Are ultrahigh energy cosmic rays heavy nuclei?

A.A. Mikhailov a ∗

aYu.G. Shafer Institute of Cosmophysical Research and Aeronomy, 31 Lenin Ave., 677980 Yakutsk,Russia

A new approach to estimate the composition of cosmic rays is proposed. It is found that the zenith angledistributions and muon components of extensive air showers observed by the Yakutsk and AGASA arrays forenergies E > 1019 eV and E > 4 × 1019 eV differ from each other. It is suggested that the primary cosmic raysat E > 4× 1019eV are heavier than those at E ∼ 1019 eV. In our method we selected one variant to estimate theshower energy from two variants, as suggested by physicists of the SUGAR array. According to the ”Hillas-E”model, the SUGAR array has detected 8 showers with energy E > 1020 eV.

1. INTRODUCTION

The composition of cosmic rays is the impor-tant characteristic to solve the problem of theirorigin. As the most sensitive parameter to thechange of primary cosmic ray composition themuon shower component can play an essentialrole. The analysis of the muon component ofextensive air showers (EAS) using the AGASAarray data (Japan) shows that for cosmic rays atE > 1019 eV the light nuclei are dominant [1].Results obtained at the HIRES array (USA) bydata of the shift rate of shower development max-imum depending on the energy show that cos-mic rays at E ∼ 2.5 × 1019 eV consist of lightnuclei [2]. The cosmic ray composition derivedfrom the Yakutsk EAS array using Cherenkovradiation points to the fact that cosmic rays atE ∼ 3×1019 eV consist mainly of protons [3]. Un-fortunately, the model calculations used in thesepapers to interpret the experimental data con-sider NN - and pN - interactions of very high-energy particles whose cross-sections have beenextrapolated from the accelerator region. Inaccu-racies can be created in this extrapolation, andthe experiments are also difficult and errors notexcluded. We suggest a new method to estimatethe cosmic ray composition on the basis of clearlyestablished experimental data.

∗grants Russian FBR 04-02-16287, Ministry of EducationRF 01-30

2. EXPERIMENT

The shower distributions for primary energies,E > 1019 eV, as a function of zenith angle, θ,for (a) - Yakutsk, and (b) - Haverah Park [4] areshown in Fig.1. The number of showers is 458 and144, respectively. The dashed line is the expectednumber of events according to [5]. The Pirson χ2

criterion shows that there is fairly good agree-ment between observed and expected numbers ofshowers. As seen in Fig.1, inclined showers arepredominant for showers with E > 1019 eV asexpected.

The shower distributions for primary energies,E > 4 × 1019 eV, are shown in Fig.2 for (a) -Yakutsk and (b) - AGASA [6]. The number ofshowers is 29 and 47, respectively. The dashedline is the expected number of events. The ob-served and expected shower distributions havebeen compared using the χ2 test. The numberof showers observed by the Yakutsk array doesnot contradict the expected number (the proba-bility that χ2 - value exceeds 5% is P ∼ 0.15).A similar result is observed by the AGASA array(Fig. 2b), but the 5% probability is P ∼ 0.07. Ifthese two shower distributions are combined, theχ2 test gives a probability P ∼ 0.03. Hence theobserved number of showers contradicts the ex-pected number, and in the zenith angular range20◦−30◦ the observed number of showers exceedsthe number expected by 2.3 standard deviations.

Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 249–252

0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved.

www.elsevierphysics.com

doi:10.1016/j.nuclphysbps.2007.11.007

Figure 1. Distribution of showers with E > 1019

eV in zenith angle θ : a-Yakutsk, b-Haverah Park.The dashed line is the expected number of show-ers.

Next let us consider the zenith angular showerdistribution of the SUGAR data. There are twovariants in Ref [7] used to estimate the showerenergy: the “Sydney” model and the “Hillas -E” model. Fig.3a shows the shower distribu-tions with E > 1019 eV using the “Sydney”model. Fig.3b shows the shower distribution forE > 4 × 1019 eV using this model, which con-tradicts the expected number of events. Accord-ing to the “Hillas - E” model, all showers inFig.3a have energies E > 4 × 1019 eV, and thismodel does not contradict the expected numberof events. On this basis, one can conclude thatthe estimation of the shower energy by the ”Hillas- E” model is more correct, and according to this

Figure 2. Distribution of showers with E > 4 ×1019 eV: a-Yakutsk, b-AGASA. The dashed lineis the expected number of showers.

model the SUGAR array has registered 8 showerswith energies E > 1020 eV.

