Search for Rare Decays of the B S Meson at the Tevatron Ralf Bernhard University of Zurich XLIst Rencontres de Moriond QCD 19 th March 2006

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Outline  Motivation for rare decays  Tevatron  CDF & DØ Detector  Search for the Decay B s → μ + μ -  Search for the Decay B s →  μ + μ -  Summary & Conclusions

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Purely leptonic B decay  B->l + l - decay is helicity suppressed FCNC  SM: BR(B s ->     ) ~ 3.4   depends only on one SM operator in effective Hamiltonian, hadronic uncertainties small  B d relative to B s suppressed by |V td /V ts | 2 ~ 0.04 if no additional sources of flavor violation  reaching SM sensitivity: present limit for B s ->  +  - comes closest to SM value Br(B d  l + l - )Br(B s  l + l - ) l = e3.4 × × l= μ 1.0 × × l= τ 3.1 × × SM expectations: C.L. 90% Br(B d  l + l - )Br(B s  l + l - ) l = e< 6.1 ·10 -8 < 5.4 ·10 -5 l= μ < 8.3 ·10 -8 <1.5 x l= τ < 2.5%< 5.0% Current published limits:

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Purely leptonic B decay  excellent probe for many new physics models  particularly sensitive to models w/ extended Higgs sector  BR grows ~tan 6  in MSSM  2HDM models ~ tan 4   mSUGRA: BR enhancement correlated with shift of (g-2)   also, testing ground for  minimal SO(10) GUT models  R p violating models, contributions at tree level  (neutralino) dark matter … Two-Higgs Doublet models: R p violating:

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Tevatron performance  excellent performance of Tevatron in 2005 and early 2006  machine delivered more than 1500 pb -1 up to now !!  recorded (DØ/CDF)  1.2/1.4 fb -1  record luminosity of 1.7  cm -2 /s in January 2006  high data taking efficiency ~85%  current dataset reconstructed and under analysis  ~1000 pb -1  compare with ~100 pb -1 Run I

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, CDF detector  Silicon Tracker SVX  up to |  <2.0  SVX fast r-  readout for trigger  Drift Chamber  96 layers in |  |<1  particle ID with dE/dx  r-  readout for trigger  tracking immersed in Solenoid 1.4T  Time of Flight  → particle ID

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, DØ detector  2T Solenoid  hermetic forward & central muon detectors  excellent coverage |  |<2  Fiber Tracker  8 double layers  Silicon Detector  up to |  <2.5

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, B production at the Tevatron  bb cross section orders of magnitude larger than at B-factories  (4S) or Z σ (pp → bb) = 150 μ b at 2TeV σ (e + e - → Z → bb) = 7nb σ (e + e - → Υ (4S) → bb) = 1nb  all kinds of b hadrons produced:  B d, B s, B c, B**,  b,  b, …  However:  QCD background overwhelming, b- hadrons hidden in 10 3 larger background  events complicated, efficient trigger and reliable tracking necessary  crucial for B physics program:  good vertexing & tracking  triggers w/ large bandwidth, strong background rejection  muon system w/ good coverage Lots going on in Si detector e.g., integrated cross sections for |y|<1:  (B +, p T  6 GeV/c)~4  b

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, “classical” triggers: robust and quiet di-muon and single- muon triggers working horse for masses, lifetimes, rare decays etc. keys to B physics program at DØ “advanced” triggers using silicon vertex detectors exploit long lifetime of heavy quarks displaced track + leptons for semileptonic modes two-track trigger (CDF) – all hadronic mode two oppositely charged tracks with impact parameter p T (B)  5 GeV L xy  450  m Decay length L xy Triggers for B physics

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Experimental search  CDF:  780 pb -1 di-muon triggered data  two separate search channels  central/central muons  central/forward muons  extract B s and B d limit  DØ:  240 pb -1 (update 300 pb -1 ) di-muon triggered data (limit)  Combined sensitivity for 700 pb -1 of recorded data (300 pb pb -1 )  both experiments:  blind analysis to avoid experimenter’s bias  side bands for background determination  use B + -> J/  K + as normalization mode  J/  ->     cancels     selection efficiencies blinded signal region: DØ: < m  < GeV/c 2 ; ±2  wide,  =90 MeV CDF: < m  < GeV/c 2 ; covering B d and B s ;  =25 MeV

