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We report a search for a heavy neutral lepton (HNL) that mixes predominantly with${\nu }_{\tau }$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy$e^{+}{e}^{-}$collider. The data sample was collected at and just below the center-of-mass energies of the$\mathrm{\Upsilon }(4S)$and$\mathrm{\Upsilon }(5S)$resonances and has an integrated luminosity of$915{\mathrm{fb}}^{-1}$, corresponding to$(836\pm 12)\times {10}^6$$e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$events. We search for production of the HNL (denoted$N$) in the decay${\tau }^{-}\to {\pi }^{-}N$followed by its decay via$N\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }$. The search focuses on the parameter-space region in which the HNL is long-lived, so that the${\mu }^{+}{\mu }^{-}$originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the$\tau$neutrino as a function of the HNL mass.

We report on a search for a resonance$X$decaying to a pair of muons in$e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}X$events in the$0.212–9.000\mathrm{GeV}/c^2$mass range, using$178{\mathrm{fb}}^{-1}$of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of$X$beyond the standard model: a$Z^{\prime }$vector boson in the$L_{\mu }-L_{\tau }$model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the$L_{\mu }-L_{\tau }$model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below$6\mathrm{GeV}/c^2$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the$L_{\mu }-L_{\tau }$model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.

ProtoDUNE Single-Phase (ProtoDUNE-SP) is a 770-ton liquid argon time projection chamber that operated in a hadron test beam at the CERN Neutrino Platform in 2018. We present a measurement of the total inelastic cross section of charged kaons on argon as a function of kaon energy using 6 and$7\mathrm{GeV}/c$beam momentum settings. The flux-weighted average of the extracted inelastic cross section at each beam momentum setting was measured to be$380\pm 26\mathrm{mbarns}$for the$6\mathrm{GeV}/c$setting and$379\pm 35\mathrm{mbarns}$for the$7\mathrm{GeV}/c$setting.

We measure the tau-to-light-lepton ratio of inclusive$B$-meson branching fractions$R(X_{\tau /\ell })\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, where$\ell$indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed$B$meson and a charged lepton candidate from$189{\mathrm{fb}}^{-1}$of electron-positron collision data collected with the Belle II detector. We find$R(X_{\tau /\ell })=0.228\pm 0.016(\mathrm{stat})\pm 0.036(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of$R(X_{\tau /\ell })$.