Bs→μμ search strategy in LHCb: Exclusion/discovery potential and SUSY implications

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

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

3. 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< 4.7x10-8 @ 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. 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. 10 -7 2x10 -8 5x10 -9 Non Universal Higgs Masses (NUHM) • 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/2007-138 [arXiv:0709.0098v1 [hep-ph] ] (2007) J.Ellis et. al. CERN-TH/2002-81 [arXiv:hep-ph/0204192] (2002) 36th ITEP Winter School of Physics (2008)

6. 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/2007-136 [ arXiv:0708.2079v1 [hep-ph] ] (2007) 36th ITEP Winter School of Physics (2008)

7. 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 Bsμμ events ~45 per n.y (if SM BR) (Interaction Point) 36th ITEP Winter School of Physics (2008)

8. Invariant Mass PID Geometry 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) 36th ITEP Winter School of Physics (2008)

9. 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. 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+) 36th ITEP Winter School of Physics (2008)

11. 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. 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. “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 < 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 Each Bin : (Bkg Expected – Observed)/ max(1, sqrt(Observed) ) 36th ITEP Winter School of Physics (2008)

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

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

16. 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 ) 36th ITEP Winter School of Physics (2008)

17. BR observed at 3σ In case of Bs  μμObservation 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. Backup Slides 36th ITEP Winter School of Physics (2008)

19. 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)

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21. 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)

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23. s1 s2 s3 . sn b1 b2 b3 . bn x1 x2 x3 . xn n input variables (IP, DOCA…) 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: • 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)