Searches for rare B s Decays with the DØ Detector

Searches for rare Bs Decays with the DØ Detector Ralf Bernhard University of Zürich HEP Seminar University of Freiburg May 10th 2006

Outline • Motivation for FCNC searches • FNAL and DØ Detector • Search for the Decay Bs→μ+μ- • Search for the Decay Bs→ μ+μ- • Observation of the Decay Bs→ ψ(2S) • Summary

History of FCNC  d Z s  • Particle physics until the '70 knew only three light quarks (u,d,s) which could mix due to the Cabibbo angle θc • As a consequence s → d transitions can occur because • This was in contradiction with experimental situation in '64 – '70, no such transition in Kaon decays were observed (limits order of 10-6) • In 1970 Glashow, Iliopoulos, Maiani (GIM) proposed a new quark (charm) to cancel the unobserved FCNC transitions (at tree level). • Historically the GIM mechanism allowed charm mass prediction before it was observed in J/ψ (cc) resonances 1974

Purely leptonic B decay • B->l+ l- decay is helicity suppressed FCNC • SM: BR(Bs->m+m-) ~ 3.410-9 • depends only on one SM operator in effective Hamiltonian, hadronic uncertainties small • Bd relative to Bs suppressed by |Vtd/Vts|2 ~ 0.04 if no additional sources of flavor violation • reaching SM sensitivity: present limit for Bs -> m+m- comes closest to SM value SM expectations: Current published limits:

Purely leptonic B decay Two-Higgs Doublet models: • excellent probe for many new physics models • particularly sensitive to models w/ extended Higgs sector • BR grows ~tan6b in MSSM • 2HDM models ~ tan4b • mSUGRA: BR enhancement correlated with shift of (g-2)m • also, testing ground for • minimal SO(10) GUT models • Rp violating models, contributions at tree level • (neutralino) dark matter … Rp violating:

Motivation for FCNC searches • FNAL and DØ Detector • Search for the Decay Bs→μ+μ- • Search for the Decay Bs→ μ+μ- • Observation of the Decay Bs→ ψ(2S) • Summary

TeVatron • New Main Injector and Antiproton Reycler • Increase number of bunches 6×6→ 36× 36 • Reduce bunch spacing 3.5μs → 396ns • Increase beam energy 900 GeV → 980 GeV • Projected integrated luminosity per experiment: • ≈ 2 fb-1 2006 • ≈ 8 fb-1 2009 • Highest initial luminosity so far 1.7×1032 cm-2 s-1 • 1.18fb-1 recorded per experiment • Data taking efficiency: 85-90%

Integrated Luminosity

B production at the TeVatron e.g., integrated cross sections for |y|<1: (B+, pT  6 GeV/c)~4 mb • bb cross section orders of magnitude larger than at B-factories (4S) or Z • σ(pp → bb) = 150μb at 2TeV • σ(e+e-→ Z → bb) = 7nb • σ(e+e- → Υ(4S) → bb) = 1nb • all kinds of b hadrons produced: • Bd, Bs, Bc, B**, b, b, … • However: • QCD background overwhelming, b-hadrons hidden in 103 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

The DØ Experiment SMT SMT SMT • Excellent coverage of Tracking and Muon Systems • Forward muon system with |η|<2 and good shielding • 4-layer Silicon and 16-layer Fiber Trackers in 2 T magnetic field

Tracking System • small tracking volume w/ radius ~0.5 m • impact parameter resolution: • ~50 m at pT ~ 1 GeV/c • ~10 m at higher pT • 2nd vertex resolution • ~40 m (r,) • ~80 m (r, z)

Muon System • 3 layer of drift tube + scintillators ( < 2) • Toroid magnet between 1st and 2nd layer allows stand-alone momentum measurement • Central Proportional Drift Tubes • 6624 drift cells (10.1 cm  5.5 cm) • Stacked in 3- and 4- deck chambers • Forward Mini Drift Tubes • 6080 8-cell tubes (9.4mm  9.4 mm) • Provides fast L1 trigger signal • Scintillation Counters (forward and central) • 4214 forward, 630 central counters • Segmentation 0.1mm * 4.5mm in  • Provide fast L1 trigger signal

Triggers for B physics • robust and quiet di-muon and single-muon triggers • keys to B physics program at DØ • large coverage ||<2, p>1.5-5 GeV – depends on Luminosity and trigger • variety of triggers based on • Level 1/2: based on Muon hits aided by Fiber Tracker (hardware/hybrid) • Level 3: flexible and fast reconstruction of full event • typical total rates at medium luminosity (7 x 1031 s-1cm-2) • di-muons : 75 Hz / 20 Hz / 2 Hz @ L1/L2/L3 • single muons : 120 Hz / 100 Hz / 50 Hz @ L1/L2/L3 (has to be prescaled) • muon purity @ L1: 90% - all physics! • Current total trigger bandwidth (input ~1.6 MHz) • 1800 Hz / 800 Hz / 50 Hz @ L1/L2/L3

