The fluxes of cosmic-ray electrons (e−) and positrons (e+) reaching the Earth are orders of magnitude lower than the fluxes of protons and other nuclei, as can be seen in Figure 1.1. The mean free path of electrons in the Galaxy is also much smaller.
Conventional cosmic-ray propagation models suggest that most of these electrons (and all the positrons) are of secondary origin, resulting from the interaction of protons and other nuclei in the interstellar medium. However, recent measurements from different experiments have questioned the validity of those models. Figure 1.2 shows the electron and positron spectra measured by different experiments. The spectral index of the positron flux increases above 10 GeV, while the spectral index of the electron spectrum decreases. As a result, the ratio of positrons to electrons (or to all electrons, e−+e+) increases, which is not consistent with being of secondary origin, at least within the frame of the conventional cosmic-ray propagation models [179]. Many new theories have been proposed to explain this anomaly. Still, some of them hold the idea that electrons are mainly of secondary origin and that the anomaly can be explained without
1.3. COSMIC-RAY ELECTRONS 13
Figure 1.2: Electron (top) and positron (bottom) spectra reported by AMS-02, compared with measurements from other instruments. Figure from [109]
adding a primary source of positrons [76]. But more commonly it is explained by setting a new primary source, both for electrons and positrons. Pulsars [180] and Dark Matter annihilation/decay [190] are the most popular candidates for those sources. A precise measurement of the positron spectrum between hundreds of GeV and a few TeV could be essential to favour (or disfavour) one of these hypotheses. The central discussion of this thesis is focused only in the question about the origin of the (hadronic) cosmic rays and in the following chapters I will “forget” about the electrons. A rather unique and creative, but unfortunately also very challenging, strategy to measure this spectrum that was studied as part of this thesis is described in Appendix C.
Chapter 2
Searching for sources of cosmic rays
On their journey from their hypothetical source to the Earth, galactic cosmic rays diffuse through the ISM, changing their trajectories as dictated by the magnetic fields they encounter. Hence, when they reach the Earth their arrival direction has lost all information about their original source. Actually, at least up to energies close to the knee, the incoming cosmic-ray flux is nearly isotropic1. Furthermore, cosmic rays cannot reach the Earth surface, as they interact with the atmosphere way above the ground, which makes their detection a challenging task.
In this chapter I will briefly discuss the different detection methods of cosmic rays that were used to obtain the well known spectrum presented of Figure 1.1. Then I will show how gamma rays represent the best opportunity to identify cosmic-ray sources. I will also discuss the connection between accelerated charged particles and gamma rays and how the latter can be detected. But since gamma rays are not the only cosmic-ray messengers, the last section discusses the possibilities that neutrino astronomy might provide in order to help unveiling the mystery of the origin of cosmic rays.
2.1 Direct and indirect detection of cosmic rays
As cosmic rays interact high in the atmosphere, their direct detection is only possible near its top or in the outer space. With balloons like BESS [30], CREAM [25] or the more recent SuperTIGER [72] or with space-born instruments like PAMELA [164], AMS-02 [19] or CALET [16] it is possible to measure the chemical and isotopic com-position of cosmic rays up to a few tens of TeV. A good understanding of the spectra of the different species can be essential to understand the nature of the acceleration processes. Different detection techniques are used in these experiments to infer the energy and charge of the incoming particle. The main drawback of these kind of mea-surements is their limited detection area and/or flight duration in the case of balloons.
This becomes critical at energies starting at ∼100 TeV, where the cosmic-ray event rate is∼5 m−2 sr−1 day−1 [103]. Thus, to access the energies close to the knee we are interested in, different strategies are needed.
All cosmic-ray measurements at the highest energies come from ground-based ex-periments that indirectly detect cosmic rays from the products of their interaction with
1There is a small but still measurable spatial anisotropy. The relative amplitude of this anisotropy is 10−4−10−3 (see, for instance [9, 189])
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pions. The products of the interaction will also create more particles and so on. A sin-gle primary cosmic ray can produce several thousands of secondary particles that move downwards in the atmosphere developing a cascade, known as Extensive Air Shower (EAS, Figure 2.1). An EAS develops over hundreds of meters in width and several kilometers in length and achieve their maximum, in terms of number of particles, at heights of 8 to 12 kilometers above seal level.
Charged pions may interact with other nuclei or decay into muons and neutrinos.
As it will be shown in Section 2.2.1, neutral pions decay quickly into two gamma rays that can initiate a special type of particle cascade known as electromagnetic shower (left panel in Figure 2.1). The interaction between a gamma ray and the atmosphere creates an electron-positron pair. These new particles emit new gamma rays through bremsstrahlung (see Section 2.2.1) that will produce more electron-positron pairs and so on. The created particles are less energetic on every step. When their energy is such that the bremsstrahlung probability is lower than the energy losses through ionization the shower stops producing new particles. When the primary particles are gamma rays, electrons or positrons, they develop this kind of showers.
There is one extra key feature about EAS. As a large fraction of the created parti-cles travel faster than the speed of light in the atmosphere, a flash of Cherenkov light is produced by the medium. Indirect cosmic-ray detection techniques work by detecting from ground some of the secondary particles produced in the showers and/or by col-lecting the Cherenkov light. Typically from the number of detected electrons Ne and muons Nµit is possible to reconstruct the energy of the primary cosmic ray, while from the ratio Ne/Nµ one can estimate its mass [131]. Experiments like KASCADE [52], KASCADE-Grande [53] or EAS-TOP [92] use shielded and unshielded scintillators to discriminate between electrons and muons. The Pierre Auger Observatory [166], that looks at UHECRs, uses water Cherenkov detectors: relativistic particles that encounter Auger water tanks, produce Cherenkov light that is then collected with a photomulti-plier tube (PMT). In that case the discrimination between muons and electrons come from the number of Cherenkov pulses produced within the same event and their inten-sity. IceTop [3] uses ice instead of water and measures the electromagnetic component of the showers in coincidence with muon detectors in IceCube. A combination between surface and underground detectors is also used by EAS-TOP to identify the nature of the detected particles. Other techniques aim to measure the lateral (HEGRA [123], CASA-BLANCA[100]) and longitudinal (fluorescence detectors of Pierre Auger [165], Telescope Array [187], HiRes[5]) development of the showers.
In any case, in indirect detection methods the ability to deduce nuclear composi-tion relies on the theoretical understanding of the shower development and the hadronic interactions that occur within the cascade [103]. The measured observables are inter-preted in terms of a primary mass and energy by comparison to air shower Monte Carlo simulations. Then, the energy estimation of the primary particle depends on the models of hadronic interactions considered in the simulations. Considering this fact it is remarkable that all the experiments exhibit a similar spectral shape near the knee, even despite the different absolute normalization and detection methods employed by each of them [129].
Thanks to indirect cosmic-ray detectors, the existence of the knee has been largely