Search for Rare Decays of the BS Meson at the Tevatron
Ralf BernhardUniversity of Zurich
Moriond QCD19th March 2006
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -2-
Outline
Introduction Tevatron CDF & DØ Detector Search for the Decay Bs → μ+μ-
Search for the Decay Bs → μ+μ-
Summary & Conclusions
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -3-
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 1.71032 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
D0 & CDF Run II Integrated Luminosity
0.0
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1.0
1.1
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Feb-02 M ay-02 Aug-02 Nov-02 Feb-03 M ay-03 Aug-03 Nov-03 Feb-04 M ay-04 Aug-04 Nov-04 Feb-05 M ay-05 Aug-05 Nov-05 Feb-06 M ay-06
Luminosity (fb
-1)
CDF Delivered (from February 9th 2002)
D0 Delivered (from April 19th 2002)
CDF Recorded (from February 9th 2002)
D0 Recorded (from April 19th 2002)
through 18 February 2006
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -4-
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 – Moriond QCD - March 18, 2006 -5-
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 – Moriond QCD - March 18, 2006 -6-
B production at the Tevatron
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 g
oing o
n
in S
i det
ecto
r
e.g., integrated cross sections for |y|<1:(B+, pT 6 GeV/c)~4 b
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -7-
• “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
pT(B)5 GeVLxy450 m
Decay length Lxy
Triggers for B physics
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -8-
Purely leptonic B decay
B->l+ l- decay is helicity suppressed FCNC
SM: BR(Bs->) ~ 3.410-9
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 -> +- comes closest to SM value
Br(Bdl+l-) Br(Bsl+l-)
l = e 3.4 × 10-15 8.0 × 10-14
l=μ 1.0 × 10-10 3.4 × 10-9
l=τ 3.1 × 10-8 7.4 × 10-7
SM expectations:C.L. 90%
Br(Bdl+l-) Br(Bsl+l-)
l = e < 6.1 ·10-8 < 5.4 ·10-5
l=μ < 8.3 ·10-8 <1.5 x 10-7
l=τ < 2.5% < 5.0%
Current published limits:
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -9-
Purely leptonic B decay
excellent probe for many new physics models
particularly sensitive to models w/ extended Higgs sector BR grows ~tan6 in MSSM 2HDM models ~ tan4 mSUGRA: BR enhancement correlated with shift of (g-2)
also, testing ground for minimal SO(10) GUT models Rp violating models, contributions at tree level
(neutralino) dark matter …
Two-Higgs Doublet models:
Rp violating:
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -10-
Experimental search CDF:
364 pb-1 di-muon triggered data two separate search channels
central/central muons central/forward muons
extract Bs and Bd limit DØ:
240 pb-1 (update 300 pb-1) di-muon triggered data
Sensitivity for 1fb-1 of recorded data
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: 5.160 < m < 5.520 GeV/c2; ±2 wide, =90 MeVCDF: 5.169 < m < 5.469 GeV/c2;
covering Bd and Bs; =25 MeV
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -11-
Pre-selection Pre-selection DØ:
4.5 < m< 7.0 GeV/c2
muon quality cuts pT()>2.5 GeV/c ()| < 2 pT(Bs cand.)>5.0 GeV/c good vertex
Pre-Selection CDF: 4.669 < m< 5.969 GeV/c2
muon quality cuts pT()>2.0 (2.2) GeV/c CMU (CMX) pT(Bs cand.)>4.0 GeV/c (Bs)| < 1 good vertex 3D displacement L3D between primary and secondary vertex (L3D)<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
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -12-
Optimization I DØ: optimize cuts on three discriminating
variables angle between and decay length
vector (pointing consistency) transverse decay length significance
(Bs has lifetime): Lxy/σ(Lxy) isolation in cone around Bs candidate
Random Grid Search
maximize /(1.+B)
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -13-
Optimization II CDF: discriminating variables
pointing angle between and decay length vector
isolation in cone around Bs candidate
proper decay length probability p() = exp(- Bs)
construct likelihood ratio to optimize on “expected upper limit”
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -14-
Unblinding the signal region
CDF: central/central: observe 0, expect 0.81 ± 0.12
Central/forward: observe 0, expect 0.66 ± 0.13
DØ (300 pb-1): observe 4, expect 4.3 ± 1.2
CDF
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -15-
DØ Sensitivity 1fb-1
Expect 2.2 ± 0.7 background events
Cut Values changedonly 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 – Moriond QCD - March 18, 2006 -16-
Normalization
relative normalization is done to B+ -> J/ K+
advantages: selection
efficiency same high statistics BR well known
disadvantages: fragmentation b->Bu
vs. b-> Bs
DØ: apply same values of discriminating cuts on this mode
CDF: no likelihood cut on this mode
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -17-
Master equation
R = BR(Bd)/BR(Bs) is small due to |Vtd/Vts|^2 B+ /Bs relative efficiency of normalization to signal channel Bd /Bs relative efficiency for Bd-> versus Bs-> events in Bs search
channel (for CDF~0, for DØ ~0.95) fs/fu fragmentation ratio (in case of Bs limit) - use world average with 15%
uncertainty
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -18-
The present (individual) limits
DØ mass resolution is not sufficient to separate Bs from Bd. Assume no Bd contribution (conservative)
CDF sets separate limits on Bs & Bd channels all limits below are 95% C.L. Bayesian incl. sys. error, DØ
also quotes FC limit
Bd limit x2 better than published Babar
limit w/ 111 fb-1
CDF Bs-> 176 pb-1 7.5×10-7 Published
DØ Bs-> 240 pb-1 5.1×10-7 Published
DØ Bs-> 300 pb-1 4.0×10-7 Prelim.
