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.


Introduction
After one century of experimental study, since Victor Hess'discovery in 1912, cosmic rays observations still play a preminent role in reconstructing the puzzle about the origin and evolution of the Universe. By means of balloon, satellite-borne and ground large size experiments, it has been possible to explore energy spectra and chemical composition of the cosmic radiation over an extremely wide energy range, spanning 21 orders of magnitude. As a consequence, the experimental results obtained in this field were and are able to shed light on several aspects of cosmology and astrophysics, from the low energy effects of Solar phenomena to the extremely energetic physics of the GZK region.
In the intermediate energy region before the so called "knee" an important topic is the evaluation of the antiparticle content. Antiprotons and positrons are a natural component of the cosmic radiation, being produced in the interaction between cosmic rays and the interstellar matter. The detection of antimatter of primary origin in the radiation would be a fundamental discovery because of the strict connection with the baryon-antibaryon asimmetry in the Universe. Moreover, an excess of antiparticles in our Galaxy could be a signature of dark matter annihilations.
The pioneering experiments of Golden [1] and Bogomolov [2], resulting in historical measurements of an antiproton excess, paved the way for several successors, either on balloons, performed mainly by the WIZARD, BESS and HEAT collaborations, and on-board the Space Shuttle, by the AMS-01 collaboration. Although the excess was not confirmed, the search for antimatter resulted in a better understanding of the background, mainly due to the estimation of secondary positrons and electrons. Nontheless, all these experiments suffered of important limitations of statistical and systematic nature, depending on the short duration of flights and large background signals from Cosmic-Ray interactions with the residual atmosphere.
New satellite long-duration experiments have been devised with the aim of measuring antiprotons and positrons at unprecedent precision level. PAMELA [3] is the first of this new class of apparatus.

The PAMELA mission
The core of PAMELA apparatus is a magnetic spectrometer [4], consisting of a permanent magnet and a six-planes silicon tracking system, to measure the sign and the rigidity (momentum over charge) of charged particles through their deflection in the uniform 0.43 T magnetic field. A Time-of-Flight system [5], composed of a set of three double-layer planes of segmented scintillators, arranged two above and one under the magnetic spectrometer, provides a fast signal for triggering the data acquisition and for measuring the time-of-flight and ionization energy losses (dE/dx) of traversing particles. The separation between the leptonic and hadronic components is mainly carried out by an imaging silicon-tungsten calorimeter [6], 16 radiation length deep with a rejection power of 10 5 for protons versus positrons, placed under the spectrometer. A neutron detector, at the bottom of the instrument, improves the rejection power by counting the neutrons produced in the shower initiated in the calorimeter by the incident particles and more abundant for hadrons than for leptons [7]. A large scintillator under the calorimeter, acting as a shower tail catcher, and an anticoincidence system, useful to reject false trigger and multiparticle events produced by secondary particles generated inside the apparatus, complete the PAMELA instrument. A schematic drawing of the PAMELA detector is shown in Fig. 1. Since July 2006 PAMELA is daily delivering about 16 Gigabytes of data to the Ground Segment in Moscow, for a total of ∼ 5×10 9 recorded triggers.

