$B\to\pi\ell\ell$ form factors for new-physics searches from lattice QCD

TL;DR

This paper presents the first ab initio lattice QCD calculation of the $B\to\pi$ tensor form factor, providing precise Standard Model predictions for the rare decay $B\to\pi\ell^+\ell^-$, crucial for new physics searches.

Contribution

The work provides the first lattice QCD calculation of the $B\to\pi$ tensor form factor, enhancing the theoretical understanding of $B\to\pi\ell^+\ell^-$ decays for new physics investigations.

Findings

Calculated the $B\to\pi$ tensor form factor $f_T$ from lattice QCD.

Predicted the $B^+\to\pi^+\mu^+\mu^-$ branching ratio as $20.4(2.1)\times10^{-9}$.

Achieved the most precise theoretical prediction to date, consistent with experimental data.

Abstract

The rare decay $B\to\pi\ell^+\ell^-$ arises from $b\to d$ flavor-changing neutral currents and could be sensitive to physics beyond the Standard Model. Here, we present the first $ab$-$initio$ QCD calculation of the $B\to\pi$ tensor form factor $f_T$. Together with the vector and scalar form factors $f_+$ and $f_0$ from our companion work [J. A. Bailey $et~al.$, Phys. Rev. D 92, 014024 (2015)], these parameterize the hadronic contribution to $B\to\pi$ semileptonic decays in any extension of the Standard Model. We obtain the total branching ratio ${\text{BR}}(B^+\to\pi^+\mu^+\mu^-)=20.4(2.1)\times10^{-9}$ in the Standard Model, which is the most precise theoretical determination to date, and agrees with the recent measurement from the LHCb experiment [R. Aaij $et~al.$, JHEP 1212, 125 (2012)]. Note added: after this paper was submitted for publication, LHCb announced a new measurement of the differential decay rate for this process [T. Tekampe, talk at DPF 2015], which we now compare to the shape and normalization of the Standard-Model prediction.