On the decay mode ${\Lambda }_b\to X_s\gamma$

Abstract

We study the inclusive $H_b\to X_s\gamma$ decay with $H_b$ a beauty baryon, in particular ${\Lambda }_b$, employing an expansion in the heavy quark mass at $O(m_b^{-3})$ at leading order in ${\alpha }_s$, keeping the dependence on the hadron spin. For a polarized baryon we compute the distribution $\frac{d^2\Gamma }{dyd\cos {\theta }_P}$, with $y=2E_{\gamma }/m_b$, $E_{\gamma }$ the photon energy and ${\theta }_P$ the angle between the baryon spin vector and the photon momentum in the $H_b$ rest-frame. We discuss the correlation between the baryon and photon polarization, and show that effects of physics beyond the Standard Model can modify the photon polarization asymmetry. We also discuss a method to treat the singular terms in the photon energy spectrum obtained by the OPE.

Comments: Latex 23 pages, 3 figures. Matches the published version