Di-Muon Data Sample 300 pb-1 Signal Region (not able to separate Bs and Bd)

Strategy Used in previous analysis Still blind! • Published a limit using 240pb-1 • Using a slightly larger data set (300pb-1) for a Tevatron combination note (which yielded the current best published limit) • Using additional recorded data • Obtain sensitivity (blind analysis) with additional data set w/o changing the analysis procedure • Combine sensitivity with existing published limit

Selection Cuts 300 pb-1 blinded signal region: 5.160 < m < 5.520 GeV/c2; ±2 wide, =90 MeV Sideband regions: 540 MeV/c2 each • Cut on Mass region of di-muon sample 4.5 < m < 7 GeV/c2 • Two good muons with a net charge of zero and a pT greater than 2.5 GeV • The triggered muons have reconstructed tracks in the tracker with • at least 3 hits in the Silicon tracker • at least 4 hits in the Fiber tracker • Good reconstructed vertex • Cut on the uncertainty of the transverse decay length (Lxy) < 150 m • A minimum pT of the Bs candidate of 5 GeV is required 38k events remain

Searching the Needle! • 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 Using discriminating variables!

Discriminating Variables • Opening angle between the vertex direction and the muon pair "Pointing consistency" • Decay length significance (Lxy/σ(Lxy)) • Isolation of the B candidate with

Optimisation Procedure • Optimise cuts on a data sub sample data and keep signal region as blind box • Performed random grid search of the 3 discriminating variables • Maximise sensitivity of searches for new signals (physics/030863) Define α as significance of the test a is the number of sigmas for α (i.e 95% →2σ→a = 2)

Optimization Procedure II • Correct statistical practice requires to decide before the experiment the values of  and CL • S/√B may push the experiment efficiency down to very small values, e.g. 0.1 expected signal events with a background of 10-5 over 10 signal expect and 1background event • S/√(S+B) cannot be maximized without knowing the x-section of the searched signal • Independent of the expectations for a signal to be present thus allowing an unbiased optimization • No dependence on metric or priors • Independent of choice of a limit setting algorithm • Punzi’s proposal can be for setting limits and discovery, by setting the constant a

Optimisation Results Decay length significance: > 18.5 Opening angle: α < 0.2 rad Isolation: Iso > 0.56 Expect 4.3 ± 1.2 background events Observe 4 events in Signal Region

Normalisation Channel B+→J/ψK+ • Use the decay of the decay J/ψ→μ+μ- to cancel μ+μ- efficiencies • Vertex an additional track to the di-muon pair • Additional cuts on the Kaon and B candidate are: • Kaon pT > 0.9 GeV/c • Collinearity of > 0.9 is required • χ2of the vertex fit contribution not more than 10, together not more than 20 Fit of a Gaussian as signal plus a quadratic function as background.

DØ Sensitivity 400 pb-1 Cut Values changed only slightly! Expect 2.2 ± 0.7 background events additional

Limit Calculation • R = BR(Bd)/BR(Bs) is small due to |Vtd/Vts|^2 • eB+ /eBs relative efficiency of normalization to signal channel • eBd /eBs relative efficiency for Bd-> m+ m- versus Bs-> m+ m- events in Bs search channel (~0.95) • fs/fu fragmentation ratio (in case of Bs limit) - use world average with 15% uncertainty all limits below are 95% C.L. Bayesian incl. sys. uncertainty

Systematic Uncertainties • Efficiency ratio determined from MC with checks in data on trigger/tracking etc. • Large uncertainty due to fragmentation ratio • Background uncertainty from interpolating fit

Tevatron limit combination I • correlated uncertainties: • BR of B± -> J/(->mm) K± • fragmentation ratio b->Bs/b->Bu,d • quote also an average expected upper limit and single event sensitivity • fragmentation ratio b->Bs/b->Bu,d • standard PDG value as default • Tevatron only fragmentation (from CDF) improves limit by 15% • uncorrelated uncertainties: • uncertainty on eff. ratio • uncertainty on background DØ has larger acceptance due to better h coverage, CDF has greater sensitivity due to lower background expectations

Combination II 2-Higgs Doublet Model world-best limit, only factor 35 away from SM BR(Bs-> m+ m- ) < 1.2 (1.5) × 10-7 @ 90% (95%) C.L. Combined TeVatron Limit: R. Bernhard et al. hep-ex/0508058

