Search for: All records

Abstract West Antarctica has experienced dramatic ice losses contributing to global sea-level rise in recent decades, particularly from Pine Island and Thwaites glaciers. Although these ice losses manifest an ongoing Marine Ice Sheet Instability, projections of their future rate are confounded by limited observations along West Antarctica’s coastal perimeter with respect to how the pace of retreat can be modulated by variations in climate forcing. Here, we derive a comprehensive, 12-year record of glacier retreat around West Antarctica’s Pacific-facing margin and compare this dataset to contemporaneous estimates of ice flow, mass loss, the state of the Southern Ocean and the atmosphere. Between 2003 and 2015, rates of glacier retreat and acceleration were extensive along the Bellingshausen Sea coastline, but slowed along the Amundsen Sea. We attribute this to an interdecadal suppression of westerly winds in the Amundsen Sea, which reduced warm water inflow to the Amundsen Sea Embayment. Our results provide direct observations that the pace, magnitude and extent of ice destabilization around West Antarctica vary by location, with the Amundsen Sea response most sensitive to interdecadal atmosphere-ocean variability. Thus, model projections accounting for regionally resolved ice-ocean-atmosphere interactions will be important for predicting accurately the short-term evolution of the Antarctic Ice Sheet. 

Results are presented for the measurement of large-scale anisotropies in the arrival directions of ultra–high-energy cosmic rays detected at the Pierre Auger Observatory during 19 yr of operation, prior to AugerPrime, the upgrade of the observatory. The 3D dipole amplitude and direction are reconstructed above 4 EeV in four energy bins. Besides the established dipolar anisotropy in R.A. above 8 EeV, the Fourier amplitude of the 8–16 EeV energy bin is now also above the 5σdiscovery level. No time variation of the dipole moment above 8 EeV is found, setting an upper limit to the rate of change of such variations of 0.3% yr⁻¹at the 95% confidence level. Additionally, the results for the angular power spectrum are shown, demonstrating no other statistically significant multipoles. The results for the equatorial dipole component down to 0.03 EeV are presented, using for the first time a data set obtained with a trigger that has been optimized for lower energies. Finally, model predictions are discussed and compared with observations, based on two source emission scenarios obtained in the combined fit of spectrum and composition above 0.6 EeV.

 

The flux of ultra-high energy cosmic rays reaching Earth above the ankle energy (5 EeV) can be described as a mixture of nuclei injected by extragalactic sources with very hard spectra and a low rigidity cutoff.Extragalactic magnetic fields existing between the Earth and the closest sources can affect the observed CR spectrum by reducing the flux of low-rigidity particles reaching Earth. We perform a combined fit of the spectrum and distributions of depth of shower maximum measured with the Pierre Auger Observatory including the effect of this magnetic horizon in the propagation of UHECRs in the intergalactic space.We find that, within a specific range of the various experimental and phenomenological systematics, the magnetic horizon effect can be relevant for turbulent magnetic field strengths in the local neighbourhood in which the closest sources lieof order B_rms≃ (50–100) nG (20 Mpc/d_s)( 100 kpc/L_coh)^1/2, with d_sthe typical intersource separation and L_cohthe magnetic field coherence length. When this is the case,the inferred slope of the source spectrum becomes softer and can be closer to the expectations of diffusive shock acceleration, i.e., ∝ E^-2.An additional cosmic-ray population with higher source density and softer spectra, presumably also extragalactic and dominating the cosmic-ray flux at EeV energies, is also required to reproduce the overall spectrum and composition results for all energies down to 0.6 EeV.

 

We measure the branching fraction of the decay$B^{-}\to D^0\rho (770{)}^{-}$using data collected with the Belle II detector. The data contain 387 million$B\overline{B}$pairs produced in$e^{+}{e}^{-}$collisions at the$\mathrm{\Upsilon }(4S)$resonance. We reconstruct$8360\pm 180$decays from an analysis of the distributions of the$B^{-}$energy and the$\rho (770{)}^{-}$helicity angle. We determine the branching fraction to be$(0.939\pm 0.021(\mathrm{stat})\pm 0.050(\mathrm{syst}))\%$, 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 

We report on a search for a resonance$X$decaying to a pair of muons in$e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}X$events in the$0.212–9.000\mathrm{GeV}/c^2$mass range, using$178{\mathrm{fb}}^{-1}$of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of$X$beyond the standard model: a$Z^{\prime }$vector boson in the$L_{\mu }-L_{\tau }$model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the$L_{\mu }-L_{\tau }$model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below$6\mathrm{GeV}/c^2$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the$L_{\mu }-L_{\tau }$model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.

Published by the American Physical Society2024 

We test the predictions of hadronic interaction models regarding the depth of maximum of air-shower profiles,$X_{\max }$, and ground-particle signals in water-Cherenkov detectors at 1000 m from the shower core,$S(1000)$, using the data from the fluorescence and surface detectors of the Pierre Auger Observatory. The test consists of fitting the measured two-dimensional ($S(1000)$,$X_{\max }$) distributions using templates for simulated air showers produced with hadronic interaction models pos-, et--04, 2.3d and leaving the scales of predicted$X_{\max }$and the signals from hadronic component at ground as free-fit parameters. The method relies on the assumption that the mass composition remains the same at all zenith angles, while the longitudinal shower development and attenuation of ground signal depend on the mass composition in a correlated way. The analysis was applied to 2239 events detected by both the fluorescence and surface detectors of the Pierre Auger Observatory with energies between${10}^{18.5}\mathrm{eV}$to${10}^{19.0}\mathrm{eV}$and zenith angles below 60°. We found, that within the assumptions of the method, the best description of the data is achieved if the predictions of the hadronic interaction models are shifted to deeper$X_{\max }$values and larger hadronic signals at all zenith angles. Given the magnitude of the shifts and the data sample size, the statistical significance of the improvement of data description using the modifications considered in the paper is larger than$5\sigma$even for any linear combination of experimental systematic uncertainties.

Published by the American Physical Society2024 

We report a measurement of decay-time-dependent charge-parity ($CP$) asymmetries in$B^0\to K_S^0{K}_S^0{K}_S^0$decays. We use$387\times {10}^6$$B\overline{B}$pairs collected at the$\mathrm{\Upsilon }(4S)$resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the$CP$-violating parameters$S$and$C$from a fit to the distribution of the decay-time difference between the two$B$mesons. The resulting confidence region is consistent with previous measurements in$B^0\to K_S^0{K}_S^0{K}_S^0$and$B^0\to (c\overline{c})K^0$decays and with predictions based on the standard model.

Published by the American Physical Society2024