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G. P. Vacca

Publications and source records attributed to G. P. Vacca.

At least 37 records · Page 2Linked to original sources
arXiv2010

Stochastic growth of quantum fluctuations during slow-roll inflation

We compute the growth of the mean square of quantum fluctuations of test fields with small effective mass during a slowly changing, nearly de Sitter stage which took place in different inflationary models. We consider a minimally coupled scalar with a small mass, a modulus with an effective mass $ \propto H^2$ (with $H$ as the Hubble parameter) and a massless non-minimally coupled scalar in the test field approximation and compare the growth of their relative mean square with the one of gauge-invariant inflaton fluctuations. We find that in most of the single field inflationary models the mean square gauge invariant inflaton fluctuation grows {\em faster} than any test field with a non-negative effective mass. Hybrid inflationary models can be an exception: the mean square of a test field can dominate over the gauge invariant inflaton fluctuation one on suitably choosing parameters. We also compute the stochastic growth of quantum fluctuation of a second field, relaxing the assumption of its zero homogeneous value, in a generic inflationary model; as a main result, we obtain that the equation of motion of a gauge invariant variable associated, order by order, with a generic quantum scalar fluctuation during inflation can be obtained only if we use the number of e-folds as the time variable in the corresponding Langevin and Fokker-Planck equations for the stochastic approach. We employ this approach to derive some bounds in the case of a model with two massive fields.

hep-th↗
arXiv2010

Asymptotic Safety in Einstein Gravity and Scalar-Fermion Matter

Within the functional renormalization group approach we study the effective QFT of Einstein gravity and one self-interacting scalar coupled to N_f Dirac fermions. We include in our analysis the matter anomalous dimensions induced by all the interactions and analyze the highly non linear beta functions determining the renormalization flow. We find the existence of a non trivial fixed point structure both for the gravity and the matter sector, besides the usual gaussian matter one. This suggest that asymptotic safety could be realized in the gravitational sector \emph{and} in the standard model. Non triviality in the Higgs sector might involve gravitational interactions.

hep-th↗
arXiv2010

Gravitational corrections to Yukawa systems

We compute the gravitational corrections to the running of couplings in a scalar-fermion system, using the Wilsonian approach. Our discussion is relevant for symmetric as well as for broken scalar phases. We find that the Yukawa and quartic scalar couplings become irrelevant at the Gaussian fixed point.

hep-th↗
arXiv2009

PT Symmetry and Renormalization in a Pomeron Model

A novel perturbative analysis for the 2+1 local supercritical field theory of pomerons is developed. It is based on the PT symmetry of the model which allows to study a similar Hamiltonian with the same real perturbative spectrum. In the lowest non trivial order of perturbation theory the pomeron interactions are shown to lead to the renormalization of the slope. The appearance of a non local interaction for two pomeron states is such that at small coupling only scattering states are present and the spectrum of two particle states is not affected.

hep-ph↗
arXiv2009

Generation of fluctuations during inflation: comparison of stochastic and field-theoretic approaches

We prove that the stochastic and standard field-theoretical approaches produce exactly the same results for the amount of light massive scalar field fluctuations generated during inflation in the leading order of the slow-roll approximation. This is true both in the case for which this field is a test one and inflation is driven by another field, and the case for which the field plays the role of inflaton itself. In the latter case, in order to calculate the average of the mean square of the gauge-invariant inflaton fluctuation, the logarithm of the scale factor $a$ has to be used as the time variable in the Fokker-Planck equation in the stochastic approach. The implications of particle production during inflation for the second stage of inflation and for the moduli problem are also discussed. The case of a massless self-interacting test scalar field in a de Sitter background with a zero initial renormalized mean square is also considered in order to show how the stochastic approach can easily produce results corresponding to diagrams with an arbitrary number of scalar field loops in the field-theoretical approach (explicit results up to 4 loops inclusive are presented).

hep-th↗
arXiv2008

PT Symmetry and Hermithean Hamiltonian in the Local Supercritical Pomeron Model

The local reggeon field theory is studied perturbatively taking advantage of the PT symmetry in the Hamiltonian formulation. In the lowest non trivial order we show that the pomeron interactions renormalize the slope. In the same order we find a non local pair potential acting between pomerons, which has a singular structure. However the analysis of the scattering operator shows that at small coupling constant bound states do not appear so that the two-particle spectrum is not changed.

hep-ph↗
arXiv2008

Inclusive 1-jet Production Cross Section at Small x in QCD: Multiple Interactions

We study corrections due to two Pomeron exchanges to the inclusive 1-jet production cross section in the Regge limit of perturbative QCD for a finite number of colors. By considering deep inelastic scattering on a weakly bound two-nucleon system, we carefully follow the logic of the AGK cutting rules and show, for the single inclusive cross section, that, due to the reggeization of the gluon, modifications of the AGK cutting rules appear. As our main result, we investigate and calculate the jet production vertex in the presence of a two-Pomeron cut correction. Compared to previous studies, we find a novel structure of the jet vertex which has not been considered before. We discuss a few implications of this new piece.

