Minimum Mass Bound Established For Thermal Dark Matter

The most popular category of dark matter particle theories are thermal dark matter theories. A new analysis sets rigorous minimum mass limits on dark matter particles in such theories that most importantly place a bound too high to be consistent with warm dark matter theories that are among the most promising thermal dark matter theories.

Refined Bounds on MeV-scale Thermal Dark Sectors from BBN and the CMB

Nashwan Sabti, James Alvey, Miguel Escudero, Malcolm Fairbairn, Diego Blas

(Submitted on 3 Oct 2019)

New light states thermally coupled to the Standard Model plasma alter the expansion history of the Universe and impact the synthesis of the primordial light elements. In this work, we carry out an exhaustive and precise analysis of the implications of MeV-scale BSM particles in Big Bang Nucleosynthesis (BBN) and for Cosmic Microwave Background (CMB) observations. We find that, BBN observations set a lower bound on the thermal dark matter mass of mχ>0.4MeV at 2σ. This bound is independent of the spin and number of internal degrees of freedom of the particle, of the annihilation being s-wave or p-wave, and of the annihilation final state. Furthermore, we show that current BBN plus CMB observations constrain purely electrophilic and neutrinophilic BSM species to have a mass, mχ>3.7MeV at 2σ. We explore the reach of future BBN measurements and show that upcoming CMB missions should improve the bounds on light BSM thermal states to mχ>(10−15)MeV. Finally, we demonstrate that very light BSM species thermally coupled to the SM plasma are highly disfavoured by current cosmological observations.

Comments:	28 pages, 10 figures, 6 tables
Subjects:	High Energy Physics - Phenomenology (hep-ph); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Report number:	KCL-2019-75
Cite as:	arXiv:1910.01649 [hep-ph]
	(or arXiv:1910.01649v1 [hep-ph] for this version)