Редкие B-распады на установке ATLAS Н.В.Никитин (к Лекциям по B-физике)

Эксперимент LHC LHC – Large Hadron Collider – Большой адронный коллайдер строится в настоящее время в CERNе: pp-столкновения (14 ТэВ). Планируемое начало работы: весна 2007 г. Четыре детектора: ATLAS, CMS – для поиска бозона Хиггса, суперсимметрии и других задач, в том числе исследовании свойств b-адронов; LHCb – оптимизирован под B-физику; ALICE – для исследования свойств КГП.

Мы сконцентрируемся на поиске редких В-распадов на установке ATLAS только потому, что группа МГУ активно принимает участие в данной работе. Следует иметь ввиду, что коллаборации CMS и, особенно, LHCb имеют не менее обширную программу по поиску редких распадов

Introduction - I Physics: b → d, s transitions (FCNC) are forbidden at the tree level in SM and occur at the lowest order through one-loop-diagrams “penguin” and “box”. Main points for study: a) The good test of SM and its possible extensions - SUSY, Two Higgs-doublet, LR, Extra Dimensions; b) Information of the long-distance QCD effects; c) Determination of the │V td │and │V ts │; d) Some of rare decays as BG to other rare decays (for example: B 0 d → π 0 µ + µ - as BG to B 0 s →µ + µ - γ ). 2

Introduction - II Branching Ratios Hierarchi in SM: Br(B 0 d →µ + µ - ) ~ “a few” * Br(B 0 d →µ + µ - γ ) ~ “a few” * Br(B 0 s →µ + µ - ) ~ “a few” * Br(B 0 s →µ + µ - γ ) ~ “a few” * Br(B 0 d → π 0 µ + µ - ) ~ “a few” * Br(B 0 d → ρ 0 µ + µ - ) ~ “a few” * Br(B 0 d → K µ + µ - ) = (4.8 ± 1.2) * (BaBar, Belle, ‘02) Br(B 0 s → φ µ + µ - ) ~ “a few” * Br(B 0 d → K * µ + µ - ) = (1.17 ± 0.33) * (BaBar, Belle, ‘03) Br(B 0 d → K * γ) = (4.3 ± 0.4) * (CLEO, ‘93) 3

Which new measurements can LHC make comparing with B-factories? a) The rare decays of B 0 s – meson (B 0 s →φγ, B 0 s →φ µ + µ -, B 0 s →µ + µ - γ and B 0 s →µ + µ - ) and Λ b - baryon; b) Differencial distributions for rare semileptonic B- meson decays (dimuon-mass spectra, forward- backward asymmetries) – very sensitive to the SM extensions; c) Branching fractions of rare muonic and rare radiative muonic B-meson decays – good sensitivity to the SM extensions. 4

The basic theoretical description -I Effective Hamiltonian for b → d,s transition: H eff (b → q)~ G F V * tq V tb ∑ C i (µ) O i (µ), includes the lowest EW-contributions and perturbative QCD corrections for Wilson coefficients C i (µ). µ - scale parameter ~ 5 GeV : separates SD (perturba- tive) and LD (nonperturbative) contributions of the strong interactions. SM NLO: A.Buras, M.Munz, PRD52, p.182, 1995 SM NNLO: C.Bobeth et al., JHEP 0404, 071, 2004 MSSM NNLO: C.Bobeth et al., hep-ph/

The basic theoretical description -II O i (µ) – set of the basic operators (specific for each model: SM, MSSM, LR and others); LD (nonperturbative) contribution of the strong inte- ractions are contained in the hadronic matrix elements: and are described in the terms of relativistic invariant function - transition formfactors. Need the nonperturbative methods (SR, QM, Lat). 6

The accuracy of calculations Stability of the Wilson coefficients to the choice of m t and μ [m b /2, 2m b ]: SM NLO: approximately 15% ; SM NNLO: approximately 6% - 7% ; MSSM NNLO: ≈ 30% depends from the parameters set. Accuracy of the nonperturbative calculations: depends on a method, but it’s not less, than 15%. For SM calculations – NLO, for MSSM – NNLO. 7

Simulations of rare B-decays for ATLAS detector 8

B 0 d,s →µ + µ - decays in ATLAS Points for study in ATLAS: Branching ratio - sensitive to the SUSY. Simulations: Full Inner detector simulation and reconstruction at low and nominal LHC luminosity 1) for TDR layout signal + background (ATLAS TDR 15, Vol.II, 1999) 2) for Initial layout only signal (ATL-COM-PHYS ). 3) signal + background in DC2. Results: Using SM teoretical predictions: Br(B 0 d, →µ + µ - ) ≈ 1.5* and Br(B 0 d, →µ + µ - ) ≈ 3.5*10 -9 we obtaned the following sensitivities for ATLAS After 3 year LHC work at L=10 33 cm -2 s -1 (30 fb -1 ) will be expected B 0 d : 4 signal ev., B 0 s : 27 signal ev., 93 BG ev. common to both After 1 year LHC work at L=10 34 cm -2 s -1 (100 fb -1 ) will be expected B 0 d : 14 signal ev., B 0 s : 92 signal ev., 660 BG ev. common to both We expect the 3*10 10 sensitivities at CL 95% for rare leptonic decays. 9

