scienza in primo piano

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1 scienza in primo piano PAMELA: a high-precision cosmic-ray observatory in space Marco Casolino 1, Paolo Papini 2, Roberta Sparvoli 1,3 1 INFN Structure of Rome Tor Vergata, Rome, Italy 2 INFN Structure of Florence, Florence, Italy 3 University of Rome Tor Vergata, Rome, Italy The instrument PAMELA has recently passed the milestone of three years of life, providing revolutionary results on the constituents of matter and antimatter in cosmic rays and paving the way for a new generation of precision experiments in space. The instrument PAMELA, in orbit since June 15 th, 2006 on board the Russian satellite Resurs DK1, is an apparatus designed to study charged particles in the cosmic radiation, with a particular focus on antiparticles as a probe of the baryon-antibaryon asymmetry in the Universe or a possible signature of dark-matter annihilation in the galactic halo; the combination of a magnetic spectrometer and different detectors, namely an imaging calorimeter, a time-of-flight system, a neutron detector and a hodoscope of anticoincidende scintillators, allows antiparticles to be reliably identified from a large background of other charged particles. PAMELA is delivering to ground 14 gigabytes of data per day, and the analysis on the different scientific goals of the mission is going on. Results on the antiproton-to-proton and positron to all-electron ratios over a wide energy range (1 100 GeV) have been recently released by the PAMELA collaboration. The data heavily attracted the attention of the theorists community: while the antiproton-to-proton ratio does not show particular differences from an antiparticle standard secondary production, indeed, in the positron to all-electron ratio an enhancement is clearly seen at energies above 10 GeV. This effect needs some primary source in terms of the annihilation of dark-matter particles as well as in terms of standard astrophysical sources to be fully explained. 1 An historical introduction About one hundred years ago Cosmic Rays (CR) offered to physicists projectiles with energies exceeding by more than three orders of magnitude those available by natural radioactivity. The structure of the nuclei could be investigated, mesons were discovered and studied, and nuclear physics was born. It took four decades of technological efforts to reach CR energies and intensities with vol25 / no5-6 / anno2009 > 5

2 accelerators. After three more decades the use of detectors and technologies born for particle detectors allows the study of cosmic rays in space with unprecedented precision, and on ground at energies again largely exceeding those supplied by accelerators. The need of a quantum leap in measurement quality and statistics of charged and neutral cosmic rays, for an undestanding of the astrophysical and cosmological processes taking place in our Galaxy, was recognized in the decade A longterm cosmic-ray research program based on ground, balloon and space experiments paralleled by theoretical studies was planned by NASA. Part of this program was the construction of the superconducting magnetic spectrometer ASTROMAG [1, 2], to be used as a facility for CR research up to energies beyond one PeV/nucleon. ASTROMAG was designed for the FREEDOM Space Station (FSS), at the time under planning and expected to go into service in In the space program envisaged by NASA, the observation of the energy spectra of antiparticles and the search for antinuclei was considered of fundamental importance. The argument was particularly important after the first observations of antiprotons in balloonborne experiments by Golden et al. [3] and Bogomolov et al. [4], suggesting an antiproton/proton ratio higher than foreseen by secondary production in the interstellar medium and hinting at the presence of unknown sources such as annihilating dark matter or primordial black holes. The first experiment selected for the ASTROMAG facility in 1988, nicknamed WIZARD [5, 6], was proposed by an international collaboration gathered around an Italian-USA core and was devoted specifically to the study of antiparticle spectra and search for antinuclei. The experiment was built around a magnetic spectrometer, made by a superconducting magnet and a system of high-precision layers of scienza in primo piano tracking. ASTROMAG can be considered the grandfather of all spectrometer devices realized afterwards in space. The fate of the ASTROMAG facility, unfortunately, was sad. The tragic explosion of the Challenger Shuttle in 1986, the Gulf War, as well as the end of the USA-Russian competition of the cold war period, slowed down the FSS program, which was terminated in The WIZARD collaboration had to rescale its projects, starting a program of balloon-borne experiments (with missions MASS89 and MASS91, Ts93, Caprice94, 97 and 98) devoted to the same scientific objectives of ASTROMAG. Space-borne research continued with the inclusion, since 1993, of several Russian institutions to form the Russian Italian Mission (RIM) program. With the collaboration of the Russian Space Agency, the WIZARD group performed several experiments in space, namely the satellite missions NINA (in 1995 on Resurs-01 n. 4) and NINA-2 (in 2000, on the first ASI satellite MITA), devoted to low-energy cosmic 1 6 < il nuovo saggiatore

