1. Bs  μ+μ- in LHCb Diego Martínez Santos
Universidade de Santiago de Compostela (USC)
PROGRAMA NACIONAL DE
BECAS FPU
36th ITEP Winter School of Physics (2008)

2. 36th ITEP Winter School of Physics (2008)
Motivation
LHCb conditions
Bs  μμ search strategy
Exclusion/discovery potential of LHCb
SUSY implications examples ?
36th ITEP Winter School of Physics (2008)

3. 36th ITEP Winter School of Physics (2008)
Motivation: BR (Bs  μμ ) sensitive to New Physics (NP)
BR (Bs  μμ ) in SM: (3.55 ± 0.33)x10-9 (for Bd : (1.00 ± 0.14)x10-10)
Current limit (Tevatron) : BR< 90% C.L, (for Bd :BR < 1.5x10-8)
 One order of magnitude from current upper limit to SM prediction !!
36th ITEP Winter School of Physics (2008)

4. 36th ITEP Winter School of Physics (2008)
Motivation: BR (Bs  μμ ) sensitive to New Physics (NP) ? +
This BR can be affected by New Physics, in particular SUSY models.
MSSM: Enhancement ~tanβ6  larger value
 Measurement/exclusion of BR (Bs  μμ ) implies constraints on NP (SUSY) parameters !!
36th ITEP Winter School of Physics (2008)

5. Best Chi2 of SUSY fit BR ~ 10-8 Non Universal Higgs Masses (NUHM)
10 -7
2x10 -8
5x10 -9
Simliar to CMSSM, but avoiding the condition of universal Higgs masses
 generalization of CMSSM useful for study the Higgs sector
Best Chi2 of SUSY fit
BR ~ 10-8
J.Ellis et. al. CERN-PH-TH/ [arXiv: v1 [hep-ph] ] (2007)
J.Ellis et. al. CERN-TH/ [arXiv:hep-ph/ ] (2002)
36th ITEP Winter School of Physics (2008)

6. 36th ITEP Winter School of Physics (2008)
Maximal CP violation Minimal Flavor Violation (MCPMFV)-MSSM
m1/2 = 250 GeV
m0 = 100 GeV
A0 = 100 GeV
ΦAGUT = 0
FCNC’s mediated only by CKM (i.e., vanish for CKM = I3)
Then, added maximum number of CP violating phases
Useful for CPV studies in SUSY
Enhancement for large tanβ
BR < SM allowed, (special case of cancellation between SM contribution and NP pseudoscalar contribution)
Even < than SM !!
[i] J.Ellis et. al. CERN-PH-TH/ [ arXiv: v1 [hep-ph] ] (2007)
36th ITEP Winter School of Physics (2008)

7. 36th ITEP Winter School of Physics (2008)
LHCb conditions
b physics experiment
Low angle spectrometer, but at center of masses
b produced at low angle
~5 x 1011 bb/fb-1
( ~ per n.year)
Trigger dedicated to select b events ( ~90% for reconstructed Bs  μμ)
Total number of Bsμμ events ~45 per n.y (if SM BR)
(Interaction Point)
36th ITEP Winter School of Physics (2008)

8. 36th ITEP Winter School of Physics (2008)
Bs  μμ search strategy (I)
Reconstruction of μ+ μ- combinations on triggered events
Selection: apply some cuts on discriminant variables to remove the most important amount of background
Classify each event using three properties (bins in a 3D phase space):
Particle Identification (PID): Probability to be muons
Geometrical properties
Invariant Mass
Get the number of expected bkg. in the Bs mass region
using sidebands (events outside ±60 MeV of Bs Mass)
Use of control channels to get the probability, for
a signal event, to fall in each bin:
Geometry & Mass: Use of B h+h-
PID: Calibration muons (MIPs in calorimeter,
J/Ψ muons)
Geometry
Invariant Mass
PID
36th ITEP Winter School of Physics (2008)

9. 36th ITEP Winter School of Physics (2008)
Bs  μμ search strategy (II)
Compare expected bkg and signal with observed  No. of observed
signal events or compatibility with only bkg.
Translate No. of evts  BR using a known control channel (B+  J/Ψ K+)
 (= normalization)
fraction of reconstruction / selection & trigger efficiencies
Hadronization fractions
( fBs = P(b Bs) )
Main source of uncertainty (13 %) for normalization with B+  J/Ψ K+ (or any other B+/Bd channel)
36th ITEP Winter School of Physics (2008)

10. 36th ITEP Winter School of Physics (2008)
Bs  μμ Event Selection
A (almost) common selection for Bs  μμ, B+ J/Ψ K+& Bh+h- is used:
Avoid different biases in geometrical properties for B h+h- & Bs  μμ
Reduces uncertainties in the fraction of selection efficiencies when computing BR
Uses basicaly B hadron properites & 2 track vertex properties
Also IPS of muons (Bs  μμ), hadrons (B  hh) or kaon (B+  J/ΨK+)
Some (necessary) differences : J/Ψ invariant mass cut (B+  J/ΨK+), hits in muon chambers (Bs  μμ, B+  J/Ψ (μμ)K+)
Cut
Eff (B+  J/Ψ K+)
B mass(500 MeV)
97.3 %
JPsi Chi2 < 14
98.3 %
B ips < 6
97.4 %
DOFS(JPsi) > 12
74.0 %
Bpt > 700 MeV
98.4 %
K+ IPS > 3.5
92.1 %
JPsiMass (60 MeV)
94.2 %
Cut
Eff (Bs  μμ)
B mass(600 MeV)
97.3 %
BsChi2 < 14
97.9 %
B ips < 6
98.8 %
DOFS(Bs) > 12
75.5 %
Bs pt > 700 MeV
97.7 %
mu+ ,mu- IPS > 3.5
93.0 %
36th ITEP Winter School of Physics (2008)

