Search for FCNC Decays Bs(d) → μ+μ-
Matthew Herndon, University of Wisconsin Madison
University of Illinois HETEP Seminar, March 2010
M. Herndon, Illinois HETEP Seminar March 2010
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Why Beyond Standard Model
Standard Model predictions validated to high precision, however
Standard Model fails to answer many fundamental questions
Many of those questions come from Astrophysics and Cosmology
Gravity not a part of the SM
What is the very high energy behaviour?
At the beginning of the universe?
Dark Matter?
Astronomical observations of indicate that there is
more matter than we see
Where is the Antimatter?
Why is the observed universe mostly matter?
Colliders will allow us to establish the nature of this
new physics in the laboratory and study it in detail
M. Herndon, Illinois HETEP Seminar March 2010
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Searches For New Physics
How do you search for new physics at a collider?
Direct searches for production of new particles
Particle-antipartical annihilation: top quark
Indirect searches for evidence of new particles
Within a complex process new particles can occur virtually
LHC is now the energy frontier
Tevatron is at an intensity frontier
billions B and Charm events on tape
So much data that we can look for some very unusual processes
Where to look
Many weak processes involving B hadrons are very low probability
Look for contributions from other low probability processes – Non Standard Model
Rare Decays present unique opportunity to find and study new physics
M. Herndon, Illinois HETEP Seminar March 2010
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Bs(d) → μ+μ- Beyond the SM
Look at processes that are suppressed in the SM
Excellent place to spot small contributions from non SM contributions
Bs(d) → μ+μ-
SM:
No tree level decay
GIM, CKM and helicity
suppressed
BF(Bs → μ+μ-) = 3.8x10-9
˜

New Physics:
Loop: MSSM: mSugra, Higgs Doublet
Rate tan6β/(MA)4
3 orders of magnitude enhancement
˜


Same particles/vertices occur in both B decay diagrams
and in dark matter scattering or 
annihilation diagrams
M. Herndon, Illinois HETEP Seminar March 2010
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
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
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Tevatron and CDF
Tevatron: 2TeV pp collider
CDF properties
Silicon Tracker
EXCELLENT TRACKING
|η|<2, 90cm long, rL00 =1.3 - 1.6cm
Results in this talk uses 3.7fb-1
Drift Chamber(COT)
96 layers between 44 and 132cm
Muon coverage
|η|<1.5
Triggered to |η|<1.0
TRIGGERED TO 1.5 GeV/c
Outer chambers: high purity muons
Bs(d) → μ+μ- benefits from more data
and the excellent CDF detector
M. Herndon, Illinois HETEP Seminar March 2010
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Bs → μμ:Experimental Challenge
Primary problem is large background at hadron colliders
Analysis selection must effectively reduce the large background around
mBs = 5.37GeV/c2 to find a possible handful of events
Key elements of the analysis: Design an effective discriminant,
determine the efficiency for signal and estimating the background level
M. Herndon, Illinois HETEP Seminar March 2010
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Data Sample
TRIGGERS ARE CRITICAL
Several Billion B and Charm Events on Tape
Di-muon CMU-CMU(X)trigger with 5.0 GeV scaler sum pT
CMU: pT(μ) > ~2.0 GeV, |η| < ~0.6, CMX: pT(μ) > ~2.2 GeV, 0.6<|η|<1.0
460M Events
pT cuts: restrict to a well understood trigger region
Apply basic quality cuts
Drift chamber tracks with hits in 3 silicon layers
Likelihood based muon Id and dE/dx to reject hadrons
Vertex quality
Loose preselection on analysis cuts
PT(μ+μ-) > 4.0 GeV/c, 3D Decay length significance > 2
Loose isolation and pointing (defined later)
55K Events
Sample still background dominated
Expect < 20 Bs(d) → μ+μ- events: based on previous limits
M. Herndon, Illinois HETEP Seminar March 2010
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Bs(d) → μ+μ- Method
Relative normalization search
Measure the rate of Bs(d) → μ+μ- decay
relative to B+ J/K+, J/→μ+μApply same sample selection criteria
Systematic uncertainties will cancel
out in the ratios of the normalization
Example: muon trigger eff same for
J/ or Bs s for a given pT
(N cand  N bg )  B + B + f u
BF (Bs    ) 

