604 research outputs found

    Measurement of branching fractions and direct CP asymmetries for B→Kπ and B→ππ decays at Belle II

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    We report measurements of the branching fractions and direct CP asymmetries of the decays B0→K+π−, B+→K+π0, B+→K0π+, and B0→K0π0, and use these for testing the standard model through an isospin-based sum rule. In addition, we measure the branching fraction and direct CP asymmetry of the decay B+→π+π0 and the branching fraction of the decay B0→π+π−. The data are collected with the Belle II detector from e+e− collisions at the Υ(4S) resonance produced by the SuperKEKB asymmetric-energy collider and contain 387×106 bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain −0.03±0.13±0.04 for the sum rule in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations

    Search for dark-matter–Nucleon interactions via Migdal effect with DarkSide-50

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    International audienceDark matter elastic scattering off nuclei can result in the excitation and ionization of the recoiling atom through the so-called Migdal effect. The energy deposition from the ionization electron adds to the energy deposited by the recoiling nuclear system and allows for the detection of interactions of sub-GeV/c2 mass dark matter. We present new constraints for sub-GeV/c2 dark matter using the dual-phase liquid argon time projection chamber of the DarkSide-50 experiment with an exposure of (12 306±184)  kg d. The analysis is based on the ionization signal alone and significantly enhances the sensitivity of DarkSide-50, enabling sensitivity to dark matter with masses down to 40  MeV/c2. Furthermore, it sets the most stringent upper limit on the spin independent dark matter nucleon cross section for masses below 3.6  GeV/c2

    Measurement of the τ\tau-lepton mass with the Belle II experiment

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    We present a measurement of the τ\tau-lepton mass using a sample of about 175 million e+eτ+τe^+e^- \to \tau^+\tau^- events collected with the Belle II detector at the SuperKEKB e+ee^+e^- collider at a center-of-mass energy of 10.579GeV10.579\,\mathrm{Ge\kern -0.1em V}. This sample corresponds to an integrated luminosity of 190fb1190\,\mathrm{fb^{-1}}. We use the kinematic edge of the τ\tau pseudomass distribution in the decay τππ+πντ{\tau^-\to\pi^-\pi^+\pi^-\nu_\tau} and measure the τ\tau mass to be 1777.09±0.08±0.11MeV ⁣/c21777.09 \pm 0.08 \pm 0.11 \,\mathrm{Me\kern -0.1em V\!/c^2}, where the first uncertainty is statistical and the second systematic. This result is the most precise to date

    Precise measurement of the Ds+D^+_s lifetime at Belle II

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    7 pages, 4 figures, to be submitted to Physical Review LettersInternational audienceWe measure the lifetime of the Ds+D_s^+ meson using a data sample of 207 fb1^{-1} collected by the Belle II experiment running at the SuperKEKB asymmetric-energy e+ee^+ e^- collider. The lifetime is determined by fitting the decay-time distribution of a sample of 116×103116\times 10^3 Ds+ϕπ+D_s^+\rightarrow\phi\pi^+ decays. Our result is \tau^{}_{D^+_s} = (498.7\pm 1.7\,^{+1.1}_{-0.8}) fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements

    Search for a long-lived spin-0 mediator in b→s transitions at the Belle II experiment

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    Additional spin-0 particles appear in many extensions of the standard model. We search for long-lived spin-0 particles S in B-meson decays mediated by a b→s quark transition in e+e− collisions at the Υ(4S) resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of 189  fb−1, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions B(B0→K∗(892)0(→K+π−)S)×B(S→x+x−) and B(B+→K+S)×B(S→x+x−), where x+x− indicates e+e−,μ+μ−,π+π−, or K+K−, as functions of S mass and lifetime at the level of 10−7

    Search for dark matter particle interactions with electron final states with DarkSide-50

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    We present a search for dark matter particles with sub-GeV/c2c^2 masses whose interactions have final state electrons using the DarkSide-50 experiment's (12306 ±\pm 184) kg d low-radioactivity liquid argon exposure. By analyzing the ionization signals, we exclude new parameter space for the dark matter-electron cross section σˉe\bar{\sigma}_e, the axioelectric coupling constant gAeg_{Ae}, and the dark photon kinetic mixing parameter κ\kappa. We also set the first dark matter direct-detection constraints on the mixing angle Ue42\left|U_{e4}\right|^2 for keV sterile neutrinos.Comment: 6 pages, 2 figure