3 m. casolino: pamela: a high-precision cosmic-ray observatory in space ray observations, and the experiments on space stations, namely the SilEye-1 (1995), SilEye-2 (1998) on the MIR, the SilEye-3 (2002), LAZIO/SiRad(2005), the ASI Facility Altea (2006) on the International Space Station, all devoted to life sciences and dosimetry in space. Many experiments specifically dedicated to antimatter searches followed the pioneer ones by Golden [3] and Bogomolov [4], performed on balloon mainly by the WIZARD, BESS and HEAT collaborations, and on board the Space Shuttle by the AMS-01 collaboration. Although the first historical results about an antiproton excess were not confirmed, the main road was opened. In parallel to the aforementioned activities, the collaboration continued planning and realization of the satellite experiment PAMELA [7] (a Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), devoted to antimatter studies up to the hundreds GeV region. In June 2006 PAMELA was launched in orbit by a Soyuz-U rocket from the Baikonur cosmodrome in Kazakhstan (fig. 1). With respect to all previous antimatter missions, PAMELA is considered a new-generation experiment: designed for space, in the absence of residual air, it has size and duration to allow accurate measures of neutral and charged components of cosmic rays up to the TeV region, making in fact particle astrophysics a science of precision both from a experimental (reduction of measurement errors) and theoretical (refining models) point of view. 2 PAMELA apparatus The PAMELA apparatus is composed of the following subdetectors, arranged as in fig. 2, from top to bottom: a time-of-flight system (ToF (S1, S2, S3)); an anticoincidence system (CARD, CAT, CAS); a magnetic spectrometer; an electromagnetic imaging calorimeter; a shower tail catcher scintillator (S4); a neutron detector. The ToF system comprises six layers of fast plastic scintillators arranged in three double planes (S1, S2 and S3). It provides a fast signal for triggering the data acquisition and measures the time-of-flight and ionization energy losses (de/dx) of traversing particles. The measured time-of-flight resolution of ~ 300 ps allows e (e + ) to be separated from anti-p (p) up to 1 GeV/c. Albedo particles can also be rejected with a significance of 30 standard deviations. The central part of the PAMELA apparatus is the magnetic spectrometer, consisting of a 0.43 T permanent magnet and a silicon tracking system. The tracking system is composed by six equidistant planes of double-sided microstrip silicon detectors (made of six 300 mm thick, cm 2 wide sensors), each providing two independent impact coordinates. The dimensions of the permanent magnet define the geometrical factor of the PAMELA experiment to be 21.5 cm 2 sr. The spectrometer measures the rigidity 2 Fig. 1 Left. PAMELA on the launch pad, the same where Sputnik and Yuri Gararin were launched in 1957 and 1961, 14 June 2006, Baikonur, Kazakhstan. Right. PAMELA on the launch pad, minutes before the launch 15 June 2006, Baikonur, Kazakhstan (photos M.C.). Fig. 2 Left: a picture of the PAMELA instrument. Right: schematic overview of PAMELA. The different subdetectors are clearly identified. The apparatus is approximately 1.3 m high, has a mass of 470 kg and an average power consumption of 355 W. vol25 / no5-6 / anno2009 > 7

4 scienza in primo piano (momentum over charge) of charged particles and the sign of the electric charge through their deflection (inverse of rigidity) in the magnetic field. During flight the spatial resolution is observed to be 3 mm, corresponding to a Maximum Detectable Rigidity (MDR) exceeding 1 TV. Ionization losses are measured in the TOF scintillator planes, the silicon planes of the tracking system and the first silicon plane of the calorimeter, allowing the absolute charge (Z) of traversing particles to be determined at least up to Z = 8. The spectrometer is surrounded by a plastic scintillator veto shield, aiming to identify false triggers and multiparticle events generated by secondary particles produced in the apparatus. Additional information to reject multiparticle events comes from the segmentation of the TOF planes in adjacent paddles and from the tracking system. The sampling electromagnetic calorimeter comprises 44 single-sided silicon planes (made of nine 380 mm thick, 8 8 cm 2 wide sensors) interleaved with 22 plates of tungsten absorber. The total depth of the calorimeter is 16.3 interaction lenghts (0.6 nuclear interaction lengths). The main task of the calorimeter is to select e + and anti-p from the background of p and e -, respectively. The longitudinal and transverse segmentation of the calorimeter, combined with the measurement of the particle energy loss in each silicon strip, allows a high identification (or rejection) power for electromagnetic showers against interacting and noninteracting hadrons. A plastic scintillator system mounted beneath the calorimeter aids in the identification of high-energy electrons and is followed by a neutron detection system. This neutron counter is made of 36 3 He proportional counters, surrounded