Rare decays of Λb→Λγ and Λb→Λl+l- in universal extra-dimension model

The exclusive weak decay of Λb→Λγ and Λb→Λl+l - are investigated in the Appelquist-Cheng-Dobrescu model, which is an extension of the standard model in the presence of universal extra dimensions. Employing the transition form factors obtained in the light-cone sum rules, we analyze how the invariant-mass distribution, the forward-backward asymmetry and the polarization asymmetry of the Λ baryon of these decay modes can be used to constrain the only additional free parameter with respect to the standard model, namely, the radius, R, of the extra dimension. Our results indicate that the Kaluza-Klein modes can lead to approximately 25% suppression of the branching ratio of Λb→Λγ; however, their contributions can bring about 10% enhancement to the decay rate of Λb→Λl+l-. It is shown that in the present scenario the zero position of the forward-backward asymmetry of Λb→Λμ+μ- is sensitive to the compactification parameter R, while the measurement of polarizations of Λ baryon in the Λb decays are not a useful tool to provide any valuable information for the universal extra-dimension model. (orig.)