High Energy Hadronic Interactions and Cosmic Ray Physics
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High Energy Hadronic Interactions and Cosmic Ray Physics G. Mitsuka Solar-Terrestrial Environment Laboratory, Nagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan Interpretation of observations of ultra-high-energy cosmic rays relies on the knowledge about forward particle productions in hadronic interaction. Especially the choice of the hadronic interaction generator (model) in the Monte Carlo simulations of air shower development sig- nificantly affects the simulated results, and accordingly accelerator data on the production of very forward emitted particles are indispensable for constraining the hadronic interaction models. In this paper, recent progress in understanding of high energy hadronic interactions is discussed from the viewpoint of the cosmic ray observations. 1 Introductions Measurements of the energy spectrum, chemical composition and arrival direction of ultra- 18 high-energy cosmic rays (E & 10 eV) are indispensable for understanding their origin and the high energy phenomena occurred in the universe. Huge experiments for observing extensive air 14 showers, E & 10 eV, have been operated and also provided valuable data so far. However the interpretation of these experimental air shower data to the primary cosmic ray parameters largely relies on our relatively poor knowledge of hadronic interaction in the earth's atmosphere at the corresponding energy scale. A biggest open problem is the long standing question of the origin and the acceleration mechanism of the galactic cosmic rays 1. In the \standard" scenario, supernova remnants can be a source of the galactic cosmic rays and the acceleration limit is thought to be Z 1015 × eV where Z is the charge of the primary cosmic ray. This scenario predicts a Z dependence of the cutoff energy for galactic cosmic rays. Determination of the chemical composition above E > 1015 eV is important for confirming the standard scenario. The Pierre Auger Observatory 2, Telescope Array 3 and HiRes 4 have carried out precise measurements of the shower particles on the ground, however their interpretation highly depends on the choice of hadronic interaction generator employed in the air shower Monte Carlo (MC) simulations. Figures 1 show the Xmax and RMS(Xmax) distribution as a function of reconstructed cosmic ray energy together with the predictions of MC simulations. It is found that, for example, predicted Xmax varies ∼ 50 g/cm2 for proton primaries and 30 g/cm2 for iron primaries at 4 1019 eV and such variation ∼ × propagates into an uncertainty of the determination of chemical composition. Even in other problems, i.e. energy spectrum and arrival direction, cosmic ray studies based on a large number of experimental data could be misinterpreted due to the uncertainty of the hadronic interaction generators in air shower MC simulations. These uncertainties can only be reduced by precise theoretical understanding of hadronic interaction and high energy accelerator experiments in the forward direction. P. FACAL SAN LUIS et al. THE DISTRIBUTION OF SHOWER MAXIMA OF UHECR AIR SHOWERS ] 2 EPOSv1.99 60 data: RMS=19±1.1 g/cm2 QGSJET01 p 850 SIBYLL2.1 MC: RMS=19±0.1 (stat.) +2 (syst.) g/cm2 QGSJETII -1 50 > [g/cm events max 800 40 <X 47 186 106 750 143 331 230 188 30 619 457 998 781 1251 700 1407 Fe 20 10 650 18 19 0 10 10 P. FACAL SAN LUIS et al. THE DISTRIBUTION-80 -60OFSHOWER -40 -20 0 MAXIMA 20 40 60 OF UHECR 80 100 AIR SHOWERS E [eV] -2 ] ΔX / 2 [g/cm ] ] 2 max EPOSv1.99 2 2 70 60 data: RMS=19±1.1 g/cm Figure 1: The resolutionQGSJET01 of Xmax obtained using events recordedp SIBYLL2.1 +2 2 850 p MC: RMS=19±0.1 (stat.) (syst.) g/cm simultaneously from twoQGSJETII FD stations, compared to a detailed -1 60 ) [g/cm 50 > [g/cm events Monte Carlo simulation. max max 800 50 1251 1407 998 781 619 457 40 <X 331 47 40 750 186 106 230 188 face Detector (SD) has 1660 water detector143 stations ar- RMS(X 331 230 188 30 619 457 143 186 998 781 30 ranged in a 1.5 km triangular grid and sensitive to the 106 1251 47 700 1407 Fe 20 shower particles at the ground. The FD has 27 tele- 20 scopes overlooking the SD, housed in 5 different stations, Fe 10 recording UV650 light emitted in the de-excitation of nitro- 10 gen molecules in the18 atmosphere after the19 passage of the 0 0 10 10 1018 1019 -80 -60 -40 -20 0 20 40 60 80 100 charged particles of a shower. The shower geometryE is [eV] re- E [eV] -2 ΔXmax/ 2 [g/cm ] ] constructed2 from the arrival times of the data. The number Figure 2: Xmax (top panel) and RMS (Xmax) (bottom panel) 70 ! " Figure 1: The resolution of Xmax obtained using events recordedof fluorescenceFigure 1: photonsXmax (left) emitted and is RMS( proportionalXmax) (right) to the distribution en- as a function