James Unwin

Fitting the Galactic Center Gamma-Ray Excess with Cascade Annihilations

The apparent excess of gamma rays in an extended region in the direction of the galactic center has a spatial distribution and amplitude that are suggestive of dark matter annihilations. If this excess is indeed due to dark matter annihilations, it would indicate the presence of both dark matter and an additional particle beyond the Standard Model that mediates the interactions between the dark matter and Standard Model states. We introduce reference models describing dark matter annihilation to pairs of these new mediators, which decouples the SM-mediator coupling from the thermal annihilation cross section and easily explains the lack of direct detection signals. We determine the parameter regions that give good descriptions of the gamma ray excess for several motivated choices of mediator couplings to the SM. We find fermion dark matter with mass 7-26 GeV and a dark vector mediator, or scalar dark matter in the 10-50 GeV range (Higgs portal mediator) or 10-65 GeV range (gluophilic mediator) can provide a comparable or improved fit, compared to the case of direct annihilation. We demonstrate that these models can easily satisfy all constraints from collider experiments, direct detection, and cosmology.

The impact of heavy-quark loops on LHC dark matter searches

A bstractIf only tree-level processes are included in the analysis, LHC monojet searches give weak constraints on the dark matter-proton scattering cross section arising from the exchange of a new heavy scalar or pseudoscalar mediator with Yukawa-like couplings to quarks. In this letter we calculate the constraints on these interactions from the CMS 5.0 fb−1 and ATLAS 4.7 fb−1 searches for jets with missing energy including the effects of heavy-quark loops. We find that the inclusion of such contributions leads to a dramatic increase in the predicted cross section and therefore a significant improvement of the bounds from LHC searches.

X-ray lines from R -parity violating decays of keV sparticles

If R-parity is only mildly violated then the lightest supersymmetric particle (LSP) can be stable over cosmologically time-scales and still account for the dark matter relic density. We examine the possibility of generating detectable X-ray lines from R-parity violating decays of keV-scale LSP dark matter to neutrino-photon pairs. Specifically, we consider scenarios in which the LSP is a light gravitino, bino, or hidden sector photino. Potential signals are discussed in the context of recent claims of an unidentified 3.5 keV X-ray line in studies of stacked galaxy clusters. We comment on the difficulties in obtaining the observed relic density for keV scale bino or hidden photino dark matter and some possible resolutions.

R-symmetric High Scale Supersymmetry

Recent results from ATLAS [1] and CMS [2] based on luminosities of 1 2.3 fb−1 suggest a possible Higgs resonance around 143 GeV. Notably, Hall & Nomora, et al. [3] predicted a Higgs boson mass of mH = 141 ± 2 GeV in a class of supersymmetric standard models with environmentally selection (E-SSM), for large tanβ. This precise prediction of the mass of the Higgs boson was obtained by matching the Higgs quartic coupling λH with the supersymmetric boundary conditionIntroducing an R-symmetry to models of high scale supersymmetry (SUSY) can have interesting consequences, and we focus on two aspects. If Majorana masses are forbidden by an R-symmetry and the main source of electroweak gaugino masses are Dirac terms, then the Higgs quartic coupling vanishing at the SUSY scale and the Higgs boson mass will be near 125 GeV. Moreover, using an R-symmetry, models with only one Higgs doublet in the UV can be constructed and we argue that, since we desire only a single Higgs at the weak scale, this scenario is more aesthetic than existing models. We subsequently present a model which draws on both of these features. We comment on neutrino masses and dark matter in these scenarios and discuss how the models presented can be discerned from alternative constructions with high scale SUSY, including Split SUSY.

UltraViolet freeze-in

A bstractIf dark matter is thermally decoupled from the visible sector, the observed relic density can potentially be obtained via freeze-in production of dark matter. Typically in such models it is assumed that the dark matter is connected to the thermal bath through feeble renormalisable interactions. Here, rather, we consider the case in which the hidden and visible sectors are coupled only via non-renormalisable operators. This is arguably a more generic realisation of the dark matter freeze-in scenario, as it does not require the introduction of diminutive renormalisable couplings. We examine general aspects of freeze-in via non-renormalisable operators in a number of toy models and present several motivated implementations in the context of Beyond the Standard Model (BSM) physics. Specifically, we study models related to the Peccei-Quinn mechanism and Z′ portals.

