Observation of the GZK Cutoff by the Hi

Observation of the GZK Cutoff by the Hi. Res Experiment Gordon Thomson Rutgers University GZK-40 Workshop

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

Outline • • • Introduction The Hi. Res 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 e. V. – Learn about galactic sources. • Big change expected at higher energies (>1017 e. V. ): – 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 6 x 1019 e. V (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 e. V – QSO’s, AGN’s are violent, give higher Emax (>1020 e. V) • 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 (Hi. Res) Experiment: Has the World’s Highest Exposure • Hi. Res is a fluorescence experiment studying UHE cosmic rays. • Fluorescence yield: – – Charged particles excite N 2 molecules. Emit ~5 photons/mip/meter. 300 -400 nm wavelength. High energy showers are bright. • Hi. Res has two detectors located atop desert mountains in westcentral 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 e. V), best statistics. • Stereo: best resolution, covers 1018. 5 < E < 1020. 5 e. V.

High Resolution Fly’s Eye (Hi. Res) Collaboration S. Ben. Zvi, 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 Hi. Res Detectors • Hi. Res 1: atop Five Mile Hill • 21 mirrors, 1 ring (3<altitude<17 degrees). • Sample-and-hold electronics (pulse height and trigger time). • Hi. Res 2: Atop Camel’s Back Ridge • 12. 6 km SW of Hi. Res 1. • 42 mirrors, 2 rings (3<altitude<31 degrees). • FADC electronics (100 ns period).

Mirrors and Phototubes • 4. 2 m 2 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 Hi. Res 1, 2, fire pattern of shots into the air, covering the field of view. – Scattered light viewed by Hi. Res 2, 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. • Hi. Res has an excellent site.

Measurement of Fluorescence Yield • Three published results: Kakimoto et al. , Nagano et al. , and T 461. • Ratio of fit to (Kakimoto, Nagano, and T 461) to fit to Kakimoto = 1. 00 ± 0. 06

Monocular Data Analysis • Pattern recognition. • Fit SDP. • Time fit (Hi. Res 2), 5 o resolution. • Profile plot. • Gaisser-Hillas fit. • Profile-constrained time fit (Hi. Res 1), 7 o resolution.

Stereo Analysis • Intersection of showerdetector 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.

Aperture Calculation acceptance • Need complete simulation of detector: create MC sample identical to the data. – Put in spectrum, composition, as measured by Fly’s Eye, Hi. Res. MIA, Hi. Res 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.

Compare to High Energy Physics Acceptance Calculation Quantity • • • Beam definition Decay/Interaction Trigger Resolution Effect of cuts • Result: KTe. V Hi. Res hard easy medium easy very 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; Hi. Res/Mia and Hi. Res 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).

“Test Beam” of High Energy Events • • Laser at Terra Ranch 35 km from Hi. Res-2 Vertical, 355 nm Fires at five energies, as bright as 40 -125 Ee. V showers. • Efficiency for good-weather nights. • Excellent trigger + reconstruction efficiency above 6 x 1019 e. V. • We see high energy events with good efficiency. GZK Cutoff

Constant–Aperture Study: a “feeling” for the data. • Cut at 10 km, 15 km • Plot histogram of energies, weighted by E 2 to see spectral features. • See the “ankle”, high energy suppression, in the raw data.

Monocular Spectra Hi. Res 1: 7/97 -5/05 Hi. Res 2: 12/99 -8/04 We observe: ankle; high-energy suppression; second knee?

