TAUP 2005: Zaragoza Observations of Ultra-high Energy Cosmic Rays Alan Watson University of Leeds Spokesperson for Pierre Auger Observatory a.a.watson@leeds.ac.uk.
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TAUP 2005: Zaragoza
Observations of Ultra-high Energy Cosmic Rays
Alan WatsonUniversity of Leeds
Spokesperson for Pierre Auger Observatory
a.a.watson@leeds.ac.uk
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
km2 sr years > 3 EeV >10 EeV
AGASA: closed in January 2004: 1600 7000 827
HiRes I: monocular ~5000 1616 403
(HiRes II: monocular 670 95
HiRes: stereo (PRELIMINARY) ~2500 ~3000 ~500
HiRes apertures are strongly energy-dependent (later)HiRes will cease operation in March 2006
Yakutsk: ~900 1303 171
Auger: data taking since Jan 2004 1750 3525 444
Telescope Array: plan is for 760 km2 with three fluorescence detectors
Array of water → Cherenkov detectors
Fluorescence →
The Pierre Auger Observatory design marries twowell-established techniques
The ‘HYBRID’ technique
11
The Pierre Auger Observatory as planned
Surface Array 1600 detector stations 1.5 km spacing 3000 km2
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 tracesFlash ADC traces
Lateral density distribution
Typical flash ADC trace
Detector signal (VEM) vs time (ns)
PMT 1
PMT 2
PMT 3
-0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 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 3
-0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 µs
Fitted Electromagnetic Shower
from Fly's Eye 1985
Tim
e μ
sec
Angle Χ in the shower-detector plane
Same Hybrid Eventθ~ 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)< 1.4º for 5 or more station events (E>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 DataLaser Data
Core position resolution:Hybrid: < 60 m Surface array: < 200 m
Laser position – Hybrid and FD only (m)
-500
+500
501
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/E
eV
) 10EeV
1 EeVHybrid Events with strict event selection:
track length > 350g cm-2
Cherenkov contamination <10%
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 ~ A0.14
So, more muons in Fe showers
(i) Variation of Depth of Maximum with Energy
Elongation Rate (Linsley 1977, Linsley and Watson 1981)
dXmax/ dlog E < 2.3Xo g cm-2 /decade
from Heitler model Xmax = ln (Eo/c )/ ln 2
Methods of Inferring the Primary Mass
HADRONIC MODELS REQUIRED
FOR INTERPRETATION
Claim: Consistent with proton dominant component
19 19.5 20 20.5
Log(Energy [eV])
−2
−1
0
1
Log(
Muo
n de
nsity
@10
00m
[m–2
])
Muon measurements with the AGASA array
Kenji Shinosaki: 129 events > 1019 eV
HiRes Spectrum MeasurementsHiRes Spectrum Measurements
StereoHR1 and HR2 Monocular
•Evidence for structure in Monocular Spectra•Ankle at 1018.5 eV•GZK cutoff
• reports by Bergman and Mannel at ICRC 2005
Weather and geometrical uncertainty cuts applied1018 1019
Comparison of Various Spectra on JE3 vs E plots – NOT RECOMMENDED as these are very misleading, as usually presented, and do the data a disservice.
HiRes I and II and StereoAGASA, Auger and HiRes I and II
NB: Provisional HiRes Stereo Spectrum is not so different from AGASA !!!!
Stereo and monocularin poor agreement
• Fit to power law.
• Single index gives poor Χ2
• Evidence for changing index
1019 1020
HiRes Stereo Flux
Springer et al. ICRC 2005F
lux x
1029
log E
Ratio of Aperturescomputed with SIBYLLand QGSJET
Sensitivity of HiRes II aperture to shower model
Zech et al. HiRes Collaboration: ICRC 2005
Spectrum measured with Auger Observatory
The function is
F=(30.9±1.7)(E/EeV)-1.84±0.03
with Χ2 = 2.4 per degree of freedom
Issues of aperture, massand hadronic interactionsunder control – systematic uncertainties being assessed
Summary
Arrival Directions: No convincing evidence for anisotropyPossibility 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 hasbeen 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 combinationof 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?
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