Bounds on the compactification scale of two universal extra dimensions from exclusive $b\to s\gamma$ decays

The exclusive radiative $B\to K^{\ast }\gamma$, $B\to K_2^{\ast }\gamma$, $B_s \to \phi \gamma$and$\Lambda_b \to \Lambda \gamma$ decays are studied in a new physics scenario with two universal extra dimensions compactified on a chiral square. The computed branching fractions depend on the size $R$ of the extra dimensions, and a comparison with the available measurements allows us to put bounds on such a fundamental parameter. From the mode $B^0\to K^{\ast 0}\gamma$ we obtain the most stringent bound: $\frac{1}{R>710}$ GeV.