Searches for rare B s Decays with the DØ Detector
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Transcript of 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-
Search for the Decay Bs → μ+μ-
Observation of the Decay Bs → ψ(2S)
Summary
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -3-
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
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -4-
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:
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -5-
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 – HEP Seminar- May 10, 2006 -6-
Motivation for FCNC searchesFNAL and DØ DetectorSearch for the Decay Bs → μ+μ-
Search for the Decay Bs → μ+μ-
Observation of the Decay Bs → ψ(2S)
Summary
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -7-
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%
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D0 & CDF Run II Integrated Luminosity
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Lum
inos
ity (f
b-1
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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
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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
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i det
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e.g., integrated cross sections for |y|<1:(B+, pT 6 GeV/c)~4 b
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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
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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)
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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
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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
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Motivation for FCNC searchesFNAL and DØ DetectorSearch for the Decay Bs → μ+μ-
Search for the Decay Bs → μ+μ-
Observation of the Decay Bs → ψ(2S)
Summary
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -15-
Di-Muon Data Sample
300 pb-1
Signal Region (not able to separate Bs and B
d)
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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!
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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
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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
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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
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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)
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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
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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
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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.
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DØ Sensitivity
Expect 2.2 ± 0.7 background events
Cut Values changedonly slightly!
400 pb-1
additional
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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
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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
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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
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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
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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
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Ms vs Bs +-
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Prospects
Expectation for Bs → μ+μ-
DØ TeVatron
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Motivation for FCNC searchesFNAL and DØ DetectorSearch for the Decay Bs → μ+μ-
Search for the Decay Bs → μ+μ-
Observation of the Decay Bs → ψ(2S)
Summary
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -33-
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.
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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 -> +-
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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 -> +-
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Discriminating Variables
Opening angle: α < 0.1 rad
Isolation: Iso > 0.72
Decay length significance: > 10.3
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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
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Expected limit Bs -> +-
expected limit at 95% C.L. for Bs ->
+-
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Motivation for FCNC searchesFNAL and DØ DetectorSearch for the Decay Bs → μ+μ-
Search for the Decay Bs → μ+μ-
Observation of the Decay Bs → ψ(2S)
Summary
Ralf P. Bernhard – HEP Seminar- May 10, 2006 -40-
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)
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Control Channel
Comparison with BaBar
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Bs → ψ(2S) Candidates
Use of Discriminating Variables
Loose selection of candidates
Significance of 6σExpect 1.8 ±1.3 events
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Calculation of the Ratio
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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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Maybe....
2fb-1
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SPARE
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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.
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ImplicationsExample: SO(10) symmetry
breaking model
Contours of constant Br(Bsμ+μ-)
R. Dermisek et al. hep-ph/0507233