Kavli Affiliate: T. Higuchi

| First 5 Authors: Belle II Collaboration, F. Abudinén, I. Adachi, R. Adak, K. Adamczyk

| Summary:

We present a reconstruction in early data of the semileptonic decay $B^+ to

pi^0 ell^+ nu_ell$, and first results of a reconstruction of the decays

$B^+ to rho^0 ell^+ nu_ell$ and $B^0 to rho^- ell^+ nu_ell$ in a

sample corresponding to 62.8 fb$^{-1}$ of Belle II data using hadronic

$B$-tagging via the full-event-interpretation algorithm. We determine the total

branching fractions via fits to the distribution of the square of the missing

mass, and find $mathcal{B}(B^+ to pi^0 ell^+ nu_ell)$ = (8.29 $pm$

1.99(stat) $pm$ 0.46(syst)) $times 10^{-5}$. We obtain $95%$ CL upper limits

on the branching fractions with $mathcal{B}(B^0 to rho^- ell^+ nu_ell) <

3.37 times 10^{-4}$ and $mathcal{B}(B^+ to rho^0 ell^+ nu_ell) < 19.7

times 10^{-5}$. We also obtain an updated branching fraction for the $B^0 to

pi^- ell^+ nu_ell$ decay, $mathcal{B}(B^0 to pi^- ell^+ nu_ell)$ =

(1.47 $pm$ 0.29(stat) $pm$ 0.05(syst)) $times 10^{-4}$, based on the sum of

the partial branching fractions in three bins of the squared momentum transfer

to the leptonic system.

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