arXiv subjects

Paolo Lipari

Publications and source records attributed to Paolo Lipari.

At least 37 records · Page 2Linked to original sources

arXiv2016

Absorption of Very High Energy Gamma Rays in the Milky Way

Galactic Gamma ray astronomy at very high energy (E > 30 TeV) is a vital tool in the study of the non-thermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions, where the targets are the radiation fields present in interstellar space. For gamma rays with energy E > 300 TeV the attenuation is mostly due to the photons of the Cosmic Microwave Background Radiation (CMBR). At lower energy the most important target are infrared photons with wavelengths in the range 50-500 micron emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for E around 150 TeV, and can be as large as P_abs around 0.45 for distant sources on lines of sight that pass close to the Galactic Center. The systematic uncertainties on the absorption probability are estimated as Delta P_abs < 0.08.

astro-ph.HE↗

arXiv2014

Solar modulations by the regular heliospheric electromagnetic field

The standard way to model the cosmic ray solar modulations is via the Parker equation, that is as the effect of diffusion in the turbulent magnetic of an expanding solar wind. Calculations performed with this method that do not include a description of the regular magnetic field in the heliosphere predict, in disagreement with the observations, equal modulations for particles and antiparticles. The effects of the regular heliospheric field, that break the the particle/anti-particle symmetry, have been included in the Parker equation adding convection terms associated to the magnetic drift velocity of charged particles moving in non--homogeneous magnetic field. In this work we take a completely different approach and study the propagation of charged particles in the heliosphere assuming only the presence of the regular magnetic field, and completely neglecting the random component. Assuming that the field is purely magnetic in the wind frame, one can deduce the existence of a large scale electric field, that results in important energy losses for charged particles that traverse the heliosphere and reach the Earth. The energy loss (Delta E) due to the large scale regular electric field depends on the particle electric charge, is proportional to the absolute value of the electric charge, and to a good approximation is independent from the particle energy and direction. We speculate that this deterministic energy loss is an important, or perhaps even the leading contribution to the solar modulation effects.

astro-ph.HE↗

arXiv2014

The lifetime of cosmic rays in the Milky Way

The most reliable method to estimate the residence time of cosmic rays in the Galaxy is based on the study of the suppression, due to decay, of the flux of unstable nuclei such as beryllium-10, that have lifetime of appropriate duration. The Cosmic Ray Isotope Spectrometer (CRIS) collaboration has measured the ratio between the fluxes of beryllium-10 and beryllium-9 in the energy range E_0 \simeq 70-145 MeV/nucleon, and has used the data to estimate an escape time tau_{ esc} = 15.0 +- 1.6 Myr. This widely quoted result has been obtained in the framework of a simple leaky-box model where the distributions of escape time and age for stable particles in the Galaxy are identical and have exponential form. In general, the escape time and age distributions do not coincide, they are not unique (because they depend on the injection or observation point), and do not have a simple exponential shape. It is therefore necessary to discuss the measurement of the beryllium ratio in a framework that is more general and more realistic than the leaky-box model. In this work we compute the escape time and age distributions of cosmic rays in the Galaxy in a model based on diffusion that is much more realistic than the simple leaky-box, but that remains sufficiently simple to have exact analytic solutions. Using the age distributions of the model to interpret the measurements of the beryllium-10 suppression, one obtains a cosmic ray residence time that is significantly longer (a factor 2 to 4 depending on the extension of the cosmic ray halo) than the leaky-box estimate. This revised residence time implies a proportional reduction of the power needed to generate the galactic cosmic rays.

