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New Results from the
HiRes Experiment
Gordon Thomson
Rutgers University
UHECR Workshop - in honour of
Alan Watson
Best Wishes to Alan from the
Fly’s Eye and HiRes Gang
Outline
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Introduction
The HiRes experiment
Spectrum of UHE Cosmic Rays
Stereo Composition Measurement
Anisotropy Searches
p-air Cross Section at 1018.5 eV
Summary
HiRes Collaboration
J.A. Bellido, R.W. Clay, B.R. Dawson
University of Adelaide
S. BenZvi, J. Boyer, B. Connolly, C.B. Finley, B. Knapp, E.J. Mannel, A. O’Neill, M. Seman, S. Westerhoff
Columbia University
J.F. Amman, M.D. Cooper, C.M. Hoffman, M.H. Holzscheiter, C.A. Painter, J.S. Sarracino, G. Sinnis, T.N. Thompson, D. Tupa
Los Alamos National Laboratory
J. Belz, M. Kirn
University of Montana
J.A.J. Matthews, M. Roberts
University of New Mexico
D.R. Bergman, G. Hughes, D. Ivanov, L.R. MacLynne, L. Perera, S.R. Schnetzer, S. Stratton, G.B. Thomson, A. Zech
Rutgers University
N. Manago, M. Sasaki
University of Tokyo
R.U. Abbasi, T. Abu-Zayyad, G. Archbold, K. Belov, Z. Cao, W. Deng, W. Hanlon, P. Huentemeyer, C.C.H. Jui, E.C. Loh, K. Martens,
J.N. Matthews, K. Reil, R. Riehle, J. Smith, P. Sokolsky, R.W. Springer, B.T. Stokes, J.R. Thomas, S.B. Thomas, L. Wiencke
University of Utah
Cosmic Rays over a Wide Energy Range
• At lower energies, spectrum of
cosmic rays is almost
featureless.
– Only the “knee” at 1015.5 eV.
– Learn about galactic sources.
• Big change expected at higher
energies (>1017 eV.):
– Change from galactic to
extragalactic sources.
– Expect features due to
interactions between CR
protons and CMBR photons.
– Learn about extragalactic
sources; and propagation over
cosmic distances.
Introduction (continued.)
• HiRes is a fluorescence experiment studying UHE cosmic
rays.
• Mono: wider energy range (1017.4 < E < 1020.5 eV), best
statistics.
• Stereo: best reconstruction, covers 1018.5 < E < 1020.5 eV.
• In this energy range expect to see:
– Transition from galactic to extragalactic sources via
transition from heavy to light composition.
– Two spectral features due to interactions between cosmic ray protons
and CMBR photons:
• Suppression above threshold (1019.8 eV) for pion production (GZK
suppression).
• Feature near 1018 eV due to e+e- pair production.
• We have evidence for all these effects.
The Two HiRes Detectors
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HiRes1: atop Five Mile Hill
21 mirrors, 1 ring (3<altitude<17
degrees).
Sample-and-hold electronics (pulse
height and trigger time).
HiRes2: Atop Camel’s Back Ridge
12.6 km SW of HiRes1.
42 mirrors, 2 rings (3<altitude<31
degrees).
FADC electronics (100 ns period).
Basin and Range geological
province;
Desert; N/S mountain ranges with
narrow valleys in between.
Mirrors and Phototubes
• 4.2 m2 spherical mirror
• 16 x 16 array of phototubes, .96 degree pixels.
Calibrations: Photon Scale
• Photon Scale
– Absolute calibration: Xenon
flasher (stable to 2%) carried
from mirror to mirror; runs
done monthly.
– Two analysis methods agree:
absolute light level, and
photoelectron statistics.
– Calibrated via NIST-traceable
photodiodes.
– Checked with HPD.
– Night-to-night relative
variations monitored with YAG
laser.
• Achieve 10% accuracy.
Calibrations:
Atmospheric Monitoring
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Molecular scattering is known:
– Include seasonal effects; then
radiosonde data shows a remaining
variation of ~5 g/cm2
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Measure aerosols:
– Two 355 nm lasers, located at
HiRes1, 2, fire pattern of shots into
the air, covering the field of view.
– Scattered light viewed by HiRes2,1.
– Measure VAOD, HAL, aerosol phase
function.
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Very clear, stable skies:
– 2/3 of nights are cloudless.
– Low aerosol levels.
– Aerosols vary slowly: typically
constant over several nights.
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HiRes has an excellent site.
Aperture at 35 km.
• Laser 35 km from HiRes2.
– Vertical shots, at ~1019.5 eV
equivalent.
– Tests distant part of aperture.
– See 100% of shots when
VAOD~0.12 or less, which is
99% of cloudless nights.
• Pixel size is important:
determines aperture for high
energy events, not event
brightness or aerosols.
– 1.0 deg  35 km.
Monocular Data Analysis
• Pattern recognition.
• Fit SDP.
• Time fit (HiRes2),
5o resolution.
• Profile plot.
• Gaisser-Hillas fit.
• Profile-constrained fit
(HiRes1),
7o resolution.
