Modelling the models

By analysing the production of mesons in the forward region of LHC proton-proton collisions, the LHCf collaboration has provided key information needed to calibrate extremely high-energy cosmic ray models.


Average transverse momentum (pT) as a function of rapidity loss ∆y. Black dots represent LHCf data and the red diamonds represent SPS experiment UA7 results. The predictions of hadronic interaction models are shown by open boxes (sibyll 2.1), open circles (qgsjet II-03) and open triangles (epos 1.99). Among these models, epos 1.99 shows the best overall agreement with the LHCf data.

LHCf is dedicated to the measurement of neutral particles emitted at extremely small angles in the very forward region of LHC collisions. Two imaging calorimeters – Arm1 and Arm2 – take data 140 m either side of the ATLAS interaction point. “The physics goal of this type of analysis is to provide data for calibrating the hadron interaction models – the well-known ‘Monte Carlo codes’ – that are used in the study of extremely high-energy cosmic rays,” explains Lorenzo Bonechi, an INFN researcher based in Florence and a member of the LHCf collaboration.

Around the world, several experiments are dedicated to these kinds of measurements, but none of them can reach the precision of LHCf. At the current LHC energy, the LHCf detectors can measure forward high-energy inclusive-particle-production cross-sections of photons, neutrons, and other neutral mesons and baryons with an unprecedented accuracy.

Among the many neutral particles that LHCf can detect, the π0 mesons are the most sensitive to nuances in proton-proton interactions. Thus, high priority has been given to analysing forward π0 production data in order to provide key information for hadron interaction models at the TeV energy scale.

So, based on experimental data obtained on 15 and 16 May 2010 during proton-proton collisions at 7 TeV, the LHCf collaboration analysed the production of these rich-in-information mesons. They identified them by their decay into two photons: the opening angle between the photons from π0 decay is detected by the LHCf detectors, constrained by an angle of ≤ 0.4 mrad for Arm1 and ≤ 0.6 mrad for Arm2.  They then compared their data to different hadronic interaction models in order to improve them: “We don’t know which model is the best one for high-energy cosmic rays,” explains Hiroaki Menjo, LHCf collaboration member from the Kobayashi-Maskawa Institute (Nagoya University). “LHCf is a vital tool for theoretical physicists, as it provides the only real data that can be used to calibrate their models for these high-energy phenomena.”

“At the beginning of next year, we will operate with lead-proton collisions,” adds Lorenzo Bonechi. “At the moment, we are already updating our detectors for the even more distant future: operation with proton-proton collisions at 14 TeV in 2015.” The goal: to provide even more invaluable input for the Monte Carlo codes used for modeling cosmic ray interactions in the Earth's atmosphere.

Click here to read the LHCf paper published on arXiv on 3 October.

by Anaïs Schaeffer