astro-ph.HE↗

arXiv2014

GAMMA-LIGHT: High-Energy Astrophysics above 10 MeV

High-energy phenomena in the cosmos, and in particular processes leading to the emission of gamma- rays in the energy range 10 MeV - 100 GeV, play a very special role in the understanding of our Universe. This energy range is indeed associated with non-thermal phenomena and challenging particle acceleration processes. The technology involved in detecting gamma-rays is challenging and drives our ability to develop improved instruments for a large variety of applications. GAMMA-LIGHT is a Small Mission which aims at an unprecedented advance of our knowledge in many sectors of astrophysical and Earth studies research. The Mission will open a new observational window in the low-energy gamma-ray range 10-50 MeV, and is configured to make substantial advances compared with the previous and current gamma-ray experiments (AGILE and Fermi). The improvement is based on an exquisite angular resolution achieved by GAMMA-LIGHT using state-of-the-art Silicon technology with innovative data acquisition. GAMMA-LIGHT will address all astrophysics issues left open by the current generation of instruments. In particular, the breakthrough angular resolution in the energy range 100 MeV - 1 GeV is crucial to resolve patchy and complex features of diffuse sources in the Galaxy as well as increasing the point source sensitivity. This proposal addresses scientific topics of great interest to the community, with particular emphasis on multifrequency correlation studies involving radio, optical, IR, X-ray, soft gamma-ray and TeV emission. At the end of this decade several new observatories will be operational including LOFAR, SKA, ALMA, HAWK, CTA. GAMMA-LIGHT will "fill the vacuum" in the 10 MeV-10 GeV band, and will provide invaluable data for the understanding of cosmic and terrestrial high-energy sources.

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arXiv2013

Establishing the astrophysical origin of a signal in a neutrino telescope

Recently the IceCube collaboration has reported the observation of 28 contained events with a visible energy in the interval between 60 TeV and 1.5 PeV, and has argued that this detection is evidence, with a statistical significance of more than four standard deviations, for the existence of an astrophysical neutrino flux that accounts for a large fraction of the events. In this work we analyze the arguments that allow to identify a component of astrophysical origin in the high energy neutrino flux separating it from atmospheric neutrinos. An astrophysical origin for a large fraction of the IceCube contained events is the simplest and most natural explanation of the data but, conservatively, an atmospheric origin cannot yet be entirely ruled out. This ambiguity should soon be resolved.

astro-ph.HE↗

arXiv2013

Interpretation of the measurements of total, elastic and diffractive cross sections at LHC

Recently at LHC one has obtained measurements of the total, elastic and diffractive cross sections in pp collisions at very high energy. The total cross section is in good agreement with predictions based on a leading behavior σ_{tot} (s) \propto (\ln s/s_0)^2, on the other hand the elastic cross section is lower than most expectations and the diffractive cross section is higher. It is remarkable that the ratio (σ_{el} + σ_{diff})/σ_{tot} calculated combining the results of the TOTEM and ALICE detectors is 0.495^{+0.05}_{-0.06}, very close to the maximum theoretically allowed value of 1/2 known as the Miettinen Pumplin bound. In this work we discuss these results using the frameworks of single and multi--channel eikonal models, and outline the main difficulties for a consistent interpretation of the data.

hep-ph↗

arXiv2011

OPERA-reassessing data on the energy dependence of the speed of neutrinos

We offer a preliminary exploration of the two sides of the challenge provided by the recent OPERA data on superluminal neutrinos. On one side we stress that some aspects of this result are puzzling even from the perspective of the wild quantum-gravity literature, where arguments in favor of the possibility of superluminal propagation have been presented, but not considering the possibility of such a sizeable effect for neutrinos of such low energies. We feel this must encourage particularly severe scrutiny of the OPERA result. On the other side, we notice that the OPERA result is reasonably consistent with $μ$-neutrino-speed data previously obtained at FERMILAB, reported in papers of 2007 and 1979. And it is intriguing that these FERMILAB79 and FERMILAB07 results, when combined with the new OPERA result, in principle provide a window on $μ$-neutrino speeds at different energies broad enough to compare alternative phenomenological models. We test the discriminating power of such an approach by using as illustrative examples the case of special-relativistic tachyons, the case of "Coleman-Glashow-type" momentum-independent violations of the special-relativistic speed law, and the cases of linear and quadratic energy dependence of the speed of ultrarelativistic muon neutrinos. Even just using $μ$-neutrino data in the range from $\sim$ 3 GeVs to $\sim$ 200 GeVs the special-relativistic tachyon and the quadratic-dependence case are clearly disfavoured. The linear-dependence case gives a marginally consistent picture and the Coleman-Glashow scenario fits robustly the data. We also comment on Supernova 1987a and its relevance for consideration of other neutrino species, also in relation with some scenarios that appeared in the large-extra-dimension literature.

