Search for the decay $B^+ \rightarrow K^+τ^+τ^-$ using data from the Belle and Belle II experiments

Abstract

We report a search for the rare decay $B^{+} \rightarrow K^{+} τ^{+} τ^{-}$ using $1.2 \times 10^9$ $Υ(4S)$ mesons produced near threshold in electron-positron collisions and collected by the Belle and Belle~II experiments. We fully reconstruct the hadronic decay of one $B$ meson produced in the $Υ(4S)\rightarrow B^{+} B^{-}$ decay, and search for $B^{\pm}\rightarrow K^{\pm} τ^{+}τ^{-}$ candidates among the remaining collision products, reconstructing a charged kaon and leptonic decays of the $τ$ leptons. We optimize the selection for best sensitivity and look for an excess over background at low values of the residual energy detected in the calorimeter after full event reconstruction. We observe no significant excess and set the limit $\mathcal{B}(B^{+}\rightarrow K^{+}τ^{+}τ^{-})< 0.56\times 10^{-3}$ at the 90% confidence level, improving on the only previous result by a factor of four.

Figures (8)

• Figure 1: Distribution, with simulation overlaid, of opposite-charge kaon-lepton mass in Belle II events. Logarithmic vertical scale in the inset assists visualization of the high kaon-lepton mass analysis region.
• Figure 2: Distributions of $E_{\text{extra}}$ for (circles) data and (histograms) simulation (red) before and (blue) after the linear-scaling fit for (top) Belle and (bottom) Belle II events populating the partner-$B$$\rm M_{bc}$ sideband.
• Figure 3: Distributions of $E_{\text{extra }}$ after final selections in (top) Belle and (bottom) Belle II data. Expectations based on corrected simulation for backgrounds and for $10^4\times$-enhanced SM signal Bouchard:2013mia are overlaid.
• Figure 4: Distribution, with simulation overlaid, of missing mass squared in (top) Belle and (bottom) Belle II events restricted to the $m(K^+\ell^-)> 1.9$ GeV/$c^2$ region.
• Figure 5: Distribution, with simulation overlaid, of c.m. momentum of the signal lepton with same charge as the signal kaon in (top) Belle and (bottom) Belle II events restricted to the $m(K^+\ell^-)> 1.9$ GeV/$c^2$ region.