HiRes1 Energy Reconstruction
• Test HiRes1 PCF
energy reconstruction
using events seen in
stereo.
• Reconstructed energy
using mono PCF
geometry vs. energy
using stereo geometry.
• Get same answer.
Stereo Analysis
• Intersection of
shower-detector planes
determines geometry,
0.60 resolution.
• Timing does as well
for parallel SDP’s.
• Two measurements of
energy, Xmax. Allows
measurement of
resolution.
Back of Envelope
Energy Calculation
dE
E  area 
dx
1
MeV
2
E  N max  1000 g / cm  2
2
g / cm 2
E  1  10 9 N max (actually 1.3  109 )
• Energy determination is robust.
• Based on center of shower, not tails.
• Easy to Monte Carlo.
Monocular Spectra:
Data / Monte Carlo Comparisons
Inputs to Monte Carlo:
Fly’s Eye stereo spectrum; HiRes/Mia and HiRes Stereo composition;
library of Corsika showers.
Detailed nightly information on trigger logic and thresholds, live mirrors, etc.
Analyze MC with exact programs used for data.
Result: excellent simulation of the data.
Spectrum Calculation
D( E )
T (E)
J (E) 
A( E ) Area  tdE
• Resolution is correctly modeled;
D(E)/A(E) = constant;
shape of spectrum comes mostly from
T(E).
• First order correction for resolution.
• Possible bias: GZK appears in data, but
not in MC.
• Second order correction:
 T ( E , noGZK ) T ( E , GZK ) 
 D( E )
b( E )  

 A( E , noGZK ) A( E , GZK ) 
• Bias is smaller than statistical
uncertainties; correction reduces J(E).
Monocular Spectra
HiRes1: 7/97-2/03
Hi/res2: 12/99-9/01
We observe: ankle;
GZK suppression at correct energy;
second knee?
Second Knee at
• Yakutsk, Akeno, Fly’s Eye Stereo,
HiRes Prototype/MIA all saw flat
spectrum followed by a steepening
in the power law. The break is
called the second knee.
• Correct for varying energy scales:
all agree on location of the second
knee.
• There are THREE spectral features
in the UHE regime.
• The ULTIMATE experiment is
one which would see the three
UHE cosmic ray features with
good statistics!
17.6
10
eV
Two Spectra:
HiRes Mono and Fly’s Eye Stereo
• Fly’s Eye Stereo
spectrum shows
second knee at 1017.6
eV.
• HiRes cannot claim
observation of second
knee.
Main Systematic Uncertainties
• Phototube calibration: 10%
• Fluorescence yield: 10% (to be measured by the Flash
experiment, among others)
• Unobserved energy in shower: 5%
• Modeling of the atmosphere: 15%
• Energy scale: 21%
• Flux: 31%
Effect of Average-Atmosphere
Approximation
09/00 - 03/01
clear nights
• Atmospheric
Database:
• Aerosol VAOD
measurement using vertical
laser tracks.
<VAOD> ~ 0.034
Preliminary
09/00 - 03/01
clear nights
• Aerosol Horizontal
Extinction Length from
horizontal laser shots.
<1/hxl> -1 ~ 20.8 km
Energy Resolution (MC study)
~ 15.9 %
Energy Resolution for
MC with atmos. database,
reconstructed with database
~ 17.5 %
Energy Resolution for
MC with atmos. database,
reconstructed with average
The Aperture (MC Study);
and the Data
Ratio of Apertures:
• numerator: using MC with
atmos. db. , reconstructed with
atmos. db.
• denominator: using MC with
atmos. db. , reconstructed with
average
Data: High Energy Events:
Average: 4 above 1019.5 eV
DataBase: 3 above 1019.5 eV
Result: Almost no change.
Does the Spectrum Continue Unabated
as a Power Law?
Fit from ankle to pion production
threshold.
Extend beyond:
Expect 29.0 events, see 11,
Poisson probability = 1x10-4
Suppression is significant.
We have good sensitivity,
but the events are not there.
Composition
• Stereo measurement
of Xmax vs. energy
• Elongation rate
changes from ~90 to
~50 g/cm2/decade at
1018.0 eV.
• Marks transition
from galactic to
extragalactic CR’s.
Resolution in HiRes Stereo Composition
• Pulls in energy and
Xmax are centered.
Energy
• Resolution:
– 20% in E and
– 15 g/cm2 in Xmax.
Xmax
• Systematic uncertainty
in Xmax ~ 15 g/cm2.
Atmospherics in Composition
Measurements
• HiRes Stereo:
– Atmospheric database used for aerosols.
– Seasonal atmospheric density profiles used for molecular
component.
– Radiosonde data from SLC, DEN show fluctuations of ~5 g/cm2.
• HiRes Prototype – MIA Hybrid Experiment:
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Constant aerosol content used, VAOD=0.05
3<Rp<4.5 km from the prototype – at all energies.
Aerosol correction is small.
Including aerosol variation might help the resolution, but will not
move the curve.
Fit Spectrum to Two Component
Toy Model
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Galactic: iron
Extragalactic: protons
Energy loss: DeMarco et al.
Parameters: extragalactic power law (at source);
relative intensity fixed to composition
measurements.
