Results of AGASA Experiment __Energy spectrum & Chemical composition __ Kenji SHINOZAKI Max-Planck-Institut für Physik (Werner-Heisenberg-Institut) Munich, Germany Max-Planck-Institut für Physik (Werner-Heisenberg-Institut) Munich, Germany on behalf of AGASA Collaboration The Highest Energy Cosmic Rays and Their Sources 21 – 23 May, Moscow

Outline Physics motivation Activities at Akeno Observatory Energy determination & spectrum –Shower properties & analysis –Systematic error in energy estimation –Comparison with other results (HiRes & A1) Muon component & chemical composition –Gamma-ray shower properties –Chemical composition & gamma-ray flux limit estimation Summary & outlook

Physics motivation Understanding nature & origin of UHECRs (>10 19 eV) – Energy spectrum – Arrival direction distribution – Chemical composition Super GZK particles incl. highest energy cosmic rays (>10 20 eV) – Bottom-up scenarios AGNs / GRBs / Galactic clusters etc. ⇒ Hadronic primaries predicted – Top-Down scenarios Topological defects Super heavy dark matter Z-burst ⇒ Gamma-ray + nucleon 1ries predicted Source location still not identified, …..but ….. p UHECR γ CMB → N π + (E 0 ~5x10 19 eV)

Arrival direction distribution (>4x10 19 eV; θ<50º) Small scale anisotropy – Event clustering (>4x10 19 eV within 2.5º) 6 doublets (○) &1 triplet (○) observed Against expected 2.0 doublets (P ch <0. 1%) There must be ~ 250 EHECR sources (185–340) :4x10 19 – eV:>10 20 eV

Log E> σ Space angle distribution of events Significant 0 degree – implying presence of compact EHECR sources Log E> σ

Institute for Cosmic Ray Research, University of Tokyo (Kashiwa) –Masaki Fukushima, Naoaki Hayashida, Hideyuki Ohoka, Satoko Osone, Makoto Sasaki, Masahiro Takeda, Reiko Torii Kinki University (Osaka) –Michiyuki Chikawa University of Yamanashi (Kofu) –Ken Honda, Norio Kawasumi, Itsuro Tsushima Saitama University (Saitama) –Naoya Inoue Musashi Institute of Technology (Tokyo) –Kenji Kadota Tokyo Institute of Technology (Tokyo) –Fumio Kakimoto Nishina Memorial Fundation (Tokyo) –Koichi Kamata Hirosaki University (Hirosaki) –Setsuo Kawaguchi Osaka City University (Osaka) –Saburo Kawakami RIKEN (Wako) – Yoshiya Kawasaki, Naoto Sakaki, Hirohiko M. Shimizu Ehime University (Matsuyama) – Satoko Mizobuchi, Hisashi Yoshi Fukuki University of Technology (Fukui) – Motohiko Nagano Communication Research Laboratory (Tokyo) – Masahiko Sasano National Institute of Radiological Sciences (Chiba) – Yukio Uchihori Chiba University (Chiba) – Nobuyuki Sakurai, Shigeru Yoshida Max-Planck-Institute for Physics (Munich) – Kenji Shinozaki, Masahiro Teshima University of Chicago (Chicago) – Tokonatsu Yamamoto AGASA Collaborators We are INTERCONTINETAL collaboration among 31 (all Japanese) scientists from 17 institutes in 3 nations Japan Federal Republic of Germany United States of America

Akeno Observatory – Inst. for Cosmic Ray Research, Univ. of Tokyo – Akeno,Yamanashi Japan (100km west of Tokyo) – Lat. 35º47’N, Long. 138º30’E Altitude 900m – Atom. depth 920 g/cm 2 – Ave. pressure 910hPa – Temp. –10 — +30 ℃ Muon detector station × Tokyo 東京 Vladivostok × ウラジオストク Yakutsk × ヤクーツク Sea of Okhotsk オホーツク 海 Pacific Ocean 太平洋 TA Prototype Tsukuba つくば × Akeno 明野 × M ｔ.Fuji 富士山 Leadburger Main Building Cosmic Ray Imaging System AUGER Water Tank

