands and test
    background predictions in orthogonal samples

10
M. Herndon
CMU Seminar
11
Data Sample
  • Start the with di-muon trigger
  • 2 CMU muons or 1 CMU and 1CMX muon with ?pTµ gt
    5.0GeV/c
  • CMU pT(µ) gt 1.5 GeV/c, ? lt 0.6, CMX pT(µ) gt
    2.0 GeV/c, 0.6 lt ? lt 1.0
  • 1 CMUP muon pT(µ) gt 3.0 GeV/c and 1 CMU(X) muon
  • Apply basic quality cuts
  • Track, vertex and muon quality cuts
  • Loose preselection on analysis cuts
  • PT(µµ-) gt 4.0 GeV/c, 3D Decay length
    significance gt 2
  • In the mass region around the Bs 4.669 lt Mµµ lt
    5.969 GeV/c2
  • Blind region 4s(Mµµ), 5.169 lt Mµµ lt 5.469 GeV/c2
  • Sideband region 0.5 GeV/c2 on either side of the
    blinded region
  • Sample still background dominated
  • Expect lt 10 Bs(d) ? µµ- events to pass these
    cuts based on previous limits

11
M. Herndon
CMU Seminar
12
Normalization Data Sample
  • Normalize to the number of B ?J/?K
    events
  • Relative normalization analysis
  • Some systematic uncertainties will cancel out in
    the ratios of the normalization
  • Example muon trigger efficiency the same for J/?
    or Bs muons for a given pT
  • Apply same sample selection criteria as for Bs(d)
    ? µµ-

12
M. Herndon
13
Signal vs. Background
  • Need to discriminate signal from background
  • Reduce background by a factor of gt 1000
  • Signal characteristics
  • Final state fully reconstructed
  • Bs is long lived (ct 438 µm)
  • B fragmentation is hard few additional tracks
  • Background contributions and characteristics
  • Sequential semi-leptonic decay b ? cµ-X ? µµ-X
  • Double semileptonic decay bb ? µµ-X
  • Continuum µµ-
  • µ fake, fakefake
  • Partially reconstructed, short lived,
    has additional tracks

-
13
M. Herndon
CMU Seminar
14
Discriminating Variables
  • Mass Mmm
  • choose 2.5s window s 25MeV/c2
  • exp(?ct/ctBs)
  • ?a fB fvtx in 3D
  • Isolation pTB/( ?trk pTB)
  • exp(?), ?a and Iso used in
    likelihood ratio
  • Optimization
  • Unbiased optimization
  • Based on simulated signal and data sidebands
  • Optimize based on likely physics result
  • a priori expected BF limit
  • 4 primary discriminating variables

14
M. Herndon
CMU Seminar
15
CDF 3D Silicon Vertex Detector
  • 8 layer silicon detector
  • 4 double sided layers with stereo strips at a
    small angle
  • 3 double sided layers layers with strips at 90
  • 3D vertexing is a powerful discriminant
  • 2D ?? from previous analysis

15
M. Herndon
CMU Seminar
16
Likelihood Ratio
  • Probability distributions used to construction
    the likelihood ratio
  • Use binned histograms to estimate the
    probabilities
  • Resulting likelihood with signal MC and data
    sidebands

16
M. Herndon
CMU Seminar
17
Background Estimate
  • Estimate the background for any given set of
    requirements
  • Typical method apply all cuts, see how many
    sideband events pass
  • Optimal cuts may be chosen to reject 1-2 unusual
    events
  • Flat background distribution allows use of wide
    mass window
  • Need to show that mass is
    distribution
    is linear
  • Need to investigate backgrounds
    that
    may peak in signal region B ? hh
  • Design crosschecks to double check background
    estimates

17
M. Herndon
CMU Seminar
18
Background Estimate
  • Expected number of combinatorial background in a
    60 MeV/c2 signal window
  • Extrapolation from sideband
  • 0.99 expected to be near optimal cut, 1
    background event typical of optimal search

18
M. Herndon
CMU Seminar
19
Background Cross-Checks
  • Use independent data samples to test background
    estimates
  • OS- opposite sign muons, negative lifetime
    (signal sample is OS)
  • SS and SS- same sign muons,
    positive and negative lifetime
  • FM OS- and OS fake µ enhanced,
    one µ fails the muon
    reconstruction quality cuts
  • Compare predicted vs. observed of bg. events
    3 sets of cuts
  • Loose LR gt 0.5
  • Medium LR gt 0.9
  • Tight LR gt 0.99

19
M. Herndon
CMU Seminar
20
Bg. Cross-Checks Cont.
OS vs. OS-
OS vs. SS
  • OS- Excellent control sample
  • SS and fake muon could represent some bb
    backgrounds

-
20
M. Herndon
CMU Seminar
21
Bg. Cross-Checks Cont.
  • OS- and Fake µ samples
  • SS,SS- expect and find 0 events for tighter cuts
  • Using 150 MeV/c2 mass window for cross-check
  • Combined CMU-CMU(X)
  • Think about results with tight cuts and FM
  • Investigated B ? hh carefully
  • Generated inclusive MC to look for anomalous
    background sources - no surprises found

21
M. Herndon
CMU Seminar
22
B ? hh Background
  • Clearly peaks in signal region
  • Sideband estimates not useful
  • Observed/measured at CDF
  • Convolute known branching ratios and acceptance
    with K and ? fake rates. (estimated for LH gt 0.99)

22
M. Herndon
CMU Seminar
23
Acceptance Efficiencies
  • ? efficiency - from data where possible
  • Using J/? ? µµ- and B ? J/?K data and special
    unbiased J/? triggers
  • Most efficiencies relative Exception - and ?LH
    and ?K

23
M. Herndon
CMU Seminar
24
Example SVX Efficiency
  • Measurement of the efficiency of adding silicon
    hits to COT tracks
  • Use J/? ? µµ- data

(2001-2002 data)
(2001-2002 data)
  • eSVX 74.5 0.3(stat) 2.2(sys)(2001-2003
    data)
  • Average efficiency for adding silicon to two
    tracks In silicon fid.

