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Results from the Pierre Auger Observatory

Results from the Pierre Auger Observatory. J. R. T. de Mello Neto University of Chicago & Universidade Federal do Rio de Janeiro for the Pierre Auger Collaboration. Outline. Introduction: the UHECRs The Pierre Auger Observatory – an hybrid detector Energy calibration

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Results from the Pierre Auger Observatory

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  1. Results from the Pierre Auger Observatory J. R. T. de Mello Neto University of Chicago& Universidade Federal do Rio de Janeiro for the Pierre Auger Collaboration

  2. Outline • Introduction: the UHECRs • The Pierre Auger Observatory – an hybrid detector • Energy calibration • The model-independent energy spectrum • Hadronic models • Photon fraction limit • Anisotropy studies • Perspectives • Auger South enhancements • North Site • Auger contributions in the proceedings of ICRC 07 – Merida, Mexico

  3. Cosmic rays flux vs. Energy • (nearly) uniform power-law spectrum spanning 10 orders of magnitude in E and 32 in flux! • structures : • ~ 3 – 5 1015 eV: knee • change of source? new physics? • ~ 3 1018 eV: ankle • transition galactic – extragalatic? • change in composition? • UHECR • one particle per century per km2 • many interesting questions S. Swordy

  4. Open questions • How cosmic rays are accelerated at ? • What are the sources? • How can they propagate along astronomical distances at such high energies? • Are they substantially deflected by magnetic fields? • Can we do cosmic ray astronomy? • What is the mass composition of cosmic rays?

  5. Detection techniques • Particles at ground level • large detector arrays (scintillators, water Cerenkov tanks, etc) • detects a small sample of secondary particles (lateral profile) • 100% duty cicle • aperture: area of array (independent of energy) • primary energy and mass composition are model dependent (rely on Monte Carlo simulations based on extrapolations of the hadronic models constrained at low energies by accelerator physics) ex: AGASA

  6. Detection techniques • Fluorescence of N2 in the atmosphere • calorimetric energy measurement as function of atmospheric depth • only for E > 1017 eV • only for dark nights (10% duty cicle) • requires good knowledge of atmospheric conditions • aperture grows with energy, varies with atmosphere ex: HiRes

  7. The Auger Observatory: Hybrid design • A large surface detector array combined with fluorescence detectors results in a unique and powerful design; • Simultaneous shower measurement allows for transfer of the nearly calorimetric energy calibration from the fluorescence detector to the event gathering power of the surface array. • A complementary set of mass sensitive shower parameters contributes to the identification of primary composition. • Different measurement techniques force understanding of systematic uncertainties in each.

  8. The Pierre Auger Collaboration Aim: To measure properties of UHECR with unprecedented statistics and precision

  9. Pierre Auger South Observatory 3000 km2 1438 deployed 1400 filled 1364 taking data 090707 ~ 85% All 4 fluorescence buildings complete, each with 6 telescopes 1st 4-fold on 20 May 2007 AIM: 1600 tanks HYBRID DETECTOR

  10. 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

  11. The fluorescence detector Los Leones telescope

  12. The fluorescence telescope 30 deg x 30 deg view per telescope

  13. First 4-fold hybrid on 20 May 2007 First hybrid qudriple event! Signal in all four FD detectors and 15 SD stations! 20 May 2007 E ~ 1019 eV

  14. Typical flash ADC trace at about 2 km Detector signal (VEM) vs time (µs) Lateral density distribution PMT 1 PMT 2 PMT 3 θ~ 48º, ~ 70 EeV 18 detectors triggered Flash ADC traces Flash ADC traces -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 µs

  15. Hybrid Event longitudinal profile

  16. Inclined Events offer additional aperture  = 79 °

  17. Energy spectrum from Auger Observatory • Based on fluorescence and surface detector data • First model- and mass-independent energy spectrum • Power of the statistics and well-defined exposure of the surface detector • Hybrid data confirm that SD event trigger is fully efficient above 3x1018 eV for θ<60o • Uses energy scale of the fluorescence detector (nearly calorimetric, model independent energy measurement) to calibrate the SD energy.

  18. Energy calibration • SD parameter S1000: interpolated tank signal at 1000 meters from the lateral distribution function • Determined for each SD event • It is proportional to the primary energy • Reduced measurement uncertainty (shower fluctuations dominate) • VEM = vertical equivalent muons from self calibration of the tank signal (from ambient muons)

  19. Energy calibration (constant intensity cut) • How to relate S(1000 m) to E? • It depends on the atmospheric depth --> shower zenith angle,  • =0 one atm, =90 36 atm, shower is attenuated depending on the zenith angle; • Showers with the same energy developing at differente zenith angles produce different S1000 signals at ground level • The corresponding grammage of atmosphere along the shower axis (shower age) is different • Choose a reference zenith angle 38° (median of the Auger data set) • Make use of the isotropy of the observed CR flux • For a fixed I0 find S(1000) at each θ such that I(>S(1000)) = I0

  20. Constant intensity cut Integral number of events for cos2(θ) for the indicated minimum value of S(1000) Same value of S1000 at higher zenith angle correspond to a higher energy • Derived attenuation curve, CIC(θ), fitted with a quadratic function. • Normalized so that CIC(38°) = 1; Define energy parameter S38= S(1000)/CIC(θ) for each shower : “the S(1000) it would have produced if it had arrived at 38o zenith angle”

  21. S38 (1000)vs. E(FD) 4 x 1019 eV Nagano et al, FY used 387 hybrid events

  22. Energy calibration Fractional difference between the SD and FD energy for the hybrid events; • Small relative dispersion • includes uncertainties in both the FD energy and the SD signal • S(1000) is intrinsecally a very good energy estimator • Reliable energy measurements when properly calibrated