AGASA (Akeno Giant Air Shower Array) ~8km Detector station – 111 surface detectors Effective area ~100km 2 Optical fibre cable connection to observatory – 27 muon detectors Southern region ~30km 2 coverage Operation – Feb. 1990–Dec separate-array operation – Dec. 1995–Jan.2004 Unified operation SB NB AB TB

Surface detector –5cm thick scintillator –Hamamatsu 5” R1512 PMT Muon detectors (2.8–10m 2 ;south region) –14–20 Proportional counters –Shielded by 30cm Fe or 1m concrete Threshold energy: 0.5GeVxsecθ –Triggered by accompanying surface detector

Shower front structure (empirical) Modified from Linsley formula –Delay time behind shower plane T d (R) [ns] = 2.6 ( 1 + R/30 [m] ) 1.5 ρ(R) -0.5 –Shower front thickness T s (R) [ns] = 2.6 ( 1 + R/30 [m] ) 1.5 ρ(R) -0.3

Lateral distribution (empirical) Modified Linsley formula ρ(R) = C (R/R M ) –α (1+R/R M ) –(η–α) {1+(R/1000) 2 } –δ C: Normalisation constant, α=1.2, δ=0.6 R M : Moliere Akeno (=91.6m) η = (3.97±0.13) – (1.79±0.62) (secθ – 1) Fluctuation of observed particle number σ ρ 2 = ρ ρ 2 + ρ (= σ scin 2 + σ rest 2 + σ stat 2 ) secθ≤1.1 S(600)=10,30[m 2 ]

Energy estimating relationships Energy vs. S(600) for vertical showers –Dai et al.’s MC result by COSMOS+QCDJET (1988) E 0 [eV] = 2.03×10 17 S 0 (600) S(600) Attenuation curve –Empirical relationship (equi-intensity cut method) S θ (600)=S 0 (600) ・ exp{–X 0 / Λ 1 (secθ–1) –X 0 / Λ 2 (secθ–1) 2 } X 0 : Atmospheric AKeno (920 g/cm 2 ) Λ 1 = 500 g/cm 2 Λ 2 = 594 g/cm 2 2×10 19 eV 1×10 19 eV

Event reconstruction 1.Centre of gravity in ρ ch distribution →a priori core location 2.Arrival direction optimisation (fitting shower front structure) 3.Core location estimation (fitting lateral distribution) 4.Iterative recalculation of Steps 2 & 3 5. S θ (600)→S 0 (600) translation 6.Energy estimation by S 0 (600) vs. E 0 relation

Event sample

Event selection criteria (standard) Dataset: February 1990 – January Energy: ≥10 17 eV (≥ eV for spectrum) 2.Zenith angle: ≤45° 3.Core location: inside AGASA boundary 4.Number of hit detector ≥ 6 5.Good reconstruction χ 2 ≤5for arrival direction fitting χ 2 ≤1.5for core location fitting

Core location distribution (> eV) Before & after unification Aperture: ~110km 2 sr extended to ~160 km 2 sr ’95.12—’04.01’90.2—’95.12

Exposure (up to May 2003) AGASA Exposure – 5.4x10 16 m 2 sec sr above ~10 19 eV within θ<45º – AGASA has higher exposure than HiRes below ~3x10 19 eV AGASA detector

Reconstruction accuracy (Energy resolution, Angular resolution) Energy resolution – ΔE 0 /E eV – ΔE 0 /E 0 20 eV Angular resolution – 19.5 eV – 20 eV ΔLog(Energy[eV]) – – Counts [%/bin] Log(Energy[eV]) 90% 68% Open angle Δθ[º]

Energy spectrum (θ<45º) Super GZK-particles exist – 11events above eV Expected 1.9 event on GZK assumption for uniform sources

