579 research outputs found
Thermal production of ultrarelativistic right-handed neutrinos: Complete leading-order results
The thermal production of relativistic right-handed Majorana neutrinos is of
importance for models of thermal leptogenesis in the early Universe.
Right-handed neutrinos can be produced both by 1 2 decay or inverse decay
and by 2 -> 2 scattering processes. In a previous publication, we have studied
the production via 1 2 (inverse) decay processes. There we have shown that
multiple scattering mediated by soft gauge boson exchange also contributes to
the production rate at leading order, and gives a strong enhancement. Here we
complete the leading order calculation by adding 2 -> 2 scattering processes
involving either electroweak gauge bosons or third-generation quarks. We find
that processes with gauge interactions give the most important contributions.
We also obtain a new sum rule for the Hard Thermal Loop resummed fermion
propagator.Comment: 27 pages, 7 figures. Error in the matrix element for the
(subdominant) subprocess with s-channel fermion exchange corrected. This
changes the corresponding phase space integral and the constant c_V.
Numerically it increases the total 2 -> 2 rate by about 2 percent and the
complete rate by about 1 percent. The main results and conclusions are
unaffecte
Expectation values of four-quark operators in pions
The values of four-quark operators averaged over pions are expressed through
those averaged over vacuum. The specific values are obtained in the framework
of the factorization assumption. For the condensates of the light quarks of the
same flavour \bar q\Gamma q\bar q\Gamma q the scalar condensate is shown to be
an order of magnitude larger than the other ones. The condensates containing
the strange quarks \bar q q\bar s s appear to be only about twice smaller than
those of the light quarks. The degeneracy of the ground state in the
Nambu--Jona--Lasinio model is shown explicitly.Comment: 9 pages, no figures, typos correcte
Finite density QCD sum rules for nucleons
It is shown how the QCD sum rules can be applied for the investigation of the
density dependence of the nucleon parameters. These characteristics can be
expressed through the expectation values of QCD operators in nuclear matter. In
certain approximations the expectation values are related to the observables.
First applications of the approach reproduced some of the basic features of
nuclear physics, providing also a new knowledge. The program of the future work
is presented. The difficulties of the approach are discussed.Comment: 22 pages, Lecture at the International School on Nuclear Physics,
Erice,200
Two- and Three-Point Functions in the Extended NJL Model
The two-point functions in generalized Nambu--Jona-Lasinio models are
calculated to all orders in momenta and quark masses to leading order in
. The use of Ward identities and the heat-kernel expansion allows for a
large degree of regularization independence. We also show how this approach
works to the same order for three-point functions on the example of the
vector-pseudoscalar-pseudoscalar three-point function. The inclusion of the
chiral anomaly effects at this level is shown by calculating the
pseudoscalar-vector-vector three-point function to the same order. Finally we
comment on how (vector-)meson-dominance comes out in the presence of explicit
chiral symmetry breaking in both the anomalous and the non-anomalous sectors.Comment: Latex, 42 pages, 3 latex figures, 7 postscript figures included,
NORDITA 94/11 N,P. Improvement in the regularization procedure for the PVV
three point functio
Lectures on Chiral Disorder in QCD
I explain the concept that light quarks diffuse in the QCD vacuum following
the spontaneous breakdown of chiral symmetry. I exploit the striking analogy to
disordered electrons in metals, identifying, among others, the universal regime
described by random matrix theory, diffusive regime described by chiral
perturbation theory and the crossover between these two domains.Comment: Lectures given at the Cargese Summer School, August 6-18, 200
Low--Energy Behavior of Two--Point Functions of Quark Currents
We discuss vector, axial-vector, scalar and pseudoscalar two-point functions
at low and intermediate energies. We first review what is known from chiral
perturbation theory, as well as from a heat kernel expansion within the context
