Cosmic-ray Antiparticles Research Articles

Application of Machine Learning to the Particle Identification of GAPS

GAPS is an international balloon-borne project that contributes to solving the dark-matter mystery through a highly sensitive survey of cosmic-ray antiparticles, especially undiscovered antideuterons. To achieve a sufficient sensitivity to rare antideuterons, a novel particle identification method based on exotic atom capture and decay has been developed. In parallel to utilizing this unique event signature in a conventional likelihood-based event identification scheme, we have begun investigating a complementary approach using a machine learning technique. In this new approach, a deep-learning package is trained on a large amount of input data from simulated antiparticle events through a multi-layered neural network. By applying this unbiased approach, we expect to mine unknown patterns and give feedback to the conventional method. In this paper, we report results from exploratory investigations that illustrate the promise of this new approach.

The results from BESS-Polar experiment

The balloon-borne experiment with a superconducting spectrometer (BESS) instrument was developed as a high-resolution, high-geometric-acceptance magnetic-rigidity spectrometer for sensitive measurements of cosmic-ray antiparticles, searches for antinuclei, and precise measurements of the absolute fluxes of light GCR elements and isotopes. The original BESS experiment flew 8 times over Lynn Lake, Canada and once from Fort Sumner, USA during the period of 1993 through 2002, with continuous improvement in the instrument. Based on the instrument concept inherited from the BESS spectrometer, a very low instrumental energy cutoff for antiprotons was achieved with a new thin-walled superconducting magnet and removal of the outer pressure vessel for BESS-Polar project. The first and second scientific flights called BESS-Polar I/II were successfully performed, over Antarctica in 2004 December and 2007 December respectively. We report the scientific results, focusing on the long-duration flights of BESS-Polar I (2004) and BESS-Polar II (2007–2008).

Time variations of proton flux in Earth inner radiation belt during 23/24 solar cycles based on the PAMELA and the ARINA data

The PAMELA and the ARINA experiments are carried out on the board of satellite RESURS-DK1 since 2006 up to now. Main goal of the PAMELA instrument is measurements of high energy antiparticles in cosmic rays while the ARINA instrument is intended studying high energy charged particle bursts in the magnetosphere. Both of these experiments have a possibility to study trapped particles in the inner radiation belt. Complex of these two instruments covers proton energy range from 30 MeV up to trapping limit (E= ∼2 GeV). Continuous measurements with the PAMELA and the ARINA spectrometers include falling and rising phases of 23/24 solar cycles and maximum of 24th one. In this report we present temporal profiles of proton flux in the inner zone of the radiation belt (1.11 < L < 1.18, 0.18 < B < 0.22G). Dependence of proton fluxes on a magnitude of the solar activity was studied for various phases of 23/24 solar cycles. At that it was shown that proton fluxes at the solar minimum are several times greater than at the solar maximum.

Cosmic Ray Electrons and Protons, and Their Antiparticles

Cosmic rays are a sample of solar, galactic, and extragalactic matter. Their origin, acceleration mechanisms, and subsequent propagation toward Earth have intrigued scientists since their discovery. These issues can be studied via analysis of the energy spectra and composition of cosmic rays. Protons are the most abundant component of the cosmic radiation, and many experiments have been dedicated to the accurate measurement of their spectra. Complementary information is provided by electrons, which comprise about 1 % of the cosmic radiation. Because of their low mass, electrons experience severe energy losses through synchrotron emission in the galactic magnetic field and inverse Compton scattering of radiation fields. Electrons therefore provide information on the local galactic environment that is not accessible from the study of the cosmic ray nuclei. Antiparticles, namely antiprotons and positrons, are produced in the interaction between cosmic ray nuclei and the interstellar matter. They are therefore intimately linked to the propagation mechanisms of the parent nuclei. Novel sources of primary cosmic ray antiparticles of either astrophysical (e.g., positrons from pulsars) or exotic origin (e.g., annihilation of dark matter particles) may exist. The nature of dark matter is one of the most prominent open questions in science today. An observation of positrons from pulsars would open a new observation window on these sources. Several experiments equipped with state-of-the art detector systems have recently presented results on the energy spectra of electrons, protons, and their antiparticles with a significant improvement in statistics and better control of systematics. The status of the field will be reviewed, with a focus on these recent scientific results.

The PAMELA experiment and antimatter in the universe

On the 15th of June 2006, the PAMELA satellite-borne experiment was launched from the Baikonur cosmodrome and it has been collecting data since July 2006. The primary scientific goal is the measurement of the antiproton and positron energy spectra. Antiparticles are a natural component of the cosmic radiation being produced in the interaction between cosmic rays and the interstellar matter. They have been shown to be extremely interesting for understanding the propagation mechanisms of cosmic rays. Furthermore, novel sources of primary cosmic-ray antiparticles of either astrophysical or exotic origin (e.g. annihilation of dark matter particles) can also be probed. In this paper we review the PAMELA antiparticle results and their significance for the field of astroparticle physics.

