arXiv subjects

Gian Paolo Vacca

Publications and source records attributed to Gian Paolo Vacca.

43 records · Page 3Linked to original sources

arXiv2013

Gauge invariant backreaction in general single field models of inflation

In a general single field inflationary model we consider the effects of long wavelength scalar fluctuations on the effective expansion rate and equation of state seen by a class of free falling observers, using a physical gauge invariant formulation. In a previous work we showed that for a free massive inflaton no backreaction is observed within some constraints. In this paper we extend the validity of our previous results to the case of an arbitrary self-interacting inflation potential, working to second order in cosmological perturbation theory and to all order in slow-roll approximation. For these general inflationary models, we also show the equivalence of the free falling observers to the ones comoving with the inflaton field.

gr-qc↗

arXiv2013

Cosmological Backreaction for a Test Field Observer in a Chaotic Inflationary Model

In an inhomogeneous universe, an observer associated with a particular matter field does not necessarily measure the same cosmological evolution as an observer in a homogeneous and isotropic universe. Here we consider, in the context of a chaotic inflationary background model, a class of observers associated with a "clock field" for which we use a light test field. We compute the effective expansion rate and fluid equation of state in a gauge invariant way, taking into account the quantum fluctuations of the long wavelength modes, and working up to second order in perturbation theory and in the slow-roll approximation. We find that the effective expansion rate is smaller than what would be measured in the absence of fluctuations. Within the stochastic approach we study the bounds for which the approximations we make are consistent.

hep-th↗

arXiv2003

QCD Corrections to Electroweak Vector Boson Scattering at Small Scattering Angles

We investigate the role of a certain class of QCD corrections to electroweak vector boson scattering at small scattering angles and large energies. These are present since, from the perturbative analysis, the vector bosons may dissociate into quark-antiquark pairs giving rise to colour dipoles interacting through gluon exchanges. After the computation of the vector boson impact factors, we present expressions for the lowest order QCD scattering amplitude and for the leading logarithmic BFKL amplitude. Particularly we discuss numerical results for the process $γγ\to ZZ$. The QCD corrections to the cross section resulting from the interference with the electroweak ones are estimated and compared with the leading pure electroweak part. Corrections resulting from the leading log BFKL amplitude are of the order of few percent already at the $0.5 - 1$ TeV energy range.

hep-ph↗

arXiv1999

The 2nd order corrections to the interaction of two reggeized gluons from the bootstrap

The 2nd order corrections are obtained to both forward and nonforward interaction of reggeized gluons in the octet colour channel using as a basis the bootstrap relation and a specific ansatz to solve it. The obtained forward kernel coincides with the logarithmic term plus two first non-logarithmic terms in the Pomeron 2nd order kernel. Both forward and nonforward kernels are found to be infrared finite.

hep-ph↗

arXiv1997

Evolution of the gluon density in x with a running coupling constant

This is a draft describing the calculation of the evolution of the gluon density in $x$ from an initial value $x=x_{0}=0.01$ to smaller values, up to $x=10^{-8}$ in the hard pomeron formalism with a running coupling introduced on the basis of the bootstrap equation. The obtained gluon density is used to calculate the singlet part of the proton structure function. Comparison with experiment and the results following from the fixed coupling evolution is made.

hep-ph↗

arXiv1997

Jet production from the perturbative QCD pomeron with a running coupling constant

An analysis of minijet production from the hard pomeron with a running coupling constant is performed. Two supercritical pomerons found in the numerical study are taken into account. The calculated inclusive jet production rate is finite at small $k_{\bot}$ and behaves like $1/k^{-4}_{\bot}$ at high $k_{\bot}$ modulo factors coming from logarithmic terms. The average $k_{\bot}$ is found to be very large ($\sim 10-13 GeV/c$) and practically independent of energy. This is interpreted as an indication that at present energies we are still far from the asymptotics and that, apart from supercritical pomerons, other states contribute significantly.

hep-ph↗

arXiv1996

Properties of the hard pomeron with a running coupling constant and the high-energy scattering

The equation for the hard pomeron with a running coupling introduced on the basis of the bootstrap requirement is solved numerically. Two supercritical pomerons are found with the intercept minus 1 of the leading one of the order 0.35-0.5 and that of the subleading one half as large. The contribution of multipomeron exchanges is found to be essential only at extremely high energies of the order of 100 TeV. Comparison of the cross-sections and structure functions to the present experimental data seems to indicate that the asymptotical regime has not yet been achieved.

hep-ph↗