Direct Measurements of Cosmic Rays (up to ^ TeV and beyond)

Transcription

1 Direct Measurements of Cosmic Rays (up to ^ TeV and beyond) Stephane Coutu*, Denis Cabezas^ C. J. Solano Salinas**'* and Reyna Xoxocotzi^ * Department of Physics, Penn State University, University Park, PA 16802, USA ^ Observatorio de Ancon, Lima, Peru **Institute de Fisica, Universidad Nacional de Ingenieria, Lima, Peru ^Institute Nacional de Investigacion y Capacitacion de Telecomunicaciones, UNI, Lima, Peru ^Facultad de Ciencias Fisico-Matemdticas, Benemerita Universidad Autonoma de Puebla, Puebla, Pue., CP: 72570, Mexico Abstract. The objective of this paper is to provide a brief overview of selected experimental measurements in the field of cosmic rays. Galactic cosmic rays are composed of primary and secondary particles. Primary cosmic rays are thought to be energized by supernova shocks within our Galaxy. The cosmic rays that eventually arrive at the Earth are mainly protons and atomic nuclei, but also contain electrons. Secondary cosmic rays are produced in collisions of primary particles with the diffuse interstellar gas and are scarce but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, antiprotons and positrons; these may also come Irom unusual sources and could possibly provide a window into new physics. For example, in the Galactic halo there could take place annihilations of heavy supersymmetric dark matter particles, which could lead to positrons or antiprotons. Keywords: cosmic ray measurements, positron, electron, antimatter, HEAT, CREAM. PACS: Sa, Ry, De. INTRODUCTION Among the many particles which travel trough the universe are the cosmic rays, which have been the subject of intense study for nearly a century. [1, 2] Cosmic rays consist of high-energy particles, which are thought to have been accelerated within the Galaxy, or possibly in extragalactic sources at the highest energies. These particles constantly bombard the Earth, generating large numbers of secondary particles in the atmosphere. Cosmic rays consist of protons (86%), a particles (11%), electrons (2%) and heavier nuclei of elements such as carbon, oxygen, magnesium, silicon, and iron (1%), as well as a majority of elements in the periodic table. The mechanisms responsible for the origin and acceleration of cosmic rays are still a subject of intense interest. Supernova shocks in our Galaxy, or else quasars, active galactic nuclei (AGN) and gamma ray bursts (GRB) at the highest energies are considered candidate sources of cosmic rays. A difficulty is that, unlike neutral astrophysical messengers such as photons and neutrinos, cosmic rays are deflected by magnetic fields, and thus do not provide directional information about their acceleration sites. Cosmic-ray spectral features known as the knee and the ankle are likely to be the reflection of changes in sources, propagation, and acceleration. The knee is a transition CP1123, Cosmic Rays and Astrophysics, edited by C. J. Solano Salinas, D. Wahl, J. Bellido, and O. Saavedra 2009 American Institute of Physics /09/$