The prototype GAPS (pGAPS) experiment

The General Antiparticle Spectrometer (GAPS) experiment is a novel approach for the detection of cosmic ray antiparticles. A prototype GAPS (pGAPS) experiment was successfully flown on a high-altitude balloon in June of 2012. The goals of the pGAPS experiment were: to test the operation of lithium drifted silicon (Si(Li)) detectors at balloon altitudes, to validate the thermal model and cooling concept needed for engineering of a full-size GAPS instrument, and to characterize cosmic ray and X-ray backgrounds. The instrument was launched from the Japan Aerospace Exploration Agency's (JAXA) Taiki Aerospace Research Field in Hokkaido, Japan. The flight lasted a total of 6h, with over 3h at float altitude (~33km). Over one million cosmic ray triggers were recorded and all flight goals were met or exceeded.

The pGAPS experiment: An engineering balloon flight of prototype GAPS

The General Anti-Particle Spectrometer (GAPS) project is being carried out to search for primary cosmic-ray antiparticles especially for antideuterons produced by cold dark matter. GAPS plans to realize the science observation by Antarctic long duration balloon flights in the late 2010s. In preparation for the Antarctic science flights, an engineering balloon flight using a prototype of the GAPS instrument, “pGAPS”, was successfully carried out in June 2012 in Japan to verify the basic performance of each GAPS subsystem. The outline of the pGAPS flight campaign is briefly reported.

Study of Elementary Particle Phenomena in Early Universe through Observation of Cosmic-ray Antiparticles(The Centenary of the Discovery of Cosmic Rays)

Study of Elementary Particle Phenomena in Early Universe through Observation of Cosmic-ray Antiparticles(The Centenary of the Discovery of Cosmic Rays)

The PAMELA space mission for antimatter and dark matter searches in space

The PAMELA satellite-borne experiment has presented new results on cosmic-ray antiparticles that can be interpreted in terms of DM annihilation or pulsar contribution. The instrument was launched from the Baikonur cosmodrome and it has been collecting data since July 2006. The combination of a permanent magnet silicon strip spectrometer and a silicon-tungsten imaging calorimeter allows precision studies of the charged cosmic radiation to be conducted over a wide energy range with high statistics. The primary scientific goal is the measurement of the antiproton and positron energy spectrum in order to search for exotic sources. PAMELA is also searching for primordial antinuclei (anti-helium), and testing cosmic-ray propagation models through precise measurements of the antiparticle energy spectrum and precision studies of light nuclei and their isotopes. This talk illustrates the most recent scientific results obtained by the PAMELA experiment.

Search for primordial antiparticles with BESS

The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) has been carried out to search for primordial antiparticles in cosmic rays. In ten flights from 1993 to 2004, it measured the cosmic-ray antiproton spectrum in the energy range 0.1–4.2 GeV at various solar activity conditions. It also searched for antideuterons and antihelium nuclei, and it made precise measurement of cosmic-ray particle spectra. The BESS program has been extended to long duration balloon (LDB) flights in Antarctica (BESS-Polar) with the goal of achieving unprecedented sensitivity in the search for primordial antiparticles. This report describes recent results from BESS and progress of the BESS-Polar program.

Galactic antiproton spectrum at high energies: Background expectation versus exotic contributions

A new generation of upcoming space-based experiments will soon start to probe the spectrum of cosmic-ray antiparticles with an unprecedented accuracy and, in particular, will open up a window to energies much higher than those accessible so far. It is thus timely to carefully investigate the expected antiparticle fluxes at high energies. Here, we perform such an analysis for the case of antiprotons. We consider both standard sources as the collision of other cosmic rays with interstellar matter, as well as exotic contributions from dark matter annihilations in the galactic halo. Up to energies well above 100 GeV, we find that the background flux in antiprotons is almost uniquely determined by the existing low-energy data on various cosmic-ray species; for even higher energies, however, the uncertainties in the parameters of the underlying propagation model eventually become significant. We also show that if the dark matter is composed of particles with masses at the TeV scale, which is naturally expected in extra-dimensional models as well as in certain parameter regions of supersymmetric models, the annihilation flux can become comparable to---or even dominate---the antiproton background at the high energies considered here.

Status of the PAMELA silicon tracker

PAMELA is a composite particle detector which will be launched during the first half of 2006 on board the Russian satellite Resurs DK-1 from Baikonur cosmodrome in Kazakhstan. This experiment is mainly conceived for the study of cosmic-ray antiparticles and for the search for light antinuclei, but other issues related to the cosmic-ray physics will be investigated. In this work the structure of the whole apparatus is shortly discussed with particular attention to the magnetic spectrometer, which has been designed and built in Firenze.