by a polyethylene moderator enveloped in a thin cadmium layer. It complements the electron-proton discrimination capabilities of the calorimeter, by detecting the increased neutron production associated with hadronic showers compared to electromagnetic ones in the calorimeter. Furthermore, the calorimeter can also operate in self-trigger mode to perform, in combination with the neutron detector, an independent measurement of the lepton component up to ~ 2 TeV (see table 1). More technical details about the entire PAMELA instrument and launch preparations can be found in [8]. The apparatus is installed inside a pressurized container attached to the Russian Resurs DK1 Earth-observation satellite and launched into space in a semi-polar (70 ) elliptical ( km) orbit on the 15 th of June The mission is foreseen to last at 8 < il nuovo saggiatore

5 m. casolino et al.: pamela: a high-precision cosmic-ray observatory in space Fig. 3 Hillas Plot of the various possible cosmic-rays sources. The sources are classified by means of their dimension and magneticfield intensity and red lines show the maximum energy achievable. Also the characteristic of some man-made detectors are plotted in the figure (PAMELA, LHC, Tevatron, LEP). Particle Antiprotons Positrons Electrons Protons Electrons + positrons Light nuclei (He/Be/C) Antinuclei search Energy range 80 MeV 190 GeV 50 MeV 300 GeV up to 500 GeV up to 700 GeV up to 2 TeV (from calorimeter) up to 200 GeV sensitivity of in anti-he/he Tab. 1 Design of PAMELA performance. Search for dark matter Search for primordial antimatter Study of cosmic-ray origin and propagation Study of solar physics and solar modulation Study of terrestrial magnetosphere Tab. 2 List of the PAMELA scientific objectives. least till the end of PAMELA was first switched on on June 21 st After a brief period of commissioning, during which several trigger and hardware configurations have been tested, PAMELA has been in a continuous data-taking mode since July 11 th. The average trigger rate of the experiment is ~ 25 Hz, varying from ~ 20 Hz at the equatorial region to ~ 30 Hz at the poles. The average fractional live time of the experiment is ~ 73%. About 14 GB of data are transferred to ground via a few down-link sessions every day. The receiving station is located at the Research Center for Earth Operative Monitoring (NTs OMZ) in Moscow, Russia. After receiving the data, a dedicated computer facility unpacks and transfers them to various institutions for further data processing and analysis. 3 Scientific objectives and first results PAMELA is a cosmic-ray observatory in orbit around the Earth: its various scientific objectives are summarized in table 2. The source of cosmic rays investigated are shown in fig. 3. The design of the PAMELA apparatus has been optimized to measure with an unprecedented precision and in a large energy interval fluxes of antiprotons and positrons coming from outside the Earth. A good measurement of these high-energy rare particles is of fundamental importance to understand their origin and to discover if exotic sources are present somewhere in the Universe. The physical mechanisms of these hypothetical sources could be connected with the new physics and their study could lead to a new insight of the natural laws. In general the astrophysical sources of high-energy particles forming cosmic-rays flux can be of different nature and located in different places: in our Solar System, in the Milky Way or in extragalactic regions. In fig. 3 the possible astrophysical sources of cosmic rays are characterized by their size and intensity of the magnetic field. This plot, prepared for the first time by Hillas to study particle acceleration at the highest energies, shows the various sources and the maximum acceleration energy, determined by size and magnetic-field strength. Since the processes of acceleration and propagation are similar on different scales, PAMELA can investigate various aspects of cosmic-ray physics. Although the energies involved are extremely different, the processes on a scale more easily accessible, such as those occurring in the interplanetary space or in the radiation belt, help vol25 / no5-6 / anno2009 > 9

6 scienza in primo piano Fig. 4 The energy content of our Universe. Dark matter remains an enigma, as is also dark energy. to understand phenomena that occur in the most distant objects and at higher energies (such as in pulsars or in supernovae explosion). Characteristic of PAMELA is therefore the study of cosmic rays in its various aspects, ranging from the physics of fundamental interactions to the mechanisms of production, acceleration and propagation of particles in the Galaxy, in our Solar System and around the Earth. 