ANALYSIS OF THE PHOTON SPECTRUM IN INCLUSIVE B-]X(S)GAMMA DECAYS

Using a combination of the operator product and heavy quark expansions, we resum the leading nonperturbative contributions to the inclusive photon spectrum in B --> X(s) gamma decays. The shape of the spectrum is determined by a universal structure function, which describes the distribution of the light-cone momentum of the b quark inside the B meson. The moments of this function are proportional to forward matrix elements of higher-dimension operators. As a by-product, we obtain the bound lambda1 < 0 for one of the parameters of the heavy quark effective theory. An integral over the B --> X(s) gamma structure function is related to the shape function that governs the fall off of the lepton spectrum close to the end point in B --> X(u)lnuBAR decays. A measurement of the photon spectrum in rare B decays can therefore help to obtain a model-independent determination of V(ub).