arXiv subjects

Nicola Tomassetti

Publications and source records attributed to Nicola Tomassetti.

At least 37 records · Page 2Linked to original sources

arXiv2018

Production of antimatter nuclei in Galactic cosmic rays

Antimatter nuclei in cosmic rays (CRs) are a promising tool for the indirect detection of dark-matter annihilation signatures. However, the search of new-physics signals in CRs relies on our knowledge of the astrophysical antimatter background which, in turns, depends critically on the several fragmentation cross-sections that regulate production and destruction of antiparticles in the interstellar medium. In this work, we have re-evaluated the astrophysical background of CR antiproton, antineutron, and antihelium nuclei in Galactic CRs using improved calculations. The production cross-sections of individual antinucleons are constrained using updated calculations that make use of recent accelerator data. The production of antideuteron and antihelium nuclei is calculated using an improved model of nuclear coalescence that accounts for the asymmetry in antineutron and antiproton production. We discuss the cross-section induced uncertainties and show that they are dominating in comparison with other uncertainties of astrophysical origin.

astro-ph.HE↗

arXiv2018

Secondary antinuclei from supernova remnants and background for dark matter searches

We compute the spectra of cosmic-ray (CR) nuclei and antinuclei under a scenario where hadronic interaction processes inside supernova remnants (SNRs) can produce a diffusively-shock-accelerated "source component" of secondary particles. This scenario is able to explain the recent measurements reported by AMS on the antiproton/proton ratio, that is found to be remarkably constant at ~60-450 GeV of kinetic energy. However, as we will show, this explanation is ruled out by the new AMS data on the B/C ratio, which is found to decrease steadily up to TeV/n energies. With the constraints provided by the two ratios, we calculate conservative (B/C driven) and speculative (pbar/p driven) SNR-induced flux contribution for the spectra of antideuteron and antihelium in CRs, along with their standard secondary component expected from CR collisions in the interstellar gas. We found that the SNR component of anti-nuclei can be significantly large at high-energy, above a few ~10 GeV/n, but it is always sub-dominant at sub-GeV/n energies, that is, the energy region where dark-matter induced signals may exceed the standard astrophysical background. Furthermore, the total antinuclei flux from insterstellar spallation plus SNR-component is tightly bounded by the data, so that hadronic production in SNRs has a minor impact on the astrophysical background for dark matter searches.

astro-ph.HE↗

arXiv2018

Solar Modulation of Galactic Cosmic Rays: Physics Challenges for AMS-02

The Alpha Magnetic Spectrometer (AMS) is a new generation high-energy physics experiment installed on the International Space Station in May 2011 and operating continuously since then. Using an unprecedently large collection of particles and antiparticles detected in space, AMS is performing precision measurements of cosmic ray energy spectra and composition. In this paper, we discuss the physics of solar modulation in Galactic cosmic rays that can be investigated with AMS my means of dedicated measurements on the time-dependence of cosmic-ray proton, helium, electron and positron fluxes.

astro-ph.HE↗

arXiv2018

Testing universality of cosmic-ray acceleration with proton/helium data from AMS and Voyager-1

The AMS experiment has recently measured the proton and helium spectra in cosmic rays (CRs) in the GeV-TeV energy region. The two spectra are found to progressively harden at rigidity $R = pc/Z >\,$200 GV, while the p/He ratio is found to fall off steadily as $p/He\sim\,R^{-0.08}$. The p/He decrease is often interpreted in terms of particle-dependent acceleration, which is in contrast with the universal nature of DSA mechanisms. A different explanation is that the p-He anomaly reflects a flux transition between two components: a sub-TeV flux component (L) provided by hydrogen-rich supernova remnants with soft acceleration spectra, and a multi-TeV component (G) injected by younger sources with amplified magnetic fields and hard spectra. In this scenario the universality of particle acceleration is not violated because both sources provide composition-blind injection spectra. The present work is aimed at testing this model using the low-energy CR flux which is expected to be L-dominated. However, at $E\sim\,$0.5-10 GeV, the fluxes are affected by energy losses and solar modulation for which a proper modeling is required. To set the properties of the L-source, I have used the Voyager-1 data collected in the interstellar space. To compare my calculations with the AMS data, I have performed a determination of the force-field modulation parameter using neutron monitor measurements. I will show that the recent $p-He$ data reported by AMS and Voyager-1 are in good agreement with the predictions of such a scenario, supporting the hypothesis that CRs are injected in the Galaxy by universal, composition-blind accelerators. At energies below $\sim\,$0.5 GeV/n, however, the model is found to underpredict the data collected by PAMELA from 2006 to 2010. This discrepancy is found to increase with increasing solar activity, reflecting an expected breakdown of the force-field approximation.

