New Physics in $ b \to s τ^+τ^-$ Processes and Correlations with $R(D^{(*)})$ and $B\to K^{(*)}ν\barν$: An (SM)EFT Analysis

Abstract

Tauonic flavour-changing neutral-current transitions ($b\to sτ^+τ^-$) are sensitive probes of New Physics connected to third-generation quarks and leptons. Motivated by the persistent anomalies in $R(D^{(*)})$ and the indications of enhanced $B^+\to K^{+}ν\barν$ rates, we revisit and update the predictions for $b\to sτ^+τ^-$ processes. Building upon a previous work on a data-driven approach to include the dominant $ψ(2S)$ resonances, we provide predictions for $B\to K^{(*)}τ^+τ^-$ ($B_s\toϕτ^+τ^-$) over the full kinematic range. To include possible New Physics contributions within the Weak Effective Theory, we provide semi-analytic expressions for the corresponding branching ratios. We then perform a comprehensive dimension-6 SMEFT analysis of New Physics realised at the TeV scale. We identify regions of parameter space preferred by current data where the branching ratios of $b\to sτ^+τ^-$ processes can be enhanced by several orders of magnitude. While current experimental sensitivities remain far above the SM rates, the enhancements suggested by present flavour data imply that forthcoming limits from LHCb, CMS, and Belle~II will probe significant regions of the relevant SMEFT parameter space. Furthermore, we highlight that while different scenarios can lead to the same predictions for $B\to K^{(*)}ν\barν$ branching ratios, they can be disentangled by measuring $B\to K^{(*)}τ^+τ^-$, $B_s\toτ^+τ^-$ and $B_s\toϕτ^+τ^-$.