2 region where the all-particle spectrum steepens from dn/de ^ E~'^-'^ below 10^^ ev to ^ E~^-^ above 10^^ ev, and the ankle is where the spectrum flattens back to ^ E~'^-'^ at ^ 10^^ ev. The coincidence of the knee with the highest energies expected from diffusive shock acceleration in supernova remnants is intriguing. Measurements (some very recent) of the positron fraction e+/(e+ + e~) in the energy range from 1 to 10 GeV, indicate that positrons are compatible with a secondary origin (as a result of cosmic-ray interactions with the interstellar medium), followed by Galactic propagation which can be simply described by a leaky box model, or alternately using more complex galactic diffusion simulation algorithms. At energies about 10 GeV and higher, measurements indicate a significant excess of positrons over the fraction expected from secondary sources. This gives rise to a variety of interpretations. For instance, it has been suggested that at high energies the production of primary electrons is inefficient, or alternately the existence of new positrons sources has been postulated. For example, the annihilation of heavy supersymmetric dark matter particles in the Galactic halo could lead to positrons or antiprotons with different energy signatures. The data suggest that indeed an additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. The observed all-electron flux is dominated by negative electrons from primary acceleration sites, with a component of about 10% of the total flux in secondary e+, and presumably a similar number of secondary e~ (indeed, interactions between the nuclear cosmic rays and nuclei in the ISM generate electrons and positrons in about equal numbers). At energies above 10 GeV, observations indicate that the positron fraction, e+/(e+ + e~) increases. Elucidating the nature of the electron sources would have a profound impact on our understanding of cosmic-ray sources in general. Cosmic rays in the energy range 10^^ 10^^ ev have been measured indirectly through observations of air showers in the atmosphere, but such measurements are at a disadvantage in that it is not possible to unambiguously identify the primary particle which originated the air shower. However, cosmic rays can be measured directly by flying particle detectors to the top of the atmosphere, for example using high-altitude balloons. In this way the identity and energy of the parent particle can be established unambiguously, but the maximum energy reach depends on the detector size and exposure time. At energies around 10^^ ev the energy spectra of primary cosmic rays have been studied with good accuracy, but above this energy the composition and spectra are not known directly. Electrons are a unique probe of cosmic-ray confinement and source distribution in our Galaxy because electrons and positrons interact with the ISM through electromagnetic processes, such as synchrotron radiation and inverse Compton scattering, which do not affect hadronic cosmic rays to the same extent. Measurements of the detailed energy dependence of the elemental spectra to as high an energy as possible is key to understanding the acceleration and galactic propagation for the bulk of cosmic rays, i.e., those at energies below the knee in the all particle spectrum. 52

3 EXPERIMENTAL TECHNIQUES FOR THE DIRECT MEASUREMENT OF COSMIC-RAY NUCLEI AND ANTIPARTICLES Various techniques are employed to make direct measurements of cosmic rays. In this paper we focus only on two representative payloads, namely the High-Energy Antimatter Telescope (HEAT) [3] instrument, and the Cosmic Ray Energetics And Mass (CREAM) [4, 5] apparatus. We describe the first configuration of HEAT which was optimized for the detection of cosmic-ray electrons and positrons below 100 GeV, and the first configuration of CREAM, targeted at elemental measurements of hadronic cosmic rays up to about 100 TeV. High-Energy Antimatter Telescope (HEAT) This instrument was developed in order to measure the abundance and energy spectra of positrons/electrons, and antiprotons/protons. Common to all of these investigations is the use of a rigidity spectrometer which employs a superconducting magnet. This spectrometer is combined with specific particle detectors to meet the individual objectives of successive high-altitude balloon flights. Electrons and positrons are a relatively rare component of the galactic cosmic radiation. Electrons constitute less than 10~^ of the total cosmic-ray flux at a given energy, while the relative abundance of positrons is well below ^ 10~^. Nevertheless, the origin of these particles is of specific astrophysical importance, and the fact that they are the only component in the charged-particle radiation that does not interact hadronically but is affected instead by electromagnetic interactions with the interstellar medium, makes electrons and positrons interesting probes of the propagation of particles through interstellar space. The majority of negative electrons originate in primary cosmic-ray acceleration sites, possibly the same that also produce the nuclear cosmic radiation. In addition, some electrons and positrons (in about equal proportions) must be generated as secondary particles resulting from collisions of cosmic-ray nuclei with the interstellar gas. The HEAT instrument was therefore designed to achieve a high rejection power, and reached an overall proton rejection of nearly 10^. This was accomplished by combining a high-resolution magnet spectrometer with time-of-flight scintillators, a transitionradiation detector, and an electromagnetic calorimeter. Fig. 1 shows a schematic crosssection of the instrument. A particle entering the top of the instrument encounters in sequence: (1) a time-of-flight (TOE) scintillator which measures the magnitude of the particle charge and starts the measurement of the time-of-flight of the particle through the instrument, (2) a transitions-radiation detector (TRD) which discriminates between positrons and protons and consists of six pairs of plastic fiber radiators and multiwire proportional chambers (MWPCs) containing xenon for X-ray detection, (3) a drift-tube hodoscope (DTH) filling the bore of the magnet (^ IT magnetic field) which measures the rigidity and charge sign of each traversing particle, and (4) an electromagnetic calorimeter (EC) which consists of 10 layers of lead and plastic scintillators which 53