astro-ph.HE↗

arXiv2018

Bayesian analysis of spatial-dependent cosmic-ray propagation: astrophysical background of antiprotons and positrons

The AMS-02 experiment has reported a new measurement of the antiproton/proton ratio in Galactic cosmic rays (CRs). In the energy range $E\sim\,$60-450 GeV, this ratio is found to be remarkably constant. Using recent data on CR proton, helium, and carbon fluxes, 10Be/9Be and B/C ratios, we have performed a global Bayesian analysis based on a Markov-Chain Monte-Carlo sampling algorithm under a "two halo model" of CR propagation. In this model, CRs are allowed to experience a different type of diffusion when they propagate in the region close of the Galactic disk. We found that the vertical extent of this region is about 900 pc above and below the disk, and the corresponding diffusion coefficient scales with energy as $D\sim\,E^{0.15}$, describing well the observations on primary CR spectra, secondary/primary ratios and anisotropy. Under this model we have carried out improved calculations of antiparticle spectra arising from secondary CR production and their corresponding uncertainties. We made use of Monte-Carlo generators and accelerator data to assess the antiproton production cross-sections and their uncertainties. While the positron excess requires the contribution of additional unknown sources, we found that the new AMS-02 antiproton data are consistent, within the estimated uncertainties, with our calculations based on secondary production.

astro-ph.HE↗

arXiv2018

Solar and nuclear physics uncertainties in cosmic-ray propagation

Recent data released by the AMS experiment on the primary spectra and secondary-to-primary ratios in cosmic rays (CRs) can pose tight constraints to astrophysical models of CR acceleration and transport in the Galaxy, thereby providing a robust baseline of the astrophysical background for dark matter search via antimatter. However, models of CR propagation are affected by other important sources of uncertainties, notably from solar modulation and nuclear fragmentation, that cannot be improved with the sole use of the AMS data. The present work is aimed at assessing these uncertainties and their relevance in the interpretation of the new AMS data on the boron-to-carbon (B/C) ratio. Uncertainties from solar modulation are estimated using improved models of CR transport in the Heliosphere constrained against various type of measurements: monthly-resolved CR data collected by balloon-born or space missions, interstellar flux data from the Voyager-1 spacecraft, and counting rates from ground-based neutron monitor detectors. Uncertainties from nuclear fragmentation are estimated using semiempirical cross-section formulae constrained by measurements on isotopically-resolved and charge-changing reactions. We found that a proper data-driven treatment of solar modulation can guarantee the desired level of precision, in comparison with the improved accuracy of the recent data on the B/C ratio. On the other hand, nuclear uncertainties represent a serious limiting factor over a wide energy range. We therefore stress the need for establishing a dedicated program of cross-section measurements at the $\mathcal{O}$(100 GeV) energy scale.

astro-ph.HE↗

arXiv2018

Production of cosmic-ray antinuclei in the Galaxy and background for dark matter searches

