Muon-induced neutrons do not explain the DAMA data

We present an accurate model of the muon-induced background in the DAMA/LIBRA experiment. Our work challenges proposed mechanisms which seek to explain the observed DAMA signal modulation with muon-induced backgrounds. Muon generation and transport are performed using the MUSIC/MUSUN code, and subsequent interactions in the vicinity of the DAMA detector cavern are simulated with Geant4. We estimate the total muon-induced neutron flux in the detector cavern to be $\Phi_n^\nu = 1.0\times10^{-9}$ cm$^{-2}$ s$^{-1}$. We predict $3.49\times10^{-5}$ counts/day/kg/keV, which accounts for less than $0.3\%$ of the DAMA signal modulation amplitude.Comment: 6 pages, 4 figures, accepted for publication in PR

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

Characterizing surface deformation throughout a full earthquake cycle is a challenge due to the lack of high‐resolution geodetic observations of duration comparable to that of characteristic earthquake recurrence intervals (250–10,000 years). Here we approach this problem by comparing long‐term geologic slip rates with geodetically derived fault slip rates by sampling only a short fraction (0.001%–0.1%) of a complete earthquake cycle along 15 continental strike‐slip faults. Geodetic observations provide snapshots of surface deformation from different times through the earthquake cycle. The timing of the last earthquake on many of these faults is poorly known, and may vary greatly from fault to fault. Assuming that the underlying mechanics of the seismic cycle are similar for all faults, geodetic observations from different faults may be interpreted as samples over a significantly larger fraction of the earthquake cycle than could be obtained from the geodetic record along any one fault alone. As an ensemble, we find that geologically and geodetically inferred slip rates agree well with a linear relation of 0.94±0.09. To simultaneously explain both the ensemble agreement between geologic and geodetic slip‐rate estimates with observations of rapid postseismic deformation, we consider the predictions from simple two‐layer earthquake‐cycle models with both Maxwell and Burgers viscoelastic rheologies. We find that a two‐layer Burgers model, with two relaxation timescales, is consistent with observations of deformation throughout the earthquake cycle, whereas the widely used two‐layer Maxwell model with a single relaxation timescale, is not, suggesting that the earthquake cycle is effectively characterized by a largely stress‐recoverable rapid postseismic stage and a much more slowly varying interseismic stage.Earth and Planetary Science

Curvature invariants in type N spacetimes

Scalar curvature invariants are studied in type N solutions of vacuum Einstein's equations with in general non-vanishing cosmological constant Lambda. Zero-order invariants which include only the metric and Weyl (Riemann) tensor either vanish, or are constants depending on Lambda. Even all higher-order invariants containing covariant derivatives of the Weyl (Riemann) tensor are shown to be trivial if a type N spacetime admits a non-expanding and non-twisting null geodesic congruence. However, in the case of expanding type N spacetimes we discover a non-vanishing scalar invariant which is quartic in the second derivatives of the Riemann tensor. We use this invariant to demonstrate that both linearized and the third order type N twisting solutions recently discussed in literature contain singularities at large distances and thus cannot describe radiation fields outside bounded sources.Comment: 17 pages, to appear in Class. Quantum Gra