39,148 research outputs found
Observational Constraints on Transverse Gravity: a Generalization of Unimodular Gravity
We explore the hypothesis that the set of symmetries enjoyed by the theory
that describes gravity is not the full group of diffeomorphisms Diff(M), as in
General Relativity, but a maximal subgroup of it, TransverseDiff(M), with its
elements having a jacobian equal to unity; at the infinitesimal level, the
parameter describing the coordinate change, xi^mu (x), is transverse, i.e.,
partial_mu(xi^mu)=0. Incidentally, this is the smaller symmetry one needs to
propagate consistently a graviton, which is a great theoretical motivation for
considering these theories. Also, the determinant of the metric, g, behaves as
a "transverse scalar", so that these theories can be seen as a generalization
of the better-known unimodular gravity. We present our results on the
observational constraints on transverse gravity, in close relation with the
claim of equivalence with general scalar-tensor theory. We also comment on the
structure of the divergences of the quantum theory to the one-loop order.Comment: Prepared for the First Mediterranean Conference on Classical and
Quantum Gravity, MCCQG, Kolymbari (Crete, Greece), 14-18 September, 2009;
also, ERE2009: Gravitation in the Large, Bilbao (Spain), 7-11 September, 200
Desperately Seeking Chiral Fermions
Chiral fermions can (presumably) be constructed by introducing two
regulators, one for the gauge fields (e.g. a lattice), and another for the
fermion functional integrals in a fixed (regulated) gauge field. This talk
discusses cutoff effects arising from the regulator of the fermions.Comment: 4 pages, contribution to Lattice '95 Postscript at
http://www-theory.fnal.gov/people/ask/TeX/lat95/chiral.p
Neutrino interactions with nucleons and nuclei at intermediate energies
We investigate neutrino-nucleus collisions at intermediate energies
incorporating quasielastic scattering and Delta(1232) excitation as elementary
processes, together with Fermi motion, Pauli blocking and mean-field potentials
in the nuclear medium. A full coupled-channel treatment of final state
interactions is achieved with a semiclassical BUU transport model. Results for
inclusive reactions and nucleon knockout are presented.Comment: Proceedings of PANIC'05, 3 pages, 3 figure
Neutral current neutrino-nucleus interactions at intermediate energies
We have extended our model for charged current neutrino-nucleus interactions
to neutral current reactions. For the elementary neutrino-nucleon interaction,
we take into account quasielastic scattering, Delta excitation and the
excitation of the resonances in the second resonance region. Our model for the
neutrino-nucleus collisions includes in-medium effects such as Fermi motion,
Pauli blocking, nuclear binding, and final-state interactions. They are
implemented by means of the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU)
coupled-channel transport model. This allows us to study exclusive channels,
namely pion production and nucleon knockout. We find that final-state
interactions modify considerably the distributions through rescattering,
charge-exchange and absorption. Side-feeding induced by charge-exchange
scattering is important in both cases. In the case of pions, there is a strong
absorption associated with the in-medium pionless decay modes of the Delta,
while nucleon knockout exhibits a considerable enhancement of low energy
nucleons due to rescattering. At neutrino energies above 1 GeV, we also obtain
that the contribution to nucleon knockout from Delta excitation is comparable
to that from quasielastic scattering.Comment: 16 pages, 10 figures; v2: version to be published in Phys. Rev. C,
comparison with results of Paschos et al. remove
Some remarks about intrinsic parity in Ryder's derivation of the Dirac equation
This work is a comment on Ryder's derivation of the Dirac equation, with
emphasis on the physical contents of this equation: the notion of particles and
antiparticles according to the Stueckelberg-Feynman interpretation, the
opposite intrinsic parity between particles and antiparticles, and the spin.Comment: 4 pages, Revte
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