Journal Club

Seminar Room, Mondays at 14:00

Monday 18th of April, 2016

Condensation of Galactic Cold Dark Matter

We consider the steady-state regime describing the density profile of a dark matter halo, if dark matter is treated as a Bose-Einstein condensate. We first solve the fluid equation for "canonical" cold dark matter, obtaining a class of density profiles which includes the Navarro-Frenk-White profile, and which diverge at the halo core. We then solve numerically the equation obtained when an additional "quantum pressure" term is included in the computation of the density profile. The solution to this latter case is finite at the halo core, possibly avoiding the "cuspy halo problem" present in some cold dark matter theories. Within the model proposed, we predict the mass of the cold dark matter particle to be of the order of M_chi c2 = 10^-2 eV, which is of the same order of magnitude as that predicted in ultra-light scalar cold dark matter models. Finally, we derive the differential equation describing perturbations in the density and the pressure of the dark matter fluid.
Comments: 19 pages, 4 figures, submitted to JCAP
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:1509.05871 [astro-ph.CO]
  (or arXiv:1509.05871v2 [astro-ph.CO] for this version)

 

Presented by M. Masip

 

The High-Energy Tail of the Galactic Center Gamma-Ray Excess

Observations by the Fermi-LAT have uncovered a bright, spherically symmetric excess surrounding the center of the Milky Way galaxy. The spectrum of the gamma-ray excess peaks sharply at an energy ~2 GeV, exhibiting a hard spectrum at lower energies, and falls off quickly above an energy ~5 GeV. The spectrum of the excess above ~10 GeV is potentially an important discriminator between different physical models for its origin. We focus our study on observations of the gamma-ray excess at energies exceeding 10 GeV, finding: (1) a statistically significant excess remains in the energy range 9.5-47.5 GeV, which is not degenerate with known diffuse emission templates such as the Fermi Bubbles, (2) the radial profile of the excess at high energies remains relatively consistent with data near the spectral peak (3) the data above ~5 GeV prefer a slightly greater ellipticity with a major axis oriented perpendicular to the Galactic plane. Using the recently developed non-Poissonian template fit, we find mild evidence for a point-source origin for the high-energy excess, although given the statistical and systematic uncertainties we show that a smooth origin of the high-energy emission cannot be ruled out. We discuss the implication of these findings for pulsar and dark matter models of the gamma-ray excess. Finally we provide a number of updated measurements of the gamma-ray excess, utilizing novel diffuse templates and the Pass 8 dataset.
Comments: 40 pages, 36 figures, To Be Submitted to PRD
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:1604.01026 [astro-ph.HE]
  (or arXiv:1604.01026v1 [astro-ph.HE] for this version)

 

Presented by M. Masip

 

A First-Order Electroweak Phase Transition in the Standard Model from Varying Yukawas

We show that the dynamics responsible for the variation of the Yukawa couplings of the Standard Model fermions generically leads to a very strong first-order electroweak phase transition, assuming that the Yukawa couplings are large and of order 1 before the electroweak phase transition and reach their present value afterwards. There are good motivations to consider that the flavour structure could emerge during electroweak symmetry breaking, for example if the Froggatt-Nielsen field dynamics were linked to the Higgs field. In this paper, we do not need to assume any particular theory of flavour and show in a model-independent way how the nature of the electroweak phase transition is completely changed when the Standard Model Yukawas vary at the same time as the Higgs is acquiring its vacuum expectation value. The thermal contribution of the fermions creates a barrier between the symmetric and broken phase minima of the effective potential, leading to a first-order phase transition. This offers new routes for generating the baryon asymmetry at the electroweak scale, strongly tied to flavour models.
Comments: 6 pages, 5 figures
Subjects: High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:1604.04526 [hep-ph]
  (or arXiv:1604.04526v1 [hep-ph] for this version)

 

Presented by J. Santiago