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Register a new userAstrophysical Limits on Massive Dark Matter
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Annihilations of weakly interacting dark matter particles provide an important signature for the possibility of indirect detection of dark matter in galaxy halos. These self-annihilations can be greatly enhanced in the vicinity of a massive black hole. We show that the massive black hole present at the centre of our galaxy accretes dark matter particles, creating a region of very high particle density. Consequently the annihilation rate is considerably increased, with a large number of $e^+e^-$ pairs being produced either directly or by successive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field, and are able to constrain the allowed values of masses and cross sections of dark matter particles.
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We explore the model-independent constraints from cosmology on a dark-matter particle with no prominent standard model interactions that interacts and thermalizes with other particles in a hidden sector. Without specifying detailed hidden-sector particle physics, we characterize the relevant physics by the annihilation cross section, mass, and temperature ratio of the hidden to visible sectors. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be nonrelativistic. Rather, freeze-out may also occur when dark matter particles are semirelativistic or relativistic. We solve the Boltzmann equation to find the conditions that hidden-sector dark matter accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density, and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. We show that for masses <1.5 keV no region of parameter space satisfies all these constraints. This is a gravitationally-mediated lower bound on the dark-matter mass for any model in which the primary component of dark matter once had efficient interactions -- even if it has never been in equilibrium with the standard model.
I consider constraints from observations on a cutoff scale in clustering due to free streaming of the dark matter in a warm dark matter cosmological model with a cosmological constant. The limits are derived in the framework of a sterile neutrino warm dark matter universe, but can be applied to gravitinos and other models with small scale suppression in the linear matter power spectrum. With freedom in all cosmological parameters including the free streaming scale of the sterile neutrino dark matter, limits are derived using observations of the fluctuations in the cosmic microwave background, the 3D clustering of galaxies and 1D clustering of gas in the Lyman-alpha (Ly-alpha) forest in the Sloan Digital Sky Survey (SDSS), as well as the Ly-alpha forest in high-resolution spectroscopic observations. In the most conservative case, using only the SDSS main-galaxy 3D power-spectrum shape, the limit is m_s > 0.11 keV; including the SDSS Ly-alpha forest, this limit improves to m_s > 1.7 keV. More stringent constraints may be placed from the inferred matter power spectrum from high-resolution Ly-alpha forest observations, which has significant systematic uncertainties; in this case, the limit improves to m_s > 3.0 keV (all at 95% CL).
We discuss how to consistently use Effective Field Theories (EFTs) to set universal bounds on heavy-mediator Dark Matter at colliders, without prejudice on the model underlying a given effective interaction. We illustrate the method for a Majorana fermion, universally coupled to the Standard Model quarks via a dimension-6 axial-axial four-fermion operator. We recast the ATLAS mono-jet analysis and show that a considerable fraction of the parameter space, seemingly excluded by a naive EFT interpretation, is actually still unexplored. Consistently set EFT limits can be reinterpreted in any specific underlying model. We provide two explicit examples for the chosen operator and compare the reach of our model-independent method with that obtainable by dedicated analyses.
We analyze the effects of capture of dark matter (DM) particles, with successive annihilations, predicted in the minimal walking technicolor model (MWT) by the Sun and the Earth. We show that the Super-Kamiokande (SK) upper limit on excessive muon flux disfavors the mass interval between 100-200 GeV for MWT DM with a suppressed Standard Model interaction (due to a mixing angle), and the mass interval between 0-1500 GeV for MWT DM without such suppression, upon making the standard assumption about the value of the local DM distribution. In the first case, the exclusion interval is found to be very sensitive to the DM distribution parameters and can vanish at the extreme of the acceptable values.
We investigate the hypothesis that Coulomb-type interactions between dark matter (DM) and baryons explain the anomalously low 21cm brightness-temperature minimum at redshift z ~ 17 that was recently measured by the EDGES experiment. In particular, we reassess the validity of the scenario where a small fraction of the total DM is millicharged, focusing on newly derived constraints from Planck 2015 cosmic microwave background (CMB) data. Crucially, the CMB power spectrum is sensitive to DM-baryon scattering if the fraction of interacting DM is larger than (or comparable to) the fractional uncertainty in the baryon energy density. Meanwhile, there is a mass-dependent lower limit on the fraction for which the required interaction to cool the baryons sufficiently is so strong that it drives the interacting-DM temperature to the baryon temperature prior to their decoupling from the CMB. If this occurs as early as recombination, the cooling saturates. We precisely determine the viable parameter space for millicharged DM, and find that only a fraction (m_chi/MeV) 0.0115% <~ f <~ 0.4% of the entire DM content, and only for DM-particle masses between 0.5 MeV - 35 MeV, can be charged at the level needed to marginally explain the anomaly, without violating limits from SLAC, CMB, Big-Bang nucleosynthesis (BBN), or stellar and SN1987A cooling. In reality, though, we demonstrate that at least moderate fine tuning is required to both agree with the measured absorption profile and overcome various astrophysical sources of heating. Finally, we point out that a ~0.4% millicharged DM component which is tightly coupled to the baryons at recombination may resolve the current 2-sigma tension between the BBN and CMB determinations of the baryon energy density. Future CMB-S4 measurements will be able to probe this scenario directly.
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