Academic literature on the topic 'Particle Astrophyics'

Abstract Since 1984 INFN and University of Pisa scientists performing experiments at Fermilab have been running a two-month summer training program for Italian students at the lab. In 1984 the program involved only a few physics students from the University of Pisa, but it was later extended to other INFN groups and to engineering students. Since 2004 the program has been supported in part by the US Department of Energy (DOE) in the frame of an exchange agreement with INFN and has been run by the Cultural Association of Italians at Fermilab (CAIF). In 2007 the Sant’Anna School of Advanced Studies (Pisa) established an agreement with Fermilab to share the cost of four engineering students each year. In the almost 40 years of its history, the program has hosted at Fermilab approximately 550 Italian students from more than 20 Italian universities and from some non-Italian universities. In addition, in the years 2010-2019, with the support of the Italian National Institute of Astrophyics (INAF), the Italian Space Agency (ASI), and CAIF, 30 students were hosted in other US laboratories and universities. The Fermilab training programs spanned from data analysis to design and construction of particle detectors and accelerator components, R/D on superconductive elements, theory of accelerators, and analysis of astrophysical data. At the other US laboratories the offered training was on Space Science. In 2015 the University of Pisa endorsed the program as one of its own Summer Schools. The interns are enrolled as Pisa students for the duration of the internship. They are required to write summary reports published in the Fermilab and University of Pisa web pages. Upon positive evaluation by a University board, students are acknowledged 6 ECTS credits. The entire program is expected to expand further under CAIF management. An agreement has been signed between ASI and CAIF, for ASI to support yearly three two-months fellowships in US space science. In the following we inform on student recruiting, training programs, and final evaluation

In this paper we discuss the impact on cosmology of recent results obtained by the LHC (Large Hadron Collider) experiments in the 2011-2012 runs, respectively at √<span style="text-decoration: overline;">s</span> = 7 and 8 TeV. The capital achievement of LHC in this period has been the discovery of a spin-0 particle with mass 126 GeV/c², very similar to the Higgs boson of the Standard Model of Particle Physics. Less exciting, but not less important, negative results of searches for Supersymmetric particles or other exotica in direct production or rare decays are discussed in connection with particles and V.H.E. astronomy searches for Dark Matter.

We present a particle method for the simulation of three dimensional compressible hydrodynamics based on a hybrid Particle-Mesh discretization of the governing equations. The method is rooted on the regularization of particle locations as in remeshed Smoothed Particle Hydrodynamics (rSPH). The rSPH method was recently introduced to remedy problems associated with the distortion of computational elements in SPH, by periodically re-initializing the particle positions and by using high order interpolation kernels. In the PMH formulation, the particles solely handle the convective part of the compressible Euler equations. The particle quantities are then interpolated onto a mesh, where the pressure terms are computed. PMH, like SPH, is free of the convection CFL condition while at the same time it is more efficient as derivatives are computed on a mesh rather than particle-particle interactions. PMH does not detract from the adaptive character of SPH and allows for control of its accuracy. We present simulations of a benchmark astrophysics problem demonstrating the capabilities of this approach.

AbstractCosmic rays with energies above 1018 eV are currently of considerable interest in astrophysics and are to be further studied in a number of projects which are either currently under construction or the subject of well-developed proposals. This paper aims to discuss some of the physics of such particles in terms of current knowledge and information from particle astrophysics at other energies.

The acceleration of electrons and charged nuclei to high energies is a phenomenon occuring at many sites throughout the universe, including the galaxy, pulsars, quasars, and around black holes. In the heliosphere, large solar flares and the often associated coronal mass ejections (CMEs) are the most energetic natural particle accelerators, occasionally accelerating protons to GeV and electrons to tens of MeV energies. The observation of these particles offers the unique opportunity to study fundamental processes in astrophysics. Particles that escape into interplanetary space can be observed in situ with particle detectors on spacecraft. In particular, particle spectra can be diagnostic of flare acceleration processes. On the other hand, energetic processes on the sun can be studied indirectly, via observations of the electromagnetic emissions (radio, X-ray, gamma-ray) produced by the particles in their interactions with the solar atmosphere. The purpose of this article is to give a brief overview on current models on particle acceleration and the present status of observations of solar energetic particles.

