Cosmic ray composition and hadronic interactions in the knee region

Nuclear Physics B (Proc. Suppl.) 151 (2006) 79–82 www.elsevierphysics.com

Cosmic ray composition and hadronic interactions in the knee region G. Navarra for the EAS-TOP Collaborationa a Dipartimento di Fisica Generale dell’Universita’ and INFN, Torino, Italy via P. Giuria, 1, 10125 Torino, Italy

We present the EAS-TOP experimental data on the cosmic ray composition in the energy range 10 14 − 1016 eV, focusing on the connections with the hadronic interaction models used for the analysis.

2. Results (a) The ”knee”, as observed in the Ne size spectrum, is shown in fig. 1 [6]. The shift of the knee positions vs the zenith angle (i.e. atmospheric depth) is clearly visible and shows that the data are compatible with a feature occurring at fixed energy for a given primary nucleus. Compatible 0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.07.014

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The possibility of testing the influence of the hadronic interaction models used for the composition studies is a parallel requirement for Extensive Air Shower experiments. This has been realized in EAS-TOP by means of multi-component detectors. EAS-TOP (Campo Imperatore, National Gran Sasso Laboratories, 2000 m a.s.l., in operation between 1989 and 2000), included: a) an e.m. detector[1]: 35 scintillator modules, 10 m2 each, distributed over an area Aef f ≈ 105 m2 , for the measurement of the shower size (Ne ), core location and arrival direction; b) a muonhadron detector[2]: 140 m2 calorimeter, operating for hadron calorimetry for Eh > 30 GeV, and for muon counting for Eμ > 1 GeV; c) a Cherenkov light detector[3]: 8 telescopes having 0.5 m2 area light collectors equipped with imaging devices and wide angle optics. Moreover, the array operated in coincidence with the deep underground MACRO and LVD muon detectors ≈ 1000 (Eμ > 1.3 TeV; full effective area ATeV μ m2 ). The measurements have been interpreted through simulations based on QGSJET[4] as implemented in CORSIKA[5].

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1. Introduction

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Figure 1. Shower size spectra measured at different zenith angles (i.e. atmospheric depths).

breaks (concerning the intensities and the spectral indexes) are observed in the muon number (Nμ ) spectra[7]1 . (b) We can therefore work on the hypothesis 1 As

examples, the indexed of the Ne and Nμ power spectra, for events with zenith angles θ < 20o , are γe = 2.56 ± 0.02, γμ = 3.21 ± 0.06 below the knee, and γe = 2.96 ± 0.06, γμ = 3.42 ± 0.10 above the knee, giving values of α (exponent of the relation: Nμ ∝ Ne α ) of 0.70 ± 0.03 and 0.80 ± 0.07 respectively, in agreement with the expectations from the interaction models (see item (f)).

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Figure 2. Experimental muon number spectrum compared with the expectations from individual primaries, whose fluxes I(E0 ) reproduce the shower size spectrum in the region of the knee.

that we are observing in both spectra the same dominating primary nucleus, that can therefore be identified. In fig. 2, the experimental muon number (Nμ ) spectrum is compared with the expectations from individual nuclei, whose fluxes I(E0 ) reproduce the shower size (Ne ) spectrum in the region of the knee following QGSJET. For each nucleus the band shows the uncertainty related to the hadronic interaction model (higher values for VENUS, lower for NEXUS). Such analysis leads therefore to the conclusion (that can be hardly modified using a different interaction model, as we see from fig. 2) of helium dominating in the knee region[7]. (c) An independent approach to the study of the dominating component at the knee is provided by the combined TeV muon MACRO2 and EAS-TOP Cherenkov light measurements[8]. From fig. 3, showing the average number of 2 The collaboration with MACRO has been determinant for reaching part of these results.

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Figure 3. Average muon numbers at the MACRO depth, from primary protons (full circle), He (open circle) and CNO (diamond) as a function of energy.

muons reaching the MACRO depth for different nuclei, we deduce that at primary energy E0 ≈ 80 TeV the muon contributions from p and He coincide (the same holds at E0 ≈ 250 TeV also for CNO). Therefore from an associated energy measurement, performed through the atmospheric Cherenkov light yield, the p+He (or p+He+CNO) flux is obtained (the event geometry is fully defined by the TeV muon coordinates). Photon densities between 103.55 and 103.75 /m2 (268 events) correspond to about 80 TeV, and between 104.15 and 104.35 /m2 (125 events) to about 250 TeV (for core distances 125 < r < 145 m). By subtracting from the (p+He) and the (p+He+CNO) fluxes the p-flux[9] we obtain the He and CNO ones, their relative ratio at 250 TeV being: J(p):J(He):J(CNO)=(0.20 ± 0.08):(0.58 ± 0.19):(0.22 ± 0.17). Such data confirm, with new observables, the dominance of helium primaries at these energies. (d) Above 1015 eV the spectra of the light (”p” or ”50% p + 50% He”), intermediate (CNO), and

