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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
trajectory 1
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trajectory 2
trajectory 2
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trajectory 2
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overlappingsignals
trajectory 2
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trajectory 2
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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…