1. Searches for rare Bs Decays with the DØ Detector
Ralf Bernhard
University of Zürich
HEP Seminar
University of Freiburg
May 10th 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

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

4. Purely leptonic B decay
B->l+ l- decay is helicity suppressed FCNC
SM: BR(Bs->m+m-) ~ 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 -> m+m- comes closest to SM value
SM expectations:
Current published limits:
Br(Bdl+l-)
Br(Bsl+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
C.L. 90%
Br(Bdl+l-)
Br(Bsl+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%

5. 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)m
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:

6. 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

7. TeVatron New Main Injector and Antiproton Reycler
Increase number of bunches 6×6→ 36× 36
Reduce bunch spacing μs → 396ns
Increase beam energy GeV → 980 GeV
Projected integrated luminosity per experiment:
≈ 2 fb
≈ 8 fb
Highest initial luminosity so far 1.7×1032 cm-2 s-1
1.18fb-1 recorded per experiment
Data taking efficiency: 85-90%

8. Integrated Luminosity

9. 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
e.g., integrated cross sections
for |y|<1:
(B+, pT  6 GeV/c)~4 mb
Lots going on
in Si detector

10. 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

11. 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)

12. 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
6624 drift cells (10.1 cm  5.5 cm)
Stacked in 3- and 4- deck chambers
Forward Mini Drift Tubes
cell tubes (9.4mm  9.4 mm)
Provides fast L1 trigger signal
Scintillation Counters (forward and central)
4214 forward, 630 central counters
Segmentation 0.1mm * 4.5mm in 
Provide fast L1 trigger signal

13. 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 : Hz / 20 Hz / 2 L1/L2/L3
single muons : 120 Hz / 100 Hz / 50 L1/L2/L3 (has to be prescaled)
muon L1: 90% - all physics!
Current total trigger bandwidth (input ~1.6 MHz)
1800 Hz / 800 Hz / 50 L1/L2/L3

14. 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

15. Di-Muon Data Sample Signal Region (not able to separate Bs and Bd)
300 pb-1
Signal Region (not able to separate Bs and Bd)

16. 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!

17. 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 < GeV/c2;
±2 wide, =90 MeV
Sideband regions:
540 MeV/c2 each

18. 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
Using discriminating variables!

19. 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

20. 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)

21. 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
No dependence on metric or priors
Independent of choice of a limit setting algorithm
Punzi’s proposal can be for setting limits and discovery, by setting the constant a

22. Optimisation Results Decay length significance: > 18.5
Opening angle: α < 0.2 rad
Isolation: Iso > 0.56
Expect 4.3 ± 1.2
background events
Observe 4 events
in Signal Region

23. 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:
Kaon pT > 0.9 GeV/c
Collinearity of > 0.9 is required
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.

24. DØ Sensitivity 400 pb-1 additional Cut Values changed only slightly!
Expect 2.2 ± 0.7
background events
additional
400 pb-1

25. 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-> m+ m- versus Bs-> m+ m- events in Bs search channel (~0.95)
fs/fu fragmentation ratio (in case of Bs limit) - use world average with 15% uncertainty
DØ Bs->mm
240 pb-1
5.1×10-7
Published
300 pb-1
4.0×10-7
Prelim.
DØ <Bs->mm
700 pb-1
<2.3×10-7>
Sensitivity
all limits below are 95% C.L. Bayesian incl. sys. uncertainty

26. 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

27. 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/(->mm) 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 h coverage,
CDF has greater sensitivity due to lower background expectations

28. world-best limit, only factor 35 away from SM
Combination II
2-Higgs Doublet Model
world-best limit, only factor 35 away from SM
BR(Bs-> m+ m- ) < 1.2 (1.5) × 90% (95%) C.L.
Combined TeVatron Limit: R. Bernhard et al. hep-ex/

29. Constraining dark matter
mSUGRA model: strong correlation between BR(Bs->m+m-) 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

30. Ms vs Bs  +-

31. Prospects
Expectation for Bs → μ+μ- DØ
TeVatron

32. 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

33. Search for Bs ->  m+m-
long-term goal: investigate b -> s l+ l- FCNC transitions in Bs meson
exclusive decay: Bs ->  m+m-
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: 95% C.L.

34. Search for Bs ->  m+m-
Dilepton mass spectrum
in b -> s l l decay
300 pb-1 of dimuon data
normalize to resonant decay Bs -> J/y f
cut on mass region 0.5 < M(mm) < 4.4 GeV/c2 excluding J/y & y’
two good muons, pt > 2.5 GeV/c
two additional oppositely charged tracks pt>0.5 GeV/c for f
f candidate in mass range < M(f) < GeV/c2
good vertex
pt(Bs cand.) > 5 GeV/c
non-resonant decay: cut out J/ and ’
J/y
Y(2S)

35. Search for Bs ->  m+m-
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/y f with same cuts as normalization
Gaussian fit with quadratic background: 73 ± 10 ± 4 Bs-> J/y f resonant decays

36. Discriminating Variables
Decay length significance: > 10.3
Opening angle: α < 0.1 rad
Isolation: Iso > 0.72

37. Limit on Bs ->  m+ m- submited to PRL hep-ex/0604015
expected background from sidebands: 1.6 ± 0.4 events
observe zero events in signal region
BR(Bs -> f m+m-)/BR(Bs -> J/y f) < 4.4 × 95% C.L.
Using central value for BR(Bs -> J/y f) = 9.3×10-4 PDG2004:
BR(Bs -> f m+m-) < 95% C.L.
x10
improvement
w.r.t previous limit
submited to PRL
hep-ex/

38. Expected limit Bs ->  m+m-
expected limit at 95% C.L. for Bs ->  m+m-

39. 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

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:
The decay B+  (2S) K+ was observed at ARGUS 1990
B  (2S) K*0 was observed in CDF Run I in 1998
B  (2S) Ks and B+  (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/
The relative branching ratio Bs  (2S) / Bs  J/ was now recently measured by CDF (they published before us)
Strategy:
Use B+  (J/,(2S)) K+ as control channel
Reconstruct the decay Bs  J/
Move the Di-Muon mass window to the (2S) resonance (3.45 GeV/c2 < m< 3.95 GeV/c2)

41. Control Channel
Comparison with BaBar

42. Bs →  ψ(2S) Candidates Significance of 6σ
Loose selection of candidates
Use of Discriminating Variables
Significance of 6σ
Expect 1.8 ±1.3 events

43. Calculation of the Ratio

44. 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).

45. Maybe....
2fb-1

46. SPARE

47. 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
A Limit on the decay Bs → μ+μ- has been presented. Improving the current published value by a factor of 10.

48. Example: SO(10) symmetry breaking model
Implications
Example: SO(10) symmetry breaking model
R. Dermisek et al.
hep-ph/
Contours of constant Br(Bsμ+μ-)