Search for Axionlike Particles Produced in e⁺ e⁻ Collisions at Belle II

International audienceWe present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e+e-→γa, a→γγ in the mass range 0.2

Search for Axionlike Particles Produced in e+e- Collisions at Belle II

We present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e+e-→γa, a→γγ in the mass range 0.2<9.7 GeV/c2 using data corresponding to an integrated luminosity of (445±3) pb-1. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axionlike particles or ALPs) are frequently postulated in extensions of the standard model. We find no evidence for ALPs and set 95% confidence level upper limits on the coupling strength gaγγ of ALPs to photons at the level of 10-3 GeV-1. The limits are the most restrictive to date for 0.2<1 GeV/c2

Precise Measurement of the D0^{0} and D+^{+} Lifetimes at Belle II

We report a measurement of the D$^{0}$ and D$^{+}$ lifetimes using D$^{0}$→K$^{-}$π$^{+}$ and D$^{+}$→K$^{-}$π$^{+}$π$^{+}$ decays reconstructed in e$^{+}$e$^{-}$→$\overline{cc}$ data recorded by the Belle II experiment at the SuperKEKB asymmetric-energy e$^{+}$e$^{-}$ collider. The data, collected at center-of-mass energies at or near the Υ(4S) resonance, correspond to an integrated luminosity of 72 fb$^{-1}$. The results, τ(D$^{0}$)=410.5±1.1(stat)±0.8(syst)  fs and τ(D$^{+}$)=1030.4±4.7(stat)±3.1(syst) fs, are the most precise to date and are consistent with previous determinations

Measurement of the integrated luminosity of the Phase 2 data of the Belle II experiment

From April to July 2018, a data sample at the peak energy of the γ(4S) resonance was collected with the Belle II detector at the SuperKEKB electron-positron collider. This is the first data sample of the Belle II experiment. Using Bhabha and digamma events, we measure the integrated luminosity of the data sample to be (496.3 ± 0.3 ± 3.0) pb-1, where the first uncertainty is statistical and the second is systematic. This work provides a basis for future luminosity measurements at Belle II

Observation of B→D(∗)K−KS0{B\to D^{(*)} K^- K^{0}_S} decays using the 2019-2022 Belle II data sample

We present a measurement of the branching fractions of four $B^{0,-}\to D^{(*)+,0} K^- K^{0}_S$ decay modes. The measurement is based on data from SuperKEKB electron-positron collisions at the $\Upsilon(4S)$ resonance collected with the Belle II detector and corresponding to an integrated luminosity of ${362~\text{fb}^{-1}}$. The event yields are extracted from fits to the distributions of the difference between expected and observed $B$ meson energy to separate signal and background, and are efficiency-corrected as a function of the invariant mass of the $K^-K_S^0$ system. We find the branching fractions to be: $$\text{B}(B^-\to D^0K^-K_S^0)=(1.89\pm 0.16\pm 0.10)\times 10^{-4},$$ $$\text{B}(\overline B{}^0\to D^+K^-K_S^0)=(0.85\pm 0.11\pm 0.05)\times 10^{-4},$$ $$\text{B}(B^-\to D^{*0}K^-K_S^0)=(1.57\pm 0.27\pm 0.12)\times 10^{-4},$$ $$\text{B}(\overline B{}^0\to D^{*+}K^-K_S^0)=(0.96\pm 0.18\pm 0.06)\times 10^{-4},$$ where the first uncertainty is statistical and the second systematic. These results include the first observation of $\overline B{}^0\to D^+K^-K_S^0$, $B^-\to D^{*0}K^-K_S^0$, and $\overline B{}^0\to D^{*+}K^-K_S^0$ decays and a significant improvement in the precision of $\text{B}(B^-\to D^0K^-K_S^0)$ compared to previous measurements