In order to clarify why the shower distributionat E > 4 × 1019 eV of the Yakutsk data contra-dicts the expected value (Fig.2), we have analyzedthese showers. We show, as an example from allthe data, the electron-photon and muon compo-nents of two inclined showers with angles and en-ergies: θ1 ∼ 58.7◦, E1 ∼ 1.2 × 1020 eV (Fig.4) ;θ2 ∼ 54.5◦ and E2 ∼ 2 × 1019 eV (Fig.5). Theseshowers were detected on May 7, 1989 and De-cember 2, 1996 at the Yakutsk EAS array, andthe axes of both showers were inside the arrayperimeter. As seen in Fig.4, the particle densitiesin the scintillator detectors (registration thresh-old for electrons is 3 MeV) and in the muon de-

A.A. Mikhailov / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 249–252250

Figure 3. Distribution of showers using the Sugararray data. The ”Sydney” model: a-E > 1019

eV, b-E > 4× 1019 eV. The ”Hillas-E” model: a-E > 4× 1019 eV. The dashed line is the expectednumber of showers.

tectors (threshold is 1 GeV) become equal, i.e.the shower with E1 ∼ 1.2 × 1020 eV consists ofmuons only. The shower with E2 ∼ 2 × 1019 eVat the same zenith angle, θ, has both electron-photon and muon components (Fig.5). The factthat the fraction of muons in inclined showers in-creases with energy is established over all data in[8].

3. DISCUSSION

Thus, two facts have been established:1) the distribution in arrival direction for showersin Yakutsk and AGASA data at energies E >4 × 1019 eV contradicts the expected number ofevents;2) the muon component in the Yakutsk inclinedshowers begins to predominate, and at E ∼ 1020

eV dominates compared with other components.These facts can be interpreted as a change in themass composition of galactic cosmic rays at E >

102

103

10-1

100

101

102

103

(r),

(m-2)

�

r, (m)

Figure 4. Particle density ρ(r) versus the dis-tance, r, to a shower core, E1 = 1.2 × 1020 eV, •- electrons and photons, o - muons, the solid lineis the expected densities of the electron-photoncomponent.

102

103

10-1

100

101

102

103

(r),

(m-2)

r, (m)

�

Figure 5. Particle density ρ(r) versus the dis-tance, r, to a shower core: E2 = 2 × 1019 eV,• - electrons and photons, o - muons, the solidand dashed lines are the expected densities of theelectron-photon component and muons.

A.A. Mikhailov / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 249–252 251

4 × 1019 eV to heavier particles.Earlier we concluded that cosmic rays with

E > 1019 eV were most likely iron nuclei andgalactic in origin [9]. Probably, cosmic rays withenergies E > 4 × 1019 eV are heavier than iron.

4. CONCLUSION

The estimate of the shower energy E > 4×1019

eV relative to a zenith angle by Yakutsk andAGASA data is not correct. According to our es-timation the SUGAR array has detected 8 show-ers with energy E > 1020 eV. Cosmic rays withenergies E > 4 × 1019 eV are most likely heavynuclei and galactic in oigin.

REFERENCES

1. K. Shinozaki, M. Chikawa, M. Fukushima etal., Proc. 28-th ICRC, Tsukuba, 1 (2003) 401.

2. G. Archbold, P.V. Sokolsky et al., Proc. 28-thICRC, Tsukuba, 1 (2003) 405.

3. S.P. Knurenko, V.A. Kolosov, Z.E. Petrov,I.Ye. Sleptsov, Proc. 27-th ICRC, Hamburg,1, (2001) 145.

4. J. Linsley, R.J.O. Reid, A.A. Watson,M.Wada, Catalogue of Highest Energy Cos-mic Rays, Tokyo, 1 (1980) 97.

5. N.N. Efimov, M.I. Pravdin, A.A. Mikhailov,Proc. 18-th ICRC, Bangalore, 2 (1983) 149.

6. M. Takeda, N. Hayashida, K. Honda et al.,Astro-ph/9902239.

7. M.M. Winn, J. Ulrichs, L.S. Peak et al., J.Phys. G: Nucl. Phys. 12 (1986) 653.

8. N.N. Efimov, T.A. Egorov, A.V. Glushkovet al. Proc. ICRR International Symposium:Astrophysical Aspects of the Most EnergeticCosmic Rays, Kofu, (1990) p.20.

9. A.A. Mikhailov, JETF Letters, 72 (2000) 160.

A.A. Mikhailov / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 249–252252