arXiv subjects

G. Sartori

Publications and source records attributed to G. Sartori.

13 recordsLinked to original sources

arXiv2016

Considerations on the radio emission from extended air showers

The process of radio emission from extended air showers produced by high energy cosmic rays has reached a good level of comprehension and prediction. It has a coherent nature, so the emitted power scales quadratically with the energy of the primary particle. Recently, a laboratory measurement has revealed that an incoherent radiation mechanism exists, namely, the bremsstrahlung emission. In this paper we expound why bremsstrahlung radiation, that should be present in showers produced by ultra high energy cosmic rays, has escaped detection so far, and why, on the other side, it could be exploited, in the 1--10~GHz frequency range, to detect astronomical $γ$-rays. We propose an experimental scheme to verify such hypothesis, which, if correct, would deeply impact on the observational $γ$-ray astronomy.

astro-ph.IM↗

arXiv2014

Experimental study of the microwave emission from electrons in air

We searched for the emission of microwave radiation in the Ku band generated by a 95 keV electron beam in air. We unequivocally detected the radiation, and measured its yield and angular dependence. Both the emitted power and its angular pattern are well described by a model, where microwave photons are generated via bremsstrahlung in the free-electron atomic-nucleus collisions, during the slowdown of the electrons. As a consequence, the radiation is not isotropic but peaked in the forward direction. The emission yield scales proportionally with the number of electrons. This contrasts a previous claim that the yield scales with the number squared, due to coherence. With a Monte Carlo simulation we extrapolate our results to the Ultra High Energy Cosmic Ray energy range.

astro-ph.HE↗

arXiv2011

Testing the neutrality of matter by acoustic means in a spherical resonator

New measurements to test the neutrality of matter by acoustic means are reported. The apparatus is based on a spherical capacitor filled with gaseous SF$_6$ excited by an oscillating electric field. The apparatus has been calibrated measuring the electric polarizability. Assuming charge conservation in the $β$ decay of the neutron, the experiment gives a limit of $ε_\text{p-e}\lesssim1\cdot10^{-21}$ for the electron-proton charge difference, the same limit holding for the charge of the neutron. Previous measurements are critically reviewed and found incorrect: the present result is the best limit obtained with this technique.

physics.atom-ph↗

arXiv2010

Measurement of the near-infrared fluorescence of the air for the detection of ultra-high-energy cosmic rays

We have investigated the fluorescence emission in the Near Infrared from the air and its main components, nitrogen and oxygen. The gas was excited by a 95kV electron beam and the fluorescence light detected by an InGaAs photodiode, sensitive down to about 1700nm. We have recorded the emission spectra by means of a Fourier Transform Infrared spectrometer. The light yield was also measured by comparing the Near Infrared signal with the known Ultraviolet fluorescence, detected by a Si photodiode. The possibility of using the Near Infrared fluorescence of the atmosphere to detect Ultra-High-Energy Cosmic Rays is discussed, showing the pros and the cons of this novel method.

astro-ph.IM↗

arXiv2004

Symmetry allowed, but unobservable, phases in renormalizable Gauge Field Theory Models

In Quantum Field Theory models with spontaneously broken gauge invariance, renormalizability limits to four the degree of the Higgs potential, whose minima determine the vacuum state at tree-level. In many models, this bound has the intriguing consequence of preventing the observability, at tree-level, of some phases that would be allowed by symmetry. We show that, generally, the phenomenon persists also if one-loop radiative corrections are taken into account. The tree-level unobservability of some phases is characteristic in two-Higgs-doublet extensions of the Standard Model with additional discrete symmetries (to protect against neutral current flavor changing effects, for instance). We show that an extension of the scalar sector through suitable singlet fields can resolve the {\em unnatural} limitations on the observability of all the phases allowed by symmetry.

hep-th↗

arXiv2003

Allowed and observable phases in two-Higgs-doublet Standard Models

In Quantum Field Theory models of electro-weak interactions with spontaneously broken gauge invariance, renormalizability limits to four the degree of the Higgs potential, whose minima determine the possible vacuum states in tree approximation. Through the discussion of some simple variants of the Standard Model with two Higgs doublets, we show that, in some cases, the technical limit imposed by renormalizability can prevent the observability of some phases of the system, that would be otherwise allowed by the symmetry of the Higgs potential. An extension of the scalar sector through suitable SU$_2$ singlet particle fields can resolve this {\em unnatural} limitation.

hep-ph↗

arXiv2003

Rational parametrization of strata in orbit spaces of compact linear groups

Functions which are covariant or invariant under the transformations of a compact linear group $G$ acting in a euclidean space $\real^n$, can be profitably studied as functions defined in the orbit space of the group. The orbit space is the union of a finite set of strata, which are semialgebraic manifolds formed by the $G$-orbits with the same symmetry. In this paper we provide a simple recipe to obtain rational parametrizations of the strata. Our results can be easily exploited, in many physical contexts where the study of covariant or invariant functions is important, for instance in the determination of patterns of spontaneous symmetry breaking, in the analysis of phase spaces and structural phase transitions (Landau's theory), in covariant bifurcation theory, in crystal field theory and in most areas of solid state theory where use is made of symmetry adapted functions. An example of utilization of the recipe is also discussed at the end of the paper.

