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Searching for Bs→µ+µ- and Bd→µ+µ- Decays at CDF

Investigating Bs→µ+µ- and Bd→µ+µ- decays at CDF for clues of new physics beyond the Standard Model. Analyzing rare B di-muon triggers to enhance Bs and Bd decay signals, suppressing backgrounds with discriminant variables and likelihood ratio discriminant method.

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Searching for Bs→µ+µ- and Bd→µ+µ- Decays at CDF

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  1. Search for Bsm+m- and Bdm+m- Decays at CDF Cheng-Ju S. Lin (Fermilab) The 13th International Conference on Supersymmetry and Unification of Fundamental Interactions IPPP Durham 18 July 2005

  2. Introduction • In the Standard Model, the FCNC decay of B m+m- is heavily • suppressed SM prediction  (Buchalla & Buras, Misiak & Urban) • Bdmm is further suppressed by CKM coupling (vtd/vts)2 • SM prediction is below the sensitivity of current experiments • (CDF+D0): SM  Expect to see 0 events at the Tevatron Any signal would indicate new physics!! Cheng-Ju Lin (Fermilab) SUSY05

  3. BEYOND STANDARD MODEL m b R-parity violating SUSY ~ n l’i23 l i22 m s • In many SUSY models, the BR could be enhanced by many • orders of magnitude: • For examples: • - MSSM: Br(Bmm) is proportional • to tan6b. BR could be as large as • ~100 times the SM prediction • - Tree level diagram is allowed in • R-parity violating (RPV) SUSY • models. Possible to observe decay • even for low value of tanb. • In context of mSUGRA, Br(Bmm) search complements • direct SUSY searches: (A. Dedes et al, hep-ph/0207026) • Low tan(b)  observation of trilepton events • Large tan(b)  observation of Br(Bmm)

  4. PROBE OF NEW PHYSICS Monte Carlo • New physics may enhance Bs and • Bdmm differently • Minimal-flavor-violation (MFV) • assumption in SUSY yields SM • relations between Bs and Bdmm • decays • Can observe both Bs and Bd: unique to • Tevatron • CDF has the mass resolution to • distinguish two decays, s(Mmm)~23MeV : • unique to CDF • Either observation or null search, results will provide important • clues about possible scenarios of new physics beyond SM M(Bs)-M(Bd)~90MeV

  5. TEVATRON • Tevatron is the highest energy collider in the world • Ecm(pp) = 1.96 TeV • RunII physics run began in Mar 2001 (>1 fb-1 delivered so far) • Expect 8fb-1 (design spec) by 2009. • B production cross-sec • is ~30mb at Tevatron • (1nb at PEPII) • All B species are produced • (B+ : Bd : Bs : Lb,) DØ CDF Tevatron Main Injector

  6. CDF II Detector Important components relevant to the analysis highlighted: Central Muon Extension: CMX (0.6< |h| < 1.0) Central Drift Chamber (COT) Silicon Vertex Detector (L00, SVXII, ISL) Superconducting Solenoid (1.4T) Central Muon Chambers: CMU, CMP (|h| < 0.6)

  7. Data Sample • Using 364pb-1 of data (Feb 02 – Aug 04) from di-muon triggers: • - CMU(P) + CMU (central-central) • - CMU(P) + CMX (central-extended) • Central-central and central-extended channels treated independently • in this analysis (background and efficiencies are different) Rare B di-muon triggers requires additional cuts to reduce background relative to inclusive J/y di-muon trigger Search region Cheng-Ju Lin (Fermilab) SUSY05

  8. Ingredients of the Analysis Overall picture: - Reconstructing di-muon events in the B mass window - Measure the branching ratio or set a limit Normalized to BJ/y K decays Key elements in the analysis: - Construct discriminant to select Bs signal and suppress bkg - understanding the background - accurately measure the acceptance and efficiency ratios Analysis optimization: Figure of merit  expected 90% C.L. upper limit Performed unbiased optimization Cheng-Ju Lin (Fermilab) SUSY05

  9. Reconstruct “normalization mode” Count the # of B+m+m-K+ candidates • Selection Requires: • pT(B)>4 GeV && |y(B)|<1 • pT(K+)>1GeV • good vertex fit quality • l/s(l) > 2 • l = proper decay length • [l (B+) = ~502mm] CMU-CMU B+mmK+ in rare B trigger Sample: N(CMU/P-CMU) = 1785±60 N(CMU/P-CMX) = 696±39

  10. Bs Sample Selection “baseline cuts” • For Bsm+m-: • Pre-selection requires: • pT(B)>4 GeV && |y(B)|<1 • l/s(l) > 2 • good vertex fit quality CMU-CMU • [l (Bs) = ~502mm] Bsmm Search Sample: N(CMU-CMU) = 22459 N(CMU-CMX) = 14305 (completely Bgd dominated) Background shapes are linear for both channels “Baseline” cuts are loose cuts to reject events that are clearly background Cheng-Ju Lin (Fermilab) SUSY05