4 1 -.i^m: FIGURE 1. Cross section drawing of the HEAT instrument. record the shower profile to determine the energy of electrons and positrons, and to provide additional proton rejection, as well as generating a TOP stop signal. This arrangement provided a highly redundant set of measurements: while the primary trajectory measurement was performed with the DTH, the position sensitivity of the TRD helped to reject erroneous trajectories; a particle identified as an electron or positron by the TRD must have had a charge one in the top scintillator and must have exhibited a clear electromagnetic shower profile in the EC; moreover, the energy measured with the EC must be consistent with the momentum determined in the magnet spectrometer. Rigidity spectrometer The successful operation of a rigidity spectrometer on a balloon-borne instrument requires the simultaneous optimization of many parameters, including weight, aperture, magnetic field intensity, tracking accuracy, maximum detectable rigidity (MDR) and the amount of multiple Coulomb scattering due to material in the path of accepted particles. The MDR is defined so that the error hrin measured rigidity R = pc/ze be given by: dr _ 1^^ R MDR (1) The sign of a particle's charge can reliably be determined for rigidities up to ^ ^MDR. For a track of total length / through a magnetic field B, sampled at n point each with a positional accuracy o, the MDR is given by MDR = 0.03 Bdl {continuous tracking approximation) (2) 54

5 or MDR = 0.03 W / Bdl(optimum spacing approximation) (3) o V 256 J In the first case, the n measurement are made at uniform spacings throughout the magnetic field region, whereas in the second «/4 measurement are made at the beginning of the track, n/2 measurements are made near 1/2, and «/4 measurement are made near /. In the above expressions the units are kgm for jbdl and GV for MDR. The overall performance of the spectrometer is characterized by the MDR distribution for all trajectories through its aperture. Transition-radiation detector (TRD) The characteristic dependence of the intensity of X-ray transition radiation (TR) on the Lorentz factor y of the primary particle makes a transition radiation detector wellsuited to discriminate between electrons and hadrons throughout the energy interval addressed by the HEAT measurements. Typically, TR is not observable for y < 10^ and saturates for y > 10^. Thus, for the energy internal 5-50 GeV, electrons and positrons produce no TR. To efficiently detect the emitted TR X-ray photons, gas counters with thin windows and a gas mixture comprised mostly of xenon are required. The radiator which produces the primary TR photons must be a dielectrically inhomogeneous material. Previous studies have shown that plastic fiber blankets provide a simple and inexpensive radiator. Time-of-flight (TOF) The time-of-flight scintillators trigger the experiment, reject upward-moving albedo particles, which might otherwise be interpreted as antiparticles, and provide a measurement of the charge of each cosmic ray accepted by the flight trigger. They determine the particle velocity by measuring, in conjunction with the top three layers of the EC, the time-of-flight for particles accepted by the trigger. The TOF scintillators consist of five rectangular modules each measuring 100cm x 25cm X 1cm constructed from Bicron BC-408 plastic scintillator. Each end of a module is viewed through a Lucite light guide by a Hamamatsu R2490 photomultiplier tube (PMT). These phototubes are oriented approximately parallel to the fringe field of the superconducting magnet. The light guides maintain this alignment so that gain losses are less than 20% and iron magnetic shields are not needed. The top three counters of the EC also provide the bottom TOF system. These are also Bicron BC-408 plastic scintillators, each of dimension 50cm x 50cm x 1cm, and each viewed by one Hamamatsu R2490 PMT. Light pipes consisting of optical fiber bundles are used to maintain proper alignment between the PMTs and the magnetic field. Gain losses are less that 4%, as verified in magnet-on and magnet-off measurements with cosmic-ray muons. 55

