The Auger Observatory was built to find the origin of UHE Cosmic Rays

Interesting because it is a genuine high energy frontier

Detects UHECRs by Extensive Air Showers EAS.

What can you measure with EAS ? :

Energy Spectrum

Arrival Directions

Composition

This is all we’ve got to work with to solve the problem of origins.

Scientific Progress Seems hopeless :

But look at the Cosmic Micro-wave Background

Spectrum - Thermal

Composition - Photons

Directionality - Slight anisotropy

What we know about CRs now.Spectrum – Non-Thermal power law.

Isotropic – with a hint at some anisotropy at the highest energies

Composition - ????

Doesn’t tell us much without composition – at the moment.

We are only able to measure the same three things for incoming MBR photons.

Age of Universe

Geometry of the space

Baryon fractionetc

Surface Array 1600 detector stations 1.5 Km spacing 3000 Km2

Fluorescence Detectors 4 Telescope enclosures 6 Telescopes per

enclosure 24 Telescopes total

The Auger Cosmic Ray Observatory

The Auger Hybrid Observatory

10 m2 water cherenkov tank

Fluorescence Detector

6 Telescopes – 30ox30o field of view

EAS viewed by all 4 fluorescence detectors and the surface array.

May 20, 2007

θ~ 48º, ~ 70 EeV

Flash ADC tracesFlash ADC traces

Lateral density distribution

Typical flash ADC trace

at about 2 km

Detector signal (VEM) vs time (µs)

PMT 1

PMT 2

PMT 3

18 detectors triggered

SD Data

FD Data

An EAS seen simultaneously with the Surface Detector and Fluorescence Detector.

Two Divisions in UHECR theories of originTop-Down Bottom-up

(Z-Burst Model). UHE s annihilate on relic s and produce a Z boson, which decays into UHE photons and light hadrons.

(SHDM Models ) Decay of super-heavy dark matter particles. [relic metastable particles] (SHDM Models) . Berezinsky (1997)

Decay of relic Topological Defects

(TD models)

Cosmic Strings decay into UHE photons

Large Photon flux at High Energies

Stochastic acceleration(shock acceleration)

Charged particle flux

Possibility of heavy nuclei

How can composition measurements help in our quest for cosmic ray origins?

Look for the photons predicted by the Top Down models.

Composition information is reflected in shower development behavior. (Also Muon content)

Different primary particle types produce showers that maximize at different depths. (different “Xmax’s”.

Look for showers with deep Xmax.

(Example)Vertical 1019 eV Corsika Simulations

For a fixed energy.

Surface Detector parameters that correlate with Xmax :

I. Tank signal Rise-time t1/2 (at 1000m from the core)

Xmax

Xmax

Shallow Xmax, t1/2 small.Deep Xmax, ta-tb ~ t1/2 large.

a

b

a

b

Can we do this with the much larger SD data set?

t1/2 is weakly correlated with Xmax

Shower size grows to ½ maximum

Shower size grows to ½ maximum

Surface Station

Surface Station

Shallow Xmax, Large rc

II. Shower Front Radius of Curvature rc.

rc

Xmax

rc

Xmax

Deep Xmax, Smaller rc

Look for photon-induced showers witha) Large t1/2 andb) Small rc

Plan : Use the much more prolific SD data to search for photons. 10x the data as FD but less accurate.

rc is anti-correlated with Xmax

Shower axis

Ground level

Use Data from 1January 2004 to 31 December 2006.

Zenith Angles between 30o and 60o

Integrated Aperture = 3130 km2 sr yr ~ 1 full array year

( (1000), )

( (1000), )

x x S

S

For each event

form the deviation from the expectation for a photon shower.(X is either t1/2 or Rc)

( (1000), )x S Is the average of an ensemble of simulated photon showers with the same S(1000) and as the recordedd event.

Energies above 10 EeV

Procedure :

2761 events pass the quality cuts.

Apply this same procedure to a spectrum-generated set of Monte Carlo photon

showers. ( )P E dE E dE

= 2

Find principle axis using first 5% of the data.

You get the greatest separation between photons and nuclears along this axis.

Remaining 95% of data is projected on to the principle axis.

Mean of photon distribution

Photon candidates lie above the dashed line.

There are no Photon candidates in the events above 10 EeV..

Photon Flux Limits

10EeV 20 EeV 40EeV

3.8x10-3 2.5x10-3 2.2x10-3 km-2sr-1yr-1

Photon Fraction 2.0% 5.1% 31%

Limits on Photon Fraction -

Most Top Down Models are disfavored but not ruled out

Top Down models clearly disfavored at all measured energies.

Photon Flux Limits compared to Top Down Model estimates

Bottom up theories – share acceleration mechanism but differ on what gets accelerated and what makes it into our detectors.

1. If the acceleration/injection region is benign (low radiation density) we might expect a nearly solar composition of nuclei to be accelerated and propagated.

2. Otherwise we might have only protons.

Measurement of Nuclear composition might thus lead us to an understanding of the injection/acceleration process.

Both cases predict the same features in the energy spectrum!

“dip” due to changing source distribution

Cut-off due toGZK effect andNuclear breakup

(Allard et. Al.)

Extra-galactic flux must be mixed to match all the features.

“Dip” due to pair productionOff the MBR.

Cut-off due toGZK effect

(Berezinsky et al)

The separation between nuclear species is quite small compared with photons.

max lnX A

Requires the accuracy of the Hybrid detector and direct measurement of EAS development.

Plan : Look at the average Xmax as a function of energy. Compare with Monte Carlo estimates.

Use hybrid data from December 2004 through April 2007

FD showers with at least one triggered SD station.

Quality cuts :

Xmax in field of view

Profile fit 2 pdf. <= 2.5

Xmax < 40 gm cm-2

E/E < 20%

Min Viewing Angle > 20o

Geometrical cuts – minimize composition biases due to different detection efficiencies for

different species.

4329 events remain after the cuts.

Data compared to EPOS1.6 Pure Composition Predictions

[Constant]

[Getting Heavier]

Best fit : two linear regions with a break in slope at 1018.35 eV

Mixed composition everywhere.

Constant comp. low energy : At high energy , getting heavier

Data Compared to QGSJET pure composition predictions

[Getting lighter] [Constant]

Mixed composition everywhere

Getting lighter at low energy : Constant at high energy.

Bad news : composition behavior depends on what interaction model you use, and the different models do not agree.

Good news : All interaction models agree on a mixed composition everywhere.

Will have to use some additional composition handle to sort this out.eg Dispersion in Xmax or Average Muon content.

The pure proton model is disfavored – even at this early stage.

Summary

Scientific progress has been made on finding the origin of UHE cosmic rays by composition studies

Searches for photons have produced flux limits which disfavor most Top-Down theories.

An analysis of average Xmax has found a mixed composition which disfavors a class of pure

proton source models.

Future studies have to be performed to establish the mixed composition conclusion in a model-indepentent way.