1. University of Liverpool for the CDF Collaboration ICHEP 28th July 2006
Rare B Decays at CDF
Sinéad M. Farrington
University of Liverpool
for the CDF Collaboration
ICHEP
28th July 2006

2. Outline Bd,s  m+m- Bs  Ds Ds Will show two analyses from CDF:
very rare decay (10-9 in Standard Model)
strong probe of new physics scenarios
Briefly also show:
Bs  Ds Ds
not so rare (10-3)
interesting CP properties 2

3. Bd,s  mm 3

4. B  mm in the Standard Model
In Standard Model FCNC decay B  mm heavily suppressed
Standard Model predicts
A. Buras Phys. Lett. B 566,115
Bd  mm further suppressed by CKM coupling (Vtd/Vts)2
Both are below sensitivity of Tevatron experiments
SUSY scenarios (MSSM,RPV,mSUGRA) boost the BR by up to 100x
Observe no events  set limits on new physics
Observe events  clear evidence for new physics 4

5. The Challenge pp collider: trigger on dimuons Mass resolution:
search region
Mass resolution:
separate Bd, Bs
s(Mmm)~24MeV
Large combinatorial background
Key elements are
select discriminating variables
determine efficiencies
estimate background 5

6. Methodology Vertex muon pairs in Bd/Bs mass windows
Unbiased optimisation, signal region blind
Aim to measure BR or set limit:
Reconstruct normalisation
mode (B+J/y K+)
Construct likelihood discriminant to select B signal and suppress dimuon background
Measure remaining background
Measure the acceptance and efficiency ratios 6

7. B  mm Selection Discriminating variables: Proper decay length (l)
Pointing (Da) |f B – f vtx|
Isolation (Iso) 7

8. Likelihood Ratio Discriminant
Likelihood discriminant more powerful than cuts alone
i: index over all discriminating variables
Psig/bkg(xi): probability for event to be signal or background for a given measured xi
Obtain probably density functions of variables using
background: Data sidebands
signal: Pythia Monte Carlo sample 8

9. Optimisation Likelihood ratio discriminant: Optimise likelihood
for best 90% C.L. limit
Bayesian approach
include statistical and systematic
errors
Optimal cut: Likelihood ratio >0.99 9

10. Unblinded Results BR(Bsmm) < 1.0×10-7 @ 95% CL
central-central
central-extension
1 event found in Bs search window
(expected background: 0.880.30)
2 events found in Bd search window
(expected background: 1.860.34)
(In central-central
Muon sample only)
BR(Bsmm) < 95% CL
< 90% CL
BR(Bdmm) < 95% CL
< 90% CL 10
(These are currently world best limits)

11. Bs  DsDs 11

12. Bs  Ds Ds
Results of Bs mixing analysis at CDF have yielded measurement of Dms (see talk at this conference by Stefano Giagu)
DG/G gives complementary insight into CKM matrix
1/G(CP even) <1/G(CP odd) since the fast transition b ccs is mostly CP even
Biggest contribution to lifetime difference comes from
Bs  Ds Ds (pure CP even)
Can constrain DG/G by measuring branching fractions 12

13. Relative BR Measurement
BR (Bs  Ds Ds ) measured relative to B0 Ds D-
3 Ds modes
Reconstructed
Total yield:395
Control mode:
3 Ds modes
Reconstructed
Total yield:23
Bs  Ds Ds: 13

14. Summary Made BR measurement of CP mode Bs Ds Ds
Bd,s m+m- are a powerful probe of new physics
Could give first hint of new physics at the Tevatron
These are world best limits
Impacting new physics scenarios’ phase space
SO(10)
Constrained
MSSM
hep-ph/
Phys. Lett B624, 47, 2005 14

15. Back-up 15

16. Searching for New Physics
Two ways to search for new physics:
direct searches – seek e.g. Supersymmetric particles
indirect searches – test for deviations from Standard Model predictions e.g. branching ratios
In the absence of evidence for new physics
set limits on model parameters l+ Z*
c02 q l-
BR(B mm)<1x10-7 q
c01 q c+ l+ W+ n
Trileptons 16

17. Expected Background
Extrapolate from data sidebands to obtain expected events
Scale by the expected rejection from the likelihood ratio cut
Also include contributions from charmless B decays
Bhh (h=K/p) (use measured fake rates) 17

18. Limits on BR(Bd,s  mm) BR(Bsmm) < 1.0×10-7 @ 90% CL
BR(Bdmm) < 90% CL
< 95% CL
These are currently world best limits
The future:
Need to reoptimise after 1fb-1 for
best results
Assume linear background
scaling
now 18

