TAUP 2005: Zaragoza Observations of Ultra-high Energy Cosmic Rays Alan Watson University of Leeds Spokesperson for Pierre Auger Observatory

Outline: Present Status of Detectors The Issues: i Arrival Directions -Galactic Centre?, BL Lac associations? ii Hadronic Interactions changes are relevant - effect on mass composition iii Energy Spectrum – is there a GZK-effect? Summary

Exposure and Event Numbers from various Instruments km 2 sr years > 3 EeV >10 EeV AGASA: closed in January 2004: HiRes I: monocular ~ (HiRes II: monocular HiRes: stereo (PRELIMINARY) ~2500 ~3000 ~500 HiRes apertures are strongly energy-dependent (later) HiRes will cease operation in March 2006 Yakutsk: ~ Auger: data taking since Jan Telescope Array: plan is for 760 km 2 with three fluorescence detectors

Array of water → Cherenkov detectors Fluorescence → The Pierre Auger Observatory design marries two well-established techniques The ‘HYBRID’ technique 11

The Pierre Auger Observatory as planned Surface Array 1600 detector stations 1.5 km spacing 3000 km 2 Fluorescence Detectors 4 Telescope enclosures 6 Telescopes per enclosure 24 Telescopes total

905 surface detector stations deployed Three fluorescence buildings complete each with 6 telescopes Status

θ~ 48º, ~ 70 EeV Flash ADC traces Lateral density distribution Typical flash ADC trace Detector signal (VEM) vs time (ns) PMT 1 PMT 2 PMT µs

Lateral density distribution θ~ 60º, ~ 86 EeV Flash ADC traces Flash ADC Trace for detector late in the shower PMT 1 PMT 2 PMT µs

Flash ADC traces Lateral density distribution Hybrid Event θ~ 30º, ~ 8 EeV

Fitted Electromagnetic Shower from Fly's Eye 1985 Time μ sec Angle Χ in the shower-detector plane Same Hybrid Event θ~ 30º, ~ 8 EeV Tanks Pixels

Angular Resolution Surface array Angular resolution (68% CL) <2.2º for 3 station events (E< 3EeV, θ < 60º ) < 1.7º for 4 station events (3<E<10 EeV) 10 EeV) Hybrid Angular resolution (68% CL) 0.6 degrees (mean) Hybrid-SD only space angle difference Hybrid Data Angle in laser beam /FD detector plane Laser Beam Entries 269 σ(ψ) ~ 1.24º

Resolution of Core Position Hybrid – SD only core position Hybrid Data Laser Data Core position resolution: Hybrid: < 60 m Surface array: < 200 m Laser position – Hybrid and FD only (m)

Energy Determination: Step 1 The detector signal at 1000 m from the shower core – called the ground parameter or S(1000) - is determined for each surface detector event using the lateral density function. S(1000) is proportional to the primary energy. The energy scale is determined from the data and does not depend on a knowledge of interaction models or of the primary composition. Zenith angle ~ 48º Energy ~ 70EeV

Energy Determination: step 2 The energy converter: Compare ground parameter S(1000) with the fluorescence detector energy Use energy converter for surface array log S(1000) * log (E/EeV) 10EeV 1 EeV Hybrid Events with strict event selection: track length > 350g cm -2 Cherenkov contamination <10%

A Big Event - One that got away! Shower/detector plane Fluorescence Mirror Energy Estimate >140 EeV

HiRes stereo events > 10 EeV plus AGASA events above 40 EeV HiRes Collaboration: ICRC 2005: Westerhoff et al.

( ii) Muon Content of Showers:- N  (>1 GeV) = AB(E/A   )p (depends on mass/nucleon) N  (>1 GeV) = 2.8A(E/A   ) 0.86 ~ A 0.14 So, more muons in Fe showers (i) Variation of Depth of Maximum with Energy Elongation Rate (Linsley 1977, Linsley and Watson 1981) dX max / dlog E < 2.3X o g cm -2 /decade from Heitler model Xmax = ln (E o /  c )/ ln 2 Methods of Inferring the Primary Mass HADRONIC MODELS REQUIRED FOR INTERPRETATION

Heck and Ostapchenko: ICRC 2005 New hadronic model: QGSJETII

Heck and Ostapchenko: ICRC 2005 X max vs. Energy for different models compared with data

Heck and Ostapchenko: ICRC 2005 SIBYLL Muon Number Ratio for different models and masses

Claim: Consistent with proton dominant component Log(Energy [eV]) −2−2 −1−1 0 1 Log(Muon –2 ]) Muon measurements with the AGASA array Kenji Shinosaki: 129 events > eV

Pierog et al. ICRC 2005 Ratio of total energy to electromagnetic energy for fluorescence detector

HiRes Spectrum Measurements StereoHR1 and HR2 Monocular Evidence for structure in Monocular Spectra Ankle at eV GZK cutoff reports by Bergman and Mannel at ICRC 2005 Weather and geometrical uncertainty cuts applied

Comparison of Various Spectra on JE 3 vs E plots – NOT RECOMMENDED as these are very misleading, as usually presented, and do the data a disservice. HiRes I and II and Stereo AGASA, Auger and HiRes I and II NB: Provisional HiRes Stereo Spectrum is not so different from AGASA !!!! Stereo and monocular in poor agreement

HiRes I and HiRes II

Fit to power law. Single index gives poor Χ 2 Evidence for changing index HiRes Stereo Flux Springer et al. ICRC 2005 Flux x log E

Ratio of Apertures computed with SIBYLL and QGSJET Sensitivity of HiRes II aperture to shower model Zech et al. HiRes Collaboration: ICRC 2005

Auger Aperture AGASA aperture

Spectrum measured with Auger Observatory The function is F=(30.9±1.7)  (E/EeV)-1.84±0.03 with Χ 2 = 2.4 per degree of freedom Issues of aperture, mass and hadronic interactions under control – systematic uncertainties being assessed

Summary Spectrum above 2 EeV aaw/Sept 2005

Summary Arrival Directions: No convincing evidence for anisotropy Possibility of BL Lac association should be clarified in ~ 2 years New Hadronic Interaction Model: suggests that there could be a heavier mass > 10 EeV than has been supposed by many in the past Spectrum: Auger: ~ 5 to 7 X AGASA by 2007 Spectrum that is largely mass and model independent AGASA/HiRes could – possibly – be understood through combination of improved understanding of HiRes aperture (composition/spectrum) and AGASA choice of models and mass assumptions ALL GROUPS HAVE REPORTED EVENTS ABOVE 100 EeV QUESTION IS: WHAT IS THE DETAILED SHAPE OF THE SPECTRUM?