Main research areas
Phenomenology of Galactic cosmic rays — Within the field of particle astrophysics, my research is about analyzing, interpreting and making sense of observations, with the goal of establishing a comprehensive and agreeable theory for the origin of cosmic rays, as well as for their propagation in the Milky Way.
Detection of charged particles in space — I am active in the analysis of cosmic-ray data collected by the Alpha Magnetic Spectrometer (AMS) experiment in the International Space Station. Within the AMS Collaboration, I have contributed to the measurements of proton and helium spectra, to the charge and isotopic composition of light nuclei, and their temporal variations.
Charged radiation in the Heliosphere — This research is aimed at predicting the temporal evolution and spatial distribution of energetic charged radiation in the interplanetary space. I am active at developing data-driven models for the transport of charged particles in the heliosphere, and analyzing multichannel data on particle fluxes and solar activity. These tasks address a prerequisite for understanding the effects of cosmic radiation and space weather hazards in space missions.
Nuclear fragmentation and coalescence — The search of dark matter signatures in cosmic-ray data requires a precise knowledge of the several fragmentation cross-sections that regulate production and destruction of Galactic particles interacting with the interstellar gas. In my research, I study the investigation fragmentation and coalescence processes that involve the production of nuclei and anti-nuclei in the Galaxy.
Antimatter and dark matter — I am active in the investigation and modeling of the physics mechanisms that lead to the production of antimatter in the Galaxy, such as positrons, antiprotons, and light anti-nuclei. I am focused on the collisions of cosmic rays with the interstellar gas, supernova explosions, or annihilation of dark matter particles in the galactic halo.
2021: Ironing Out Cosmic Rays: the Fe spectrum [PRL]
With data of the Alpha Magnetic Spectrometer (AMS) in the International Space Station, we have measured, with percent-level precision, the rigidity spectrum of Iron in cosmic rays. Iron is the heaviest element yet to be accurately characterized in the cosmic radiation. The measurement is made in the rigidity range 2.65 GV to 3.0 TV with 620,000 nuclei collected by AMS since 2011 to 2020. We found that above 80.5 GV of rigidity, the rigidity dependence of the cosmic ray Fe flux is identical to the rigidity dependence of other primary cosmic-ray fluxes such as He, C, and O. At these rigidities, the Fe/O ratio is constant at 0.155 ± 0.006 . This result shows that unexpectedly Fe and He, C, and O belong to the same class of primary cosmic rays which is different from the primary cosmic rays Ne, Mg, and Si class.
Physics Focus: Ironing Out Cosmic Rays
2020: The heavy side of cosmic rays… Ne-Mg-Si [PRL]
In this research, we report the observation of new properties of primary cosmic rays, neon (Ne), -magnesium (Mg), and silicon (Si), measured in the rigidity range 2.15 GV to 3.0 TV. We use a sample of 1.8 million Ne, 2.2 million Mg, and 1.6 million Si nuclei collected by the Alpha Magnetic Spectrometer experiment on the International Space Station. The Ne and Mg spectra have identical rigidity dependence above 3.65 GV. The three spectra have identical rigidity dependence above 86.5 GV, deviate from a single power law above 200 GV, and harden in an identical way. Unexpectedly, above 86.5 GV the rigidity dependence of primary cosmic rays Ne, Mg, and Si spectra is different from the rigidity dependence of primary cosmic rays He, C, and O. This shows that the Ne, Mg, and Si and He, C, and O are two different classes of primary cosmic rays.
Physics Focus: New Data Reveal the Heavy Side of Cosmic Rays.
2019: Isotopic composition of cosmic ray helium nuclei [PRL]
This paper presents new AMS measurements on the 3He and 4He fluxes in cosmic rays. The measurements are based on 100 million 4He nuclei in the rigidity range from 2.1 to 21 GV, and 18 million 3He from 1.9 to 15 GV, collected from May 2011 to November 2017. We observed that the the fluxes of 3He and 4He exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the 3He/4He flux ratio is measured for the first time. Below 4 GV, the ratio is found to have a significant long-term time dependence. Above 4 GV, the flux ratio is found to be time independent and its rigidity dependence is well described by a single power law, in agreement with the B/O and B/C spectral indices at high energies.