3.1 Antimatter and dark matter The first evidence of the presence of antimatter in Nature happened with the discovery of the positrons by C. D. Anderson in 1932 (using cosmic rays as a particle beam). The existence of this electron antiparticle was predicted previously by P. A. M. Dirac in 1928 within his electron theory in which the concepts of relativity and quantum mechanics where combined together. Today we can produce a small quantity of antimatter in our laboratory or observe few traces of it during the collision of cosmic rays with the Earth. Observing the world around us, the abundance of antimatter with respect to ordinary matter appears very poor. However, the most accredited theories of evolution of the Universe state that an equal amount of matter and antimatter have been generated at the origin of our Universe, just after the Big-Bang. Matter and antimatter would then almost immediately annihilate each other. From this process amazingly appears to have survived only the matter that forms stars, planets, ourselves and everything we know. The question, now unsolved, is how it was possible to have the complete disappearance of antimatter and/or whether in some other portion of the Universe there is an abundance of antimatter of primordial origin, to compensate what we observe locally. The search of antimatter in cosmic rays could probe the baryon-antibaryon asymmetry in the Universe. Close to this fascinating question, another one appeared in the last two decades, even more mysterious: as represented in fig. 4, today we know that the Universe consists only of 4% of the matter familiar to us, made of protons, neutrons and electrons (and of small amounts of antimatter). It is estimated that about 73% of what exists in the cosmos is made up of an invisible and homogenous substance called dark energy. The last about 23% would be made up of particles much different from ordinary matter, which do not aggregate in celestial bodies, which do not emit electromagnetic radiation and therefore are not directly visible ( dark matter ). The presence of dark matter, inferred from the gravitational effects it has on the motion of celestial bodies, has been highlighted since the 1930s, but only in recent decades various measures of astrophysical cosmological importance (abundance of light elements, cosmic radiation background, evolution of galactic structures) have clearly established its importance in the budget of energy that composes our Universe, allowing also to outline its fundamental characteristics. The fundamental question about what is the nature of the particles that make up the dark matter still remains. 3.2 Origin and propagation of the cosmic rays Both the baryon-antibaryon asymmetry and the nature of dark matter can be addressed observing the antiparticle components in the cosmic rays. It is thought that the bulk of the cosmic rays in our Galaxy are accelerated by events, presumably supernovae explosions, which provide energy to the particles (protons, nuclei and electrons). During the propagation and the diffusion of these high-energy particles in the galactic magnetic field, interactions occur with nuclei of the interstellar medium. This means that there are secondary nuclei due to the break (or spallation) of primary nuclei accelerated directly from the sources. The nuclear composition observed on Earth is then modified with respect to the original one present at the sources. Also the shape of the energy spectrum of cosmic rays (which extends up to energies around ev, see fig. 5) is affected by the propagation, due to the fact that high-energy 10 < il nuovo saggiatore

7 m. casolino et al.: pamela: a high-precision cosmic-ray observatory in space cosmic rays escape more easily from the Galaxy than lowenergy cosmic rays. During their propagation, cosmic-rays nuclei also produce a small percentage of antiprotons and positrons, which joins the flow of cosmic rays (fig. 6). The rarity of these antiparticles (one antiproton every about protons and one positron every about 10 electrons) is another proof of the asymmetry between matter and antimatter, at least in our Galaxy. Thanks to our knowledge of fundamental interactions, of the interstellar gas composition and of the nature of cosmic rays, it is possible to predict both the intensity of secondary antiparticle components and their energy spectra. The crucial point is that any excess of antiparticles in cosmic rays could be a signature of the dark-matter existence (or antimatter presence) in the Universe. Depending on the various models on the nature and mass of the elusive particles that form this kind of matter, it is possible to predict distortions and increases of the spectra of antiprotons and positrons Nature of dark matter From the cosmological point of view, the most promising candidate of dark matter is identified with a particle without electric charge or color, massive and that weakly interacts (Weakly Massive Interactive Particle WIMP). The absence of electrical charge prevents it to emit electromagnetic radiation and justifies its darkness. The absence of colour charge makes the WIMPs non-interactive in the domain of strong interactions, preventing them from forming anomalous nuclear states, which is absent in our Universe. The weakness of the interaction limits the rate with which WIMPs can destroy themselves, thus ensuring their survival over billions of years of evolution of the Universe. The presence of a mass of at least a few dozen GeV/c 2 guarantees the relativistic nature of these particles when they are decoupled from the thermal bath of the early universe, in agreement with the models of cold dark matter required by the process of evolution of structures in our Universe. Fig. 5 Differential spectra of cosmic rays. Galactic protons are the most abundant species, with helium making up about 10% of the particles. During Solar Particle events or inside the inner radiation belt the flux of protons can increase by several orders of magnitude. Units of measure for electrons and positrons are: part/m 2 sr s MeV. Fig. 6 Schematic representation of the secondary production of antiprotons in the interstellar medium. 6 vol25 / no5-6 / anno2009 > 11

