A
bstract
Studies of dark matter annihilation through an
s
-channel resonance are often based on recipes such as a narrow width approximation or real intermediate state subtraction. We review a ...recipe-free formalism that can be implemented at the NLO level in the full theory, and ensures the cancellation of mass singularities. Its basic ingredients can be formulated in the relativistic regime, but we show that the procedure simplifies if we go to the non-relativistic one and assume the presence of kinetic equilibrium. The latter case is illustrated for scalar singlet dark matter with
m
φ
≃ 60 GeV, freezing out at
T
≃ (1–3) GeV, re-confirming the viability of this scenario with couplings tiny enough to evade experimental constraints.
A
bstract
Studying the diffusion and kinetic equilibration of heavy quarks within a hot QCD medium profits from the knowledge of a coloured Lorentz force that acts on them. Starting from the spatial ...components of the vector current, and carrying out two matching computations, one for the heavy quark mass scale (
M
) and another for thermal scales
MT
T
, we determine 1-loop matching coefficients for the electric and magnetic parts of a Lorentz force. The magnetic part has a non-zero anomalous dimension, which agrees with that extracted from two other considerations, one thermal and the other in vacuum. The matching coefficient could enable a lattice study of a colour-magnetic 2-point correlator.
A
bstract
The production rate of right-handed neutrinos from a Standard Model plasma at a temperature above a hundred GeV is evaluated up to NLO in Standard Model couplings. The results apply in the ...so-called relativistic regime, referring parametrically to a mass
M
~
πT
, generalizing thereby previous NLO results which only apply in the non-relativistic regime
M
≫
πT
. The non-relativistic expansion is observed to converge for
M
≳ 15
T
, but the smallness of
any
loop corrections allows it to be used in practice already for
M
≳ 4
T
. In the latter regime any non-covariant dependence of the differential rate on the spatial momentum is shown to be mild. The loop expansion breaks down in the ultrarelativistic regime
M
≪
πT
, but after a simple mass resummation it nevertheless extrapolates reasonably well towards a result obtained previously through complete LPM resummation, apparently confirming a strong enhancement of the rate at high temperatures (which facilitates chemical equilibration). When combined with other ingredients the results may help to improve upon the accuracy of leptogenesis computations operating above the electroweak scale.
A
bstract
Starting from operator equations of motion and making arguments based on a separation of time scales, a set of equations is derived which govern the non-equilibrium time evolution of a ...GeV-scale sterile neutrino density matrix and active lepton number densities at temperatures
T
≳ 130 GeV. The density matrix possesses generation and helicity indices; we demonstrate how helicity permits for a classification of various sources for leptogenesis. The coefficients parametrizing the equations are determined to leading order in Standard Model couplings, accounting for the LPM resummation of 1 +
n
↔ 2 +
n
scatterings and for all 2 ↔ 2 scatterings. The regime in which sphaleron processes gradually decouple so that baryon plus lepton number becomes a separate non-equilibrium variable is also considered.
A
bstract
The co-annihilation rate of heavy particles close to thermal equilibrium, which plays a role in many classic dark matter scenarios, can be “simulated” in QCD by considering the pair ...annihilation rate of a heavy quark and antiquark at a temperature of a few hundred MeV. We show that the so-called Sommerfeld factors, parameterizing the rate, can be defined and measured non-perturbatively within the NRQCD framework. Lattice measurements indicate a modest suppression in the octet channel, in reasonable agreement with perturbation theory, and a large enhancement in the singlet channel, much above the perturbative prediction. The additional enhancement is suggested to originate from bound state formation and subsequent decay. Making use of a Green’s function based method to incorporate thermal corrections in perturbative co-annihilation rate computations, we show that qualitative agreement with lattice data can be found once thermally broadened bound states are accounted for. We suggest that our formalism may also be applicable to specific dark matter models which have complicated bound state structures.
A
bstract
Recently many investigations have considered Majorana dark matter co-annihilating with bound states formed by a strongly interacting scalar field. However only the gluon radiation ...contribution to bound state formation and dissociation, which at high temperatures is subleading to soft 2 → 2 scatterings, has been included. Making use of a non-relativistic effective theory framework and solving a plasma-modified Schrödinger equation, we address the effect of soft 2 → 2 scatterings as well as the thermal dissociation of bound states. We argue that the mass splitting between the Majorana and scalar field has in general both a lower and an upper bound, and that the dark matter mass scale can be pushed at least up to 5…6TeV.
A
bstract
The vector channel spectral function and the dilepton production rate from a QCD plasma at a temperature above a few hundred MeV are evaluated up to next-to-leading order (NLO) including ...their dependence on a non-zero momentum with respect to the heat bath. The invariant mass of the virtual photon is taken to be in the range
~ (
πT
)
2
~ (1GeV)
2
, generalizing previous NLO results valid for
≫ (
πT
)
2
. In the opposite regime 0
<
≪ (
πT
)
2
the loop expansion breaks down, but agrees nevertheless in order of magnitude with a previous result obtained through resummations. Ways to test the vector spectral function through comparisons with imaginary-time correlators measured on the lattice are discussed.
A
bstract
We apply a formalism accounting for thermal effects (such as modified Sommerfeld effect; Salpeter correction; decohering scatterings; dissociation of bound states), to one of the simplest ...WIMP-like dark matter models, associated with an “inert” Higgs doublet. A broad temperature range
T
∼
M/
20
. . . M/
10
4
is considered, stressing the importance and less-understood nature of late annihilation stages. Even though only weak interactions play a role, we find that resummed real and virtual corrections increase the tree-level overclosure bound by 1
. . .
18%, depending on quartic couplings and mass splittings.
We estimate the momentum diffusion coefficient of a heavy quark within a pure SU(3) plasma at a temperature of about 1.5T sub(c). Large-scale Monte Carlo simulations on a series of lattices extending ...up to 192 super(3) x 48 permit us to carry out a continuum extrapolation of the so-called color-electric imaginary-time correlator. The extrapolated correlator is analyzed with the help of theoretically motivated models for the corresponding spectral function. Evidence for a nonzero transport coefficient is found and, incorporating systematic uncertainties reflecting model assumptions, we obtain kappa = (1.8-3.4)T super(3). This implies that the "drag coefficient," characterizing the time scale at which heavy quarks adjust to hydrodynamic flow, is (ProQuest: Formulae and/or non-USASCII text omitted) = (1.8-3.4)(T sub(c)/T) super(2)(M/1.5 GeV) fm/c, where M is the heavy quark kinetic mass. The results apply to bottom and, with somewhat larger systematic uncertainties, to charm quarks.
A
bstract
We compute the production rate of the energy density carried by gravitational waves emitted by a Standard Model plasma in thermal equilibrium, consistently to leading order in coupling ...constants for momenta
k
∼
πT
. Summing up the contributions from the full history of the universe, the highest temperature of the radiation epoch can be constrained by the so-called
N
eff
parameter. The current theoretical uncertainty ∆
N
eff
≤
10
−
3
corresponds to
T
max
≤
2
×
10
17
GeV. In the course of the computation, we show how a subpart of the production rate can be determined with the help of standard packages, even if subsequently an IR subtraction and thermal resummation need to be implemented.
PDF
