FOM-program proposal 2007

"The origin of cosmic rays", (Nikhef, IMAPP, KVI)

Objectives and focus of the program

(extracted from the proposal)

One of the key unresolved issues in astroparticle physics concerns the unknown origin of cosmic rays. Cosmic rays are a collective name for a variety of subatomic particles that are bombarding the outer layers of the Earth’s atmosphere continuously. These particles are originating from unknown distant sources in the universe, and reach energies well in excess of the highest energies that can be produced in the most advanced man-made particle accelerators. In fact, cosmic rays are the only observed ultra high-energy (UHE) particles on Earth, i.e. with energies in excess of 1016 eV. Still, the origin of these cosmic rays and the mechanism(s) leading to such ultra-high energies are largely unknown. We propose to use two novel and complementary techniques to search for the origin of ultra high-energy cosmic rays. These observational techniques will be exploited (and further developed) in an effort to discover (for the first time) extra-galactic point sources of cosmic rays in the universe and measure their energy spectrum. This is the main objective and focus of the present proposal.

The proposed research program will enable us to study several frontier questions in the emerging field of astroparticle physics:

  • Ultra-high energies. The measured cosmic-ray energy spectrum extends to energies of 1020 eV and beyond. How do cosmic rays acquire such energies? It seems natural to associate these energies with the most energetic explosions in the universe such as Active Galactic Nuclei, Gamma Ray Bursts, Micro-quasars or Supernova Remnants. Evidence in support of this hypothesis can be obtained by searching for ultra-high energy cosmic rays originating from (optically known) sources of this kind.
  • Dark Matter. The motion of stars within galaxies and the motion of galaxies within clusters provide evidence for the existence of large amounts of dark matter in the Universe. In fact, all current estimates, including those derived from satellite-based observations of the cosmic microwave background radiation indicate that Dark Matter constitutes about 23 ± 4% of the total energy content of the Universe. Can part of the (high-energy) cosmic rays be attributed to the annihilation of two dark matter particles? As it is becoming increasingly likely that Dark Matter is composed of particles not contained in the Standard Model of particles and fields, such particles may decay or annihilate giving rise to the production of photons, quarks and neutrinos that can be observed on Earth.
  • GZK limit. Does the cosmic-ray energy spectrum extend beyond 5 1019 eV, which is the highest possible energy a proton can maintain before being slowed down due to collisions with the cosmic microwave background radiation? If cosmic rays are observed well above this so-called Greisen-Zatsepin-Kuzmin (GZK) limit, they will provide evidence for entirely novel phenomena such as relatively nearby cosmic accelerators or physical effects beyond the realm of the Standard Model.

To address these questions the sources of cosmic rays need to be identified, and the energy spectrum of the cosmic rays needs to be determined up to the highest possible values. Given the large range of the energies involved two complementary probes are needed: neutrinos in the energy domain up to 1016 eV and air showers induced by protons or heavier atomic nuclei for energies above 1018 eV. In practice, the proposed searches will be carried out with the Antares deep-sea neutrino telescope, which is presently being built on the bottom of the Mediterranean Sea near Toulon at a depth of 2480 m, and the almost completed Pierre Auger air-shower Observatory (or Auger for short) in western Argentina near the city of Malargüe. As Antares and Auger are largely viewing the same part of the (southern) sky, these observatories do not only cover a complementary energy domain, but also allow for the observation of the same cosmic sources. In fact, the detection of neutrinos and ultra-high energy protons from the same cosmic source would represent a profound breakthrough and issue – at the same time – the start of particle astronomy.