Thermal decoupling of WIMPs from first principles

Bringmann, Torsten; Hofmann, Stefan

doi:10.1088/1475-7516/2007/04/016

Cited by 186 publications

(233 citation statements)

References 32 publications

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“…Assuming that kinetic decoupling happens suddenly, like depicted in the right-hand panel of Fig. 3 (shown in red), gives a reasonable approximation to the full evolution of y -though we find that for very small lepton couplings, and negligible φ-scattering, the kinetic decoupling process may be more delayed than for standard WIMPs [9,10]: since more scattering events are necessary to keep the DM in thermal equilibrium in this case, it also takes longer before scattering becomes sufficiently inefficient so that y remains completely constant. Therefore, sudden decoupling works very well as an approximation to Y (the lines are completely obscured by the full solution in Fig.…”

Section: B Decoupling On-and Off-resonancementioning

confidence: 75%

“…T χ ∝ a −2 , which simply reflects the redshift of the WIMP momenta due to the expansion of the universe. The fact that the transition between these two regimes happens on a rather short timescale [9,10] allows to conveniently define the temperature of kinetic decoupling as…”

Section: B Kinetic Decouplingmentioning

confidence: 99%

“…Unfortunately, however, one can no longer use the fact that the momentum transfer is small in this case and a substantial extension to the formalism presented in Refs. [9,10] would be required, which is beyond the scope of this article.…”

Section: B Dark Matter Self-scatteringmentioning

confidence: 99%

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Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates

2012

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We consider dark matter consisting of weakly interacting massive particles (WIMPs) and revisit in detail its thermal evolution in the early universe, with a particular focus on models where the annihilation rate is enhanced by the Sommerfeld effect. After chemical decoupling, or freeze-out, dark matter no longer annihilates but is still kept in local thermal equilibrium due to scattering events with the much more abundant standard model particles. During kinetic decoupling, even these processes stop to be effective, which eventually sets the scale for a small-scale cutoff in the matter density fluctuations. Afterwards, the WIMP temperature decreases more quickly than the heat bath temperature, which causes dark matter to reenter an era of annihilation if the cross-section is enhanced by the Sommerfeld effect. Here, we give a detailed and self-consistent description of these effects. As an application, we consider the phenomenology of simple leptophilic models that have been discussed in the literature and find that the relic abundance can be affected by as much two orders of magnitude or more. We also compute the mass of the smallest dark matter subhalos in these models and find it to be in the range O(10 −10 M ) − O(10 M ); even much larger cutoff values are possible if the WIMPs couple to force carriers lighter than about 100 MeV. We point out that a precise determination of the cutoff mass allows to infer new limits on the model parameters, in particular from gamma-ray observations of galaxy clusters, that are highly complementary to existing constraints from g − 2 or beam dump experiments.PACS numbers: 95.35.+d, 98.80.Cq, 95.30.Tg

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Section: B Decoupling On-and Off-resonancementioning

confidence: 75%

Section: B Kinetic Decouplingmentioning

confidence: 99%

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Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates

2012

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“…Therefore, kinetic decoupling (the departure from Maxwell-Boltzmann phase space distribution) occurs long after chemical decoupling (the freeze-out of number changing interactions); see for example Refs. [35][36][37]. For coscattering, elastic scattering, χφ → χφ, generically decouples before inelastic scattering, χφ → ψφ, because of the small coupling of χ to the thermal bath.…”