Detector calibration PWD monitored every RUN (~10h) –Information taken into account in analysis Stability of detector –Gain variation (peak of PWD) :±0.7% –Linearity variation(slope of PWD):±1.6% Linearity variation (11yr) Pulse width distri. (~10hr) Gain variation (11yr) a: Slope t 1 :Peak Cf. Δτ/ =–Δa/ Channel [0.5ns]

Detector simulation (GEANT) Detector container (0.4mm iron roof) – Detector box (1.6mm iron) Scintillator (5cm thick) Earth (backscattering) Detector response understood at ±5% accuracy

Energy conversion Muon / neutrino Ele. Mag 90% 90% primary energy carried by EM component – primary particle & model ~a few % dependence S(600) depending less on primary particle / model AIRES + QGSJET98 / SIBYLL for p & Fe Energy dispersion in atmosphere

Energy conversion factor Ref.Model1ryab Dai et al. ’88 COSMOSQCDJETp Single=electron (900m) Nagano et al. ’99 (CORSIKA5.621)QGSJET98p Single= PH peak (900m)Fe SIBYLL1.6p Fe Sakaki et al. ’01 (AIRES2.2.1)QGSJET98p Single= PW peak (667m)Fe SIBYLL1.6p Fe E 0 = a [10 17 eV]x S(600) b Presently assigned primary energy: – 10% ±1 2% – Most conservative (We need to push up current energy)

S(600) attenuation curve 45º AIRES code + QGSJET / SIBYLL model for p / Fe S(600) attenuating rather slowly – Correction factor less than 2 up to 45º zenith angle S(600) attenuation curve consistent between data & MC – Depending less on 1ry particles or interaction models – Error on energy estimation: ± 5% 45º

Shower phenomenology effects (shower front thickness/ delaying particles) Shower front thickness Particle arrival time (2x10 20 eV) Delaying particles Overestimation effects – Important far away from core Data between several 100m – 1km dominant in energy estimation – Effect of shower front thickness +5% ± 5% – Effect of delaying particles +5% ± 5%

Major systematics in AGASA energy Detector Absolute gain±0.7% Linearity±7% Detector response (container, box backscattering) ±5% Energy estimator S(600) Interaction model, primary particles, altitude–10%±12% Shower Phenomenology Lateral distribution±7% S(600) attenuation±5% Shower front structure+5%±5% Late arriving particles+5%±5% Total±18% Systematics is energy independent above eV Feature of spectrum can hardly change that extends beyond GZK cutoff.

Consistency check in different aperture Inside array Well inside array (~2/3 AGASA) No systematic found in different apertures EHECR spectrum extension beyond GZK cut-off

Recent spectra (AGASA vs. ICRC) HiRes: Bergman et al. ’03 ~2.5 sigma discrepancy between AGASA & HiRes Energy scale difference by 25% vs. HiRes-stereo vs. HiRes-I vs. HiRes-II

Comparison of N e vs. S(600) in Akeno 1km 2 array E 0 = 8.5×10 18 [eV] –by N e = 5.13×10 9 E 0 = 9.3×10 18 [eV] –by S(600) = 45.7 [/m 2 ] E 0 [eV] = 3.9×10 15 (N e /10 6 ) 0.9 –Derived from attenuation curve comparison with Chacalaya (5200m; 540g/cm 2 ) experiment Fairly good agreement between experiment & MC

AGASA vs. A1 comparison

Chemical composition study UHECR composition is key discriminator of models ⇒ Muons in giant air shower are key observable for AGASA Presence of Super-GZK particles – No location identified as their sources – Possibilities of Top-down models (TDs, Z-burst, SHDM…)

Gamma-ray shower properties Fewer muon content (photoproduced muon) Landau-Pomeranchuk-Migdal (LPM) effect (>~3x10 19 eV) –‘Slowing down’ shower development Interaction in geomagnetic field (>several x eV) –‘Accelerating’ shower development –LPM effect extinction –Incident direction dependence 2000 g/cm 2 0 g/cm g/cm eV Gamma-ray (geomag. Interacted) eV Proton eV Gamma-ray (LPM effect) 1000 g/cm 2 Simulated with MC by Stanev & Vankov

