Inflaton Dark Matter

The initial density of dark matter (DM) particles, otherwise secluded from the standard model (SM), may be generated at reheating, with an initial temperature ratio for internal thermalizations, ξi=TDM,i/TSM,i. This scenario necessarily implies inflaton-mediated scatterings between DM and SM after reheating, with a rate fixed by the relic abundance of DM and the reheat temperature. These scatterings can be important for an inflaton mass and reheat temperature as high as O(107 GeV) and O(109 GeV), respectively, since the thermally averaged collision terms become approximately independent of the inflaton mass when the bath temperature is larger than the mass. The impact of these scatterings on DM cosmology is studied modeling the perturbative reheating physics by a gauge-invariant set of inflaton interactions up to dimension 5 with the SM gauge bosons, fermions, and the Higgs fields. It is observed that an initially lower (higher) DM temperature will rapidly increase (decrease), even with very small couplings to the inflaton. There is a sharp lower bound on the DM mass below which the relic abundance cannot be satisfied due to faster backscatterings depleting DM quanta to SM particles. For low DM masses, the cosmic microwave background constraints become stronger due to the collisions for ξi<1, probing values as small as O(10−4), and weaker for ξi>1. The big-bang nucleosynthesis constraints become stronger due to the collisions for lower DM masses, probing ξi as small as O(0.1), and weaker for higher DM mass. Thus inflaton-mediated collisions with predictable rates, relevant even for high-scale inflation models, can significantly impact the cosmology of light DM. Published by the American Physical Society 2024

micrOMEGAs5.0 : Freeze-in

The freeze-in mechanism describes the out-of-equilibrium production of dark matter (DM) particles via feeble couplings or non-renormalisable interactions with large suppression scales. In the latter case, predictions suffer from a strong sensitivity to the initial conditions of the universe, such as the details of reheating. In this work, we investigate how this sensitivity is altered in the presence of a cosmological first-order phase transition. We show that freeze-in via non-renormalisable interactions is not always dominated by the highest temperatures of the Standard Model (SM) thermal bath, but instead may be governed by the period immediately after the phase transition, during which the decaying scalar field transfers its energy density to the SM radiation. We refer to this alternative production regime as DM phase-in. Using numerical and approximate analytical solutions of the relevant Boltzmann equations, we determine the conditions under which phase-in or conventional freeze-in production dominates the final DM abundance in terms of the type of interaction between the DM and SM particles, the amount of supercooling before and the evolution of the scalar field after the phase transition. In the phase-in regime, the DM abundance is correlated with the peak frequency of the gravitational wave signal associated with the phase transition, opening up new observational possibilities.

We investigate a recently proposed non-thermal mechanism for dark matter production, in which a small initial dark matter (χ) number density undergoes exponential growth through scatterings with bath particles (ϕ) in the early universe (χϕ → χχ). The process ends when the scattering rate becomes Boltzmann suppressed. The analysis, in literature, is performed on the simplifying assumption of the dark matter phase space tracing the equilibrium distribution of either standard model or a hidden sector bath. Owing to the non-thermal nature of the production mechanism, this assumption may not necessarily hold. In this work, we test the validity of this assumption by numerically solving the unintegrated Boltzmann equation for the dark matter distribution. Our results, independent of the initial conditions, show that after exponential growth ceases, the dark matter distribution exhibits equilibrium-like behaviour at low comoving momentum, especially for higher couplings. While full kinetic equilibrium-like behaviour is not reached across all momentum modes, the scaled equilibrium approximation provides reasonable estimates for the dark matter abundance. However, for more accurate results in scenarios where dark matter is not in kinetic equilibrium with the thermal bath or does not have sufficiently strong self-interactions with itself that can lead to thermalization, the full unintegrated Boltzmann equation must be solved.

