Cosmic Ray International Seminar 13-17 September 2010 Catania - Italy Mass composition results from the Pierre Auger Observatory Simone Riggi on behalf of the Pierre Auger Collaboration C.S.F.N.S.M. University of Catania INFN Catania
The role of composition measurements Impact of composition information in CR understanding Explanation of the spectral features 1 → i.e. the nature of the knee, ankle and flux suppression above 5 × 10 19 eV Strong discrimination observable among different CR models 2 → i.e. significant photon fluxes in top-down scenarios vs astrophysical scenarios Support/cross-check to anisotropy analysis 3 → benefit for correlation analysis with astrophysical objects Composition measurements rather uncertain above 10 19 eV 2 / 43
The Pierre Auger Observatory ◮ Southern Observatory (Malargüe - Argentina): area ∼ 3000 km 2 → Hybrid design, completed in March 2008, in data taking since 2004 → Surface Detector (SD): 1660 water Cherenkov tanks → Fluorescence Detector (FD): 4 sites × 6 telescopes → Atmospheric monitoring devices: 4 Lidar stations, Central Laser Facility (CLF), weather stations, radio balloon soundings, IR cloud cameras, . . . → R&D activities: high elevation telescopes (HEAT), additional array with finer granularity (AMIGA), radio antennas and muon counters ◮ Northern Observatory (Colorado - USA): area ∼ 21000 km 2 3 / 43
The Surface Detector (SD) ◮ 12 tonnes of deionized water ◮ Tank signal calibration in Vertical Equiva- ◮ 3 Photonis PMTs (diameter 12 cm) lent Muons (VEMs) ◮ FADC sampling rate 40 MHz ◮ 5 SD trigger levels, T1&T2 @ PMT and tank level ◮ Solar panels for power supply trigger ◮ Tank event rate 3 kHz − − − − → 3 events/day ◮ Time tagging with GPS system ( ∼ 8 ns res- olution), antenna for data transfer 4 / 43
SD Data Reconstruction A real SD event (AugerId 200733602278) view from top sample ADC trace Event footprint: group of triggered tanks ◮ Signal-weighted barycenter of triggered tanks → core location ◮ Fit to tank timings with a shower front model → shower axis ◮ Fit LDF to tank signals ⇒ S 1000 FD calibration − − − − − − − − − − → Energy 5 / 43
SD Data Reconstruction lateral profile Event footprint: group of triggered tanks ◮ Signal-weighted barycenter of triggered tanks → core location ◮ Fit to tank timings with a shower front model → shower axis ◮ Fit LDF to tank signals ⇒ S 1000 FD calibration − − − − − − − − − − → Energy 6 / 43
The Fluorescence Detector (FD) ◮ Absolute and relative calibration with UV ◮ 6 telescopes/FD site ⇒ 24 telescopes; LED sources ◮ Atmosphere calibrated with many devices ◮ Telescope aperture: 30 ◦ × 30 ◦ (Lidar, CLF, radio soundings, . . . ) ◮ Camera: 22 × 20 Photonis PMTs ◮ 3 FD trigger levels, FLT @ PMT level, ◮ FADC sampling rate: 10 MHz SLT&TLT @ camera level, T3 hybrid trigger @ FD level ◮ Time tagging with GPS system ◮ Hybrid event rate ∼ 5-10 events/hour 7 / 43
Composition studies in Auger H YBRID D ETECTOR S URFACE D ETECTOR F LUORESCENCE D ETECTOR → Signal rise time → X max → Signal rise time asymmetry → RMS(X max ) 8 / 43
Composition from hybrids Depth of shower maximum X max 8 400 ) entries 9 Number of charged particles (x 10 19 proton, E=10 eV 7 19 350 iron, E=10 eV 19 gamma, E=10 eV 6 300 5 250 4 200 3 150 2 100 1 50 0 0 200 300 400 500 600 700 800 900 1000 1100 500 600 700 800 900 1000 1100 1200 2 2 slant depth X [g/cm ] X [g/cm ] max MC profiles for p, Fe and γ X max distributions for p, Fe and γ → protons develop deeper in atmosphere and fluctuate more than nuclei → average X max and its fluctuations measured with great precision 9 / 43
Composition from hybrids: X max reconstruction A real FD event (AugerId 200530502095) 3D view camera view Event footprint: sequence of hit PMTs forming a track in the camera ◮ signal-weighted fit to PMT directions → Shower Detector Plane (SDP) ◮ signal-weighted fit to PMT timings → shower axis 10 / 43
Composition from hybrids: X max reconstruction longitudinal profile X max reconstruction bias How to reconstruct X max ? ◮ light profile at the telescope aperture → energy deposit profile ◮ Gaisser-Hillas fit to profiles → X max X max reconstruction systematics ◮ including rec algorithm, choice of longitudinal fitting function & lateral distribution ◮ systematic uncertainty < 8 g/cm 2 (@10 18 eV) 11 / 43
