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) 28th 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 (2.2m2) Covering ~100km2 area 27 muon detectors (south region) 14–20 Proportional counters (2.8–10m2) Shielded by 30cm Fe or 1m concrete Threshold energy: 0.5GeVxsecθ Triggered by accompanying surface detector Akeno Observatory 35.7ºN 138.5ºE 900m asl. 920g/cm2 Surface detectors x111 Muon detectors x27

Lateral distribution of muons No significant change in lateral distribution shape up to 1020eV rm(R)=C(R/R0)-1.2(1+R/R0)-2.52(1+(R[m]/800)3)-0.6 ,E0=1017.5–1019eV R0: Characteristic distance (280m @q=25o) Lateral distribution function obtained by A1 Experiment (Hayashida et al. 1995)

Primary mass estimator E0 =1.8x1020 [eV] rm(1000) = 2.4 [1/m2] Lateral distribution Muon density at 1000m rm(1000) Fitting muon data in R=800-1600m to LDM Meas. error~±40% Muon: Empirical formulae Charged particle:

Event selection Dataset (13 December 1995 – 31 December 2002) E0 ≥1019eV Zenith angle: q≤36º Normal event quality cuts ≥2 muon detectors in R=800m–1600m ⇒ rm(1000) Statistics 129 events above 1019eV 19 events above 1019.5eV

Simulations Proton / iron primaries AIRES+QGSJET Gamma-ray primaries (Geomag. + LPM) Geomagnetic field effect Significant above 1019.5eV Code by Stanev &Vankov LPM effect Significant above 1019.0eV Included in AIRES Detector configuration & analysis process

rm(1000) distribution >1019eV Consistent with proton dominant component Average relationship rm (1000)[m−2]= (1.26±0.16)(E0[eV]/1019)0.93±0.13 1 Log(Muon density@1000m[m–2]) −1 −2 19 19.5 20 20.5 Log(Energy [eV])

Akeno 1km2 (A1): Hayashida et al. ’95 (Interpreted by AIRES+QGSJET) Iron fraction (p+Fe 2comp. assumption) Present result (@90% CL) Fe frac.: <35% (1019 –1019.5 eV) <76% (above 1019.5eV) A1: PRELIMINARY Akeno 1km2 (A1): Hayashida et al. ’95 (Interpreted by AIRES+QGSJET) Gradual decrease of Fe fraction between 1017.5 & 1019eV VERY PRELIMINARY Haverah Park (HP): Ave et al. ’03 Volcano Ranch (VR): Dova et al. (present conf.) HiRes (HiRes): 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% (>1019eV) (g/p<0.45) 56% (>1019.5eV) (g/p<1.27) Topological defects (Sigl et al. ‘01) (Mx=1016[eV]; flux normalised@1020eV ) Z-burst model(Sigl et al. ‘01) (Flux normalised@1020eV) SHDM-model (Berezinski ‘03) (Mx=1014[eV]; flux normalised@1020eV ) SHDM-model (Berezinski et al. ‘98) (Mx=1014[eV]; flux normalised@1019eV )

Summary AGASA muon data in showers above 1019eV (q<36o) No significant change in lateral distribution shape up to 1020eV rm(1000)[m−2]= (1.26±0.16)(E0[eV]/1019) 0.93±0.13 UHECR composition interpreted by AIRES+QGSJET (p+Fe 2 composition) E0 = 1017.5 −1019eV (Akeno 1km2 data; VERY PRELIMINARY) Primary mass: gradual decrease from middle heavy to light Above 1019eV Proton dominance favoured & consistent with extrapolation from lower energies Fe fraction less than 40%@90%CL (>1019eV) Gamma-ray flux in UHECRs No evidence for gamma-ray dominance Upper limit@90%CL on gamma-ray fraction mixed with proton <34% above 1019eV <56% above 1019.5eV 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 (>3x1019eV) Geomagnetic effect (>5x1019eV) 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 45o q=24.6° GMF LPM

