Chemical Composition of UHECRs Observed by AGASA Kenji

Chemical Composition of UHECRs Observed by AGASA Kenji Shinozaki Max-Planck-Institut für Physik, Munich for AGASA Collaboration M. Chikawa (Kinki); M. Fukushima, N. Hayashida, K. Mase, H. Ohoka, S. Osone, N. Sakurai, M. Sasaki, R. Torii (ICRR-Tokyo); K. Honda, N. Kawasumi, I. Tsushima (Yamanashi); N. Inoue (Saitama); K. Kadota (Musashi Inst. Tech. ); F. Kakimoto (TITech); K. Kamata (Nishina Fund. ); S. Kawaguchi (Hirosaki); S. Kawakami (Osaka C. ); Y. Kawasaki, N. Sakaki, M. Takeda, H. M. Shimizu (RIKEN); S. Mizobuchi, H. Yoshii (Ehime); M. Nagano (Fukui U. Tech); K. Shinozaki, M. Teshima (MPI-Munich); Y. Uchihori (NIRS); T. Yamamoto (Chicago); S. Yoshida (Chiba) 28 th International Cosmic Ray Conference (Tsukuba) August 2, 2003

Introduction • Presence of Super-GZK particles – No location identified as their sources • Origin models – Bottom-up scenarios • AGNs / GRBs / Galactic clusters etc. ⇒ Hadronic primaries predicted – Top-Down scenarios • Topological defects / SHDM / Z-burst ⇒ Gamma-ray + nucleon primaries predicted UHECR composition is important to understand origin character ⇒ Muons in giant air shower are key observable for AGASA

AGASA (Akeno Giant Air Shower Array) • 111 surface detector (2. 2 m 2) – Covering ~100 km 2 area • 27 muon detectors (south region) Akeno Observatory 35. 7ºN 138. 5ºE 900 m asl. 920 g/cm 2 Surface detectors x 111 Muon detectors x 27 – 14– 20 Proportional counters (2. 8– 10 m 2) – Shielded by 30 cm Fe or 1 m concrete • Threshold energy: 0. 5 Ge. Vxsecθ – Triggered by accompanying surface detector

Lateral distribution of muons No significant change in lateral distribution shape up to 1020 e. V rm(R)=C(R/R 0)-1. 2(1+R/R 0)-2. 52(1+(R[m]/800)3)-0. 6 , E 0=1017. 5– 1019 e. V R 0: Characteristic distance (280 m @q=25 o) Lateral distribution function obtained by A 1 Experiment (Hayashida et al. 1995)

Primary mass estimator E 0 =1. 8 x 1020 [e. V] rm(1000) = 2. 4 [1/m 2] Lateral distribution • Muon density at 1000 m rm(1000) – Fitting muon data in R=800 -1600 m to LDM – Meas. error~± 40% Muon: Charged particle: Empirical formulae

Event selection • Dataset (13 December 1995 – 31 December 2002) – E 0 ≥ 1019 e. V – Zenith angle: q≤ 36º – Normal event quality cuts – ≥ 2 muon detectors in R=800 m– 1600 m ⇒ rm(1000) • Statistics 129 events above 1019 e. V 19 events above 1019. 5 e. V

Simulations • Proton / iron primaries – AIRES+QGSJET • Gamma-ray primaries (Geomag. + LPM) – Geomagnetic field effect • Significant above 1019. 5 e. V • Code by Stanev &Vankov – LPM effect • Significant above 1019. 0 e. V • Included in AIRES • Detector configuration & analysis process

rm(1000) distribution >1019 e. V Consistent with proton dominant component Log(Muon density@1000 m[m– 2]) Average relationship rm (1000)[m− 2]= (1. 26± 0. 16)(E 0[e. V]/1019)0. 93± 0. 13 1 0 − 1 − 2 19 19. 5 20 Log(Energy [e. V]) 20. 5

Iron fraction (p+Fe 2 comp. assumption) Present result (@90% CL) Fe frac. : <35% (1019 – 1019. 5 e. V) <76% (above 1019. 5 e. V) A 1: PRELIMINARY Akeno 1 km 2 (A 1): Hayashida et al. ’ 95 (Interpreted by AIRES+QGSJET) Gradual decrease of Fe fraction between 1017. 5 & 1019 e. V VERY PRELIMINARY Haverah Park (HP): Ave et al. ’ 03 Volcano Ranch (VR): Dova et al. (present conf. ) Hi. Res (Hi. Res): Archbold et al. (present conf. )

