Transcript Why HiRes Was Able to Observe the GZK Cutoff

Douglas Bergman
University of Utah
CCAPP Inaugural Symposium
12 October 2009
Introduction
 The High Resolution Fly’s Eye (HiRes) experiment has
recently finished its 10 year data taking run.
 Good chance to summarize our knowledge of ultra-high
energy cosmic rays as seen from the northern hemisphere
 The recent results from HiRes, final analyses, cover all
three of the basic types of cosmic ray measurements
 Spectrum (now in stereo)
 Composition
 Anisotropy (in particular, correlation with the local mass
structure of the universe)
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The HiRes Experiment
 HiRes was a stereo
fluorescence detector, operated
from 1997-2006 on Dugway
Proving Grounds in Utah
 Observe the air-showers
created by CR’s by collecting
fluorescence light
HiRes-I
HiRes-II
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The HiRes Experiment
 Light collected by 5 m2
mirrors onto an array of
256 (16×16) of PMT’s
 Each PMT sees 1° cone
 Each PMT records time
and amount of light seen
 Reconstruct shower
geometry by stereo
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Sample HiRes Event
Nmax = (7.1 ± 0.5) × 109
Xmax = 779 ± 26 g/cm2
E = 8.6 ± 0.6 EeV
χ2/DOF = 19.5/17
Nmax = (6.14 ± 0.13) × 109
Xmax = 812 ± 5 g/cm2
E = 8.4 ± 0.2 EeV
χ2/DOF = 100/54
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Stereo Spectrum Measurement
 To find spectrum:
 Collect data
 Find energy of each event
 Bin events in energy bins
 Calculate the aperture
(that’s the hard part)
 Calculate aperture by
simulation of detector
 Verify by data/simulation
comparisons
Reduce systematic by
finding “fully efficient” area
at each energy
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Stereo Spectrum Measurement
 To find spectrum:
 Collect data
 Find energy of each event
 Bin events in energy bins
 Calculate the aperture
(that’s the hard part)
 Calculate aperture by
simulation of detector
 Verify by data/simulation
comparisons
 Reduce systematics by
finding “fully efficient” area
at each energy
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The UHECR Energy Spectrum
 The “geo-constrained”
spectrum is not
systematically different
than the full spectrum, so
we use the full spectrum
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The UHECR Energy Spectrum
 The stereo spectrum
confirms the observation
of the GZK we observed
with out monocular
analyses
 In the southern
hemisphere, Auger see a
similar (but with perhaps
slightly different slopes
and a different cutoff
energy)
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UHECR Composition Measurement
 Xmax grow logarithmically
with energy as the shower
branches more
 Heavier CR’s (more
nucleons) act like a
superposition of lower
energy proton showers
 E

X max  R  ln  ln A   C
 Ec

 X ( A) 
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 X ( p)
A
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UHECR Composition Measurement
 Measure composition by
Protons in QGSJetII
finding average Xmax vs
energy
 Not gaussian: mean
subject to biases
 Different models give
different averages, but
similar slopes (elongation
rate)
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Iron in QGSJetII
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
 Make acceptance
correction based on
QGSJetII protons
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
 Make acceptance
correction based on
QGSJetII protons
 Compare to other results
 Combined with
HiRes/MIA, heavier at
low energies, mostly light
by 1 EeV
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
 Make acceptance
correction based on
QGSJetII protons
 Compare to other results
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UHECR Composition Measurement
 Here’s the HiRes data
 Looks mostly like protons
 Make acceptance
correction based on
QGSJetII protons
 Compare to other results
 Note that uncorrected
average is very close to
Auger
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UHECR Composition Measurement
 Also look at the width of
showers
 HiRes width agrees with
predicted width for
protons
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UHECR Composition Measurement
 Also look at the width of
showers
 HiRes width agrees with
predicted width for
protons
 Compare to Auger
(without detector
resolution removed)
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UHECR Correlation with LSS
 HiRes data indicates:
 UHECR’s are protons
 Many come from far away


Otherwise no GZK
Beyond 50 Mpc
 Trajectories rigid enough to point back to origin
 Look for correlations with various objects (say AGN as
Auger has done)
 Or look for correlation with mass structure out to 250 Mpc
using flux limited samples (2MASS)
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UHECR Correlation with LSS
 Start with 2MASS to
create LSS model
57 EeV
 Smear by variable angle
 Limit distance by energy
 Convolve with HiRes
40 EeV
exposure
 Perform K-S test based on
density of LSS model
10 EeV
Smearing angle of 6°
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UHECR Correlation with LSS
10 EeV
40 EeV
57 EeV
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UHECR Correlation with LSS
 Plot K-S probability for
both isotropic and LSS
models
 Choose 95% CL a priori
 Good agreement with
isotropy
 Poor agreement at small
scattering angles for LSS
 No correlation at 95% CL
for E > 40 EeV and θs < 10°
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Conclusions
 HiRes has observed the GZK cutoff in both monocular
and stereo modes
 HiRes finds the composition of UHECR’s above 1 EeV to
be predominantly light, as one might expect from the
presence of the GZK cutoff
 HiRes observes no correlation with the local, large-scale
structure of the universe
 The lack of correlations is surprising since magnetic field
smearings are only expected to be at the 5° level
 The Telescope Array is currently operating in the North, and
will provide much more anisotropy data
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