Possible instability of the vacuum in a strong magnetic field

The possibility that a static magnetic field may decay through production of electron positron pairs is studied. The conclusion is that this decay cannot happen through production of single pairs, as in the electric case, but only through the production of a many-body state, since the mutual magnetic interactions of the created pairs play a relevant role. The investigation is made in view of the proposed existence of huge magnetic field strengths around some kind of neutron stars.Comment: TeX 7pages No figures. Submitted to Modern Physics letters

Disentangling correlations in Multiple Parton Interactions

Multiple Parton Interactions are the tool to obtain information on the correlations between partons in the hadron structure. Partons may be correlated in all degrees of freedom and all different correlation terms contribute to the cross section. The contributions due to the different parton flavors can be isolated, at least to some extent, by selecting properly the final state. In the case of high energy proton-proton collisions, the effects of correlations in the transverse coordinates and in fractional momenta are, on the contrary, unavoidably mixed in the final observables. The standard way to quantify the strength of double parton interactions is by the value of the effective cross section and a small value of the effective cross section may be originated both by the relatively short transverse distance between the pairs of partons undergoing the double interaction and by a large dispersion of the distribution in multiplicity of the multi-parton distributions. The aim of the present paper is to show how the effects of longitudinal and transverse correlations may be disentangled by taking into account the additional information provided by double parton interactions in high energy proton-deuteron collisions.Comment: 23 pages, 8 figure

Pair Production in a Time Dependent Magnetic Field

The production of electron-positron pairs in a time-dependent magnetic field is estimated in the hypotheses that the magnetic field is uniform over large distances with respect to the pair localization and it is so strong that the spacing of the Landau levels is larger than the rest mass of the particles. This calculation is presented since it has been suggested that extremely intense and varying magnetic fields may be found around some astrophysical objects.Comment: 11 pages, Plain TeX, no figures. Submitted to Modern Physics Letter

Production of light particles by very strong and slowly varying magnetic fields

The possibility that around some astrophysical objects there are non-static magnetic fields of enormous intensity suggests that in these situations real particles may be produced. The slowness of the variation is compensated by the huge intensity. The main issue is the production of e+,e- pairs annihilating into photons and the direct production of photons, as one of the concurrent process in the GRB (gamma ray bursts). Then some simple effects due to the presence of the intense gravity are studied and finally a look is given to the production of other kinds of particles.Comment: 3 pages ISMD (2006) Conference - Paraty, R.J. Brazi

Parton fluxes and virtual pions in heavy nuclei

The partonic flux originated from a heavy nucleus is not the mere sum of the fluxes coming from the individual nucleons. There are various effects that give rise to modifications. One of these effects is here investigated i.e. the presence of a cloud of virtual pions co-moving with the nucleus. It is found that the contribution of these virtual particles to the total parton flux should be rather small even if one includes the contribution of resonances.Comment: 14 pages, no figure

Double Parton Scatterings in High-Energy Proton-Nucleus Collisions and Partonic Correlations

The joint study of Double Parton Scatterings, in high energy proton-proton and proton-nucleus collisions, can provide a lot of information on multi-parton correlations. The multi-parton structure is in fact probed in a different way by DPS, in $p$-$p$ and in $p$-$A$ collisions. In $p$-$A$ collisions the interpretation of the experimental results may be however complicated by the presence of interference terms, which are missing in $p$-$p$. A suitable reaction channel, where interference terms are absent, is $WJJ$ production. By studying $WJJ$ production in $p$-$Pb$ collisions, we estimate that the fraction of events due to DPS may be larger by a factor 3 or 4, as compared to $p$-$p$, while the amount of the increased fraction can give a direct indication on the importance of different correlation terms.Comment: 27 pages, 9 figure

GLOBAL POSITIONIG SYSTEM AND RELATIVITY

La presente nota introduce il sistema di posizionamento globale su base satellitare (Global Positioning System – GPS). Dopo un breve inquadramento storico della tecnologia, la cui origine si può far risalire al lancio del primo satellite artificiale russo Sputnik (ottobre 1957), ed al successivo sistema Transit, della marina militare americana, viene presentato il funzionamento del sistema, con particolare riguardo alle correzioni relativistiche cui è sottoposto il segnale per arrivare all'attuale precisione del posizionamento sul territorio. Al fine di comprendere il fondamento teorico di tali correzioni, sono brevemente ricordate le celebri Teorie della relatività ristretta e generale di Albert Einstein, sottolineando le connessioni della seconda con la geometria non euclidea, che, come è noto, ne ha permesso la formulazione matematica.This paper deals with the Global Positioning System technology (GPS). After a brief historical introduction on GPS origins (from the Russian artificial satellite Sputnik launch in orbit – October 1957 – to the following Transit system of the United States Navy), the system main working is introduced. Particular attention is given to the so called “relativistic effects”, which are corrected by the methodology in order to achieve the nowadays ground positioning precision. Albert Einstein Special and General Relativity theories are then briefly introduced, in order to make the reader understand the theoretic grounding of the relativistic corrections in GPS