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The E vtGen-based Model for the MonteCarlo Generation of the Rare Radiative Leptonic B-decays
A. V. Popov N. V. Nikitin, D. V. Savrina
(SINP, Lomonosov Moscow State University, ITEP)


Introduction
The main goal of this talk is to present the new Monte-Carlo model for the simulation of the rare radiative leptonic B-mesons decays in the framework of the program package EvtGen. This model is based on the thorough theoretical calculations of these decays in the SM with CPviolation effects. In addition, in this model it is possible to easily change all the input parameters (including CKM-matrix elements in the Wolfenstein parametrization).
Alexander Popov QFTHE P' 2010 , Golitsyno, September 14

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Program package E vtGen
The EvtGen package has been created for the simulation of the bhadrons decays at B-factories. Now this package is used by LHC Collaborations (LHCb, ATLAS, CMS). The basic idea of EvtGen is that each decay is described in terms of helicity amplitudes which are forming a density matrix. So EvtGen makes it possible to correctly simulate the angular and spin correlations in the entire decay chain. The EvtGen package provides a service in which new decays can be simply added as new modules. These modules, which perform the simulation of decays, are called models in EvtGen.

Alexander Popov QFTHE P' 2010 , Golitsyno, September 14

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Rare radiative leptonic B-decays
Rare radiative leptonic decays are induced by flavor changing neutral currents (FCNC) bd, s, which are forbidden at the tree level in the framework of the SM and occur starting from the lowest order only through the one-loop "penguin" and "box" diagrams.
Lowest order SM contributions to the decays.

Channel

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I.Balakireva, D.Melikhov, N.Nikitin, D.Tlisov, Phys.Rev. D81, 054024 (2010). D.Melikhov, N.Nikitin, K.Toms, Phys.Rev. D70, 114028 (2004). F.Kruger, D.Melikhov, Phys.Rev. D67, 034002 (2003).
Alexander Popov QFTHE P' 2010 , Golitsyno, September 14

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Theoretical review
The bq transitions q={d,s} are described using the effective Hamiltonian in the Wilson expansion form with CP-violation effects:

where GF is the Fermi constant, Vtq and Vtb are the CKM matrix elements, u(q)=VubV*uq/VtbV*tq. The set of Wilson coefficients Ci(µ) depends on the chosen model. The scale parameter µ (approximately equal to the b-quark mass ~5 GeV) separates the perturbative and nonperturbative contributions of the strong interactions. Oi(µ) is the set of basic operators. The nonperturbative contributions of the strong interactions are contained in the matrix elements of this operators: It can be described in terms of Lorentz-invariant form factors and structures composed of 4-momenta of the initial and final particles, metrical tensor gµ and Levi-Civita symbol µ.
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E vtGen model for rare radiative leptonic B-decays
We prepare the new EvtGen model BSTOGLLMNT for rare radiative leptonic B-mesons decays. In this model: decay channels of Bd- and Bs-mesons are included: Bd(s)l+l- where l={e,µ,}; th e th e th e c on a re th e form factors are calculated using the dispersion relation of QM and the vector-meson dominance approach; µ-dependence of the Wilson coefficients Ci and the tribution from , , `` etc. vector resonances in the SM i nc l uded; CP-violation effects are included.
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Input parameters for BSTOGLLMNT
There are 9 input parameters specified in BSTOGLLMNT model: mu is the scale parameter µ ~5 GeV; Nf is the number of "effective" quark flavours, used for calculation of the strong interaction running constant S(MZ) (Nf=5 by default); Res_swch is the parameter of switching on/off the resonant contribution (with resonances by default). The area of J/ and 'resonances is excluded in the matrix element; ias defines a choice of the strong interaction running constant S(MZ) value. Egamma is the photon energy cut (20 MeV by default) in the Bmeson rest frame. A, lambda, barrho and bareta are the CKM matrix parameters corresponding the Wolfenstein parametrisation: A, , , and .

Alexander Popov QFTHE P' 2010 , Golitsyno, September 14

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Classes structure of BSTOGLLMNT

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E xample of the

2-distribution q

The normalized distribution for the BSTOGLLMNT (left) and the theoretical prediction from Phys.Rev. D70, 114028 . (2004) (right) for the decays
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E xample of the A FB-distribution

The AFB-distributions for the BSTOGLLMNT (left) and the theoretical prediction from Phys.Rev. D81, 054024 (2010) (right) for the decay .
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Conclusion
We have prepared the EvtGen-based Monte-Carlo generator model for the description of the rare r a d ia tiv e le p to n ic d e c a y s , which includes the resonant contribution and the CPv io la tio n e ffe c ts . We have found a good agreement between the theoretical predictions and the MC results.

Alexander Popov QFTHE P' 2010 , Golitsyno, September 14

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