Second Knee at • Yakutsk, Akeno, Fly’s Eye Stereo, Hi. Res 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. 17. 6 ~10 e. V

Spectrum with Systematic Uncertainty from Composition • • Composition determines whether <Xmax> is in Hi. Res’ field of view, or above. Different apertures for Corsika/QGSJet protons and iron; leads to systematic uncertainty below 1018 e. V, which is larger than statistical uncertainty. Hi. Res 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

Hi. Res and Other Experiments Hi. Res, Fly’s Eye Stereo, and Hi. Res/MIA Hi. Res, AGASA, Auger(2005)

5σ Observation of the GZK Suppression • Broken Power Law Fits – No Break Point • Chi 2/DOF = 162/39 – One BP • Chi 2/DOF = 68. 2/37 • BP = 18. 63 – Two BP’s • Chi 2/DOF = 34. 7/35 • 1 st BP = 18. 63 • 2 nd BP = 19. 75 – Two BP with Extension • Expect 51. 1 events • Observe 15 events • Poisson probability: P(15; 51. 1) = 3 x 10 -9 (5. 8 s) – Independent statistics: P(14; 44. 9)=7 x 10 -8 (5. 2σ)

Integral Spectra • Want to test E½ with integral spectra • Use 2 BP Fit with Extension for the comparison • log 10 E½ = 19. 73 ± 0. 07 • Berezinsky et al. : log 10 E½ = 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 e. V. • Heavy (galactic) nuclei decrease, give way to light (extragalactic) composition. • Galactic/extragalactic transition is complete by about 1018 e. V. • 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 e. V, ρ=const*(1+z)m. Energy loss from interactions with CMBR and uniform expansion. • Best fit: – m = 1. 95 – g = -2. 47 – Chi 2 = 49. 7/41

Interpretation of Extragalactic Spectrum • Pion-production pileup causes the bump at 1019. 5 e. V. • e+e- pair production excavates the ankle. • Pileup from e+eproduction 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 ~ 3 x 1018 e. V. – Berezinsky et al. describe a change in diffusion regimes, when r. L = lc This would cause a break in the extragalactic spectrum near Ec If B=1 n. G and lc=1 Mpc, then Ec=0. 9 x 1018 e. V, 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 e. V, 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 SDSS QSO density histograms: Croom et al. , Schneider et al. Lines: (1+z)3 QSO luminosity density, optical Boyle and Terlevich AGN luminosity density, 2 -8 ke. V X-rays. Barger et al.

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 e. V. • • 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, 108 o 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 e. V.

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 e. V. • Infill array for hybrid observation at the lowest energies.

TALE Detectors: 6 -km Stereo Detectors • Move Hi. Res mirrors to form two stereo detectors with TA FD’s. • Observe the ankle with a flat aperture. • Increase stereo aperture at 1018 e. V over Hi. Res by an order of magnitude. 1019 e. V 1018 e. V 1020 e. V

TALE Detectors: the Tower Detector • Two methods of lowering the minimum energy: – Use bigger mirrors. – Look higher up. • Tower detector with 3 x larger mirrors: – 750 cm radius of curvature. – Use Hi. Res-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 e. V. • Study the second knee, composition, in hybrid mode.

TALE Detectors: the Infill Array • 110 scintillation counters, spacing of 400 m, shown in red. • 10 x Hi. Res/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 km 2 ster, 3 x. Hi. Res • half SD, • half fluorescence (in mono, stereo, hybrid, stereo hybrid). – 10 x Hi. Res stereo aperture at 1018 e. V. – 10 x Hi. Res/MIA hybrid aperture, E < 1018 e. V. – Extend Emin down to 1016. 5 e. V. • 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 e. V). – Observe light composition above 1018 e. V. • Do cosmology.

TA/TALE Anisotropy • Hi. Res sees correlations with BL Lac’s. • Point source figures of merit at 1019 e. V: (Hi. Res has 31 events above bkg correlated with BL Lac’s) Experiment Aperture (km 2 ster) Resolution Figure of Merit (A/Resolution 2) Hi. Res 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 260 0. 1 26000 TA/TALE hybrid stereo 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 e. V). • 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 • Hi. Res 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 6 x 1019 e. V (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 e. V QSO’s, AGN’s are violent, give higher Emax (>1020 e. V) 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 e. V. • Extragalactic sources: – – All source claims must be confirmed. Breaks in evolution may show up in extragalactic spectrum in the 1017 e. V decade. γ and m can be measured. The TA/TALE experiment may be able to observe source evolution.