The Observation of UltraHigh Energy Cosmic Rays using

The Observation of Ultra-High Energy Cosmic Rays using the Hi. Res Detector R. Wayne Springer University of Utah Physics of Ultra-High Energy Cosmic Rays Detection of Ultra-High Energy Cosmic Rays Description of the Hi. Res Detector Results from the Hi. Res Detector University of Utah Physics Department Colloquium September 4, 2003

Ultra High Energy Cosmic Rays (UHECR) What are they? Where do they come from? Learn something about fundamental physics by studying them….

Sources of Cosmic Rays The Sun Solar Wind Low Energy < 10 Ge. V Supernovae Capable of accelerating particles to 1015 e. V AGNs / GRBs … Possible sources for UHECRs particles with energies in excess of 1015 e. V

Physics of Ultra-High Energy Cosmic Rays (UHECR) Basic Questions: How are such energetic (E>1017 e. V) particles produced? Astrophysical sources such as supernovae are NOT believed to be able to accelerate particles to > 1015 e. V. Where are they produced? Perhaps related to how they are produced and the following. . . Further Complication If they are produced in distant (>50 Mp. C ) sources how do they propagate through the ubiquitous cosmic microwave background? GZK Cutoff” in Energy Spectrum “

Possible Production Mechanisms for UHECR Astrophysical Sources Active Galactic Nuclei(AGN) Gamma Ray Bursters(GRB) ? ? Requires large regions of space with sufficient magnetic fields and shock waves to ACCELERATE particles. . . “Top-Down” Sources Decay of Superheavy quasi-stable particles produced in early stages of universe Decay of topological defects produced during non-thermal phase transitions during expansion/inflation of universe. ? ? ? Exotic non-acceleration models requiring “new physics” where UHECRs are produced via the DECAY of MASSIVE particles. . .

Requirements on acceleration region size and strength Diagonal lines indicate Requirement to achieve 1020 e. V

“Top Down” Sources Decay of X particles m. X > 1012 e. V Density of X particles to account for flux of UHECR spectrum from X-particle decay. . . êTopological Defects Domain walls Strings

Propagation through Universe Galactic Magnetic Fields ~ 25 kpc BGalactic~ 2 x 10 -6 gauss = 2 x 10 -10 T r=radius of curvature in B field At what momentum does r = 12. 5 kpc? pc(in e. V)=cr. B pc(in e. V) =3 x 108 m/s * 1. 2 x 1020 m * 2 x 10 -10 T Momentum = p = 7. 2 x 1018 e. V/c Energy ~ 7. 2 x 1018 e. V/c © Gary Walker M 51 Expect confinement if Energy Less than this…

Propagation through Universe COBE map of microwave background For protons with energy exceeding EGZK=5 x 1019 e. V, s> mpc 2 for collisions between the proton and cosmic microwave background photons and pion photoproduction becomes possible. . . GZK Cutoff The Greisen-Zatsepin-Kuzmin cutoff results in the degradation of the energy of protons after a distance of 50 Mpc.

Schematic of Cosmic Ray Spectrum Expected cutoff at EGZK

The Paradox of the GZK cutoff The GZK cutoff limits the region of the universe where the observed UHECR with energies above the GZK cutoff could have been produced. . . If the sources of UHECR are cosmological in origin, then a cutoff in the measured energy spectrum would be expected since the volume of the universe being sampled at E>EGZK has been significantly reduced Observed events above the GZK cutoff do NOT point back to “interesting astrophysical objects…. NEW PHYSICS? ? ?

Evading the GZK cutoff Exotic Solutions (from Bhattacharjee, Sigl…) “Top-Down” production of UHECR within GZK cutoff distance UHECR are supersymmetric particles such as the S 0 (uds-gluino) which have a higher threshold for pion photoproduction … Extremely high energy neutrinos E>1021 annihilate through the Z 0 resonance with relic massive neutrinos “nearby” to produce hadrons with E>EGZK (Weiler, 1997) Violations of Lorentz Invariance, principle of equivalence and/or quantum gravity effects would modify the kinematical constraints of the collision between the proton and the microwave photon thereby eliminating pion photoproduction. . . Simple Solution (Farrar, Piran) Extragalactic Magnetic Field B>1 m. G UHECR need no longer point back to source that produced it. . Implies that UHECR are produced in local neighboorhood. BUT CAN THIS EXPLAIN THE SPECTRUM?

