the Pierre Auger Observatory(Cosmic Rays of Ultra-High Energy)

• The puzzle of UHECR• Principle and advantages of an hybrid detector• Present status of the Observatory• Sensitivity to hadronic modelling at UHE• Perspectives

Pierre Billoir, LPNHE (CNRS/univ. Paris 6/Paris 7)(Auger Collaboration)

EDS05, Blois, may 2005

Energy Spectrum of Cosmic Rays

kn

ee

an

k le

? ?

Galactic ? Extragalactic ?

Open issue: the end of the spectrum

GZK cutoff

Below GZK:AGASA (ground array) and HiRes (fluorescence) almost agree(30 % systematic on E ?)

Above GZK:unexplained divergence !

AUGER has to find thetruth…with an hybrid detector

after Douglas Bergman

Modelling of shower development (1)

• hadronic cascade: ~1/3 lost at each step (0 2 ) ~2/3 re-interacts until decaying into muons (E ~ a few GeV)

• electromagnetic cascade: mainly: pair production and bremsstrahlungsupposed to be well known)1 atmosphere = many

steps most of the energy goes into e.m. cascade muon rate: related to Nstep

down to Edecay

(larger if primary is heavy)

Principle and aims of the Observatory

• Large area on two sites (both hemispheres) 2 x 3000 km2 → few tens of events/year/site above 1020 eV ( if spectrum extrapolates in 1/E with ~ 3 , i.e. no GZK cutoff) → no statistical ambiguity on the spectrum around 1020 → full sky coverage (point sources and extended structures)

• Hybrid detection (ground array + fluorescence telescope) - better geometrical reconstruction (<1 deg) - cross-calibration of energy (sources of systematic errors are different !)

• More possibilities for primary identification - traditional use of Xmax from fluorescence profile - structure of the front at ground → stage of evolution (again, possibilities of cross-checks) - window to “exotic” primaries (photon, neutrino)

The end…

Layout of the Southern Observatory

Surface Array 3000 km2

1600 water tanks (1.5 km spacing)

Fluorescence Detector 4 sites 6 Telescopes per site (30x30 deg2)

Water Cherenkov tanks

Communications antenna

Electronics enclosure Solar panels

Battery box

3 – nine inchphotomultipliertubes

Plastic tank with 12 tons of water

GPS antenna

Optical system

corrector lens(aperture x2)

aperture boxshutterfilter UV passsafety curtain

segmented spherical mirror

440 PMT camera1.5° per pixel

Status of the Array (May 2005)

~ 800 tanks deployed (~ 730 sending data)

stable running most of the time

2 telescopes in activity(10 % of the time)1 more soon

Calibration (very simplified !)

SD: use the Vertical Equivalent Muon - directly measured with hodoscope - indirectly from”muon hump” in the field (very large statistics !) - electron from muon decay within tankAlso studied: dependence on atm. conditions,water level, etc…Problem: response to photons, electrons

vertical muons

all muons

FD: various tools - lidar (probing along a laser beam) - infrared cloud monitor - central laser facility (seen from all tel.) - ballons (atm. param.) - drum calibration (uniform illumination)etc…

Thick cloud

Clear sky linear behavior

Thin layer LIDAR DATA

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Geometric improvement using hybrid detection

Shower detector plane: well defined

Position within SDP: problem if small angular range:

Solution(s) - stereo view by 2 telescopes (big showers only !) - one more constraint: time at ground (1 tank is enough !)

better than 1 deg (direction) and 100 m (position)achievable in hybrid mode, even at low energy

t() = t0 + Rp/c tan((3 param. to be measured

An example of hybrid reconstruction

SD points

Geometrical fit from FD only Hybrid fit

extrapolation

A big stereo hybrid event !

[Fick Plots]

Profile reconstruction with FD

this event: intial viewing angle 15°, i.e. large direct Cherenkov contributioniterative procedure, converges in <4 steps; suggested energy here 2 EeV

Direct Ch.

Scattered Ch.

total observedGaisser-Hillas fit

Cherenkov subtraction

Energy reconstruction with SDusing the lateral profile to evaluate S(1000)

S(1000) to energy: under tuning (simulation+hybrid events)

Stat

istical e

rror

s on

ly !

The biggest SD event (up to now…)

zenith angle 60 deg Energy of the order of 1020 eV (large uncertainty !)

First SD-FD energy comparison

Preliminary

Not yet very precise, but clear correlation…

“young” and “old” showers as seen by SD

“vertical” (13 deg)(long signal with multiple peaks)

“horizontal” (76 deg) (muonic tail: very short peak)

Evaluation of “age” + precise value of zenith angle (1 deg) indirect measurement of Xmax (less precise than FD, but more statistics !)

Sky coverage (galactic coord.)

Present rates

Per day, in present configuration of SD < 60 deg, excluding edges and regions around “holes”) ~500 events ~100 above 1018 eV ~2 above 1019 eV most of them fully reconstructible (but: preliminary energy scale !!! )

Hybrid: about 5 % of SD reconstructed events(regions not yet covered in front of telescopes) + many (FD & 1 or 2 SD stations) at low energy(with improved geometry from timing in SD station)

Modelling of shower development (2)

Firs

t in

tera

ctio

n

First steps: bigshower-to-showerfluctuations(model dependent !)

Large Npart : quasi-deterministicevolution of densities(“universal” shape)

Main effect of initial fluctuations:• Global translation of e.m. cascade• Modulation of muon rate

Electromagnetic cascade+ muons (+ a few hadrons)

1 atmosphere depthX = 0 X ~ 1000 g/cm2

Different models…

Fraction of energy carried by the “leader” (most energetic secondary)

QGSJET gives more“nearly elastic” interactions

Xmax is moredelayed w.r.t. Xfirst

(from S. Ranchon’s thesis)

Many secondaries, with low energies

One “leader” with a large xlab

First interaction :two extreme situations

A possible parametrization of Xmax distribution

Xmax-Xfirst int.

(from S. Ranchon’s thesis)

Xmax

AIRES simulation packagehadronic model : QGSJET01 protons, 1019 eV

Gaussian part : high multiplicityExponential tail : contribution of “nearly elastic” processes

Simulated protons at 2.1018 eVFitting a convolution: gaussian * exponentialexp = (1+CR-air

: contribution of the tail of the Xmax-Xfirst distributionto 0model dependent !)

Remark: if large (e.g. proton), is not too much sensitive to measurement error…

Dependence on the primary• Longitudinal profile :

<Xmax> increases with Eprim (~ 50 g/cm2 per decade) decreases with A (~ 80 g/cm2 between p and Fe)Problem : modelling uncertainties on <Xmax> are comparable to this difference ! (and the bias may depend on energy…)

Can the exponential tail give an useful information ?Two unknown functions of E : composition of CR and cross sections !

• Muon content : - Again : differences between models comparable with p – Fe difference (~ 30 % ?) - difficult to measure (signal from muons mixed with electromagnetic contribution)

Conclusion• The components work well• Deployment in good shape (at least in summer)• Multiple tools for calibration• Large statistics; window at “low” energy

• The hybrid concept is validated • still work to fully inter-calibrate• northern site to be built !

• modelling errors to be controlled: possible bias on energy; identification is difficult can we hope a stabilization of UHE model predictions ?

first physics results this summer…