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Pre-selection  Pre-selection DØ:  4.5 < m  < 7.0 GeV/c 2  muon quality cuts  p T (  )>2.5 GeV/c  (  )| < 2  p T (B s cand.)>5.0 GeV/c  good vertex  Pre-Selection CDF:  < m  < GeV/c 2  muon quality cuts  p T (  )>2.0 (2.2) GeV/c CMU (CMX)  p T (B s cand.)>4.0 GeV/c  (B s )| < 1  good vertex  3D displacement L 3D between primary and secondary vertex  (L 3D )<150  m  proper decay length 0 < < 0.3 cm e.g. DØ: about 38k events after pre-selection Potential sources of background: continuum  Drell-Yan sequential semi-leptonic b->c->s decays double semi-leptonic bb->  X b/c->  x+fake fake + fake 300 pb -1

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Optimization I  DØ:  optimize cuts on three discriminating variables  angle between     and decay length vector (pointing consistency)  transverse decay length significance (B s has lifetime): L xy / σ (L xy )  isolation in cone around B s candidate  Random Grid Search  maximize  /(1.+  B)

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Optimization II  CDF: discriminating variables  pointing angle between     and decay length vector  isolation in cone around B s candidate  proper decay length probability p( ) = exp(-  Bs )  construct likelihood ratio to optimize on “expected upper limit”

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Unblinding the signal region  CDF:  central/central: observe 1, expect 0.88 ± 0.30  Central/forward: observe 0, expect 0.39 ± 0.21  DØ ( 300 pb -1 ):  observe 4, expect 4.3 ± 1.2 CDF

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, DØ Sensitivity 1f b -1 Expect 2.2 ± 0.7 background events Cut Values changed only slightly! Used in previous analysis Used now in addition! Obtain a sensitivity (w/o unblinding) w/o changing the analysis Combine “old” Limit with obtained sensitivity

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Normalization  relative normalization is done to B + -> J/  K +  advantages:     selection efficiency same  high statistics  BR well known  disadvantages:  fragmentation b->B u vs. b-> B s  DØ: apply same values of discriminating cuts on this mode  CDF: no likelihood cut on this mode 300 pb -1

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Master equation  R = BR(B d )/BR(B s ) is small due to |V td /V ts |^2  B + /  Bs relative efficiency of normalization to signal channel  Bd /  Bs relative efficiency for B d ->      versus B s ->     events in B s search channel (for CDF~0, for DØ ~0.95)  f s /f u fragmentation ratio (in case of B s limit) - use world average with 15% uncertainty

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, The present (individual) limits  DØ mass resolution is not sufficient to separate B s from B d. Assume no B d contribution (conservative)  CDF sets separate limits on B s & B d channels  all limits below are 95% C.L. Bayesian incl. sys. error, DØ also quotes FC limit B d limit x3 better than published Babar limit w/ 111 fb -1 CDF B s ->  176 pb ×10 -7 Published DØ B s ->  240 pb ×10 -7 Published DØ B s ->  300 pb ×10 -7 Prelim. DØ  700 pb -1 Prelim. Sensitivity CDF B s ->  364 pb ×10 -7 Published CDF B s ->  780 pb ×10 -7 Prelim. CDF B d ->  364 pb ×10 -8 Published CDF B d ->  780 pb ×10 -8 Prelim.

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Search for B s ->  +  -  long-term goal: investigate b -> s l + l - FCNC transitions in B s meson  exclusive decay: B s ->  +  -  SM prediction:  short distance BR: ~1.6×10 -6  about 30% uncertainty due to B->  form factor  2HDM: enhancement possible, depending on parameters for tan  and M H+  presently only one published limit  CDF Run I: 6.7×10 95% C.L.