obtained of by the the energy. measurements Data (points) by theare shown fluorescence with the simultaneously from two FD stations, compared to a detailed p predictions for proton and iron for several hadronic interaction ergy depositeddetector60 ofin the Pierreatmosphere Auger by Observatory. the shower. Measured Using the data (closed circle) are shown with the predictions for proton ) [g/cm Monte Carlo simulation. and iron primary cosmic ray for several hadronic interactionmodels. generators. The number Theof events number in each of bin events is indicated. in each Systematic energy shower geometrymax and correcting for the attenuation of the 50 1251 uncertainties are indicated as a band. 1407 998 781 619 bin is also shown. Shaded457 area indicates the systematic uncertainties. Figures are taken from the Proceedings of light between the shower and the331 detector, the longitudinal 5 40 ICRC 2011 . profile of the shower can be reconstructed.230 188 This profile is face Detector (SD) has 1660 water detector stations ar- RMS(X 143 186 fitted to a Gaisser-Hillas30 function [7] to determine Xmax we require data from at least one SD station, we place an ranged in a 1.5 km triangular grid and sensitive to the 106 shower particles at the ground. The FD has 27 tele-and the energy of the shower [8]. 47 energy dependent cut on both the shower zenith angle and This20 paper is organized as follows. Section 2 discusses how much high energy hadron pro- the distance of the SD station to the reconstructed core so scopes overlooking the SD, housed in 5 different stations,We followduction the analysis cross alreadysection reported can affect in [6]. a We determination considerFe of the origin of cosmic rays. In Section 3, an recording UV light emitted in the de-excitation of nitro-only showers10 reconstructed using FD data and that have at the trigger probability of a single station at these energies application of inclusive photon spectra 4 at the TeV energy scale to air shower simulation is gen molecules in the atmosphere after the passage of theleast a signal in0 one of the SD stations measured in coinci- is saturated for both proton and iron primaries. discussed.10 Instead18 Section is1019 assigned to the discussion of low-energy pion productions. Finally charged particles of a shower. The shower geometry is re-dence. The geometry for these events is determined withE [eV] an Finally, requiring that the shower maximum is observed constructed from the arrival times of the data. The number Figurethis paper 2: Xmax is summarized(top panel) and inRMS Section (Xmax) 5.(bottom panel) angular uncertainty! of" 0.6◦ [9]. The aerosol content in the means that, for some shower geometries, we could intro- of fluorescence photons emitted is proportional to the en-atmosphereas a function is monitored of the constantly energy. Data during (points) data are taking shown [10] with theduce a composition dependent bias in our data. This is ergy deposited in the atmosphere by the shower. Using the predictions for proton and iron for several hadronic interaction and onlymodels.2 events Hadron The for number which production of events a reliable in each crossmeasurement bin is indicated. section of Systematic the avoided using only geometries for which we are able to shower geometry and correcting for the attenuation of theaerosoluncertainties optical depth are indicated exists are as considered. a band. Also the cloud observe the full range of the Xmax distribution. light between the shower and the detector, the longitudinal content is monitoredPossible effects nightly ofacross an uncertaintythe array and ofperiods hadronAt production the end 6744 cross events section (42% of on those the that determination pass the quality profile of the shower can be reconstructed. This profile is with excessive cloud coverage are rejected. Furthermore, cuts) remain above 1018 eV.Thesystematicuncertainty6 fitted to a Gaisser-Hillas function [7] to determine X weof requireXmax and data from RMS(2 atX leastmax one) are SD investigated station, we place by an R. Ulrich, R. Engel and M. Unger . In this maxwe reject events with a χ /Ndf greater than 2.5 when the in the energy reconstruction of the FD events is 22% The and the energy of the shower [8]. energystudy, dependent artificial cut modifications on both the shower are applied zenith angle to the and secondary multiplicity, hadron production cross profile is fitted to a Gaisser-Hillas, as this could indicate the resolution in X13 is at the level of 20 g/cm2 over the en- thesection, distance elasticity of the SD and station pion to charge-ratio the reconstructed in the coresibyll so model maxand a large number of air showers We follow the analysis already reported in [6]. We considerpresence of residual clouds. The total statistical uncertainty ergy range considered.