2019: Numerical models for cosmic protons and helium in the heliosphere [ASR]
Thanks to space-borne experiments of cosmic-ray (CR) detection, such as the AMS and PAMELA missions in low-Earth orbit, or the Voyager-1 spacecraft in the interstellar space, a large collection of multi-channel and time-resolved CR data has become available. Recently, the AMS experiment has released new precision data, on the proton and helium fluxes in CRs, measured on monthly basis during its first six years of mission. The data reveal a remarkable long-term behavior in the temporal evolution of the proton-to-helium ratio at rigidity R below 3 GV. As we have argued in a recent work, such a behavior may reflect the transport properties of low-rigidity CRs in the inteplanetary space. In particular, it can be caused by mass/charge dependence of the CR diffusion coefficient. In this paper, we present our developments in the numerical modeling of CR transport in the Milky Way and in the heliosphere. Within our model, and with the help of approximated analytical solutions, we describe in details the relations between the properties of CR diffusion and the time-dependent evolution of the proton-to-helium ratio.
Precision measurements of cosmic ray positrons and electrons are presented up to the TeV scale. The measured positron flux, based on 1.9 million positrons collected by AMS on the ISS, is well described by the sum of a term associated with the positrons produced in the collision of cosmic rays, which dominates at low energies, and a new source term of positrons, which dominates at high energies. A finite energy cutoff of the source term of about 800 GeV is established with a significance of more than 4-sigma. These experimental data on cosmic ray positrons show that, at high energies, they predominantly originate either from dark matter annihilation or from other astrophysical sources. The measured electron flux, based on 28 millions electrons, exhibits a complex energy dependence in the entire energy range. The different behavior of the electrons and positrons measured by AMS is clear evidence that most high energy electrons originate from different sources than high energy positrons.
2019: Penetrating particle ANalyzer – The PAN project [ASR]
PAN is a new concept of cosmic ray detector in deep space and interplanetary missions. By design, the PAN instrument is able to precisely measure and monitor the flux, composition, and direction of charged cosmic-ray particles. The science program of the PAN project is multi- and cross-disciplinary, covering cosmic-ray astrophysics, solar physics, space weather and space travel. The PAN experiment will fill an observation gap of Galactic cosmic rays in the GeV region, and provide precise information of the spectrum, composition and emission time of energetic particle originated from the Sun.
2018: A new observational test for diffusion of cosmic rays in the heliosphere [PRL]
After six years of continuous observations in space, the Alpha Magnetic Spectrometer experiment has released new data on the temporal evolution of the proton and helium fluxes in cosmic rays. These data revealed that the ratio between proton and helium fluxes at the same value of rigidity R=p/Z (momentum/charge ratio) is not constant at rigidity below 3 GV. In particular, the ratio is found to decrease steadily during the descending phase of Solar Cycle 24 toward the next minimum. We show that such a behavior is a remarkable signature of the βxλ(R) dependence in the diffusion of cosmic rays in heliosphere, where β is their adimensional speed and λ(R) is their mean free path, a universal function of rigidity for all nuclei. This dependence is responsible for distinctive charge/mass dependent effects in the time-dependent modulation of low-rigidity particles.
2018: New AMS data of the temporal dependence of CR protons and helium [PRL]
Based on the analysis of one billion particles collected by AMS, we have obtained the time-resolved measurements of the proton and helium fluxes in a wide range of energy (0.5 to 50 GeV/n) and over 72 months of observation time (May 2011 – May 2017). Our results have been published in PRL. They show that the proton flux and the helium fluxes show nearly identical fine structures in both time and relative amplitude. The amplitudes of the flux structures decrease with increasing energy. Moreover, these amplitudes are reduced during the time period after the 2014 Solar Maximum, when both fluxes are found to increase steadily. Remarkably, a puzzling long-term behavior has been observed in the ratio p/He between proton and helium fluxes. This behavior may reflect fundamental properties of CR diffusion in the heliospheric turbulence. More details on these results can be found in the MAtISSE project page.
2018: New AMS data of the temporal dependence of electrons and positrons [PRL]
In this paper, we have reported the detailed time and energy dependence of the electron (e-) and positron (e+) fluxes, along with their ratio e+/e-, at energy from 0.5 to 50 GeV/n and over 72 months of observation time (May 2011 – May 2017). The data show that the e+/e- ratio follows a smooth transition from one value to another across the last solar maximum of 2014. This transition reflects the physics of the magnetic reversal. Further deailed information can be extracted from the time-dependence of the absolute fluxes at different energies, that are currently being investigated. With these data, for the first time, the charge-sign dependent modulation during solar maximum has been characterized in detail by leptons alone. More details on these results can be found in the MAtISSE project page.