Performance of an Ultra-Thin Superconducting Solenoid for Particle Astrophysics

An extremely thin superconducting solenoid with a main diameter of 0.9 m and a length of 1.4 m has been fabricated for a balloon borne experiment in Antarctica to study anti-particles in cosmic rays. The solenoid has a 0.8 m diameter warm bore where a magnetic field of 0.8-1.0 T is induced. The coil was wound with mechanically advanced aluminum stabilized superconductor recently developed by using micro-alloying Ni into a pure aluminum base and by cold-work hardening, and consequently the electromagnetic force may be fully supported by the coil itself without any additional support structure. The solenoid was successfully charged up to 1.05 T without any premature quenches. Despite measured strains were beyond 1500 micro-strain, the coil behaved elastically. Because of relatively small RRR of 110, the quench energy is distributed rather unevenly, and a temperature difference of over 100 K was observed. Nevertheless, it was found to be safe to quench the magnet.

The PAMELA silicon tracker

The silicon tracker of the PAMELA apparatus has been assembled and it is ready to fly on-board the Russian satellite Resurs DK for a 3-year mission. The experiment will study, mainly, spectra of particles and antiparticles in cosmic rays. The magnetic spectrometer's primary goal is to precisely measure momenta of charged particles, whose trajectories have been bent by a permanent magnet. The detector is composed of 6 planes of double-sided silicon microstrip detectors, inserted between adjacent modules of a permanent magnet which produces an almost uniform magnetic field inside a rectangular cavity that particles cross. The spatial resolution of the detectors is about 3 μm for the bending coordinate. The development of such detectors required a complex manufacturing procedure in order to preserve the physical performance in a device suitable for a space mission. In the construction phase data originating from both beam tests and simulation helped to check the detector's characteristics and to optimize the achievable spatial resolution. The development and the final assembling of these detectors are described in this paper.

The silicon microstrip detectors of the PAMELA experiment: simulation and test results

The PAMELA detector will fly at the beginning of 2004 on board the Russian satellite Resurs–DK for a 3-year mission designed to study mainly antiparticles in cosmic rays. The core of the apparatus is a magnetic spectrometer in which silicon microstrip detectors are employed. A dedicated simulation study, tuned on beam test data, is presented: it allows to determine the best position finding algorithm for different incidence angles.

A thin superconducting solenoid magnet for particle astrophysics

An extremely thin superconducting solenoid magnet is being developed to investigate cosmic-ray antiparticles in the Universe. The uniform solenoidal field is provided in a particle detector system to analyze the particle momentum. The solenoid coil is wound with advanced aluminum stabilized superconductor recently developed by using micro-alloying with Ni, followed by cold-work mechanical hardening. It is designed with a central magnetic field of 1.2 T in a volume of 0.9 m in diameter and 1.4 m in length. The radiation thickness of the coil is to be 0.056 X/sub 0/ with a physical coil thickness of 3.4 mm. This paper describes the conceptual design and progress of basic development work.

The search for antimatter in cosmic rays

The search for antimatter in the universe is a page in the history of the Ioffe Physicotechnical Institute (IPTI). Experiments on spacecraft and high-altitude balloons, begun in the 1960s, yielded information on to the presence or absence of antimatter stars or galaxies according to evidence arising in explosive processes in these objects. Antiprotons with energies of 2–5 GeV in galactic cosmic rays were observed at the end of the 1970s in balloon experiments by the Cosmic Spectrometry Laboratory at the IPTI. These studies were done using a magnetic spectrometer at altitudes with a residual pressure of 10 g/cm2 with a threshold geomagnetic rigidity of 3 GV. High-latitude experiments in the 1980s, yielding the first measurements of the flux of galactic antiprotons with energies of 0.2–2 GeV, gave some indication of the mechanism by which they are generated. The measured ratios of the fluxes of antiprotons and protons in the cosmic rays are 2.4−1.3+2.4×10−4 and 6−5+14×10−5 at energies of 2–5 and 0.2–2 GeV, respectively. Subsequent balloon-borne experiments employing magnetic spectrometers by groups from the USA and Japan have confirmed the results obtained by the IPTI. Experimental and theoretical work on the search for antiparticles in cosmic rays is summarized and the astrophysical consequences of this research are discussed. Experimental data on the detection of antiparticles in galactic cosmic rays indicate that there are no objects made of antimatter within the local group of galaxies.

Non-GUT baryogenesis and large-scale structure of the Universe

We discuss a mechanism for generating baryon density perturbations and study the evolution of the baryon charge density distribution in the framework of the low temperature baryogenesis scenario. This mechanism may be important for the large scale structure formation of the Universe and particularly, may be essential for understanding the existence of a characteristic scale of 130ft Mpc in the distribution of the visible matter. The detailed analysis showed that both the observed very large scale of the visible matter distribution in the Universe and the observed baryon asymmetry value could naturally appear as a result of the evolution of a complex scalar field condensate, formed at the inflationary stage. Moreover, according to our model, at present the visible part of the Universe may consist of baryonic and antibaryonic sheila, sufficiently separated, so that annihilation radiation is not observed. This is an interesting possibility as far as the observational data of antiparticles in cosmic rays do not rule out the possibility of antimatter superclusters in the Universe.