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UHE Cosmic Ray Flux: The Auger Results

C. Di Giulio for the Pierre Auger Collaborationa)Università degli Studi di Roma Tor Vergata

b)INFN Roma Tor Vergata.

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0 4km

AGASA100 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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ArgentinaAustraliaBrazilBolivia*MexicoUSAVietnam**Associate Countries

~300 PhD scientists from ~70 Institutions and 17 countries

Czech Republic France Germany ItalyNetherlandsPoland PortugalSlovenia Spain United Kingdom

Aim: To measure properties of UHECR with unprecedentedstatistics 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 stationCommunications

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

Cerenkovlight

, e±

1.2 m ~ 3 X

o

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SD: shower reconstructionThe dependence of the particle density on the distance from the shower axis is fitted by a lateral distribution function (LDF).

1700700

1000)1000()( rrSrS

size parameter

slope parameter

distance from the core

(β) 2-2.5)

S(1000)

distance from the core (m)

Sign

al

(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 calorimetricmeasurement

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FD TelescopeSchmidt 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 diaphragm

Edep Nγ(λ) Photons in FD FOV ADC counts

Fluorescence yield(from laboratorymeasurements)

Detector calibration

Geometry

A Ri

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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 /ADC10% error

FD Absolute CalibrationDrum: a calibrate light source

uniformly illuminates the FD camera

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355 nmsteerable

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

dXdE

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 atmospherefor the same energy and massS(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

Cerenkovlight

, e± 1.2 m

~ 3 Xo

• -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 freeparameters

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?