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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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

d
 As a consequence s → d transitions can occur because
Z
s

 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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Purely leptonic B decay
 B->l+ l- decay is helicity suppressed FCNC
 SM: BR(Bs->+-) ~ 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 -> +- comes closest to SM value
SM expectations:
Current published limits:
Br(Bdl+l-)
Br(Bsl+l-)
C.L. 90%
Br(Bdl+l-)
Br(Bsl+l-)
l=e
3.4 × 10-15
8.0 × 10-14
l=e
< 6.1 ·10-8
< 5.4 ·10-5
l=μ
1.0 × 10-10
3.4 × 10-9
l=μ
< 8.3 ·10-8
<1.5 x 10-7
l=τ
3.1 × 10-8
7.4 × 10-7
l=τ
< 3.1·10-3
< 5.0%
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Purely leptonic B decay
 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)
 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
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 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
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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%
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Integrated Luminosity
D0 & CDF Run II Integrated Luminosity
through 18 February 2006
2.0
CDF Delivered (from February 9th 2002)
1.9
1.8
D0 Delivered (from April 19th 2002)
1.7
1.6
CDF Recorded (from February 9th 2002)
D0 Recorded (from April 19th 2002)
1.3
1.2
1.1
1.0
0.9
0.8
0.7
Luminosity (fb -1)
1.5
1.4
0.6
0.5
0.4
0.3
0.2
0.1
0.0
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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
May-06
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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
e.g., integrated cross sections
for |y|<1:
(B+, pT  6 GeV/c)~4 b
 all kinds of b hadrons produced:
 Bd, Bs, Bc, B**, b, b, …
 However:
 QCD background overwhelming, bhadrons 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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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The DØ Experiment
 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
SMT
SMT
SMT
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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)
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
o6624 drift cells (10.1 cm  5.5 cm)
oStacked in 3- and 4- deck chambers
Forward Mini Drift Tubes
o6080 8-cell tubes (9.4mm  9.4 mm)
oProvides fast L1 trigger signal
Scintillation Counters (forward and
central)
o4214 forward, 630 central counters
oSegmentation 0.1mm * 4.5mm in 
oProvide fast L1 trigger signal
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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 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
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Di-Muon Data Sample
300 pb-1
Signal Region (not able to separate Bs and Bd)
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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!
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Selection Cuts
 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
300 pb-1
blinded signal region:
5.160 < m < 5.520 GeV/c2;
±2 wide, =90 MeV
Sideband regions:
540 MeV/c2 each
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
Using
 fake + fake
discriminating variables!
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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)
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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 optimization
o No dependence on metric or priors
o Independent of choice of a limit setting algorithm
 Punzi’s proposal can be for setting limits and discovery, by setting the
constant a
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Optimisation Results
Opening angle: α < 0.2 rad
Isolation: Iso > 0.56
Decay length
significance: > 18.5
Expect 4.3 ± 1.2
background events
Observe 4 events
in Signal Region
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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/c
o Collinearity of > 0.9 is required
o 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.
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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DØ Sensitivity
Cut Values changed
only slightly!
Expect 2.2 ± 0.7
background events
additional
400 pb-1
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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-> + - 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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Combination II
2-Higgs Doublet Model
BR(Bs-> + - ) < 1.2 (1.5) × 10-7 @ 90% (95%) C.L.
world-best limit,
only factor 35
away from SM
Combined TeVatron Limit: R. Bernhard et al. hep-ex/0508058
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Constraining dark matter
 mSUGRA model: strong
correlation between
BR(Bs->+-) 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
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Ms vs Bs  +-
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Prospects
Expectation for Bs → μ+μ-
DØ
TeVatron
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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 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
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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
tanb and MH+
 presently only one published limit
 CDF Run I: 6.7×10-5 @ 95% C.L.
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Search for Bs ->  + 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 – HEP Seminar- May 10, 2006
Y(2S)
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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 – HEP Seminar- May 10, 2006
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Discriminating Variables
Decay length significance: > 10.3
Opening angle: α < 0.1 rad
Isolation: Iso > 0.72
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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 PRL
hep-ex/0604015
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Expected limit Bs ->  +-
expected limit
at 95% C.L.
for Bs -> 
+-
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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 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
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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
o
o
o
The decay B+  Y(2S) K+ was observed at ARGUS 1990
B  Y(2S) K*0 was observed in CDF Run I in 1998
B  Y(2S) Ks and B+  Y(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/Y
o The relative branching ratio Bs  Y(2S) / Bs  J/Y was now
recently measured by CDF (they published before us)
 Strategy:
o Use B+  (J/Y,Y(2S)) K+ as control channel
o Reconstruct the decay Bs  J/Y
o Move the Di-Muon mass window to the Y(2S) resonance (3.45
GeV/c2 < m< 3.95 GeV/c2)
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Control Channel
Comparison with BaBar
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Bs →  ψ(2S) Candidates
Loose selection of candidates
Use of Discriminating Variables
Significance of 6σ
Expect 1.8 ±1.3 events
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Calculation of the Ratio
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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).
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Maybe....
2fb-1
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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SPARE
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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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.
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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Implications
Example: SO(10) symmetry breaking
model
R. Dermisek et al.
hep-ph/0507233
Contours of constant Br(Bsμ+μ-)
Ralf P. Bernhard – HEP Seminar- May 10, 2006
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