(Non-)Thermal Production of WIMPs during Kination

Abstract

Understanding the nature of the Dark Matter (DM) is one of the current challenges in modern astrophysics and cosmology. Knowing the properties of the DM particle would shed light on physics beyond the Standard Model and even provide us with details of the early Universe. In fact, the detection of such a relic would bring us information from the pre-Big Bang Nucleosynthesis (BBN) period, an epoch from which we have no direct data, and could even hint at inflation physics. In this work, we assume that the expansion rate of the Universe after inflation is governed by the kinetic energy of a scalar field φ, in the so-called "kination" model. Adding to previous work on the subject, we assume that the φ field decays into both radiation and DM particles, which we take to be Weakly Interacting Massive Particles (WIMPs). The present abundance of WIMPs is then fixed during the kination period through either a thermal "freeze-out" or "freeze-in" mechanism, or through a non-thermal process governed by the decay of φ. We explore the parameter space of this theory with the requirement that the present WIMP abundance provides the correct relic budget. Requiring that BBN occurs during the standard cosmological scenario sets a limit on the temperature at which the kination period ends. Using this limit and assuming the WIMP has a mass mχ = 100 GeV, we obtain that the thermally-averaged WIMP annihilation cross section has to satisfy the constraints 4 × 10 −16 GeV −2 σv 2 × 10 −5 GeV −2 in order for having at least one of the production mechanism to yield the observed amount of DM. This result shows how the properties of the WIMP particle, if ever measured, can yield information on the pre-BBN content of the Universe.

Probing the early Universe with axion physics and gravitational waves

“…Such a modification to the ΛCDM model at temperatures larger than T BBN would lead to several testable consequences in various phenomenological aspects. In fact, the presence of an early NSC would alter the properties at which a Weakly Interacting Massive Particle (WIMP) free zes out [16][17][18][19][20] and decouples kinetically from the primordial plasma [21,22], and open the pathway to testable modifications to the standard leptogenesis models [23]. In general, if a new relic is ever discovered, the properties of its phase-space distribution would carry crucial information on the pre-BBN era.…”

Establishing the dark matter relic density in an era of particle decays

“…Notably, the form of the initial expansion rate leaves a lasting imprint on relic densities established while H ∝ T 4 , because the value of the exponent n changes the temperature evolution of the Standard Model thermal bath The prospect of the dark matter relic abundance being established during a period of entropy injection following an early matter dominated era was originally studied in influential papers of Giudice, Kolb & Riotto [5] and Gelmini & Gondolo [6]. This was later adapted to the case of a period of decays following kination domination by Visinelli [20]. In this work we have further generalised to the case in which the initial epoch has a general expansion rate of the form H ∼ T 2+n/2 .…”

“…For n = 2 then rather X(TRH) ∝ ln(ARH), this case was studied in[20] and we will not discuss it further.-12 -� � = �� -� ��� η=�� -� η=�� -� η=���…”

Freeze-in production of dark matter prior to early matter domination

“…Unless it is driven by oscillations of the inflaton itself, it is typically preceded by a RD phase or a period with a more general equation of state. Thermal freeze-out or freeze-in at these early stages can also contribute to the DM relic abundance (for example, see [9][10][11]) and thereby affect the allowed parameter space. Since freeze-out occurs at temperatures below the DM mass m χ , pre-EMD production will only be relevant for (very) large DM masses in this case.…”