PAMELA results on antimatter
Starting from 2008 the PAMELA collaboration published data concerning many fields of charged cosmic ray physics. Studies of antimatter have been considered particulary interesting and received most attention of the scientific community.
The comparison between the antiproton-to-proton flux ratio(published in the 1-100 GeV energy range [8] and then upgraded in the wider range 60 MeV -180 GeV [9]) and the positron fraction (published in the 1.5 -100 GeV energy range [10,11]) generated many interpretations and a lively discussion.
The positron fraction measured by PAMELA is significantly above the background expected from a standard production of positrons and electrons during propagation of cosmic ray protons and nuclei in the Galaxy [12]. Conversely, in thep/p energy spectrum no excess is found with respect to the predicted background. A proper interpretation of those results requires a careful estimation of the expected antiparticle secondary production, in order to disentangle possible contributions from exotic sources of antiprotons and positrons.
The excess in the positron fraction reported by the PAMELA collaboration has been interpreted in many works as due to annihilation or decay of dark matter in the Galaxy , CAPRICE98 [20], BESS99-00 [21], BESS-POLAR04 [22], BESS-POLAR II [23], AMS-01 [24]. The dotted lines show the upper and lower limits calculated for a pure secondary production of antiprotons during the propagation of cosmic rays in the Galaxy for a Diffusion Reacceleration with Convection model [25]. The solid line shows a calculation [26] for the case of a Plain Diffusion model (e.g. [13]). Alternatively, it has been ascribed to direct production of positrons by nearby pulsars (e.g. [14]), or due to pion production during stochastic acceleration of hadronic cosmic rays in nearby sources (e.g. [15]). The PAMELA collaboration has published also the antiproton energy spectrum in the energy interval between 60 MeV and 180 GeV [9]. In this case PAMELA results are in agreement with the previous measurements, reproducing the expected peak around 2 GeV (due to the kinematic constraints on the antiproton production), and are in overall agreement with a pure secondary production. Nevertheless, the experimental uncertainties are smaller than the spread in the different theoretical curves and, therefore, the data provide important constraints on parameters relevant for secondary production calculations and set new stringent limits to dark matter annihilation and decay [16,17].
Recently, PAMELA data acquired until January 2010 were reanalysed using a calorimeter-based antiproton selection analysis using multivariate algorithms [18]. Furthermore, the requirements on the tracking information were relaxed to extend the energy range up to ∼400 GeV. This resulted in an increased antiproton selection efficiency over the entire energy range, providing a ∼ 40 % increase in the statistics of antiprotons. The resulting antiproton energy spectrum between 60 MeV and 350 GeV is shown in Fig. 2.
The most recent PAMELA result regards the precision measurement of the cosmic rays positron flux (reported in Fig. 3, together with other previous measurements) and fraction [27], the latter extending previously published measurements up to 300 GeV in kinetic energy. Such results, still showing a significant increase in the positron component at high energy, cannot be easily reconciled with purely secondary production and confirm the  [27], superimposed to other recent measurements of the positron energy spectrum: CAPRICE94 [28], HEAT94+95 [29], AMS-01 [30], Fermi [31]. Notice that PAMELA, Fermi and AMS-01 results are from space-borne experiments. The PAMELA lower limit is that for a 90 % confidence level. PAMELA data points include both statistical and systematic errors. The solid line is the original GAL-PROP calculation [12] for pure secondary production of positrons during the propagation of cosmic rays in the Galaxy. The dotted and dash-dotted lines show a recent calculation [32] for secondary and secondary plus primary (from astrophysical sources) positrons, respectively. The dashed line shows a calculation [33] for secondary plus primary positrons from dark matter annihilation necessity of accounting for additional sources of either astrophysical or exotic origin, as it is noticeable comparing measurements to a few theoretical calculations, as in Figs. 3 and 4. Innovative and useful information is also coming, and is currently a main topic for ongoing PAMELA data analysis, from the low energy part of the energy spectra and ratios, which is the region below ∼10 GeV where solar modulation dominates, for both leptons and hadrons (e.g. [36,37]). As an example, new models of the solar modulation accounting for the polarity reversal of the magnetic field of the Sun have been developed [35] to explain the trend of PAMELA positron fraction below 10 GeV, measured during a A − period, and the discrepancy with the measurement by AMS-01 [30], performed in a A + period (theoretical curves calculated for both cases are reported in Fig. 4).
Another interesting result concerning antimatter in space was obtained by PAMELA: the first detection of antiprotons trapped in the Earth's magnetosphere [38]. The existence of a significant flux of antiprotons produced in nuclear interactions of energetic cosmic rays with the terrestrial atmosphere and accumulate in the geomagnetic field, predicted by several theoretical works, has been confirmed by the measurement of the trapped antiproton energy spectrum in the South Atlantic Anomaly (SAA) region, for the kinetic energy range 60-750 MeV, and of the atmospheric sub-cutoff antiproton spectrum outside the radiation belts. PAMELA results show that the magnetospheric antiproton flux in the SAA exceeds the cosmic ray antiproton flux by three orders of magnitude at the just passed solar minimum, and exceeds the sub-cutoff antiproton flux outside radiation belts by four orders of magnitude, constituting the most abundant source of antiprotons near the Earth.  [27] and other results from space-borne experiments AMS-01 [30], Fermi [31] and AMS-02 [34]. Such new result are in agreement to those previously published by the collaboration [10,11], extending the energy range to 300 GeV. The PAMELA lower limit is that for a 90 % confidence level. Solid line is GALPROP calculation [12] for a pure secondary production of positrons. The observed increase in the fraction above 10 GeV, incompatible with a pure secondary component of positrons originating in the propagation of cosmic rays, has been confirmed by Fermi and AMS-02. Fermi results are higher than PAMELA and AMS-02 data, which might indicate a residual proton contamination in the positron sample, but shows the same increasing trend as the energy increases. The discrepancy of both PAMELA and AMS-02 measurements with those from AMS-01, below 10 GeV, is instead well explained by the switch in the polarity of the solar magnetic field: dotted and dash-dotted curves are in fact recent calculations [35] that account for polarity + (well matching AMS-01 data) and -(at which PAMELA and AMS-02 measurements were performed)

Conclusions
PAMELA has measured the cosmic-ray antiparticle energy spectra over some of the most extended energy range ever achieved with no atmospheric overburden. The positron data show interesting features: an increase in the abundance over the secondary expectation above 10 GeV and a time dependence at lower energies which is different from that observed for the electron flux. Two different scenarios have been proposed as an explanation of the positron excess: one involving standard astrophysics, e.g. nearby young pulsar or nonstandard processes in the secondary production of positrons, and the other involving more exotic explanations, like dark matter annihilations. The antiproton spectrum is in agreement with standard secondary production models. This result places strong constraints on dark matter models since they usually do not predict an asymmetry between leptonic and hadronic production.