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The High Resolution Fly’s Eye. John Matthews University of Utah Department of Physics and High Energy Astrophysics Institute. Outline. Questions Detector Calibration Atmosphere Capabilities. UHECR Questions. Energy Spectrum Sources, Anisotropy Composition Search for neutrinos.
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The High Resolution Fly’s Eye John Matthews University of Utah Department of Physics and High Energy Astrophysics Institute
Outline • Questions • Detector • Calibration • Atmosphere • Capabilities
UHECR Questions • Energy Spectrum • Sources, Anisotropy • Composition • Search for neutrinos
The Cosmic Ray Spectrum • Direct Observation below about 1014 eV • Indirect Observation at higher energies • HiRes is optimized for stereo observations above 3x1018 eV
Anisotropy and Composition • Relatively isotropy • Slight anisotropy observed by Fly’s Eye and possible sources observed by AGASA • Composition • In the 1012 – 1015 eV energy range, direct measurements are available. The composition is: • ~50% protons • ~25% alpha particles • ~13% C/N/O nuclei • <1% electrons • At higher energies, Fly’s Eye observed a composition which was heavy (iron-like) transitioning to lighter (proton-like) around 5x1018 eV • Neutrino Search • Search for deeply penetrating or upward going events….
The Atmosphere as Detector • The cosmic ray initiates an extensive air shower when it hits the atmosphere • Measure the resulting fluorescence light
The High Resolution Fly’s Eye (HiRes) • Spherical Mirrors: Area = 5.1 m2 • 256 PMT pixels/mirror: 1 degree resolution
Stereo Observation • Viewing the shower from two sites allows for precise measurement of geometry as well as a measurement of the energy resolution
Utah’s West Desert • HiRes is located in Utah’s West Desert at the Dugway Proving Ground • The two sites are separated by 12.6 km
HiRes-I: Little Granite Mountain • 22 mirrors viewing • ~360 deg azimuth • 3-17 deg elevation • S&H electronics (5.6 ms integration time) • Began data collection in 1997, completed 1998 • 2947 hrs data, 2314 hrs good weather
HiRes-II: Camel’s Back Ridge • 42 mirrors viewing ~360 deg azimuth and 3-31 deg in elevation • FADC electronics with 100ns integration time • Completed in Aug. 1999
Atmospheric Transmission • Need to know the atmospheric quality • Clouds • Haze • General transmission (horizontal extinction length and vertical scale height)
Atmospheric Instrumentation • Two Steerable YAG Lasers • Located at each of the HiRes sites • Computer/GPS controlled • Operated nightly • One Steerable Mobile YAG Laser • Computer/GPS controlled • Operated as needed • 10 Vertical Xenon Flashers • Arrayed between the two HiRes sites
Better than Standard Desert • The “Standard Desert” Model uses an aerosol horizonal extinction length of 12 km and vertical scale height of 1.2 km. Optical Depth: OD=VS/HE = 1.2/12 = 0.10 • Dugway Atmosphere is, on average, cleaner with an aerosol horizontal extinction length of 25 km and 1.0 km vertical scale height. Optical Depth: OD=VS/HE = 1.0/25 = 0.04 • In addition, the molecular part of the atmosphere has a horizontal extinction length of 18 km
A HiRes Event • Example of a cosmic ray air shower as observed by HiRes-I: • Up = North • Right = East • Center of Circle = Zenith (UP) • Outer Circle = Horizon
Capabilities of the HiRes Experment • Stereo instantaneous aperture grows to 10,000 km2-str at 1020 eV. Factor of ten over Fly’s Eye • Energy resolution 10% • Xmax resolution 30 gm/cm2 • Angular resolution better than 0.4 degreees • Measure Spectrum, Composition, and Anisotropy. Search for Neutrino flux • Expect to collect at least 5 years of data
Measurement of the UHECR Spectrum and its Structure John Matthews University of Utah Department of Physics and High Energy Astrophysics Institute
The HiRes-I Data Set • Collected June 1997- May 2001 • 2947 hours of monocular data • 2314 hours of good weather data selected for this analysis • Containing 119M triggers (mostly noise and atmospheric monitoring ie laser shots) • 4.54M downward “track-like” events selected for further processing
HiRes-I Data Cuts Events were also required to: • >25 p.e./PMT (on average) – for reliable shower profile reconstruction – avoid Poisson fluctuation problems • angular speed < 3.33 deg/ms (5 km for vertical tracks) • 10,683 events selected for reconstruction
HiRes-I Monocular Fitting • Profile Constrained Fit (assumed a Gaisser-Hillas parameterization) • Assumed • x0 = 40 gm/cm2 (shower initiation) • l = 70 gm/cm2 (elongation rate) • Allowed xmax to vary 680 < xmax < 900 gm/cm2, 35 gm/cm2 steps (expected range for p - Fe primaries) • Stepped through y in 1 deg steps (angle of shower in the plane)
HiRes-I: More Data Cuts Assuming the profile fit converged, additional cuts were made. Events were required to have: • > 8 deg of track length • First observation < 1000 gm/cm2 (highest elevation) • Inplane shower angle, y < 120 degrees (avoid events with significant Cerenkov light) • No more than 1 bin with >25% Cerenkov light • 5,264 events remain
HiRes-II Data • Good weather data Dec. 1999 – May 2000 • Due to longer tracks (two rings of mirrors) and greater timing information (FADC), HiRes-II events can be reconstructed using timing information alone.
HiRes-II Data Cuts Required: • Downward, track-like events • Angular speed < 11 deg./ms • Track-length > 10 deg. (ring 2 events) • Track-length > 7 deg. (ring 1 events) • Zenith angle < 60 degrees • Shower maximum observed • < 60% of signal Cerenkov light subtraction • 781 Events remain
