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

The exclusive radiative $B \to K^* \gamma$, $B \to K^*_2 \gamma$, $B_s \to \phi \gamma$ 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 $B^0 \to K^{*0} \gamma$ we obtain the most stringent bound: $1 \over R>710$ GeV.