990 research outputs found
Dynamical renormalization group approach to relaxation in quantum field theory
The real time evolution and relaxation of expectation values of quantum
fields and of quantum states are computed as initial value problems by
implementing the dynamical renormalization group (DRG).Linear response is
invoked to set up the renormalized initial value problem to study the dynamics
of the expectation value of quantum fields. The perturbative solution of the
equations of motion for the field expectation values of quantum fields as well
as the evolution of quantum states features secular terms, namely terms that
grow in time and invalidate the perturbative expansion for late times. The DRG
provides a consistent framework to resum these secular terms and yields a
uniform asymptotic expansion at long times. Several relevant cases are studied
in detail, including those of threshold infrared divergences which appear in
gauge theories at finite temperature and lead to anomalous relaxation. In these
cases the DRG is shown to provide a resummation akin to Bloch-Nordsieck but
directly in real time and that goes beyond the scope of Bloch-Nordsieck and
Dyson resummations. The nature of the resummation program is discussed in
several examples. The DRG provides a framework that is consistent, systematic
and easy to implement to study the non-equilibrium relaxational dynamics
directly in real time that does not rely on the concept of quasiparticle
widths.Comment: LaTex, 27 pages, 2 .ps figure
Non-equilibrium dynamics in quantum field theory at high density: the tsunami
The dynamics of a dense relativistic quantum fluid out of thermodynamic
equilibrium is studied in the framework of the Phi^4 scalar field theory in the
large N limit. The time evolution of a particle distribution in momentum space
(the tsunami) is computed. The effective mass felt by the particles in such a
high density medium equals the tree level mass plus the expectation value of
the squared field. The case of negative tree level squared mass is particularly
interesting. In such case dynamical symmetry restoration as well as dynamical
symmetry breaking can happen. Furthermore, the symmetry may stay broken with
vanishing asymptotic squared mass showing the presence of out of equilibrium
Goldstone bosons. We study these phenomena and identify the set of initial
conditions that lead to each case. We compute the equation of state which turns
to depend on the initial state. Although the system does not thermalize, the
equation of state for asymptotically broken symmetry is of radiation type. We
compute the correlation functions at equal times. The two point correlator for
late times is the sum of different terms. One stems from the initial particle
distribution. Another term accounts for the out of equilibrium Goldstone bosons
created by spinodal unstabilities when the symmetry is asymptotically
broken.Both terms are of the order of the inverse of the coupling for distances
where causal signals can connect the two points. The contribution of the out of
equilibrium Goldstones exhibits scaling behaviour in a generalized sense.Comment: LaTex, 49 pages, 15 .ps figure
Large scale magnetogenesis from a non-equilibrium phase transition in the radiation dominated era
We study the generation of large scale primordial magnetic fields by a
cosmological phase transition during the radiation dominated era. The setting
is a theory of N charged scalar fields coupled to an abelian gauge field, that
undergoes a phase transition at a critical temperature much larger than the
electroweak scale. The dynamics after the transition features two distinct
stages: a spinodal regime dominated by linear long-wavelength instabilities,
and a scaling stage in which the non-linearities and backreaction of the scalar
fields are dominant. This second stage describes the growth of horizon sized
domains. We implement a recently introduced formulation to obtain the spectrum
of magnetic fields that includes the dissipative effects of the plasma. We find
that large scale magnetogenesis is very efficient during the scaling regime.
The ratio between the energy density on scales larger than L and that in the
background radiation r(L,T) = rho_B(L,T)/rho_{cmb}(T) is r(L,T) \sim 10^{-34}
at the Electroweak scale and r(L,T) \sim 10^{-14} at the QCD scale for L \sim 1
Mpc. The resulting spectrum is insensitive to the magnetic diffusion length. We
conjecture that a similar mechanism could be operative after the QCD chiral
phase transition.Comment: LaTex, 25 pages, no figures, to appear in Phys. Rev.
Space-time evolution of heavy sterile neutrinos in cascade decays
Heavy sterile-like neutrinos may be produced resonantly from the decay of
pseudoscalar mesons and may decay into several different channels in a cascade
. In general these are
rare events with displaced vertices. We provide a non-perturbative and
manifestly unitary framework that describes the cascade decay and yields the
space-time evolution of the probabilities for sterile neutrinos, final states
and the total number of events at a far detector. The results are general,
valid for Dirac or Majorana neutrinos and only input the total decay rates and
branching ratios for the production and decay channels. We apply the general
results to two examples of "visible" decay: i) via a standard model charged current
vertex and ii) the radiative decay . For this latter cascade process we find substantial
corrections to previous assessments within the parameter space argued to solve
the anomalous excess of electron-like events at MiniBooNE. These large
corrections may help relieve the tension with recent experimental bounds on
radiative decays of heavy sterile neutrinos.Comment: 22 pages, 7 fig
Real-Time Dynamics with Fermions on a Lattice
The 1+1 dimensional abelian Higgs model with fermions is a toy model for the
theory of electroweak baryogenesis. We study the dynamics of the model with
axially coupled fermions in real-time. The model is defined on a spacetime
lattice to preserve gauge invariance and to obtain numerical stability in a
simple way. We take into account the phenomenon of lattice fermion doubling.
The dynamics is approximated by treating the inhomogeneous Bose fields
classically, which is justified in a large N_f approximation. The back reaction
on the Bose fields due to fermion field fluctuations is calculated using a mode
function expansion. We discuss and present numerical results for the response
of fermions to sphaleron transitions, the renormalizability of the effective
equations of motion and non-perturbative dynamics in the framework of
non-equilibrium quantum field theory. The long-time behaviour of the system is
discussed and we speculate about applications to finite density calculations.Comment: 34 pages + 20 eps figures, improved presentation, discussion of the
figures and figure captions expanded, references added; to appear in
Nucl.Phys.
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