6 Electromagnetic calorimeter (EC) The electromagnetic calorimeter (EC) performs three functions: electron energy measurement, hadron rejection, and bottom time-of-flight measurement. Because of weight constraints, the EC is designed with a total thickness of 9.5 radiation lengths of interleaved lead and scintillator. The EC consists of a stack of 10 identical modules. Each module is 50cm x 50cm, and consists of a 0.5cm thick hardened lead plate (98.54% Pb, 1.40% Sn, 0.061% Ca) above a 1cm thick Bicron BC408 scintillator wrapped in ^ 0.5mm aluminum foil. Three sides of each scintillator are surrounded by an aluminum frame to provide support for the lead plates. The remaining side of each scintillator is coupled to a rectangular array of 246 polymethyl methacrylate optical fibers (2mm diameter). The other end of the approximately 50cm long fibers are regrouped into a circular face and glued to a 2in diameter Lucite 90 elbow, which in turn is glued to a Hamamatsu R2490 photomultiplier tube. This light pipe design makes it possible to accurately align each PMT along the magnetic field, as required for linear operation of the PMTs without loss of gain. The detector system and all electronics are enclosed in a sealed gondola maintaining atmospheric pressure throughout the balloon flight. The data are transmitted to ground via telemetry. The Cosmic Ray Energetics And Mass (CREAM) The origin of cosmic-ray nuclei, their acceleration mechanism and propagation through the interstellar medium are some of the most fundamental problems in cosmology today. Theorists have developed a convincing model of acceleration by diffusive shock waves from supernova explosions that provide a natural explanation of the power-law spectrum observed at energies above a few times 10^^ ev and up to a few times 10^^ ev. This model predicts a sharp break at about Z x 10^^ ev, which has yet to be observed in any single element spectrum. The all-particle spectrum observed by ground-based experiments shows a kink, the so-called knee, at about 3 x 10^^ ev, and some experiments have reported an increase in the fraction of iron-like nuclei compared to proton-like nuclei around the knee, offering some support to the model. Direct measurement of a knee in the proton spectrum, however, would provide the most convincing evidence to support the model. CREAM is poised to extend accurate direct measurement by more than an order of magnitude, to above the expected proton spectrum knee. CREAM objectives The science objectives of CREAM include the measurement of elemental spectra up to iron, determining if the proton spectral index is different than that of heavier nuclei, searching for potential spectral features such as a knee in the proton spectrum, and measuring the energy dependence of the flux ratio between secondary nuclei (e.g.. 56

7 boron) and primary nuclei (e.g., carbon). To achieve these objectives, the CREAM measurement requirements are as follows: (1) elemental coverage is required from hydrogen to iron inclusive (1 < Z < 26), with a charge resolution sufficient to resolve each element in the range; (2) the energy resolution is designed to be better than 50% over the energy range, with an absolute calibration better than 10%; (3) a total exposure of about 200 days at balloon float altitudes, requiring repeated flight campaigns in Antarctica. CREAM has a larger effective geometry factor for protons and helium than any previous, current, or planned instrument until the Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS) or a similar detector is flown. Since no such instrument has been approved yet, none such is expected to be deployed until at least With its first flight CREAM has more than doubled the total world data of directly measured high-energy (> 1 TeV) cosmic-ray nuclei with reasonable energy resolution, and has the first high-statistics data sample allowing accurate boron to carbon ratio measurements to be expanded to the 100 GeV/nucleon region. The ultimate goal of the CREAM program is to collect at least 500 events each above 10^^ ev, for protons. He, CNO, Ne-Si and the Fe group, and to produce element spectra for all nuclei up to iron, with the proton and helium spectra extending above 10^^ ev. The CREAM instrument One of the greatest challenges in designing an instrument intended to directly measure high-energy cosmic-ray particles is the enormous dynamic range needed to cover the energy range, along with the limited means of calibration. For low-charge particles such as protons and helium nuclei, the only practical means of measuring energies above 10^^ ev is through the use of a calorimeter. This technique will work as well for heavier nuclei, but flight calorimeters are limited by the large mass required to allow a sufficient geometric acceptance for these lower flux particles. In addition, calorimeter calibrations require the use of accelerator beam particles of a known energy, which is limited by current technology to a few times 10^^ ev. This calibration was extended to about 10^^ ev by use of nuclear fragment beams. For nuclei with Z > 3, a TRD can measure the Lorentz factor (y), which when paired with knowledge of the particle mass, via charge identification, provides an energy measurement. The relatively low density of TRDs allows the large geometric coverage needed for lower flux particles with an acceptable detector weight. Since the TRD measures Y rather than energy, one can calibrate this type of instrument using a variety of particles with varying masses (e.g., electrons muons, pions and protons), so that a combination of beams can cover a range from y < 10^ and up to y > 10^, nicely covering the CREAM energy range of interest. However, TRDs cannot accurately measure highenergy protons and helium nuclei, and their energy resolution is strongly energy dependent, making it more difficult to extract accurate spectra. By combining a calorimeter and a TRD in the same experiment, one can cross-calibrate between the two for a large subsample of events, for optimal energy determination. Additionally, since the two detector types have different sources of systematic errors, combining the two reduces systematic 57

