6,154 research outputs found
Classical Limit for Scalar Fields at High Temperature
We study real-time correlation functions in scalar quantum field theories at
temperature . We show that the behaviour of soft, long wavelength
modes is determined by classical statistical field theory. The loss of quantum
coherence is due to interactions with the soft modes of the thermal bath. The
soft modes are separated from the hard modes by an infrared cutoff \L \ll
1/(\hbar\beta). Integrating out the hard modes yields an effective theory for
the soft modes. The infrared cutoff \L controls corrections to the classical
limit which are \cO{\hbar\beta\L}. As an application, the plasmon damping
rate is calculated.Comment: 24 pages, 7 eps figures, Late
Calculating the diffractive from the inclusive structure function
It is demonstrated that the global properties of the rapidity gap events at
HERA can be understood based on electron-gluon scattering and a
non-perturbative mechanism of colour neutralization. Using the measured
inclusive structure function to determine the parameters of the parton
model, the diffractive structure function is predicted. The ratio of
diffractive and inclusive cross sections, , is determined by the probability of the produced quark-antiquark pair to
evolve into a colour singlet state.Comment: talk at Workshop on DIS and QCD, Paris, April 1995, 3 pages LaTeX,
uses qcdparis.sty, 2 figures (uuencoded
Abelian hard thermal loops on a lattice
In Abelian theories, one can write for the hard thermal loop equations of
motions a local formulation that is more economical than the traditional Vlasov
formulation and is in explicitly canonical form. I show how this formulation
can be used for simulating non-equilibrium dynamics in the Abelian Higgs model.Comment: 3 pages. Talk given at LATTICE99 (electroweak), Pisa, Ital
Non equilibrium dynamics of mixing, oscillations and equilibration: a model study
The non-equilibrium dynamics of mixing, oscillations and equilibration is
studied in a field theory of flavored neutral mesons that effectively models
two flavors of mixed neutrinos, in interaction with other mesons that represent
a thermal bath of hadrons or quarks and charged leptons. This model describes
the general features of neutrino mixing and relaxation via charged currents in
a medium. The reduced density matrix and the non-equilibrium effective action
that describes the propagation of neutrinos is obtained by integrating out the
bath degrees of freedom. We obtain the dispersion relations, mixing angles and
relaxation rates of ``neutrino'' quasiparticles. The dispersion relations and
mixing angles are of the same form as those of neutrinos in the medium, and the
relaxation rates are given by where
are the relaxation rates of the flavor fields in
\emph{absence} of mixing, and is the mixing angle in the medium.
A Weisskopf-Wigner approximation that describes the asymptotic time evolution
in terms of a non-hermitian Hamiltonian is derived. At long time
``neutrinos'' equilibrate with the bath. The equilibrium
density matrix is nearly diagonal in the basis of eigenstates of an
\emph{effective Hamiltonian that includes self-energy corrections in the
medium}. The equilibration of ``sterile neutrinos'' via active-sterile mixing
is discussed.Comment: 28 pages, 3 figures, version to appear in PR
Leptogenesis and Low-energy Observables
We relate leptogenesis in a class of theories to low-energy experimental
observables: quark and lepton masses and mixings. With reasonable assumptions
motivated by grand unification, one can show that the CP-asymmetry parameter
takes a universal form. Furthermore the dilution mass is related to the light
neutrino masses. Overall, these models offer a natural explanation for a lepton
asymmetry in the early universe.Comment: 10 pages, revised discussion on light neutrino masse
CP violation in scatterings, three body processes and the Boltzmann equations for leptogenesis
We obtain the Boltzmann equations for leptogenesis including decay and
scattering processes with two and three body initial or final states. We
present an explicit computation of the CP violating scattering asymmetries. We
analyze their possible impact in leptogenesis, and we discuss the validity of
their approximate expressions in terms of the decay asymmetry. In scenarios in
which the initial heavy neutrino density vanishes, the inclusion of CP
asymmetries in scatterings can enforce a cancellation between the lepton
asymmetry generated at early times and the asymmetry produced at later times.
We argue that a sizeable amount of washout is crucial for spoiling this
cancellation, and we show that in the regimes in which the washouts are
particularly weak, the inclusion of CP violation in scatterings yields a
reduction in the final value of the lepton asymmetry. In the strong washout
regimes the inclusion of CP violation in scatterings still leads to a
significant enhancement of the lepton asymmetry at high temperatures; however,
due to the independence from the early conditions that is characteristic of
these regimes, the final value of the lepton asymmetry remains approximately
unchanged.Comment: 24 pages, 6 figures. One appendix added. Some numerical results and
corresponding figures (mainly fig. 3) corrected. Final version to be
published in JHE
Thermal leptogenesis in brane world cosmology
The thermal leptogenesis in brane world cosmology is studied. In brane world
cosmology, the expansion law is modified from the four-dimensional standard
cosmological one at high temperature regime in the early universe. As a result,
the well-known upper bound on the lightest light neutrino mass induced by the
condition for the out-of-equilibrium decay of the lightest heavy neutrino,
eV, can be moderated to be in the case of with the
lightest heavy neutrino mass () and the ``transition temperature''
(), at which the modified expansion law in brane world cosmology is
smoothly connecting with the standard one. This implies that the degenerate
mass spectrum of the light neutrinos can be consistent with the thermal
leptogenesis scenario. Furthermore, as recently pointed out, the gravitino
problem in supersymmetric case can be solved if the transition temperature is
low enough GeV. Therefore, even in the supersymmetric
case, thermal leptogenesis scenario can be successfully realized in brane world
cosmology.Comment: 9 pages, final versio
The importance of flavor in leptogenesis
We study leptogenesis from the out-of-equilibrium decays of the lightest
heavy neutrino in the medium (low) temperature regime, T\lsim 10^{12}
GeV ( GeV), where the rates of processes mediated by the (and
) Yukawa coupling are non negligible, implying that the effects of lepton
flavors must be taken into account. We find important quantitative and
qualitative differences with respect to the case where flavor effects are
ignored: (i) The cosmic baryon asymmetry can be enhanced by up to one order of
magnitude; (ii) The sign of the asymmetry can be opposite to what one would
predict from the sign of the total lepton asymmetry ; (iii)
Successful leptogenesis is possible even with .Comment: 27 pages, 2 figures. Added 3 reference
Flavor Delta(54) in SU(5) SUSY Model
We design a supersymmetric SU (5) GUT model using \Delta (54), a finite
non-abelian subgroup of SU (3)f . Heavy right handed neutrinos are introduced
which transform as three-dimensional repre-sentation of our chosen family
group. The model successfully reproduces the mass hierarchical mass structures
of the Standard Model, and the CKM mixing matrix. It then provides predictions
for the light neutrino with a normal hierarchy and masses such that m{\nu},1
\approx 5\times10-3 eV, m{\nu}, 2 \approx 1\times 10-2 eV, and m{\nu},3 \approx
5 \times 10-2 eV. We also provide predictions for masses of the heavy
neutrinos, and correc- tions to the tri-bimaximal matrix that fit within
experimental limits, e.g. a reactor angle of -7.31o. A simple modification to
our model is introduced at the end and is shown to also produce predictions
that fall well within those limits.Comment: 22 page
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