Annihilation Signals from Asymmetric Dark Matter

A bstractIn the simplest models of asymmetric dark matter (ADM) annihilation signals are not expected, since the DM is non-self-conjugate and the relic density of anti-DM is negligible. We investigate a new class of models in which a symmetric DM component, in the ‘low-mass’ 1-10 GeV regime favoured for linking the DM and baryon asymmetries, is repopulated through decays. We find that, in models without significant velocity dependence of the annihilation cross section, observational constraints generally force these decays to be (cosmologically) slow. These late decays can give rise to gamma-ray signal morphologies differing from usual annihilation profiles. A distinctive feature of such models is that signals may be absent from dwarf spheroidal galaxies.

Axial vector Z ′ and anomaly cancellation

Abstract Whilst the prospect of new Z ′ gauge bosons with only axial couplings to the Standard Model (SM) fermions is widely discussed, examples of anomaly-free renormalisable models are lacking in the literature. We look to remedy this by constructing several motivated examples. Specifically, we consider axial vectors which couple universally to all SM fermions, as well as those which are generation-specific, leptophilic, and leptophobic. Anomaly cancellation typically requires the presence of new coloured and charged chiral fermions, and we argue that in a large class of models masses of these new states are expected to be comparable to that of the axial vector. Finally, an axial vector mediator could provide a portal between SM and hidden sector states, and we also consider the possibility that the axial vector couples to dark matter. If the dark matter relic density is set due to freeze-out via the axial vector, this strongly constrains the parameter space.

Flooded Dark Matter and S Level Rise

A bstractMost dark matter models set the dark matter relic density by some interaction with Standard Model particles. Such models generally assume the existence of Standard Model particles early on, with the dark matter relic density a later consequence of those interactions. Perhaps a more compelling assumption is that dark matter is not part of the Standard Model sector and a population of dark matter too is generated at the end of inflation. This democratic assumption about initial conditions does not necessarily provide a natural value for the dark matter relic density, and furthermore superficially leads to too much entropy in the dark sector relative to ordinary matter. We address the latter issue by the late decay of heavy particles produced at early times, thereby associating the dark matter relic density with the lifetime of a long-lived state. This paper investigates what it would take for this scenario to be compatible with observations in what we call Flooded Dark Matter (FDM) models and discusses several interesting consequences. One is that dark matter can be very light and furthermore, light dark matter is in some sense the most natural scenario in FDM as it is compatible with larger couplings of the decaying particle. A related consequence is that the decay of the field with the smallest coupling and hence the longest lifetime dominates the entropy and possibly the matter content of the Universe, a principle we refer to as “Maximum Baroqueness”. We also demonstrate that the dark sector should be colder than the ordinary sector, relaxing the most stringent free-streaming constraints on light dark matter candidates. We will discuss the potential implications for the core-cusp problem in a follow-up paper. The FDM framework will furthermore have interesting baryogenesis implications. One possibility is that dark matter is like the baryon asymmetry and both are simultaneously diluted by a late entropy dump. Alternatively, FDM is compatible with an elegant non-thermal leptogenesis implementation in which decays of a heavy right-handed neutrino lead to late time reheating of the Standard Model degrees of freedom and provide suitable conditions for creation of a lepton asymmetry.

Superheavy thermal dark matter and primordial asymmetries

A bstractThe early universe could feature multiple reheating events, leading to jumps in the visible sector entropy density that dilute both particle asymmetries and the number density of frozen-out states. In fact, late time entropy jumps are usually required in models of Affleck-Dine baryogenesis, which typically produces an initial particle-antiparticle asymmetry that is much too large. An important consequence of late time dilution, is that a smaller dark matter annihilation cross section is needed to obtain the observed dark matter relic density. For cosmologies with high scale baryogenesis, followed by radiation-dominated dark matter freeze-out, we show that the perturbative unitarity mass bound on thermal relic dark matter is relaxed to 1010 GeV. We proceed to study superheavy asym-metric dark matter models, made possible by a sizable entropy injection after dark matter freeze-out, and identify how the Affleck-Dine mechanism would generate the baryon and dark asymmetries.

Exodus: Hidden origin of dark matter and baryons

A bstractWe propose a new framework for explaining the proximity of the baryon and dark matter relic densities ΩDM ≈ 5ΩB. The scenario assumes that the number density of the observed dark matter states is generated due to decays from a second hidden sector which simultaneously generates the baryon asymmetry. In contrast to asymmetric dark matter models, the dark matter can be a real scalar or Majorana fermion and thus presents distinct phenomenology. We discuss aspects of model building and general constraints in this framework. Moreover, we argue that this scenario circumvents several of the experimental bounds which significantly constrain typical models of asymmetric dark matter. We present a simple supersymmetric implementation of this mechanism and show that it can be used to obtain the correct dark matter relic density for a bino LSP.