The presence of turbulence in astrophysical magnetic fields can have a significant effect on the diffusion of particles and, therefore, should be taken into account when performing simulations involving particle propagation. After reviewing the constructionof the turbulent magnetic field component, we incorporate this feature in two separate projects. In the first, we consider the possible source(s) of hadronic cosmic rays thought to be responsible for the diffuse TeV gamma-ray emission in the vicinity ofthe Galactic center. Assuming a completely turbulent magnetic field with an average strength of 10-100microG, we find that relativistic protons do not travel far enough to produce gamma-rays spatially correlated with the giant molecular clouds, as seen by HESS,when injected into the interstellar medium by a single point source, such as the supermassive black hole Sagittarius A*. Increasing the number of point sources to five does improve the longitudinal extent of the emission but either shows only weak correlation with the molecular gas or highlights the source positions - both pictures areinconsistent with HESS observations. We conclude that protons must be accelerated throughout the Galactic center region via e.g. a second-order Fermi process in order to reproduce the HESS gamma-ray map if the magnetic field there is completely turbulent. Secondly, we examine the possible link between the asymmetric 511keV electron-positron annihilation emission from the inner Galactic disk and hard low mass X-ray binaries (LMXBs). Three different magnetic field configurations were considered: a completely turbulent field, a field in which the turbulent component has equal energy density as the mean component, and a strongly ordered field with little turbulence. Assuming the environment around each LMXB system is the same, we find that the LMXBs alone cannot account for all the positrons necessary to sufficiently fill the region regardless of the particular magnetic field structure chosen. Another transport mechanism (e.g. a galactic wind) in addition to the diffusive motion caused by the magnetic field fluctuations and/or allowing the LMXBs to be embedded in different phases of the interstellar medium is needed for the LMXB picture to remain a viable possibility.

A survey of the theory of neutrino oscillations in dense matter and neutrino backgrounds is presented. We discuss collective neutrino systems using the gyroscopic pendulum analogy and describe the motion that results from self-induced parametric resonances. The effects of dense matter on the flavour oscillations of neutrinos are also detailed. This theory is applied to the case of continuous supernova neutrino spectra and explanations of the spectral swapping behaviour seen in numerical studies are summarized. The results of numerical simulations of supernova oscillations in turbulent supernova backgrounds are presented and discussed. We study the motion of two example supernova neutrino spectra and examine the differences in the dynamics and flavour evolution that results from adding turbulent fluctuations to the supernova matter background. We also investigate the effect that fluctuations in the neutrino density can have on the oscillation behaviour. We find that in general the final neutrino spectra emerging from the inner supernova regions are quite robust to fluctuations in the backgrounds in our model, while the intermediate dynamics can be very strongly altered. Some significant changes in the final spectra are also found to occur when the neutrino background density fluctuations are large. We give a detailed review of the resonant matter effects that determine the survival probabilities of atmospheric muon neutrinos. The differences between various Earth density models are described, and these models are then used to predict the flux of muon-type neutrino events in the Deep Core extension to the IceCube detector. We use recent results from the detector collaboration and build on previous work which considered the sensitivity of the detector to the mass hierarchy, and show that uncertainties in the Earth's density can have a significant influence on the event rates.

In this thesis I study astrophysical and cosmological effects of axion-like particles (ALPs). ALPs are pseudo-scalar particles, which are generally very weakly-interacting, with a coupling α/M E · B to electromagnetism. They are predicted by many theories which extend the standard model (SM) of particle physics, most notably string theory. String theory compactifications also predict many scalar fields called moduli which describe the size and shape of the extra, compact dimensions. In string theory models generically the moduli fields are responsible for reheating the universe after inflation. Being gravitationally-coupled, they will also decay to any other particles or sectors of the theory, including any light ALPs, of which there are usually many. The ALPs produced by moduli decay will contribute to dark radiation, additional relativistic energy density. The amount of dark radiation is tightly constrained by observations, this bounds the branching fraction of moduli decays into ALPs, which constrains the string theory model itself. I calculate the amount of dark radiation produced in a model with one light modulus, solely responsible for reheating, called the Large Volume Scenario. I study a minimal version of this model with one ALP and a visible sector comprised of the minimal supersymmetric SM. The dominant visible sector decay mode is to two Higgses, I include radiative corrections to this decay and find that ALP dark radiation is over-produced in this minimal version of the model, effectively ruling it out. The production of ALPs from moduli decay at reheating seems to be a generic feature of string theory models. These ALPs would exist today as a homogeneous cosmic ALP background (CAB). The coupling of ALPs to electromagnetism allows ALPs to convert to photons and vice versa in a magnetic field, leading to potential observable astrophysical signals of this CAB. Observations have shown an excess in soft X-ray emission from many galaxy clusters. I use detailed simulations of galaxy cluster magnetic fields to show that a CAB can explain these observations by conversion of ALPs into X-ray photons. I simulate ALP-photon conversion in four galaxy clusters and compare to soft X-ray observations. I show the excesses (or lack thereof) can be fit consistently across the clusters for a CAB with ALP-photon inverse coupling of M = 6 - 12 x 10¹² GeV, if the CAB spectrum has energy ~ 200 eV. I also study the possibility of using galaxy clusters to search for and constrain the ALP coupling to photons using cluster X-ray emission. Conversion of X-ray photons into ALPs will cause spectral distortions to the thermal X-ray spectrum emitted by galaxy clusters. I show that the non-observation of these distortions is able to produce the strongest constraints to date on the ALP-photon inverse coupling, M ≳ 7 x 10¹¹ GeV.

