A New Tool for Detecting BSM Physics in $B\to K^{\ast }\ell \ell$ Decays

arXiv

2203.06827

Date

2022-03-14

Frontier

Rare Processes and Precision Frontier (RF)

Topical groups

• RF1

Submitted by

Alexei Sibidanov

Abstract

Flavor-changing neutral current (FCNC) processes such as $b\to s\ell \ell$ do not occur at tree-level in the Standard Model (SM), which makes them sensitive probes of physics beyond the standard model (BSM). Intriguing BSM hints in $b\to s$ transitions have been observed by multiple flavor physics experiments, LHCb, Belle, BaBar, and Belle II. We have upgraded the widely used event generator EvtGen to model $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$ with improved SM decay amplitudes and amplitudes for possible BSM physics contributions, implemented in the operator product expansion in terms of Wilson coefficients; this upgraded event generator can then be used to investigate the experimental sensitivity to the most general BSM signal resulting from dimension-six operators. We describe the advantages and potential of the newly developed 'Sibidanov Physics Generator' in improving sensitivity of searches and clarifying signatures. Advantages include properly simulating BSM scenarios, interference between SM and BSM amplitudes, and correlations between different BSM observables as well as acceptance bias. We show that exploiting such correlations substantially improves experimental sensitivity. As a demonstration of the proposed approach, we review the prospects for improved measurements with $B\to K^{\ast }\ell \ell$ decays from the expected 50 ab${}^{-1}$ dataset of the Belle II experiment with a four-dimensional unbinned maximum likelihood fit. We describe new promising experimental signatures, and novel ideas such as $\mathrm{\Delta }$-observables, which reduce uncertainties in the SM expectations due to QCD and resonance effects. The $\mathrm{\Delta }$-observables appear ideally suited for Belle II with the large data sets expected in the next decade. Belle II also has excellent sensitivity to New Physics (NP) in the Wilson coefficients $C_7$ and $C_7^{\prime }$, which appear at low $q^2$ in the di-electron channel.