8 7 In supersymmetric extensions of the Standard Model of elementary particles, there are several scenarios in which new particles can play the role of WIMPs: to determine the nature of dark matter would therefore help not only to solve a cosmological puzzle but open a new window in the panorama of fundamental physics. The search for these particles, fossils of the Big Bang and presumably inhabitants of dark halos of galaxies, stretches into two main directions: the direct search, conducted in underground laboratories aimed at revealing the interactions of WIMPs with detectors of large mass, and indirect research, carried out in space. The latter is based on the principle that WIMPs can annihilate each other and produce, after the hadronization, a number of elementary particles. Of particular interest, because of the low astrophysics background, are the possible signatures in gamma rays and cosmic antimatter fluxes: positrons, antiprotons, antideuterons. In this way, a very strong connection between the search for primordial antimatter and search for dark matter in space is created, because both require experimental techniques based on magnetic spectrometers that can identify the sign of the particle charge. The most studied WIMP is the neutralino, a combination of supersymmetric partners of the neutral gauge bosons of the Standard Model. Neutralinos are Majorana fermions that can annihilate with each other in the halo of the Galaxy, resulting in the symmetric production of particles and antiparticles, the latter providing an observable signature. Another interesting candidate, among the many proposed, is the Kaluza-Klein lightest particle in the Universal Extra Dimension framework. 3.4 Observation of antiparticles in cosmic rays As already mentioned, the main objective of the PAMELA experiment is the very accurate measurement of the antiparticle spectrum of cosmic rays. Antiprotons and positrons have the same mass and the same absolute value of the electric charge of their counterparts, protons and electrons, which are the basic constituents of atoms of our world. Their electric charges, however, have opposite sign and thus antiparticles can be identified by the opposite curvature in the magnetic field of the PAMELA spectrometer (fig. 2). Particle identification in PAMELA is based on the determination of the rigidity by the spectrometer and the 12 < il nuovo saggiatore

9 m. casolino et al.: pamela: a high-precision cosmic-ray observatory in space 8 properties of the energy deposit and interaction topology in the calorimeter. The analysis technique was validated using the PAMELA collaboration s official simulation program tuned using particle accelerator beam data. Figure 7 shows a ~ 29 GV negatively charged particle with a hadronic interaction in the calorimeter identified as an antiproton, while fig. 8 shows a ~ 92 GV positively charged particle with a typical electromagnetic shower in the calorimeter identified as a positron. In these figures different signatures in the electromagnetic calorimeter and in the neutron detector can be clearly noticed. In fig. 9 we show the measured ratio between the antiproton flux and the proton flux up to 100 GeV, an energy never achieved previously, recently published by PAMELA [9]. The figure also shows the predictions of some models on the secondary production [10 12]. The experimental results are in substantial agreement with what is expected and there is no evidence of any excess due to primary sources. We can also see that the accuracy of the measurements allows distinguishing between different propagation models of cosmic rays in the Galaxy. Figure 10 shows the measured fraction of positrons with respect to the total flux of electrons and positrons, measured by PAMELA and published in February 2009 [13]. Experimental data are compared with a typical expectation of the secondary component [14, 15]. It is important to note that at low energies, below about 10 GeV, the energy spectrum of cosmic rays at the Earth is modified by the phenomenon of the time-dependent solar modulation that depends on the sign of the particles charge as well as their energy (it will be discussed later). Fig. 7 The event display of a ~ 29 GV antiproton interacting in the calorimeter. The bending (x) and non-bending (y) views are shown on the left and on the right, respectively. A plan view of PAMELA is shown in the center. The signals as detected by PAMELA detectors are shown along with the particle trajectory (solid line) reconstructed by the fitting procedure of the tracking system. Fig. 8 The event display of a ~ 92 GV positron. The bending (x) and non-bending (y) views are shown on the left and on the right, respectively. A plan view of PAMELA is shown in the center. The signals as detected by PAMELA detectors are shown along with the particle trajectory (solid line) reconstructed by the fitting procedure of the tracking system. vol25 / no5-6 / anno2009 > 13

