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Rare B and D Decays at CDF

This research explores rare B and D decays at CDF, focusing on Bd, Bs → mm decays using dedicated dimuon triggers. We aim to set limits on or measure branching ratios, observing events to identify new physics beyond the Standard Model. By training a Neural Net with signal Monte Carlo data and data sidebands, we can further separate signal and background events. The study uses the forward-backward asymmetry in B0→ mm.K* decay to predict new physics scenarios. Results indicate significant progress in setting limits on rare decay rates. By expanding the dataset, more insights into new physics could emerge by the end of 2011.

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Rare B and D Decays at CDF

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  1. Sinéad M. Farrington University of Oxford for the CDF Collaboration HQL 15th October 2010 Rare B and D Decays at CDF

  2. Dedicated rare B and D dimuon triggers • Will show B→mm, B→ mmh, D→mm • Using all muon chambers to |h|1.1 • Tracking capability leads to good mass resolution CDF Central Muon Extension (0.6< |h| < 1.0) Central Muon Chambers (|h| < 0.6) 2

  3. Bd,smm 0 3

  4. In Standard Model FCNC decay B  mm heavily suppressed • Standard Model predicts B mm in the Standard Model 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 4

  5. 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 5

  6. 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 Neural Net to select B signal and suppress dimuon background • Measure remaining background • Measure the acceptance and efficiency ratios 6

  7. B mm Neural Net Variables • Proper decay length (l) • Pointing (Da) |fB – fvtx| • Isolation (Iso) 7

  8. Neural Net • Train NN using signal MC and • data sidebands for background • Rather than making a cut on NN value: • Combine results from several • bins of NN output value • Optimise choice of bins by • maximising expected limit on • Bs decay 8

  9. Backgrounds Combinatorial background obtained by extrapolating sidebands to signal region Check this method in same sign dimuon and negative lifetime control regions B→hh backgrounds (h=p/K) evaluated using MC and fake rates measured in data

  10. 3.7 fb-1 Results BR(Bsmm) < 4.3×10-8 @ 95% CL 3.6 90 BR(Bdmm) < 7.6×10-9 @ 95% CL 6.0 90 • World best limits • Will have ~x2 this • dataset by end of 2011 CDF public note 9892 10

  11. s s s s s s b b • B Rare Decays Bm+m- h : • B+ mm K+ • B0mm K* • Bsmmf • FCNC b  sg* • Penguin or box processes in the Standard Model: • Predicted BR(Bsmmf)=1.61x10-6 Rare B Decays: Bd,sm+m-K+/K*/f PRL103:171801,2009 PRD 79:031102,2009 PRD 79:011104,2009 observed at Babar,Belle,CDF not seen until now m- m+ m- m+ hep-ph/0303246 11

  12. Standard Model • Sensitive to Wilson coefficients: can be affected by new physics • Forward backward asymmetry in B0 mm K* decay • Predictions exist for several new physics scenarios Forward Backward Asymmetry • θ = the angle of the lepton with respect to B direction in the dilepton rest frame • s = q2/mB2 C7=-C7SM C9C10=-C9C10SM AFB=-AFBSM

  13. Forward backward asymmetry in B0 mm K* decay • Predictions exist for several new physics scenarios Forward Backward Asymmetry Babar Belle C7=-C7SM C9C10=-C9C10SM AFB=-AFBSM arXiv:0904.0770 arXiv:0804.4412 13

  14. (J/)   B h • Rare decays: B →mm K+/K*0/f • Simple topology: • Vertex two muons with hadron (h=K+/K*0/f) • Require J/y mass window clean normalisation mode • Reject J/y, y’ mass window rare decays sample • Measure the relative BR: Methodology Relative efficiency from MC 14

  15. Candidate invariant mass distributions Results: mass distributions Number of sigma statistical significance: 9.7 8.5 6.3 15

  16. 4.4 fb-1 • Forward backward asymmetry • Unbinned likelihood fit • Angular acceptances taken from MC Results: Asymmetry CDF public note 10047 Asymmetry as function of q2=mass(mm) 2 Expect zero asymmetry in B+ Compatible and competitive with the B factories 16

  17. D mm 17

  18. Highly suppressed decay • Predicted BR ~ 4x10-13 • Can be enhanced in R parity violating SUSY by up to 7 orders of magnitude D→mm Rare Decay 18

  19. Measure branching ratio relative to control channel • Measure muon fake rates in tagged D* samples • Main background is from B decays with cascade D decays D→mm Rare Decay 19

  20. 0.36 fb-1 • Measure branching ratio relative to control channel D→mm Results CDF public note 9226 20

  21. Bd,sm+m-, D m+m- are a powerful probe of new physics • Could give first hint of new physics at the Tevatron • Impacting new physics scenarios’ phase space • B→mm h first observation in Bs mode, first measurement of asymmetry in these decays at hadron collider • Data sample will be x2 by end of 2011, many more results to come! Summary SO(10) Constrained MSSM hep-ph/0507233 Phys. Lett B624, 47, 2005 21

  22. Back-up 22

  23. Gathered ~7.6 fb-1 to date CDF 23

  24. 1)First observation in Bs channel 2) Tests of Standard Model • branching ratios • kinematic distributions (with enough statistics) • Effective field theory for b  s (Operator Product Expansion) • Sensitive to Wilson coefficients, Ci • calculable for many models (e.g. SUSY, technicolor) • Decay amplitude: C9 • Dilepton mass distribution: C7, C9 • Forward-backward asymmetry: C10 Motivations for B→mmh Search 24

  25. 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 Searching for New Physics l+ Z* c02 q l- BR(Bmm)<1x10-7 q c01 q c+ l+ W+ n Trileptons 25

  26. 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/p) (use measured fake rates) 26

  27. now Limits on BR(Bd,smm) • BR(Bsmm) < 1.0×10-7 @ 90% CL • < 8.0×10-8 @ 95% CL • BR(Bdmm) < 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 27

  28. m b RPV SUSY ~ n l’i23 l i22 m s • SUSY could enhance BR by orders of magnitude • MSSM: BR(B  mm)  tan6b • may be 100x Standard Model B mm in New Physics Models • 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 28

  29. 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 29

  30. Likelihood p.d.f.s Input p.d.f.s: Isolation Pointing angle Ct significance New plots*** 30

  31. 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 CDF Central Muon Extension (0.6< |h| < 1.0) Central Muon Chambers (|h| < 0.6) 31

  32. B Rare Decays • B+ mm K+ • B0mm K* • Bsmmf • Lbmm L • FCNC b  sg* • Penguin or box processes in the Standard Model: • Rare processes: Latest Belle measurement Bd,smm K+/K*/f hep-ex/0109026, hep-ex/0308042, hep-ex/0503044 observed at Babar, Belle m m m m x10-7 32

  33. 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 Motivations 33

  34. 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)>5 GeV • Lxy>100mm • mass(m m)<6 GeV • mass(m m)>2.7 GeV Samples (CDF) 34

  35. Signal and Side-band Regions • Use events from same triggers for • B+ and Bs(d) mm reconstruction. • Search region: • - 5.169 < Mmm < 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.

  36. 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 • Bsm+m- • (signal efficiencies) • B+JK+m+m-K+ (nrmlztn efncy and xchks) • B+Jp+m+m-p+ (nrmlztn correction)

  37. 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 h2>0.13 mh<111GeV m+<104GeV 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

  38. Method: Likelihood Variable Choice Prob(l) = probability of Bsmm yields l>lobs (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)

  39. 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 LH>0.9 to • 40% for LH>0.99 • 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)

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