  11. Enhance Bs,dmm and Suppress Background We have explored various discriminating variables for selecting Bmm events and suppress bkg. The chosen ones are: • Invariant m+m- mass, Mvtx. : within +/- 60MeV (2.5s) • Proper decay-length (l): • Isolation (Iso): (fraction of pT from Bmm within DR=(h2+f2)1/2 cone of 1) • “pointing (Da)”: (3D opening angle between Bs momentum and decay axis) Cheng-Ju Lin (Fermilab) SUSY05

  12. cut cut Discriminating Variables (Sig vs BKG) To further reduce Bgd, we apply the additional cuts: Da<0.70 rad && Iso > 0.50 eff(signal) ~ 92% This leaves in m+m- data: N(CMU-CMU) = 6242 N(CMU-CMX) = 4908 ~x3 down in bkg but still… Cheng-Ju Lin (Fermilab) SUSY05

  13. Likelihood Ratio Discriminant • We use a likelihood ratio method: Ps/b is the probability for a given sig/bkg to have a value of x, where i runs over all discriminating variables. • The chosen variables are: • - isolation (iso) • - 3D pointing (Da), • - proper decay length probability [P(l)=exp(-l/lBs)] • PDF for the individual likelihood is reconstructed from the • data sideband for background and Pythia MC for signal Cheng-Ju Lin (Fermilab) SUSY05

  14. Likelihood PDFs and LH Ratio Input PDFs for Likelihood Ratio Likelihood Ratio Discriminant (CMU/P-CMU channel) (Isolation) (Pointing Angle) Prob(l) Cheng-Ju Lin (Fermilab) SUSY05

  15. Background Estimate LH CMU-CMU CMU-CMX cut pred obsv pred obsv >0.50 236+/-4 235 172+/-3 168 OS- >0.90 37+/-1 32 33+/-1 36 >0.99 2.8+/-0.2 2 3.6+/-0.2 3 >0.50 2.3+/-0.2 0 2.8+/-0.3 3 SS+ >0.90 0.25+/-0.03 0 0.44+/-0.04 0 >0.99 <0.10 0 <0.10 0 >0.50 2.7+/-0.2 1 3.7+/-0.3 4 SS- >0.90 0.35+/-0.03 0 0.63+/-0.06 0 >0.99 <0.10 0 <0.10 0 >0.50 84+/-2 84 21+/-1 19 FM+ >0.90 14.2+/-0.4 10 3.9+/-0.2 3 >0.99 1.0+/-0.1 2 0.41+/-0.03 0 • Extrapolate the number of events in the sidebands to the signal region • and scale by the expected rejection of the likelihood ratio cut • Use toy-MC to estimate bkg rejection of likelihood ratio cut • based on input distributions from data sidebands Control sample for bkg estimate cross-check: 1.) OS- : opposite-charge dimuon l < 0 2.) SS+ : same-charge dimuon l > 0 3.) SS- : same-charge dimuon l < 0 4.) FM : fake muon sample (at least one leg failed muon stub chi2 cut) Note: Using a wide ±100MeV window around B mass for bkg cross-check

  16. 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) fu/fs=3.83±0.57 (HFAG 2004)

  17. Analysis Optimization Quantity which vary in optimization Poisson prob of observing nobs when expecting nbg 90% CL UL on Nsignal when expecting nbg bkgd evts using Bayesian Method (w/ flat prior) and including uncertainties We used the set of requirements which yielded the minimum a priori expected BR Limit: • For optimization, we scan over a range of LH values • Assume 1 fb-1 of data •  Optimal cuts: LH>0.99

  18. Looking at Results For optimized cuts of LH >0.99 and pT(B) > 4GeV and a  60 MeV window around world avg B mass CMU-CMU Channel CMU-CMX Channel We observed 0 event in the signal region! Cheng-Ju Lin (Fermilab) SUSY05

  19. Bmm Limits Bs Summary: CMU-CMU: Single event sensitivity = (1.0±0.2) ×10-7 Expected # bkg (364pb-1) = 0.81 ± 0.12 CMU-CMX: Single event sensitivity = (1.6±0.3) ×10-7 Expected # bkg (336pb-1) = 0.66 ± 0.13 We observed 0 event which yields a combined limit of: Br(Bsmm) <1.5×10-7 @ 90% CL ; 2.0×10-7 @ 95% CL Bd Summary: We also observed 0 event which yields a combined limit of: Br(Bdmm) < 3.8×10-8 @ 90% CL ; 4.9×10-8 @ 95% CL Both Bs and Bd results are a x2 better than best published results! D0 PRL 94 (2005) 042001 (240pb-1) BaBar PRL 94 (2005) 221803 (111fb-1) Cheng-Ju Lin (Fermilab) SUSY05

