Searches for rare B s Decays with the DØ Detector

Searches for rare Bs Decays with the DØ Detector

Ralf BernhardUniversity of Zürich

HEP SeminarUniversity of Freiburg

May 10th 2006

Ralf P. Bernhard – HEP Seminar- May 10, 2006 -2-

Outline

Motivation for FCNC searches

FNAL and DØ Detector

Search for the Decay Bs → μ+μ-


Observation of the Decay Bs → ψ(2S)

Summary


History of FCNC 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

d

s

Z


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 < 3.1·10-3 < 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:


Motivation for FCNC searchesFNAL and DØ DetectorSearch for the Decay Bs → μ+μ-



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:

o ≈ 2 fb-1 2006

o ≈ 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%


D0 & CDF Run II Integrated Luminosity

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Feb-02 May-02 Aug-02 Nov-02 Feb-03 May-03 Aug-03 Nov-03 Feb-04 May-04 Aug-04 Nov-04 Feb-05 May-05 Aug-05 Nov-05 Feb-06 May-06

Lum

inos

ity (f

b-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

Integrated Luminosity


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


The DØ Experiment

SMTSMT

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 measurementCentral Proportional Drift Tubes

o6624 drift cells (10.1 cm 5.5 cm)oStacked in 3- and 4- deck chambers

Forward Mini Drift Tubeso6080 8-cell tubes (9.4mm 9.4 mm)oProvides fast L1 trigger signal

Scintillation Counters (forward and central)

o4214 forward, 630 central countersoSegmentation 0.1mm * 4.5mm in oProvide fast L1 trigger signal


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

Triggers for B physics





Summary


Di-Muon Data Sample

300 pb-1

Signal Region (not able to separate Bs and B

d)


Strategy 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

Used in previous analysis Still blind!


Selection Cuts

38k events remain

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

300 pb-1

blinded signal region:5.160 < m < 5.520 GeV/c2; ±2 wide, =90 MeV

Sideband regions:540 MeV/c2 each


Searching the Needle!

Using discriminating variables!

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


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 optimizationo No dependence on metric or priorso 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

Opening angle: α < 0.2 rad Isolation: Iso > 0.56Decay length significance: > 18.5

Expect 4.3 ± 1.2 background events

Observe 4 eventsin 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:

o Kaon pT > 0.9 GeV/co Collinearity of > 0.9 is requiredo χ2 of 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

Expect 2.2 ± 0.7 background events

Cut Values changedonly slightly!

400 pb-1

additional


Limit Calculation

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 (~0.95)

fs/fu fragmentation ratio (in case of Bs limit) - use world average with 15% uncertainty

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

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

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 sensitivity

DØ has larger acceptance due to better coverage, CDF has greater sensitivity due to lower background

expectations


Combination II

Combined TeVatron Limit: R. Bernhard et al. hep-ex/0508058

world-best limit, only factor 35

away from SM

BR(BBR(Bss-> -> ) < 1.2 (1.5) × 10 ) < 1.2 (1.5) × 10-7 -7 @ 90% (95%) C.L@ 90% (95%) C.L..

2-Higgs Doublet Model


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

CDMS

CDF & DØ

CDF & DØ


Ms vs Bs +-


Prospects

Expectation for Bs → μ+μ-

DØ TeVatron





Summary


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.


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/ (2S)

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

Search for Bs -> +-


Discriminating Variables

Opening angle: α < 0.1 rad

Isolation: Iso > 0.72

Decay length significance: > 10.3


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

submited to PRLhep-ex/0604015


Expected limit Bs -> +-

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

+-





Summary


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:o The decay B+ (2S) K+ was observed at ARGUS 1990o B (2S) K*0 was observed in CDF Run I in 1998

o B (2S) Ks and B+ (2S)K*+ by CLEO in 2000

o Measurements show that the rates of B+ and B0 mesons decay to (2S) states is approximately 60% of the analogues decay to J/

o The relative branching ratio Bs (2S) / Bs J/ was now recently measured by CDF (they published before us)

Strategy:o Use B+ (J/,(2S)) K+ as control channel

o Reconstruct the decay Bs J/

o 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

Use of Discriminating Variables

Loose selection of candidates

Significance of 6σExpect 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).


Maybe....

2fb-1


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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.


ImplicationsExample: SO(10) symmetry

breaking model

Contours of constant Br(Bsμ+μ-)

R. Dermisek et al. hep-ph/0507233

Searches for rare B s Decays with the DØ Detector

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