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Figure 4. Energy spectra of the three mass groups obtained from the Ne and GeV muon data.

heavy (Fe) primaries are obtained by fitting the muon number distributions in intervals of Ne [7]. The results, as fluxes (vs. E0 ), are reported in fig. 4: the energy range between 1015 and 1016 eV is thus characterized by average spectra: i) steeper for the light component (γp,He > 3.1); ii) harder for the intermediate one (γCN O  2.75); iii) with constant slope for the heavy primaries (γF e  2.3 ÷ 2.7, consistent with the direct data). (e) An independent composition study is performed by means of a similar muon number distribution analysis in intervals of shower size, using the Eμ > 1.3 TeV muons recorded by MACRO. The obtained relative spectra of the light and heavy components are reported in fig. 5 [10]. The observed steepening of the light component at the knee agrees with the result of the GeV muon analysis, showing the consistency of QGSJET in describing the EAS properties for secondaries produced in the central and fragmentation regions3. 3 The

agreement with KASCADE[11] is also quite significant in such context, due to the different atmospheric depth of the two arrays (of about 200 gr/cm2 ) and there-

Figure 5. Light (p+He) and heavy (Mg+Fe) primary spectra as obtained from the fits to the TeV muon multiplicity distributions (by MACRO) in different bins of Ne (by EAS-TOP).

(f) To discuss the influence of the interaction model on the general conclusions, average values of muon numbers (< ρμ >∝ Nμ ) in intervals of shower size are shown in fig. 6 for the experimental data and simulated single elements spectra. The increasing average primary mass is seen from the drift of the experimental data from average helium to CNO. In fig. 7 the slopes of the Nμ -Ne relations for different interaction models implemented in CORSIKA[12–14] are compared for primary protons with the expectations of QGSJET. The properties of the interaction model can be represented in terms of the parameter α characteristic of the Nμ -Ne relation: Nμ ∝ Ne α (see footnote 1). While its experimental value (from fig. 6) is αexp = 0.907 ± 0.004, for the different models we have: αQGSJET = 0.792 ± 0.007, αV EN US = 0.820 ± 0.007, αN EXUS = 0.77 ± 0.02, αDP MJET = 0.789 ± 0.008. These are therefore the slopes expected following the different models for a ”constant” primary composifore the different stages of development of the cascades.

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Figure 6. < ρμ > at rcore between 180 and 210 m vs Ne relation (measured and expected from QGSJET for individual elements).

tion4 . None of the models can therefore explain the experimental value of α without requiring an additional increasing of the average primary mass around the knee. 3. Conclusions The experimental data on the EAS e.m. and muon components with GeV and TeV energy thresholds (i.e. from secondaries produced in the central and fragmentation regions) are consistent with each other and with the expectations from QSGJET. The knee (observed in the Ne and Nμ spectra) is associated to breaks in the energy spectra of the lighter components. The general conclusions of increasing average mass with primary energy, and of helium dominating at the knee do not depend on the specific interaction model used for the analysis inside CORSIKA. 4 The

increasing mass with primary energy up to 10 15 eV reported e.g. by JACEE would increase such α values of about 0.006, i.e. inside the uncertainties.

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Figure 7. Comparison between the Nμ -Ne relationships obtained for different interaction models (proton primaries) and for the experimental data, normalized to QGSJET.

REFERENCES 1. EAS-TOP Coll, N.I.M., A 336 (1993) 310 2. EAS-TOP Coll, N.I.M., A 420 (1999) 117 3. EAS-TOP Coll, Il Nuovo Cimento, 105A (1992) 1807 4. N.N. Kalmykov et al., Nucl. Phys. B, 52B (1997) 17 5. D. Heck et al, FZK-Report 6019 (1998) 6. EAS-TOP Coll, Astrop. Phys., 10 (1999) 1 7. EAS-TOP Coll, Astrop. Phys., 21 (2004) 583 8. EAS-TOP and MACRO Colls, Astrop. Phys., 21 (2004) 223 9. EAS-TOP Coll, Astrop. Phys., 19 (2003) 329 10. EAS-TOP and MACRO Colls, Astrop. Phys., 20 (2004) 641 11. KASCADE Coll, Astrop. Phys., 16 (2002) 373 12. H.J. Drescher et al., Phys. Rep. 350 (2001) 93 13. J. Ranft, Phys. Rev. D51 (1995) 64 ; hepph/9911232 (1999) 14. K. Werner, Phys. Rep. 232 (1993) 87