32 research outputs found
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 - and in - collisions. In - collisions the
interpretation of the experimental results may be however complicated by the
presence of interference terms, which are missing in -. A suitable
reaction channel, where interference terms are absent, is production. By
studying production in - collisions, we estimate that the fraction
of events due to DPS may be larger by a factor 3 or 4, as compared to -,
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 relativitĂ 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