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 knee ankle ?? 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 the truth…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 bremsstrahlung  supposed to be well known) 1 atmosphere = many steps most of the energy goes into e.m. cascade muon rate: related to N step down to E decay (larger if primary is heavy)

Principle and aims of the Observatory Large area on two sites (both hemispheres) 2 x 3000 km 2 → few tens of events/year/site above eV ( if spectrum extrapolates in 1/E  with  ~ 3, i.e. no GZK cutoff) → no statistical ambiguity on the spectrum around → 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 X max 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 km water tanks (1.5 km spacing) Fluorescence Detector 4 sites 6 Telescopes per site (30x30 deg 2 )

Water Cherenkov tanks Communications antenna Electronics enclosure Solar panels Battery box 3 – nine inch photomultiplier tubes Plastic tank with 12 tons of water GPS antenna

Optical system corrector lens (aperture x2) aperture box shutter filter UV pass safety curtain segmented spherical mirror 440 PMT camera 1.5° 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 tank Also 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(  ) = t 0 + R p /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 contribution iterative procedure, converges in <4 steps; suggested energy here 2 EeV Direct Ch. Scattered Ch. total observed Gaisser-Hillas fit Cherenkov subtraction

Energy reconstruction with SD using the lateral profile to evaluate S(1000) S(1000) to energy: under tuning (simulation+hybrid events) Statistical errors only !

The biggest SD event (up to now…) zenith angle 60 deg Energy of the order of 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 X max (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 eV ~2 above 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) First interaction First steps: big shower-to-shower fluctuations (model dependent !) Large N part : quasi-deterministic evolution 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 = 0X ~ 1000 g/cm 2

Different models… Fraction of energy carried by the “leader” (most energetic secondary) QGSJET gives more “nearly elastic” interactions X max is more delayed w.r.t. X first (from S. Ranchon’s thesis) Many secondaries, with low energies One “leader” with a large x lab First interaction : two extreme situations

A possible parametrization of X max distribution X max -X first int. (from S. Ranchon’s thesis) X max AIRES simulation package hadronic model : QGSJET01 protons, eV Gaussian part : high multiplicity Exponential tail : contribution of “nearly elastic” processes Simulated protons at eV Fitting a convolution: gaussian * exponential  exp = (1+  CR-air  : contribution of the tail of the X max -X first distribution to 0model dependent !) Remark: if large (e.g. proton),  is not too much sensitive to measurement error…

Dependence on the primary Longitudinal profile : increases with E prim (~ 50 g/cm 2 per decade) decreases with A (~ 80 g/cm 2 between p and Fe) Problem : modelling uncertainties on 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…