Alpha Magnetic Spectrometer Research Articles

A comparison of deep learning models for proton background rejection with the AMS electromagnetic calorimeter

Abstract The alpha magnetic spectrometer (AMS) is a high-precision particle detector onboard the International Space Station containing six different subdetectors. The transition radiation detector and electromagnetic calorimeter (ECAL) are used to separate electrons/positrons from the abundant cosmic-ray proton background. The positron flux measured in space by AMS falls with a power law which unexpectedly softens above 25 GeV and then hardens above 280 GeV. Several theoretical models try to explain these phenomena, and a more accurate measurement of positrons at higher energies is needed to help test them. The currently used methods to reject the proton background at high energies involve extrapolating shower features from the ECAL to use as inputs for boosted decision tree and likelihood classifiers. We present a new approach for particle identification with the AMS ECAL using deep learning (DL). By taking the energy deposition within all the ECAL cells as an input and treating them as pixels in an image-like format, we train an MLP, a CNN, and multiple ResNets and convolutional vision transformers (CvTs) as shower classifiers. Proton rejection performance is evaluated using Monte Carlo (MC) events and ISS data separately. For MC, using events with a reconstructed energy between 0.2–2 TeV, at 90% electron accuracy, the proton rejection power of our CvT model is more than five times that of the other DL models. Similarly, for ISS data with a reconstructed energy between 50–70 GeV, the proton rejection power of our CvT model is more than 2.5 times that of the other DL models.

Identification of positrons and electrons at TeV energy scale using the AMS tracker, time-of-flight counters, and electromagnetic calorimeter

The Alpha Magnetic Spectrometer, AMS, is successfully collecting data since its installation on the International Space Station on May 19, 2011. One of the main AMS objectives is the measurement of cosmic ray positrons and electrons at the highest energies. We present a new technique for the identification of electrons and positrons, which is used in the AMS data analysis in the energy range up to a few TeV. The focus of this article is on the rejection of the proton background using the AMS silicon tracker, the time-of-flight counters, and the electromagnetic calorimeter.

Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica

A technique has recently been developed for tracking short-term spectral variations in Galactic cosmic rays (GCRs) using data from a single neutron monitor (NM), by collecting histograms of the time delay between successive neutron counts and extracting the leader fraction L as a proxy of the spectral index. Here we analyze L from four Antarctic NMs from 2015 March to 2023 September. We have calibrated L from the South Pole NM with respect to a daily spectral index determined from published data of GCR proton fluxes during 2015–2019 from the Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station. Our results demonstrate a robust correlation between the leader fraction and the spectral index fit over the rigidity range 2.97–16.6 GV for AMS-02 data, with uncertainty of 0.018 in the daily spectral index as inferred from L. In addition to the 11 yr solar activity cycle, a wavelet analysis confirms a 27 day periodicity in the GCR flux and spectral index corresponding to solar rotation, especially near sunspot minimum, while the flux occasionally exhibits a strong harmonic at 13.5 days. The magnetic field component along a nominal Parker spiral (i.e., the magnetic sector structure) is a strong determinant of such spectral and flux variations, with the solar wind speed exerting an additional, nearly rigidity-independent influence on flux variations. Our investigation affirms the capability of ground-based NM stations to accurately and continuously monitor cosmic-ray spectral variations over the long-term future.

Charge sign dependence of recurrent Forbush decreases in 2016–2017

Context. This study investigates the periodicities of galactic cosmic ray flux attributed to corotating interaction regions (CIRs) using Alpha Magnetic Spectrometer (AMS-02) data from late 2016 to early 2017. Aims. We determine the rigidity dependence of recurrent Forbush decrease (RFD) amplitudes induced by CIRs for different particles with a focus on charge sign. Methods. We carried out a frequency analysis using a Lomb-Scargle algorithm and superposed epoch analysis for all particles. For protons and helium, we compared the results with a single Forbush decrease (FD) analysis. Results. Our results reveal that the rigidity dependence of proton amplitudes attributed to the northern coronal hole is in qualitative agreement with previous findings. In contrast, the amplitudes attributed to the southern coronal hole show no rigidity dependence. Furthermore, the amplitude of the helium modulation exceeds that of protons, in line with the observation for long-term modulation. For positrons, statistical limitations stand in the way of any definitive conclusions. In comparison to the positively charged particles, the modulation behavior of electrons reveals a different pattern.