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arXiv2011

Sibyll with charm

The cosmic ray interaction event generator Sibyll is widely used in extensive air shower simulations for cosmic ray and neutrino experiments. Charmed particle production has been added to the Monte Carlo with a phenomenological, non-perturbative model that properly accounts for charm production in the forward direction. As prompt decays of charm can become a significant background for neutrino detection, proper simulation of charmed particles is very important. We compare charmed meson and baryon production to accelerator data.

astro-ph.HE↗

arXiv2010

Atmospheric lepton fluxes at very high energy

The observation of astrophysical neutrinos requires a detailed understanding of the atmospheric neutrino background. Since neutrinos are produced in meson decays together with a charged lepton, important constraints on this background can be obtained from the measurement of the atmospheric muon flux. Muons, however, can also be produced as mu+ mu- pairs by purely electromagnetic processes. We use the Z-moment method to study and compare the contributions to the atmospheric muon and neutrino fluxes from different sources (pi/K decay, charmed and unflavored hadron decay, and photon conversion into a muon pair). We pay special attention to the contribution from unflavored mesons (eta, eta', rho0, omega and phi). These mesons are abundant in air showers, their lifetimes are much shorter than those of charged pions or kaons, and they have decay branching ratios of order 10^-4 into final states containing a muon pair. We show that they may be the dominant source of muons at E_mu >10^3 TeV.

astro-ph.HE↗

arXiv2009

Cosmic ray event generator Sibyll 2.1

The cosmic ray interaction event generator Sibyll is widely used in extensive air shower simulations. We describe in detail the properties of Sibyll 2.1 and the differences with the original version 1.7. The major structural improvements are the possibility to have multiple soft interactions, introduction of new parton density functions, and an improved treatment of diffraction. Sibyll 2.1 gives better agreement with fixed target and collider data, especially for the inelastic cross sections and multiplicities of secondary particles. Shortcomings and suggestions for future improvements are also discussed.

hep-ph↗

arXiv2009

Multiple Parton Interactions in Hadron Collisions and Diffraction

Hadrons are composite objects made of quarks and gluons, and during a collision one can have several elementary interactions between the constituents. These elementary interactions, using an appropriate theoretical framework, can be related to the total and elastic cross sections. At high c.m. energy it also becomes possible to identify experimentally a high pt subset of the parton interactions and to study their multiplicity distribution. Predictions of the multiple interactions rates are difficult because in principle one needs to have a knowledge of the correlated Parton Distribution Functions that describe the probability to find simultaneously different partons in different elements of phase space. In this work we address this question and suggest a method to describe effectively the fluctuations in the instantaneous configuration of a colliding hadron. This problem is intimately related to the origin of the inelastic diffractive processes. We present a new method to include the diffractive cross section in an eikonal formalism that is equivalent to a multi-channel eikonal. We compare with data and present an extrapolation to higher energy.

hep-ph↗

arXiv2008

The Concepts of "Age" and "Universality" in Cosmic Ray Showers

The concept of "age" as a parameter for the description of the state of development of high energy showers in the atmosphere has been in use in cosmic ray studies for several decades. In this work we briefly discuss how this concept, originally introduced to describe the average behavior of electromagnetic cascades, can be fruitfully applied to describe individual showers generated by primary particles of different nature, including protons, nuclei and neutrinos. Showers with the same age share three different important properties: (i) their electron size has the same fractional rate of change with increasing depth, (ii) the bulk of the electrons and photons in the shower (excluding high energy particles) have energy spectra with shapes and relative normalization uniquely determined by the age parameter, (iii) the electrons and photons in the shower have also the same angular and lateral distributions sufficiently far from the shower axis. In this work we discuss how the properties associated with the shower age can be understood with simple arguments, and how the shapes of the electron and photon spectra and the relative normalization that correspond to a certain age can be calculated analytically.

