Forbidden Dark Matter

Dark Matter (DM) may be a thermal relic that annihilates into heavier states in the early Universe. This Forbidden DM framework accommodates a wide range of DM masses from keV to weak scales. An exponential hierarchy between the DM mass and the weak scale follows from the exponential suppression of the thermally averaged cross section. Stringent constraints from the cosmic microwave background are evaded because annihilations turn off at late times. We provide an example where DM annihilates into dark photons, which is testable through large DM self-interactions and direct detection.Comment: 6 Pages, 3 Figures; Version published in PR

Displaced Vertices from X-ray Lines

We present a simple model of weak-scale thermal dark matter that gives rise to X-ray lines. Dark matter consists of two nearly degenerate states near the weak scale, which are populated thermally in the early universe via co-annihilation with slightly heavier states that are charged under the Standard Model. The X-ray line arises from the decay of the heavier dark matter component into the lighter one via a radiative dipole transition, at a rate that is slow compared to the age of the universe. The model predicts observable signatures at the LHC in the form of exotic events with missing energy and displaced leptons and jets. As an application, we show how this model can explain the recently observed 3.55 keV X-ray line.Comment: 20 pages, 7 figure

The Origin of the Spatial Distribution of X-ray luminous AGN in Massive Galaxy Clusters

We study the spatial distribution of a 95% complete sample of 508 X-ray point sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations of 51 massive galaxy clusters found in the MACS survey. Covering the redshift range z=0.3-0.7, our cluster sample is statistically complete and comprises all MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4 keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain clusters. We find the XPS surface density, computed in the cluster restframe, to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The excess, believed to be caused by AGN in the cluster, is significant at the 8.0 sigma confidence level compared to the XPS density observed at the field edges. No significant central excess is found in the control fields. To investigate the physical origin of the AGN excess, we study the radial AGN density profile for a subset of 24 virialized clusters. We find a pronounced central spike (r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad secondary excess centered at approximately the virial radius of the host clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects increased nuclear activity triggered by close encounters between infalling galaxies and the giant cD-type elliptical occupying the very cluster center. By contrast, the secondary excess at the cluster-field interface is likely due to black holes being fueled by galaxy mergers. In-depth spectroscopic and photometric follow-up observations of the optical counterparts of the XPS in a subset of our sample are being conducted to confirm this picture.Comment: ApJ Letters, accepted (4 pages, 3 figures, uses emulateapj

Charged Fermions Below 100 GeV

How light can a fermion be if it has unit electric charge? We revisit the lore that LEP robustly excludes charged fermions lighter than about 100 GeV. We review LEP chargino searches, and find them to exclude charged fermions lighter than 90 GeV, assuming a higgsino-like cross section. However, if the charged fermion couples to a new scalar, destructive interference among production channels can lower the LEP cross section by a factor of 3. In this case, we find that charged fermions as light as 75 GeV can evade LEP bounds, while remaining consistent with constraints from the LHC. As the LHC collects more data, charged fermions in the 75-100 GeV mass range serve as a target for future monojet and disappearing track searches.Comment: 35 pages, 11 figures, 2 table

A Fourth Exception in the Calculation of Relic Abundances

We propose that the dark matter abundance is set by the decoupling of inelastic scattering instead of annihilations. This coscattering mechanism is generically realized if dark matter scatters against states of comparable mass from the thermal bath. Coscattering points to dark matter that is exponentially lighter than the weak scale and has a suppressed annihilation rate, avoiding stringent constraints from indirect detection. Dark matter upscatters into states whose late decays can lead to observable distortions to the blackbody spectrum of the cosmic microwave background.Comment: 8 pages, 6 figures. V3: figure adde