 
 BsBs
NB +
fs
+

BR(B +  J /K + )  BR(J /   + )
M. Herndon, Illinois HETEP Seminar March 2010
3 X 108 Bs events
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Bs(d) → μ+μ- Method
Estimate all basic selection acceptances and efficiencies.
Identify variables that discriminate signal and background
Validate modelling using B+
Design multivariate discriminant, NN, for background rejection
Unbiased optimization based on Pythia signal MC and part of mass
sidebands
Validate performance on B+ data
Estimate combinatoric background level from sidebands
Separately estimate B→hh
Validate background prediction method in
control regions designed to be enhanced in expected backgrounds
Check low significance signal regions before highest significance region
M. Herndon, Illinois HETEP Seminar March 2010
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Signal vs. Background
+
Need to discriminate signal from background
Reduce background by a factor of ~ 10000
L3D
55K Events
Signal characteristics
L3D
Final state fully reconstructed
P()
-
di-muon vertex
Bs is long lived (cτ = 438 μm)
B fragmentation is hard: few additional tracks
primary vertex
Background contributions and characteristics
+
Sequential semi-leptonic decay: b → cμ-X → μ+μ-X
P()
-
Double semileptonic decay: bb → μ μ X
+ -
Continuum μ+μ-
L3D
L3D
di-muon vertex
μ + fake, fake+fake
Partially reconstructed, lower pT, short lived, doesn’t
primary vertex
point to the primary vertex, and has additional tracks
Cut on mass, lifetime, pT , how well p points to the vertex and isolation10
M. Herndon, Illinois HETEP Seminar March 2010
Discriminating Variables
7 primary discriminating variables
Mass m 2.5σ window: σ = 24MeV/c2
λ=cτ/cτBs, λ/λ, α : |φB – φvtx| in 3D
Isolation: pTB/( trk + pTB)
pTBs, pTlow
Combine in
NN
Unbiased optimization based on simulated
signal and data sidebands: 2fb-1 optimization
Extensively tested for mass bias
Set limits using 3NN bins and 5 mass bins
M. Herndon, Illinois HETEP Seminar March 2010
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Basic Validation
Validation vs. published dataset and B+→J/ψK+ MC
New data ~1.7fb-1
Upgrade: new trigger acceptance muons cross dead region of tracker
Effective 2x
upgrade
Very stable
performance
with time
M. Herndon, Illinois HETEP Seminar March 2010
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Detailed Validation
Validation vs. published dataset and B+→J/ψK+ MC
All preselection variables and discriminating variables
pT and iso not expected to agree. Reweighed to match B+ and Bs data
M. Herndon, Illinois HETEP Seminar March 2010
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NN Validation
Discriminating variable validation using: B+→J/ψK+ MC
pT and isolation reweighing applied
NN validation
Compare performance on
B+→J/ψK+ data and MC
~4% difference assigned as a
systematic uncertainty
~4% uncertainty from pT and iso
reweighing
Can reliably estimate efficiency of NN
M. Herndon, Illinois HETEP Seminar March 2010
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Control Regions
Use independent data samples to test
background estimates
+

