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Massimo Pietroni

Publications and source records attributed to Massimo Pietroni.

52 records · Page 3Linked to original sources
arXiv2005

Dark Energy Condensation

The two most popular candidates for dark energy, i.e. a cosmological constant and quintessence, are very difficult to distinguish observationally, mostly because the quintessence field does not have sizable fluctuations. We study a scalar field model for dark energy in which the scalar field is invariant under reflection symmetry, phi -> -phi. Under general assumptions, there is a phase transition at late times (z < 0.5). Before the phase transition, the field behaves as a cosmological constant. After the phase transition, a time-dependent scalar condensate forms, the field couples with dark matter and develops sizable perturbations tracking those of dark matter. The background cosmological evolution is in agreement with existing observations, but might be distinguished from that of a cosmological constant by future Supernovae surveys. The growth of cosmological perturbations carries the imprint of the phase transition, however a non-linear approach has to be developed in order to study it quantitatively.

astro-ph↗
arXiv2003

Brane Worlds and the Cosmic Coincidence Problem

Brane world models with `large' extra dimensions with radii in the r_l ~ 0.01- 0.1 mm range and smaller ones at r_s < (1 TeV)^(-1) have the potential to solve the cosmic coincidence problem, i.e. the apparently fortuitous equality between dark matter and dark energy components today. The main ingredient is the assumption of a stabilization mechanism fixing the total volume of the compact submanifold, but allowing for shape deformations. The latter are associated with phenomenologically safe ultra-light scalar fields. Bulk fields Casimir energy naturally plays the role of dark energy, which decreases in time because of expanding r_l. Stable Kaluza Klein states may play the role of dark matter with increasing, O(1/r_s), mass. The cosmological equations exhibit attractor solutions in which the global equation of state is negative, the ratio between dark energy and dark matter is constant and the observed value of the ratio is obtained for two large extra dimensions. Experimental searches of large extra dimensions should take into account that, due to the strong coupling between dark matter and radii dynamics, the size of the large extra dimensions inside the galactic halo may be smaller than the average value.

hep-ph↗
arXiv2002

The Cosmic Coincidence Problem in the Brane World

The so called `cosmic coincidence' problem seems to suggest a coupling between dark energy and dark matter. A possibility could be given by dark matter particles with masses depending exponentially on the scalar field associated to dark energy. If the latter also has an exponential potential, attractor solutions exist with constant Omega_{DM} and Omega_{DE} and negative effective equation of state. Due to the very low mass of the dark energy field, its coupling to baryons should be strongly suppressed. Here we present a natural realization of this scenario in a model with large extra dimensions.

hep-ph↗
arXiv2001

Hard Non-commutative Loops Resummation

The non-commutative version of the euclidean $g^2ϕ^4$ theory is considered. By using Wilsonian flow equations the ultraviolet renormalizability can be proved to all orders in perturbation theory. On the other hand, the infrared sector cannot be treated perturbatively and requires a resummation of the leading divergencies in the two-point function. This is analogous to what is done in the Hard Thermal Loops resummation of finite temperature field theory. Next-to-leading order corrections to the self-energy are computed, resulting in $O(g^3)$ contributions in the massless case, and $O(g^6\log g^2)$ in the massive one.

hep-th↗
arXiv2001

Wilsonian Renormalization Group and the Non-Commutative IR/UV Connection

We study the IR/UV connection of the four-dimensional non-commutative phi^4 theory by using the Wilsonian Renormalization Group equation. Extending the usual formulation to the non-commutative case we are able to prove UV renormalizability to all orders in perturbation theory. The full RG equations are finite in the IR, but perturbative approximations of them are plagued by IR divergences. The latter can be systematically resummed in a way analogous to what is done in finite temperature field theory. As an application, next-to-leading order corrections to the two-point function are explicitly computed. The usual Wilsonian picture, i.e. the insensitivity of the IR regime to the UV, does not hold in the non-commutative case. Nevertheless it can be partially recovered by a matching procedure, in which a high-energy theory, defined in the deep non-commutative regime, is connected at some intermediate scale to a commutative low-energy theory. The latter knows about non-commutativity only through the boundary conditions for two would-be irrelevant couplings.

hep-th↗
arXiv2000

Boltzmann Suppression of Interacting Heavy Particles

Matsumoto and Yoshimura have recently argued that the number density of heavy particles in a thermal bath is not necessarily Boltzmann-suppressed for T << M, as power law corrections may emerge at higher orders in perturbation theory. This fact might have important implications on the determination of WIMP relic densities. On the other hand, the definition of number densities in a interacting theory is not a straightforward procedure. It usually requires renormalization of composite operators and operator mixing, which obscure the physical interpretation of the computed thermal average. We propose a new definition for the thermal average of a composite operator, which does not require any new renormalization counterterm and is thus free from such ambiguities. Applying this definition to the model of Matsumoto and Yoshimura we find that it gives number densities which are Boltzmann-suppressed at any order in perturbation theory. We discuss also heavy particles which are unstable already at T=0, showing that power law corrections do in general emerge in this case.