In this work we study localization of a Kalb-Ramond tensorial gauge field on a membrane described by real scalar fields. The membrane is embedded in an AdS-type five-dimensional bulk space, which mimics a Randall-Sundrum scenario. First, we consider a membrane described by only a single real scalar field. In that scenario we find that there is no localized tensorial zero mode. When we take into account branes described by two real scalar fields with internal structures, we obtain again a nonlocalized zero mode for a Kalb-Ramond tensorial gauge field. After modifying our model of one single scalar field by coupling the dilaton to the Kalb-Ramond field, we find that this result is changed. Furthermore, we analyze Kaluza-Klein massive modes and resonance structures.

We explore the production of thermal dark matter (DM) candidates (WIMPs, SIMPs, ELDERs and Cannibals) during cosmic reheating. Assuming a general parametrization for the scaling of the inflaton energy density and the standard model (SM) temperature, we study the requirements for kinetic and chemical DM freeze-out in a model-independent way. For each of the mechanisms, up to two solutions that fit the entire observed DM relic density exist, for a given reheating scenario and DM mass. As an example, we assume a simple particle physics model in which DM interacts with itself and with SM through contact interactions. We find that low-temperature reheating can accommodate a wider range of couplings and larger masses than those permitted in the usual instantaneous high-temperature reheating. This results in DM solutions for WIMPs reaching masses as high as 1014 GeV, whereas for SIMPs and ELDERs, we can reach masses of 1013 GeV. Interestingly, current experimental data already constrain the enlarged parameter space of these models with low-reheating temperatures. Next-generation experiments could further probe these scenarios.

The dynamics of cosmic reheating, that is, on how the energy stored in the inflaton is transferred to the standard model (SM) thermal bath, is largely unknown. In this work, we show that the phenomenology of the nonbaryonic dark matter (DM) ultraviolet freeze-in production strongly depends on the dynamics of the cosmic-reheating era. Using a general parametrization for the Hubble expansion rate, and SM temperature, we thoroughly investigate DM production during reheating, not only recovering earlier findings that focused on specific cases but also exploring alternative scenarios. Additionally, we derive a generalized framework for DM production via inflaton decays and identify the viable parameter space, while simultaneously addressing constraints from cosmic microwave background observations. As illustrative examples, we explore gravitational DM production through scatterings of SM particles or inflatons, deriving well-defined parameter regions for these scenarios.

The search for singlet fermion dark matter at high-energy colliders is\ncommonly analyzed with a singlet scalar mediator, which however violates the\nstandard model (SM) gauge invariance, renormalizability and unitarity. These\nproblems can be cured by introducing a mixing between the singlet scalar $s$\nand the SM Higgs boson $h$. Thus one has to consider two scalar mediators $h_1$\nand $h_2$, where $h_1$ is identified as the discovered 125 GeV Higgs boson. As\na specific example, we consider the dark matter (DM) search in the $t \\bar{t} +\nE_{T}^{\\text{miss}}$ channel. According to the masses of dark matter and two\nscalar mediators, we classify the process into four cases. By investigating the\ntotal cross sections and differential distributions, we find that the\ncontribution of the 125~GeV Higgs boson $h_1$ cannot be neglected in all cases\nand can even dominate once $h_1$ is on-shell in dark matter production.\nFurther, we study the impact of $h_1$ on the LHC bounds of dark matter searches\nin the hadronic and semileptonic channels with the integrated luminosity of\n$36~\\text{fb}^{-1}$. Finally we make a brief comment that $h_1$ should be also\nconsidered in the vector DM search at high-energy colliders.\n