Revisit of GeV Gamma-Ray Emission from Orion B with the Fermi Large Area Telescope

We revisit the γ-ray emission above 300 MeV towards the massive star-forming region of Orion B by adopting 14 yr observations with the Fermi Large Area Telescope and utilizing the updated software tools. The extended γ-ray emission region around Orion B is resolved into two components (region I and region II). The γ-ray spectrum of region I agrees with the predicted γ-ray spectrum assuming the cosmic ray (CR) density is the same as that of Alpha Magnetic Spectrometer (AMS-02) measured locally. The γ-ray emissivity of region II appears to be deficit at low energy band (E < 3 GeV). Through modeling we find that CR densities exhibit a significant deficit below 20 GeV, which may be caused by a slow diffusion inside the dense region. This is probably caused by an increased magnetic field whose strength increases with the gas density.

Advancing Precision Particle Background Estimation for Future X-Ray Missions: Correlated Variability between the Alpha Magnetic Spectrometer and Chandra/XMM-Newton

Galactic cosmic-ray (GCR) particles have a significant impact on the particle-induced background of X-ray observatories, and their flux exhibits substantial temporal variability, potentially influencing background levels. In this study, we present 1 day binned high-energy reject rates derived from the Chandra-ACIS and XMM-Newton EPIC-pn instruments, serving as proxies for the GCR particle flux. We systematically analyze the ACIS and EPIC-pn reject rates and compare them with the AMS proton flux. Our analysis initially reveals robust correlations between the AMS proton flux and the ACIS/EPIC-pn reject rates when binned over 27 day intervals. However, a closer examination reveals substantial fluctuations within each 27 day bin, indicating shorter-term variability. Upon daily binning, we observe finer temporal structures in the data sets, demonstrating the presence of recurrent variations with periods of ∼25 days and 23 days in the ACIS and EPIC-pn reject rates, respectively, spanning the years 2014–2018. Notably, during the 2016–2017 period, we additionally detect periodicities of ∼13.5 days and 9 days in the ACIS and EPIC-pn reject rates, respectively. Intriguingly, we observe a time lag of ∼6 days between the AMS proton flux and the ACIS/EPIC-pn reject rates during the second half of 2016. This time lag is not visible before 2016 and after 2017. The underlying physical mechanisms responsible for this time lag remain a subject of ongoing investigation.

Improvement in the electron/positron proton separation based on the Electromagnetic Calorimeter of the Alpha Magnetic Spectrometer

Improvement in the electron/positron proton separation based on the Electromagnetic Calorimeter of the Alpha Magnetic Spectrometer

Properties of Cosmic Deuterons Measured by the Alpha Magnetic Spectrometer.

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of the deuteron (D) flux are presented. The measurements are based on 21×10^{6} D nuclei in the rigidity range from 1.9 to 21GV collected from May 2011 to April 2021. We observe that over the entire rigidity range the D flux exhibits nearly identical time variations with the p, ^{3}He, and ^{4}He fluxes. Above 4.5GV, the D/^{4}He flux ratio is time independent and its rigidity dependence is well described by a single power law ∝R^{Δ} with Δ_{D/^{4}He}=-0.108±0.005. This is in contrast with the ^{3}He/^{4}He flux ratio for which we find Δ_{^{3}He/^{4}He}=-0.289±0.003. Above ∼13 GV we find a nearly identical rigidity dependence of the D and p fluxes with a D/p flux ratio of 0.027±0.001. These unexpected observations indicate that cosmic deuterons have a sizable primarylike component. With a method independent of cosmic ray propagation, we obtain the primary component of the D flux equal to 9.4±0.5% of the ^{4}He flux and the secondary component of the D flux equal to 58±5% of the ^{3}He flux.