2018: New results on lithium, beryllium and boron in cosmic rays [PRL]
This paper reports the observation of new properties of so-called “secondary” cosmic rays Li, Be, and B. These elements have been detected at unprecedently high energy (up to 3.3 TV of rigidity) by AMS during the first five years of operation aboard the International Space Station. The rigidity (momentum per unit charge) ranges from 1.9 GV to 3.3 TV. The three energy spectra are found to deviate from a single power law above 200 GV in an identical way. This behavior of secondary cosmic rays has also been observed in the AMS measurement of primary cosmic rays He, C, and O but the rigidity dependences of primary cosmic rays and of secondary cosmic rays are distinctly different. In particular, above 200 GV, the secondary cosmic rays harden more than the primary cosmic rays.
Synopsis: Space Measurements of Secondary Cosmic Rays.
2017: Fresh Insights on Cosmic-Ray Propagation from the new AMS Data [RNAAS]
The Alpha Magnetic Spectrometer 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.
2017: Measurements of helium, carbon, and oxygen in CRs with AMS-02 [PRL]
In this paper, we report the observation of new properties of primary cosmic rays He, C, and O measured in the rigidity (momentum/charge) range 2 GV to 3 TV with 90 million helium 8.4 million carbon, and 7 million oxigen nuclei collected by the Alpha Magnetic Spectrometer during the first five years of operation. Above 60 GV, these three spectra have identical rigidity dependence. They all deviate from a single power law above 200 GV and harden in an identical way.
2017: Evidence for a Time Lag in Solar Modulation of Galactic Cosmic Rays [ApJL]
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 DT = 8.1 +/- 0.9 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.
This work is featured as “research highlight” by the American Astronomical Society.More details on these results can be found in the MAtISSE project page.
2017: Solar and nuclear physics uncertainties in cosmic-ray propagation [PRD]
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 O(100 GeV) energy scale. More details on these results can be found in the MAtISSE project page.
2017: Production and acceleration of antinuclei in supernova shockwaves [ApJL]
We have calculated 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 â€œsource componentâ€ 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 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 dark-matter searches.
2017: Production of antimatter nuclei in Galactic cosmic rays [ICRC]
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.
2017: The curious case of high-energy deuterons in Galactic cosmic rays [ApJL]
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. 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 supernova remnants 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.
2017: Testing universality in CR acceleration using AMS-02 and Voyager-1 data [ASR]
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~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~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 ~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. More details on these results can be found in the MAtISSE project page.
2016: Bayesian analysis of CR propagation: antimatter background [PRD]
The AMS-02 experiment has reported a new measurement of the antiproton/proton ratio in Galactic cosmic rays (CRs). In the energy range E~60-450 GeV, this ratio is found to be remarkably constant. Using recent data on CR proton, helium, carbon, 10Be/9Be, and B/C ratio, 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~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. Our work got the attention of the science communication agency MEDIA-INAF. Figure 15 of our paper has been selected by the PRD editors as Kaleidoscope. More details on these results can be found in the MAtISSE project page.
2016: Measurement of B/C ratio with AMS-02 [PRL]
Precise knowledge of the boron/carbon (B/C) ratio is essential for understanding the processes of cosmic-ray transport and interactions in the Galaxy. This paper reports the precise measurement of the B/C ratio from 1.9 GV to 2.6 TV of rigidity, based on 2.3 million boron and 8.3 million carbon nuclei collected by AMS during the first 5 years of operation. The B/C ratio is found not to show significant structures, in contrast to many models that require such structures at high rigidities. Above 65 GV of rigidity, the B/C ratio is well described by a single power-law function with log-slope 0.33, in agreement with the asymptotic behaviour expected from Kolmogorov theory of interstellar turbulence.