UHECR From Source to Detector

Detection of Ultra-High Energy Cosmic Rays ULTRA LOW FLUX For E>1017 e. V flux < 10 -10 particle/m 2/sr/sec ==> a 1 m 2 2 p sr detector would collect only 1 event/ 50 years !!!! ULTRA LARGE DETECTOR Need detectors with very large apertures (~10, 000 km^2 sr) to compensate for low flux… Atmospheric Calorimeter Exploit Extensive Air Showers using the atmosphere as part of your detector system. . .

Flux roughly follows Power law Flux~E-3. 0 Flux Cosmic Ray Flux Structure in Spectrum Flux varies by 32 orders of magnitude over energy range 108 e. V 1020 e. V Cosmic Ray Particles with energies extending beyond 108 Te. V? ? ? Where does it stop? At energies above 1020 e. V the flux is extremely low!!! Namely 1 particle/km 2 steradian/century What’s this? Limit to Supernova Acceleration Mechanisms LHC Energy scale Long wait here!!! Energy

Extensive Air Showers êHadronic shower initiated by primary êElectromagnetic Shower produced from gammas from p 0 decays….

Using the Atmosphere as a Calorimeter Shower Development of hadronic and electromagnetic showers at Ultra. High energies… reconstruct primary UHECR’s energy and composition UV fluorescence light yield Atmospheric Monitoring Need to know how UV light is attenuated in atmosphere Energy reconstruction Aperture determination Detector Response and Calibration Optics PMT response Electronics Absolute and Relative calibrations of detector system. . .

The Air Fluorescence Technique Detector Response and Calibration Atmospheric Monitoring Need to Know Fluorescence. Yield Need to understand Shower Development Atmospheric Monitoring Detector Response and Calibration Shower Development

Calibration Ideally we would have a 1018 e. V proton accelerator In geo-synchronous orbit over Utah…. Simulate shower development using HEP inspired Monte Carlo Simulation code… We perform piece-wise calibration of optics, PMTs and electronics also perform end-to-end calibration using laser beam…

The Air Fluorescence Technique The fluorescence technique was first investigated as a means for estimating yields of atmospheric nuclear tests.

FLASH experiment at SLAC How about 1010 28 Ge. V Electrons instead? Measure Fluorescence Yield Study Shower Development using thick targets Test beam in 6/2002 Experimental Program 6/2003 T-461 Experimental Setup

Atmospheric Monitoring Atmospheric monitoring is realized by observing the scattered light from beamed light sources such as lasers and radio-controlled vertical flashers

What can Hi. Res determine about Ultra High Energy Cosmic Rays (UHECR) Energy Spectrum (Flux vs. Energy) Composition (on a statistical basis, also including neutrino and gamma searches) Arrival Directions Charged Particle Astronomy?

Description of the Hi. Res Detector Sites Two “eyes” separated by 12. 6 km “Eyes” located ~500 feet above Desert floor. Located in West Desert of UTAH, elevation ~4800 feet (870 g/cm 2). Excellent visibility Aperture ~10, 000 km 2 -sr for E>1020 e. V Duty Cycle ~ 10% Viewing Distance up to 30+ km Detector Components Optics 5. 1 m 2 mirrors (21 @ Hi. Res 1 covering 3 -15 deg) (42 @ Hi. Res 2 covering 3 -31 deg) PMT field of view 1 x 1 degree (arranged in ~ 16 x 16 cluster at focal plane of each mirror for a total of 16128 PMTs) Readout Electronics Hi. Res 1: Sample and Hold Hi. Res 2: FADC 100 ns clock

The Hi. Res Detector Volume Photograph of the “Camels’ back” site (Hi. Res 2) looking Northeast…

Photograph of Hi. Res Mirror and PMT cluster (prototype)

The Measurement of the Energy Spectrum Important to understand the following… Energy Measurement Detector Calibration Shower Geometry (STEREO HELPS!!) Atmospheric Conditions Aperture Detector Calibration Trigger Thresholds Reconstruction Atmospheric Conditions Need to ensure that there are no tails in Energy distribution!!!! Note that the atmosphere has greatest effect on the aperture at enegies below 10 Ee. V!!!!