In this work we study the hybrid kind of dark matter (DM) production mechanism where both thermal and non-thermal contribution at two different epochs set the DM relic abundance. This hybrid set up in turn shifts the parameter space of DM in contrast to pure thermal DM scenario. We review such production mechanism in the context of the SU(2)_L lepton doublet dark matter (Psi ) augmented with an additional singlet dark scalar (S). The neutral component of the dark doublet can serve as a stable DM candidate and in pure thermal scenario, it is under-abundant as well as excluded from direct detection constraints due to its strong gauge interactions in the sub-TeV mass regime. However, in addition to the thermal contribution, the late time non-thermal DM production from the decay of the long-lived dark scalar S helps to fulfill the deficit in DM abundance. On the other hand, the strong gauge mediated direct detection constraint can be evaded with the help of a SU(2)_L triplet scalar(with Y=2), resulting a pseudo-Dirac DM. To realize our proposed scenario we impose a discrete {mathcal {Z}}_2 symmetry under which both Psi and S are odd while rest of the fields are even. We find the lepton doublet pseudo-Dirac DM with mass sim 450{-}1200 GeV, compatible with the observed relic density, direct, indirect, and existing collider search constraints.

We study a class of unified dark matter (UDM) models based on generalized K-essence, where a single scalar field with non-canonical kinetic terms accounts for dark radiation, dark matter, and dark energy. Starting from the purely kinetic Lagrangian proposed by Scherrer (2004 Phys. Rev. Lett. 93 011301), we extend the analysis to quadratic and exponential scalar potentials and explore their phenomenology. All models are implemented in a modified version of Hi_CLASS (a Horndeski capable extension of CLASS) and confronted with data from Planck 2018, DESI DR1, and Big Bang Nucleosynthesis. The scenarios reproduce the full sequence of cosmic epochs: an early radiation-like phase, a matter-dominated era, and late-time accelerated expansion. The new models predict slightly higher values of the Hubble constant compared to ΛCDM, thereby partially alleviating the respective tensions from ∼ 4.4 σ to ∼ 3.4 σ . The quadratic potential requires an ultralight mass that makes it effectively indistinguishable from the Scherrer solution. Overall, generalized K-essence provides a minimal (single field and minimally coupled) and observationally viable realization of UDM, offering a unified description of the dark sector with distinctive signatures in both early- and late-time cosmology.

In this paper, we consider the processes of single top-quark production in association with dark matter particles through the t-channel in simplified models with scalar and pseudoscalar mediators. Within the framework of these models, we analyze the angular correlations that arise in the top quark production process in association with dark matter, comparing them with the production of a top quark in the Standard Model. We consider angular correlations of kinematic variables, which define admissible regions in momentum space and can be used experimentally to distinguish between the fraction of the total momentum carried away by the scalar or pseudoscalar mediator. We demonstrate that scalar mediator significantly changes the rest frame of the well-known angular correlation observable and changes the distributions. In this case, this variable provides clear separation between the SM and DM contribution and can be used to increase the sensitivity of future DM analyses.

Many models of dark matter place their dark matter candidate inside a broader dark sector with other particles and fields for theoretical and phenomenological reasons. The cosmological evolution of these dark sectors can differ significantly from standard scenarios, in particular due to their interplay with the Standard Model of particle physics. In this thesis, we study various dark sector cosmologies, including their evolution and resulting constraints. In view of ever more stringent limits from direct and indirect searches on dark matter as a thermal relic from the primordial plasma encompassing the SM particles, scenarios of a dark sector decoupled from the Standard Model receive increasing interest. Interestingly, the corresponding dark matter production mechanism of thermal freeze-out can also occur entirely in a decoupled dark sector. Still, certain modifications to the standard treatment need to be taken into account. We study these changes and find significant deviations of the annihilation cross-section required to obtain the observed dark matter abundance, in particular if the dark matter particle and its annihilation product are similar in mass. After the first gravitational waves were observed by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), interest in primordial black holes as a candidate for dark matter has been renewed. As gravitational waves can be produced in primordial black hole binary mergers, this setup can be considered a dark sector with purely gravitational interactions. Large initial primordial black hole clustering has been suggested to circumvent the strong limits on this scenario. We show that instead, gravitational wave constraints are enhanced by large clustering such that highly clustered primordial black holes with the masses corresponding to the LIGO events cannot account for all of the dark matter in the Universe. A major part of this thesis studies Big Bang Nucleosynthesis (BBN) as a probe for MeVscale dark sectors. Due to the remarkable agreement of predictions for the primordial light element abundances from BBN with observations, alterations are generally strongly constrained. Dark sectors can change the predictions for the primordial light element abundances by the influence of their cosmological evolution during BBN itself and subsequent disintegration processes, e.g. photodisintegration. The former proceed via alterations of the Hubble rate, neutrino decoupling, the time-temperature relation, and the best-fit value for the baryon-to-photon ratio. The latter are due to late-time high-energy electromagnetic injections into the Standard Model plasma, which induce an electromagnetic cascade producing an abundance of non-thermal photons that can disintegrate light nuclei. We derive these constraints for annihilations of MeV-scale dark matter, electromagnetic decays of MeV-scale dark sector particles, and axion-like particles coupled to photons.

