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G. P. Vacca

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

At least 55 records · Page 3Linked to original sources
arXiv2005

Representations of the LL BFKL Kernel and the Bootstrap

Different forms of the BFKL kernel both in coordinate and momentum representations may appear as a result of different gauge choices and/or inner scalar products of the color singlet states. We study a spectral representation of the BFKL kernel not defined on the Moebius space of functions but on a deformation of it, which provides the usual bootstrap property due to gluon reggeization. In this space the corresponding symmetry is made explicit introducing a deformed realization of the sl(2,C) algebra.

hep-ph↗
arXiv2005

Deformed Spectral Representation of the BFKL Kernel and the Bootstrap for Gluon Reggeization

We investigate the space of functions in which the BFKL kernel acts. For the amplitudes which describe the scattering of colorless projectiles it is convenient to define, in transverse coordinates, the Moebius space in which the solutions to the BFKL equation vanish as the coordinates of the two reggeized gluons coincide. However, in order to fulfill the bootstrap relation for the BFKL kernel it is necessary to modify the space of functions. We define and investigate a new space of functions and show explicitly that the bootstrap relation is valid for the corresponding spectral form of the kernel. We calculate the generators of the resulting deformed representation of the sl(2,C) algebra.

hep-ph↗
arXiv2005

AGK Cutting Rules and Multiple Scattering in Hadronic Collisions

We discuss the AGK rules for the exchange of an arbitrary number of reggeized gluons in perturbative QCD in the high energy limit. Results include the cancellation of corrections to single jet and double jet inclusive cross sections, both for hard and soft rescattering contributions.

hep-ph↗
arXiv2005

Adiabatic regularization of the graviton stress-energy tensor in de Sitter space-time

We study the renormalized energy-momentum tensor of gravitons in a de Sitter space-time. After canonically quantizing only the physical degrees of freedom, we adopt the standard adiabatic subtraction used for massless minimally coupled scalar fields as a regularization procedure and find that the energy density of gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the Hubble parameter, but with a positive sign. According to this result the scalar expansion rate, which is gauge invariant in de Sitter space-time, is increased by the fluctuations. This implies that gravitons may then add to conformally coupled matter in driving the Starobinsky model of inflation.

gr-qc↗
arXiv2004

Small x evolution: BFKL vs Dipole Picture

The relation between the two approaches is discussed both in the linear and nonlinear regimes. It is connected to the gauge invariance and Moebius symmetry in LL perturbative QCD. First corrections beyond the large N approximation are discussed for the nonlinear case.

hep-ph↗
arXiv2004

Pomeron Vertices in Perturbative QCD in Diffractive Scattering

We analyse the momentum space triple Pomeron vertex in perturbative QCD. In addition to the standard form of this vertex which is used in the context of total cross-sections at high energies and in the QCD reggeon field theory, there exists an alternative form which has to be used in the study of high-mass diffraction. We review and analyse the relation between these two versions. We discuss some implications for the BK-equation. In the second part of our paper we extend this analysis to the Pomeron-Odderon-Odderon vertex.

hep-ph↗
arXiv2004

Small x QCD

A review of some theoretical aspects of small x QCD physics is given, with a particular emphasis to the relation between the BFKL and the colour dipole approaches. The nonlinear evolution equations one may construct, as a better approximation beyond the linear analysis, are discussed together with their relation to a possible saturation regime.

hep-ph↗
arXiv2004

Interactions of Reggeized Gluons in the Möbius Representation

We investigate consequences of the Möbius invariance of the BFKL Hamiltonian and of the triple Pomeron vertex. In particular, we show that the triple Pomeron vertex in QCD, when restricted to the large $N_c$ limit and to the space of Möbius functions, simplifies and reduces to the vertex used in the Balitsky-Kovchegov (BK) equation. As a result, the BK equation for the dipole density appears as a special case of the nonlinear evolution equation which sums the fan diagrams for BFKL Green's functions in the Möbius representation. We also calculate the corrections $O(1/(N_c^2-1)$ to the triple Pomeron vertex in the space of Möbius functions, and we present a generalization of the BK-equation in the next-to-leading order approximation in the $1/N_c$ expansion.

hep-ph↗
arXiv2004

Energy-Momentum Tensor of Cosmological Fluctuations during Inflation

We study the renormalized energy-momentum tensor (EMT) of cosmological scalar fluctuations during the slow-rollover regime for chaotic inflation with a quadratic potential and find that it is characterized by a negative energy density which grows during slow-rollover. We also approach the back-reaction problem as a second-order calculation in perturbation theory finding no evidence that the back-reaction of cosmological fluctuations is a gauge artifact. In agreement with the results on the EMT, the average expansion rate is decreased by the back-reaction of cosmological fluctuations.