2002/2003
In questo lavoro ho affrontato lo studio della produzione dal vuoto di particelle (elettroni, posi troni e fotoni) in presenza di campi magnetici intensi e lentamente variabili nel tempo. Per "campi magnetici intensi" intendo campi magnetici la cui intensità è molto maggiore del valore Ber = m2c3 /(ne) = 4.4 x 1013 gauss che corrisponde al valore minimo dell'ampiezza che un campo magnetico deve avere affinché risulti energeticamente possibile la creazione dal vuoto di una coppia e- - e+. Tali campi magnetici non possono essere prodotti in laboratorio, tuttavia, come mostrano certe evidenze indirette e simulazioni numeriche, essi possono essere presenti attorno a certi oggetti astrofisici compatti (stelle di neutroni estremamente magnetizzate dette magnetar o buchi neri massicci). Per questo motivo, nel presente lavoro ho assunto che le sorgenti dei campi magnetici in gioco sono sempre oggetti astrofisici compatti del tipo appena descritto. In particolare, ho tentato di applicare i miei risultati ai cosiddetti Gamma-Ray Bursts (GRB) e ai loro spettri energetici. I G RB sono impulsi molto intensi di raggi gamma soft che sono rivelati in media una volta al giorno dai nostri satelliti e che, si pensa, sono originati proprio attorno a sorgenti astrofisiche come buchi neri massicci o, secondo alcuni modelli, magnetar. Il mio punto di vista è quello di un fisico teorico e non di un astrofisico e, pertanto, i modelli che utilizzo sono versioni molto semplificate della realtà. Tuttavia, alcuni degli spettri di fotoni che ho calcolato mostrano somiglianze qualitative con i corrispondenti spettri energetici sperimentali dei GRB. Da un punto di vista dei risultati, la tesi può essere divisa in tre parti distinte: la prima riguarda la produzione di coppie e- -e+ in presenza di un campo magnetico intenso e lentamente variabile in varie configurazioni, la seconda riguarda la produzione di fotoni in presenza di un campo magnetico intenso e lentamente rotante e, infine, la terza riguarda gli effetti che il campo gravitazionale dell'oggetto astrofisico compatto induce sulla produzione di coppie e- - e+. Nella prima parte ho calcolato la probabilità per unità di volume che una coppia e- - e+ venga creata dal vuoto in presenza di un campo magnetico intenso e lentamente variabile per mezzo della teoria delle perturbazioni adiabatiche al primo ordine. Inizialmente, ho mostrato analiticamente che se il campo magnetico cambia direzione allora vengono innescati meccanismi di produzione molto più efficienti rispetto a quelli innescati in presenza di un campo magnetico variabile solo in modulo. Il motivo fisico di questo fatto va ricercato nell'esistenza di stati di singola particella elettronici e positronici la cui energia non dipende dal campo magnetico. Infatti, questi stati, detti transverse ground states (TGS), hanno, in presenza di un campo magnetico intenso, un'energia molto più bassa di quella degli altri stati e solo se il campo magnetico varia in direzione è possibile creare una coppia in cui sia l'elettrone che il positrone sono in un TGS. Un'altra conclusione di questa prima parte riguarda il ruolo che il campo elettrico indotto dalla variazione nel tempo del campo magnetico gioca nel fenomeno della produzione. Infatti, si vede che la creazione della coppia è possibile (ovviamente) solo se tale campo elettrico è presente e, in particolare, che la probabilità di creazione per unità di volume è proporzionale al quadrato del campo elettrico stesso. A vendo in mente una possibile applicazione dei calcoli agli spettri dei GRB, nella seconda parte della tesi ho calcolato lo spettro dei fotoni emessi da elettroni e positroni presenti in un campo magnetico intenso e puramente rotante in seguito alla loro annichilazione o come radiazione di sincrotrone. In entrambi i casi lo spettro finale è stato calcolato numericamente. Mentre lo spettro di annichilazione presenta un picco pronunciato in corrispondenza della massa dell'elettrone, lo spettro di sincrotrone mostra due andamenti differenti attorno ad un valore di energia rv 1-3 Me V. In generale, la forma dello spettro di sincrotrone somiglia qualitativamente a quella di alcuni spettri di G RB mentre lo spettro di annichilazione è decisamente diverso. In particolare, è risultato che analogamente agli spettri sperimentali l'andamento dello spettro di sincrotrone per piccole energie è inversamente proporzionale all'energia del fotone. Infine, ho anche calcolato analiticamente lo spettro dei fotoni emessi direttamente dal vuoto in seguito all'interazione non lineare del vuoto stesso col campo magnetico rotante ma i risultati mostrano che il nun1ero di fotoni così prodotti è decisamente inferiore a quello dei fotoni prodotti attraverso gli altri due meccanismi e la loro presenza può essere trascurata. Come ho detto all'inizio, i campi magnetici che considero sono prodotti da stelle di neutroni o da buchi neri. Per questo, può risultare importante tenere in considerazione anche la presenza del campo gravitazionale prodotto dall'oggetto compatto. Ho fatto questo nell'ultima parte della tesi in cui ho visto come le energie e gli stati elettronici e positronici di singola particella e, di conseguenza, le probabilità di produzione di una coppia vengono modificate dalla presenza di un campo gravitazionale debole trattato perturbativamente o dalla presenza di uno intenso trattato non perturbativamente. Nel primo caso, il risultato più interessante è che in presenza di un campo gravitazionale (seppur debole) perpendicolare al campo magnetico è possibile creare coppie con l 'elettrone e il positrone in un TGS anche se il campo magnetico varia solo in modulo. Invece, il trattamento del caso non perturbativo è risultato completamente diverso per il fatto che i livelli energetici dell'elettrone e del positrone, a differenza che nello spaziotempo di Minkowski, sono individuati da un numero quantico continuo e indipendente dagli altri numeri quantici e dal campo magnetico. In questo caso, ho mostrato come gli effetti del campo gravitazionale sulla probabilità di creazione sono effettivamente molto importanti, tanto da non poter essere trascurati. In particolare, elettroni e positroni con energie molto alte vengono creati in numero maggiore in presenza di un campo gravitazionale intenso che nello spaziotempo di Minkowski.