hep-ph↗
arXiv1999

Scalar-Tensor Gravity and Quintessence

Scalar fields with inverse power-law effective potentials may provide a negative pressure component to the energy density of the universe today, as required by cosmological observations. In order to be cosmologically relevant today, the scalar field should have a mass $m_ϕ= O(10^{-33} {\mathrm eV})$, thus potentially inducing sizable violations of the equivalence principle and space-time variations of the coupling constants. Scalar-tensor theories of gravity provide a framework for accommodating phenomenologically acceptable ultra-light scalar fields. We discuss non-minimally coupled scalar-tensor theories in which the scalar-matter coupling is a dynamical quantity. Two attractor mechanisms are operative at the same time: one towards the tracker solution, which accounts for the accelerated expansion of the Universe, and one towards general relativity, which makes the ultra-light scalar field phenomenologically safe today. As in usual tracker-field models, the late-time behavior is largely independent on the initial conditions. Strong distortions in the cosmic microwave background anisotropy spectra as well as in the matter power spectrum are expected.

hep-ph↗
arXiv1999

SUSY QCD and Quintessence

Recent data point in the direction of a cosmological constant dominated universe. We investigate the role of supersymmetric QCD with N_f < N_c as a possible candidate for dynamical cosmological constant (``quintessence''). When N_f > 1, the multiscalar dynamics is fully taken into account, showing that a certain degree of flavor symmetry in the initial conditions is necessary in order to attain the required late-time ``tracking'' attractor behavior. A possible solution to the problem of long-range interactions of gravitational strength is proposed. Finally we show how, as a consequence of the interaction with the Standard Model fields, the early cosmological evolution of the scalar fields may be dramatically altered, and the approach to the tracking behavior may become much more efficient.

hep-ph↗
arXiv1999

Critical Slowing Down and Defect Formation

The formation of topological defects in a second order phase transition in the early universe is an out-of-equilibrium process. Condensed matter experiments seem to support Zurek's mechanism, in which the freezing of thermal fluctuations close to the critical point (critical slowing down) plays a crucial role. We discuss how this picture can be extrapolated to the early universe, pointing out that new scaling laws may emerge at very high temperatures and showing how critical slowing down emerges in the context of a relativistic quantum field theory.

hep-ph↗
arXiv1998

Critical Slowing Down from $T\neq 0$ Wilson RG

The Thermal Renormalization Group can be employed to study the dynamics of $T\neq 0$ Quantum Field Theories close to second order phase transitions, where neither resummed perturbation theory nor first principle lattice simulations can be employed. As an example, I discuss the computation of the plasmon damping rate in the scalar field theory from $T\gg T_C$ down to $T_C$. As the critical point is approached, the lifetime of long wavelength thermal fluctuations diverges. Taking this effect into account, the notion of quasiparticle, and kinetic approaches, can be extended close to the critical regime. The consequences on the scenarios for topological defect formation in the early universe are also discussed.

hep-ph↗
arXiv1998

The Plasmon Damping Rate for T -> T_C

The plasmon damping rate in scalar field theory is computed close to the critical temperature. It is shown that the divergent result obtained in perturbation theory is a consequence of neglecting the thermal renormalization of the coupling. Taking this effect into account, a vanishing damping rate is obtained, leading to the critical slowing down of the equilibration process.

hep-ph↗
arXiv1997

Screening Masses in SU(N) from Wilson Renormalization Group

We apply a gauge invariant formulation of Wilson Renormalization Group (RG) to the computation of the Debye and transverse gluon masses in pure gauge SU(N) at high temperature. Following the Hard Thermal Loop effective field theory as a guideline, we develop an approximation scheme to the exact evolution equations. The Debye mass receives sizable corrections compared to the leading order perturbative result, mainly due to the infrared singular behavior in the transverse gluon sector. A non-vanishing mass for the transverse gluons is found, which acts as an infrared regulator though not efficiently enough as to restore the validity of perturbation theory. Indeed, discussing the role of higher dimensional operators, we show that the gauge coupling for the transverse modes typically flows to non-perturbative values unless extremely high temperatures are reached. After comparing our results with recent lattice simulations, we comment on the possibility of using this formulation of the RG as a tool to construct an effective field theory for the non-perturbative, long wavelength, transverse modes.

hep-ph↗
arXiv1997

Gauge-Invariant Renormalization Group at Finite Temperature and Debye Screening

I describe an application of Wilson Renormalization group to the real time formalism of finite temperature field theory. The approach has two nice features: 1) the RG flow equations describe non-perturbatively the effect of thermal fluctuations only, and, 2) the flow is gauge invariant. I then describe the application of the method to the study of the gluon self-energy in SU(N), and present results for the computation of the Debye and magnetic screening masses.

hep-ph↗
arXiv1996

Wilson Renormalization Group formulation of Real Time thermal field theories

We apply Renormalization Group techniques to the Real Time formulation of thermal field theory. Due to the separation between the $T=0$ and the $T\neq 0$ parts of the propagator in this formalism, one can derive exact evolution equations for the Green functions describing the effect of integrating out thermal fluctuations of increasing wavelengths, the initial conditions being the renormalized Green functions of the $T=0$ theory. As a first application, we study the phase transition for the real scalar theory, computing the order of the transition, the critical temperature, and critical exponents, in different approximations to the evolution equations for the scalar potential.

hep-ph↗