UHECR Workshop - in honour of Alan Watson New Results from the HiRes Experiment Gordon Thomson Rutgers University

Best Wishes to Alan from the Fly’s Eye and HiRes Gang

Outline Introduction The HiRes experiment Spectrum of UHE Cosmic Rays Stereo Composition Measurement Anisotropy Searches p-air Cross Section at 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 eV. –Learn about galactic sources. Big change expected at higher energies (>10 17 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 ( < E < eV ), best statistics. Stereo: best reconstruction, covers < E < 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 ( eV) for pion production (GZK suppression). Feature near eV due to e + e - pair production. We have evidence for all these effects.

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). Basin and Range geological province; Desert; N/S mountain ranges with narrow valleys in between.

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. –Night-to-night relative variations monitored with YAG laser. Achieve 10% accuracy.

Calibrations: Atmospheric Monitoring Molecular scattering is known: –Include seasonal effects; then radiosonde data shows a remaining variation of ~5 g/cm 2 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: –2/3 of nights are cloudless. –Low aerosol levels. –Aerosols vary slowly: typically constant over several nights. HiRes has an excellent site.

Aperture at 35 km. Laser 35 km from HiRes2. –Vertical shots, at ~ 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), 5 o resolution. Profile plot. Gaisser-Hillas fit. Profile-constrained fit (HiRes1), 7 o 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, resolution. Timing does as well for parallel SDP’s. Two measurements of energy, X max. Allows measurement of resolution.

Back of Envelope Energy Calculation 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 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).

Monocular Spectra We observe: ankle; GZK suppression at correct energy; second knee? HiRes1: 7/97-2/03 Hi/res2: 12/99-9/01

Second Knee at 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. The ULTIMATE experiment is one which would see the three UHE cosmic ray features with good statistics!

Two Spectra: HiRes Mono and Fly’s Eye Stereo Fly’s Eye Stereo spectrum shows second knee at 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%

Atmospheric Database: Aerosol VAOD measurement using vertical laser tracks. Aerosol Horizontal Extinction Length from horizontal laser shots. Effect of Average-Atmosphere Approximation 09/ /01 clear nights Preliminary ~ ~ 20.8 km

Energy Resolution (MC study) Energy Resolution for MC with atmos. database, reconstructed with database Energy Resolution for MC with atmos. database, reconstructed with average ~ 15.9 % ~ 17.5 %

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 eV DataBase: 3 above 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 X max vs. energy Elongation rate changes from ~90 to ~50 g/cm 2 /decade at eV. Marks transition from galactic to extragalactic CR’s.

Resolution in HiRes Stereo Composition Pulls in energy and Xmax are centered. Resolution: –20% in E and –15 g/cm 2 in Xmax. Systematic uncertainty in Xmax ~ 15 g/cm 2. Energy Xmax

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/cm 2. HiRes Prototype – MIA Hybrid Experiment: –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 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.

Ankle location comes from average source spectrum and distance. MAGENTA: m=0: static universe (sources are closer) BLACK: m=3: Hubble expansion (sources are farther away) The position of the ankle tells distance to sources.

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 HiRes1 mono autocorrelation: None seen. Stereo autocorrelation: scan in energy and angular scale. None found: most significant point has P chance =.52 HiResAgasa

Search for Sources of Constant Intensity 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 The 6 Agasa clusters are NOT sources of constant intensity. Caveat: if 2 Agasa clusters are of random origin, then joint probability is Stereo analysis under way.

Large Scale Anisotropy Search: Dipole Enhancement (suggested by Biermann et al., and Farrar et al.) Source Location α Galactic Center.01 ±.05 Centaurus A -.02 ±.06 M ±.03

p-air Cross Section at eV 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.

Summary: HiRes Physics Results HiRes mono spectra: –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 to –Change in elongation at about 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: to eV. See the second knee, ankle, and GZK suppression all in one experiment. Characteristics: –Spectrum: need excellent resolution. Fluorescence detectors are necessary. –Composition: Seeing X max is very important. Again need fluorescence. –A large ground array is necessary. –Ground array great for anisotropy above eV. 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 Large ground array. Powerful fluorescence detector: –TA and HiRes fluorescence detectors combined. –Fluorescence aperture > Ground array aperture. –Energy range from below to 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. 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.