Antiproton Flux Research Articles

Cosmic-ray antiprotons in the galaxy

A key issue in fundamental physics is the nature of non-baryonic dark matter in the universe. Among the candidates proposed by the particle physics community are neutralinos whose annihilation in the Galactic halo can produce antiprotons with a characteristic spectrum. Several balloon and space borne experiments have been actively looking to detect the neutralino signature in cosmic antiprotons. This task is however hampered by the existence of a secondary “background” flux of antiprotons produced in the interactions of cosmic-ray particles in the interstellar medium. A notable difficulty is that a self-consistent and complete model of cosmic-ray propagation in the Galaxy has thus far been elusive. When subtracting the secondary background from the observed data, it is therefore critical that all viable models of secondary cosmic-ray antiproton propagation be thoroughly examined, and all uncertainties stemming from plurality of the models be duly considered. In this paper we concentrate on three particular models of Diffusion, Galactic Wind, and Distributed Stochastic Reacceleration, and discuss the predicted antiproton spectrum in each model.

PAMELA: a satellite experiment for antiparticles measurement in cosmic rays

PAMELA is a satellite-borne experiment that will study the antiproton and positron fluxes in cosmic rays in a wide range of energy (from 80 MeV up to 190 GeV for antiprotons and from 50 MeV up to 270 GeV for positrons) and with high statistics, and that will measure the antihelium/helium ratio with a sensitivity of the order of 10/sup -8/. The detector will fly on-board a polar orbiting Resurs DK1 satellite, which will be launched into space by a Soyuz rocket in 2004 from Baikonur cosmodrome in Kazakhstan, for a 3-year-long mission. Particle identification and energy measurements are performed in the PAMELA apparatus using the following subdetectors: a magnetic spectrometer made up of a permanent magnet equipped with double-sided microstrip silicon detectors, an electromagnetic imaging calorimeter composed of layers of tungsten absorber and silicon detectors planes, a transition radiation detector made of straw tubes interleaved with carbon fiber radiators, a plastic scintillator time-of-flight and trigger system, a set of anticounter plastic scintillator detectors, and a neutron detector. The features of the detectors and the main results obtained in beam test sessions are presented.

Propagation of secondary antiprotons and cosmic rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper, we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni ( Z ⩽ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three “radioactive clock” isotopes in CR, 10Be, 26Al, 36Cl, are all consistent and indicate a halo size z h ∼ 4 kpc based on the most accurate data taken by the ACE spacecraft.

Precise measurements of the cosmic ray antiproton spectrum with BESS including the effects of solar modulation

The Balloon Borne Experiment with a Superconducting Spectrometer (BESS) has measured the energy spectrum of cosmic-ray antiprotons between 0.18 and 4.20 GeV in eight flights between 1993 and 2002. Above about 1 GeV, models in which antiprotons are secondary products of the interactions of primary cosmic rays with the interstellar gas agree with the BESS antiproton spectrum. Below 1 GeV, the data show a possible excess antiproton flux compared to secondary model predictions, suggesting the presence of an additional source of antiprotons. The antiproton/proton ratios measured between 1993 and 1999, during the Sun’s positive-polarity phase, are consistent with simple models of solar modulation. However, results from the 2000 flight, following the solar magnetic field reversal, show a sudden increase in the antiproton/proton ratio and tend to favor a charge-sign-dependent drift model. To extend BESS measurements to lower energies, an evolutionary instrument, BESS-Polar, is under construction for polar flight in 2004.

Antiprotons in cosmic rays from neutralino annihilation

We calculate the antiproton flux due to relic neutralino annihilations, in a two-dimensional diffusion model compatible with stable and radioactive cosmic ray nuclei. We find that the uncertainty in the primary flux induced by the propagation parameters alone is about two orders of magnitude at low energies, and it is mainly determined by the lack of knowledge of the thickness of the diffusive halo. On the contrary, different dark matter density profiles do not significantly alter the flux: a Novarro-Frenk-White distribution produces fluxes which are at most 20% higher than an isothermal sphere. The most conservative choice for propagation parameters and dark matter distribution normalization, together with current data on cosmic antiprotons, cannot lead to any definitive constraint on the supersymmetric parameter space, either in a low-energy effective minimal supersymmetric standard model, or in a minimal supergravity scheme. However, if the best choice for propagation parameters---corresponding to a diffusive halo of $L=4\mathrm{kpc}$---is adopted, some supersymmetric configurations with the neutralino mass ${m}_{\ensuremath{\chi}}\ensuremath{\lesssim}100\mathrm{GeV}$ should be considered as excluded. An enhancement flux factor---due for instance to a clumpy dark halo or a higher local dark matter density---would imply a more severe cut on the supersymmetric parameters.