gr-qc↗
arXiv2003

The Process gamma* + p -> eta_c + X: A Test for the Perturbative QCD Odderon

The rates of inclusive photo- and electroproduction of the eta_c meson: gamma* + p -> eta_c + X are calculated in the triple Regge region, integrated over the diffractive mass X. For the Regge exchanges we use the hard Pomeron and Odderon, both being calculated in the framework of perturbative QCD. The integrated cross-section depends upon the coupling of the BFKL pomeron to two $C=-1$ odderons, and it is found to be of the order 60 pb for photoproduction and 1.5 pb at Q^2=25 GeV^2.

hep-ph↗
arXiv2003

Jet Vertex in the Next-to-Leading log(s) Approximation

The next-to-leading corrections to the jet vertex which is relevant for the Mueller-Navelet jets production in hadronic collisions and for the forward jet cross section in lepton-hadron collisions are presented in the context of a k_t factorizazion formula which resums the leading and next-to-leading logarithms of the energy.

hep-ph↗
arXiv2003

The Inflaton and Time in the Matter-Gravity System

The emergence of time in the matter-gravity system is addressed within the context of the inflationary paradigm. A quantum minisuperspace-homogeneous minimally coupled inflaton system is studied with suitable initial conditions leading to inflation and the system is approximately solved in the limit for large scale factor. Subsequently normal matter (either non homogeneous inflaton modes or lighter matter) is introduced as a perturbation and it is seen that its presence requires the coarse averaging of a gravitational wave function (which oscillates at trans-Planckian frequencies) having suitable initial conditions. Such a wave function, which is common for all types of normal matter, is associated with a ``time density'' in the sense that its modulus is related to the amount of time spent in a given interval (or the rate of flow of time). One is then finally led to an effective evolution equation (Schroedinger Schwinger-Tomonaga) for ``normal'' matter. An analogy with the emergence of a temperature in statistical mechanics is also pointed out.

gr-qc↗
arXiv2002

Jets at high energies, factorization and jet vertex in NL ln(s)

The next-to-leading corrections to the jet vertex which is relevant for the Mueller-Navelet jets production in hadronic collisions and for the forward jet cross section in lepton-hadron collisions are presented in the context of a k_t factorizazion formula which resums the leading and next-to-leading logarithms of the energy. Both the quark- and gluon-initiated contribution are now computed. This completes the framework for a full phenomenological analysis of Mueller-Navelet jets in NL log(s) approximation. Forward jets phenomenology still requires the NL photon impact factor.

hep-ph↗
arXiv2002

On the Triple Pomeron Vertex in Perturbative QCD

We estimate the size of the triple Pomeron vertex in perturbative QCD and compare with the phenomenological value extracted from Regge fits to experimental data. For simplicity, the results of the QCD analysis are taken in the large-N_c limit. We find that the perturbative triple Pomeron coupling is of the same order of magnitude as the observed one. We also estimate the size of the Pomeron self energy and its contribution to the renormalization of the Pomeron intercept. The effect is very small, in agreement with previous nonperturbative estimates.

hep-ph↗
arXiv2001

The NLO Jet Vertex for Mueller-Navelet and Forward Jets: the Quark Part

We calculate the next-to-leading corrections to the jet vertex which is relevant for the Mueller-Navelet-jets production in p-pbar collisions and for the forward jet cross section in e-p collisions. In this first part we present the results of the vertex for an incoming quark. Particular emphasis is given to the separation of the collinear divergent part and the central region of the produced gluon.

hep-ph↗
arXiv2001

Energy-Momentum Tensor of Field Fluctuations in Massive Chaotic Inflation

We study the renormalized energy-momentum tensor (EMT) of the inflaton fluctuations in rigid space-times during the slow-rollover regime for chaotic inflation with a mass term. We use dimensional regularization with adiabatic subtraction and introduce a novel analytic approximation for the inflaton fluctuations which is valid during the slow-rollover regime. Using this approximation we find a scale invariant spectrum for the inflaton fluctuations in a rigid space-time, and we confirm this result by numerical methods. The resulting renormalized EMT is covariantly conserved and agrees with the Allen-Folacci result in the de Sitter limit, when the expansion is exactly linearly exponential in time. We analytically show that the EMT tensor of the inflaton fluctuations grows initially in time, but saturates to the value H^2 H(0)^2, where H is the Hubble parameter and H(0) is its value when inflation has started. This result also implies that the quantum production of light scalar fields (with mass smaller or equal to the inflaton mass) in this model of chaotic inflation depends on the duration of inflation and is larger than the usual result extrapolated from the de Sitter result.

gr-qc↗