The Science of LHCf

Cosmic ray illustration
Illustration of high-energy cosmic rays striking the Earth's atmosphere. Click image for larger version. Image © CERN
The LHCf collaboration may only have 25 members, but its work could have a big impact on cosmic-ray research. LHCf scientists study proton-proton collisions at the LHC that are similar to the collisions of ultra-high-energy cosmic rays with the earth's atmosphere. Their results contribute to the measurement of the energy of these cosmic rays by the large cosmic-ray experiments at the Pierre Auger Observatory in Argentina and the Telescope Array in Utah.

Cosmic rays are charged particles—mainly protons—that travel through space. Most of these particles are well understood, but a tiny number with ultra-high energies (1018 eV to above 1020 eV) remain a mystery. Ultra-high-energy cosmic rays with energies above 1020 eV are rare—only about one per square kilometer per century hits the earth. Experiments like the Pierre Auger Observatory and the Telescope Array hope to resolve the mysteries of how and where these ultra-high-energy cosmic rays are produced. Scientists do not know if such cosmic rays can be understood in terms of our existing knowledge of physics or if solving the mystery will require new physics.

LHCf scientists make key measurements that the Pierre Auger Observatory and the Telescope Array will use to simulate and interpret the air showers—cascades of secondary particles—produced by ultra-high-energy cosmic rays.

When cosmic rays enter the earth's atmosphere, the charged particles collide with nuclei in the upper atmosphere and produce secondary particles, including neutral pions, which in turn generate air showers. These shower particles travel forward from the original collision in approximately the same direction as the original ray. Similarly, many of the proton-proton collisions at the LHC produce neutral pions in the same forward direction as the proton beam.

The LHCf detectors are placed on either side of the ATLAS experiment about 140 meters from the interaction point at a zero degree collision angle. The detectors, made of tungsten plates and plastic scintillators, can accurately measure the number and energy of neutral pions produced in the forward direction in ATLAS collisions.

The LHCf experiment will explore cosmic-ray-like collisions equivalent to a cosmic ray of 1017 eV striking the atmosphere, nearly 1,000 times more energy than in previous experiments.