The LHCf collaboration has measured the production spectrum of photons using the highest-energy accelerator beams in the world, at CERN’s LHC machine. With proton beams at 3.5 TeV the total collision energy is equivalent to when protons of 2.5 × 1016 eV strike a stationary target, which is an energy region that is of interest to cosmic-ray physicists.
The LHCf experiment consists of two independent calorimeters installed on either side of the ATLAS interaction point at the LHC. Using data obtained in 2010 during proton runs at 7 TeV in the centre-of-mass, the collaboration has measured the photons emitted into two very forward regions, that is, close to zero degrees to the beam direction, in the pseudo-rapidity ranges from 8.81 to 8.99 and from 10.94 to infinity (Adriani et al. 2011). To minimize contamination from beam-gas background and pile-up events, the team chose a limited but best dataset corresponding to an integrated luminosity of 0.68 nb–1. After selecting single photon-like events in common pseudo-rapidity ranges, they obtained consistent energy spectra from two detectors.
The collaboration has compared its data with the predictions from various hadron interaction models used in the study of cosmic-ray air showers, together with PYTHIA 8.145, which is popular in the high-energy-physics community. As the figure shows, there is significant deviation between the data and model above 2 TeV in the higher rapidity region. Three well known models – DPMJET 3.04, QGSJET II-03 and PYTHIA 8.145 – predict significantly higher photon yields than the experiment finds above 2 TeV, but agree reasonably well with the data at 0.5–1.5 TeV. The other models – SIBYLL 2.1 and EPOS 1.99 – do not predict such high photon yields but predict a smaller yield over the whole energy range. The difference is less marked in the lower rapidity region, but nevertheless none of the models shows perfect agreement with data.
The energy spectra of collision products at high-rapidities are crucial to understand correctly the development of cosmic-ray-induced air showers. Following recent notable improvements in observations of ultra-high-energy cosmic rays (UHECR), it is becoming increasingly important to reduce the uncertainty. The impact of the current LHCf results on cosmic-ray physics is now under study as the collaboration works together with theorists on further analyses of the data on neutral pions and neutrons. The data will also cast light on the energy dependence of hadron interactions and the extrapolation into the UHECR energy range. At the same time, the collaboration is studying the feasibility of data-taking during ion collisions (ion–ion and/or proton–ion), which would give a better simulation of cosmic-ray-air collisions.