Search for Rare Decays of the B S Meson at the Tevatron

Search for Rare Decays of the BS Meson at the Tevatron

Ralf BernhardUniversity of Zurich

XLIst Rencontres de Moriond QCD19th March 2006

Ralf P. Bernhard – XLIst Rencontres de Moriond QCD - March 19, 2006 -2-

Outline

Motivation for rare decays

Tevatron CDF & DØ Detector Search for the Decay Bs → μ+μ-

Search for the Decay Bs → μ+μ-

Summary & Conclusions


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:


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:


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

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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


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


DØ detector

2T Solenoid hermetic forward

& central muon detectors excellent coverage ||<2

Fiber Tracker 8 double layers

Silicon Detector up to |<2.5


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


• “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


Experimental search CDF:

780 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 (limit)

Combined sensitivity for 700 pb-1 of recorded data (300 pb-1 + 400 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: 5.160 < m < 5.520 GeV/c2; ±2 wide, =90 MeVCDF: 5.169 < m < 5.469 GeV/c2;

covering Bd and Bs; =25 MeV


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

300 pb-1


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)


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”


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


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


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 300 pb-1


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


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 x3 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-> 700 pb-1 <2.3×10-7>Prelim.

Sensitivity

CDF Bs-> 364 pb-1 2.0×10-7 Published

CDF Bs-> 780 pb-1 1.0×10-7 Prelim.

CDF Bd-> 364 pb-1 4.9×10-8 Published

CDF Bd-> 780 pb-1 3.0×10-8 Prelim.


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.


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/ ’


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


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


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

R. Dermisek et al., hep-ph/0507233 (2005)


SPARE


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


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


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


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.


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


expected limit Bs -> +-

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


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

Search for Rare Decays of the B S Meson at the Tevatron

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