Limits on gamma-ray fraction Assuming 2 -comp. (p+gamma-ray) primaries • Gamma-ray fraction upper limits (@90%CL) to observed events – 34% (>1019 e. V) Topological defects (Sigl et al. ‘ 01) (Mx=1016[e. V]; flux normalised@1020 e. V ) Z-burst model(Sigl et al. ‘ 01) (Flux normalised@1020 e. V) SHDM-model (Berezinski ‘ 03) (Mx=1014[e. V]; flux normalised@1020 e. V ) SHDM-model (Berezinski et al. ‘ 98) (Mx=1014[e. V]; flux normalised@1019 e. V ) (g/p<0. 45) – 56% (>1019. 5 e. V) (g/p<1. 27)

Summary • AGASA muon data in showers above 1019 e. V (q<36 o) – No significant change in lateral distribution shape up to 1020 e. V – rm(1000)[m− 2]= (1. 26± 0. 16)(E 0[e. V]/1019) 0. 93± 0. 13 • UHECR composition interpreted by AIRES+QGSJET (p+Fe 2 composition) – E 0 = 1017. 5 − 1019 e. V (Akeno 1 km 2 data; VERY PRELIMINARY) • Primary mass: gradual decrease from middle heavy to light – Above 1019 e. V • Proton dominance favoured & consistent with extrapolation from lower energies – Fe fraction less than 40%@90%CL (>1019 e. V) • Gamma-ray flux in UHECRs – No evidence for gamma-ray dominance – Upper limit@90%CL on gamma-ray fraction mixed with proton <34% above 1019 e. V <56% above 1019. 5 e. V to observed UHECRs These limits can be possible constraints against origin models

Another approach (Energy underestimation for gamma-rays) • Effects on UHE Gamma-ray – LPM effect (>3 x 1019 e. V) – Geomagnetic effect (>5 x 1019 e. V) • Possible anisotropy in the sky expected for UHE gamma-rays – No indication found for UHE gamma-rays (present low statistics) • Possible approach for future large-scale experiments Akeno sky up to 45 o

Introduction • Presence of Super-GZK particles – No location identified as their sources • Origin models – Bottom-up scenarios • AGNs / GRBs / Galactic clusters etc. ⇒ Hadronic primaries predicted – Top-Down scenarios • Topological defects / SHDM / Z-burst ⇒ Gamma-ray + nucleon primaries predicted UHECR composition is key discriminator of models ⇒ Muons in giant air shower are key observable for AGASA

Introduction • Presence of Super-GZK particles – No location identified as their sources • Origin models – Bottom-up scenarios • AGNs / GRBs / Galactic clusters etc. ⇒ Hadronic primaries predicted – Top-Down scenarios • Topological defects / SHDM / Z-burst ⇒ Gamma-ray + nucleon primaries predicted UHECR composition is key discriminator of models ⇒ Muons in giant air shower are key observable for AGASA

Limits on gamma-ray fraction Assuming 2 -comp. (p+gamma-ray) primaries • Gamma-ray fraction upper limits (@90%CL) – 34% (>1019 e. V) (g/p<0. 45) – 56% (>1019. 5 e. V) Topological defects (Sigl et al. ‘ 01) (Mx=1016[e. V]; flux normalised@1020 e. V ) Z-burst model(Sigl et al. ‘ 01) (Flux normalised@1020 e. V) SHDM-model (Berezinski et al. ‘ 98) (Mx=1014[e. V]; flux normalised@1019 e. V ) (g/p<1. 27) to observed events

Limits on gamma-ray fraction Assuming 2 -comp. (p+gamma-ray) primaries • Gamma-ray fraction upper limits (@90%CL) to observed events – 34% (>1019 e. V) (g/p<0. 45) Topological defects (Sigl et al. ‘ 01) (Mx=1016[e. V]; flux normalised@1020 e. V ) Z-burst model(Sigl et al. ‘ 01) (Flux normalised@1020 e. V) SHDM-model (Berezinski et al. ‘ 98) (Mx=1014[e. V]; flux normalised@1019 e. V ) – 56% (>1019. 5 e. V) (g/p<1. 27)