19. B  mm in New Physics Models
SUSY could enhance BR by orders of magnitude
MSSM: BR(B  mm)  tan6b
may be 100x Standard Model m n
l’i23
l i22 ~ b s
RPV SUSY
R-parity violating SUSY: tree level diagram via sneutrino
observe decay for low tan b
mSUGRA: B  mm search complements direct SUSY searches
Low tan b  observation of trilepton events
High tan b  observation of B  mm
Or something else!
A. Dedes et al, hep-ph/ 19

20. Expected Background
Tested background prediction in several control regions and find good agreement
OS-: opposite sign muon, negative lifetime
SS+: same sign muon, positive lifetime
SS-: same sign muon, negative lifetime
FM+: fake muon, positive lifetime 20

21. Likelihood p.d.f.s Input p.d.f.s: Isolation Pointing angle
Ct significance
New plots*** 21

22. CDF six dedicated rare B triggers using all muon chambers to |h|1.1
Tracking capability leads to good mass resolution
Use two types of muon pairs:
central-central
central-extension
Central Muon Extension
(0.6< |h| < 1.0)
Central Muon Chambers
(|h| < 0.6) 22

23. Bd,s  mm K+/K*/f B Rare Decays B+  mm K+ B0  mm K* Bs  mm f
Lb  mm L
FCNC b  sg*
Penguin or box processes in the Standard Model:
Rare processes: Latest Belle measurement
hep-ex/ ,
hep-ex/ ,
hep-ex/
observed at Babar, Belle m m m m
x10-7 23

24. Motivations 1) Would be first observations in Bs and Lb channels
2) Tests of Standard Model
branching ratios
kinematic distributions (with enough statistics)
Effective field theory for b  s (Operator Product Expansion)
Rare decay channels are sensitive to Wilson coefficients which are calculable for many models (several new physics scenarios e.g. SUSY, technicolor)
Decay amplitude: C9
Dilepton mass distribution: C7, C9
Forward-backward asymmetry: C10 24

25. Samples (CDF) Dedicated rare B triggers in total six Level 3 paths
Two muons + other cuts
using all chambers to |h|1.1
Use two types of dimuons:
CMU-CMU
CMU-CMX
Additional cuts in some
triggers:
Spt(m)>5 GeV
Lxy>100mm
mass(m m)<6 GeV
mass(m m)>2.7 GeV 25

26. Signal and Side-band Regions
Use events from same triggers for
B+ and Bs(d) mm reconstruction.
Search region:
< Mmm < GeV
- Signal region not used in
optimization procedure
s(Mmm)~24MeV
Monte Carlo
Search region
Sideband regions:
- 500MeV on either side of search region
- For background estimate and analysis
optimization.

27. Pythia MC MC Samples Tune A default cdfSim tcl
realistic silicon and beamline
pT(B) from Mary Bishai
pT(b)>3 GeV && |y(b)|<1.5
Bsm+m-
(signal efficiencies)
B+JK+m+m-K+
(nrmlztn efncy and xchks)
B+Jp+m+m-p+
(nrmlztn correction)

28. SO(10) Unification Model
R. Dermisek et al.,
hep-ph/
tan(b)~50 constrained by unification
of Yukawa coupling
All previously allowed regions (white)
are excluded by this new measurement
Unification valid for small M1/2
(~500GeV)
New Br(Bsmm) limit strongly
disfavors this solution for
mA= 500 GeV
h2>0.13
mh<111GeV
m+<104GeV
Red regions are excluded by either theory or experiments
Green region is the WMAP preferred region
Blue dashed line is the Br(Bsmm) contour
Light blue region excluded by old Bsmm analysis
Excluded by this
new result

29. Method: Likelihood Variable Choice
Prob(l) = probability of
Bsmm yields l>lobs
(ie. the integral of the
cumulative distribution)
Prob(l) = exp(-l/438 mm)
yields flat distribution
reduces sensitivity to
MC modeling inaccuracies
(e.g. L00, SVX-z)

30. Step 4: Compute Acceptance and Efficiencies
Most efficiencies are determined directly from data using inclusive
J/ymm events. The rest are taken from Pythia MC.
a(B+/Bs) = / (CMU-CMU)
= / (CMU-CMX)
eLH(Bs): ranges from 70% for LH>0.9 to
40% for LH>0.99
etrig(B+/Bs) = / (CMU-CMU)
= / (CMU-CMX)
Red = From MC
Green = From Data
Blue = combination of MC
and Data
ereco-mm(B+/Bs) = / (CMU-CMU/X)
evtx(B+/Bs) = / (CMU-CMU/X)
ereco-K(B+) = / (CMU-CMU/X)