The Pierre Auger ObservatoryNicolás G. Busca

Fermilab-University of Chicago

FNAL User’s Meeting, May 2006

(1 particle per km2-century)

FNAL

LHC

Ultra High Energy CR (UHECR) spectrum

Ankle

Trans GZK events?

by S. Swordy

Cosmic Ray (CR) Spectrum

The GZK puzzle: UHECR lose energy very quickly by collisions with the CMB, so they should come from nearby sources. But there are no known nearby sources of CR!

The Detection principle: observe the CR shower in the atmosphere.

Such a low flux can only be detected with extremely large detectors: the atmosphere is an essential component!

We try to observe 1 particle per km2 per century.

The Pierre Auger Observatory

• Is the largest ground array detector of cosmic rays

• It’s goal is to study UHECR (E>1018eV)

• It’s design ensures high statistics and a experimentally based energy calibration

• Detector resolutions are also experimentally determined. Systematic uncertainties are traceable.

• What are UHECR?

• Where do they come from?

• Are they accelerated or do they come from super heavy particle decay?

• Shape of the spectrum…

Questions addressed by Auger:

The Hybrid Detector

Auger combines two detection techniques:• Ground array of water cherenkov detectors (the surface detector or “SD”)

The SD provides high statistics ~100% duty cycle.

• 24 fluorescence telescopes (the fluorescence detector or “FD”)

The FD has only a 10% duty cycle: operates only on clear moonless nights.

It provides a direct measurement of the primary energy.

PAO current deployment status

• 950 stations operating, >1200 deployed (no electronics yet)

• 18 fluorescence telescopes

• Aperture ~ 2000 km2sr

• Official data taking started in Jan. 2004

• Expect deployment completion in 2007

60 k

m

The SD1600 water cherenkov detectors, 3000 km2

radio antenna

solar panel

battery box

GPS

electronics

PMT

purified water

Water tank features:•12 tons of purified water

• 3 PMTs measure light deposited by shower particles

• electronics 25ns FADC

• self-calibration using atmospheric muon flux

• power supplied by solar panels

• GPS timing and positioning

~60km

An example SD event

E ~ 80EeV

Timing information is used to reconstruct the shower direction

The SD samples a transverse section of the shower. Timing allows for a direction measurement. The signal amplitude characterizes the energy of the shower.

Event 1225537

A fit to a lateral distribution function determines amplitude

shower front

SD stations

6km

The FD

Sketch of a fluorescence telescope:

spherical mirror

camera

dia

phra

gm

The camera:

• Array of 440 pmt (1.5deg each)

4 buildings with 6 telescopes each

An example hybrid event

Highest signal tank

pixels

dXdX

dEEem

SD-FD energy calibration

Calibration curve from hybrid events:

)log(06.179.0)log( 38SE

The current Auger Spectrum (first 2 years of data)

Features:• No composition assumption for 3EeV<E<25EeV

• No assumption on hadronic interaction models

• Assumes no dramatic composition change occurs above 25EeV

• 30% energy systematic uncertainty at low energies and 50% at high energies (due to extrapolation of calibration curve)

)sec()/log(

211 msreVEd

dF

other Results I didn’t have time to talk about:

Sources and anisotropy studies:

• No excess from the Galactic Centre as claimed by AGASA and SUGAR (Lettesier-Selvon et al., 2005).

• No excess from galactic/super-galactic plane

Photon Flux:

• < 26% at above 10EeV

Auger as a gamma-ray burst observatory:

• Detection capabilities complementary to satellite detectors (Allard et al., 2005)

Some answers and prospects…

• Is there a GZK suppression in the spectrum?

Not enough statistics to address this question yet. Aperture

will multiply by ~3 once the observatory is finished.

• Where do CR come from?

No evidence for individual sources, correlations with

the galactic plane or global anisotropy.

• What are CR anyways?

< 26% of photons above 1019eV. Prospects to study

composition by exploiting other capabilities of the detector.

• Prospects

Auger North: site selected, R&D phase.