Precision calculations of $B\to K^*$ form factors from SCET sum rules beyond leading-power contributions

TL;DR

This paper improves theoretical predictions for $B\to K^*$ form factors by calculating subleading-power corrections using light-cone sum rules, and applies these results to predict rare decay branching fractions and polarization fractions.

Contribution

It systematically computes subleading-power corrections to $B\to K^*$ form factors within SCET sum rules, enhancing the precision of theoretical predictions beyond leading power.

Findings

Subleading-power contributions are about 30% of leading-power results.

Predicted branching fractions for $B \to K^* \nu\bar{\nu}$ decays.

Estimated longitudinal polarization fraction $F_L=0.44(4)$.

Abstract

We construct light-cone sum rules (LCSR) for the $B\to K^*$ form factors in the large recoil region using vacuum-to-$B$-meson correlation functions, and systematically calculate subleading-power corrections to these form factors at tree level, including next-to-leading power contributions from the hard-collinear propagator, the subleading effective current $\bar{q}\Gamma[i\slashed{D}_{\perp}/(2m_b)]h_v$, and twist-five/six four-particle higher-twist effects. By incorporating the available leading-power results at $\mathcal{O}(\alpha_s)$ and the corrections to higher-twist $B$-meson light-cone distribution amplitudes from our previous work, we improve the precision of theoretical predictions for $B\to K^*$ form factors and find that the subleading-power contributions amount to 30\% of the corresponding leading-power results. Employing the Bourrely-Caprini-Lellouch (BCL) parametrization, we determine the numerical results for $B\to K^*$ form factors across the full kinematic range through a combined fit of LCSR predictions in the large recoil region and lattice QCD results in the small recoil region. Using the newly obtained $B\to K^*$ form factors, we compute the branching fractions for the rare decays $B \to K^* \nu_\ell\bar{\nu}_\ell$ in the Standard Model, obtaining $\mathcal{BR}(\bar{B}^0 \to \bar{K}^{*0} \nu_\ell\bar{\nu}_\ell)=8.09(96)\times 10^{-6}$ and $\mathcal{BR}(\bar{B}^+ \to \bar{K}^{*+} \nu_\ell\bar{\nu}_\ell)=9.95(1.05)\times 10^{-6}$. Additionally, we predict that the longitudinal $K^*$ polarization fraction is $F_L=0.44(4)$.