  20. Dedes, Dreiner, Nierste, PRL 87(2001) 251804 mSUGRA M0 vs M1/2 Excluded • We are beginning to carve • into mSUGRA space • For mh~115GeV implies • 10-8<Br(Bsmm)<3×10-7 M0 [GeV] Excluded Solid red = excluded by theory or experiment Dashed red line = light Higgs mass (mh) Dashed green line = (dam)susy (in units of 10-10) Black line = Br(Bsmm)

  21. Dedes, Dreiner, Nierste, PRL 87(2001) 251804 mSUGRA M0 vs M1/2 Excluded • We are beginning to carve • into mSUGRA space • For mh~115GeV implies • 10-8<Br(Bsmm)<3×10-7 M0 [GeV] Excluded by this new result Excluded Solid red = excluded by theory or experiment Dashed red line = light Higgs mass (mh) Dashed green line = (dam)susy (in units of 10-10) Black line = Br(Bsmm)

  22. SO(10) Unification Model R. Dermisek et al., JHEP 0304 (2003) 037 • tan(b)~50 constrained by • unification of Yukawa coupling • White region is not excluded • Unification valid for small M1/2 • (~500GeV) h2>0.13 mh<111GeV m+<104GeV 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

  23. SO(10) Unification Model R. Dermisek et al., JHEP 0304 (2003) 037 • 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

  24. Summary • Bsmm is a powerful probe of new physics. Could potentially • provide the first hint of SUSY at the Tevatron • Using 364pb-1 of data, CDF has obtained world best limits on • Bs and Bd channels: • CDF: Br(Bs) < 1.5×10-7 (2.0×10-7) @ 90% (95%) CL • Br(Bd) < 3.8×10-8 (4.9×10-8) @ 90% (95%) CL • TEVNPWG is combining CDF and D0 limits. Expect ~20% • improvements over CDF limits alone • The limits are now starting to constrain interesting regions • of SUSY parameter space • We have covered an order of magnitude since Run I. Will • cover at least another order of magnitude before the end of RunII Cheng-Ju Lin (Fermilab) SUSY05

  25. BACKUP SLIDES

  26. Correlations Between Variables Correlations between discriminating variables are negligible:  straighforward to construct likelihood discriminant (see next slide)

  27. Checking MC Modeling of Signal LH Compare B+ Data and MC • For CMU-CMU: • MC reproduces Data • efficiency vs LHood cut • to 10% or better • Assign 10% (relative) • systematic • CMU-CMX MC vs Data • agreement is better

  28. Checking MC Modeling of Signal LH • For CMU-CMX: • MC reproduces Data • efficiency vs LHood cut • to 5% or better • Assign 5% (relative) • systematic for • CMU-CMX

  29. eLH(Bs) cut CMU-CMU CMU-CMX LH>0.90 (70+/-1)% (66+/-1)% LH>0.92 (67+/-1)% (65+/-1)% LH>0.95 (61+/-1)% (60+/-1)% LH>0.98 (48+/-1)% (48+/-1)% LH>0.99 (38+/-1)% (39+/-1)% Likelihood Ratio Efficiency for Bs Signal • determined from Bsmm MC • MC modeling checked by comparing eLH(B+) • between MC and sideband subtracted Data (stat uncertainties only)

  30. Estimate LH Bkg Rejection (RLH) Since discriminating variables are uncorrelated, use toy MC to estimate RLH based on input distributions from data SB Likelihood Ratio Data vs Toy MC Likelihood Ratio Rejection from Toy MC RLH Cut CMU-CMU CMU-CMX LH>0.85 0.0245+/-0.0005 0.0226+/-0.0005 LH>0.92 0.0130+/-0.0004 0.0120+/-0.0003 LH>0.99 0.0014+/-0.0001 0.0015+/-0.0001 (Errors are stat only) LH strongly suppresses bkg KS-Prob(CMU)=11% KS-Prob(CMX)=5%

  31. m b R-parity violating SUSY ~ n l’i23 l i22 m s RPV SUSY EXCLUSION B. Dutta et al, PLB 538 (2002) 121 • Possible to exclude phase space • even for small tan(b) • Exclusion strongly depends on the • coupling. Excluded

  32. Bmm Sensitivity To Heavy Higgs MFV MSSM (tanb=50) (A. Dedes et al, hep-ph/0407285) • Br(Bsmm) is sensitive • to the mass of heavy Higgs • If the branching ratio is • measured  sets an upper • limit on the mass of the • heavy Higgs • mA mass limit is where • BR crosses the green curve • (same argument hold of • any tanb≤ 50) • The mass limit is fairly • model independent (Excluded by Bsmm)

  33. BsLimit Projection • Extrapolate based on the • current analysis which was • optimized for 1/fb • Assume background and • single-event-sensitivity • scale linearly with luminosity • Will need to re-optimize the • analysis for > 3/fb

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