Journal Club
Seminar Room
If you want to propose a paper, you can contact Supratim Das Bakshi (sdb AT ugr.es)
Monday 24th of February, 2014
Constraining Axion Dark Matter with Big Bang Nucleosynthesis
(Submitted on 24 Jan 2014 (v1), last revised 5 Feb 2014 (this version, v2))
We show that Big Bang Nucleosynthesis (BBN) significantly constrains axion-like dark matter. The axion acts like an oscillating QCDθ angle that redshifts in the early universe, increasing the neutron-proton mass difference at neutron freeze-out. An axion-like particle that couples too strongly to QCD results in the underproduction of 4He during BBN and is thus excluded. The BBN bound overlaps with much of the parameter space that would be covered by proposed searches for time-varying neutron EDMs. The QCD axion does not couple strongly enough to affect BBN.
Submission history
From: Benjamin Safdi [view email]
[v1] Fri, 24 Jan 2014 21:05:43 GMT (309kb,D)
[v2] Wed, 5 Feb 2014 19:33:14 GMT (309kb,D)
Presented by Adrián Ayala
Criteria for Natural Hierarchies
(Submitted on 11 Feb 2014)
With the discovery of a particle that seems rather consistent with the minimal Standard Model Higgs boson, attention turns to questions of naturalness, fine-tuning, and what they imply for physics beyond the Standard Model and its discovery prospects at run II of the LHC. In this article we revisit the issue of naturalness, discussing some implicit assumptions that underly some of the most common statements, which tend to assign physical significance to certain regularization procedures. Vague arguments concerning fine-tuning can lead to conclusions that are too strong and perhaps not as generic as one would hope. Instead, we explore a more pragmatic definition of the hierarchy problem that does not rely on peeking beyond the murky boundaries of quantum field theory: we investigate the fine-tuning of the electroweak scale associated with thresholds from heavy particles, which is both calculable and dependent on the nature of the would-be ultraviolet completion of the Standard Model. We discuss different manifestations of new high-energy scales that are favored by experimental hints for new physics with an eye toward making use of fine-tuning in order to determine natural regions of the new physics parameter spaces.
Presented by J. Santiago
Far from equilibrium dynamics of Bose-Einstein condensation for Axion Dark Matter
(Submitted on 19 Feb 2014)
Axions and similar very weakly interacting particles are increasingly compelling candidates for the cold dark matter of the universe. Having very low mass and being produced non-thermally in the early Universe, axions feature extremely high occupation numbers. It has been suggested that this leads to the formation of a Bose-Einstein condensate with potentially significant impact on observation and direct detection experiments. In this note we aim to clarify that if Bose-Einstein condensation occurs for light and very weakly interacting dark matter particles, it does not happen in thermal equilibrium but is described by a far-from-equilibrium state. In particular we point out that the dynamics is characterized by two very different timescales, such that condensation occurs on a much shorter timescale than thermalization.
Presented by M. Masip
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