Limits on gamma-ray fraction Assuming 2 -comp. (p+gamma-ray) primaries • Gamma-ray fraction upper limits (@90%CL) to observed events – 34% (>1019 e. V) (g/p<0. 45) Topological defects (Sigl et al. ‘ 01) (Mx=1016[e. V]; flux normalised@1020 e. V ) Z-burst model(Sigl et al. ‘ 01) (Flux normalised@1020 e. V) SHDM-model (Berezinski et al. ‘ 98) (Mx=1014[e. V]; flux normalised@1019 e. V ) – 56% (>1019. 5 e. V) (g/p<1. 27)

Limits on gamma-ray fraction Assuming 2 -comp. (p+gamma-ray) primaries • Gamma-ray fraction upper limits (@90%CL) to observed events – 34% (>1019 e. V) Topological defects (Sigl et al. ‘ 01) (Mx=1016[e. V]; flux normalised@1020 e. V ) Z-burst model(Sigl et al. ‘ 01) (Flux normalised@1020 e. V) SHDM-model (Berezinski ‘ 03) (Mx=1014[e. V]; flux normalised@1020 e. V ) SHDM-model (Berezinski ‘ 98) (Mx=1014[e. V]; flux normalised@1019 e. V ) (g/p<0. 45) – 56% (>1019. 5 e. V) (g/p<1. 27)

Muon component No significant change in shape of LDM up to 1020 e. V • Muon density@1000 m rµ (1000) – ~20% to total charged particles – Feasible mass estimator for UHECRs rm(R)=C(R/R 0)-1. 2(1+R/R 0)-2. 52(1+(R[m]/800)3)-0. 6 , E 0=1017. 5– 1019 e. V R 0: Characteristic distance (280 m @q=25 o) Lateral distribution function obtained by A 1 Experiment (Hayashida et al. 1995)

S(600) vs. E 0 LPM q=24. 6° GMF

S(600) vs. E 0 LPM q=24. 6° GMF

Fraction of iron Assuming 2 -comp. (p+Fe) 1 ries Muon density or Xmax ® Frac. of iron (AIRES+QGSJET) 19 Present result: Frac. of Fe 14 +14 – 16 % above 10 e. V Compiled by Nagano & Watson ‘ 00 Haverah Park: Ave et al. ’ 03 Valcano Ranch: Dova et al. (present conf. )

Average primary mass (Iron fraction for p+Fe 2 comp assumption) Assuming 2 -comp. (p+Fe) primaries Present result (●) +16 Fe frac. : 14– 14 % above 1019 e. V cf. Present data interpreted by AIRES+SIBYLL Fe frac. : 78 +22 % above 1019 e. V – 15 Akeno 1 km 2 (A 1) From: rm (600) vs. S(600) relationship Haverah Park: Ave et al. ’ 03 Hayashida et al. ’ 95 Volcano Ranch: Dova et al. (present conf. ) (preliminary re-interpretation by MC) Hi. Res: Archbold et al. (present conf. )

ρµ(600) vs. E 0

Analysis & Simulation • Dataset (13 December 1995 – 31 December 2002) – E 0≥ 1019 e. V estimated by S(600) E 0[e. V]=2 x 1017 S(600) for θ=0º – Zenith angle: q≤ 36º – Good fitting on core location & arrival direction (n hit ≥ 6; χ2 cuts) – Requiring ≥ 2 muon detectors in R=800 m– 1600 m from core ⇒ rm(1000) determined by fitting with LDM (~40% error by MC) – Statistics 133 events above 1019 e. V 25 events above 1019. 5 e. V • Simulations (incl. detector configuration & analysis process) – Proton / iron primaries • AIRES code +QGSJET model – gamma-ray primaries • UHE gamma-ray interaction with geomagnetic field (Significant effect working above ~3 x 1019 e. V) – Implemented with MC code developed by Stanev &Vankov Shower in atmosphere is followed by AIRES+QGSJET

Analysis • Dataset (13 December 1995 – 31 December 2002) – E 0 ≥ 1019 e. V – Zenith angle: q≤ 36º – Normal event quality cuts – ≥ 2 muon detectors in R=800 m– 1600 m ⇒ rm(1000) – Statistics 129 events above 1019 e. V 19 events above 1019. 5 e. V 5 events above 1020 e. V