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arXiv2008

Problems in High Energy Astrophysics

This contribution discusses some of the main problems in high energy astrophysics, and the perspectives to solve them using different types of "messengers": cosmic rays, photons and neutrinos

astro-ph↗

arXiv2008

Proton and Neutrino Extragalactic Astronomy

The study of extragalactic sources of high energy radiation via the direct measurement of the proton and neutrino fluxes that they are likely to emit is one of the main goals for the future observations of the recently developed air showers detectors and neutrino telescopes. In this work we discuss the relation between the inclusive proton and neutrino signals from the ensemble of all sources in the universe, and the resolved signals from the closest and brightest objects. We also compare the sensitivities of proton and neutrino telescopes and comment on the relation between these two new astronomies.

astro-ph↗

arXiv2007

Flavor Composition and Energy Spectrum of Astrophysical Neutrinos

The measurement of the flavor composition of the neutrino fluxes from astrophysical sources has been proposed as a method to study not only the nature of their emission mechanisms, but also the neutrino fundamental properties. It is however problematic to reconcile these two goals, since a sufficiently accurate understanding of the neutrino fluxes at the source is needed to extract information about the physics of neutrino propagation. In this work we discuss critically the expectations for the flavor composition and energy spectrum from different types of astrophysical sources, and comment on the theoretical uncertainties connected to our limited knowledge of their structure.

astro-ph↗

arXiv2006

Perspectives of High Energy Neutrino Astronomy

This work discusses the perspectives to observe fluxes of high energy astrophysical neutrinos with the planned km3 telescopes. On the basis of the observations of GeV and TeV gamma-rays, and of ultra high energy cosmic rays, it is possible to construct well motivated predictions that indicate that the discovery of such fluxes is probable. However the range of these predictions is broad, and the very important opening of the ``neutrino window'' on the high energy universe is not guaranteed with the current design of the detectors. The problem of enlarging the detector acceptance using the same (water/ice Cherenkov) or alternative (acoustic/radio) techniques is therefore of central importance.

astro-ph↗

arXiv2003

Cosmic Ray Astrophysics and Hadronic Interactions

Research in cosmic rays is now nearly a century old, but most of the fundamental questions in this field remain unanswered, on the other hand the perspectives of future studies in the next decade are very bright. New detectors will provide higher quality data in the entire energy range from 10^8 to 10^{20} eV (or more if particles of higher energy have non negligible fluxes), moreover cosmic ray astrophysics must now be considered, together with gamma, neutrino and gravitational wave astronomy, as one of the subfields of high energy astrophysics, and using information from these four "messengers" there is the potential of a detailed understanding of the origin of the high energy radiation in the universe. High energy cosmic rays are measured indirectly observing the showers they generate in the atmosphere, and a correct and detailed interpretation of these measurements will require an improved understanding of the properties of hadronic interactions. The new collider experiments, and in particular the LHC project at CERN offer the unique possibility to perform measurements of great value for cosmic ray astrophysics. It is of great importance for cosmic research that this possibility is fully exploited with the appropriate instrumentation and analysis.

astro-ph↗

arXiv2002

Neutrino oscillation studies and the neutrino cross section

The present uncertainties in the knowledge of the neutrino cross sections for E_nu \sim 1 GeV, that is in the energy range most important for atmospheric and long baseline accelerator neutrinos, are large. These uncertainties do not play a significant role in the interpretation of existing data, however they could become a limiting factor in future studies that aim at a complete and accurate determination of the neutrino oscillation parameters. New data and theoretical understanding on nuclear effects and on the electromagnetic structure functions at low Q^2 and in the resonance production region are available, and can be valuable in reducing the present systematic uncertainties. The collaboration of physicists working in different subfields will be important to obtain the most from this available information. It is now also possible, with the facilities developed for long baseline beams, to produce high intensity and well controlled neutrino beams to measure the neutrino interaction properties with much better precision that what was done in the past. Several projects and ideas to fully exploit these possibilities are under active investigation. These topics have been the object of the first neutrino interaction (NUINT01) workshop.

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