OS-: opposite sign muons, negative lifetime
(signal sample is OS+)
SS+ and SS-: same sign muons,
positive and negative lifetime. No trigger matching
** OS-, SS: Opposite side B hadrons
FM: OS- and OS+: fake μ enhanced,
one μ fails the muon Id cuts
primary
vertex
loose vertex cuts
** FM: False muon backgrounds
Fake di-muon
vertex
Compare predicted vs. observed # of bg. events:
For multiple NN cuts
M. Herndon, Illinois HETEP Seminar March 2010
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Control Regions
Comparison of control with signal
region NN input distributions
Do not expect perfect agreement
Isolation different for events
where muons originate from
different b hadrons.
Test background prediction method by
using sidebands to make predictions
in extended signal region
Extended to maximize statistics
Extended signal region: 4σ(mμμ),
5.169 < mμμ < 5.469 GeV
Sideband region: 0.5 GeV on
either side of the signal region
M. Herndon, Illinois HETEP Seminar March 2010
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Control Regions
Background predictions and observed background in control regions
Errors based on statistics of sideband region.
24 Independent checks of the background estimation method
M. Herndon, Illinois HETEP Seminar March 2010
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Expected Sensitivity
Efficiencies and acceptances
NN efficiencies. 0.8<NN<0.95, 12%, 0.95<NN<0.995, 22%, NN>0.995, 44%
We expect substantial signal!
NN>0.8, 1.2 events
0.7 events with NN>0.995
M. Herndon, Illinois HETEP Seminar March 2010
Have reached single event
sensitivity to the SM
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Expected Background
Combinatoric backgrounds: from linear fit to sidbands.
Highest NN bin. Compare to p0 and exponitial fit for systematic uncertainty.
B→hh
Use Bs(d) → μ+μ efficiencies with analytic model of B→hh mass shape
Convolute with muon fake rates measured in D* data
M. Herndon, Illinois HETEP Seminar March 2010
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B  hh Background
Clearly peaks in signal region
Sideband estimates not useful
Convolute known branching ratios and
acceptance with K and  fake rates.
All decays observed/measured at CDF
NN>0.995
N Bs Mass
N Bd Mass
Window
Window
0.074
0.81
Small for Bs
Order of magnitude larger for Bd
B  hh background small but not negligible
M. Herndon, Illinois HETEP Seminar March 2010
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Expected and Observed Data
M. Herndon, Illinois HETEP Seminar March 2010
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DiMuon Mass vs. NN
Mass distributions in three NN bins and vs NN for UU and UX combined
Bs NN>0.995, 6 background expected, 7 events observed, signal 0.7
M. Herndon, Illinois HETEP Seminar March 2010
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Bs(d) → μ+μ- Limits
Set limits using CLs methodology
Systematic uncertainties included
Cross check using Bayesian method, consistent at 5% level
Limit 4.3x10-8 (3.3x10-8 expected)
PVAL 23%, +0.73 sigma
D0 expected sensitivity with 5fb-1: 5.3x10-8
Previous CDF result with 1.9fb-1: 5.8x10-8 (4.8x10-8 expected)
CDF continues to have the world’s best limits
Analysis now background limited and reaching a sensitivity where SM
signal is substantial!
M. Herndon, Illinois HETEP Seminar March 2010
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History and Future
M. Herndon, Illinois HETEP Seminar March 2010
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Bs → μ+μ-: Physics Reach
BF(Bs  +- ) < 4.3x10-8 at 95% CL
Excluded!
A previous result circa 2005
BF(Bs  +- ) = 1.0x10-7
BF(Bs  +- ) = 5x10-8
Dark matter constraints
L. Roszkowski et al. JHEP 0509 2005 029
Strongly limits specific SUSY models:
Typical example of SUSY Constraints
SUSY SO(10) models
Allows for massive neutrino
Incorporates dark matter results
M. Herndon, Illinois HETEP Seminar March 2010
However, large amount of recent
work specifically on dark matter
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Bs → μ+μ- and Dark Matter
Bs → μ+μ- correlated to dark matter searches
CMSSM supergravity model
S. Baek, D.G. Cerdeno Y.G. Kim, P.
Ko, C. Munoz, JHEP 0506 017, 2005
Bs → μ+μ- and neutralino scattering cross sections
are both a strong function of tanβ
In focus point region , high tanβ(50),
positive μ, favoured in CDM allowed fits
Current bounds on Bs → μ+μ- exclude parts
of the dark matter parameter space
Excluded by new
Bs → μ+μtan=50
M. Herndon, Illinois HETEP Seminar March 2010
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Conclusions
B(s,d) → μ+μ- results
BF(Bs  +- ) < 4.3x10-8 at 95% CL
BF(Bd  +- ) < 7.6x10-9 at 95% CL
Worlds Best Limits!
Best Bs and Bd results: well ahead of D0 and the B factories
Limit excludes allowed parameter space of SO(10) models
Expanding sensitivity to interesting areas of MSSM parameter space
Results correlated with some of the other most interesting topics in
physics such as Higgs searches and dark matter!
M. Herndon, Illinois HETEP Seminar March 2010
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Backup
M. Herndon, Illinois HETEP Seminar March 2010
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B Physics and Dark Matter
B Physics constraints impact dark matter in two ways
Dark matter annihilation rates
Interesting for indirect detection experiments
Annihilation of neutralinos
Dark matter scattering cross sections
Interesting for direct detection experiments
Nucleon neutralino scattering cross sections
Models are (n,c)MSSM models with constraints to simplify the parameter space:
Key parameters are tanβ and MA as in the flavour sector along with m1/2
Two typical programs of analysis are performed
Calculation of a specific property: Nucleon neutralino scattering cross sections
Constraints from Bs(d) → μ+μ- and b  s as well as g-2, lower bounds on the Higgs mass, precision
electroweak data, and the measured dark matter density.
General scan of allowed SUSY parameter gives ranges of allowed values
Results can then be compared to experimental sensitivities
M. Herndon, Illinois HETEP Seminar March 2010
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SUSY and Dark Matter
What’s consistent with the constraints?
There are various areas of SUSY
parameter space that are allowed by
flavour, precision electroweak, WMAP
Stau co-annihilation
Funnel 2m ˜  m A