Measuring Energy Spectrum Count Particles vs. Energy Observe and measure energy Correct energy for atmospheric effects “Fill histogram” Number v Energy Determine Exposure vs. Energy Evaluate on-time exposure taking into account dead mirrors etc. Aperture Determination for each exposure Correct aperture for Atmospheric Effects Divide! Energy Spectrum

Reconstruction Techniques Monocular Reconstruction Uses only one “eye” ==> Monocular Curvature of timing profile can be used to resolve ambiguity in distance and orientation of shower geometry. Constraining shower profile to give reasonable value of Xmax helps ease problem of fitting for geometry ==> Can’t determine Xmax when this is done however Stereo Reconstruction Uses two “eyes” ==> Stereo Shower geometry determined in straightforward manner by interesecting the shower-detector planes… Much more robust and reliable method to determine shower geometry and hence energy. . .

Data Processing and Analysis Filter events (remove noise and artificial light source events) Determine shower-detector plane Binning of signal (Angular, time-based at HR 2) Bin Signal = (K *N)*(Tatmos)*(FY*MR*FT*QE)*G K=energy conversion ; N=Number Shower particles ; Tatmos=atmospheric transmission; FY=fluorescence yield; FT=filter transmission QE=quantum efficiency of PMT ; G=Gain of PMT Profile fitting Energy Reconstruction

Hi. Res 1 Event Gallery TZA

Hi. Res 2 Event Gallery: Event 1 • FADC readout • Time binning Mirror Display NPE v Time v Channel

A 25 Microsecond Movie (playback at 1/500, 000 speed)

Air Shower at Hi. Res I Detector Simpson Springs 12. 6 km Dugway Proving Grounds Hi. Res II Detector

Stereoscopic Event Reconstruction Determination of Shower Geometry The geometry of the air shower is determined simply by finding the intersection of the showerdetector planes Reduced Uncertainty in Energy Determination

Stereoscopic Event Reconstruction Determination of shower profile Hi. Res-I binning ➢ 1. 5 degree angular bins ➢Ray tracing to determine detector acceptance Hi. Res-II binning ➢ Signal ➢ Time based binning ➢Measure intensity and direction of light spot every 100 ns ➢Ray tracing to determine detector acceptance ➢ Profile fit ➢ Signal fit to shower profile function ➢Cerenkov correction calculated based on geometry. ➢d. E/d. X determined from fit ➢Primary particle total energy calculated using “standard” relationship between EM and total energy. . ➢ Integral of shower profile determines energy of UHECR Depth [g/cm 2]

Stereoscopic Event Reconstruction Energy uncertainty/energy <5. 0 ➢Xbottom-Xtop>100 g/cm ➢Xmax>Xtop-200 g/cm ➢ 400 g/cm <Xmax<1200 g/cm ➢Zenith angle<70 degrees Entries=8758 RMS=62. 5 Sigma=25. 5 Num events Choose best of HR 1 or HR 2 ➢Basic cuts: ➢Profile chi 2/d. o. f<15 ➢Nbin>3 ➢Xmax<Xbottom+100 g/cm ➢Xmax>Xtop 300 g/cm ➢Tight cuts ➢ Num events Energy Resolution HR 1 no cuts Entries=8828 RMS=57. 9 Sigma=15. 8 HR 2 no cuts ➢ Cuts need to OPTIMIZE Energy and Statistics ➢ Entries=8006 RMS=62. 3 Sigma=25. 5 Num events % fractional resolution Basic Cuts % fractional resolution Entries=7002 RMS=62. 3 Sigma=25. 5 Tight Cuts % fractional resolution

Number events Hi. Res Stereo Flux Measurement Energy Distributions No. Weather cuts 1291 hours 1944 events Log E(e. V) Good Weather cuts 1006 hours 1588 events Log E(e. V)