Composition from hybrids: event selection Hybrid data from December 2004 - March 2009, Energy > 10 18 eV ◮ C ALIBRATION S ELECTION → no bad pixels ◮ A TMOSPHERE S ELECTION → small cloud coverage and optimal aerosol conditions ◮ G EOMETRY S ELECTION → d tank − axis < 2 km, θ view >20 ◦ → precise measurement of the shower axis ( ∼ 0.1 ◦ ) ◮ P ROFILE S ELECTION → optimal GH fit, small X max uncertainties ( < 40 g/cm 2 ) → no gaps in profiles → X max observed in field of view (FoV) → unbiased measurement of � X max � ∼ 1.5 × 10 6 raw hybrids selection − − − − − − − − → ∼ 3754 hybrids for physics analysis 12 / 43
Composition from hybrids: event selection 2 Efficiency Ratio Eff /Eff p Fe 1.8 Eff /Eff He Fe Eff /Eff O Fe 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 18 19 10 10 log [E/eV] 10 Requiring X max in FOV reject very deep and shallow showers → bias the event selection towards light primaries How to avoid biases in selection? → Viewable depths depend on shower geometry (telescope FOV, distance from the FD), energy, atmosphere → Find a range of detectable depths on a event-by-event basis → Require X max within the fiducial boundary → Guarantee an unbiased selection in � X max � ( < 5 g/cm 2 ) and RMS ( < 3 g/cm 2 ) 13 / 43
Composition from hybrids: event selection Good and bad events. . . Good! X max outside FoV! Cloudy conditions! Bad aerosol conditions! 14 / 43
Composition from hybrids: X max resolution resolution check with stereo data total resolution ◮ The resolution deteriorates towards low energies (less light) ◮ 10 18 eV: ∼ 27 g/cm 2 , 10 19 eV: < 20 g/cm 2 ◮ Cross-checked with stereo data 15 / 43
Nuclear composition from hybrids Elongation rate → Change of composition around the ankle → Increase of the average mass up to 59 EeV → No zenith angle bias 16 / 43
Nuclear composition from hybrids Elongation rate 850 ] 2 > [g/cm QGSJET01 QGSJETII proton Sibyll 2.1 800 EPOS 1.99 max <X 750 700 Auger - PRL 2009 ± σ 1 contour 650 syst iron HiRes - 2009 600 18 19 10 10 E/eV → A UGER and H I R ES results compatible within systematics 17 / 43
Nuclear composition from hybrids X max RMS → Increase of the average mass up to 59 EeV → No zenith angle bias 18 / 43
Nuclear composition from hybrids X max RMS ] -2 RMS [g cm 70 60 50 max 40 X 30 20 10 18 19 10 10 E/eV → A UGER and H I R ES results compatible within systematics 19 / 43
Composition reconstruction with the SD Rise time t 1 / 2 in surface stations Signal [VEM peak] Signal [VEM peak] 120 120 µ ± 100 100 ± γ e total 80 80 60 60 40 40 20 20 0 0 60 60 65 65 70 70 75 75 80 80 85 85 90 90 95 95 100 100 105 105 t [25 ns] t [25 ns] ◮ t 1 / 2 sensitive to shower development higher particle production heights (shallow showers) → narrow time pulses (smaller t 1 / 2 ) ◮ t 1 / 2 sensitive to electron/muon content muons produce narrow pulses in tanks → muon-rich showers (nuclei) have smaller t 1 / 2 ◮ t 1 / 2 linearly correlates with X max N t i 1 / 2 − t 1 / 2 ( r , θ, E ref ) benchmark � ∆ � = 1 � σ i N 1 / 2 ( r , θ, S ) i = 1 20 / 43
Composition reconstruction with the SD Asymmetry of rise time in surface stations ◮ em component absorption in late region → early-late asymmetry (dependence on azimuth ξ ) → muons dominate in late regions → smaller t 1 / 2 in late regions ◮ em absorption increases with zenith θ → muon component almost asymmetry-free → asymmetry decreases with θ ◮ Asymmetry profile maximum as composition indicator � t 1 / 2 � = a+bcos ξ r → asymmetry profile b a ( sec θ ) → asymmetry maximum linearly correlates to X max 21 / 43
Composition reconstruction with the SD Use hybrids to calibrate t 1 / 2 & AsymmMax with X max t 1 / 2 vs X max AsymmMax vs X max 22 / 43
Nuclear composition from SD AsymmMax vs energy from SD data → Increase of the average mass with energy → Completely independent SD results! 23 / 43
Nuclear composition from SD Average X max from SD data → Increase of the average mass with energy → Support FD results 24 / 43
Summary Results from Auger using data collected during construction ◮ Independent measurement of composition with 2 detectors ◮ Parameter reconstruction and selection under control ◮ Different sources of systematics studied → Increase of the average nuclear mass with energy (both FD & SD!) More to come soon on composition. . . ◮ increasing statistics ◮ composition from X max distributions ◮ low energy enhancements ( → 10 17 eV) ◮ composition with muons counters and radio antennas 25 / 43
Backup slides
X max resolution 27 / 43
X max systematics 28 / 43
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