Using the full data sample of 980 fb−1 collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays Ξc0→Ξ0ℓ+ℓ− (ℓ=e or μ). No significant signals are observed in the Ξ0ℓ+ℓ− invariant-mass distributions. Taking the decay Ξc0→Ξ−π+ as the normalization mode, we report 90% credibility upper limits on the branching fraction ratios B(Ξc0→Ξ0e+e−)/B(Ξc0→Ξ−π+)<6.7×10−3 and B(Ξc0→Ξ0μ+μ−)/B(Ξc0→Ξ−π+)<4.3×10−3 based on the phase-space assumption for signal decays. The 90% credibility upper limits on the absolute branching fractions of B(Ξc0→Ξ0e+e−) and B(Ξc0→Ξ0μ+μ−) are found to be 9.9×10−5 and 6.5×10−5, respectively. Published by the American Physical Society 2024

We have analyzed 121.4 fb-1 of data collected at the Gamma(5S) resonance by the Belle experiment using the KEKB asymmetric-energy e(+)e(-) collider to search for the decay B-s(0) -> J/psi pi(0). We observe no signal and report an upper limit on the branching fraction B(B-s(0) -> J/psi pi(0)) of 1.21 x 10(-5) at 90% confidence level. This result is the most stringent, improving the previous bound by 2 orders of magnitude.

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 $\Upsilon(4S)$ and $\Upsilon(5S)$ resonances and has an integrated luminosity of $915~\textrm{fb}^{-1}$, 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.

Using the full data sample of 980fb(-1) collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays Xi(0)(c)->Xi 0l & thorn;l-(l 1/4 eor mu). No significant signals are observed in the Xi 0l & thorn;l-invariant-mass distributions. Taking the de fraction ratios B( Xi(0)(c)->Xi(e0)& thorn;e-& THORN;=B & eth;Xi 0c ->Xi-pi & thorn;& THORN;<6.7x10-3andB & eth;Xi 0c ->Xi 0 mu & thorn;mu-& THORN;=B & eth;Xi 0c ->Xi-pi & thorn;& THORN;<4.3x10-3based on the phase-space assumption for signal decays. The 90% credibility upperlimits on the absolute branching fractions of B & eth;Xi 0c ->Xi 0e & thorn;e-) and B & eth;Xi(0)c ->Xi(0)mu(+)mu(-)) are found to be9.9x10(-5 )and 6.5x10(-5), respectively.

Using the full data sample of 980 fb^-1 collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays Ξ_c^0 →Ξ^0ℓ^+ℓ^- (ℓ=e or μ). No significant signals are observed in the Ξ^0ℓ^+ℓ^- invariant-mass distributions. Taking the decay Ξ_c^0 →Ξ^- π^+ as the normalization mode, we report 90% credibility upper limits on the branching fraction ratios B (Ξ_c^0 →Ξ^0 e^+ e^-) / B(Ξ_c^0→Ξ^-π^+) < 6.7 × 10^-3 and B (Ξ_c^0 →Ξ^0 μ^+ μ^-) / B(Ξ_c^0→Ξ^-π^+) < 4.3 × 10^-3 based on the phase-space assumption for signal decays. The 90% credibility upper limits on the absolute branching fractions of B (Ξ_c^0 →Ξ^0 e^+ e^-) and B (Ξ_c^0 →Ξ^0 μ^+ μ^-) are found to be 9.9 × 10^-5 and 6.5 × 10^-5, respectively.

We have analyzed 121.4 fb^-1 of data collected at the Υ(5S) resonance by the Belle experiment using the KEKB asymmetric-energy e^+e^- collider to search for the decay B_s^0→ J/ψπ^0. We observe no signal and report an upper limit on the branching fraction ℬ(B_s^0→ J/ψπ^0) of 1.21× 10^-5 at 90% confidence level. This result is the most stringent, improving the previous bound by two orders of magnitude.