Constraints from the first LHC data on hadronic event generators for ultra-high energy cosmic-ray physics

TL;DR

This study benchmarks hadronic interaction models used for ultra-high-energy cosmic-ray air showers against the first LHC proton-proton data, focusing on midrapidity observables dNch/dη, <pT>, and P(Nch). It finds no single model describes all observables across the LHC energy range, though Reggeon Field Theory–based models capture central densities reasonably well, while collider-oriented tunes vary in their agreement, underscoring the need to retune MPI and saturation implementations. The results constrain extrapolations to GZK energies and generally support conventional explanations for the cosmic-ray knee, though forward and baryon-production physics remain critical uncertainties for EAS interpretation. The paper emphasizes the value of forthcoming LHC measurements, especially forward and pPb data, to tighten these constraints and improve cosmic-ray physics predictions.

Abstract

The determination of the primary energy and mass of ultra-high-energy cosmic-rays (UHECR) generating extensive air-showers in the Earth's atmosphere, relies on the detailed modeling of hadronic multiparticle production at center-of-mass (c.m.) collision energies up to two orders of magnitude higher than those studied at particle colliders. The first Large Hadron Collider (LHC) data have extended by more than a factor of three the c.m. energies in which we have direct proton-proton measurements available to compare to hadronic models. In this work we compare LHC results on inclusive particle production at energies sqrt(s) = 0.9, 2.36, and 7 TeV to predictions of various hadronic Monte Carlo (MC) models used commonly in cosmic-ray (CR) physics (QGSJET, EPOS and SIBYLL). As a benchmark with a standard collider physics model we also show PYTHIA (and PHOJET) predictions with various parameter settings. While reasonable overall agreement is found for some of the MC, none of them reproduces consistently the sqrt(s) evolution of all the observables. We discuss implications of the new LHC data for the description of cosmic-ray interactions at the highest energies.

Figures (12)

• Figure 1: Data on the all-particle flux of cosmic rays. The flux has been scaled by $E^{2.5}$ to make the features clearly visible. The axis at the top indicates the equivalent c.m. energy if the cosmic ray particles were protons. The nominal collider c.m. energy for different accelerators is also shown. References to the data can be found in Bluemer:2009zf.
• Figure 2: Pseudorapidity distributions of charged hadrons, $h^\pm \equiv (h^++h^-)$, measured in NSD $p$-$p$ events at the LHC ($\sqrt{s}$ = 0.9, 2.36 and 7 TeV) by ALICE Aamodt:2010ftAamodt:2010pp and CMS Khachatryan:2010xsKhachatryan:2010us (and by UA5 Alner:1986xu in $p$-$\bar{p}$ at 900 GeV) compared to three different versions of pythia and to the phojet MC. The dashed band is the systematic uncertainty of the CMS experiment which is similar to those of the two other measurements.
• Figure 3: Pseudorapidity distributions of charged hadrons, $h^\pm \equiv (h^++h^-)$, measured in NSD $p$-$p$ events at the LHC (0.9, 2.36 and 7 TeV) by ALICE Aamodt:2010ftAamodt:2010pp and CMS Khachatryan:2010xsKhachatryan:2010us (and by UA5 Alner:1986xu in $p$-$\bar{p}$ at 900 GeV) compared to the predictions of qgsjet 01 and II, sibyll, and epos. The dashed band is the systematic uncertainty of the CMS experiment which is similar to those of the two other measurements.
• Figure 4: Collision-energy dependence of the midrapidity charged hadron invariant yields in non single-diffractive (NSD, left panel) and inelastic (right panel) $p$-$p$ and $p$-$\bar{p}$ collisions compared to different tunes of pythia 6 and 8 and to phojet 1.12.
• Figure 5: Collision-energy dependence of the midrapidity charged hadron invariant yields in non single-diffractive (NSD, left panel) and inelastic (right panel) $p$-$p$ and $p$-$\bar{p}$ collisions compared to the predictions of qgsjet 01 and II, sibyll, and epos.