10 scienza in primo piano At high energies, however the spectrum observed on Earth is the same as outside the Solar System. Above 10 GeV, i.e. in the region where the effect of solar modulation is negligible, there is a clear overabundance over predictions. Various hypotheses about the nature of this exciting and unexpected increase at high energy have been proposed. The most interesting is precisely that connected with the annihilation of dark matter, even if astrophysical sources such as pulsars could contribute in part to the observed flux of positrons. It should also be noted that any hypothesis of primary source that justifies the observed positron signal must be such as not to produce an antiproton flux observable by PAMELA. This constrains significantly the assumptions on primary sources of antimatter, and in particular on the possible models of dark matter. In addition, it should be noted that every possible source of positrons is actually also a source of electrons. Indeed, the physical process of creating a positron also implies the creation of a corresponding electron. Therefore a source of positrons in our Galaxy is also a source of electrons. The electron cosmic flux is then composed of three main components: a primary component produced by supernovae, a secondary component produced by the cosmic-ray flux interaction with the interstellar medium and this new 14 < il nuovo saggiatore

11 m. casolino et al.: pamela: a high-precision cosmic-ray observatory in space kind of primary source. Measuring the electron flux at high energy with good precision, it should be possible to observe structures in the spectrum that correspond to this last component. Work in progress of the PAMELA experiment is to provide measurement of the electron flux. PAMELA is also looking for antimatter nuclei, namely nuclei composed by antiprotons and antineutrons. The identification of a single nucleus of antihelium would have a major impact on theories of primordial baryogenesis, i.e. during the first few minutes after the Big Bang, while the identification of heavier antinuclei, produced only through nuclear fusion of antiprotons in hypothetical antistars, would be proof of the presence of antimatter areas in the Universe. 3.5 Galactic and heliospheric cosmic rays Our Sun is in constant change, passing every 11 years from a state of minimum to a state of maximum activity. In the former case the sunspot number is small and the field is approximately dipolar. At the solar maximum the field is chaotic and strongly variable, with a large number of sunspots producing several solar particle events. After 11 years the Sun is back to solar minimum but with the dipole in an inverted configuration. Thus it takes 22 years for the Sun to return in the same magnetic configuration. The Sun is constantly emitting a solar wind of speed between hundreds and thousand of km/s. The solar wind expands in the heliosphere and determines its configuration, affecting through the process of solar modulation the particle flux below 10 GeV and its temporal evolution. In fig. 11 the variation of galactic protons as a function of time is shown. At the time of PAMELA launch, the solar activity was higher and the flux was lower. As we progressed deeper in the solar minimum, the particle flux increased. The solar-modulation parameter in spherical approximation can be determined from data assuming a rigidity power law interstellar spectrum: in the case of PAMELA we go from 500 MV in 2006 to 400 MV in This model is however not capable of taking into account the effect of charge-dependent solar modulation, which is due to charge-sign dependence of the drift effects. This produces different time behavior of the proton flux as a function of time according to the solar polarity. In the current polarity for instance the positron flux is lower below 10 GeV. More detailed models take into account the three-dimensional structure of the heliosphere and are able to reproduce this structure and its temporal evolution. One way to observe the charge-dependent solar-modulation effect is to measure the positron fraction content in the cosmic rays in different periods of time. If the solar modulation affects the electron flux in the same way as the positron flux, then the positron fraction should be constant. This is not the case, in fact what PAMELA observes is shown in fig. 12 where the PAMELA measurement at low energy Fig. 9 Measurement of the ratio between the antiproton flux and the proton flux performed by the PAMELA experiment. Data are compared with different expectation for the secondary production in the interstellar medium [10, 11, 12]. Fig. 10 PAMELA measurement of the positron fraction with respect to the total electron plus positron flux. The experimental data are compared to a couple of typical expectations from secondary production in the interstellar medium. The calculations of the positron flux take into account the solar effect with a simple model of the solar modulation. A > 0 refers to the Moskalenko and Strong calculation [14] in case of the previous solar cycle and A < 0 refers to the Grimani calculation [15] in case of the present solar cycle. Fig. 11 Differential spectrum of galactic protons in various periods of the PAMELA mission. Note how solar modulation affects the flux at low energies. At solar maximum (see also BESS data) the solar modulation is higher and particle flux is lower (as seen in July 2006). As we progress deeper in the solar minimum particle flux increases. The lower curves show the helium flux. Fig. 12 PAMELA measurement of the positron fraction with respect to the total electron plus positron flux at low energy (< 10 GeV). Data are compared with other experiments (Clem&Evenson [16], AMS [17], CAPRICE94 [18], HEAT [19]). 12 vol25 / no5-6 / anno2009 > 15