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Search for B s ->   +  -  DØ: 300 pb -1 of dimuon data  normalize to resonant decay B s -> J/   cut on mass region 0.5 < M(  ) < 4.4 GeV/c 2 excluding J/  &  ’  two good muons, p t > 2.5 GeV/c  two additional oppositely charged tracks p t >0.5 GeV/c for   candidate in mass range < M(  ) < GeV/c 2  good vertex  p t (B s cand.) > 5 GeV/c  non-resonant decay: cut out J/  and  ’ Dilepton mass spectrum in b -> s l l decay J/  ’’

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Search for B s ->   +  -  Blind analysis: optimization with following variables in random grid search  Pointing angle  Decay length significance  Isolation  Background modeled from sidebands  Use resonant decay B s -> J/  with same cuts as normalization  Gaussian fit with quadratic background: 73 ± 10 ± 4 B s -> J/  resonant decays

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Limit on B s ->   +  -  expected background from sidebands: 1.6 ± 0.4 events  observe zero events in signal region BR(B s ->     )/BR(B s -> J  ) < 4.4 × 10 95% C.L. Using central value for BR(B s -> J  ) = 9.3×10 -4 PDG2004: BR(B s ->     ) < 4.1×10 95% C.L. x10 improvement w.r.t previous limit

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Conclusions  CDF & DØ provide world best limits on purely leptonic decays B d,s ->      limit important to constrain new physics  With more statistics to come enhance exclusion power/discovery potential for new physics  Improved DØ limit on exclusive B s ->  +  -  decay shown, about 2x above SM  Tevatron is doubling statistics every year - stay tuned for many more exciting results on B physics R. Dermisek et al., hep-ph/ (2005)

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, SPARE

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Future Prospects for B s ->      assuming unchanged analysis techniques and reconstruction and trigger efficiencies are unaffected with increasing luminosity  for 8fb -1 /experiment an exclusion at 90%C.L. down to 2  is possible  both experiments pursue further improvements in their analysis

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Tevatron limit combination I  fragmentation ratio b->B s /b->B u,d  standard PDG value as default  Tevatron only fragmentation (from CDF) improves limit by 15%  uncorrelated uncertainties:  uncertainty on eff. ratio  uncertainty on background  correlated uncertainties:  BR of B ± -> J/  (->  ) K ±  fragmentation ratio b->B s /b->B u, d  quote also an average expected upper limit and single event sensitivity hep-ex/ DØ has larger acceptance due to better  coverage, CDF has greater sensitivity due to lower background expectations

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Combination II  combined CDF & DØ limit:  BR(B s ->     )     ) < 1.2 (1.5) × 10 90% (95%) C.L.  world-best limit, only factor 35 away from SM  important to constrain models of new physics at tan   e.g. mSO(10) model is severely constraint Example: SO(10) symmetry breaking model Contours of constant Br(B s  μ + μ - ) R. Dermisek et al. hep-ph/

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, FCNC & new physics  flavor-changing neutral current processes  in SM forbidden at tree level  at higher order occur through box- and penguin diagrams  sensitive to virtual particles in loop, thus can discern new physics  GIM-suppression for down-type quarks relaxed due to large top mass  observable SM rates lead to tight constraints of new physics  corresponding charm decays are less scrutinized and largely unexplored  smaller BRs, more suppressed by GIM-mechanism, long-distance effects also dominating  nevertheless large window to observe new physics beyond SM exists: R p -violating models, little Higgs models w/ up-like vector quark etc.

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Systematic uncertainties  systematics for DØ (CDF very similar)  efficiency ratio determined from MC with checks in data on trigger/tracking etc.  large uncertainty due to fragmentation ratio  background uncertainty from interpolating fit

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, expected limit B s ->  +  -   expected limit at 95% C.L. for B s ->  +  - 

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, Constraining dark matter  mSUGRA model: strong correlation between BR(B s ->     ) with neutralino dark matter cross section especially for large tan   constrain neutralino cross section with less than, within and greater than 2  of WMAP relic density universal Higgs mass parameters non-universal Higgs mass Parameters,  H u =1,  H d =-1 S. Baek et al., JHEP 0502 (2005) 067