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UHE Cosmic Ray Flux:
The Auger ResultsC. Di Giulio for the Pierre Auger Collaboration
a)Università degli Studi di Roma Tor Vergata
b)INFN Roma Tor Vergata.
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0 4km
AGASA
100 km2
LOW STATISTIC!!
10 events
above GZK
Status:
HiRes Group: astro-ph/0703099
γ = 5.1 ± 0.7
J = J0 E - γ
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Argentina
Australia
Brazil
Bolivia*
Mexico
USA
Vietnam**Associate Countries
~300 PhD scientists from
~70 Institutions and 17
countries
Czech Republic
France
Germany
Italy
Netherlands
Poland
Portugal
Slovenia
Spain
United Kingdom
Aim: To measure properties of UHECR with unprecedented
statistics and precision.
The Pierre Auger Collaboration
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The Pierre Auger Observatory:
Hybrid Detector!
FD
SD
Surface Detector (SD):•detection of the shower front at ground
(-) Shower size at ground E
(+) Duty cicle ~ 100% (important for UHECR)
Fluorescence Detector (FD):•fluorescence light:
300-400 nm light from the de-excitation
of atmospheric nitrogen (~ 4 /m/electron)
(+) Longitudinal shower development
calorimetric measurement of E (Xmax)
(-) Duty cicle ~ 10%
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Malargue - Argentina
Lat.: 35o S Long.:69o W
Pampa Amarilla
1400 m a.s.l.
875 g/cm2
• Low population density
(< 0.1 / km2)
• Good atmospheric conditions
(clouds, aerosol…)
The Pierre Auger Observatory:
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Total area 3000 km2
SD 1600 water Cherenkov
detectors on a 1.5 km
triangolar grid
FD 4 x 6 fluorescence
telescopes
50 km
~ 1550 are
operational
The Auger Hybrid Detector
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A surface array station
Communications antenna GPS antenna
Electronicsenclosure
Solar panels
Battery box
3 photomultiplier tubes looking into the water collect light left
by the particles
Plastic tank with 12 tons of very pure water
Online calibration with background muons.
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SD: shower reconstructionThe calibration of the water Cherenkov detector is provided by the muons entering the tanks in the vertical direction (VEM: vertical equivalent muon ).
PMT MuonVertical
scintillator
The tanks activated by the event record the particle density in unit of VEM and the time of arrival.
This data are used to determine the axis of the shower.
1.5 km
shower front
diffusive
Tyvek
PMT
water
Cerenkov
light
, e±
1.2
m ~
3 Xo
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SD: shower reconstruction
The dependence of the particle density on the distance from the shower axis is fitted by a lateral distribution function (LDF).
1700
700
1000)1000()(
rrSrS
size parameter
slope parameter
distance from the core
(β) 2-2.5)
S(1000)
distance from the core (m)
Sig
nal
(VE
M)
vertical equivalent muon = VEM
34 tanks
core
The fit allows determining the particle density S(1000) at the distance of 1000 m from the axis.
This quantity is our energy estimator.
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S(1000): is the energy estimator for the Auger array
less sensible to signal fluctuations
S(1000)
Energy
Simulation
(?)
SD: shower energy estimator
In the Auger Detector the energy scale is determined from the data and does not depend on a knowledge of interaction models or of the primary composition – except at level of few %.
FD calorimetric
measurement
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FD Telescope
Schmidt optics
Camera (sferical surface)
30ox30o FOV
440 PMTs 1.5o light
spot: 15 mm (0.5o)
Spherical mirror, 3.4m
radius of curvature
2.2 m diameter diaphragm,
corrector ring
+ UV optical filter
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bin=100 nsFD Event:
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Nγ(λ)
Ri
AT(λ)
Edep
FD Longitudinal Profile
Photons at
diaphragmEdep Nγ(λ) Photons in
FD FOVADC counts
Fluorescence yield
(from laboratory
measurements)
Detector
calibration
Geometry
A
Ri2
Atmosphere
T(λ)
Drum.Lidar, CLF, ballon lunch etc etc...
5.05 ± 0.71 photons/MeV
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Dru
m
Mirror reflectivity,
PMT sensitivity etc.,
are all included!
~ 5 /ADC
10% error
FD Absolute CalibrationDrum: a calibrate light source
uniformly illuminates the FD
camera
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355 nm
steerable
laser
Central
laser facilityCLF laser track seen
by FD
Estimation of the aerosol
content of the atmosphere
Atmospheric Monitoring
Many instruments to check the atmosphere.
Balloon launches
(p, T, humidity..)
~30 km
Aerosols: clouds, dust, smoke and other pollutants
1 LIDAR per eye
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Ttank + Rtank / c ≈ TFD
R tank
Ttank
TFD
SD
FD
mono fit
hybrid fit
Hybrid Geometry:
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o
Expected photons
fluorescence
cherenkov
The signal, after correcting for attenuation of fluorescence light due to Rayleigh and aerosol scattering, is proportional to the number of fluorescence photons emitted in the field of view of the pixel.
Cherenkov light produced at angles close to the shower axis can be scattered towards the FDs and this contamination is accounted in the reconstruction procedure.
Using the Fluorescence Yield information we convert the light profile in the energy deposit profile.
Light Profile
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Longitudinal Profile
Xmax~ 810 g/cm2
E ~ 3.5 1019 eV
dX
dE
Nucl. Instr. Meth. A588 (2008) 433-441.