math-ph↗

arXiv2003

Tools in the orbit space approach to the study of invariant functions: rational parametrization of strata

Functions which are equivariant or invariant under the transformations of a compact linear group $G$ acting in an euclidean space $\real^n$, can profitably be studied as functions defined in the orbit space of the group. The orbit space is the union of a finite set of strata, which are semialgebraic manifolds formed by the $G$-orbits with the same orbit-type. In this paper we provide a simple recipe to obtain rational parametrizations of the strata. Our results can be easily exploited, in many physical contexts where the study of equivariant or invariant functions is important, for instance in the determination of patterns of spontaneous symmetry breaking, in the analysis of phase spaces and structural phase transitions (Landau theory), in equivariant bifurcation theory, in crystal field theory and in most areas where use is made of symmetry adapted functions. A physically significant example of utilization of the recipe is given, related to spontaneous polarization in chiral biaxial liquid crystals, where the advantages with respect to previous heuristic approaches are shown.

math-ph↗

arXiv2001

Possible ground states of D-wave condensates in isotropic space

A complete and rigorous determination of the possible ground states for D-wave pairing Bose condensates is presented. Using an orbit space approach to the problem, we find 15 allowed phases (besides the unbroken one), with different symmetries, that we thoroughly determine, specifying the group-subgroup relations between bordering phases.

cond-mat.supr-con↗

arXiv2001

Geometric invariant theory approach to the determination of ground states of D-wave condensates in isotropic space

A complete and rigorous determination of the possible ground states for D-wave pairing Bose condensates is presented, using a geometrical invariant theory approach to the problem. The order parameter is argued to be a vector, transforming according to a ten dimensional real representation of the group $G=${\bf O}$_3\otimes${\bf U}$_1\times <{\cal T}>$. We determine the equalities and inequalities defining the orbit space of this linear group and its symmetry strata, which are in a one-to-one correspondence with the possible distinct phases of the system. We find 15 allowed phases (besides the unbroken one), with different symmetries, that we thoroughly determine. The group-subgroup relations between bordering phases are pointed out. The perturbative sixth degree corrections to the minimum of a fourth degree polynomial $G$-invariant free energy, calculated by Mermin, are also determined.

cond-mat.supr-con↗

arXiv2000

Determination of the orbit spaces of non-coregular compact linear groups with one relation among the basic polynomial invariants in the $\hat P$-matrix approach

Invariant functions under the transformations of a compact linear group $G$ acting in $\real^n$ can be expressed in terms of functions defined in the orbit space of $G$. We develop a method to determine the isotropy classes of the orbit spaces of all the real linear groups whose integrity bases (IB) satisfy only one independent relation. The method is tested for IB's formed by 3 (independent) basic invariants. The result is obtained through a metric matrix $\hat P(p)$, defined only from the scalar products between the gradients of a minimal IB. We determine the matrices $\wP(p)$ from a universal differential equation, which satisfy new convenient additional conditions, which fit for the non-coregular case. Our results may be relevant in physical contexts where the study of covariant or invariant functions is important, like in the determination of patterns of spontaneous symmetry breaking in quantum field theory, in the analysis of phase spaces and structural phase transitions (Landau's theory), in covariant bifurcation theory, in crystal field theory and so on.

hep-th↗

arXiv1995

Four Dimensional Orbit Spaces of Compact Coregular Linear Groups

All four dimensional orbit spaces of compact coregular linear groups have been determined. The results are obtained through the integration of a universal differential equation, that only requires as input the number of elements of an integrity basis of the ideal of polynomial invariants of the linear group. Our results are relevant and lead to universality properties in the physics of spontaneous symmetry breaking at the classical level.

hep-th↗

arXiv1995

Orbit spaces of reflection groups with 2, 3, and 4 basic polynomial invariants

Covariant or invariant functions under a compact linear group can be expressed in terms of functions defined in the orbit space of the group. The semialgebraic relations defining the orbit spaces of all finite coregular real linear groups with at most 4 basic invariants are determined. For each group $G$ acting in $\real^n$, the results are obtained through the computation of a metric matrix $\widehat P(p)$, which is defined only in terms of the scalar products between the gradients of a set of basic polynomial invariants $p_1(x),\dots p_q(x),\x\in\real^n$ of $G$; the semi-positivity conditions $\widehat P(p)\ge 0$ are known to determine all the equalities and inequalities defining the orbit space $\real^n/G$ of $G$ as a semi-algebraic variety in the space $\real^q$ spanned by the variables $p_1,\dots ,p_q$. In a recent paper, the $\widehat P$-matrices, for $q\le 4$, have been determined in an alternative way, as solutions of a universal differential equation;the present paper yields a partial, but significant, check on the correctness and completeness of these solutions. Our results can be widely exploited,e.g. in the determination of patterns of spontaneous symmetry breaking, in the analysis of structural phase transitions (Landau's theory),in covariant bifurcation theory,in crystal field theory and in solid state theory where symmetry adapted functions are used.

hep-th↗