bs μ + μ - in lhcb

36th ITEP Winter School of Physics (2008)

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Bs μ+μ- in LHCb

PROGRAMA NACIONAL DEBECAS FPU

Diego Martínez Santos(ДИЕГО МАРТИНЕЗ САНТОC)

Universidade de Santiago de Compostela (USC)


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• Motivation

• LHCb conditions

• Bs μμ search strategy

• Exclusion/discovery potential of LHCb

• SUSY implications examples

?


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Motivation: BR (Bs μμ ) sensitive to New Physics (NP)

•BR (Bs μμ ) in SM:SM: (3.55 (3.55 ± 0.33)x10± 0.33)x10-9-9 (for Bd : (1.00 ± 0.14)x10-10)

•Current limit (Tevatron) : BR< 4.7x10BR< 4.7x10-8-8 @ 90% C.L, @ 90% C.L, (for Bd :BR < 1.5x10-8)

One order of magnitude from current upper limit to SM prediction !!


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Motivation: BR (Bs μμ ) sensitive to New Physics (NP)

+

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This BR can be affected by New Physics, in particular SUSY models.MSSM: Enhancement ~tanβ6 larger value Measurement/exclusion of Measurement/exclusion of BR (Bs BR (Bs μμμμ ) implies constraints on NP (SUSY) ) implies constraints on NP (SUSY) parameters !!parameters !!


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10 -7

2x10 -8

5x10 -9

Non Universal Higgs Masses (NUHM)

J.Ellis et. al. CERN-PH-TH/2007-138 [arXiv:0709.0098v1 [hep-ph] ] (2007)J.Ellis et. al. CERN-TH/2002-81 [arXiv:hep-ph/0204192] (2002)

•Simliar to CMSSM, but avoiding the condition of universal Higgs masses

generalization of CMSSM useful for study the Higgs sector

Best Chi2 of SUSY fitBR ~ 10-8


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Maximal CP violation Minimal Flavor Violation (MCPMFV)-MSSM

[i] J.Ellis et. al. CERN-PH-TH/2007-136 [ arXiv:0708.2079v1 [hep-ph] ] (2007)

Even < than SM !!

m1/2 = 250 GeVm0 = 100 GeVA0 = 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)


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LHCb conditions

•b physics experiment

•Low angle spectrometer, but at center of masses

•b produced at low angle

•~5 x 1011 bb/fb-1 •( ~ 10 12 per n.year)

•Trigger dedicated to select b events ( ~90% for reconstructed Bs μμ)

•Total number of Bsμμ events ~45 per n.y (if SM BR)

(Interaction Point)


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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 regionusing 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


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•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)

Bs μμ search strategy (II)

nBs

nTRIGSELREC

TRIGn

SELn

RECnn

N

N

f

fBRBR

Hadronization fractions

( fBs = P(b Bs) )

Main source of uncertainty (13 %) for normalization with B+ J/Ψ K+ (or any other B+/Bd channel)

fraction of reconstruction / selection & trigger efficiencies


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Bs μμ Event Selection

• A (almost) common selection for Bs μμ, B+ J/Ψ K+& Bh+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 %


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Geometrical Variables

• lifetime• muon Impact Parameter Significant (IPS)

• 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?

(arbitrary normalization)

Red: signalBlue: bb inc.Black: b μ b μGreen: Bc+ J/Ψμν

less μIPS

Bs IP (mm)

Isolation

lifetime (ps)DOCA (mm)


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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)

SR

Expected events per nominal LHCb year in SR:

Signal (SM) : 22.8Bkg: 150

Note: 22.8/ sqrt(150) = 1.9


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“One day” experiment

Each Bin : (Bkg Expected – Observed)/ max(1, sqrt(Observed) )

SR

•Example of ~one nominal day experiment (full simulation of bb dimuon, only bkg)

•Expected bkg from sidebands

•0 events observed in SR

•Nsig < 4.5 @ 95 % CL

•Nsig < 3.5 @ 90% CL

BR < 1.2 x 10-7 @ 95 % CL

BR < 9.4 x 10-8 @ 90 % CL


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LHCb potential

0.01 0.1

10

50

BR

(x10

-9)

Luminosity (fb -1)

0.5

SM prediction

BR excluded at 90 % CL

Expected limit at the end of Tevatron

0 2 4 6 8 101

10

BR

(x1

0-9)

Luminosity (fb-1)

SM prediction

BR observed at 3σ

Note: 1 nominal year = 2 fb-1


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0.01 0.1

10

50

BR

(x10

-9)

Luminosity (fb -1)

0.5

SM prediction

BR excluded at 90 % CL

Expected limit at the end of Tevatron

LHCb potential

10 -7

2x10 -8

5x10 -9


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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 = 80.398 ± 0.025 GeV

calculations using the program SoftSUSY from Ben Allanch (Cambridge) ; BR’s computed using program from Athanasios Dedes (Durham )


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In case of Bs μμ Observation

mSUGRA Phase Space is strongly reduced as function of the BR seen (and its accuracy) BR ~ 3x10 -9

Phase Space region compatible with BR(Bsμμ) ~ 1x10 -8

0 2 4 6 8 101

10

BR

(x10

-9)

Luminosity (fb-1)

SM prediction

BR observed at 3σ


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Backup Slides


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Correlation for signal (very small for background)

signal independentGaussian variables (for signal)

signal independentGaussian variables (for background)

Same procedure making a 2D Gaussian for Background


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CMSSM and relation with g-2

•CMSSM: Relation with Muon Anomalous Magnetic Dipole Momentaμ = (g -2)/2Current 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 )


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Method for variable-combination

•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:

s1s2s3 .sn

b1b2b3 .bn

x1x2x3 .xn

n input variables(IP, DOCA…)

n variables which, for signal, are independent and Gaussian (sigma 1) -distributed

χ2S = Σ si

2

same, but for background

χ2B = Σ bi

2

χ2 = χ2S - χ2

B

And make it uniform for signal (flat distribution)

bs μ + μ - in lhcb

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