Antimatter nuclei in cosmic rays (CR) represent a promising discovery channel for the indirect search of dark matter. We present astrophysical background calculations of CR antideuteron ($\overline{d}$) and antihelium ($\overline{He}$). These particles are produced by high-energy collisions of CR protons and nuclei with the gas nuclei of the interstellar medium. In our calculations, we also consider production and shock acceleration of antinuclei in the shells of supernova remnants (SNRs). The total flux of $\overline{d}$ and $\overline{He}$ particles is constrained using new AMS measurements on the boron/carbon (B/C) and antiproton/proton ($\bar{p}/p$) ratios. The two ratios leads to different antiparticle fluxes in the high-energy regime $E\gtrsim$ 10 GeV/n where, in particular, $\bar{p}/p$-driven calculations leads to a significantly larger antiparticle flux in comparison to predictions from conventional B/C-driven constraints. On the other hand, both approaches provide consistent results in the sub-GeV/n energy window, which is where dark matter induced signal may exceed the astrophysical background. In this region, the total antinuclei flux, from interaction in the insterstellar gas and inside SNRs, is tightly bounded by the data. Shock-acceleration of antiparticles in SNRs has a minor influence in the astrophysical background for dark matter searches.

astro-ph.HE↗

arXiv2018

The curious case of high-energy deuterons in Galactic cosmic rays

A new analysis of cosmic ray (CR) data collected by the SOKOL experiment in space found that the deuteron-to-helium ratio at energies between 500 and 2000 GeV/nucleon takes the value d/He ~ 1.5. As we will show, this result cannot be explained by standard models of secondary CR production in the interstellar medium and points to the existence of a high-energy source of CR deuterons. To account for the deuteron excess in CRs, we argue that the only viable solution is hadronic interaction processes of accelerated particles inside old supernova remnants (SNRs). From this mechanism, however, the B/C ratio is also expected to increase at energy above ~50 of GeV/nucleon, in conflict with new precision data just released by the AMS-02 experiment. Hence, if this phenomenon is a real physical effect, hadronic production of CR deuterons must occur in SNRs characterized by low metal abundance. In such a scenario, the sources accelerating C-N-O nuclei are not the same as those accelerating helium or protons, so that the connection between d/He ratio and B/C ratio is broken, and the latter cannot be used to place constraints on the production of light isotopes or antiparticles.

astro-ph.HE↗

arXiv2018

Production and acceleration of antinuclei in supernova shockwaves

We compute the energy spectra of antideuterons and antihelium in cosmic rays (CRs) in a scenario where hadronic interactions inside supernova remnants (SNRs) can produce a diffusively-shock-accelerated "source component" of secondary antinuclei. The key parameters that specify the SNR environment and the interstellar CR transport are tightly constrained with the new measurements provided by the AMS experiment on the B/C ratio and on the antiproton/proton ratio. The best-fit models obtained from the two ratios are found to be inconsistent with each other, as the antiproton/proton data require enhanced secondary production. Thus, we derive conservative (i.e., B/C-driven) and speculative (antiproton/proton-driven) upper limits to the SNR flux contributions for the d and He spectra spectra in CRs, along with their standard secondary component expected from CR collisions in the interstellar gas. We find that the source component of antinuclei can be appreciable at kinetic energies above a few ~10 GeV/n, but it is always sub-dominant below a few GeV/n, that is the energy window where dark matter (DM) annihilation signatures are expected to exceed the level of secondary production. We also find that the total (standard + SNR) flux of secondary antinuclei is tightly bounded by the data. Thus the presence of interaction processes inside SNRs does not critically affect the total background for DM searches.

astro-ph.HE↗

arXiv2018

Evidence for a Time Lag in Solar Modulation of Galactic Cosmic Rays

The solar modulation effect of cosmic rays in the heliosphere is an energy-, time-, and particle-dependent phenomenon which arises from a combination of basic particle transport processes such as diffusion, convection, adiabatic cooling, and drift motion. Making use of a large collection of time-resolved cosmic-ray data from recent space missions, we construct a simple predictive model of solar modulation which depends on direct solar-physics inputs: the number of solar sunspots and the tilt angle of the heliospheric current sheet. Under this framework, we present calculations of cosmic-ray proton spectra, positron/electron and antiproton/proton ratios and their time dependence in connection with the evolving solar activity. We report evidence for a time-lag $Δ{T}=8.1\pm\,1.2$ months, between solar activity data and cosmic-ray flux measurements in space, which reflects the dynamics of the formation of the modulation region. This result enables us to forecast the cosmic-ray flux near Earth well in advance by monitoring solar activity