Trapped antiprotons produced by cosmic rays in the Earth's magnetosphere

The existence of significant fluxes of antiparticles in the Earth magnetosphere has been predicted on theoretical considerations in this article. These antiparticles (positrons or antiprotons) at several hundred kilometers of altitudes, we believe are not of direct extraterrestrial origin, but are the natural products of nuclear reactions of the high energy primary cosmic rays (CR) and trapped protons (TP) confined in the terrestrial radiation belt, with the constituents of terrestrial atmosphere. Extraterrestrial positrons and antiprotons born in nuclear reactions of the same CR particles passing through only 5–7 g/cm 2 of interstellar matter, exhibit lower fluxes compared to the antiprotons born at hundreds of g/cm 2 in the atmosphere, which when confined in the magnetic field of the Earth (in any other planet), get accumulated. We present the results of the computations of the antiproton fluxes at 10 MeV to several GeV energies due to CR particle interactions with the matter in the interstellar space, and also with the residual atmosphere at altitudes of ∼1000 km over the Earth's surface. The estimates show that the magnetospheric antiproton fluxes are greater by two orders of magnitude compared to the extraterrestrial fluxes measured at energies <1–2 GeV.

Secondary proton flux induced by cosmic ray interactions with the atmosphere

The atmospheric secondary antiproton flux is studied for detection altitudes extending from sea level up to about three earth radii, by means of a three-dimensional Monte Carlo simulation, successfully applied previously to other satellite and balloon data. The calculated antiproton flux at mountain altitude is found to be in fair agreement with the recent BESS measurements. The flux obtained at balloon altitude is also in agreement with calculations performed in previous studies and used for the analysis of balloon data. The flux at sea level is found to be significant. The antineutron flux is also evaluated. The antiproton flux is prospectively explored up to around $2\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{km}$ from the Earth. The results are discussed in the context of the forthcoming measurements by large acceptance experiments.

Secondary antiproton flux induced by cosmic ray interactions with the atmosphere

The atmospheric secondary antiproton flux is studied for detection altitudes extending from sea level up to about three earth radii, by means of a three-dimensional Monte Carlo simulation, successfully applied previously to other satellite and balloon data. The calculated antiproton flux at mountain altitude is found to be in fair agreement with the recent BESS measurements. The flux obtained at balloon altitude is also in agreement with calculations performed in previous studies and used for the analysis of balloon data. The flux at sea level is found to be significant. The antineutron flux is also evaluated. The antiproton flux is prospectively explored up to around $2\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{km}$ from the Earth. The results are discussed in the context of the forthcoming measurements by large acceptance experiments.

The BALLOON-borne and PAMELA experiments for the study of the antimatter component in cosmic rays

The PAMELA experiment is based on a satellite-borne equipment actually in the final integration phase. It will be installed on board of the Russian satellite Resurs DK1 and launched in a quasi-polar orbit from the Baikonur cosmodrom at the beginning of next year. PAMELA will measure the antiproton and positron fluxes in cosmic rays with high statistics and in a large energy range (80 MeV–190 GeV for antiprotons and 50 MeV–270 GeV for positrons), extending to never investigated energies the measurements of several balloon borne experiments performed by the same PAMELA collaboration in last decade. This will make achievable sensitive tests of cosmic ray propagation models in the Galaxy and the search, in an energy range never investigated before, of possible structures in the fluxes. These structures, related to the presence of primary antiparticle sources, could be signals of “new physics”, connected with open problems like dark matter existence and matter/antimatter symmetry in the Universe. The detector consists of a very precise magnetic spectrometer, several scintillation counter hodoscopes to measure the energy losses and times of flight, and a high granularity and deep Si–W calorimeter, augmented by a very compact transition radiation detector and a He3 neutron detector hodoscope, and protected around and on the top by an anticoincidence system.

Antiprotons confined in the earth’s inner magnetosphere

The possible existence of noticeable fluxes of antiparticles in the Earth magnetosphere is considered theoretically in this article. These antiparticles (antiprotons in this paper) that are confined by geomagnetic field at the altitudes of several hundred kilometers are predominantly not of immediate extraterrestrial origin, but rather are the products of nuclear reactions of the high energy primary cosmic rays (CR) with constituents of the terrestrial atmosphere. Direct extraterrestrial antiprotons impinging upon the Earth’s magnetosphere are themselves also secondary in origin, i.e. they are born in nuclear reactions of the same CR passing through 5–7 g/cm 2 of interstellar matter. These exhibit lower fluxes compared to the magnetospheric antiprotons that are born at a pass length of hundreds g/cm 2 in the residual Earth atmosphere. Such locally generated antiprotons can be confined by the magnetic field of the Earth (or equivalently by another planet) and so accumulated in the magnetosphere. We here present the results of numerical simulation of antiproton fluxes in the energy range from 10 MeV to several GeV produced by CR in the Earth’s atmosphere at altitudes of about 1000 km, and we compare this to antiprotons born in interstellar matter. The estimates presented herein show a significant (up to two orders of magnitude) excess of magnetospheric antiproton fluxes over those formed in the interstellar media at energies <1 to 2 GeV.

