Heavy sterile neutrinos in $B$ decays and new QCD corrections to their semi-hadronic decay rates

TL;DR

Heavy sterile neutrinos (HSN) are explored as extensions to the Standard Model motivated by neutrino masses, leptogenesis, and dark matter. The authors model HSN production in $B \rightarrow D^{*} \ell N$ using a dimension-6 operator framework and analyze the impact on Belle II angular distributions, while also computing inclusive hadronic decays $N \rightarrow \ell + \mathrm{had.}$ with QCD corrections up to $\alpha_s^4$ via gauge-boson correlators, establishing perturbative validity thresholds. Their fits to Belle II data show no evidence for new physics up to $m_N \lesssim 62.5\,\mathrm{MeV}$, and a bump at $m_N \approx 354\,\mathrm{MeV}$ in a missing-mass-squared distribution is not statistically significant. The paper provides a robust, high-order QCD-calculated framework for the hadronic decay width, enabling reliable branching-ratio predictions and strengthening constraints on HSN scenarios in flavor physics contexts.

Abstract

In modern experiments on flavour physics it is possible to search for the decays of $B$ or $D$ mesons or $τ$ leptons into final states with heavy neutrinos $N$ (a.k.a. heavy neutral leptons). I present a common study of theorists and experimentalists from Belle II on constraints on $B \rightarrow D^{*} \ell N$. Next I discuss the status of the theory predictions of the various $N$ decay rates. In scenarios in which $N$ interacts with SM particles only through sterile-active neutrino mixing, the dependence of the lifetime on the relevant mixing angles is important to determine whether $N$ decays in the detector or outside. To calculate the inclusive decay rate into semi-hadronic final states reliably one needs to include radiative QCD corrections. I present analytic results for the QCD-corrected decay rates and discuss their phenomenological impact.