Study of $B \to KK\ell\nu_\ell$ decays at BelleLubej, Matic (Avtor) Zupanc, Anže (Mentor) detektor Bellemezoni Bsemileptonski razpadirazvejitveno razmerjeWe present the branching fraction measurement of the charmless semileptonic decay $B^+ \to K^+K^-\ell^+\nu_\ell$. The measurement has been performed on a data sample corresponding to 710 $\rm{fb^{-1}}$ of integrated luminosity, collected with the Belle detector at the KEKB asymmetric-energy $e^+$$e^-$ collider in Tsukuba, Japan. We present the results obtained with the B2BII data format converter. This is the first measurement of the decay, where we obtain the branching fraction of $\mathcal{B}(B^+ \to K^+K^-\ell^+\nu) = (3.04 \pm 0.51 \pm ^{+0.67} _{-0.66}) \times 10^{-5}$. With the signal significance of 4.6$\sigma$, this measurement counts as the first evidence for the decay.20182018-11-29 11:45:31Doktorsko delo/naloga105432VisID: 95460COBISS_ID: 3271012sl