GZK-40 Workshop

Observation of the GZK Cutoffby the HiRes Experiment

Gordon Thomson

Rutgers University

The HiRes Collaboration congratulates Georgy Zatsepin and Vadim Kuzman on the occasion of the 40th anniversary of their epoch making prediction of the high-energy cutoff of the cosmic ray flux.

Outline

• Introduction

• The HiRes experiment

• Calculating the spectrum

• Results

• Information about sources

• Future studies

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. (there are one-too-many features.)

– Learn about extragalactic sources; and propagation over cosmic distances.

Important Questions about UHE Cosmic Rays

• Do extragalactic cosmic ray protons interact with photons of the CMBR?– Does pion production suppresses the flux above 6x1019 eV (the

GZK cutoff)?– Does e+e- pair production leave a signature?

• Where does the flux of galactic sources give way to that of extragalactic sources?– The Milky Way is a milquetoast galaxy; Emax< 1018.5 eV– QSO’s, AGN’s are violent, give higher Emax (>1020 eV)

• What are the extragalactic sources? What are their properties?– QSO’s? AGN’s? GRB’s? ???– Can we point to any sources?– Spectral index and Emax distribution?– Evolution of the sources?

High Resolution Fly’s Eye (HiRes) Experiment:

Has the World’s Highest Exposure

• HiRes is a fluorescence experiment studying UHE cosmic rays. • Fluorescence yield:

– Charged particles excite N2 molecules.– Emit ~5 photons/mip/meter.– 300-400 nm wavelength.– High energy showers are bright.

• HiRes has two detectors located atop desert mountains in west-central Utah. Operated from 5/1997 to 4/2006.

• Collected data on moonless nights: about 10% duty factor. • Mono: wider energy range (1017.2 < E < 1020.5 eV), best statistics.

• Stereo: best resolution, covers 1018.5 < E < 1020.5 eV.

High Resolution Fly’s Eye (HiRes) Collaboration

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, S.R. Schnetzer, L. Scott, S. Stratton, G.B. Thomson

Rutgers University

N. Manago, M. Sasaki

University of Tokyo

R.U. Abbasi, T. Abu-Zayyad, G. Archbold, K. Belov, A. Blake, Z. Cao, W. Deng, W. Hanlon, P. Huentemeyer, C.C.H. Jui, E.C. Loh, K. Martens, J.N. Matthews, D. Rodriguez, J. Smith, P. Sokolsky, R.W. Springer, B.T. Stokes, J.R. Thomas, S.B. Thomas, L. Wiencke

University of Utah

The Two HiRes Detectors

• 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).

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 and 4 km laser shots.

– Night-to-night relative variations monitored with YAG laser.

• Achieve 10% accuracy.

Calibrations:Atmospheric Monitoring

• Atmospheric Scattering: molecular portion measured by radiosonde balloons at two nearby airports.

• 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.• Very clear, stable skies:

– ~70% of nights are cloudless.– Low aerosol levels: <VAOD>=.04– Aerosols vary slowly: typically

constant over several nights.

• HiRes has an excellent site.

Measurement of Fluorescence Yield

• Three published results: Kakimoto et al., Nagano et al., and T461.

• Ratio of fit to (Kakimoto, Nagano, and T461) to fit to Kakimoto

= 1.00 ± 0.06

Monocular Data Analysis

• Pattern recognition.• Fit SDP.• Time fit (HiRes2),

5o resolution.• Profile plot.• Gaisser-Hillas fit.• Profile-constrained

time fit (HiRes1), 7o resolution.

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

• Energy determination is robust.

• Based on center of shower, not tails.

• Easy to Monte Carlo.

)101.3(actually 101

/2/1000

2

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9max

9

22

max

NE

cmg

MeVcmgNE

dx

dEareaE

Aperture Calculation

• Need complete simulation of detector: create MC sample identical to the data.– Put in spectrum, composition, as measured by Fly’s Eye, HiRes-

MIA, HiRes stereo experiments; use actual Corsika showers.– Shower development– Light collection– Trigger and readout electronics

• Write out MC in same format as data.• Analyze both with same program.• Compare histograms of data and MC to judge success (or

failure) of simulation.

AEThr

EAcc

)(

)(Aperture

acceptance

Compare to High Energy Physics Acceptance Calculation

Quantity KTeV HiRes• Beam definition hard easy• Decay/Interaction easy medium• Trigger medium medium• Resolution hard easy• Effect of cuts easy easy

• Result: very accurate accurate

Details of Simulation• Gaisser-Hillas formula:

data and MC mean profiles.

• Corsika Sibyll/QGSJet aperture ratio.

• Trigger threshold change by 5%.

Compare Data to Monte Carlo: Judge success of simulation and

acceptance calculation.

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, and an accurate aperture calculation.

(Steeply Falling) Spectrum Calculation

• 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:

• Bias is smaller than statistical uncertainties; correction reduces J(E).

tdEArea

ET

EA

EDEJ

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GZKEA

GZKET

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“Test Beam” of High Energy Events

• Laser at Terra Ranch • 35 km from HiRes-2• Vertical, 355 nm• Fires at five energies, as bright

as 40-125 EeV showers.• Efficiency for good-weather

nights.• Excellent trigger +

reconstruction efficiency above 6 x 1019 eV.

