Hunting for the X_b via Radiative Decays

Abstract: In this paper, we study radiative decays of X_b, the counterpart of the famous X(3872) in the bottomonium-sector as a candidate for meson-meson molecule, into the \gamma \Upsilon(nS) (n=1, 2, 3). Since it is likely that the X_b is below the B\bar B^* threshold and the mass difference between the neutral and charged bottom meson is small compared to the binding energy of the X_b, the isospin violating decay mode X_b\to \Upsilon (nS)\pi^+\pi^- would be greatly suppressed. This will promote the importance of the radiative decays. We use the effective Lagrangian based on the heavy quark symmetry to explore the rescattering mechanism and calculate the partial widths. Our results show that the partial widths into \gamma \Upsilon(nS) are about 1 keV, and thus the branching fractions may be sizeable, considering the fact the total width may also be smaller than a few MeV like the X(3872). These radiative decay modes are of great importance in the experimental search for the X_b particularly at hadron collider. An observation of the X_b will provide a deeper insight into the exotic hadron spectroscopy and is helpful to unravel the nature of the states connected by the heavy quark symmetry.