LHCf sheds new light on cosmic rays

The energy spectrum of the single photon obtained using data from the LHCf experiment has turned out to be very different from that predicted by the theoretical models used until now to describe the interactions between very high-energy cosmic rays and the earth's atmosphere. The consequences of this discrepancy for cosmic ray studies could be significant.


Artistic impression of cosmic rays entering Earth's atmosphere. (Credit: Asimmetrie/Infn).

It took physicists by surprise when analysis of the data collected by the two LHCf calorimeters in 2010 showed that high-energy cosmic rays don't interact with the atmosphere in the manner predicted by theory.

The LHCf detectors, set up 140 metres either side of the ATLAS interaction point, are dedicated to the study of the secondary particles emitted at very small angles during proton-proton collisions in the LHC, with energies comparable to cosmic rays entering the earth's atmosphere at 2.5x1016 eV. The aim of the experiment is to refine the models currently used to study very high-energy cosmic radiation. And according to the recent results of the LHCf experiment, these models will indeed require some changes. LHCf Deputy Spokesman Oscar Adriani explains: "We have used the data recorded to measure the energy spectrum of the single photon, which derives from the decay of a neutral pion appearing in the particle shower formed when very high-energy cosmic rays interact with atmospheric gas." The researchers can use studies of the single photon to extrapolate information on the physical processes induced by cosmic radiation. 

A comparison between the different Monte-Carlo models and experimental data gathered by LHCf in 2010. 

The results of this work have caused quite a stir because of discrepancies with respect to the results predicted by the most common Monte-Carlo models used for the study of cosmic rays. The discrepancies appeared in the single photon spectrum for energies above 1.5 TeV. Beyond this value, the energy distribution no longer corresponds to that anticipated by the models. "Thanks to the LHC, we've been able to explore a hitherto inaccessible energy region," Oscar Adriani relates. "Given the significant disparities between the theoretical predictions and our experimental data, I believe that physicists specialising in this research field will be obliged to re-visit their results in the light of this new information." 

Although the jury is obviously still out, the members of the LHCf collaboration expect this news to cause some upheaval in the field of cosmic rays in the not-too-distant future.


by Anaïs Schaeffer