Light-front quark model and analysis of the spectrum of photons from inclusive B -> X-S gamma decays

The electroweak-decay width Gamma (B --> X(s)gamma) is investigated in a li ght-front (LF) constituent quark model. A new partonlike formula is derived that establishes a simple relation between Gamma (B --> X(s)gamma) and the b --> s gamma decay width. A treatment of the b quark as an on-mass-shell particle and the inclusion of effects that arise from the transverse motion of the b quark in the B meson ale basic features of this approach. Adoptin g different b-quark LF distribution functions, both phenomenological ones a nd those that are derived fi om constituent quark models, and neglecting pe rturbative corrections, we compute the photon energy spectra and the moment s of the shape function. It is shown that the LF approach can be matched co mpletely with a heavy-quark expansion (HQE), provided that the constituent b-quark mass is redefined in a way similar to that used in HQE to define th e pole mass of the b quark. In this way, the correction to first order in 1 /m(b) can be eliminated from the total width in agreement with the general statement of HQE. We also show that the photon energy spectra calculated in the LF approach agree well with those obtained in the model of Altarelli e t al., provided that the same distribution function is used as an input in both cases. Despite the simplicity of the model, our results are in fairly good agreement both with HQE predictions and with available experimental da ta. (C) 2000 MAIK "Nauka/Interperiodica".