Measurement of $B^+\toτ^+ν_τ$ branching fraction with a hadronic tagging method at Belle II

TL;DR

The study measures the branching fraction of $B^+\rightarrow\tau^+\nu_\tau$ using hadronic tagging with Belle II, reconstructing the accompanying $B$ via the Full Event Interpretation and identifying the signal in the recoil across four one-prong $\tau$ decays. A simultaneous 2D maximum-likelihood fit to $E_\text{ECL}^\text{extra}$ and $M_\text{miss}^2$ is performed, with data corresponding to $\mathcal{L}=365\ \text{fb}^{-1}$ at the $\Upsilon(4S)$ resonance. The result, $\mathcal{B}(B^+\rightarrow\tau^+\nu_\tau)=(1.24\pm0.41\text{(stat)}\pm0.19\text{(syst)})\times10^{-4}$, has a significance of $3.0\sigma$ and is consistent with SM predictions and the world average, providing a competitive constraint on $|V_{ub}|$ and on charged-Higgs-type new physics. The analysis employs extensive data-driven calibrations of tagging efficiency, background shapes, and neutral-cluster multiplicities, achieving robust agreement between data and MC across multiple control samples.

Abstract

We present a measurement of the branching fraction of $B^+\toτ^+ν_τ$ decays using $(387\pm6)\times 10^6$ $Υ(4S)$ collected between 2019 and 2022 with the Belle II detector at the SuperKEKB $e^+e^-$ collider. We reconstruct the accompanying $B^-$ meson using the hadronic tagging method, while $B^+\toτ^+ν_τ$ candidates are identified in the recoil. We find evidence for $B^+\toτ^+ν_τ$ decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is $\mathcal{B}(B^+\toτ^+ν_τ) = [1.24 \pm 0.41 (\text{stat.}) \pm 0.19 (\text{syst.})] \times 10^{-4}$.

Figures (11)

• Figure 1: Distributions of $|\cos \theta_\text{T}\xspace|$ (top) and $M_\text{bc}$ (bottom) in off-resonance data and continuum simulation before (left) and after (right) the continuum MC reweighting. The $\tau\xspace^+\tau\xspace^-$ component is negligible after the requirement on $R_2$.
• Figure 2: First row: distributions of $n_{\gamma\mathrm{extra}}$ (a) and $E_\text{ECL}^\text{extra}$ (b) in data and simulation for $E_\text{ECL}^\text{extra}$$<1\mathrm{\,Ge V}\xspace$. Second row: distributions of $E_\text{ECL}^\text{extra}$ with $n_{\gamma\mathrm{extra}}\xspace = 3$ (c) and $n_{\gamma\mathrm{extra}}\xspace = 5$ (d). The number of events in simulation is scaled to the data for (c) and (d) to compare the shapes. The $B^+\rightarrow\xspace\tau^+\nu_\tau$ signal events are a small component of the full sample.
• Figure 3: Distributions of $n_{\gamma\mathrm{extra}}$ (left) and $E_\text{ECL}^\text{extra}$ (right) in data and simulation for $E_\text{ECL}^\text{extra}$$<1\mathrm{\,Ge V}\xspace$ after applying the $n_{\gamma\mathrm{extra}}$ calibration.
• Figure 4: Two-dimensional PDFs of $E_\text{ECL}^\text{extra}$ and $M_{\mathrm{miss}}^2$ from simulation for signal (top) and background (bottom) in the $\tau\xspace^+$$\rightarrow$$e\xspace^+$$\nu\xspace\xspace_e$$\overline{\nu\xspace}\xspace_\tau$ channel (left) (similar for the $\tau\xspace^+$$\rightarrow$$\mu\xspace^+$$\nu\xspace\xspace_\mu$$\overline{\nu\xspace}\xspace_\tau$ channel) and in the $\tau\xspace^+$$\rightarrow$$\pi\xspace\xspace^+$$\overline{\nu\xspace}\xspace_\tau$ (right) (similar for the $\tau\xspace^+$$\rightarrow$$\rho\xspace^+$$\overline{\nu\xspace}\xspace_\tau$ channel). The color represents the PDF probability in each bin.
• Figure 5: First row: distributions of $E_\text{ECL}^\text{extra}$ (left) and $M_{\mathrm{miss}}^2$ (right) with the fit results superimposed. The signal MC is scaled by a factor of 30 to make it visible. Second row: distributions of $E_\text{ECL}^\text{extra}$ with the fit results superimposed for the leptonic channels in the signal enriched region $M_{\mathrm{miss}}^2\xspace>10\ \mathrm{GeV^2/c^4}$. Third row: distributions of $E_\text{ECL}^\text{extra}$ with the fit results superimposed for the hadronic channels in the signal enriched region $M_{\mathrm{miss}}^2\xspace>0.8\ \mathrm{GeV^2/c^4}$.