Cosmic Rays

Viktor Hess and Carl David Anderson were awarded the 1936 Nobel prize in physics for the discovery of Cosmic Rays¹ (CRs) and the experimental verification of the existence of the positron predicted by P.A.M. Dirac in 1928 (Dirac [1928]). Viktor Hess deployed an electrometer in an air balloon to observe that the intensity of ionisating radiation increases with height above the earth surface (Hess [1912]). This result was found independently in other experiments and was in conflict with the hypothesis that natural radioactivity in the earth is the source of the observed ionizing radiation. In contrast, the measurements pointed towards the existence of previously unexpected exotic sources of high energy radiation in outer space. After establishing that there is a source of ionizing radiation outside the earth in the 1920s, the measurements of the composition and the question of the nature of the ionizing radiation received much attention in the 1930s.

Carl D. Anderson discovered the positron in a Wilson cloud chamber installed in a strong magnetic field while trying to measure the mass and charge of CRs by means of their deflection in a magnetic field (Anderson [1932]). Subsequently also the muon (initially named mesotron) was detected in 1937 with a platinum shielded cloud chamber by C.D.

Anderson and S. Neddermeyer (Neddermeyer and Anderson [1937]) and it became clear that the investigation of CRs is a prime source of information on particles and firmly established the field of particle physics. Around the time of the discovery of the muon, two essentially equivalent models of how secondary particles can be produced in so called air showers by primary CRs were published (Carlson and Oppenheimer [1937], Bhabha

1The expression Cosmic Rays was coined by R. A. Millikan who interpreted them as high energy photons (Millikan [1925]).

and Heitler [1937]). These suggested that the detected CRs are secondary products of the primary CRs interacting in the earth atmosphere. The models of air showers predicted that the extension of air showers increases with energy and enabled Pierre Auger to measure CR events with energies of ∼ 10¹⁵ eV in the late 1930s (Auger et al. [1939]) by deploying Geiger counters in coincidence circuits. The order of magnitude of the inferred primary cosmic ray energy was highly surprising as it was ’actually impossible to imagine a single process able to give a particle such an energy’ (Auger et al. [1939]).

The nature and origin of CRs became even more mysterious and the newly discovered particles as well as their energy spectra attracted the work of many physicists by the end 1930s. Investigations of the influence of the earth magnetic field on CRs showed∼1941 that the majority of CRs are protons. The intensive investigation of CRs triggered the isolation of problems and formation of new branches of physics. Astroparticle physics evolved to be more concerned with the question of the origin and nature of primary CRs.

Particle physics in contrast developed from the investigation of the reaction products and constitution of CRs. However, still there is a considerable overlap of particle and astroparticle physics. The search for dark matter certainly defines one of the overlapping regions as will be discussed in the course of this thesis.

The origin of CRs is by now still not found, however, important steps for answering this puzzling question have been achieved. The establishment that there is a non-vanishing magnetic field in the inter stellar medium of the Milky Way in the early 1950s destroyed the initial hope of detecting the sources of CRs by measuring their arrival direction as CRs are deflected by the galactic magnetic field and thus their arrival direction is nearly isotropic²after many deflections, except for low energy CRs (E .1 GeV) shielded by the solar and earth magnetic field and possibly for ultra high energies E 100 GeV. The energy spectra of primary cosmic rays follow in general a power lawdΦ(E)/dE∼E^−Γ. This energy dependence is a major hint in favor of modeling the acceleration of CRs by the Fermi theory of CR acceleration (Fermi [1949]) and derived models (see Hillas [2005]

for a review). Those models naturally lead to power law spectra where the spectral index reflects the physical properties of the acceleration and propagation conditions.

The primary CR proton energy spectrum with E & 30 GeV, i.e. the particle flux Φ differential in energy above the solar wind influence, up to∼100 TeV, i.e. below the so called ’knee’, is described by

dΦ(E)

dE = (1.8±0.1) protons

m²s sr GeV·10⁴(E/GeV)^−2.7±0.1. (1.1)

2Anisotropies in the∼100 GeV to the∼10 TeV energy range are at the level of∼0.3 % for the dipole amplitude (see IceCube Col. [2010] and references therein).

1.1 Astroparticle andγ-ray Astrophysics Compatible within the given errors³ on the spectral index is the energy dependence of the primary cosmic ray helium flux, however, primary protons constitute ∼ 80% and primary helium∼15% (Eidelman et al. [2004]) of the cosmic ray flux. The residual flux is from heavier elements as well as electrons⁴. Electrons follow a steeper spectrum than hadronic CRs reflecting their fast energy loss mainly due to synchrotron radiation in the galactic magnetic fields and inverse Compton scattering on low energy radiation fields.

The CR electron spectrum in the energy range ∼10 GeV−40 TeV is well described by a broken power law

a rapid transition of the spectral index from Γ₁ to Γ₂ occurs in the measured electron spectrum at the energy Eb = (0.9±0.1) TeV. For more information on the electron cosmic ray spectrum measurement see H.E.S.S. Col. [2008] and H.E.S.S. Col. [2009].

The stated precision of the flux normalizations and spectral indices agrees in order of magnitude for the proton and electron spectrum. The more complicated structure of the electron spectrum regarding the spectral break hints towards a more complicated production mechanism and propagation history for CR electrons compared to CR pro-tons. No convergence in modeling the CR electron spectrum has yet been reached but convincing results can be obtained by assuming that the CR electron spectrum up to energies of ∼ 1 TeV is produced in an ensemble of nearby pulsar wind nebulae where the cooling time of the electrons due to inverse Compton scattering and synchrotron radiation limits the diffusion length of CR electrons to a few hundred parsec (Grasso et al. [2009]) from their source. For electrons with an energy above∼1 TeV, the secondary production of electrons and positrons in decays of charged pions produced in interactions of hadronic CRs with ambient gas can lead to the harder spectrum (see Grasso et al.

[2009] for this but also for alternative interpretations).

The study of the spectrum and the composition are main sources of direct information about the origin and nature of cosmic rays. At the highest energies, also the anisotropy in the primary CR arrival direction could help to identify their astrophysical sources.

However, in the∼TeV scale the CR arrival directions are as stated above very uniformly distributed and indirect methods to study the CR origin have to be deployed. Particles traveling without deflection from astrophysical particle accelerators can back-trace the acceleration mechanism and origin of CRs. The detection of neutrinos orγ-rays produced

3The errors do not represent the best achieved precision of individual experiments sensitive to dif-ferent energy ranges. However, within the given errors many difdif-ferent experiments operating with independent techniques and different circumstances can be well described. Individual experiments reach a factor ∼ 10 better precision and recently also report spectral breaks in the energy range

∼1 GeV−1 TeV (PAMELA Col. [2011]) and significant differences in the spectral index of the helium and proton CR spectra in the quoted energy range. Those effects are, however, not significant within the error on the spectral index given above.

4Electrons are in this special case understood as electrons and positrons. The positron fraction in the primary CR flux is∼10 % at 100 GeV (PAMELA Col. [2009], Fermi Col. [2012], AMS-II Col. [2013]).

f.i. in charged or neutral pion decays, which can in turn be produced in interactions of hadronic CRs with interstellar gas, is an important technique to yield information on the origin of CR. Recently, the detection of γ-rays with energies around the kinematic cut off for neutral pion production in proton matter interactions towards two supernovae remnants with the Fermi satellite gave the first ’direct evidence that cosmic-ray protons are accelerated in SNRs’ (Ackermann et al. [2013]).