3,706 research outputs found
Dispersive and chiral symmetry constraints on the light meson form factors
The form factors of the light pseudoscalar mesons are investigated in a
dispersive formalism based on hadronic unitarity, analyticity and the OPE
expansion of the QCD Green functions. We propose generalizations of the
original mathematical techniques, suitable for including additional low energy
information provided by experiment or Chiral Perturbation Theory (CHPT). The
simultaneous treatment of the electroweak form factors of the and
mesons allows us to test the consistency with QCD of a low energy CHPT theorem.
By applying the formalism to the pion electromagnetic form factor, we derive
quite strong constraints on the higher Taylor coefficients at zero momentum,
using information about the phase and the modulus of the form factor along a
part of the unitarity cut.Comment: 29 pages, 3 figure
Stochastic background of gravitational waves from cosmological sources
Gravitational waves (GW) can constitute a unique probe of the primordial
universe. In many cases, the characteristic frequency of the emitted GW is
directly related to the energy scale at which the GW source is operating in the
early universe. Consequently, different GW detectors can probe different energy
scales in the evolution of the universe. After a general introduction on the
properties of a GW stochastic background of primordial origin, some examples of
cosmological sources are presented, which may lead to observable GW signals.Comment: Proceedings of LISA Symposium X, accepted for publication in Journal
of Physics: Conference Series. Typos corrected, two references adde
Adding helicity to inflationary magnetogenesis
The most studied mechanism of inflationary magnetogenesis relies on the
time-dependence of the coefficient of the gauge kinetic term
. Unfortunately, only extremely finely tuned versions
of the model can consistently generate the cosmological magnetic fields
required by observations. We propose a generalization of this model, where also
the pseudoscalar invariant is multiplied by a
time dependent function. The new parity violating term allows more freedom in
tuning the amplitude of the field at the end of inflation. Moreover, it leads
to a helical magnetic field that is amplified at large scales by
magnetohydrodynamical processes during the radiation dominated epoch. As a
consequence, our model can satisfy the observational lower bounds on fields in
the intergalactic medium, while providing a seed for the galactic dynamo, if
inflation occurs at an energy scale ranging from to GeV. Such
energy scale is well below that suggested by the recent BICEP2 result, if the
latter is due to primordial tensor modes. However, the gauge field is a source
of tensors during inflation and generates a spectrum of gravitational waves
that can give a sizable tensor to scalar ratio even if
inflation occurs at low energies. This system therefore evades the Lyth bound.
For smaller values of , lower values of the inflationary energy scale are
required. The model predicts fully helical cosmological magnetic fields and a
chiral spectrum of primordial gravitational waves.Comment: 17 pages, 4 figures. Minor changes to match the version accepted for
publication in JCA
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