12 scienza in primo piano 13 (below 10 GeV) is compared with other measurements [16 19]. The PAMELA positron fraction is systematically lower than the measurements done in the previous 11 years solar cycle (black points) and it is in agreement with the only measurement done in the same solar period of PAMELA (green points also if the statistical errors of these data are very large due to the short collecting time of this balloon experiment). PAMELA observations are thus taking place during the solar minimum of cycle XXIII. This minimum is lasting longer than expected, hinting to the presence of lower harmonics ( years) in the solar modulation. At the time of writing there is no consensus on how the Sun will proceed toward the next maximum. It is expected that PAMELA will monitor the transition toward solar maximum XXIV and the associated solar particle events, helping in a more detailed understanding of Solar processes and how they affect the Earth s magnetosphere. 3.6 Solar particle events In addition to the long-term solar modulation, the Sun can manifest dramatically producing Solar Flares and Coronal Mass Ejections (CMEs). These eruptions, visible in various photon frequencies, can release an enormous quantity of energy due to the annihilation of the magnetic-field lines. The particles thus accelerated expand in the interplanetary space, reaching up to several tenths of AU in size. Proton and helium flux during these events can increase for some days by several orders of magnitude producing a hazard in interplanetary space. In fig. 13 we show one series of major solar events detected by PAMELA in December The characteristics of PAMELA allow real-time measurements of different particle spectra at energies and with a precision previously unreached in space. These measurements are crucial in understanding the acceleration and propagation mechanisms which take place at the Sun and in the heliosphere. 16 < il nuovo saggiatore

13 m. casolino et al.: pamela: a high-precision cosmic-ray observatory in space Fig. 13 Flux of protons emitted during the solar particle event of 13 December Before the event (black curve) flux was three orders of magnitude lower than the event. The eruption and corresponding CME (visible in the inset of the figure) accelerated particles, detected by Pamela in its first crossing in the southern region (red curve). The event accelerated particles up to 3 GeV. As the event progressed particle flux at high energies decreased, with lower energy increasing due to the longer time required for slower particles to reach the Earth. 14 Fig. 14 Particle map of trapped cosmic rays (E > 35 MeV p; E > 3.5 MeV e - ). It is possible to distinguish the inner Van Allen belt (protons) and the electron belt in the south pole. Fig. 15 Measurement of the trapped protons in the inner radiation belt. The regions selected are shown in the inset of the figure. Note the galactic component at high energies (> 10 GeV), the reentrant albedo component measured at the same cutoff region but outside the anomaly (black curve) and the increasing flux (up to four orders of magnitude) toward the core of the anomaly Trapped and secondary particles The high-inclination orbit of Resurs allows PAMELA to use the geomagnetic field as a huge spectrometer to study particles of different origin and nature. Earth has two radiation belts, named Van Allen belts from the name of their discoverer. They consist of two regions, one with protons and closer to the Earth and one farther away, located at high latitudes and dominated by electrons (fig. 14). Since the magnetic field is conservative, the proton belt is composed by neutrons which decay to protons and find themselves trapped. In the case of electrons the trapping occurs during periods of solar or magnetospheric activity, which perturb the outer geomagnetic field. At intermediate latitudes there is an additional component of short-lived particles of secondary origin, produced in the interaction of galactic cosmic rays with the atmosphere. These particles are an important source of radiation to crew on board the International Space Station. A precise measurement of this component is also used to measure the high-energy proton-nucleus interaction cross-section, needed in the secondary particle production in the Galaxy, and as an input to the calculation of the spectrum for the atmospheric neutrino flux. In fig. 15 the measurements of the flux at the core of the Van Allen belt performed by PAMELA are shown. In this region the particle flux can be up to four orders of magnitude higher than in the same geomagnetic regions but outside the belt. The flux exhibits a rigidity-dependent power law spectrum where particles are trapped up to some GeV. The lower peak to the left is due to particles of galactic nature, above the geomagnetic cutoff. vol25 / no5-6 / anno2009 > 17