• We see high energy events with good efficiency.

GZK CutoffGZK Cutoff

Constant–Aperture Study:a “feeling” for the data.

• Cut at 10 km, 15 km• Plot histogram of

energies, weighted by E2 to see spectral features.

• See the “ankle”, high energy suppression,

in the raw data.

Monocular Spectra

We observe: ankle; high-energy suppression; second knee?

HiRes1: 7/97-5/05HiRes2: 12/99-8/04

Second Knee at ~1017.6 eV

• 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. Location of second knee is not known accurately.

Spectrum with Systematic Uncertainty from Composition

• Composition determines whether <Xmax> is in HiRes’ field of view, or above.

• Different apertures for Corsika/QGSJet protons and iron; leads to systematic uncertainty below 1018 eV, which is larger than statistical uncertainty.

• HiRes can’t say much about the second knee.

• We need an experiment, with wide enough energy range, which would see all three UHE cosmic ray features with good statistics!

Add the Stereo Spectrum

HiRes and Other Experiments

HiRes, Fly’s Eye Stereo, and HiRes/MIA HiRes, AGASA, Auger(2005)

5σ Observation of the GZK Suppression

• Broken Power Law Fits– No Break Point

• Chi2/DOF = 162/39

– One BP• Chi2/DOF = 68.2/37• BP = 18.63

– Two BP’s• Chi2/DOF = 34.7/35• 1st BP = 18.63• 2nd BP = 19.75

– Two BP with Extension• Expect 51.1 events• Observe 15 events• Poisson probability:

P(15;51.1) = 3x10-9

(5.8)

– Independent statistics: P(14;44.9)=7x10-8 (5.2σ)

Integral Spectra

• Want to test E½ with integral spectra

• Use 2BP Fit with Extension for the comparison

• log10E½ = 19.73 ± 0.07• Berezinsky et al.:

log10E½ = 19.72, for wide range of conditions.

• Suppression is at the right energy for the GZK cutoff.

The GZK cutoff is present.We see the Ankle clearly.

What’s next?

• Where is the galactic/extragalactic transition?• Composition gives an indication.

• Is there a sign of e+e- pair production?• What are the properties of the extragalactic sources?

– Power law?– Distribution of Emax values?– Evolution of the sources?

• Fit the spectrum to a model which contains both galactic and extragalactic sources.

<Xmax> Composition Galactic/Extragalactic Transition

• There is a model-independent break in slope at about 1018 eV.

• Heavy (galactic) nuclei decrease, give way to light (extragalactic) composition.

• Galactic/extragalactic transition is complete by about 1018 eV.

• Fits, plus QGSJet predictions, yield model of proton fraction as a function of energy.

• All fluorescence measurements of Xmax are consistent.

• Only fluorescence experiments SEE Xmax.

Fit Spectrum and Composition Simultaneously

• Fit composition and spectrum simultaneously. – Heavy = nuclei, inert propagation– Light = extragalactic protons.

• Extragalactic proton model: – Spectral index γ, – Emax = 1021 eV, – ρ=const*(1+z)m.– Energy loss from interactions with

CMBR and uniform expansion.• Best fit:

– m = 1.95– = -2.47– Chi2 = 49.7/41

Interpretation of Extragalactic Spectrum

• Pion-production pileup causes the bump at 1019.5 eV.

• e+e- pair production excavates the ankle.

• Pileup from e+e- production below ankle.

• Fractionation in distance and energy; e.g., z=1 dominates at second knee.

• Evolution of the sources is important. D. Bergman’s plot of shells in z

The Ankle is Important

• Ankle is due to e+e- pair production:– Berezinsky: better evidence of

CMBR interactions than GZK.– Shows that composition is

mostly protons [+ some Helium (Hillas); or “light” (Allard et al.)].

• Astrophysics: tells about cosmic ray sources. – Ankle region spectral index

at source. – fall to ankle evolution

parameter, m.

What Causes the Second Knee?• Possibilities:

– A second type of galactic source? Must have Emax ~ 3x1018 eV. – Berezinsky et al. describe a change in diffusion regimes, when rL = lc This would

cause a break in the extragalactic spectrum near Ec If B=1 nG and lc=1 Mpc, then Ec=0.9x1018 eV, just above the second knee.

– The break in QSO and AGN evolution at z~1.6 might show up in the UHECR spectrum as a spectral break. It would be located at the energy of the second knee. (see below)

• Tests:– Second type of galactic source not supported by current <Xmax> data.– Collect better data: one experiment needs to cover from 1016.5 to 1020.5 eV, and

observe all three spectral features. Measure the energy of the second knee, and its sharpness.

– Composition is important. • Kascade-Grande results will be interesting, but model-dependent. • Measure <Xmax> throughout the region a fluorescence experiment is needed. Must

test Kascade-Grande results against fluorescence results.– Perform a correlated study: measure whether the second knee occurs in events at

low or high <Xmax>. Low <Xmax> galactic; high <Xmax> extragalactic.