11. 36th ITEP Winter School of Physics (2008)
Geometrical Variables
lifetime
muon Impact Parameter Significant (IPS)
Red: signal
Blue: bb inc.
Black: b μ
b μ
Green:
Bc+  J/Ψμν
Isolation
DOCA: distance between tracks making the vertex
B Impact Parameter (IP ) to PV
Isolation: Idea: muons making fake Bs→μμ might came from another SV’s  For each muon; remove the other μ and look at the rest of the event: How many good - SV’s (forward, DOCA, pointing) can it make?
less μIPS
(arbitrary
normalization)
Bs IP (mm)
DOCA (mm)
lifetime (ps)
36th ITEP Winter School of Physics (2008)

12. 36th ITEP Winter School of Physics (2008)
Geometrical Likelihood
Those variables combined in a likelihood (procedure in backup slides)
Geometry Likelihood (GL) for signal (& B h+h-) is flat from 0 to 1
Bkg peaks at 0
High discriminant power
Sensitive region (SR): ~ GL > 0.5, (& Invariant mass inside ± 60 MeV window)
(arbitrary
normalization)
Expected events per nominal LHCb year in SR:
Signal (SM) : 22.8
Bkg: 150
Note: 22.8/ sqrt(150) = 1.9 SR
36th ITEP Winter School of Physics (2008)

13. 36th ITEP Winter School of Physics (2008)
“One day” experiment SR
Example of ~one nominal day experiment (full simulation of bb dimuon, only bkg)
Expected bkg from sidebands
0 events observed in SR
Nsig < 95 % CL
Nsig < 90% CL
 BR < 1.2 x 95 % CL
 BR < 9.4 x 90 % CL
Each Bin :
(Bkg Expected – Observed)/ max(1, sqrt(Observed) )
36th ITEP Winter School of Physics (2008)

14. 36th ITEP Winter School of Physics (2008)
LHCb potential
BR observed at 3σ
BR excluded at 90 % CL
Expected limit at the
end of Tevatron
Note: 1 nominal year = 2 fb-1
36th ITEP Winter School of Physics (2008)

15. 36th ITEP Winter School of Physics (2008)
LHCb potential
BR excluded at 90 % CL
Expected limit at the
end of Tevatron
10 -7
2x10 -8
5x10 -9
36th ITEP Winter School of Physics (2008)

16. 36th ITEP Winter School of Physics (2008)
mSUGRA-implications example
CMSSM parameter values chosen:
m1/2 in [0, 1400 GeV]
m0 in [0,1400 GeV]
A0 = 0
μ >0
Other constraints:
h0 > 114 GeV
mW = ± GeV
calculations using the program SoftSUSY from Ben Allanch (Cambridge) ; BR’s computed using program from Athanasios Dedes (Durham )
36th ITEP Winter School of Physics (2008)

17. 36th ITEP Winter School of Physics (2008)
In case of Bs  μμ Observation
BR observed at 3σ
Phase Space region compatible with BR(Bsμμ) ~ 1x10 -8
mSUGRA Phase Space is strongly reduced as function of the BR seen (and its accuracy)
BR ~ 3x10 -9
36th ITEP Winter School of Physics (2008)

18. 36th ITEP Winter School of Physics (2008)
Backup Slides
36th ITEP Winter School of Physics (2008)

19. 36th ITEP Winter School of Physics (2008)
Correlation for signal (very small for background)
signal independent
Gaussian variables (for signal)
signal independent
Gaussian variables (for background)
 Same procedure making a 2D Gaussian for Background
36th ITEP Winter School of Physics (2008)

20. 36th ITEP Winter School of Physics (2008)

21. 36th ITEP Winter School of Physics (2008)
CMSSM and relation with g-2
CMSSM: Relation with Muon Anomalous Magnetic Dipole Moment
aμ = (g -2)/2
Current value of aμ - aμ(SM)  if tanß ~ 50 gaugino mass are in ~400 – 600 GeV  BR(Bs  μμ) ~ 1-4 x 10 -8
Sensitive to several other models
aμ - aμ(SM)
BR (Bs  μμ )
36th ITEP Winter School of Physics (2008)

22. 36th ITEP Winter School of Physics (2008)

23. 36th ITEP Winter School of Physics (2008)
Method for variable-combination s1 s2 s3 . sn b1 b2 b3 bn x1 x2 x3 xn
n input variables
(IP, DOCA…)
For constructing Geometry & PID likelihoods, we have made some operations over the input variables. Trying to make them uncorrelated
A very similar method is described by Dean Karlen,
Computers in Physics Vol 12, N.4, Jul/Aug 1998
The main idea:
n variables which, for signal, are independent and Gaussian (sigma 1) -distributed
χ2S = Σ si2
same, but for background
χ2B = Σ bi2
χ2 = χ2S - χ2B
And make it uniform for signal (flat distribution)
36th ITEP Winter School of Physics (2008)