Download presentation
Presentation is loading. Please wait.
Published byEmmeline Bridges Modified over 9 years ago
1
Sinéad M. Farrington University of Liverpool for the CDF Collaboration ICHEP 28 th July 2006 Rare B Decays at CDF
2
2 Outline Will show two analyses from CDF: B d,s very rare decay (10 -9 in Standard Model) strong probe of new physics scenarios Briefly also show: B s D s D s not so rare (10 -3 ) interesting CP properties 0 0
3
3 B d,s 0
4
4 B in the Standard Model In Standard Model FCNC decay B heavily suppressed Standard Model predicts A. Buras Phys. Lett. B 566,115 B d further suppressed by CKM coupling (V td /V ts ) 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
5
5 The Challenge Large combinatorial background Key elements are select discriminating variables determine efficiencies estimate background search region pp collider: trigger on dimuons )~24MeV Mass resolution: separate B d, B s
6
Vertex muon pairs in B d /B s mass windows Unbiased optimisation, signal region blind Aim to measure BR or set limit: Reconstruct normalisation mode (B + J/ K + ) Construct likelihood discriminant to select B signal and suppress dimuon background Measure remaining background Measure the acceptance and efficiency ratios 6 Methodology
7
Proper decay length ( ) Pointing ( ) | B – vtx | Isolation (Iso) 7 B Selection Discriminating variables:
8
8 Likelihood Ratio Discriminant Likelihood discriminant more powerful than cuts alone i: index over all discriminating variables P sig/bkg (x i ): probability for event to be signal or background for a given measured x i Obtain probably density functions of variables using background: Data sidebands signal: Pythia Monte Carlo sample
9
9 Optimisation Likelihood ratio discriminant: Optimise likelihood for best 90% C.L. limit Bayesian approach include statistical and systematic errors Optimal cut: Likelihood ratio >0.99
10
10 Unblinded Results 1 event found in B s search window (expected background: 0.88 0.30) 2 events found in B d search window (expected background: 1.86 0.34) (In central-central Muon sample only) BR(B s ) < 1.0×10 -7 @ 95% CL < 8.0×10 -8 @ 90% CL BR(B d ) < 3.0×10 -8 @ 95% CL < 2.3×10 -8 @ 90% CL central-central central-extension (These are currently world best limits)
11
11 Bs DsDsBs DsDs 0
12
12 B s D s D s Results of B s mixing analysis at CDF have yielded measurement of m s (see talk at this conference by Stefano Giagu) gives complementary insight into CKM matrix 1/ (CP even) <1/ (CP odd) since the fast transition b ccs is mostly CP even Biggest contribution to lifetime difference comes from B s D s D s (pure CP even) Can constrain by measuring branching fractions
13
13 Relative BR Measurement BR (B s D s D s ) measured relative to B 0 D s D - Control mode: B s D s D s : 3 D s modes Reconstructed Total yield:23 3 D s modes Reconstructed Total yield:395
14
14 Summary Made BR measurement of CP mode B s D s D s B d,s 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 SO(10) Constrained MSSM hep-ph/0507233 Phys. Lett B624, 47, 2005
15
15 Back-up
16
16 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 BR(B 1x10 -7 Trileptons l+l+ l-l- l+l+ q q Z* W+W+ q
17
17 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 (h=K/ ) (use measured fake rates)
18
now 18 Limits on BR(B d,s ) BR(B s ) < 1.0×10 -7 @ 90% CL < 8.0×10 -8 @ 95% CL BR(B d ) < 3.0×10 -8 @ 90% CL < 2.3×10 -8 @ 95% CL These are currently world best limits The future: Need to reoptimise after 1fb-1 for best results Assume linear background scaling
19
19 B in New Physics Models SUSY could enhance BR by orders of magnitude MSSM: BR(B ) tan 6 may be 100x Standard Model R-parity violating SUSY: tree level diagram via sneutrino observe decay for low tan mSUGRA: B search complements direct SUSY searches Low tan observation of trilepton events High tan observation of B Or something else! ’ i23 i22 b s RPV SUSY A. Dedes et al, hep-ph/0207026
20
20 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
21
21 Likelihood p.d.f.s Input p.d.f.s: Isolation Pointing angle Ct significance New plots***
22
six dedicated rare B triggers using all muon chambers to | | 1.1 Tracking capability leads to good mass resolution Use two types of muon pairs: central-central central-extension 22 CDF Central Muon Extension (0.6< | | < 1.0) Central Muon Chambers (| | < 0.6)
23
23 B d,s K + /K*/ B Rare Decays B + K + B 0 B s b FCNC b s * Penguin or box processes in the Standard Model: Rare processes: Latest Belle measurement observed at Babar, Belle hep-ex/0109026, hep-ex/0308042, hep-ex/0503044 x10 -7
24
24 Motivations 1) Would be first observations in B s and b 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: C 9 Dilepton mass distribution: C 7, C 9 Forward-backward asymmetry: C 10
25
25 Samples (CDF) Dedicated rare B triggers in total six Level 3 paths Two muons + other cuts using all chambers to | | 1.1 Use two types of dimuons: CMU-CMU CMU-CMX Additional cuts in some triggers: p t ( )>5 GeV L xy >100 m mass( )<6 GeV mass( )>2.7 GeV
26
Signal and Side-band Regions Use events from same triggers for B + and Bs(d) reconstruction. Search region: - 5.169 < M < 5.469 GeV - Signal region not used in optimization procedure Sideband regions: - 500MeV on either side of search region - For background estimate and analysis optimization. Search region Monte Carlo )~24MeV
27
MC Samples Pythia MC Tune A default cdfSim tcl realistic silicon and beamline pT(B) from Mary Bishai pT(b)>3 GeV && |y(b)|<1.5 – B s (signal efficiencies) – B+ JK+ K+ (nrmlztn efncy and xchks) – B+ J + (nrmlztn correction)
28
R. Dermisek et al., hep-ph/0304101 SO(10) Unification Model tan( )~50 constrained by unification of Yukawa coupling All previously allowed regions (white) are excluded by this new measurement Unification valid for small M 1/2 (~500GeV) New Br(Bs limit strongly disfavors this solution for m A = 500 GeV Red regions are excluded by either theory or experiments Green region is the WMAP preferred region Blue dashed line is the Br(Bs ) contour Light blue region excluded by old Bs analysis h 2 >0.13 m + <104GeV m h <111GeV Excluded by this new result
29
Method: Likelihood Variable Choice Prob( ) = probability of B s yields > obs (ie. the integral of the cumulative distribution) Prob( ) = exp(- /438 m) yields flat distribution reduces sensitivity to MC modeling inaccuracies (e.g. L00, SVX-z)
30
Step 4: Compute Acceptance and Efficiencies Most efficiencies are determined directly from data using inclusive J/ events. The rest are taken from Pythia MC. (B+/Bs) = 0.297 +/- 0.008 (CMU-CMU) = 0.191 +/- 0.006 (CMU-CMX) LH (Bs): ranges from 70% for LH>0.9 to 40% for LH>0.99 trig (B+/Bs) = 0.9997 +/- 0.0016 (CMU-CMU) = 0.9986 +/- 0.0014 (CMU-CMX) reco- (B+/Bs) = 1.00 +/- 0.03 (CMU-CMU/X) vtx (B+/Bs) = 0.986 +/- 0.013 (CMU-CMU/X) reco-K (B+) = 0.938 +/- 0.016 (CMU-CMU/X) Red = From MC Green = From Data Blue = combination of MC and Data
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.