DØ Bs-> 1 fb-1 2.3×10-7
Prelim.Sensitivity
CDF Bs-> 364 pb-1 2.0×10-7 Published
CDF Bs-> 1 fb-1 1.0×10-7 Prelim.
CDF Bd-> 364 pb-1 4.9×10-8 Published
updates on limits/sensitivities expected soon
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -19-
Tevatron limit combination I
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
correlated uncertainties:
BR of B± -> J/(->) K±
fragmentation ratio b->Bs/b->Bu,d
quote also an average expected upper limit and single event sensitivityhep-ex/0508058
DØ has larger acceptance due to better coverage, CDF has greater sensitivity due to lower background
expectations
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -20-
Combination II
combined CDF & DØ limit:
BR(BBR(Bss-> -> ) < 1.2 (1.5) ) < 1.2 (1.5) × 10× 10-7 -7 @ 90% (95%) @ 90% (95%) C.LC.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(Bsμ+μ-)
R. Dermisek et al. hep-ph/0507233
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -21-
Future Prospects for Bs->
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 210-
8 is possible both experiments
pursue further improvements in their analysis
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -22-
Search for Bs -> +-
long-term goal: investigate b -> s l+ l- FCNC transitions in Bs meson
exclusive decay: Bs -> +-
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 MH+
presently only one published limit CDF Run I: 6.7×10-5 @ 95% C.L.
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -23-
Search for Bs -> +-
DØ: 300 pb-1 of dimuon data normalize to resonant decay Bs ->
J/ cut on mass region 0.5 < M() <
4.4 GeV/c2 excluding J/& ’ two good muons, pt > 2.5 GeV/c two additional oppositely charged
tracks pt>0.5 GeV/c for candidate in mass range 1.008 <
M() < 1.032 GeV/c2
good vertex pt(Bs cand.) > 5 GeV/c non-resonant decay: cut out J/
and ’
Dilepton mass spectrum in b -> s l l decay
J/ ’
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -24-
Search for Bs -> +-
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/with same cuts as normalization Gaussian fit with quadratic background: 73 ± 10 ± 4 Bs-> J/resonant decays
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -25-
Limit on Bs -> + -
expected background from sidebands: 1.6 ± 0.4 events observe zero events in signal region
BR(Bs -> )/BR(Bs -> J) < 4.4 × 10-3 @ 95% C.L.
Using central value for BR(Bs -> J) = 9.3×10-4 PDG2004:
BR(Bs -> ) < 4.1×10-6 @ 95% C.L.
x10 improvement
w.r.t previous limit
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -26-
Conclusions CDF & DØ provide world best limits on purely leptonic decays Bd,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 Bs -> +-
decay shown, about 2x above SM Tevatron is doubling statistics every year - stay tuned for many more exciting results on B physics
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -27-
SPARE
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -28-
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: Rp-violating models, little Higgs models w/ up-like vector quark etc.
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -29-
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 – Moriond QCD - March 18, 2006 -30-
expected limit Bs -> +-
expected limit at 95% C.L. for Bs -> +-
Ralf P. Bernhard – Moriond QCD - March 18, 2006 -31-
Constraining dark matter
mSUGRA model: strong correlation between BR(Bs->) 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, Hu=1, Hd=-1 S. Baek et al.,
JHEP 0502 (2005) 067
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