8 uncertainties of the energy measurement. The CREAM instrument was launched as a Long Duration Balloon (LDB) payload from McMurdo Station, Antarctica on December 16, 2004 and flew for a recordbreaking 42 days, with a second CREAM flight one year later (and two more flights since). The CREAM design is unique in that it obtains two independent energy measurements using a tungsten/scintillator sampling calorimeter and Transition Radiation Detector (TRD) with up to four independent charge measurements of incident particles using a novel Timing-based scintillator Charge Detector (TCD), a plastic Cherenkov Detector (CD), scintillating fiber hodoscopes, and Silicon Charge Detector (SCD). MEASUREMENTS AND THEIR INTERPRETATION Measurements of positron flux and interpretation As mentioned above, cosmic rays consist of protons (86%), a particles (11%), electrons (1 2%) and heavier elements such as carbon, oxygen, magnesium, silicon, and iron (1%), etc. Cosmic-ray electrons interact with the Galactic interstellar medium (ISM) exclusively through electromagnetic processes, such as synchrotron radiation and inverse Compton scattering, which do not significantly affect the nucleonic cosmic-ray components. For this reason electrons are a unique probe of cosmic-ray confinement and source distribution in the Galaxy. The primary e^ flux is dominated by negative electrons, from primary acceleration sites. Hadronic interactions between the nuclear cosmic rays and nuclei in the interstellar medium produce again about 10% as much in secondary negative and positive electrons (positrons), in equal proportion. Studying the flux of antiparticles such as positrons therefore carries information on cosmic-ray propagation through the Galaxy. Cosmic-ray positrons and electrons have been studied by balloon-borne intruments (like HEAT [3]), and recently by the new PAMELA satellite experiment [6]. Figure 2 shows recent measurements of the positron fraction e'^/{e~ + e+) for different experiments [7, 8, 9, 10, 11, 12]. Secondary production Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. In the low-energy region the positron fraction deviates from the predictions of a purely secondary production mechanism, the leaky-box prediction [13]. A possible explanation of this effect would come from a solar modulation mechanism that depends on the charge of the particle, and changes from one solar cycle to the next [14]. Figure 3.a shows expectations from secondary production under a solar-modulated leaky-box model. The new PAMELA results, together with older HEAT results, show a significant increase of the positron fraction in the energy range IQGeV < E < loogev, 58

9 "T 1 \ I I "T I I I I I I "T I I I I I 'd) d) 0.2 0) o (0 c o "In o Q o Muller& Tang MASS 19S9 * TS93 «HEAT94+95 ^ CAPRICE94 AMS98 A HEATOO * Clem & Evenson 2001 PAMELA 0.0 I I \ I 1 1 Energy (GeV) FIGURE 2. Positron fraction as a function of energy measured by several experiments. even though, in a purely secondary production prediction, the positron fraction is expected to decrease as energy increases. To understand this anomalous behavior it may be necessary to postulate new primary sources of positrons [15]. Solar-modulaied leaky box 380GeV/c^WIMP, KT 336GeV/c^WIMP,BE BOGeV/c^WIMP.BB i.0.02 Pulsar yrays YY interaclions Giant molecular clouds 10 Energy (GeV) FIGURE 3. HEAT positron fraction data compared with best-fit model predictions. 59

