Contributions of vector-like quarks to radiative B meson decay
  We study the decay B\to X_s\gamma in a minimal extension of the standard model with extra up- and down-type quarks whose left- and right-handed components are both SU(2) singlets. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix are obtained from the experimental results for the branching ratio. Even if the extra quarks are too heavy to be detected in near-future colliders, the branching ratio could have a value which is non-trivially different from the prediction of the standard model. 
Mayumi Aoki, Eri Asakawa, Makiko Nagashima, Noriyuki Oshimo, Akio   Sugamoto