2016: Measurement of high-energy antiprotons with AMS-02 [PRL]
In this work, we have performed a precision measurement the antiproton flux and the antiproton-to-proton flux ratio in cosmic rays in the rigidity range from 1 to 450 GV. The measurement is based on a high-statistic sample of about 350,000 antiproton events, In the rigidity range from 60 to nearly 500 GV, the antiproton-to-proton ratio is found to be remarkably constant. This result is at tension with the predictions of standard model of cosmic-ray propagations, from which the ratio is expected to decrease with rigidity, roughly, as fast as the boron-to-carbon ratio does. The antiproton production and propagation in the Galaxy is far from being understood and deserve more theoretical investigation.
2015: Origin of the p/He ratio anomaly in cosmic rays [ApJL]
Recent data on Galactic cosmic rays 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/charge ratio) falls off steadily as p/He~R^D, with D=-0.08 between R~40 GV and R~2 TV. Besides, the single spectra of proton and helium are found to progressively harden at R>100GV. The p/He anomaly is generally ascribed to particle-dependent acceleration mechanisms occurring in Galactic cosmic-ray 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. In this work, using the new AMS data, I have shown 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. Remarkably, the “universality” of particle acceleration in sources is not violated in this model. A distinctive signature of my scenario is the high-energy flattening of the p/He ratio at multi-TeV energies, which is hinted by existing data and will be resolutely tested by new space experiments ISS-CREAM and CALET.
2015: Measurement of the Helium Flux in Cosmic Rays with AMS-02 [PRL]
Knowledge of the precise rigidity dependence of the helium flux is important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a single power law.
2015: Particles and antiparticles under a two-halo scenario of CR propagation [PRD(R)]
The study of antimatter particles in space excites many physicists who believe in the possibility that these particles come from annihilation of dark matter particles. This possibility is very difficult to prove because antiparticles are also created by collisions of ordinary CRs with the matter of the Galactic plane. For example, collisions of energetic protons can produce antiprotons or positrons. Hence the level of antimatter arising from CR collisions has to be carefully calculated. However, this calculation depends crucially on our knowledge of CR propagation through the Galaxy. In this paper, I have presented new calculations aimed at explaining recent conflicting observations on protons, nuclei, anisotropy, and gamma-rays. I performed calculation under a two-halo scenario of CR propagation, where CRs at higher energies tend to gather closer to the Galactic plane. As shown, this leads to an enhanced production of energetic antimatter. In comparison with standard calculations, I predict that antiprotons and positrons are approximately 5 times more abundant at the highest energies detectable by the AMS experiment in space. If the upcoming data from AMS confirms this scenario, then the current searches for dark matter signals in space will have to account for it. My paper is getting some attention by the press, see here.
2015: Wanted! Nuclear data for CR propagation physics with AMS [PRC]
High-energy Li-Be-B nuclei in CR are being measured with unprecedent accuracy by AMS. These data bring valuable information to the CR propagation physics. In particular, combined measurements of B/C and Be/B ratios may allow to break the parameter degeneracy between the CR diffusion coefficient and the size of the propagation region, which is crucial for dark matter searches. Hovewer, the parameter determination relies in the calculations of the Be and B production from collisions of heavier nuclei with the gas. These calculations depend crucially on the quality of the cross-section data. Using the available cross-section data, I have presented for the first time an evaluation of the nuclear uncertainties and their impact in constraining the propagation models. I found that the AMS experiment can provide tight constraints on the transport parameters allowing to resolutely break the degeneracy, while nuclear uncertainties in the models are found to be a major limiting factor. Once these uncertainties are accounted, the degeneracy remains poorly resolved. In particular, the BeB ratio at 1-10 GeV/n is found not to bring valuable information for the parameter extraction. On the other hand, precise B/C data at higher energy may be useful to test the nuclear physics inputs of the models. In summary, given the level of precision of AMS, nuclear uncertainties in secondary production models are found to be a major limiting factor for further progress in CR propagation. The collection of new nuclear data, within a dedicated program of cross-section measurements and modeling, would enable to fully exploit the potential of the AMS data.