Average S(600) vs. energy relationship for gamma-rays (Akeno) Gamma-ray energy underestimation 19 eV 19.5 eV (Maximum LPM effct) 20 eV (Recovered by geomag. effect)

   (R)=C(R/R 0 ) -1.2 (1+R/R 0 ) (1+(R[m]/800) 3 ) -0.6,E 0 = –10 19 eV R 0 : Characteristic distance  =25 o ) Lateral distribution function obtained by A1 Experiment (Hayashida et al. 1995) Lateral distribution of muons No significant change in shape of LDM up to eV

Empirical formulae Primary mass estimator Lateral distribution SAMPLE Charged particle: Muon: Muon density at 1000m   (1000) –Fitting muon data in R= m to LDM –Error~±40% E 0 =1.8x10 20 eV   (1000)=2.4[/m 2 ] Muon  µ (1000) – ~20% to total charged particles – Feasible mass estimator for UHECRs

Analysis Dataset (13 December 1995 – 31 December 2002) –E 0 ≥10 19 eV –Zenith angle:  ≤36º –Normal event quality cuts – ≥ 2 muon detectors in R=800m–1600m ⇒   (1000) –Statistics 129 events above eV 19 events above eV

Simulations Proton / iron primaries (AIRES2.2.1+QGSJET98) Gamma-ray primaries (Geomag. + AIRES +LPM) – Geomagnetic field effect Significant above eV Code by Stanev &Vankov – LPM effect Significant above eV Included in AIRES Detector configuration & analysis process

  (1000) distribution (E 0 >10 19 eV) Average relationship    (1000)[m −2 ]= (1.26±0.16)(E 0 [eV]/10 19 ) 0.93±0.13 Consistent with proton dominant component Log(Energy [eV]) −2−2 −1−1 0 1 Log(Muon –2 ])

Akeno 1km 2 (A1): Hayashida et al. ’95 (Interpretation by AIRES+QGSJET) Haverah Park (HP): Ave et al. ’03 Volcano Ranch (VR): Dova et al. (present conf.) HiRes (HiRes): Archbold et al. (present conf.) Present result CL) Fe frac.: <35% (10 19 – eV) <76% (above eV) Iron fraction (p+Fe 2comp. assumption) A1: PRELIMINARY Gradual decrease of Fe fraction between & eV VERY PRELIMINARY A1: Preliminary

Limits on gamma-ray fraction Gamma-ray fraction upper limits to observed events – 34% (>10 19 eV) (  /p<0.45) –56% (> eV) (  /p<1.27) Topological defects (Sigl et al. ‘01) (M x =10 16 [eV]; flux 20 eV ) Z-burst model(Sigl et al. ‘01) (Flux 20 eV) SHDM-model (Berezinski ‘03) (Mx=10 14 [eV]; flux 20 eV ) Assuming 2-comp. (p+gamma-ray) primaries SHDM-model (Berezinski et al. ‘98) (Mx=10 14 [eV]; flux 19 eV )

Summary AGASA operation –14year-observation watching 17km 2 century sr >95% live-ratio Systematic errors in energy determination – ±18% independent of energy (≥10 19 eV) Super-GZK particles do exist –11 events observed >10 20 eV against 1.9 on GZK assumption –Energy spectrum remains extending beyond GZK cut-off Conventional GZK mechanism can hardly explain!! Chemical composition –Gradual lightening between & eV –Light component 19 eV (AIRES+QGSJET) –Gamma-ray dominance negative at highest energies Fraction of gamma-rays eV) (AIRES+QGSJET)

Another approach (Energy underestimation for gamma-rays) LPM GMF  =24.6° Effects on UHE Gamma-ray – LPM effect (>3x10 19 eV) – Geomagnetic effect (>5x10 19 eV) 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 This slide was shown for talk