The LHC experiments are mostly on a very grand scale, with huge detectors and collaborations of as many as 2000 people; however, LHCf, like TOTEM, is quite special. The detectors are much smaller and LHCf has an equally small collaboration of just 22 people. The collaboration led by Yasushi Muraki, with members from Japan, Italy and the US, has just finished testing its detectors.
The focus of the experiment is to study the forward moving particles in the proton-proton collisions at the LHC. This will be used to compare the various shower models widely used to estimate the primary energy of ultra high-energy cosmic rays, with energy in the region of 1019 eV (10 billion billion electronvolts). When the proton-proton collisions occur at the LHC pions are produced just as in a cosmic ray air shower. The amount of these secondary particles produced at the LHC can be measured accurately with the LHCf detectors, since the energy and direction of the primary beam is well known. The data will then be compared with the models used by the cosmic ray community.
Although discovered as long ago as 1912 by the Austrian physicist, Victor Hess, cosmic rays remain mysterious. In particular, physicists would like to know more about the origins of the very high energy cosmic rays, up to 1020 eV that have been observed during recent decades. Some important experiments, such as the Pierre Auger Cosmic Ray Observatory in Argentina (See CERN Courier, July/August 2006), the Telescope Array experiment in the US and the HESS experiment in Namibia are dedicated to this research (See CERN Courier, February 2005). The LHCf experiment aims to give some valuable data to input into these studies; many of the physicists participating in LHCf are also involved in these and other projects related to cosmic rays.
The detectors of LHCf will be placed on either side at 140 m from the ATLAS interaction point. This location will allow for observation of particles at nearly zero degrees to the proton beam direction. The detectors consist of two towers of sampling calorimeters designed by Katsuaki Kasahara from the Shibaura Institute of Technology. Each of them is made of tungsten plates and plastic scintillators of 3 mm thickness for sampling.
Many of the physicists from LHCf have reunited from the former SPS experiment UA7, which also focused on forward physics. The LHCf experiment will be simulating cosmic ray collisions nearly 1000 times more energetic than UA7 was able to access. The energy of proton collisions in the LHC will be equivalent to a cosmic ray of 1017 eV smashing into the atmosphere. Therefore, LHCf will use the LHC beams to test the interaction models of cosmic rays to higher accuracy.
Did you know?
Cosmic rays are charged particles, mainly protons, but also alpha particles (helium nuclei) or heavier nuclei that bombard the Earth's atmosphere from outer space. These nuclei collide with the nuclei in the upper atmosphere producing many secondary particles, which in turn collide with other nuclei in the lower atmosphere. This process continues in a cascade, producing a shower of billions of particles reaching the ground.
Cosmic rays show a wide range of energy. The low energy cosmic rays are plentiful (many thousand per square metre every second), many of which come from the sun. The highest energy cosmic rays, up to 1020 electronvolts, are very rare, arriving at a rate of one per square kilometre per century! The source of ultra high energy cosmic rays remains a mystery, as the primary ray seems to come from all directions.
This, when we had thought science was at an end?
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