of the extended Nambu-Jona-Lasinio (ENJL) model of ref. \cite{12}. In this work
we derive then these two-point functions to all orders in the momenta and to
leading order in . We find an improved high-energy behaviour and a
general way of parametrizing them that shows relations between some of the
two-point functions, which are also valid in the presence of gluonic
interactions. The similarity between the shape of the experimentally known
spectral functions and the ones we derive, is greatly improved with respect to
those predicted by the usual constituent quark like models. We also obtain the
scalar mass independent of the regularization scheme. In the end,
we calculate fully an example of a nonleptonic matrix element in the
ENJL--model, the electromagnetic mass difference and find good
agreement with the measured value.Comment: 37 pages + 6 uuencoded figures, CERN TH 6924/93, CPT-93/P.2917,
NORDITA 93/43-N,
R^2 Corrections to Asymptotically Lifshitz Spacetimes
We study corrections to five-dimensional asymptotically Lifshitz
spacetimes by adding Gauss-Bonnet terms in the effective action. For the
zero-temperature backgrounds we obtain exact solutions in both pure
Gauss-Bonnet gravity and Gauss-Bonnet gravity with non-trivial matter. The
dynamical exponent undergoes finite renormalization in the latter case. For the
finite-temperature backgrounds we obtain black brane solutions perturbatively
and calculate the ratio of shear viscosity to entropy density . The KSS
bound is still violated but unlike the relativistic counterparts, the causality
of the boundary field theory cannot be taken as a constraint.Comment: 24 pages, Latex, typos fixed, accepted by JHE
Common Origin of Soft mu-tau and CP Breaking in Neutrino Seesaw and the Origin of Matter
Neutrino oscillation data strongly support mu-tau symmetry as a good
approximate flavor symmetry of the neutrino sector, which has to appear in any
viable theory for neutrino mass-generation. The mu-tau breaking is not only
small, but also the source of Dirac CP-violation. We conjecture that both
discrete mu-tau and CP symmetries are fundamental symmetries of the seesaw
Lagrangian (respected by interaction terms), and they are only softly broken,
arising from a common origin via a unique dimension-3 Majorana mass-term of the
heavy right-handed neutrinos. From this conceptually attractive and simple
construction, we can predict the soft mu-tau breaking at low energies, leading
to quantitative correlations between the apparently two small deviations
\theta_{23} - 45^o and \theta_{13} - 0^o. This nontrivially connects the
on-going measurements of mixing angle \theta_{23} with the upcoming
experimental probes of \theta_{13}. We find that any deviation of \theta_{23} -
45^o must put a lower limit on \theta_{13}. Furthermore, we deduce the low
energy Dirac and Majorana CP violations from a common soft-breaking phase
associated with mu-tau breaking in the neutrino seesaw. Finally, from the soft
CP breaking in neutrino seesaw we derive the cosmological CP violation for the
baryon asymmetry via leptogenesis. We fully reconstruct the leptogenesis
CP-asymmetry from the low energy Dirac CP phase and establish a direct link
between the cosmological CP-violation and the low energy Jarlskog invariant. We
predict new lower and upper bounds on the \theta_{13} mixing angle, 1^o <
\theta_{13} < 6^o. In addition, we reveal a new hidden symmetry that dictates
the solar mixing angle \theta_12 by its group-parameter, and includes the
conventional tri-bimaximal mixing as a special case, allowing deviations from
it.Comment: 60pp, JCAP in Press, v2: only minor stylistic refinements (added Daya
Bay's future sensitivity in Figs.2+8, shortened some eqs, added new
Appendix-A and some references), comments are welcome
Dilepton Spectra from Decays of Light Unflavored Mesons
The invariant mass spectrum of the and pairs
from decays of light unflavored mesons with masses below the -meson mass to final states containing along with a dilepton pair one
photon, one meson, and two mesons are calculated within the framework of the
effective meson theory. The results can be used for simulations of the dilepton
spectra in heavy-ion collisions and for experimental searches of dilepton meson
decays.Comment: 73 pages, 19 figures, 3 tables, REVTeX, new references adde
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