Question

What are cosmic particles

Answer 1

In August 1912, Austrian physicist Victor Hess made a historic balloon flight that opened a new window on matter in the universe. As he ascended to 5300 metres, he measured the rate of ionization in the atmosphere and found that it increased to some three times that at sea level. He concluded that penetrating radiation was entering the atmosphere from above. He had discovered cosmic rays.

These high-energy particles arriving from outer space are mainly (89%) protons – nuclei of hydrogen, the lightest and most common element in the universe – but they also include nuclei of helium (10%) and heavier nuclei (1%), all the way up to uranium. When they arrive at Earth, they collide with the nuclei of atoms in the upper atmosphere, creating more particles, mainly pions. The charged pions can swiftly decay, emitting particles called muons. Unlike pions, these do not interact strongly with matter, and can travel through the atmosphere to penetrate below ground. The rate of muons arriving at the surface of the Earth is such that about one per second passes through a volume the size of a person’s head.

A new world of particles

Studies of cosmic rays opened the door to a world of particles beyond the confines of the atom: the first particle of antimatter, the positron (the antielectron) was discovered in 1932, the muon in 1937, followed by the pion, the kaon and several more. Until the advent of high-energy particle accelerators in the early 1950s, this natural radiation provided the only way to investigate the growing particle "zoo". Indeed, when CERN was founded in 1954, its convention included cosmic rays in the list of scientific interests. But even though accelerators came to provide the best hunting ground for new particles, the physics of cosmic rays is still widely studied.

Answer 2

Showers of high energy particles occur when energetic cosmic rays strike the top of the Earth's atmosphere. Most cosmic rays are atomic nuclei: most are hydrogen nuclei, some are helium nuclei, and the rest heavier elements.