An enormous increase in atmospheric positron flux during a summer thunderstorm on Mount Aragats

On July 11, 2023, we observed a remarkable 500% increase in positron flux, coinciding with a significant Thunderstorm Ground Enhancement (TGE). The enhanced flux of electrons and gamma rays was attributed to relativistic runaway electron avalanches (RREAs) generated within the dipole formed between the main negatively charged layer in the middle of the thundercloud and the Lower Positively Charged Region (LPCR) at the bottom of the thundercloud. Concurrently, a substantial enhancement in the 511 keV gamma-ray flux resulting from electron-positron annihilation was recorded. This surge is intricately linked to the LPCR within the thundercloud. The emergence of the LPCR induces a polarity change in the atmospheric electric field (AEF) below the LPCR (fourth dipole), leading to the deceleration of electrons and the acceleration of positrons.Particle flux measurements were conducted using scintillation and NaI(TL) spectrometers. To mitigate the contamination of natural gamma radiation and refine the 511 keV flux measurements, the ORTEC spectrometer was shielded with a 4 cm thick lead filter. CORSIKA simulations corroborate the observed positron flux enhancement.Highlighting the synergy between high-energy physics in the atmosphere and astroparticle physics, we introduce a new scenario to elucidate the enigmatic large flux of galactic positrons measured by the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station (ISS).

Feature Selection Techniques for CR Isotope Identification with the AMS-02 Experiment in Space

Isotopic composition measurements of singly charged cosmic rays (CR) provide essential insights into CR transport in the Galaxy. The Alpha Magnetic Spectrometer (AMS-02) can identify singly charged isotopes up to about 10 GeV/n. However, their identification presents challenges due to the small abundance of CR deuterons compared to the proton background. In particular, a high accuracy for the velocity measured by a ring-imaging Cherenkov detector (RICH) is needed to achieve a good isotopic mass separation over a wide range of energies. The velocity measurement with the RICH is particularly challenging for Z=1 isotopes due to the low number of photons produced in the Cherenkov rings. This faint signal is easily disrupted by noisy hits leading to a misreconstruction of the particles’ ring. Hence, an efficient background reduction process is needed to ensure the quality of the reconstructed Cherenkov rings and provide a correct measurement of the particles’ velocity. Machine learning methods, particularly boosted decision trees, are well suited for this task, but their performance relies on the choice of the features needed for their training phase. While physics-driven feature selection methods based on the knowledge of the detector are often used, machine learning algorithms for automated feature selection can provide a helpful alternative that optimises the classification method’s performance. We compare five algorithms for selecting the feature samples for RICH background reduction, achieving the best results with the Random Forest method. We also test its performance against the physics-driven selection method, obtaining better results.

Antiproton flux and properties of elementary particle fluxes in primary cosmic rays measured with the alpha magnetic spectrometer

We present the latest precision measurements of the fluxes of all charged cosmic elementary particles, protons, electrons, antiprotons and positrons based on the first 10 years of data collected by the Alpha Magnetic Spectrometer on the International Space Station. These unique results, obtained with the same detector and with unprecedented precision in the uncharted energy range, provide precise experimental information and reveal new properties of cosmic charged elementary particles. In the absolute rigidity range of 60 to 525 GV, the antiproton-to-proton flux ratio is constant and the antiproton flux and proton flux have identical rigidity dependence. This behavior indicates an excess of high-energy antiprotons compared with traditional predictions of secondary antiprotons produced from the collision of cosmic rays. More importantly, from 60 to 525 GV, the antiproton flux and positron flux also show identical rigidity dependence. The positron-to-antiproton flux ratio is independent of energy, with its value of 2 established with percent-level accuracy. This unexpected observation suggests a common origin of high-energy antiprotons and positrons in the cosmos.