astro-ph.HE↗

arXiv2018

Fresh Insights on Cosmic-ray Propagation from the New AMS Data

The Alpha Magnetic Spectrometer (AMS) experiment has released new measurements of primary and secondary nuclei in cosmic rays at the TeV energy scale. Using these data, we present new results based on a global Bayesian analysis of our two halo model of CR transport.

astro-ph.HE↗

arXiv2015

Origin of the Proton-to-Helium Ratio Anomaly in Cosmic Rays

Recent data on Galactic cosmic rays (CRs) revealed that the helium energy spectrum is harder than the proton spectrum. The AMS experiment has now reported that the proton-to-helium ratio as function of rigidity $R$ (momentum-to-charge ratio) falls off steadily as p/He $\sim R^Δ$, with $Δ\approx$-0.08 between $R\sim$40 GV and $R\sim$2 TV. Besides, the single spectra of proton and helium are found to progressively harden at $R\gtrsim$100 GV. The p/He anomaly is generally ascribed to particle-dependent acceleration mechanisms occurring in Galactic CR sources. However, this explanation poses a challenge to the known mechanisms of particle acceleration since they are believed to be "universal", composition blind, rigidity mechanisms. Using the new AMS data, we show that the p/He anomaly can be simply explained in terms of a two-component scenario where the GeV-TeV flux is ascribed to a hydrogen-rich source, possibly a nearby supernova remnant, characterized by a soft acceleration spectrum. This simple idea provides a common interpretation for the p/He ratio and for the single spectra of proton and helium: both anomalies are explained by a flux transition between two components. The "universality" of particle acceleration in sources is not violated in this model. A distinctive signature of our scenario is the high-energy flattening of the p/He ratio at multi-TeV energies, which is hinted at by existing data and will be resolutely tested by new space experiments ISS-CREAM and CALET.

astro-ph.HE↗

arXiv2015

AMS-02 in Space: Physics Results

The Alpha Magnetic Spectrometer (AMS-02) is a particle physics experiment designed to study origin and nature of Galactic Cosmic Rays (CRs) up to TeV energies from space. With its high sensitivity, long exposure and excellent identification capabilities, AMS is conducting a unique mission of fundamental physics research in space. To date, more than 60 billion CR events have been collected by AMS. The new results on CR leptons and the analysis and light-nuclei are presented and discussed. The new leptonic data indicate the existence of new sources of high-energy CR leptons, that may arise either by dark-matter particles annihilation or by nearby astrophysical sources of $e^{\pm}$ pairs. Future data at higher energies and forthcoming measurements on the antiproton spectrum and the boron-to-carbon ratio will be crucial in providing the discrimination among the different scenario.

astro-ph.HE↗

arXiv2015

AMS-02 in space: physics results, overview, and challenges

The Alpha Magnetic Spectrometer (AMS-02) is a state of the art particle detector measuring cosmic rays (CRs) on the International Space Station (ISS) since May 19th 2011. AMS-02 identifies CR leptons and nuclei in the energy range from hundreds MeV to few TeV per nucleon. Several sub-detector systems allow for redundant particle identification with unprecedented precision, a powerful lepton-hadron separation, and a high purity of the antimatter signal. The new AMS-02 leptonic data from 1 to 500 GeV are presented and discussed. These new data indicate that new sources of CR leptons need to be included to describe the observed spectra at high energies. Explanations of this anomaly may be found either in dark-matter particles annihilation or in the existence of nearby astrophysical sources of $e^{\pm}$. Future data at higher energies and forthcoming measurements on the antiproton spectrum and the boron-to-carbon ratio will be crucial in providing the discrimination among the different scenario.