Evolution of QSO’s and AGN’s

QSO density histograms: Croom et al., Schneider et al.

QSO luminositydensity, optical Boyle and Terlevich

AGNluminosity density, 2-8 keV X-rays. Barger et al.

SDSS

Lines: (1+z)3

Where should the break be seen?

• Adjust evolution to match QSO’s: – m=2.6, z<1.6– Lower m, z>1.6

• Break appears at the location of the second knee.

• Need a new experiment with wider energy range to study the ankle and second knee.

• Must extend spectrum and <Xmax> measurements lower by an order of magnitude.

• TA/TALE aim: measure spectrum from 1016.5 to over 1020 eV.

2004

Upcoming Experiment:The Telescope Array (TA) and

TA Low Energy Extension (TALE)• TA surface detector: 576

scintillation counters, 1.2 km spacing.

• 3 TA fluorescence detectors overlook SD, 108o in azimuth each.

• 2 TALE fluorescence detectors plus infill array:– Observe the ankle with flat

aperture.– Tower detector + infill

array cover lower energies.

• Cover 1016.5 – 1020.5 eV.

TA/TALE Layout

• Two 6-km stereo pairs: observe the ankle with flat aperture.

• Tower detector with 3 times larger mirrors: reach down to 1016.5 eV.

• Infill array for hybrid observation at the lowest energies.

TALE Detectors: 6-km Stereo Detectors

• Move HiRes mirrors to form two stereo detectors with TA FD’s.

• Observe the ankle with a flat aperture.

• Increase stereo aperture at 1018 eV over HiRes by an order of magnitude.

1018 eV1019 eV

1020 eV

TALE Detectors: the Tower Detector

• Two methods of lowering the minimum energy:– Use bigger mirrors.– Look higher up.

• Tower detector with 3x larger mirrors:– 750 cm radius of curvature.– Use HiRes-type phototubes with

Winston cones.– Build 15 mirrors. – Look up to 71 degrees elevation.

• Lower minimum energy by an order of magnitude, to 1016.5 eV.

• Study the second knee, composition, in hybrid mode.

TALE Detectors: the Infill Array

• 110 scintillation counters, spacing of 400m, shown in red.

• 10 x HiRes/MIA hybrid aperture.

• Study the second knee in hybrid mode.

• Observe galactic – extragalactic transition.

• Considering transfer of AGASA counters.

Tower detector

TA FD

TA/TALE• Apertures:

– High energy aperture: 3000 km2 ster, 3xHiRes • half SD, • half fluorescence (in mono, stereo, hybrid, stereo hybrid).

– 10x HiRes stereo aperture at 1018 eV.– 10x HiRes/MIA hybrid aperture, E < 1018 eV. – Extend Emin down to 1016.5 eV.

• Measure all three spectral features in one experiment.• Study the ankle with flat aperture.• Study the galactic-extragalactic transition:

– Mixed composition at low energies: several heavy elements contribute; compare with Kascade and Kascade-Grande.

– Watch the heavy elements die away (~1017.5 eV).– Observe light composition above 1018 eV.

• Do cosmology.

TA/TALE Anisotropy• HiRes sees correlations with BL Lac’s.• Point source figures of merit at 1019 eV: (HiRes has 31 events above bkg correlated with BL Lac’s)

Experiment Aperture

(km2 ster)

Resolution Figure of Merit (A/Resolution2)

HiRes stereo 300 (avg) 0.5 deg 1200

TA/TALE stereo 340 0.5 1360

TA SD 1500 1.5 667

Auger SD 6600 1.5 2933

TA/TALE hybrid stereo

260 0.1 26000

Multi-energy observations are important!

TA/TALE is a Powerful Detector

• Spectrum measurements over four orders of magnitude in energy (1016.5 to 1020.5 eV).

• Composition measurements in hybrid or stereo over entire energy range.

• Flat stereo aperture in ankle region.

• World’s best point-source capability for multi-energy observations.

Summary

• HiRes has observed the GZK cutoff.

• We see two of the three spectral features in the ultrahigh energy regime: the GZK cutoff and the ankle.

• We see the galactic/extragalactic transition.

• We will continue these studies with TA/TALE.

Answers to Some Questions• Do extragalactic cosmic ray protons

interact with photons of the CMBR?– Does pion production suppresses the

flux above 6x1019 eV (the GZK suppression)?

– Does e+e- pair production leave a signature?

• Where does the flux of galactic sources give way to that of extragalactic sources?

– The Milky Way is a milquetoast galaxy; Emax< 1018.5 eV

– QSO’s, AGN’s are violent, give higher Emax (>1020 eV)

• What are the extragalactic sources? What are their properties?

– Can we point to any sources?– QSO’s? AGN’s? GRB’s? ???– Spectral index and Emax distribution?– Evolution of the sources?

• YES! CR’s interact with the CMBR.

– GZK suppression is present.– e+e- pair production excavates the ankle.

• Transition is complete by ~1018 eV.

• Extragalactic sources:– All source claims must be confirmed.– Breaks in evolution may show up in

extragalactic spectrum in the 1017 eV decade.

– γ and m can be measured. – The TA/TALE experiment may be able

to observe source evolution.