Constraining dark matter • mSUGRA model: strong correlation between BR(Bs->m+m-) with neutralino dark matter cross section especially for large tanb • constrain neutralino cross section with less than, within and greater than 2 of WMAP relic density universal Higgs mass parameters CDF & DØ CDMS non-universal Higgs mass Parameters, dHu=1, dHd=-1 CDF & DØ S. Baek et al., JHEP 0502 (2005) 067

Ms vs Bs+-

Prospects Expectation for Bs→μ+μ- DØ TeVatron

Search for Bs ->  m+m- • long-term goal: investigate b -> s l+ l- FCNC transitions in Bs meson • exclusive decay: Bs ->  m+m- • SM prediction: • short distance BR: ~1.6×10-6 • about 30% uncertainty due to B-> form factor • 2HDM: enhancement possible, depending on parameters for tanb and MH+ • presently only one published limit • CDF Run I: 6.7×10-5 @ 95% C.L.

Search for Bs ->  m+m- Dilepton mass spectrum in b -> s l l decay • 300 pb-1 of dimuon data • normalize to resonant decay Bs -> J/y f • cut on mass region 0.5 < M(mm) < 4.4 GeV/c2 excluding J/y & y’ • two good muons, pt > 2.5 GeV/c • two additional oppositely charged tracks pt>0.5 GeV/c for f • f candidate in mass range 1.008 < M(f) < 1.032 GeV/c2 • good vertex • pt(Bs cand.) > 5 GeV/c • non-resonant decay: cut out J/ and ’ J/y Y(2S)

Search for Bs ->  m+m- • Blind analysis: optimization with following variables in random grid search • Pointing angle • Decay length significance • Isolation • Background modeled from sidebands • Use resonant decay Bs -> J/y f with same cuts as normalization • Gaussian fit with quadratic background: 73 ± 10 ± 4 Bs-> J/y f resonant decays

Discriminating Variables Decay length significance: > 10.3 Opening angle: α < 0.1 rad Isolation: Iso > 0.72

Limit on Bs ->  m+ m- • expected background from sidebands: 1.6 ± 0.4 events • observe zero events in signal region BR(Bs -> f m+m-)/BR(Bs -> J/y f) < 4.4 × 10-3 @ 95% C.L. Using central value for BR(Bs -> J/y f) = 9.3×10-4 PDG2004: BR(Bs -> f m+m-) < 4.1×10-6 @ 95% C.L. x10 improvement w.r.t previous limit submited to PRL hep-ex/0604015

Expected limit Bs -> m+m- expected limit at 95% C.L. for Bs -> m+m-

Motivation for Bs→ ψ(2S) • PDG says decay has been “seen” (1 event observed at ALEPH in 1992 when they measured the Bs mass ) • Historically: • The decay B+ (2S) K+ was observed at ARGUS 1990 • B (2S) K*0 was observed in CDF Run I in 1998 • B (2S) Ks and B+(2S)K*+ by CLEO in 2000 • Measurements show that the rates of B+ and B0 mesons decay to (2S) states is approximately 60% of the analogues decay to J/ • The relative branching ratio Bs(2S) / Bs J/ was now recently measured by CDF (they published before us) • Strategy: • Use B+ (J/,(2S)) K+ as control channel • Reconstruct the decay Bs J/ • Move the Di-Muon mass window to the (2S) resonance (3.45 GeV/c2 < m< 3.95 GeV/c2)

Control Channel Comparison with BaBar

Bs→ ψ(2S) Candidates Loose selection of candidates Use of Discriminating Variables Significance of 6σ Expect 1.8 ±1.3 events

Calculation of the Ratio

Summary • A search for the FCNC decay Bs → μ+μ-has been presented. • Importance of this decay to constrain models beyond the SM. • We have more data recorded to further improve (observe) the limit (decay). • Expect an update/combination for the summer. • A search for the FCNC decay Bs → +μ-has been presented. • The obtained limit improves the published limit by a factor of 10 (with just a 1/3 of the recorded data). • This decay mode should be observable in Run II. • The observation of decay Bs → (2S)has been presented. • The results for the BR are in agreement with the expectations (around 60% with respect to the corresponding J/  mode).

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Summary • A new expected sensitivity on the decay Bs → μ+μ- has been presented. • Goal is to improve the sensitivity, using new discriminating variables and multivariate techniques, unblind if sensitive is around 1 10-7. • A Limit on the decay Bs → μ+μ- has been presented. Improving the current published value by a factor of 10.

Implications Example: SO(10) symmetry breaking model R. Dermisek et al. hep-ph/0507233 Contours of constant Br(Bsμ+μ-)

Searches for rare B s Decays with the DØ Detector