We present a search for Dark Matter (DM) particles production in the mono-\(W/Z(q\bar{q})\) channel using pp collision data at a centre-of-mass energy of \(\sqrt{s}\) = 13 TeV corresponding to an integrated luminosity of 36.1 \(\mathrm{fb}^{-1}\), recorded by the ATLAS detector at the Large Hadron Collider (LHC). No significant excess over the Standard Model (SM) prediction is observed. The search results are interpreted in terms of limits on invisible Higgs boson decays into DM particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross section for \(W/Z +\)DM production.

We propose a scenario with a fermion dark matter, where the dark matter\nparticle used to be the Dirac fermion, but it takes the form of the Majorana\nfermion at a late time. The relic number density of the dark matter is\ndetermined by the dark matter asymmetry generated through the same mechanism as\nleptogenesis when the dark matter was the Dirac fermion. After efficient dark\nmatter annihilation processes have frozen out, a phase transition of a scalar\nfield takes place and generates Majorana mass terms to turn the dark matter\nparticle into the Majorana fermion. In order to address this scenario in\ndetail, we propose two simple models. The first one is based on the Standard\nModel (SM) gauge group and the dark matter originates the $SU(2)_L$ doublet\nDirac fermion, analogous to the Higgsino-like neutralino in supersymmetric\nmodels. We estimate the spin-independent/dependent elastic scattering cross\nsections of this late-time Majorana dark matter with a proton and find the\npossibility to discover it by the direct and/or indirect dark matter search\nexperiments in the near future. The second model is based on the $B-L$ gauged\nextension of the SM, where the dark matter is a SM singlet. Although this model\nis similar to the so-called Higgs portal dark matter scenario, the\nspin-independent elastic scattering cross section can be large enough to detect\nthis dark matter in future experiments.\n

A search is performed for dark matter (DM) produced in association with a single top quark or a pair of top quarks using the data collected with the CMS detector at the LHC from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to 138 fb −1 of integrated luminosity. An excess of events with a large imbalance of transverse momentum is searched for across 0, 1 and 2 lepton final states. Novel multivariate techniques are used to take advantage of the differences in kinematic properties between the two DM production mechanisms. No significant deviations with respect to the standard model predictions are observed. The results are interpreted considering a simplified model in which the mediator is either a scalar or pseudoscalar particle and couples to top quarks and to DM fermions. Axion-like particles that are coupled to top quarks and DM fermions are also considered. Expected exclusion limits of 410 and 380 GeV for scalar and pseudoscalar mediator masses, respectively, are set at the 95% confidence level. A DM particle mass of 1 GeV is assumed, with mediator couplings to fermions and DM particles set to unity. A small signal-like excess is observed in data, with the largest local significance observed to be 1.9 standard deviations for the 150 GeV pseudoscalar mediator hypothesis. Because of this excess, mediator masses are only excluded below 310 (320) GeV for the scalar (pseudoscalar) mediator. The results are also translated into model-independent 95% confidence level upper limits on the visible cross section of DM production in association with top quarks, ranging from 1 pb to 0.02 pb.