14 4 Conclusions PAMELA is the first of a series of new satellite and space-station borne experiments which are flying or will fly in the next years. For the first time measurements of cosmic rays can be performed with the precision, statistics and temporal evolution needed to clarify many of the open problems in cosmology, astrophysics and solar terrestrial environment. Amongst the many striking results obtained by the experiment in its first three years of observations, we have the discovery of the positron to all-electron ratio increase at high energy, the direct observation of high-energy solar-particle events, and the measurement of the core of the South Atlantic Anomaly. After the reboost of the satellite orbit to reduce friction with the upper layers of the atmosphere, PAMELA mission has been extended for additional two years to continue measures toward the next solar maximum and increase statistics and energy range for antiparticle search. References [1] M. A. Green et al., IEEE Trans. Magn., MAG-23, no. 2 (1987) 1240; S. C. Kappadath et al., Proceedings of the 24 th ICCR, Rome, Vol. 2 (1995) p. 230; E. P. Mazets et al., Astrophys. and Space Science, 33 (1975) 347; S. Orito et al., Proceedings of the 24 th ICCR, Rome, Vol. 3 (1995) p. 76. [2] NASA, Astromag Definition Team, The Particle Astrophysics Magnet Facility ASTROMAG, NASA Report, May [3] R. L. Golden et al., Astrophys Lett., 24 (1984) 75. [4] E. A. Bogomolov et al., Proceedings of the 20 th ICCR, Moscow, Vol. 2 (1987) p. 72. [5] R. L. Golden et al., Wizard: a proposal to measure the cosmic rays including antiprotons, positrons, nuclei and to conduct a search for primordial antimatter (College of Engineering, NMSU) [6] P. Spillantini et al., Il Nuovo Cimento B, 103 (1989) 625; P. Spillantini, Nucl. Phys. B, 122 (2003) 66; F. W. Stecker, Nature, 273 (1978) 493; J. I. Trombka et al., Astrophys. J., 212 (1977) 925; R. S. White, Astrophys. J., 218 (1977) 920. [7] O. Adriani et al., Proceedings of the 24 th ICCR, Rome, Vol. 3 (1995) p [8] P. Picozza et al., Astropart. Phys., 27 (2007) 296. [9] O. Adriani et al., Phys. Rev. Lett., 102 (2009) [10] M. Simon, A. Molnar and S. Roesler, Astrophys. J., 499 (1998) 250. [11] F. Donato et al., Astrophys. J., 563 (2001) 172. [12] V. S. Ptuskin et al., Astrophys. J., 642 (2006) 902. [13] O. Adriani et al., Nature, 458 (2009) 607. [14] I. V. Moskalenko and A. W. Strong, Astrophys. J., 493 (1998) 694. [15] C. Grimani et al., Proceedings of the 30 th ICCR, Merida, Vol. 1 (2007) 485. [16] J. Clem and P. Evenson, Astrophys. J., 568 (1997) 216. [17] J. Alcaraz, et al., Phys. Lett. B, 484 (2000) 10. [18] M. Boezio, et al., Astrophys. J., 532 (2000) 653. [19] S. W. Barwick, et al., Astrophys. J., 482 (1997) 191. Marco Casolino Researcher at INFN Roma Tor Vergata and visiting scientist at RIKEN. He has taken part in the realization of various experiments in space. The most relevant are: the Russian space station Mir, SilEye-1 and SilEye-2, International Space Station, SilEye-3/ Alteino, Altea, Lazio-Sirad and on satellites NINA-1 on Resurs-01, NINA-2 on ASI MITA satellite, PAMELA on Resurs-DK1. He has participated in several integration and launch campaigns in Baikonur, Plesetsk and Kennedy Space Center, in training of the astronaut crew of MIR and ISS. Work on space stations involved studies of life science, radiation environment, dosimetry. He is Principal Investigator of the ESA Altcriss project, devoted to the shielding of astronauts from radiation. Satellite research is devoted to the study of matter and antimatter in cosmic rays and indirect search for dark matter. Paolo Papini Paolo Papini works as researcher at the Italian National Institute of Nuclear Physics (INFN) in Florence. His main activity concerns the cosmic-rays study focusing the attention on the antimatter component. Since 1991 he has participated in the experiments of the WiZard-RIM/ PAMELA collaboration both in stratospheric balloon experiments and in experiments onboard satellites. He also collaborates in an experiment at CERN to study forward interactions of particular interest for ultra-high-energy cosmic-ray physics. Roberta Sparvoli Roberta Sparvoli is a researcher at the University of Rome Tor Vergata and has a Research Fellowship from the Italian National Institute of Nuclear Physics (INFN). She devoted most of her scientific activity to the search for antimatter and dark-matter signals, and the study of nuclear and isotopic components of cosmic rays in a wide range of energy, by means of experiments on stratospheric balloons and onboard satellites. She also carried out research of physical/biological nature with experiments aboard two space stations: Mir and ISS. Roberta Sparvoli is part of the collaboration WiZard-RIM/PAMELA since 1993, and is responsible of the PAMELA-Rome Tor Vergata group since < il nuovo saggiatore