Antiproton radiation belt produced by cosmic rays in the Earth magnetosphere

The possible existence of noticeable fluxes of antiparticles in the Earth magnetosphere has been predicted on theoretical considerations in this article. The antiprotons expected at several hundred kilometers of altitudes, we do not believe are of direct extraterrestrial origin, but are the natural products of nuclear reactions of the high‐energy primary cosmic rays (CR) with the constituents of the terrestrial atmosphere. Extraterrestrial, galactic antiprotons are themselves of secondary in origin, i.e. they are born in nuclear reactions of the same CR particles passing through 5–7 g/cm2 of interstellar matter encountered during their lifetime in the Galaxy. We expect that the fluxes of magnetospheric antiprotons to be higher compared to the interstellar fluxes because the fluxes get accumulated due to confinement by the magnetic field of the Earth. We present the results of the computations of the antiproton fluxes at 50 MeV to several GeV energies due to the CR particle interactions with the matter in the interstellar space, and also with the residual atmosphere at altitudes of ∼1000 km over the Earth's surface. The estimates show that the magnetospheric antiproton fluxes are two orders of magnitude greater compared to the interstellar fluxes measured at energies &lt; 1 GeV.

Challenging Cosmic‐Ray Propagation with Antiprotons: Evidence for a “Fresh” Nuclei Component?

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratio (e.g., Boron/Carbon) produce too few antiprotons, while the traditional non-reacceleration models can reproduce the antiproton flux but fall short of explaining the low-energy decrease in the secondary to primary nuclei ratio. Matching both the secondary to primary nuclei ratio and antiproton flux requires artificial breaks in the diffusion coefficient and the primary injection spectrum suggesting the need for other approaches. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local unprocessed component at low energies, thus decreasing the measured secondary to primary nuclei ratio. A model reproducing antiprotons, B/C ratio, and abundances up to Ni is presented.

The status of the PAMELA experiment

The PAMELA experiment will be launched on-board of a polar orbiting Resurs DK1 satellite in mid-2003. The primary objective of PAMELA is to measure the flux of antiprotons (80 MeV – 190 GeV) and positrons (50 MeV – 270 GeV) in the cosmic radiation. The wide energy range and large statistics (O(10 4) antiprotons and O(10 5) positrons) will allow sensitive tests of cosmic ray propagation models and searches for exotic sources of antiparticles, such as cold dark matter neutralino annihilations. PAMELA is built around a permanent magnet spectrometer equipped with double-sided silicon detectors and surrounded by an active veto shield. This is complemented by a proportional straw tube / carbon fibre radiator transition radiation detector and a silicon-tungsten imaging calorimeter which is augmented by a scintillator shower tail catcher. A time-of-flight system provides the primary experimental trigger. The status of PAMELA is presented along with main results from simulations and test-beam studies.

Antiprotons from spallation of cosmic rays on ISM

We provide the first evaluation of the secondary interstellar cosmic antiproton flux that is fully consistent with cosmic ray nuclei in the framework of a two-zone diffusion model. We also study and conservatively quantify all possible sources of uncertainty that may affect that antiproton flux. Uncertainties related to propagation are shown to range between 10% and 25%, depending on which part of the spectrum is considered, while the ones related to nuclear physics stand around 22–25 % over all the energy spectrum.

Measurement of low-energy antiproton detection efficiency in BESS below [formula omitted

An accelerator experiment was performed using a low-energy antiproton beam to measure antiproton detection efficiency of BESS, a balloon-borne spectrometer with a superconducting solenoid. Measured efficiencies showed good agreement with calculated ones derived from the BESS Monte Carlo simulation based on GEANT/GHEISHA. With detailed verification of the BESS simulation, the relative systematic error of detection efficiency derived from the BESS simulation has been determined to be ±5%, compared with the previous estimation of ±15% which was the dominant uncertainty for measurements of cosmic-ray antiproton flux.

Pamela tracking system: status report

The Pamela apparatus will be launched at the end of 2002 on board of the Resurs DK Russian satellite. The tracking system, composed of six planes of silicon sensors inserted inside a permanent magnetic field was intensively tested during these last years. Results of tests have shown a good signal-to-noise ratio and an excellent spatial resolution, which should allow to measure the antiproton flux in an energy range from 80 MeV up to 190 GeV. The production of the final detector modules is about to start and mechanical and thermal tests on the tracking tower are being performed according to the specifications of the Russian launcher and satellite.