2015: Origin of the spectral upturn in the cosmic-ray C/Fe and O/Fe ratios [PRD]
The observed spectrum of Galactic CRs has several exciting features such as the rise in the positron fraction above ~10 GeV of energy and the spectral hardening of protons and helium above ~300 GeV/nucleon of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the C/Fe or O/Fe ratios, at energy ~50 GeV per nucleon. It is recognized that the observed positron excess can be explained by pion production processes during diffusive shock acceleration of CR hadrons in nearby sources. Recently, it was suggested that a scenario with nearby source dominating the GeV-TeV spectrum may be connected with the change of slope observed in protons and nuclei, which would be interpreted as a flux transition between the local component and the large-scale distribution of Galactic sources. Here I show that, under a two-component scenario with nearby source, the shape of the spectral transition is expected to be slightly different for heavy nuclei, such as iron, because their propagation range is spatially limited by inelastic collisions with the interstellar matter. This enables a prediction for the primary/primary ratios between light and heavy nuclei. From this effect, a spectral upturn is predicted in the C/Fe and O/Fe ratios in good accordance with the ATIC-2 data.
2015: Measurement of the Proton Flux in Cosmic Rays with AMS-02 [PRL]
Knowledge of the the behaviour of the proton and helium spectra at high energy is a highly demanded challenge in CR astrophysics. In 2014-2015 I have been intensively involved in the proton and helium spectra measurements in close collaboration with other institutions. In the LPSC/AMS team Grenoble, we are involved in all steps of the measurement, such as the analysis of raw data, simulation and data-driven studies of the detector response, spectral unfolding algorithm, study of spectral properties, interpretation of the data. After 1.5 years of intensive work, the first AMS result on the proton flux in the momentum range 1 GeV/c – 1.8 TeV/c have been now released. Based on 300 million proton events, the data show the detailed variation with momentum of the flux, which is found to progressively hardens above 100 GeV/c.
2014: Connection between positron fraction and CR hadrons [ApJL]
Very recently, the AMS collaboration has released new precision data on the positron fraction, e+/(e- + e+), between 0.5 and 500 GeV. The data show a rise of the fraction between 10 and 200 GeV, followed by a possible plateau at higher energies. In this new work, we established a connection between the positron fraction anomaly and the puzzling CR spectral hardening observed in proton and nuclei. The lepton anomaly indicates the existence of a nearby eÂ± source. It has been proposed by several authors that high-energy positrons can be produced inside nearby supernova remnants via interactions of CR hadrons with the ambient medium. A distinctive prediction of this mechanism is a high-energy rise of the boron-to-carbon ratio which, however, has not been observed at present. This scenario also requires that the supernovae remnants (SNRs) responsible for the e+ excess have ineffective magnetic field amplification and slow shock speed. We argue that a CR accelerator of these properties (typical of old SNRs) cannot account for the CR hadronic spectra observed up to the knee energies (5 PeV), so that this scenario is incomplete and must be revisited. We propose a new picture where, in addition to such a nearby CR accelerator, the high-energy spectrum of CR hadrons is provided by the large-scale population of SNRs, on average younger, that can efficiently accelerate CRs up to the knee. Under this scenario, the spectral hardening of CR hadrons can be naturally interpreted as the transition between the two components. As we show, our two-component model breaks the connection between the positron fraction and the boron-to-carbon ratio, which is now predicted to decrease with energy in accordance with the data. Clearly, forthcoming data from AMS on proton/helium spectra and the B/C ratio will be crucial for testing this model.
2013: High-energy Measurement of the positron fraction by AMS-02 [PRL]
The first results relased by the AMS-02 collaboration consistent in the measurement of the positron fraction (positron flux divided by the electron plus positron flux) at energy between 0.5 and 350 GeV. Within the lepton working group, my activity was focused in the estimation of the background which is crucial for selecting the positron signal at high energies. I studied the contamination induced by cosmic-ray hadrons (such as protons, helium and heavier nuclei) interacting with the top-of-instrument material and possible analysis strategies in order to suppress this background. Similarly, I also studied the effect of top-of-instrument interactions of elections. The occurrence of radiative processes such as bremstralung followed by pair convertion may produce background positrons that affect the measurement at high energy.
Physics Focus: Positron Galore
2013: Identification of light CR nuclei with AMS-02 in space [ICRC2013]
The charge identification using the several detector elements of the spectrometer is a crucial aspect of the data analysis of light nuclei in CRs, i.e., for the determination of they spectra and abundances. I developed a global likelihood estimator for the AMS charge in the full dynamic range (from Z = 1 to Z = 28). I made performance studies for the charge identification capabilities of all the AMS sub-detectors. With the combination of high statistics and good performance, measurements of CR nuclear fluxes can be obtained also for the rarer CR species such as Li, Be and B nuclei. With the AMS group in Grenoble, we did a first determination of the CR Lithium flux between 0.5 and 1000 GeV of kinetic energy per nucleon. The results have been presented to the AMS Collaboration. An extension of this work will consist in the measurement of 6Li/7Li isotopic composition up to 10 GeV/nucleon.