• Three fits:
– Best fit.
– Constant density, static universe
– Allow Hubble expansion: (1+z)m factor (expect m=3)
Best Fit
Contributions from Shells in z
• Nearby shells
show effect of pion
production.
• e+e- production
excavates the
ankle.
• Red shift moves
everything to the
left.
The position of the ankle tells distance to sources.
MAGENTA: m=0: static universe (sources are closer)
BLACK: m=3: Hubble expansion (sources are farther away)
Ankle location comes from average source spectrum and distance.
Stereo Spectrum:
Comparison and Resolution
• ffff
Good data/MC comparisons
Energy resolution
Stereo Spectrum
Stereo: black
HiRes1 mono: red
HiRes2 mono: blue
In agreement with mono,
But poorer statistics.
Anisotropy Searches
• HiRes1 mono
anisotropy:
asymmetric error bars,
7x0.5 deg. sq.,
area=14 deg. sq.
• Stereo anisotropy:
tiny error bars:
0.5x0.5 deg. sq.,
area=1 deg. sq.
• AGASA events:
area=20 deg.sq.
Anisotropy Searches:
Autocorrelation
HiRes
• HiRes1 mono
autocorrelation:
None seen.
• Stereo autocorrelation:
scan in energy and angular
scale.
None found: most
significant point has
Pchance=.52
Agasa
Search for Sources of Constant Intensity
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Promote the 6 Agasa clusters to be
sources of UHE cosmic rays.
We should see them too.
Search for HiRes1 mono overlaps
at 3 sigma, find 5 events, expect
4.2 randomly.
Consistent with chance overlaps.
Joint probability is 0.0013
The 6 Agasa clusters are NOT
sources of constant intensity.
Caveat: if 2 Agasa clusters are of
random origin, then joint
probability is 0.010
Stereo analysis under way.
Large Scale Anisotropy Search:
Dipole Enhancement
(suggested by Biermann et al., and Farrar et al.)
1 
n   cos 
2 2
Source Location
Galactic Center
Centaurus A
M87
α
.01 ± .05
-.02 ± .06
-.02 ± .03
p-air Cross Section at
• p-air total inelastic cross
section.
– Stereo measurement of
Xmax.
– Separates dependence of
Xmax on depth of first
interaction and shower
development.
– Gets shower development
from Corsika/QGSJet
(almost identical from
Sibyll).
– Search for gamma ray
events is under way.
18.5
10
eV
Summary: HiRes Physics Results
• HiRes mono spectra:
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See two (of the three) spectral features;
Two caused by CR – CMBR interactions;
Fit spectrum and composition, can find distance to sources.
Position of the ankle is important for astrophysics;
Must understand the galactic flux to understand the extragalactic
flux: composition is important.
• HiRes stereo spectrum:
– Agrees well with mono.
– Modest statistics as yet;
– Will extend energy coverage and statistics;
• Stereo composition measurement:
– Composition is light from 1018 to 1019.4
– Change in elongation at about 1018 eV.
Physics Results (cont’d.)
• HiRes1 mono anisotropy:
– No evidence for point sources yet;
– No confirmation of Agasa clusters;
– No dipole distributions seen.
• Stereo anisotropy:
– Excellent angular resolution;
– No evidence yet for point sources.
• Measuring p-air total cross section.
The “Ultimate” UHECR Experiment
• Achilles heel of UHECR experiments: varying energy scales between
experiments + narrow energy ranges covered per experiment.
• The ultimate experiment would stand alone.
• Wide energy coverage: 1017.0 to 1020.5 eV.
• See the second knee, ankle, and GZK suppression all in one
experiment.
• Characteristics:
– Spectrum: need excellent resolution. Fluorescence detectors are
necessary.
– Composition: Seeing Xmax is very important. Again need fluorescence.
– A large ground array is necessary.
– Ground array great for anisotropy above 1019 eV.
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Observe the galactic/extragalactic transition via composition change.
Measure all the effects of the CMBR on cosmic ray propagation.
Measure average properties of extragalactic sources.
Search for anisotropy.
Ultimate (continued):
TA/TALE
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Large ground array.
Powerful fluorescence detector:
– TA and HiRes fluorescence
detectors combined.
– Fluorescence aperture > Ground
array aperture.
– Energy range from below 1017.0 to
1020.5 eV.
– Higher elevation angle coverage:
lower energy threshold.
– Infill array for improved low
energy measurements.
– Excellent site: Millard Co. Utah;
has mountains for fluorescence
detectors, flat valley floor for
ground array.
– Good atmosphere, detectors above
the aerosol muck.
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Accomplish all the goals in
previous slide.
The Big Picture
We are making progress in understanding UHE cosmic rays:
– HiRes is verifying and extending previous measurements of spectrum and
composition, from Fly’s Eye and other experiments.
– We see evidence for the galactic–extragalactic transition.
– We see evidence for interactions between cosmic rays + CMBR photons.
– The spectrum does not continue unabated (GZK suppression is present).
– The position of the ankle is important.
– Cosmic ray astronomy (a.k.a. anisotropy studies) is in its infancy.
– TA/TALE is being designed and built for next-generation measurements.