B 0 d →K*(892)µ + µ - decay at ATLAS Points for study in ATLAS: Branching ratio - sensitive to the SUSY ; Differencial distributions (dimuon-mass spectra, A FB ) – very good sensitivity to the SUSY. Simulations: Full ATLAS Inner detector simulation and reconstruction at low luminosity (ATLAS TDR 15, Vol.II, 1999) using theoretical matrix element from paper D.Melikhov, N.Nikitin, S.Simula, PRD57, 6814, Results of simulation: After 3 year LHC work at L=10 33 cm -2 s -1 (30 fb -1 ) will be expected ~2000 signal events at 290 BG events 10

BdK*BdK* B s  Preliminary results Non-optimized offline cuts, resolution (>100 MeV/c 2 ) could be improved. Using DC1 simulation tools: estimations including trigger and off-line selection cuts for 1year LHC work at L=2*10 33 cm –2 s -1 (20fb -1 ) : B s  : 3200 signal ev., S/√BG > 7; B d  K *0  : 8500 signal ev., S/√BG > 5.  →     BG rejection under investigation combining π 0 / γ rejection cuts, kinematics and angle between B 0 and K + at K * rest frame cuts. Radiative penguins in ATLAS Points for study in ATLAS: Branching for B s , polarization measurements and CP-violation effects. 11

Baryonic rare decay of Λ b → Λ µ + µ - Points for study in ATLAS: Branching ratio - sensitive to the SUSY ; FB asymmetry - very sensitive to the SUSY. Generation procedure: EvtGen used to generate events using amplitudes from works ( C-H.Chen, C.Q.Geng, PRD64, , 2001 ) and ( T.M.Aliev et.al., NPB649, p , 2003 ). Results of DC1-simulation: At Br(Λ b → Λ (p π - ) µ + µ - ) ~ 2·10 -6 after 1 year LHC work at L=2*10 33 cm -2 s -1 (20 fb -1 ) will be expected ~ 1800 events of the decay Λ b → Λ (p π - ) µ + µ -. 12

B 0 d → π 0 µ + µ - as BG to B 0 d,s →µ + µ - γ and B 0 d,s →µ + µ - decays |η(μ)| 6 GeV, π 0 → γ γ, p T (π 0 ) < 4GeV The decay B 0 d →π 0 µ + µ - are essential background for the decay B 0 d,s →µ + µ - (γ) at the particle level simulation. B 0 s →µ + µ - B 0 d →µ + µ - B 0 d → π 0 µ + µ - Number of events Mµµ Number of events B 0 s →µ + µ - γ B 0 d →µ + µ - γ B 0 d → π 0 µ + µ - Mµµ 16

SUSY in rare B-decays at ATLAS detector

SUSY: main motivations for study in rare B-decays I) Many kinds of heavy (m > 1 TeV) SUSY particles. II) Only lightest of these particles can be detected on LHC. III) Other SUSY-particles give the tiny contributions as virtual particles in SM processes.To find the information on such particles it is necessary to choose the decays : a) where SM contributions are suppressed as much as possible; b) QCD nonperturbative corrections well known; c) branchings can be measured in ATLAS. Rare B 0 d,s and Λ b decays are IDEAL CHOICE for that! 18

MSSM for B 0 d,s →µ + µ - decays and ATLAS sensitivity C.Bobeth et al., PRD66, , (2002) The B 0 d,s →µ + µ - barnchings as functions of charge Higgs boson mass M H for two choice the tan β. 19 tan β = 50 tan β = 60 SM 95% CL for ATLAS sensitivity Br(B 0 q → µ + µ - ) x 10 8

MSSM in B →K * (892) µ + µ - decay and ATLAS precision Sensitivity of A FB to the choice of the Wilson coefficients in one MSSM scenario: P.Cho, M.Misiak, D.Wyller, PRD54, p.3329, Three intervals for variable q 2 /M 2 B. If in the first interval the negative average asymmetry will be measured, it will be convincing demonstration of a SM exten- sions reality. Intervalmin − − −max SM10%-14%-29% MSSM -17 – 5%-35 − -13%~ 30% ATLAS Accuracy 5% 4.5% 6.5% 20