Stereoscopic Event Reconstruction Number events MC/Data Ratio Data/Monte Carlo Comparison Zenith and Azimuth angle distributions

Determination of Stereoscopic Aperture Use Simulation to generate events Reconstruct these MC events using standard reconstruction software to determine aperture Calculated for “average atmospheric conditions” of VSH=1. 0 km and HAL=25. 0 km 76% Proton 24% Iron mixture Aperture exceeds 10, 000 km 2 -sr above 100 Ee. V

Atmospheric Effects on Aperture Atmospheric clarity influences The effective size of Hi. Res More sensitive to atmospheric Clarity at lower energies!!! Also shifts reconstructed energy

Hi. Res Stereo Flux Measurement Energy Distributions The energy distribution of stereo events after all cuts. The line is the predicted number of events using a Fly’s Eye spectrum with no GZK cutoff

Flux * 1029 m-2 s-1 steradian Stereo UHECR Flux Spectral index = -2. 95+/-0. 09 Log E

Stereo UHECR Flux Spectral index = -2. 95+/-0. 09 Agrees with Hi. Res mono spectrum

E 3 * Flux Stereo UHECR Flux Red = HR 1 Mono Blue = HR 2 Mono Black = Stereo Good Agreement!! Log E

Stereo UHECR Flux

Systematic Uncertainties Energy Scale and Atmosphere PMT calibration: 10% Fluorescence yield: 10% Unobserved energy: 5% Atmospheric absorption: most sensitive to vertical aerosol optical depth (VAOD) – – Mean VAOD = 0. 04 VAOD RMS = 0. 02 VAOD systematic is smaller. Modify MC and analysis programs to use VAOD = 0. 02 and 0. 06, reanalyze. – J(E) changes by 15% Total systematic uncertainty on energy = 21%

Systematic Uncertainties Need to propagate uncertainty into spectrum measurement. Energy is modified. Easy to deal with by just multiplication … Aperture is modified. Need to utilize hundreds of years of VAX 11/780 CPU equivalent to generate Simulated events…

E 3 J(E) Hi. Res Stereo Flux Measurement Energy**3 * Flux • Hi. Res Stereo Spectrum is consistent With HR 1 Monocular Spectrum • Change in spectral index weakly observed at an energy of 1018. 6 e. V. • STATISTICAL ERRORS ONLY • Aperture rapidly varying and sensitive to details below 3 x 1018 Log E(e. V)

Publication of Hi. Res Stereo Spectrum imminent

Hi. Res 1 Monocular Spectrum • Period: June, 1997 – May, 2001 • 50915 mirror hours. • Cuts: – – – – Clear weather. Downward going track. Track length > 7. 9 degrees Pseudodistance > 5 km. 85 < tubes/degree < 4. Photoelectrons/degree > 25 Constrained fit converges. Shower max in view • Minimum energy is 3 x 1018 e. V due to shorter tracks.

Hi. Res 2 Monocular Spectrum • Dec. , 1999 – May, 2000 (first stable Hi. Res 2 running). ~30% of data. • Consistent trigger (big change after May). • Cuts: – – – – – Clear weather. Downward going track. Track length > 7 degrees Linear fit chisquared/tube < 20 Pseudodistance > 1. 5 km. 85 < tubes/degree < 3. Photoelectrons/degree > 25 Zenith angle < 60 degrees Shower max in view

Hi. Res Monocular Spectra • Fit: E-2. 8 from 18. 7 to 19. 8; Predicts 29. 8 events, log. E>19. 8; See 11. Probability = 7. 3 x 10 -5 GZK cutoff? ? ?

UHECR Spectra Monocular Hi. Res and Fly’s Eye Note that Fly’s eye energy scale shifted down by 7% to obtain agreement

UHECR Spectra Monocular Hi. Res and AGASA Agasa sees 10 events above 1020 e. V No GZK cutoff Hi. Res Monocular sees 2 events CONTROVERSY!

AGASA Energy Rescaled by 0. 79

Conclusions The two Hi. Res detectors continue to collect data. Measured flux agrees with Fly’s Eye experiment. We see spectral features. Our monocular spectra supports the existence of the GZK cutoff Need more statistics and study of systematics