astro-ph.HE↗

arXiv2015

Fragmentation cross-sections and model uncertainties in Cosmic Ray propagation physics

Abundances and energy spectra of cosmic ray nuclei are being measured with high accuracy by the AMS experiment. These observations can provide tight constraints to the propagation models of galactic cosmic rays. In the view of the release of these data, I present an evaluation of the model uncertainties associated to the cross-sections for secondary production of Li-Be-B nuclei in cosmic rays. I discuss the role of cross section uncertainties in the calculation of the boron-to-carbon and beryllium-to-boron ratios, as well as their impact in the determination of the cosmic-ray transport parameters.

astro-ph.HE↗

arXiv2015

Consistent description of leptonic and hadroninc spectra in cosmic rays

The AMS Collaboration has recently released data on cosmic ray (CR) leptons and hadrons that can shed light on two exciting problems in CR physics: on one side, the origin of the rise of the CR positron fraction above ~10 GeV of energy, on the other side, the nature of the spectral features observed in CR protons and helium at TeV energies. Concerning heavier nuclei, The ATIC-2 experiment has recently reported an puzzling spectral upturn at energy ~50 GeV per nucleon in several primary/primary ratios involving Iron, such as the O/Fe or C/Fe ratio. In this work, the AMS data are described using a two-component scenario, where the total CR flux is provided by a mixture of fluxes accelerated by sources with different properties. Within this picture, the role of secondary CR production inside nearby supernova remnants is discussed. In particular, we present the predictions of our model for the C/Fe and O/Fe ratios, in connection with the spectral anomalies found by the ATIC-2 experiment.

astro-ph.HE↗

arXiv2015

Inhomogeneous diffusion model for recent data on high-energy cosmic rays

The AMS Collaboration has recently released precision data on cosmic ray (CR) leptons and protons at high energies. Interesting progresses have also been made on the measurement of CR nuclei, such as the boron-to-carbon ratio or the lithium spectrum, up to TeV/nucleon energies. In order to provide a description these data, I consider a diffusion model of CR propagation which allows for latitudinal variations of the CR diffusion properties in the Galactic halo. I discuss the role of high-precision data on light CR nuclei in resolutely testing this model and the key propagation parameters.

astro-ph.HE↗

arXiv2015

Identification of Light Cosmic-Ray Nuclei with AMS-02

AMS-02 is a wide acceptance (0.5 m2 sr) and long duration (up to 20 years) magnetic spectrometer operating onboard the International Space Station since May 2011. Its main scientific objectives are the indirect research of Dark Matter, searches of primitive Anti-Matter and the precise measurement of the Cosmic-Ray (CR) spectra. Among charged CR species, AMS-02 will be able to measure relative abundances and absolute fluxes of CRs nuclei from Hydrogen up to at least Iron (Z = 26) in a kinetic energy range from hundreds MeV to TeV per nucleon. The high statistics measurement of the chemical composition of CRs in this extended energy range will reveal new insights about the CRs life in the Galaxy, from their origin to the propagation in the interstellar medium, giving new constraints to astrophysical models of Galactic CRs. The nucleus absolute charge, Z, is measured several times along the trajectory of the particle inside AMS-02 using different detection techniques: in the 9 planes of the Silicon Tracker, in the 4 layers of scintillator counters of the Time-of-Flight system (TOF), in the Ring Imaging Cherenkov Counter (RICH) as well as in the 20 layers of Transition Radiation Detector (TRD) and in the upper layers of the Electromagnetic Calorimeter (ECAL). The combination of the redundant measurements delivered by the tracking system and by the TOF allows an accurate discrimination between chemical elements. The charge measurements in the detectors on top of AMS, as the Upper plane of the Tracker and in TRD, is used for the identification of the incoming nuclear specie and allows the charge-changing events background estimation. The AMS-02 different charge measurement principles are here briefly explained, and performance of each sub-detector presented. Then the AMS-02 combined charge separation capability as well as the interaction events identification principles are presented.

physics.ins-det↗