Semileptonic $B^{-}\to\pi^{+}\pi^{-}\ell^{-}\bar\nu_\ell$ decayover the full $\pi\pi$ invariant mass spectrum

Abstract The Belle Collaboration has recently reported a measurement of the branching fraction 
for the semileptonic decay $B^{-}\to\pi^{+}\pi^{-}\ell^{-}\bar\nu_\ell$, where $\ell=e$ or $\mu$.
Utilizing the newly available data over the full $\pi\pi$ invariant mass spectrum, 
we determine the non-resonant $B\to\pi\pi$ transition form factors.
We obtain a non-resonant branching fraction of
${\cal B}_N(B^{-}\to\pi^{+}\pi^{-}\ell^{-}\bar\nu_\ell)=(3.5\pm 1.4^{+4.3}_{-2.4})\times 10^{-5}$. 
This result indicates that the non-resonant contribution can be comparable 
in magnitude to the resonant components, and therefore should not be regarded 
as a negligible background in precision studies.
Our findings highlight the importance of dedicated experimental efforts 
at Belle II and LHCb to further probe the non-resonant contribution. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.