24
M. Herndon
25
New SVX Efficiency
  • Improved silicon pattern recognition code and
    detector performance

2 and flat - improved code 5 eSVX improved
code and new quality cuts
eSVX 74.5 ? 88.5 2001-2003 ? 2001-2005 data
25
M. Herndon
CMU Seminar
26
LR Efficiency Cross-Check
  • Efficiencies determined using realistic MC
  • MC efficiencies validated by comparing with data
    using B ? J/?K events
  • Pt and isolation distributions

    reweighed to match data

    Same treatment for Bs
  • 4 systematic uncertainty assigned
  • 5 from isolation reweighing

26
M. Herndon
CMU Seminar
27
Final Efficiencies Acceptances
  • LR efficiencies determined using realistic MC
  • Pt and isolation distributions reweighed to match
    data using Bs ? J/?? events
  • For LR gt 0.99
  • 39 efficiency, 7 sys
  • Factor 2000 of rejection
  • Acceptance ratio
  • a(B/Bs) 0.297 0.006 (CMUCMU), 0.191 0.008
    (CMUCMX) 7 sys - generation model
  • Most efficiency ratios near 1
  • Two absolute efficiencies
  • ?KCOT 95 and ?KSXV 96

27
M. Herndon
CMU Seminar
28
Optimization Results
  • Tried Likelihood ratios from 0.9-0.99 LR Cut 0.99
  • Systematic uncertainties
  • Bg. estimation 28
  • Bg statistical error
  • Sensitivity 16
  • 12 fs/fu
  • 7 acceptance
  • 7 LR
  • NB 5763 101
  • eLR 39
  • Backgrounds

28
M. Herndon
CMU Seminar
29
Bs(d) ? µµ- Search Result
  • Results 1(2) Bs(d) candidates observed
    consistent with background expectation
  • CDF 1 Bs result 3.0?10-6
  • CDF Bs result 365pb-1 2.0?10-7
  • D0 Bs result 240pb-1 4.1?10-7
  • BaBar Bd result 8.3?10-8(90)

PRL 95, 221805 2005
DO Note 4733-Conf
PRL 94, 221803 2005
29
M. Herndon
CMU Seminar
30
Bs ? µµ Physics Reach
  • Excluded at 95 CL
  • BF(Bs ? ??- ) 1.0x10-7
  • Dark matter constraints

R. Dermisek et al.hep-ph/0507233
R. Dermisek JHEP 0304 (2003) 037
  • Strongly limits specific SUSY models SUSY SO(10)
    models
  • Allows for massive neutrino
  • Incorporates dark matter results

30
M. Herndon
CMU Seminar
31
Bs ? µµ Physics Reach
  • In addition to limiting S0(10) models starting
    to impact standard MSSM scenarios CMSSM
  • Blue BF(Bs ? µµ-)
  • Light green b ? s?
  • Cyan dark matter constraints
  • Dashed red Higgs mass bound
  • Red brown areas excluded
  • Incorporates errors from fBs and mtop

J. Ellis Phys. Lett. B624, 47 2005
BF(Bs ? ??- ) 2.0(1.0)x10-7
31
M. Herndon
CMU Seminar
32
Conclusions
CDF B(s,d) ? µµ- results
  • Best Bs and Bd results well ahead of D0 and the
    B factories
  • Limit excludes part of parameter space allowed by
    SO(10) models
  • Expanding sensitivity to interesting areas of
    MSSM parameter space
  • Improving analysis to include selection NN and
    advanced lepton ID(from CMU Vivek)
  • Improved fs results could also improve result by
    10-20(from CMU Karen)

M. Herndon
32
CMU Seminar
33
Muon Trigger Efficiencies
  • L1 eff Use J/? ? µµ- trigger that only requires
    one muon
  • L3 eff Use autoexcept trigger
  • L1 Muon L1(pT,?), L3 1 number
  • Convolute eL1 for each muon and eL3
  • Systematic errors L1 and L3
  • Kinematic difference between J/? ? µµ-
    and Bs(d) ? µµ-
  • 2-Track correlations
  • Sample statistics
  • etrig 85 3
  • Offline muon reconstruction
  • From J/? ? µµ- L1 trigger with one muon found
  • Systematic from comparison to Z
  • emuon 95.9 1.3(stat) 0.6(sys)

33
M. Herndon
34
COT Efficiency
  • Estimated by embedding COT hits from MC muons in
    real data
  • Occupancy effects correctly accounted for
  • Efficiency driven by loss of hits when hit
    density is high
  • Demonstrate agreement between hit usage in data
    and MC
  • Tunable parameters can lower or raise the hit
    usage and the efficiency to bracket the data
  • eCOT 99.62 0.02
  • Systematic errors
  • Isolation dependence dominant systematic
  • Pt dependence
  • 2-track correlations
  • Varying the simulation tuning
  • Error 0.34 -0.91
  • Consistent with true tracking eff. using high pt
    elections identified in the calorimeter

34
M. Herndon
35
Bs ? µµ Physics Reach
  • In addition to limiting S0(10) models starting
    to impact standard MSSM scenarios mSugra
  • Blue BF(Bs ? µµ-)
  • Light green b ? s?
  • Cyan dark matter constraints
  • Dashed red Higgs mass bound
  • Red brown areas excluded
  • Incorporates errors from fBs and mt

J. Ellis Phys. Lett. B624, 47 2005
35
M. Herndon
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