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% (>1019eV) (g/p<0.45) 56% (>1019.5eV) (g/p<1.27) to observed events Topological defects (Sigl et al. ‘01) (Mx=1016[eV]; flux normalised@1020eV ) Z-burst model(Sigl et al. ‘01) (Flux normalised@1020eV) SHDM-model (Berezinski et al. ‘98) (Mx=1014[eV]; flux normalised@1019eV )

Limits on gamma-ray fraction Assuming 2-comp. (p+gamma-ray) primaries Gamma-ray fraction upper limits (@90%CL) to observed events 34% (>1019eV) (g/p<0.45) 56% (>1019.5eV) (g/p<1.27) Topological defects (Sigl et al. ‘01) (Mx=1016[eV]; flux normalised@1020eV ) Z-burst model(Sigl et al. ‘01) (Flux normalised@1020eV) SHDM-model (Berezinski et al. ‘98) (Mx=1014[eV]; flux normalised@1019eV )

Limits on gamma-ray fraction Assuming 2-comp. (p+gamma-ray) primaries Gamma-ray fraction upper limits (@90%CL) to observed events 34% (>1019eV) (g/p<0.45) 56% (>1019.5eV) (g/p<1.27) Topological defects (Sigl et al. ‘01) (Mx=1016[eV]; flux normalised@1020eV ) Z-burst model(Sigl et al. ‘01) (Flux normalised@1020eV) SHDM-model (Berezinski et al. ‘98) (Mx=1014[eV]; flux normalised@1019eV )

Limits on gamma-ray fraction Assuming 2-comp. (p+gamma-ray) primaries Gamma-ray fraction upper limits (@90%CL) to observed events 34% (>1019eV) (g/p<0.45) 56% (>1019.5eV) (g/p<1.27) Topological defects (Sigl et al. ‘01) (Mx=1016[eV]; flux normalised@1020eV ) Z-burst model(Sigl et al. ‘01) (Flux normalised@1020eV) SHDM-model (Berezinski ‘03) (Mx=1014[eV]; flux normalised@1020eV ) SHDM-model (Berezinski ‘98) (Mx=1014[eV]; flux normalised@1019eV )

Muon component No significant change in shape of LDM up to 1020eV Muon density@1000m rµ (1000) ~20% to total charged particles Feasible mass estimator for UHECRs rm(R)=C(R/R0)-1.2(1+R/R0)-2.52(1+(R[m]/800)3)-0.6 ,E0=1017.5–1019eV R0: Characteristic distance (280m @q=25o) Lateral distribution function obtained by A1 Experiment (Hayashida et al. 1995)

S(600) vs. E0 LPM GMF q=24.6°

S(600) vs. E0 LPM GMF q=24.6°

Fraction of iron Present result: Frac. of Fe 14 % above 1019eV Assuming 2-comp. (p+Fe) 1ries Muon density or Xmax ® Frac. of iron (AIRES+QGSJET) Present result: Frac. of Fe 14 % above 1019eV +14 –16 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 2comp assumption) Assuming 2-comp. (p+Fe) primaries Present result (●) Fe frac.:14 % above 1019eV +16 –14 cf. Present data interpreted by AIRES+SIBYLL Fe frac.: 78 % above 1019eV +22 –15 Akeno 1km2 (A1) From: rm (600) vs. S(600) relationship Hayashida et al. ’95 (preliminary re-interpretation by MC) Haverah Park: Ave et al. ’03 Volcano Ranch: Dova et al. (present conf.) HiRes: Archbold et al. (present conf.)

ρµ(600) vs. E0

Analysis & Simulation Dataset (13 December 1995 – 31 December 2002) E0≥1019eV estimated by S(600) E0[eV]=2x1017S(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=800m–1600m from core ⇒ rm(1000) determined by fitting with LDM (~40% error by MC) Statistics 133 events above 1019eV 25 events above 1019.5eV 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 ~3x1019eV) Implemented with MC code developed by Stanev &Vankov Shower in atmosphere is followed by AIRES+QGSJET

Analysis Dataset (13 December 1995 – 31 December 2002) E0 ≥1019eV Zenith angle: q≤36º Normal event quality cuts ≥2 muon detectors in R=800m–1600m ⇒ rm(1000) Statistics 129 events above 1019eV 19 events above 1019.5eV 5 events above 1020eV