1 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.

2 0 4km AGASA 100 km 2 LOW STATISTIC!! 10 events above GZK Status: HiRes Group: astro-ph/ γ = 5.1 ± 0.7 J = J 0 E - γ

3 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

4 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: nm light from the de-excitation of atmospheric nitrogen (~ 4  /m/electron)‏ (+) Longitudinal shower development calorimetric measurement of E (X max )‏ (-) Duty cicle ~ 10%

5 Malargue - Argentina Lat.: 35 o S Long.:69 o W Pampa Amarilla 1400 m a.s.l. 875 g/cm 2 Low population density (< 0.1 / km 2 )‏ Good atmospheric conditions (clouds, aerosol…)‏ The Pierre Auger Observatory:

6 Total area 3000 km 2 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

7 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 Online calibration with background muons.

8 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 Muon Vertical 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 X o 

9 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 slope parameter distance from the core (β  )  2-2.5)‏ S(1000)‏ distance from the core (m)‏ Signal (VEM) ‏ 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.

10 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

11 FD Telescope Schmidt optics Camera (sferical surface) 30 o x30 o FOV 440 PMTs 1.5 o light spot: 15 mm (0.5 o ) Spherical mirror, 3.4m radius of curvature 2.2 m diameter diaphragm, corrector ring + UV optical filter

12 bin=100 ns FD Event:

13 N γ (λ)‏ RiRi A T(λ)‏ E dep FD Longitudinal Profile Photons at diaphragm E dep N γ (λ)‏ Photons in FD FOV ADC counts Fluorescence yield (from laboratory measurements)‏ Detector calibration Geometry ARi2ARi2 Atmosphere T(λ)‏ Drum. Lidar, CLF, ballon lunch etc etc ± 0.71 photons/MeV

14 Drum Mirror reflectivity, PMT sensitivity etc., are all included! ~ 5  /ADC 10% error FD Absolute Calibration Drum: a calibrate light source uniformly illuminates the FD camera

nm steerable laser Central laser facility CLF 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

16 T tank + R tank / c ≈ T FD R tank T tank T FD SD FD mono fit hybrid fit Hybrid Geometry:

17 o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

18 Longitudinal Profile X max ~ 810 g/cm 2 E ~ eV Nucl. Instr. Meth. A588 (2008) 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. E tot E cal 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

19 Systematics on the Absolute Energy Scale Note: Activity on several fronts to reduce these uncertainties

20 ground XgXg X g /cos  vertical showerinclined shower Due 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. S 38 = S(1000,38 0 )‏ SD Calibration using FD Energy S38, represents the signal at 1000m the very same shower would have produced if it had arrived from a zenith angle of 38°

21 measurement of the energy resolution 16%-S 38 8%-E FD FD syst. uncertainty (22%) dominates 50 VEM ~ eV 661 hybrid events 19% SD Calibration using FD Energy

22 Full efficiency above 3x10 18 eV Aperture 7000 km 2 sr yr (3% error) ~ events above eV (~ 1 year Auger completed 4 x AGASA)‏ SD Aperture geometric quantity! 1 January 2004 to 31 August 2007

23 Exp. Observed > 4x ±3 69 > ± 1 1 Evidence of GZK cutoff UHECR Auger Flux (  <60 0 )‏

24 Detailed features of the spectrum better seen by taking difference with respect to reference shape J s = A x E Slope γ above 4x10 19 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 (  <60 0 )‏

25 Conclusion: Auger results reject the hypothesis that the cosmic-ray spectrum continues with a constant slope above 4 × 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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27 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, X max ) X max is increasing with energy (more energetic showers can develop longer before being quenched by atmospheric losses)

28 Composition from hybrid data X max resolution ~ 20 g/cm 2 Larger statistics or independent analysis of the fluctuations of Xmax and SD mass composition estimators are needed.. = 5

29 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)

30 PMT Muon Vertical scintillator 1 VEM ≈ 100 p.e. muon peak VEM peak Online calibration with background muons (2 kHz)‏ The Surface Detector Unit Calibration

31 diffusive Tyvek PMT water Cerenkov light , e ± 1.2 m ~ 3 X o   -response ~ track e/  -response ~ energy  sign. ~ e.m. sign eV simulated showers The Surface Detector Unit

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 eV activates 7-8 tanks The Shower direction using SD

33 t(χ i ) = t 0 + R p · tan [(χ 0 - χ i )/2] 1) Shower detector plane (SDP)‏ Camera pixels monocular geometry 2) Shower axis within the SDP titi χiχi ≈ line but 3 free parameters extra free parameter Large uncertainties ( )‏ (R p,  o )‏ FD Shower direction:

34 Excitation of the nitrogen molecules and their radiative dexcitation. Collisional quenching Fluorescence Yield in Air Several groups working on the measurement of the absolute yield Goal: uncert. close to 5% Air Fluorescence spectrum 3 MeV e - beam AIRFLY 357 nm 391 nm 337 nm p and T dependence Yield vs altitudine AIRFLY

35 SDP reconstruction Pulse finding Time vs χ fit Light at diaphragm Drum calibration Shower profile reconstruction (pixel selection) SD FD mono fit hybrid fit

36 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.

degrees degrees Comparison of the three Auger spectra - consistency ICRC 07 UHECR Auger Flux

38 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?