UHE Cosmic Ray Flux:
The Auger Results
C. Di Giulio for the Pierre Auger Collaboration
a)Università degli Studi di Roma Tor Vergata
b)INFN Roma Tor Vergata.
1
Status:
AGASA
100 km2
10 events
above GZK
γ = 5.1 ± 0.7
0
4km
J = J0 E - γ
HiRes Group: astro-ph/0703099
LOW STATISTIC!!
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The Pierre Auger Collaboration
Czech Republic
Argentina
France
Australia
Germany
Brazil
Italy
Bolivia*
Netherlands
Mexico
Poland
USA
Portugal
Vietnam*
Slovenia
*Associate Countries
Spain
United Kingdom
~300 PhD scientists from
~70 Institutions and 17
countries
Aim: To measure properties of UHECR with unprecedented
statistics and precision.
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The Pierre Auger Observatory:
Hybrid Detector!
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%
FD
Surface Detector (SD):
•detection of the shower front at ground
(-) Shower size at ground  E
SD
(+) Duty cicle ~ 100% (important for UHECR)
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The Pierre Auger Observatory:
Malargue - Argentina
Pampa Amarilla
Lat.: 35o S
Long.:69o W
1400 m a.s.l.
875 g/cm2
• Low population density
(< 0.1 / km2)
• Good atmospheric conditions
(clouds, aerosol…)
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The Auger Hybrid Detector
Total area 3000 km2
SD
1600 water Cherenkov
detectors on a 1.5 km
triangolar grid
~ 1550 are
operational
FD
4 x 6 fluorescence
telescopes
50 km
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A surface array station
Communications
antenna
GPS antenna
Electronics
enclosure
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
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Online calibration with background muons.
SD: shower reconstruction
The calibration of the water Cherenkov detector is provided by the muons entering the
tanks in the vertical direction (VEM: vertical equivalent muon ).
PMT
diffusive
Tyvek
PMT
Vertical
Muon
scintillator
 , e±
water
1.5 km

1.2 m ~ 3 Xo
Cerenkov
light

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.
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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).
size parameter
distance from the core


core


r
r

700




S
(
r
)

S
(
1000
)
 


1000
1700




slope parameter
(β) 2-2.5)
Signal (VEM)
The fit allows determining the particle
density S(1000) at the distance of 1000
m from the axis.
34 tanks
vertical equivalent muon = VEM
S(1000)
This quantity is our energy estimator.
distance from the core (m)
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SD: shower energy estimator
S(1000): is the energy estimator for the Auger array
less sensible to signal fluctuations
S(1000)
Energy
Simulation
(?)
FD calorimetric
measurement
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 %.
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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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FD Event:
bin=100 ns
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FD Longitudinal Profile
Edep
Nγ(λ)
T(λ)
A
Ri
Edep
Nγ(λ)
Fluorescence yield
(from laboratory
measurements)
5.05 ± 0.71 photons/MeV
Photons in
FD FOV
Geometry
A
Ri2
Photons at
diaphragm
Atmosphere
T(λ)
Lidar, CLF, ballon
lunch etc etc...
ADC counts
Detector
calibration
Drum.
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FD Absolute Calibration
Drum: a calibrate light source
uniformly illuminates the FD camera
Mirror reflectivity,
PMT sensitivity etc.,
are all included!
~ 5 /ADC
10% error
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Atmospheric Monitoring
Central
laser facility
355 nm
steerable
laser
~30 km
CLF laser track seen
by FD
Many instruments to
check the
atmosphere.
Estimation of the aerosol
content of the atmosphere
1 LIDAR per eye
Balloon launches
(p, T, humidity..)
Aerosols:
clouds, dust,
smoke and
other
pollutants
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Hybrid Geometry:
Ttank
TFD
SD
hybrid fit
mono fit
Ttank + Rtank / c ≈ TFD
FD
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Light Profile
Expected photons
co
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.
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
Longitudinal Profile
E ~ 3.5 1019 eV
Etot
Ecal
dE
dX
Xmax~ 810 g/cm2
Nucl. Instr. Meth. A588 (2008) 433-441.
Only a 10% model dependent correction
•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.
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%.

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Systematics on the Absolute Energy
Scale
Note: Activity on several fronts to reduce these uncertainties
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SD Calibration using FD Energy
vertical shower
inclined shower
Xg
Xg/cos
Due to the attenuation in the atmosphere
for the same energy and mass
S(1000;vertical)< S(1000
Attenuation curve derived with constant
intensity cut technique.
ground
for each shower determine
S38 = S(1000,380)
S38, represents the signal at 1000m
the very same shower would have
produced if it had arrived from a
zenith angle of 38°
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SD Calibration using FD Energy
50 VEM ~ 1019 eV
661 hybrid
events
FD syst. uncertainty
(22%) dominates
19%
measurement of the energy
resolution
16%-S38 8%-EFD
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SD Aperture
geometric quantity!
Aperture

trigger

area
(t)dt

Full efficiency above 3x1018 eV
1 January 2004 to 31 August 2007
Aperture 7000
km2
sr yr (3% error)
(~ 1 year Auger completed
4 x AGASA)
~20.000 events
above 3 1018 eV
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UHECR Auger Flux (<600)
Evidence of GZK cutoff
> 4x1019
Exp. Observed
167±3
69
> 1020
35±1
1
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UHECR Auger Flux (<600)
Detailed features of the spectrum better seen by taking difference with
respect to reference shape Js = A x E-2.69
Fit E-γ
γ = 2.69 ± 0.02(stat)
GZK cut off
Slope γ above
4x1019 eV:
4.2 ± 0.4(stat)
HiRes:
5.1 ± 0.7
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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
<A> = 5
Xmax resolution ~ 20 g/cm2
Larger statistics or independent analysis of the fluctuations of Xmax and SD mass
composition estimators are needed..
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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
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( data taken during construction of the observatory)
The Surface Detector Unit Calibration
muon peak

VEM peak
Online calibration with background
muons (2 kHz)
1 VEM ≈ 100 p.e.
PMT
Vertical
Muon
scintillator
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The Surface Detector Unit
diffusive
Tyvek
 , e±
Cerenkov
light
water

1.2 m ~ 3 Xo
• -response ~ track
• e/-response ~ energy
PMT
1019 eV simulated showers
 sign. ~ e.m. sign.
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The Shower direction using SD

1.5 km
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
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FD Shower direction:
1) Shower detector plane (SDP)
Camera pixels
2) Shower axis within the SDP
monocular geometry
(Rp,co)
t(χi) = t0 + Rp· tan [(χ0 - χi)/2]
extra free parameter
ti
≈ line but 3 free
parameters
Large uncertainties
(10-200)
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χi
Fluorescence Yield in Air
Air Fluorescence spectrum
Excitation of the nitrogen molecules and
their radiative dexcitation . Collisional
quenching
•
AIRFLY
337 nm
3 MeV
e- beam
p and T dependence Yield vs altitudine
AIRFLY
Several groups working on the
measurement of the absolute
yield
Goal: uncert. close to 5%
357 nm
391 nm
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Shower profile reconstruction
Pulse finding
SDP reconstruction
(pixel selection)
SD
Drum
calibration
hybrid fit
mono fit
Light at diaphragm
Time vs χ fit
FD
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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
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Berezinsky et al., Phys.Lett. B612 (2005) 147.
UHECR Auger Flux
Comparison of the three Auger spectra - consistency
0-60 degrees
60-80 degrees
ICRC 07
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Astrophysical models and the Auger
spectrum
models assume: an injection
spectral 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?
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