hep-ph↗
arXiv2008

Vacuum Energy and Standard Model Physics

The conditions for the cancellation of one loop contributions to vacuum energy (both U.V. divergent and finite) coming from the Standard Model (SM) fields are examined. It is proven that this is not possible unless one introduces besides several bosons, at least one massive fermion having mass within specific ranges. On examining one of the simplest SM extensions satisfying the constraints one finds that the mass range of the lightest massive boson is compatible with the present Higgs mass bounds. We do not consider effects associated with condensates or renormalization group running.

hep-th↗
arXiv2008

Cosmological Unparticle Correlators

We introduce and study an extension of the correlator of unparticle matter operators in a cosmological environment. Starting from FRW spaces we specialize to a de Sitter spacetime and derive its inflationary power spectrum which we find to be almost flat. We finally investigate some consequences of requiring the existence of a unitary boundary conformal field theory in the framework of the dS/CFT correspondence.

hep-th↗
arXiv2007

Small x QCD and Multigluon States: a Color Toy Model

We introduce and study a toy model with a finite number of degrees of freedom whose Hamiltonian presents the same color structure of the BKP system appearing in the studies of QCD in the Regge limit. We address within this toy model the question of the importance of finite number of color corrections with respect to the planar limit case.

hep-th↗
arXiv2007

Colored Spin Systems, BKP Evolution and finite N_c effects

Even within the framework of the leading logarithmic approximation the eigenvalues of the BKP kernel for states of more than three reggeized gluons are unknown in general, contrary to the planar limit case where the problem becomes integrable. We consider a 4-gluon kernel for a finite number of colors and define some simple toy models for the configuration space dynamics, which are directly solvable with group theoretical methods. Then we study the dependence of the spectrum of these models with respect to the number of colors and make comparisons with the large limit case.

hep-th↗
arXiv2007

Multiple interactions and AGK rules in pQCD

We review some aspects of multiple interactions in High Energy QCD; we discuss in particular AGK rules and present some results concerning multiple interactions in the context of jet production.

hep-ph↗
arXiv2007

The Impact of Ultraviolet Regularization on the Spectrum of Curvature Perturbations During Inflation

Inflationary predictions based on the linear theory of cosmological perturbations are related to the two point function of a (second quantized) real scalar free field during the accelerated stage. Such a two point function is finite, in contrast with its coincidence limit, which is divergent due to the ultraviolet divergences proper of field theory. We therefore argue that predictions of most of the inflationary models do not necessarily need a regularization scheme to leading order, i.e. tree level, which is required instead for non-linear corrections or calculations involving the energy-momentum tensor. We also discuss unpleasant features of the "would be" regularized spectrum obtained using the traditional fourth order adiabatic subtraction.

hep-th↗
arXiv2007

Vacuum energy and spectral function sum rules

We reformulate the problem of the cancellation of the ultraviolet divergencies of the vacuum energy, particularly important at the cosmological level, in terms of a saturation of spectral function sum rules which leads to a set of conditions on the spectrum of the fundamental theory. We specialize the approach to both Minkowski and de Sitter space-times and investigate some examples.

hep-th↗
arXiv2007

Non-adiabatic quantum effects from a Standard Model time-dependent Higgs vev

We consider the time-dependence of the Higgs vacuum expectation value (vev) given by the dynamics of the Standard Model and study the non-adiabatic production of both bosons and fermions, which is intrinsically non-perturbative. In the Hartree approximation, we analyse the general expressions that describe the dissipative dynamics due to the back-reaction of the produced particles. In particular, we solve numerically some relevant cases for the Standard Model phenomenology in the regime of relatively small oscillations of the Higgs vev.

hep-th↗
arXiv2006

On the 'toy model" in the Reggeon field theory

The Reggeon field theory with zero transverse dimensions is studied in the Hamiltonian formulation for both sub-and supercritical pomeron. Mathematical aspects of the model, in particular the scalar products in the space of quantum states, are discussed. Relation to reaction-diffusion processes is derived in absence of pomeron merging. Numerical calculations for different parameters of the models, $α(0)-1=μ$ and the triple pomeron coupling constant $λ$, show that the triple pomeron interaction always makes amplitudes fall with rapidity irrespective of the value of the intercept. The smaller the values of the ratio $λ/μ$ the higher are rapidities $y$ at which this fall starts, so that at small values of $λ$ it begins at asymptotically high rapidities (for $λ/μ<1/4$ the fall is noticeable only at $μy>100$). No visible singularity is seen for the critical pomeron. A perturbative treatment is proposed which may be useful for more realistic models.

hep-ph↗
arXiv2006

Second Order Gauge-Invariant Perturbations during Inflation

The evolution of gauge invariant second-order scalar perturbations in a general single field inflationary scenario are presented. Different second order gauge invariant expressions for the curvature are considered. We evaluate perturbatively one of these second order curvature fluctuations and a second order gauge invariant scalar field fluctuation during the slow-roll stage of a massive chaotic inflationary scenario, taking into account the deviation from a pure de Sitter evolution and considering only the contribution of super-Hubble perturbations in mode-mode coupling. The spectra resulting from their contribution to the second order quantum correlation function are nearly scale-invariant, with additional logarithmic corrections to the first order spectrum. For all scales of interest the amplitude of these spectra depend on the total number of e-folds. We find, on comparing first and second order perturbation results, an upper limit to the total number of e-folds beyond which the two orders are comparable.

gr-qc↗