    Measurement of branching-fraction ratios and CPCP asymmetries in B±DCP±K±B^{\pm} \to D_{CP\pm}K^{\pm} decays at Belle and Belle II

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    We report results from a study of B±DK±B^\pm \rightarrow DK^\pm decays followed by DD decaying to CPCP~eigenstates, where DD indicates a D0D^0 or Dˉ0\bar{D}^{0} meson. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle ϕ3\phi_{3}. The results are based on a combined analysis of the final data set of 772×106 BBˉ772 \times 10^6~B\bar{B} pairs collected by the Belle experiment and a data set of 198×106 BBˉ198 \times 10^6~B\bar{B} pairs collected by the Belle~II experiment, both in electron-positron collisions at the Υ(4S)\Upsilon(4S) resonance. We measure the CPCP asymmetries to be ACP+= (+12.5±5.8±1.4)%\mathcal{ A}_{CP +} =~(+12.5 \pm 5.8 \pm 1.4)\% and ACP= (16.7±5.7±0.6)%\mathcal{ A}_{CP -} =~(-16.7 \pm 5.7 \pm 0.6)\%, and the ratios of branching fractions to be RCP+= 1.164±0.081±0.036\mathcal{ R}_{CP+}=~1.164 \pm 0.081 \pm 0.036 and RCP= 1.151±0.074±0.019\mathcal{ R}_{CP-} =~1.151 \pm 0.074 \pm 0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries ACP+\mathcal{A}_{CP +} and ACP\mathcal{A}_{CP -} have similar magnitudes and opposite signs; their difference corresponds to 3.5~standard deviations. From these values we calculate 68.3% confidence intervals of (8.5<ϕ3<16.58.5^{\circ}<\phi_{3}<16.5^{\circ}) or (84.5<ϕ3<95.584.5^{\circ}<\phi_{3}<95.5^{\circ}) or (163.3<ϕ3<171.5163.3^{\circ}<\phi_{3}<171.5^{\circ}) and 0.321<rB<0.4650.321<r_{B}<0.465

    Observation of e+e−→ωχbJ(1P) and Search for Xb→ωϒ(1S) at s near 10.75 GeV

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    We study the processes eþ e− → ωχ bJ ð1PÞ (J ¼ 0, 1, or 2) using samples at center-of-mass energies pffiffiffi s ¼ 10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 fb−1 of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the ϒð4SÞ resonance. We report the first observation of ωχ bJ ð1PÞ signals at pffiffiffi pffiffiffi s ¼ 10.745 GeV. By combining Belle II data with Belle results at s ¼ 10.867 GeV, we find energy dependencies of the Born cross sections for eþ e− → ωχ b1;b2 ð1PÞ to be consistent with the shape of the ϒð10753Þ state. These data indicate that the internal structures of the ϒð10753Þ and ϒð10860Þ states may pffiffiffi differ. Including data at s ¼ 10.653 GeV, we also search for the bottomonium equivalent of the Xð3872Þ state decaying into ωϒð1SÞ. No significant signal is observed for masses between 10.45 and 10.65 GeV=c2

    Measurement of the inclusive semileptonic BB meson branching fraction in 62.8 fb1^{-1} of Belle II data

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    International audienceWe report a measurement of the branching fraction of inclusive semileptonic BB meson decays BXcνB\to X_c\ell\nu\ell in Υ(4S)BBˉ\Upsilon(4S)\to B\bar B~data recorded by the Belle II experiment at the SuperKEKB asymmetric-energy e+ee^+e^- collider and corresponding to 62.8 fb1^{-1} of integrated luminosity. Only a charged lepton (electon or muon) is reconstructed and the signal yield is determined from a fit to the lepton momentum distribution in the center-of-mass frame of the colliding beams. Averaging the result in the electron and muon channels, we find B(BXcν)=(9.75±0.03(stat)±0.47(sys))\mathcal {B} (B \rightarrow X_c \ell\nu_{\ell}) = (9.75 \pm 0.03(stat) \pm 0.47(sys))%
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