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Gravitino Warm Dark Matter with Entropy Production

Edward A. Baltz, Hitoshi Murayama

TL;DR

KeV-mass gravitinos in gauge-mediated SUSY breaking face relic-density tensions from thermal freeze-out and small-scale structure constraints. The authors propose a natural entropy-production mechanism via out-of-equilibrium messenger decays at decay temperatures around $T_D \sim 10$ MeV, diluting the gravitino relic density by a factor of order $s'/s \sim 6$–$8$. A simple analytic treatment and a full numerical evolution show that this dilution yields a viable warm dark matter candidate with $m_{3/2} \approx 1$ keV and a compatible free-streaming scale $R_{fs}$ for $m_{3/2} \sim 1$–$1.5$ keV. This approach preserves the GMSB framework while alleviating tensions with Ly$\alpha$ forest data and dwarf-scale structure, highlighting a natural cosmological role for messenger decays.

Abstract

Gravitinos with a mass in the keV range are an interesting candidate for warm dark matter. Recent measurements of the matter density of the universe and of cosmic structures at the dwarf galaxy scale rule out the simplest gravitino models with thermal freeze-out. We construct a model where the decay of the messenger particles that transmit the supersymmetry breaking to the observable sector generates the required entropy to dilute the gravitino relic density by the required factor of a few to come in line with observations. The model is natural, and requires only that the coupling of the messenger sector to the standard model be set so that the decay happens at the appropriate time.

Gravitino Warm Dark Matter with Entropy Production

TL;DR

KeV-mass gravitinos in gauge-mediated SUSY breaking face relic-density tensions from thermal freeze-out and small-scale structure constraints. The authors propose a natural entropy-production mechanism via out-of-equilibrium messenger decays at decay temperatures around MeV, diluting the gravitino relic density by a factor of order –. A simple analytic treatment and a full numerical evolution show that this dilution yields a viable warm dark matter candidate with keV and a compatible free-streaming scale for – keV. This approach preserves the GMSB framework while alleviating tensions with Ly forest data and dwarf-scale structure, highlighting a natural cosmological role for messenger decays.

Abstract

Gravitinos with a mass in the keV range are an interesting candidate for warm dark matter. Recent measurements of the matter density of the universe and of cosmic structures at the dwarf galaxy scale rule out the simplest gravitino models with thermal freeze-out. We construct a model where the decay of the messenger particles that transmit the supersymmetry breaking to the observable sector generates the required entropy to dilute the gravitino relic density by the required factor of a few to come in line with observations. The model is natural, and requires only that the coupling of the messenger sector to the standard model be set so that the decay happens at the appropriate time.

Paper Structure

This paper contains 5 sections, 17 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Gravitino mass and relic density
  3. Entropy production: simple model
  4. Entropy production: full evolution
  5. Discussion

Figures (1)

  • Figure 1: Evolution of entropy density. Here we show the decay of messengers at temperatures of 100 MeV (solid curve) and 10 MeV (dashed curve), producing an entropy increase of about a factor of eight. The onset of matter domination at a temperature of about 1 GeV (100 MeV) is clearly seen, and the decay at 100 MeV (10 MeV) is also clear.