2012: Origin of the Cosmic Ray Spectral Hardening [ApJL]
Recent data on CR protons and nuclei revealed that their energy spectrum above ~100 GeV per nucleon exibiths a remarkable change in slope which challenges the traditional descriptions of CR acceleration and propagation processes. Contrary to the existing explainations, I have proposed that such a feature is as an interstellar propagation effect originating by a Galactic diffusion coefficient that is not separable into energy and space (as previously assumed). From this scenario, I found remarkable implications for several open problems in CR acceleration/propagation physics. I gave basic predictions for primary CR spectra, secondary-to-primary ratio, and anisotropy that will be tested soon by AMS.
2012: Uncertainties in CR Propagation: H and He Isotropes [Ap&SS]
Observations of light CR isotopes provide valuable information on their origin and propagation in the Galaxy. Using the AMS-01 data in the range 0.2-1.5 GeV/nucleon, I have compared the measurements on H and He isotopes with calculations for interstellar propagation and solar modulation. The comparisons are made within the astrophysical constraints provided by the B/C ratio data and within the nuclear uncertainties arising from errors in the production cross-section data. These data are described well by a diffusive-reacceleration model with parameters that match the B/C ratio data, indicating that He and heavier nuclei such as Câ€“Nâ€“O experience similar propagation histories. The astrophysical uncertainties are expected to be dramatically reduced by the upcoming AMS-02 data, so that the nuclear uncertainties will likely represent the most serious limitation on the reliability of the model predictions. On the other hand, I have found that secondary-to-secondary ratios such as 2H/3He, 6Li/7Li or 10B/11B are barely sensitive to the key propagation parameters and can represent a useful diagnostic test for the consistency of the calculations. A figure from this work have been selected by Ap&SS for the cover image of Volume 342, Issue 1.
2012: AMS Hadron Tomography with CRs [www]
Operating in the International Space Station since May 2011, AMS-02 is performing accurate measurement of CR Hydrogen and Helium nuclei with unprecedent sensitivity. The image represents the AMS detector pictured by itself using CRs. Data on helium-to-proton ratio at high energies are used to generate a tomographic reconstruction of the top-of-payload material layer. In fact, the different interaction cross sections of H and He nuclei give rise to clear structures corresponding to inhomogenities in the material. Detector elements such as screws, electronics components or mechanical interfaces are clearly recognizable.
2012: Secondary Nuclei from SNRs and CR Propagation [A&A]
Supernova remnants (SNRs) are the most promising accelerators of primary CRs such as p, He, C, N, O, Fe. Secondary CR nuclei such as Li, Be, and B are believed to be produced in-flight by high-energy collisions of primary nuclei with the interstellar matter (ISM). Within this picture, the secondary-to-primary ratios are used to constrain the basic parameters of the CR propagation models. Beyond this picture, hadronic interactions of CRs inside SNRs and re-acceleration of pre-existing CRs occurring in SNR shocks may generate an additional source component of secondary nuclei. This component, which comes directly from SNRs, is harder than the standard one (from the ISM) and may be dominating at TeV energies. In the framework of the diffusive shock acceleration theory and the diffusion model of CR transport, we have investigated the role of these processes in observed and future data, and their impact in the determination of the CR propagation parameters. Our models provide good fits to the B/C ratio down to 2 GeV/nucleon when secondary source components are included, that may imply the necessity of a re-examination of low energy effects such as galactic wind convection or diffusive reacceleration. Unfortunately, the SNR key parameters associated with these effects are degenerated with the propagation parameters, so that more precise data are called for. Promising estimates are made for the data forthcoming by AMS.
2012: AMS-02 Physics Potential for Gamma-Ray Studies
Observations of high-energy gamma-ray sources and diffuse emission above 1 GeV of energy carry valuable information for a variety of physics studies such as CR acceleration and propagation, galactic morphology, or dark matter searches. Also the Moon and the Sun are gamma-ray sources, due to interactions of charged CRs with their surface/atmosphere and by Inverse-Compton IC scattering of electrons with solar photons in the Heliosphere. The AMS experiment, charged CR detector, is able to detect high energy photons in the GeV-TeV energy range. I have investigate the AMS capabilities in detecting the gamma-ray spectra above 1 GeV of energy, considering the galactic and extragalactic components of the diffuse gamma-ray emission as well as the lunar and solar emission.
