Neutralino dark matter in scenarios with early matter domination

Published for SISSA by Springer Received: August 26, 2018 Revised: October 24, 2018 Accepted: November 19, 2018 Published: December 7, 2018 Manuel Drees and Fazlollah Hajkarim Bethe Center for Theoretical Physics and Physikalisches Institut, Universität Bonn, Nussallee 12, D-53115 Bonn, Germany E-mail: , Abstract: We investigate the production of neutralino dark matter in a cosmological scenario featuring an early matter dominated era ending at a relatively low reheating temperature. In such scenarios different production mechanisms of weakly interacting massive particles (WIMPs), besides the well-studied thermal production, can be important. This opens up new regions of parameter space where the lightest neutralino, as the bestknown supersymmetric (SUSY) WIMP, obtains the required relic abundance. Many of these new sets of parameters are also compatible with current limits from colliders as well as direct and indirect WIMP searches. In particular, in standard cosmology bino-like neutralinos, which emerge naturally as lightest neutralino in many models, can have the desired relic density only in some finetuned regions of parameter space where the effective annihilation cross section is enhanced by co-annihilation or an s-channel pole. In contrast, if the energy density of the universe was dominated by long-lived PeV-scale particles (e.g. moduli or Polonyi fields), bino-like neutralinos can obtain the required relic density over wide regions of supersymmetric parameter space. We identify the interesting ranges of mass and decay properties of the heavy long-lived particles, carefully treating the evolution of the temperature of the thermal background. Keywords: Supersymmetry Phenomenology, Strings and branes phenomenology ArXiv ePrint: 1808.05706 Open Access, c The Authors. Article funded by SCOAP3 . https://doi.org/10.1007/JHEP12(2018)042 JHEP12(2018)042 Neutralino dark matter in scenarios with early matter domination Contents 1 2 Basic framework 3 3 Thermal neutralino dark matter 7 4 Neutralino production in non-standard cosmology 4.1 Discussion of the parameter space 4.1.1 Modulus mass 4.1.2 Branching fraction 4.2 Numerical results 4.2.1 Light moduli 4.2.2 Intermediate-mass moduli 4.2.3 Heavy moduli 13 14 14 15 16 16 18 20 5 Summary and conclusions 21 1 Introduction The lightest neutralino as lightest supersymmetric particle (LSP) is one of the oldest and most studied examples of a weakly interacting massive particle (WIMP) candidate for the cosmological Dark Matter (DM); see e.g. [1] for an early exploration of parameter space, and [2, 3] for reviews. The minimal supersymmetric extension of the Standard Model (MSSM) contains four neutralino current eigenstates: a bino, a wino, and two higgsinos. Given current collider constraints on superparticles, in particular on the masses of charginos and the heavier neutralinos, we now know that over most of parameter space, the mass eigenstates are relatively pure states, with little mixing. Most analyses of WIMP DM worked in the framework of standard cosmology, where the Universe was radiation-dominated starting at the end of inflation and ending at a temperature around 1 eV. Moreover, it is usually assumed that the post-inflationary reheat temperature was sufficiently high that WIMPs attained full thermal (chemical and kinetic) equilibrium. The WIMP relic density is then basically inversely proportional to its (effective) annihilation cross section [4, 5]. In that case higgsino-like WIMPs typically need to have a mass near 1 TeV to have the correct relic density, and a wino-like WIMP should be at least two times heavier. While it has recently been pointed out that these values might be lowered by 30% or so due to co-annihilation effects [6], the required values are still uncomfortably high when compared to estimates of weak-scale finetuning in the MSSM. In particular, while bounds on the masses of scalar tops and gluinos based on simple loop –1– JHEP12(2018)042 1 Introduction 1 This argument can be evaded [11] if there is a soft supersymmetry breaking contribution to the higgsino mass; this would not contribute to the Higgs boson masses which in turn determine finetuning. While this is technically possible, it would require a rather complicated supersymmetry breaking scenario. –2– JHEP12(2018)042 calculations [7] are somewhat controversial [8–10], it is generally agreed that higgsino, and hence LSP, masses above several hundred GeV would lead to percent level (or worse) finetuning; note that in the MSSM the higgsino mass enters the relevant finetuning condition already at tree-level.1 In standard cosmology, higgsino- or wino-like WIMPs with masses in the few hundred GeV range would have too small a relic density. In contrast, a bino-like WIMP has too large a relic density in such a scenario, unless its effective annihilation cross section is boosted by co-annihilation [12–14] or by an s-channel pole [12, 15]. A predicted underdensity of WIMP DM can be cured by adding another DM component, e.g. axions [16–18]; this can be done within the framework of minimal cosmology, and without changing TeV-scale particle physics. On the other hand, a scenario that predicts too large a relic density for a given DM candidate is clearly excluded. This argument thus disfavors bino-like WIMPs, at least within minimal cosmology. At the same time bino-like WIMPs quite easily satisfy the increasingly stringent constraints from direct WIMP searches [19, 20]; these searches exclude many scenarios where the WIMP is higgsino-like, if the latter contributes most or all of DM. Moreover, indirect searches [19, 21] now exclude models where most or all of DM consists of wino-like (higgsino-like) WIMPs with mass below ∼ 0.8 (∼ 0.4) TeV, but hardly constrain the parameter space if the LSP is bino-like. These null results therefore favor bino-like WIMPs. At the same time bino-like neutralinos often emerge as LSP in simple models where the superparticle spectrum can be described by a small number of free parameters. In particular, if gaugino masses unify at or near the same scale where the gauge couplings meet in the MSSM, the weak-scale bino mass will be about half of the wino mass. Moreover, if stop squarks and Higgs bosons have similar soft breaking masses at this very high energy scale, the weak-scale higgsino mass parameter typically comes out larger than the bino mass. These arguments motivate us to investigate a non-minimal cosmological scenario, in the hope of finding an extended region of parameter space where a bino-like WIMP obtains the required relic density. In particular, we analyse scenarios featuring an early matterdominated epoch sometime between the end of inflation and Big Bang nucleosynthesis (BBN). This is quite well motivated, since UV-complete theories like supergravity [22] and superstring theory often contain heavy but long-lived scalar particles, nowadays usually called moduli. They are long-lived sinc (...truncated)