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  1. Abstract

    Galaxy clusters are expected to be both dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay atγ-ray energies and are predicted to be sources of large-scaleγ-ray emission due to hadronic interactions in the intracluster medium (ICM).In this paper, we estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuseγ-ray emission from the Perseus galaxy cluster.We first perform a detailed spatial and spectral modelling of the expected signal for both the DM and the CRp components. For each case, we compute the expected CTA sensitivity accounting for the CTA instrument response functions. The CTA observing strategy of the Perseus cluster is also discussed.In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratioX500within the characteristic radiusR500down to aboutX500< 3 × 10-3, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index αCRp= 2.3. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure αCRpdown to about ΔαCRp≃ 0.1 and the CRp spatial distribution with 10% precision, respectively. Regarding DM, CTA should improve the current ground-basedγ-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to ∼ 5, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models withτχ> 1027s for DM masses above 1 TeV.These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.

     
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    Free, publicly-accessible full text available October 1, 2025
  2. Context.The nearby elliptical galaxy M87 contains one of only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio toγ-ray energies) took part in the second M87 EHT campaign.

    Aims.The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity.

    Methods.The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high-energy (VHE)γ-rays as well as details of the individual observations and light curves. We also conducted phenomenological modelling to investigate the basic source properties.

    Results.We present the first VHEγ-ray flare from M87 detected since 2010. The flux above 350 GeV more than doubled within a period of ≈36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image.

    Conclusions.Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHEγ-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and it emphasises the need for combined image and spectral modelling.

     
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    Free, publicly-accessible full text available December 1, 2025
  3. Aims.We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE.

    Methods.The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC,Fermi-LAT,NuSTAR,XMM-Newton,Swift, and several optical and radio telescopes to complement IXPE data.

    Results.During the IXPE exposures, the measured 0.2–1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σsignificance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2–5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, theSwift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation,NuSTARdata reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.

     
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    Free, publicly-accessible full text available April 1, 2025
  4. We measure the branching fraction of the decayBD0ρ(770)using data collected with the Belle II detector. The data contain 387 millionBB¯pairs produced ine+ecollisions at theϒ(4S)resonance. We reconstruct8360±180decays from an analysis of the distributions of theBenergy and theρ(770)helicity angle. We determine the branching fraction to be(0.939±0.021(stat)±0.050(syst))%, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  5. Abstract

    Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

     
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    Free, publicly-accessible full text available July 1, 2025
  6. We report a measurement of decay-time-dependent charge-parity (CP) asymmetries inB0KS0KS0KS0decays. We use387×106BB¯pairs collected at theϒ(4S)resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract theCP-violating parametersSandCfrom a fit to the distribution of the decay-time difference between the twoBmesons. The resulting confidence region is consistent with previous measurements inB0KS0KS0KS0andB0(cc¯)K0decays and with predictions based on the standard model.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  7. We search for the rare decayB+K+νν¯in a362fb1sample of electron-positron collisions at theϒ(4S)resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanyingBmeson inϒ(4S)BB¯events to suppress background from other decays of the signalBcandidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanyingBmeson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for theB+K+νν¯branching fraction of[2.7±0.5(stat)±0.5(syst)]×105and[1.10.8+0.9(stat)0.5+0.8(syst)]×105, respectively. Combining the results, we determine the branching fraction of the decayB+K+νν¯to be[2.3±0.5(stat)0.4+0.5(syst)]×105, providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  8. A<sc>bstract</sc>

    We report results from a study ofB±→ DK±decays followed byDdecaying to theCP-even final stateK+Kand CP-odd final state$$ {K}_S^0{\pi}^0 $$KS0π0, whereDis an admixture ofD0and$$ {\overline{D}}^0 $$D¯0states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angleϕ3. The results are based on a combined analysis of the final data set of 772×106$$ B\overline{B} $$BB¯pairs collected by the Belle experiment and a data set of 198×106$$ B\overline{B} $$BB¯pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be$$ \mathcal{A} $$ACP+= (+12.5±5.8±1.4)% and$$ \mathcal{A} $$ACP−= (16.7±5.7±0.6)%, and the ratios of branching fractions to be$$ \mathcal{R} $$RCP+= 1.164±0.081±0.036 and$$ \mathcal{R} $$RCP−= 1.151±0.074±0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries$$ \mathcal{A} $$ACP+and$$ \mathcal{A} $$ACP−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< 16.5°) or (84.5°<ϕ3< 95.5°) or (163.3°<ϕ3< 171.5°) and 0.321 <rB< 0.465.

     
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    Free, publicly-accessible full text available May 1, 2025
  9. Free, publicly-accessible full text available April 1, 2025