Search for $B^0 \to K^{\ast 0} τ^+ τ^-$ decays at the Belle II experiment

TL;DR

A search for the rare flavor-changing neutral-current decay at the SuperKEKB electron-positron collider with data collected by the Belle II experiment at the SuperKEKB electron-positron collider is presented.

Abstract

We present a search for the rare flavor-changing neutral-current decay $B^0 \to K^{\ast 0} τ^+ τ^-$ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb$^{-1}$ data sample recorded at the center-of-mass energy of the $Υ(4S)$ resonance. One of the $B$ mesons produced in the $Υ(4S)\to B^0 \bar{B}^0$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{\ast 0}$ and two $τ$ leptons of opposite charge. The $τ$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $ρ$ meson, and additional neutrinos. We set an upper limit on the branching ratio of $BR(B^0 \to K^{\ast 0} τ^+ τ^-) < 1.8 \times 10^{-3}$ at the 90% confidence level, which is the most stringent constraint reported to date.

Figures (2)

• Figure 1: Distribution of $E_\mathrm{extra}$ for events passing the nominal selection, with all the corrections applied, for the $\ell\ell$ signal category. The signal $B^0\rightarrow\xspace K^{*0} \tau\xspace^+\tau\xspace^-\xspace$ histogram is shown scaled assuming a branching fraction of $10^{-2}$.
• Figure 2: Distributions of $\eta(\mathrm{BDT})$ in the SR for the four signal categories. The fit results are shown for the two background components ($B\xspace\xspace\overline{ B\xspace}{}\xspace$ and $q\xspace\xspace\overline q\xspace\xspace\xspace$) and the $B^0\rightarrow\xspace K^{*0} \tau\xspace^+\tau\xspace^-\xspace$ signal, with a fitted branching fraction of $\left[-0.15 \pm 1.01 \right]\times 10^{-3}$. A $B^0\rightarrow\xspace K^{*0} \tau\xspace^+\tau\xspace^-\xspace$ signal distribution, scaled assuming a branching fraction of $10^{-2}$, is shown as reference. The bottom panel shows the pull distributions.