High energy neutrino, photon, and cosmic ray fluxes from VHS cosmic strings

Decaying topological defects, in particular cosmic strings, can produce a significant flux of high energy neutrinos, photons and cosmic rays. According to the prevailing understanding of cosmic string dynamics in an expanding Universe, the network of long strings loses its energy first into string loops, which in turn give off most of their energy as gravitational radiation. However, it has been suggested by Vincent, Hindmarsh, and Sakellariadou (VHS) that particle emission may be the dominant energy loss channel for the long string network. In this case, the predicted flux of high energy particles would be much larger. Here we calculate the predicted flux of high energy gamma rays, neutrinos and cosmic ray antiprotons and protons as a function of the scale of symmetry breaking $\ensuremath{\eta}$ at which the strings are produced and as a function of the initial energy ${m}_{J}$ of the particle jets which result from the string decay. Assuming the validity of the VHS scenario, we find that due to the interactions with the cosmic radiation backgrounds all fluxes but the neutrino flux are suppressed at the highest energies. This indicates that the observed events above the GZK cutoff can only be accounted for in this scenario if the primary particle is a neutrino and $\ensuremath{\eta}$ is somewhat less than the GUT scale, i.e. $\ensuremath{\eta}\ensuremath{\lesssim}{10}^{23} \mathrm{eV}.$ The domain of parameter space corresponding to GUT-scale symmetry breaking is excluded also by the current observations below the GZK cutoff. A new aspect of this work is the calculation of the spectrum of the tau neutrinos directly produced in the decay of the X particles. This significantly increases the tau neutrino signal at high energies in all cosmic string scenarios.

A Novel Antimatter Detector Based on X‐Ray Deexcitation of Exotic Atoms

We propose a novel antiparticle detector. The gaseous antiparticle spectrometer (GAPS) effects particle identification through the characteristic X-rays emitted by antiparticles when they form exotic atoms in gases. GAPS obtains particularly high grasp (effective area-solid angle product) at lower particle energies, where conventional schemes are most limited in their utility. The concept is simple and lightweight, so it can be readily employed on balloon- and space-based missions. An extremely powerful potential application of GAPS is a space-based search for the neutralino through the detection of a neutralino annihilation by-product?the antideuteron. Paradoxically, this space-based search for the neutralino is capable of achieving comparable sensitivity to as yet unrealized third-generation, underground dark matter experiments. And GAPS can obtain this performance in a very modest satellite experiment. GAPS can also provide superior performance in searches for primary antiprotons produced via neutralino annihilation and black hole evaporation and in probing subdominant contributions to the antiproton flux at low energies. In a deep space mission, GAPS will obtain higher sensitivity for a given weight and power than BGO calorimeters.

A high granularity imaging calorimeter for cosmic-ray physics

An imaging calorimeter has been designed and is being built for the PAMELA satellite-borne experiment. The physics goals of the experiment are the measurement of the flux of antiprotons, positrons and light isotopes in the cosmic radiation. The calorimeter is designed to perform a precise measurement of the total energy deposited, to reconstruct the spatial development of the showers (both in the longitudinal and in the transverse directions), and to measure the energy distribution along the shower itself. From this information, the calorimeter will identify antiprotons from an electron background and positrons in a background of protons with an efficiency of about 95% and a rejection power better than 10 −4. Furthermore, a self-trigger system has been implemented with the calorimeter that will be employed to measure high-energy (from about 300 GeV to more than 1 TeV) electrons. The instrument is composed of 22 layers of tungsten, each sandwiched between two “views” of silicon strip detectors ( X and Y). The signals are read out by a custom VLSI front-end chip, the CR1.4P, specifically designed for the PAMELA calorimeter, with a dynamic range of 7.14 pC or 1400 minimum ionizing particle (mip). We report on the simulated performance and prototype design.

The PAMELA experiment on satellite and its capability in cosmic rays measurements

The PAMELA equipment will be assembled in 2001 and installed on board the Russian satellite Resurs. PAMELA is conceived mainly to study the antiproton and positron fluxes in cosmic rays up to high energy (190 GeV for p ̄ and 270 GeV for e +) and to search antinuclei, up to 30 GeV/n, with a sensitivity of 10 −7 in the He / He ratio. The PAMELA telescope consists of: a magnetic spectrometer made up of a permanent magnet system equipped with double sided microstrip silicon detectors; a transition radiation detector made up of active layers of proportional straw tubes interleaved with carbon fibre radiators; and a silicon–tungsten imaging calorimeter made up of layers of tungsten absorbers and silicon detector planes. A time-of-flight system and anti-coincidence counters complete the PAMELA equipment. In the past years, tests have been done on each subdetector of PAMELA; the main results are presented and their implications on the anti-particles identification capability in cosmic rays are discussed here.