Bulk Region

˜ m˜  m˜

˜


 m , good for LHC
Low m0 and
1/2
Focus Point
Large m0 neutralino becomes higgsino like
Enhanced Higgs exchange scattering diagrams
TeV H. Baer et. al.
Disfavoured by g-2, but g-2 data is controversial
Informs you about what types of dark matter Interactions are interesting
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M. Herndon, Illinois HETEP Seminar March 2010
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Flavour Constraints on m
An analysis uses all available flavour constraints
Bs → μ+μ-, b  s, Bs Oscillations, B  
CMSSM - constrained so that
SUSY scalers and the Higgs
and the gauginos have a
common mass at the GUT scale:
m0 and m1/2 respectively
J. Ellis, S. Heinemeyer, K. Olive, A.M Weber
and G. Weiglein hep-ph/0706.0652
Focus Point
Stau co-annihilation
This region favoured
because of g-2
Definite preferred
neutralino masses
M. Herndon, Illinois HETEP Seminar March 2010
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B Physics and Dark Matter
Putting everything together including most recent theory work on b  s
Analysis shows a preference for
the Focus Point region, g-2
deweighted
Higgsino component of Neutralino
is enhanced.
Enhances dominant Higgs
exchange scattering diagrams
Interesting relative to light Higgs
searches at Tevatron and LHC
Probability in some regions
has gone down
R. Austri, R. Trotta, L. Roszkowski, hep-ph/0705.2012
Current experiments starting to probe interesting regions
M. Herndon, Illinois HETEP Seminar March 2010
However…
S. Baek, et.al.JHEP 0506 017, 2005 32
Bs → μ+μ- and Dark Matter
Bs → μ+μ- correlated to dark matter searches
CMSSM supergravity model
S. Baek, D.G. Cerdeno Y.G. Kim, P.
Ko, C. Munoz, JHEP 0506 017, 2005
Bs → μ+μ- and neutralino scattering cross sections
are both a strong function of tanβ
In focus point region, high tanβ(50),
positive μ, CDM allowed
Current bounds on Bs → μ+μ- exclude parts
of the dark matter parameter space
Excluded by new
Bs → μ+μtan=50
M. Herndon, Illinois HETEP Seminar March 2010
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