•A Gaisser-Hillas function is fitted to the reconstructed shower profile which provides the measurement of the energy of the shower deposited in the atmosphere.
Etot
Ecal
The estimate of this missing energy depends on the mass of the primary cosmic ray and on the hadronic model used for its computation.
The systematic uncertainty due to the lack of knowldege of the mass composition and of the hadronic interaction model is 4%.
Only a 10% model dependent correction
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Systematics on the Absolute Energy Scale
Note: Activity on several fronts to reduce these uncertainties
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ground
Xg Xg/cos
vertical shower inclined showerDue to the attenuation in the atmosphere
for the same energy and mass
S(1000;vertical)< S(1000
for each shower determine
Attenuation curve derived with constant
intensity cut technique.
S38 = S(1000,380)
SD Calibration using FD Energy
S38, represents the signal at 1000mthe very same shower would have produced if it had arrived from a zenith angle of 38°
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measurement of the energy
resolution
16%-S38 8%-EFD
FD syst. uncertainty
(22%) dominates
50 VEM ~ 1019 eV
661 hybrid
events
19%
SD Calibration using FD Energy
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(t)dtareatriggerAperture
Full efficiency above 3x1018 eV
Aperture 7000 km2 sr yr (3% error) ~20.000 events
above 3 1018 eV
(~ 1 year Auger completed 4 x AGASA)
SD Aperture
geometric quantity!
1 January 2004 to 31 August 2007
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Exp. Observed
> 4x1019 167±3 69
> 1020 35±1 1
Evidence of GZK cutoff
UHECR Auger Flux (<600)
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Detailed features of the spectrum better seen by taking difference with
respect to reference shape Js = A x E-2.69
Slope γ above
4x1019 eV:
4.2 ± 0.4(stat)
HiRes:
5.1 ± 0.7
γ = 2.69 ± 0.02(stat)
Fit E-γ
GZK cut off
UHECR Auger Flux (<600)
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Conclusion:
Auger results reject the hypothesis that the cosmic-ray spectrum continues with a constant slope above 4 × 1019 eV, with a significance of 6 standard deviations.
The flux suppression, as well as the correlation of the arrival directions of the highest-events with the position of nearby extragalactic objects, supports the GZK prediction.
A full identification of the reasons for the suppression will come from knowledge of the mass spectrum in the highest-energy region and from reductions of the systematic uncertainties in the energy scale.
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Composition from hybrid data
• UHECR: observatories detect induced showers in the atmosphere• Nature of primary: look for diferences in the shower development• Showers from heavier nuclei develop earlier in the atm with smaller
fluctuations– They reach their maximum development higher in the atmosphere (lower
cumulated grammage, Xmax )
• Xmax is increasing with energy (more energetic showers can develop longer before being quenched by atmospheric losses)
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Composition from hybrid data
Xmax resolution ~ 20 g/cm2
Larger statistics or independent analysis of the fluctuations of Xmax and SD mass composition estimators are needed..
<A> = 5
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Composition from hybrid data
• The results of all three experiments are compatible within their systematic uncertainties.• The statistical precision of Auger data already exceed that of preceeding experiments ( data taken during construction of the observatory)
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PMT MuonVertical
scintillator
1 VEM ≈ 100 p.e.
muon peak VEM peak
Online calibration with background
muons (2 kHz)
The Surface Detector Unit Calibration
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diffusive
Tyvek
PMT
water
Cerenkov
light
, e± 1
.2 m
~ 3 X
o
• -response ~ track• e/-response ~ energy
sign. ~ e.m. sign.
1019 eV simulated showers
The Surface Detector Unit
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1.5 km
shower front
Fit of the particle arrival times
with a model for the shower
front (not exactly plane)
very good
time resolution (~ 12 ns)
Vertical shower of energy
1019 eV activates 7-8 tanks
The Shower direction using SD
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t(χi) = t0 + Rp· tan [(χ0 - χi)/2]
1) Shower detector plane (SDP)
Camera pixels
monocular geometry
2) Shower axis within the SDP
ti
χi
≈ line but 3 free
parameters
extra free parameter
Large uncertainties
(10-200)
(Rp,o)
FD Shower direction:
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Excitation of the nitrogen molecules and their radiative dexcitation . Collisional quenching
Fluorescence Yield in Air
Several groups working on the measurement of the absolute yieldGoal: uncert. close to 5%
Air Fluorescence spectrum
3 MeV e- beam
AIRFLY
357 nm391 nm
337 nm
• p and T dependence Yield vs altitudine
AIRFLY
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SDP reconstruction Pulse finding
Time vs χ fit Light at diaphragm
Drum calibration
Shower profile reconstruction
(pixel selection)
SD
FD
mono fit
hybrid fit
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Auger (Feb 07)
compared to
Hires and Agasa
Fairly agreement within
systematic uncertainties
Dip explained by
CMB-interactions (e+e-) of
extragalactic protonts
Berezinsky et al., Phys.Lett. B612 (2005) 147.
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0-60 degrees
60-80 degrees
Comparison of the three Auger spectra - consistency
ICRC 07
UHECR Auger Flux
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Astrophysical models and the Auger spectrum
models assume: an injectionspectral index, an exponential cutoff at an energy of Emax times the charge of the nucleus,and a mass composition at the acceleration site as well as a distribution of sources.
Auger data: sharp suppression in the spectrum with a high confidence level!
Expected GZK effect or a limit in the acceleration process?