Insights into hadronic physics beyond LHC energies from studies of cosmic rays with the Pierre Auger Observatory

Prof Alan Watson, University of Leeds

Friday 22 January 2016, 1330-1500
Lecture Theatre 1, Fylde College

The Pierre Auger Observatory was designed to study the properties of cosmic rays with energies above 10e18 eV to beyond 10e20 eV and with it results of importance for particle physics and astrophysics have been obtained. The astrophysical interest lies in discovering the sources of these particles and for this one must measure the energy spectrum and the mass composition: the measurements rely, to a lesser and greater extent respectively, on knowledge of hadronic physics, such as cross-section, multiplicity and inelasticity, at centre-of-mass energies up to ~ 30 times above what is accessible at the LHC. After describing the Auger Observatory, I will discuss our calorimetric measurement of the primary energy which can be made with an accuracy of ~ 10%, independent of the unknown hadronic physics, and the astrophysical implications of the result which also make use of measurements of the primary mass which are challenged by our lack of knowledge of the hadronic physics, but consistent and surprising features about the variation of the mass with energy are emerging – although dramatic changes in hadronic features at extreme energies, such as a huge rise the p-air cross-section, cannot be excluded.

To help clarify the situation, a measurement of the p-air cross-section at 57 TeV has been made at the Observatory which is found to agree well with extrapolations from machine energies. However discrepancies are found between what is predicted and what is observed in the muon properties of showers. These observations call for further refinement of the extrapolations from LHC energies and may therefore lead to new insights.