One of the most pressing mysteries in astroparticle physics is the composition and origin of cosmic rays at energies around 1 EeV = 1018 eV. In contrast to charged particles, deflected in galactic and extragalactic magnetic fields, neutral particles such as neutrinos, neutrons, or photons point back to their production site. One way to learn more about sources of ultra-high energy cosmic rays is therefore to search for an accumulation of events from specific directions. This is akin to astronomical observations of a distant galaxy, say, made with an optical telescope collecting photons of visible light.
100 years after their prediction by Albert Einstein, Gravitational Waves (GW) were detected in 2015 by the LIGO detectors. With the surface detector of Auger we have searched for ultrahigh-energy neutrinos in temporal and spatial coincidence with such remarkable events.
The highest energy cosmic rays remain elusive and mysterious, and their study requires extraordinary efforts. At the Pierre Auger Observatory in Argentina, the giant air showers of particles created by these cosmic rays are detected when they slam into the ground by a large array of water tanks equipped with electronic detectors. But on dark nights they are simultaneously detected by telescopes sensitive to the faint sky glow left by the air showers. The new report by the Pierre Auger Collaboration correlates in detail the signals in the water tanks with those from the telescopes. The correlation is uniquely sensitive to the presence in the primary beam of nuclei with different masses, and is used in particular to help resolve how many types of atomic nuclei contribute to the cosmic ray flux.
Maxwell’s unified description of electric and magnetic phenomena is one of the greatest achievements of 19th century physics. Free magnetic charges and currents are not allowed in Maxwell’s equations, a consequence of their apparent absence in Nature. However, their existence would naturally explain the quantization of electric charge, as first noted by Dirac in 1931.
Cosmic rays are energetic particles, mostly atomic nuclei, raining down upon the Earth from the depths of the cosmos. Understanding their detailed nature and origins remains a primary goal in modern-day astroparticle physics.