Similarities between air showers and ATLAS forward physics
Strong interactions in a regime of low momentum transfer (so-called soft interactions) cannot be treated perturbatively, which makes the modelling of such processes strongly dependent on experimental input from hadron colliders. In hadron-hadron collisions, e.g. at the LHC, such soft hadronic interactions manifest themselves by the creation of particles in the very forward regions (which means at large rapidities). This process is the same when a cosmic particle hits the atmosphere and interacts with the atoms of the air, creating a cosmic air shower. The picture here shows a simulated air shower over the Auger observatory (courtesy Chantelauze/Staffi/Bret).
Measurements with the forward detectors of and around ATLAS
These forward-pointing collisions can be measured either by calorimeters in the ultra-forward region of ATLAS, ZDC, or the LHCf detector which can detect neutral particles or by tracking devices located close to the beam like ATLAS AFP and ALFA detectors, which are used for the detection of scattered protons. The location of these detectors can be seen in the sketch below (by C. Leitgeb). The measured particle spectra and cross-sections will help to tune the currently available phenomenological models for soft hadronic interactions. The improved accuracy of such models would benefit the simulation of air showers induced by cosmic particles as well as the understanding of strong interaction background in proton-proton collisions at the LHC (so-called pile-up). Part of the group is working on the preparations of such measurements with the aforementioned detectors for the current run at the LHC for proton-proton collisions as well as for the foreseen proton-oxygen collisions. Furthermore, we are cooperating with the DESY Zeuthen gamma-ray group to exploit synergies in that topic.