Effects of Forbush Decreases on the Global Electric Circuit

AbstractThe suppression of high‐energy cosmic rays, known as Forbush decreases (FDs), represents a promising factor in influencing the global electric circuit (GEC) system. Researchers have delved into these effects by examining variations, often disruptive, of the potential gradient (PG) in ground‐based measurements taken in fair weather regions. In this paper, we aim to investigate deviations observed in the diurnal curve of the PG, as compared to the mean values derived from fair weather conditions, during both mild and strong Forbush decreases. Unlike the traditional classification of FDs, which are based on ground level neutron monitor data, we classify FDs using measurements of the Alpha Magnetic Spectrometer (AMS‐02) on the International Space Station. To conduct our analysis, we employ the superposed epoch method, focusing on PGs collected between January 2010 and December 2019 at a specific station situated at a low latitude and high altitude: the Complejo Astronómico El Leoncito (CASLEO) in Argentina (31.78°S, 2,550 m above sea level). Our findings reveal that for events associated with FDs having flux amplitude (A) decrease ≤10%, no significant change in the PG is observed. However, for FDs with A &gt; 10%, a clear increase in the PG is seen. For these A &gt; 10% events, we also find a good correlation between the variation of Dst and Kp indices and the variation of PG.

A Comprehensive Comparison of Various Galactic Cosmic-Ray Models to the State-of-the-art Particle and Radiation Measurements

Galactic cosmic rays (GCRs) are the slowly varying background energetic particles that originate outside the solar system, are modulated by the heliospheric magnetic field, and pose ongoing radiation hazards to deep space exploration missions. To assess the potential radiation risk, various models have been developed to predict the GCR flux near Earth based on propagation theories and/or empirical functions. It is essential to benchmark these models by validating against the state-of-the-art measurements. In this work, a comprehensive model–observation comparison of the energy-dependent particle flux has been performed, by combining five typical GCR models and observational data from the Cosmic Ray Isotope Spectrometer on board the Advanced Composition Explorer spacecraft at relatively lower energies and data from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics and Alpha Magnetic Spectrometer at higher energies. The analysis shows that, out of the five models investigated in this study, the optimal model, characterized by minimal relative difference or reduced chi-square divergence from measurements, depends on the particle type, energy range, and epoch of interest. Furthermore, a silicon slab is applied to compute the absorbed dose rate using conversion factors applied to GCR model outputs, and the results are compared to measurements from the Cosmic Ray Telescope for the Effects of Radiation. The comparisons in this paper have implications for the strengths and limitations of individual GCR models, advance our comprehension of the underlying GCR transport mechanisms, and also have strong application aspects for mitigating space radiation risks.

Determination of the anisotropy of elementary particles with the Alpha Magnetic Spectrometer on the International Space Station

A measurement of the cosmic ray anisotropy on the arrival directions of elementary particles (electrons, positrons and protons) has been performed in galactic coordinates by the Alpha Magnetic Spectrometer onboard the International Space Station. The analysis is based on the sample of events collected in the first 10 years of data taking. The results are consistent with isotropy for all cosmic ray species and upper limits on the dipole amplitude have been computed. In particular, 95% credible interval upper limits of δ<1.50% and δ<0.34% are obtained for positrons and electrons, respectively, above 16 GeV. On the other hand, the upper limit to the dipole anisotropy of protons above 200 GV is found to be δ<0.30%.

Origins of cosmic positrons and electrons

The precision measurements of the cosmic-ray positron and electron fluxes collected by the Alpha Magnetic Spectrometer on the International Space Station during first ten years of operations are presented. The positron flux exhibits complex energy dependence. It is 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, which dominates at high energies and is associated with either dark matter or astrophysical origin. The positron source term also manifest itself in the measured electron spectrum. This is the first indication of the existence of an identical charge symmetric source term both in the positron and in the electron spectra. If confirmed with a higher level of confidence, these observations might suggest the existence of new physics.

