The Physics of Ultra-High Energy

Cosmic Rays (UHECR)and Their Detection with Fluorescence Detectors

Charlie Jui

ICAHTLT

Sedona, AZ,

Aug 10,1999

Outline

• Introduction to Cosmic Rays

• Acceleration Mechanisms and Possible Sources

• Detection of Cosmic Rays

• Measuring UHECR using a Fluorescence Detector

• The Fly’s Eye and its results

• The Highest Energy Event and the problem of the GZK cut-off

• Status of the High-Resolution Fly’s Eye (HiRes) experiment and preliminary results.

Cosmic Rays

• Cosmic Rays were discovered in 1912 by V. Hess: he noticed that the rate of spontaneous discharge of electroscopes became faster at higher altitudes.

Cosmic Rays

They Are:

• Atomic particles (electrons, nuclei), radiation (gamma rays), and exotic short lived particles.

• At TeV-PeV range:~50% protons

~25% alpha particles

~13% C/N/O nuclei

<1% electrons

<0.1% gammas

Cosmic Ray Spectrum

• Cosmic Rays with energies in excess of 100 EeV have been reported:

• Over the full range 1GeV-100EeV the spectrum follows roughly a single power law of spectral index ~3

• changes of slope appear at:~10 PeV (Knee)

~5 EeV (Ankle)

Mystery of Ultra-High Energy Cosmic Rays (~ 1 EeV)

• What are they?

Apparent shift from heavy to light composition at the “ankle”*.

• Where do they come from? Apparently nowhere in particular: no point sources or significant anisotropy have been observed**.

• How are they made / accelerated?

Some plausible theories: but it takes some fine tuning to achieve ~10-100 EeV energies.

BUT: any serious model must explain power law and index ~3.

Acceleration Mechanisms

• Fermi (1949): Stochastic collisions between particles and magnetic clouds in the interstellar medium:– Particles gain energy in “head-on”

collisions, lose energy in “rear-end” collisions. “Head-on” collisions are more probable.

– Leads naturally to power-law spectrum. But spectral index depends on details…difficult to conceive of universal index.

Flux of EeV Particles

Array km2 yr srAGASA (100 km2)Fly’s Eye (stereo)Fly’s Eye (monocular)Haverah Park (12 km2)Yakutsk (25 km2)

630151930270490

Total 2,471Total (Exc. FE mono) 1,541

Comparison of integrated exposure at ~ 5 x 1019 eV for different experiments.

Acceleration Mechanisms (cont’d)

• Diffusive Shock Acceleration (1st Order Fermi Acceleration):– Particles repeatedly crossing a

shock front: collisions are always “head-on”

– More efficient acceleration– leads to “universal” spectral index

of 2.0

(a) Shock front traveling at speed U

(b) seen in rest frame of shock front

(c) rest frame of downstream medium

(d) rest frame of upstream medium

Possible Sources of UHECR

• Diffusive shock acceleration (Fermi) in extended objects:– Lobes of radio galaxies (Biermann)– Galaxy cluster accretion shocks

(Kang, et. al)– Collisions of galaxies (Cesarsky)– Motion of galaxies in ISM

• Acceleration in strong fields associated with accretion disks and compact rotating galaxies (Colgate)

Exotic Mechanisms

• “Top-Down” Models: Decay or annihilation of some super-heavy particles or cosmological relics:– e.g. topological defects, relic

magnetic monopoles.

• Acceleration in Catastrophic events:– GRB’s

• New Physics?

Pierre Auger:

Discovered Extensive Air Showers

High-Resolution Fly’s Eye

• Two stations, 12.5 km apart• HiRes I - 22 mirrors, 256 tubes/mir.

S/H electronics - completed in ‘98. Monocular data taking in progress - two years of data in hand.

• HiRes II - 42 mirrors, FADC electronics - completed in July 1999. Stereo data taking began in May 1999.

• Data taking projected for five year run.

HiRes Capability

• 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 .4 deg• Measure Spectrum, Composition,

and Anisotropy. Search for Neutrino flux

HiRes/CASA/MIA

• Study of cosmic ray composition near 1017 eV using Xmax and muon multiplicity methods.

• First simultaneous measurement of shower longitudinal profile development and muon content.

• Xmax method: measure distribution of shower Xmax and compare to Monte Carlo predictions for pure proton and pure Fe flux.

• Monte Carlo uses Corsika shower simulation and detailed HiRes and MIA detector simulation. Use qgsjet and Sybil hadronic models implemented in Corsika.

• Compare Xmax and (600) elongation rates with predictions

• Data is consistent with a transition from a heavy to a light composition - in agreement with previous Fly’s Eye results. Data prefers qgsjet model.

HiRes Monocular Preliminary Results

• HiRes I monocular data from 2.0 years analyzed using constrained shower profile method.

• Monte Carlo studies indicates method works well above 1019 eV. Energy resolution tails require additional study.

• Spectrum slope and normalization consistent with F.E. results.

• Analysis of data continuing. Validation of method with Stereo data in process.

Summary

• The existence of ultra-high energy cosmic rays remains one of the enigmas of physics:– acceleration mechanism?

– Sources?

– Composition?

• Fluorescence Detectors are a powerful and cost-effective way of measuring the energy, composition, and direction of UHECR.

• The discovery of events above the theoretical GZK cut-off (Fly’s Eye, AGASA) only deepens the mystery.

• HiRes is well underway in its efforts to shed light on some of these questions.