5 Search Results

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $$426\,\text {fb}^{-1}$$ of data collected at the energy-asymmetric $$e^{+}e^{-}$$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $$\Upsilon$$ (4S).

Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles 𝜒₁ and 𝜒₂, with 𝑚⁡(𝜒₂) >𝑚⁡(𝜒₁), is induced by a dark Higgs field and a corresponding dark Higgs boson ℎ′. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a 365 fb⁻¹ data sample collected at Belle II, which operates at the SuperKEKB 𝑒⁺⁢𝑒⁻ collider,more » we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎⁡(𝑒⁺⁢𝑒⁻ → ℎ′⁡𝜒₁⁢𝜒₂), and the product of branching fractions ℬ⁡(𝜒₂ → 𝜒_1⁢𝑒⁺⁢𝑒⁻) ×ℬ⁡(ℎ′ → 𝑥⁺⁢𝑥⁻), where 𝑥⁺⁢𝑥⁻ indicates 𝜇⁺⁢𝜇⁻, 𝜋⁺⁢𝜋⁻, or 𝐾⁺⁢𝐾⁻, as functions of ℎ′ mass and lifetime at the level of 10⁻¹ fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 10⁻⁵ and on the dark photon kinetic mixing parameter at the level of 10⁻³. This is the first search for dark Higgs bosons in association with inelastic dark matter.« less

The K-long and muon detector (KLM) constitutes the outer-most volume of the Belle II spectrometer at the interaction region of the SuperKEKB collider in Tsukuba, Japan. The KLM detector was partially upgraded since the Belle experiment by replacing many of its resistive-plate chambers with scintillators containing wavelength-shifting fibers and instrumenting it with silicon photomultipliers. Here, we describe the readout electronics, firmware, and software created to control and acquire data from the scintillators and resistive-plate chambers.

We present measurements of the first to fourth moments of the lepton mass squared $$q$$² of $$B$$ → $$X_cℓ\overline{ν}_ℓ$$ decays for $ℓ = e, μ$ and with $$X_c$$ a hadronic system containing a charm quark. These results use a sample of electron-positron collisions at the $$Υ$$($4S$) resonance corresponding to 62.8 fb^-1 of integrated luminosity and collected by the Belle II 2 experiment in 2019 and 2020. To identify the $$X_c$$ system and reconstruct $$q$$², one of the $$B$$ mesons from an $$Υ$$($4S$) → $$B\overline{B}$$ decay is fully reconstructed in a hadronic decay mode using a multivariate $$B$$ tagging algorithm. Wemore » report raw and central moments for $$q$$² > 1.5 $GeV$²=$$c$$⁴ up to $$q$$² > 8.5 $GeV$²=$$c$$⁴, probing up to 77% of the accessible $$B$$ → $$X_cℓ\overline{ν}_ℓ$$ phase space. This is the first measurement of moments in the experimentally challenging range of [1.5; 2.5] $GeV$²=$$c$$⁴. The results can be used for a new determination of |$$V_{cb}$$| using inclusive $$B$$ → $$X_cℓ\overline{ν}_ℓ$$ decays.« less

An absolute measurement of the Λ$$^+_c$$ lifetime is reported using Λ$$^+_c → pK^– π^+$$ decays in events reconstructed from data collected by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The total integrated luminosity of the data sample, which was collected at center-of-mass energies at or near the $$\textit{Υ}(4S)$$ resonance, is 207.2 fb^–1. The result, $$τ(Λ^+_c)$$ = 203.20 ± 0.89 ± 0.77 fs , where the first uncertainty is statistical and the second