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confidence: 99%

Fourth Exception in the Calculation of Relic Abundances

2017

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We propose that the dark matter abundance is set by the decoupling of inelastic scattering instead of annihilations. This coscattering mechanism is generically realized if dark matter scatters against states of comparable mass from the thermal bath. Coscattering points to dark matter that is exponentially lighter than the weak scale and has a suppressed annihilation rate, avoiding stringent constraints from indirect detection. Dark matter upscatters into states whose late decays can lead to observable distortions to the blackbody spectrum of the cosmic microwave background.Introduction: Dark Matter (DM) constitutes most of the matter in our Universe, but its origin is unknown. One of the most attractive possibilities is that DM starts in thermal equilibrium in the early Universe, and its abundance is set once its annihilations become slower than the expansion rate. This framework is insensitive to initial conditions and has the further appeal of tying the DM abundance to its (potentially observable) interactions.The most widely considered possibility is that 2-to-2 annihilations to Standard Model (SM) particles set the DM relic density. This is known as the Weakly Interacting Massive Particle (WIMP) paradigm [1-4] and points to DM particles with weak scale masses and crosssections. This theoretical framework has had considerable impact shaping experimental searches for DM.However it has long been appreciated that simple variations to the cosmology of thermal relics can have dramatic consequences. In a seminal paper, Ref.[5] enumerates three "exceptions" to thermal relic cosmology: (1) mutual annihilations of multiple species (coannihilations), (2) annihilations into heaver states (forbidden channels), and (3) annihilations near a pole in the cross section. These exceptions lead to phenomenology that can differ significantly from standard WIMPs (see for example [11][12][13]34]), while sharing their appealing theoretical features.In this letter, we introduce a fourth exception. Like Ref.[5], we assume DM begins in thermal equilibrium, has its number diluted through 2-to-2 annihilations, and has a temperature that tracks the photon temperature (for studies that relax at least one of these assumptions see for example Refs. [14][15][16][17][18][19][20][21][22][23][24][25][26][27]). We consider the presence of two states charged under the symmetry that stabilizes DM: χ and ψ, where m χ < m ψ and χ is DM. We assume that χ annihilations are suppressed, and two processes are active:

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“…After freezeout, the WIMP temperature remains fixed to the temperature of the primordial plasma via frequent elastic scattering from standard-model particles. When the temperature drops below the kinetic-decoupling temperature T kd , which generally falls in WIMP models in the range T kd ∼ 10 MeV − few GeV [5,6], WIMPs kinetically decouple from the plasma, and their temperature subsequently decays with the scale factor R as R −2 , rather than R −1 . Afterwards, WIMPs are effectively collisionless; they behave in the subsequent Universe like the cold dark matter required to account for detailed measurements of the cosmic microwave background (CMB) and large-scale structure.…”

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confidence: 99%

Early Annihilation and Diffuse Backgrounds in Models of Weakly Interacting Massive Particles in Which the Cross Section for Pair Annihilation Is Enhanced by1/v

Kamionkowski

Profumo

2008

Phys. Rev. Lett.

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Several recent studies have considered modifications to the standard weakly-interacting massive particle (WIMP) scenario in which the cross section (times relative velocity v) for pair annihilation is enhanced by a factor 1/v. Since v ∼ 10 −3 in the Galactic halo, this may boost the annihilation rate into photons and/or electron-positron pairs enough to explain several puzzling Galactic radiation signals. Here we show that if the annihilation cross section scales as 1/v, then there is a burst of WIMP annihilation in the first dark-matter halos that form at redshifts z ∼ 100 − 200. If the annihilation is to gamma rays in the energy range 100 keV − 300 GeV, or to electron-positron pairs in the energy range GeV − 2 TeV, then there remains a contribution to the diffuse extragalactic gamma-ray background today. Upper limits to this background provide constraints to the annihilation cross section. If the photon or electron-positron energies fall outside these energy ranges, then the radiation is absorbed by the intergalactic medium (IGM) and thus ionizes and heats the IGM. In this case, cosmic microwave background constraints to the ionization history also put limits on the annihilation cross section. 1In the standard weakly-interacting massive particle (WIMP) scenario [1,2,3], dark matter is composed of particles that have electroweak interactions with ordinary matter. Such particles cease to annihilate to standard-model particles in the primordial plasma to produce a cosmological relic density that is generically in the ballpark of that required to account for the dark matter. The most widely studied WIMPs are the neutralino, the lightest supersymmetric particle in several supersymmetric extensions to the standard model [1], and, more recently, the lightest Kaluza-Klein particle in models with universal extra dimensions

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Thermal decoupling of WIMPs from first principles

Cited by 186 publications

References 32 publications

Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates

Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates

Fourth Exception in the Calculation of Relic Abundances

Early Annihilation and Diffuse Backgrounds in Models of Weakly Interacting Massive Particles in Which the Cross Section for Pair Annihilation Is Enhanced by1/v

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