Joint LHCb--Belle II Prospects to Constrain New Physics in $B\to D^{(*)}\tau\nu$

TL;DR

This study explores a combined analysis approach using LHCb and Belle II data to improve constraints on new physics effects in semileptonic B decays, employing event reweighting and correlated fits.

Contribution

It introduces a robust, scalable method for simultaneous EFT parameter extraction across multiple datasets, reducing biases and enhancing sensitivity.

Findings

Simultaneous fits with shared parameters outperform independent analyses.

Event reweighting enables efficient exploration of Wilson coefficient space.

Correlated treatment of form factors improves the robustness of constraints.

Abstract

Semileptonic $b\to c\tau\bar{\nu}_{\tau}$ decays are powerful probes of non-Standard-Model effects within an effective-field-theory (EFT) framework, but fully exploiting them in current and future data demands combinations that maximise sensitivity while controlling biases from Standard-Model-based modelling and from theory inputs that are shared, and therefore correlated, across analyses in different experiments. We present a first sensitivity study of a combined extraction of Wilson coefficients in $\bar{B}\to D^{(*)}\tau\bar{\nu}_{\tau}$ decays using LHCb- and Belle~II-like analysis configurations. Detector simulations for signal and backgrounds are typically generated under Standard Model assumptions; if non-SM contributions are present, this can bias the fitted Wilson coefficients. In addition, hadronic inputs such as form-factor parameters of signal and background components are common across analyses, requiring a consistent treatment of fully correlated effects in combinations. To avoid repeating large-scale detector simulation for each EFT hypothesis, we use event-by-event reweighting to map simulated samples to arbitrary combinations of Wilson coefficients. We then compare a simultaneous fit across multiple $\bar{B}\to D^{(*)}\tau\bar{\nu}_{\tau}$ channels and datasets with a combination based on post-fit averages. Sharing Wilson coefficients and common form-factor parameters in the simultaneous fit reduces model-induced biases and improves sensitivity relative to independent fits, providing a robust and scalable strategy for precision EFT constraints in $b\to c\tau\bar{\nu}_{\tau}$ transitions using forthcoming LHCb and Belle~II datasets.