  We show that the B \to X(s) gamma photon energy (E_gamma) spectrum can be
reliably computed by resummed perturbation theory. Our calculation is based on
Dressed Gluon Exponentiation (DGE) incorporating Sudakov and renormalon
resummation. It is shown that the resummed spectrum does not have the
perturbative support properties: it smoothly extends to the non-perturbative
region E_gamma > m/2, where m is the quark pole mass, and tends to zero near
the physical endpoint. The calculation of the Sudakov factor, which determines
the shape of the spectrum in the peak region, as well as that of the pole mass,
which sets the energy scale, are performed using Principal-Value Borel
summation. By using the same prescription in both, the cancellation of the
leading renormalon ambiguity is respected. Furthermore, in computing the
Sudakov exponent we go beyond the formal next-to-next-to-leading logarithmic
accuracy using the large-order asymptotic behavior of the series, which is
accurately determined from the relation with the pole mass. Upon matching the
resummed result with the next-to-leading order expression we compute the
spectrum, obtain its moments as a function of a minimum photon energy cut,
analyze sources of uncertainty and show that our predictions are in good
agreement with Belle data.

High Energy Physics - Phenomenology