A commentary on the NIM A paper by G. Ambrosi et al. on the spatial resolution of the silicon tracker of the Alpha Magnetic Spectrometer

A commentary on the NIM A paper by G. Ambrosi et al. on the spatial resolution of the silicon tracker of the Alpha Magnetic Spectrometer

Real-Time Monitoring of Solar Energetic Particles Using the Alpha Magnetic Spectrometer on the International Space Station

The International Space Station (ISS) orbits at an average altitude of 400 km, in the Low Earth Orbit (LEO) and is regularly occupied by astronauts. The material of the Station, the residual atmosphere and the geomagnetic field offer a partial protection against the cosmic radiation to the crew and the equipment. The solar activity can cause sporadic bursts of particles with energies between ∼10 keV and several GeVs called Solar Energetic Particles (SEPs). SEP emissions can last for hours or even days and can represent an actual risk for ISS occupants and equipment. The Alpha Magnetic Spectrometer (AMS) was installed on the ISS in 2011 and is expected to take data until the decommissioning of the Station itself. The instrument detects cosmic rays continuously and can also be used to monitor SEPs in real-time. A detection algorithm developed for the monitoring measures temporary increases in the trigger rates of AMS, using McIlwain’s L-parameter to characterize different conditions of the data-taking environment. A real-time monitor for SEPs has been realized reading data from the AMS Monitoring Interface (AMI) database and processing them using the custom algorithm that was developed.

New vertex reconstruction algorithm for measuring gamma-rays with the AMS tracker

Gamma-ray polarization is one of the most promising tools to probe the universe at extreme conditions and search for new physics. The Alpha Magnetic Spectrometer (AMS) is a precision particle physics detector on the International Space Station. With its high accuracy thin silicon strip detectors and permanent magnet, AMS is unique to study the polarization of high energy gamma-rays. The detection of gamma-rays above a few hundreds of MeV typically utilize the electron–positron pair production process, and the key step is to reconstruct the vertex where gamma-ray converts into the electron–positron pair. We present a new vertex reconstruction algorithm that is based on an extended cellular automaton to determine the direction and energy information of gamma-rays from the electron and positron trajectories in the AMS tracker. The vertex reconstruction efficiency is significantly improved with the presented algorithm over the entire energy range above 100 MeV and reaches >90% above 1 GeV. The fake rate is drastically reduced over the entire energy range and is reduced from ∼10% to ∼3% at 200 MeV. The presented algorithm will thus provide the basis for measuring gamma-ray polarization with AMS.

Temporal Structures in Positron Spectra and Charge-Sign Effects in Galactic Cosmic Rays.

We present the precision measurements of 11years of daily cosmic positron fluxes in the rigidity range from 1.00 to 41.9GV based on 3.4×10^{6} positrons collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The positron fluxes show distinctly different time variations from the electron fluxes at short and long timescales. A hysteresis between the electron fluxes and the positron fluxes is observed with a significance greater than 5σ at rigidities below 8.5GV. On the contrary, the positron fluxes and the proton fluxes show similar time variation. Remarkably, we found that positron fluxes are modulated more than proton fluxes with a significance greater than 5σ for rigidities below 7GV. These continuous daily positron fluxes, together with AMS daily electron, proton, and helium fluxes over an 11-year solar cycle, provide unique input to the understanding of both the charge-sign and mass dependencies of cosmic rays in the heliosphere.

Machine learning approach to the background reduction in singly charged cosmic-ray isotope measurements with AMS-02

Studying the isotopic composition of single-charge cosmic rays (CRs) provides essential data to investigate the CR propagation processes in our Galaxy. While current measurements are rare above 4 GeV/nucleon, the Alpha Magnetic Spectrometer (AMS-02) is able to measure the isotopic fluxes up to 10 GeV/n by combining the momentum measured by the silicon tracker with the precise measurements of the velocity provided by its Ring Imaging Cherenkov Detector (RICH). The correct measurement of the particles’ velocity is essential for identifying isotopes through their mass. This is particularly challenging for single-charge particles due to the low number of photons they produce in their Cherenkov rings, which makes the reconstruction easily disrupted by noise. Hence, identifying the sources and cleaning the sample from the background is essential for ensuring the quality of the rings. In this paper, we propose a novel approach to track the events whose mass is misidentified due to interactions inside the AMS-02 detector. Based on the actual location of these interactions, we propose a novel strategy to mitigate the background effectively and with high efficiency, which includes using cut-based selection criteria and a multivariate estimator based on the signals detected by the RICH.