10 Sources of primary cosmic-ray positrons Annihilating dark matter WIMPs. It has been suggested that annihilating Galactichalo dark-matter WIMPs (Weakly Interacting Massive Particles) may be a source of high-energy positrons [16, 17]. Most dark matter candidates are Majorana particles, where direct annihilation into e~e^ pairs is suppressed. One exception is a model by Kamionkowski and Turner [16] in which WIMPs with mass m^ < 80 9lGeV/c^ can annihilate through resonant production of W+W~ or Z^zP pairs. The resulting electrons and positrons are propagated in a leaky-box model. The model predicts enhancements in the positron fraction near energies of m^/2 (due to direct decays of the gauge bosons into e^). In another work by Baltz and Edsjo [17], positron production by annihilating dark matter neutralinos is revisited, and a large fraction of the Minimal Supersymmetric Standard Model (MSSM) parameter space is sampled. In figure 3.a we can see a comparison of these models. Recently, Ishiwata et al. [18] studied LSP (Lightest SuperParticle) candidates such as a Bino-like lightest neutralino and the gravitino. Pair creation near discrete sources. Primary positrons could arise when e~e^ pairs are created by electromagnetic processes, for instance through the conversion of high-energy y rays in the polar-cap region of Galactic radio pulsars [19]. Another electromagnetic process would be the interaction of very high-energy y rays with optical and/or UV radiation in the vicinity of discrete sources [20], resulting in e~e^ production. We can see the two model predictions in Fig. 3.b. Positron production in giant molecular clouds. A different possibility is the generation of electrons and positrons in hadronic processes. In one model [21], hadronic cosmic rays can enter and interact within giant molecular clouds, resulting in the secondary generation of mostly 71+ and K^, which ultimately decay into muons, and thereafter into electrons and positrons. Fermi reacceleration due to fluctuations in the magnetic field in the turbulent gas could then boost the energy of the e+. See the prediction in Fig. 3.b. Other positron sources. Different primary positron sources have been suggested as well. For example, e~e^ pair production in the magnetosphere of pulsars could be followed by particle acceleration to relativistic energies in the pulsar wind driven by low frequency electromagnetic waves [22]. Also, P+ radionuclei such as ^^Co ejected during a supernova blast, possibly followed by shock acceleration in the envelope [23], could result in an enhancement of the positron fraction. IMeasurements of antiproton flux and interpretation Antiprotons, as well as positrons, constitute a rare but interesting component of the cosmic radiation. They are mostly secondary cosmic-ray particles, generated in nuclear 60

11 interactions of high-energy cosmic rays with the interstellar medium. The kinematic threshold energy for p production in p p collisions of primary cosmic-ray protons causes a p energy spectrum and a p/p ratio that decline rapidly from a few GeV towards lower energies. Solar modulation softens this "cutoff". It is important to know if there are significant additional contributions that have a different and perhaps more exotic origin. Additional sources of antiprotons might be evaporating primordial black holes (PBH), or annihilating supersymmetric (SUSY) particles. The PBH contribution would be expected to be significant at energies below 1 GeV, while SUSY particles annihilations, like neutralinos, could also affect the antiproton intensity at higher energies, above several GeV. * [:APRICEB4 O CAPWCEOS A IMAX V MAS381 a BESS [ 95lc'97} t BESS ('98) HEAT SOOO Kinetic Energy [GeV] loo.o FIGURE 4. Observed p/p ratios by various experiments. Figure 4 shows observations by HEAT and other experiments of the p/p flux ratio at the top of the atmosphere, compared with model calculations for secondary production, such as the leaky-box model [24] (SMR), diffusion models [25] (MSR-1, MSR-2), and primary antiproton production. Possible primary antiproton contributions come from evaporating PBH [26] (MMO) and from neutralino annihilation (J&K) [27]. IMeasurements of nuclei flux and interpretation Cosmic-ray nuclei are expected to also produce secondary nuclei by spallation reactions during interstellar transport. The CREAM experiment [4, 5] measurements illustrate this effect well [28]. In Figures 5 and 6 we show CREAM measurements of the Boron to Carbon, Nitrogen to Oxygen, and Carbon to Oxygen ratios vs energy, after corrections. Also shown are older measurements of the HEAO satellite and CRN space 61

