Terms Of Dark Matter Research Articles

Composite magnetic dark matter and the 130 GeV line

We propose an economical model to explain the apparent 130 GeV gamma ray peak, found in the Fermi/LAT data, in terms of dark matter (DM) annihilation through a dipole moment interaction. The annihilating dark matter particles represent a subdominant component, with mass density 7--17% of the total DM density; and they only annihilate into $\ensuremath{\gamma}\ensuremath{\gamma}$, $\ensuremath{\gamma}Z$, and $ZZ$, through a magnetic (or electric) dipole moment. Annihilation into other standard model particles is suppressed, due to a DM mass splitting in the magnetic dipole case, or to $p$-wave scattering in the electric dipole case. In either case, the observed signal requires a dipole moment of strength $\ensuremath{\mu}\ensuremath{\sim}2/\mathrm{TeV}$. We argue that composite models are the preferred means of generating such a large dipole moment, and that the magnetic case is more natural than the electric one. We present a simple model involving a scalar and fermionic techniquark of a confining SU(2) gauge symmetry. We point out some generic challenges for getting such a model to work. The new physics leading to a sufficiently large dipole moment is below the TeV scale, indicating that the magnetic moment is not a valid effective operator for LHC physics, and that production of the strongly interacting constituents, followed by techni-hadronization, is a more likely signature than monophoton events. In particular, four-photon events from the decays of bound state pairs are predicted.

Radiative events as a probe of dark forces at GeV-scale e + e − colliders

High-luminosity e+ e- colliders at the GeV scale (flavor factories) have been recently recognized to be an ideal environment to search for a light weakly coupled vector boson U (dark photon) emerging in several new physics models able to interpret anomalous astrophysical observations in terms of dark matter. At flavor factories a particularly clean channel is the production of the U boson in association with a photon, followed by the decay of the U boson into lepton pairs. Beyond the approximations addressed in previous works, we revisit the reach potential of this channel by performing an exact lowest-order calculation of the signal and background processes. We also include the effect of initial and final state QED corrections neglected so far, to show how they affect the distributions of experimental interest. We present new results for the expected statistical significance to a dark photon signal at KLOE/KLOE-2 and future super-B factories. The calculation is implemented in a new release of the event generator BabaYaga@NLO, which is available for full event simulations and data analysis.

Gamma-ray and radio constraints of high positron rate dark matter models annihilating into new light particles

The possibility of explaining the positron and electron excess recently found by the PAMELA and ATIC collaborations in terms of dark matter (DM) annihilation has attracted considerable attention. Models surviving bounds from, e.g, antiproton production generally fall into two classes, where either DM annihilates directly with a large branching fraction into light leptons, or, as in the recent models of Arkani-Hamed et al., and of Nomura and Thaler, the annihilation gives low-mass (pseudo)scalars or vectors $\phi$ which then decay into $\mu^+\mu^-$ or $e^+e^-$. While the constraints on the first kind of models have recently been treated by several authors, we study here specifically models of the second type which rely on an efficient Sommerfeld enhancement in order to obtain the necessary boost in the annihilation cross section. We compute the photon flux generated by QED radiative corrections to the decay of $\phi$ and show that this indeed gives a rather spectacular broad peak in $E^2d\sigma/dE$, that for these extreme values of the cross section violate gamma-ray observations of the Galactic center for DM density profiles steeper than that of Navarro, Frenk and White. The most stringent constraint comes from the comparison of the predicted synchrotron radiation in the central part of the Galaxy with radio observations of Sgr A*. For the most commonly adopted DM profiles, the models that provide a good fit to the PAMELA and ATIC data are ruled out, unless there are physical processes that boost the local anti-matter fluxes more than one order of magnitude, while not affecting the gamma-ray or radio fluxes.

Dark matter and Higgs boson physics

A vectorlike colorless fermion doublet and a singlet added to the standard model allow a consistent interpretation of dark matter in terms of the lightest neutral particle, as they may help in obtaining successful gauge coupling unification. We analyze in detail the mass range of the lightest neutral particle below the W mass, i.e. in a range of the parameters where the physics of the standard model Higgs boson may be substantially affected either directly or indirectly.

Status and perspectives of indirect and direct dark matter searches

In this review article the current status of particle dark matter is addressed. We discuss the main theoretical extensions of the standard model which allow to explain dark matter in terms of a (yet undiscovered) elementary particle. We then discuss the theoretical predictions for the searches of particle dark matter: direct detection in low-background underground experiments and indirect detection of neutrinos, gamma-rays and antimatter with terrestrial and space-borne detectors. Attention will be placed also on the discussion of the uncertainties, mainly of astrophysical origin, which affect the theoretical predictions. The constraints placed by these searches on the extensions of the standard models will be briefly addressed.

Charged boson stars and vacuum instabilities

We consider charged boson stars and study their effect on the structure of the vacuum. For very compact particle like “stars”, with constituent mass m ∗ close to the Planck mass m Pl , i.e. m 2 ∗ = O(αm 2 Pl) , we argue that there is electric charge Z c , which, primarily, is due to the formation of a pion condensate ( Z c ≅ 0.5 α −1 e, where α is the fine structure constant and e is the electric charge of the positron). If the charge of the “star” is larger than Z c we find numerical evidence for a complete screening indicating a limiting charge for a very compact object. There is also a less efficient competing charge screening mechanism due to spontaneous electron-positron pair creation in which case Z c ≅ α −1 e. Astrophysical and cosmological abundances of charged compact boson stars are briefly discussed in terms of dark matter.

Cosmic separation of phases

A first-order QCD phase transition that occurred reversibly in the early universe would lead to a surprisingly rich cosmological scenario. Although observable consequences would not necessarily survive, it is at least conceivable that the phase transition would concentrate most of the quark excess in dense, invisible quark nuggets, providing an explanation for the dark matter in terms of QCD effects only. This possibility is viable only if quark matter has energy per baryon less than 938 MeV. Two related issues are considered in appendices: the possibility that neutron stars generate a quark-matter component of cosmic rays, and the possibility that the QCD phase transition may have produced a detectable gravitational signal.