In this work I have studied the production from vacuum of electrons, positrons and photons in the presence of strong and slowly-varying magnetic fields. "Strong magnetic fields" here means magnetic fields whose intensity is much larger than Ber = m2c3 / (he) = 4.4 x 1013 gauss corresponding t o the minimum strength of a magnetic field whose energy is enough to create an e- - e+ pair from vacuum. Such intense magnetic fields cannot be created in terrestrial laboratories but, as some indirect evidences and numerical simulations show, they may be present around some astrophysical compact objects (strongly magnetized neutron stars called magnetar or massive black ho l es). For this reason, in the present work I h ave assumed t ha t the sources of the magnetic fields are always such kind of astrophysical compact objects. In particular, I have tried t o apply my results to the so-called Gamma-Ray Bursts ( G RB) an d their energy spectra. G RB are very intense soft gamma-ray pulses that our satellites register on average once a day and that are thought to be originated around astrophysical objects like massive black ho l es or, following some models, magnetars. My point of view is no t astrophysical but theoretical then the models I have used are very simplified versions of the real situation. Nevertheless, some of the photon spectra I have calculated are qualitatively similar to the corresponding experimental G RBs energy spectra. The results of the thesis can be divided into three different parts: the first one concerns the production of e- - e+ pairs in the presence of a strong, slowly-varying magnetic field in various configurations, the second one concerns the production of photons in the presence of a strong and slowly-rotating magnetic field and, finally, the third one concerns how the presence of the gravitational field of the astrophysical compact object affects the production of e- -e+ pairs. In the first part I have calculated the probability per unit volume that an e- - e+ pair is created from vacuum in the presence of a strong, slowly varying magnetic field through the first-order adiabatic perturbation theory. Firstly, I have shown analytically that if the direction of the magnetic field changes with time then production mechanisms are primed that are much more efficient than those primed in the presence of a magnetic field changing only in strength. The physical reason of this fact is the existence of one particle electron and positron states whose energy does not depend on the magnetic field. In fact, these states, called transverse ground states (TGS), have, in the presence of a strong magnetic field, an energy much lower than that of the other states and only if the magnetic field changes in direction it is possible to create a pair in which both the electron and the positron are in a TGS. Another conclusion in this first part concerns the role that the electric field induced by the time variation of the magnetic field plays in the production mechanism. In fact, one sees that the pair creation is possible ( obviously) only if such an electric field is present an d, in particular, t ha t the probability per unit volume is proportional to the square of the electric field itself. Having in mind a possible application of the calculations to G RBs spectra, in the second part of the thesis I have calculated the spectrum of the photons emitted by electrons and positrons in the magnetic field as a consequence of their annihilation or as synchrotron radiation. In both cases the final spectrum has been calculated numerically. While the annihilation spectrum shows a well marked peak around the electron mass, the synchrotron spectrum shows two different behaviours around an energy value rv 1-3 Me V. In general, the form of the synchrotron spectrum is qualitatively similar to some GRBs spectra while the annihilation spectrum is completely different. In particular, analogously to the experimental spectra the low-energy behaviour of the synchrotron spectrum is proportional to the inverse of the photon energy. Finally, I have also calculated the spectrum of the photons emitted directly from vacuum as a consequence of the nonlinear interaction of the vacuum itself with the rotating magnetic field but the results show that the number of photons produced through this mechanism is definitely lower than that of the photons produced through the other mechanisms and their presence can be neglected. As I have said at the beginning, the magnetic fields considered are produced by neutron stars or black holes. For this reason, taking into account the gravitational field produced by the compact object may give relevant results. I have clone this in the last part of the thesis where I have shown how the one particle electron an d positron energies and states and, consequently, the probability production of a pair are modified by the presence of a weak gravitational field treated perturbatively or by the presence of a strong gravitational field treated non perturbatively. In the first case, the most important result is that in the presence of a ( though weak) gravitational field perpendicular to the magnetic field it is possible to create pairs with the electron and the positron both in a TGS even if the magnetic field changes only in strength. Instead, the treatment of the non perturbative case resulted completely different because the electron an d positron one particle energies, unlike in Minkowski spacetime, are characterized by a continuous quantum number independent of the other quantum numbers and of the magnetic field. In this case, I have shown how the effects of the gravitational field on the production probability are really important and that they cannot be neglected. In particular, high-energy electrons an d positrons are more likely produced in the presence of a strong gravitational field than in Minkowski spacetime.
XVI Ciclo
1974
Versione digitalizzata della tesi di dottorato cartacea.