12 shuttle experiments. The data are consistent with a propagation diffusion parameter of 0.6 (illustrated by the solid lines in the Figures), to be compared with expectations for values of 0.33 (long-dashed) or 0.7 (short-dashed). This is significant, since understanding the diffusion parameter allows the measured cosmic-ray spectrum at Earth to be deconvoluted to correct for propagation effects, and therefore obtain the cosmic-ray spectrum at the supernova shock acceleration site. Figure 6 also illustrates that the spectra are consistent with a power law c [Energy/n)~^ with a spectral index a = 2.6. Energy(GeV/n) Energy(GeV/n) FIGURE 5. CREAM measurements of ratio of nuclei. Left: B/C ratio. Right: N/0 ratio. -s 10m I I I iiiiil I I I iiiiii I I I iiiiii i r n) O o -f 10^ uj I I I I 10 10'' 10'' Energy(GeV/n) 10 10" 10" Energy(GeV/n) FIGURE 6. power law. CREAM measurements of ratio of nuclei. Left: C/0 ratio. Right: data consistent with a 62

13 SUMMARY AND CONCLUSIONS The observed cosmic-ray radiation at high energy can be generated by acceleration of stellar wind particles in supernova shocks. The data suggest that a small additional antimatter component may be present that cannot be explained by secondary production mechanisms. An interesting suggestion is that annihilating dark matter particles in the Galactic halo, perhaps supersymmetric particles, are a source of high-energy positrons. A primary contribution to the positron intensity above a few GeV cannot be ruled out. The CREAM investigation is designed to measure cosmic-ray composition to the supernova energy scale of 10^^ ev in a series of long-duration balloon flights in Antarctica. Directly measured are high-energy cosmic-ray nuclei from hydrogen to iron, in the energy range from about 1 TeV to over 1000 TeV. CREAM is unique in that it employs redundant measurements of both energy and charge of the incident nuclei. CREAM is the first instrument to have both detectors in the same payload for high energy (>TeV) cosmic-ray composition measurements. This allows in-flight cross calibration of the two techniques and, therefore, better energy determination. The CREAM TRD measures Z > 3 particles with energy resolution of ^ 15% for carbon and ^ 7% for iron. The CREAM calorimeter measures all elements with energy resolution better than 45% for all energies. The CREAM data set is expected to grow even more interesting in the coming years. The HEAT instrument was developed for a series of observations in cosmic-ray astrophysics that require the use of a superconducting magnet spectrometer. In addition to the spectrometer, a combination of time-of-flight scintillators, a transition radiation detector, and an electromagnetic shower counter, provides particle identification, energy measurement, and powerful discrimination against the large background of protons. With the HEAT balloon-borne instrument were measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Two measurements of the cosmic-ray positron fraction as a function of energy were made using the HEAT balloon-borne instrument. The results confirm that the positron fraction does not rise significantly with energy above ^10 GeV, although a small excess above purely secondary production cannot be ruled out. The recent PAMELA positron measurements seem to indicate a larger excess than seen by HEAT previously. At low energies the positron fraction is more difficult to interpret owing to possible charge dependence in the level of solar modulation. In conclusion, the HEAT positron measurements indicate a subtle feature, which cannot be caused by the atmospheric corrections applied to the data or any other known systematic effect. As confirmed and expanded by PAMELA, this feature could be evidence for a new exotic source of positrons, especially at energies beyond about 7 GeV. The exact nature of this source is likely to remain controversial for some time. ACKNOWLEDGMENTS We thank the organizing committee of the Third International School of Cosmic Rays and Astrophysics, Lima, Peru. 63

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