2011: AMS Physics Potential with Charged CRs [www]
AMS is performing accurate measurements of charged CRs. The origin and evolution of CR nuclei is crucial to understand the different aspects of their acceleration and propagation in the ISM. This is is particularly relevant in order to put constraints on the parameters of the CR astrophysical models and, as a consequence, to disentangle the faint contributions to anti-proton and positron spectra arising from dark matter annihilation or exotic sources. I have estimated the impact of AMS data on the CR astrophysics by evaluating its capability in determining the CR elemental spectra or constraining the astrophysical models of CR galactic propagation. These applet simulations give an idea of the improvement which can be expected from AMS for a series of physics analysis topics as a function of time.
2011: Isotopic Composition on Light Nuclei in Cosmic Rays [ApJ]
The variety of isotopes in CRs allows us to study different aspects of the processes that CRs undergo between the time they are produced and the time of their arrival in the heliosphere. In this work I measured the isotopic ratios 2H/4He, 3He/4He, 6Li/7Li, 7Be/(9Be+10Be) and 10B/11B in the range 0.2 – 1.4 GeV of kinetic energy per nucleon. The measurements, based on the data collected in space by AMS-01 on STS-91, agree well with the previous data (coming from balloon based experiments) and set new standard of precision.
2010: Charge Composition on Light Nuclei in Cosmic Rays [ApJ]
Measurement of the chemical and isotopic composition of CRs is essential for the precise understanding of their propagation in the Galaxy. While the model parameters are mainly determined using the B/C ratio, the study of extended sets of ratios can provide stronger constraints on the propagation models. Using the AMS-01 data, I have investingated the relative abundances of the light nuclei lithium, beryllium, boron and carbon. My work led to the measurement of the secondary to primary ratios Li/C, Be/C and B/C in the kinetic energy range 0.35 – 45 GeV/nucleon. The isotopic ratio 7Li/6Li was determined in the magnetic rigidity interval 2.5 – 6.3 GV. These results are in substantial agreement with other measurements, where they exist. A 10-15% overproduction of Be is found in the model predictions and can be attributed to uncertainties in the production cross-section data
2010: Silicon Tracker performance studies and alignment [NIMA]
This paper reports the performance of the AMS Silicon Tracker fully assemebled in flight configuration. The Tracker is characterized in terms of spatial resolution and detection efficiency. I have developed track reconstruction algorithms and a procedure for the detector alignment. These data were also useful for understanding and modeling the charge reconstruction performance of the single modules, and the dependence of the signals on the particle inclination or impact point. In these studies I gained very valuable experience that helped me in my subsequent research.
Collaborations and projects
- CRISP – Centro Ricerche Innovative per lo Spazio – the interdisciplinary group at University of Perugia founded on the framework agreement ASI-UniPG 2019-2-HH.0.
- AMS-02 – Alpha Magnetic Spectrometer collaboration – an international team of 500 scientists, who developed the AMS-02 experiment and are analyzing the data.
- AMS-01 – Alpha Magnetic Spectrometer – the precursor experiment of the AMS project, in the 10-day engineering flight on June 1998 onboard space shuttle Discovery (mission STS-91).
- ALADInO – Antimatter Large Acceptance Detector In Orbit – an international project for a future experiment of High Precision Particle Astrophysics, proposed as project for ESA-Voyage 2050 as a New Window on the Universe.
- PAN – Penetrating Particles Analyzer – an R&D project for the development of a scientific instrument, a compact particle detector suitable for deep space and interplanetary missions [White Paper].
- MAtISSE – Multichannel Investigation of Solar Modulation Effects in Galactic Cosmic Rays – The MSCA-IF action agreement N.707543 (2016-2018), with UniPG in collaboration with LIP-Lisbon, for studying the phenomenology of cosmic ray modulation.
- DRIFT – Cosmic antimatter drifting through the Solar Wind – Ongoing project (2019-2021) within local funding (FRB-2019) at University of Perugia.