AbstractIn this chapter we move from single particle motion to the statistical description of a large number of charged particles, the plasma. This discussion provides the basis for the rich flora of plasma waves that are essential for understanding the sources and losses of radiation belt particles through wave–particle interactions.

AbstractIn this chapter we discuss the concepts that govern the motion of charged particles in the geomagnetic field and the principles how they stay trapped in the radiation belts. The basic particle orbit theory can be found in most plasma physics textbooks. We partly follow the presentation in Koskinen (Physics of space storms, from solar surface to the earth. Springer-Praxis, Heidelberg, 2011). A more detailed discussion can be found in Roederer and Zhang (Dynamics of magnetically trapped particles. Springer, Heidelberg, 2014). A classic treatment of adiabatic motion of charged particles is Northrop (The adiabatic motion of charged particles. Interscience Publishers, Wiley, New York, 1963).

AbstractThe main sources of charged particles in the Earth’s inner magnetosphere are the Sun and the Earth’s ionosphere. Furthermore, the Galactic cosmic radiation is an important source of protons in the inner radiation belt, and roughly every 13 years, when the Earth and Jupiter are connected via the interplanetary magnetic field, a small number of electrons originating from the magnetosphere of Jupiter are observed in the near-Earth space. The energies of solar wind and ionospheric plasma particles are much smaller than the particle energies in radiation belts. A major scientific task is to understand the transport and acceleration processes leading to the observed populations up to relativistic energies. Equally important is to understand the losses of the charged particles. The great variability of the outer electron belt is a manifestation of the continuously changing balance between source and loss mechanisms, whereas the inner belt is much more stable.

AbstractUnderstanding the role of plasma waves, extending from magnetohydrodynamic (MHD) waves at ultra-low-frequency (ULF) oscillations in the millihertz range to very-low-frequency (VLF) whistler-mode emissions at frequencies of a few kHz, is necessary in studies of sources and losses of radiation belt particles. In order to make this theoretically heavy part of the book accessible to a reader, who is not familiar with wave–particle interactions, we have divided the treatise into three chapters. In the present chapter we introduce the most important wave modes that are critical to the dynamics of radiation belts. The drivers of these waves are discussed in Chap. 10.1007/978-3-030-82167-8_5 and the roles of the wave modes as sources and losses of radiation belt particles are dealt with in Chap. 10.1007/978-3-030-82167-8_6.