  23. Summary of systematic uncertainties Note: Activity on several fronts to reduce these uncertainties Fluorescence Detector Uncertainties Dominate • Invisible energy: fraction of the energy carried away by neutrinos and energetic muons (Monte Carlo dependent) • energy determination nearly independent of mass or model assumptions

  24. Energy spectrum from SD < 60° Exp Obs >1019.6 132 +/- 9 51 > 1020 30 +/- 2.5 2 Slope = -2.62 ± 0.03 Calibration unc. 18% FD syst. unc. 22% 5165 km2 sr yr ~ 0.8 full Auger year • sharp suppression in the spectrum is seen for the last energy decade • pure power law is rejected with 6σ ( E > 1018.6 eV ) and 4σ( E > 1019 eV )

  25. Slope = -2.7 ± 0.1

  26. Hybrid Spectrum: clear evidence of the ‘ankle’ at ~ 4 x 1018 eV - 3.1 ± 0.3

  27. Energy spectra from Auger • The agreement between the spectra derived using three diferent methods is good • It is underpinned by the common method of energy calibration based on the FD measurements.

  28. 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?

  29. 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)

  30. Composition from hybrid data Xmax resolution ~ 20 g/cm2

  31. 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)

  32. test of hadronic models • Assumption: universality of the eletromagnetic shower evolution • Test: number of muons needed to obtain a self consistent description of data Lateral distribution function Longitudinal profile

  33. Universality of the e/m shower component Sem parameterised as a function of the distance to ground DG = Xdet - Xmax Predicted signal at 1000 m: includes e/m signal for muon decays

  34. constant intensity method Cosmic ray flux isotropic Result accounting for shower fluctuations and detector resolution

  35. expected tank signal at 1019 eV from Auger hybrid data from Auger data: const. intensity method Corresponding energy scale: • within current uncertainty of fluorescence detector energy scale • it corresponds to assigning showers a ~ 30% higher energy than done in the fluorescence detector-based Auger shower reconstruction!

  36. test of hadronic models • two other methods, one using golden hybrid events and another using inclined showers, give consistent results with the constant intensity method ; • Auger hybrid data: test of hadronic interaction models up to ultra-high energy ( Elab > 1019 eV, ) • The number of muons measured in data is about 1.5 times bigger than that predicted by QGSJET II for proton showers! • Universality of eletromagnetic shower evolution indicates energy scale compatible with that of fluorescence detectors.

  37. Top down models • acceleration models (astrophysics): • active galactic nuclei, gamma-ray bursts... • not easy to reach > 100 EeV; • photon fractions typically < ~ 1% • non-acceleration models (particle physics) • UHECR: decay products of high-mass particles (> 1021eV) • super-heavy dark matter (SHDM): from early universe and concentraded on the halo of galaxies and clusters of galaxies • topological defects (TD) produced throughout the universe • UHECR produced as secondary particles (hadronization process) and are most photons and neutrinos, with minority of nucleus • photon fraction typically > ~ 10% • SHDM: CR from our galaxy, photons with a hard energy spectrum • TD: sources distributed in the universe, photons interact with CMB • (expect smaller photon fraction)

  38. UHE photons status in 2005 HP: Haverah Park Ave et al.,2000; event rates A1, A2: AGASA muons @ 1000 m Shinozaki et al., 2002; M. Risse et al., 2005 Models: ZB,SHDM,TD - Gelmini et al. 2005 SHDM' – Ellis et al., 2005 • cosmic ray photon fraction: check nonacceleration models • upper limits so far: surface detectors only !? • needed: cross check by fluorescence technique (Xmax in hybrids)

  39. variables for composition (photons) Showers with greater Xmax have a time distribution in the SD which is more spread (geometrical effect) Energetic muons ( spherical shower front) larger values of Xmax related to smaller values of Rc. Photons: greater time spread and smaller radius of curvature • Data lying above the dashed line • ( the mean of the distribution for photons) • are identfied as photon candidates. • No events meet this requirement.

  40. photon limits A = Agasa HP = Haverah Park Y = Yakutsk

  41. Angular resolution Surface detector Hybrid data: better angular resolution, ~ 0.7o @ 68% c.l. in the EeV energy range Events with E > 10 EeV : 6 or more SD stations

  42. Galactic center • Galactic Center is a “natural” site for cosmic ray acceleration • Supermassive black hole • Dense clusters of stars • Stellar remnants • SNR (?) Sgr A East • SUGAR excess is consistent with a point source, indicating neutral primaries • Neutrons would go undeflected, and neutron decay length at 1018 eV is comparable to the distance to the Galactic center (~8.5 kpc) Chandra

  43. Source at the Galactic center AGASA Significance (σ) 20o scales 1018 – 1018.4 eV 22% excess • Cuts are a posteriori • Chance probability is not well defined N. Hayashida et al., Astroparticle Phys. 10 (1999) 303

  44. Source at Galactic center SUGAR 5.5o cone 1018 – 1018.4 eV 85% excess J.A. Bellido et al., Astroparticle Phys. 15 (2001) 167

  45. results for the galactic center • test of AGASA:obs/exp = 2116/2159.5 R = 0.98 ± 0.02 ± 0.01 NOT CONFIRMED (with 3x more stats) • test of SUGAR:obs/exp = 286/289.7 R = 0.98 ± 0.06 ± 0.01 NOT CONFIRMED (with 10x more stats) • Galactic Center as a point source (σ=1.5°): obs/exp = 53.8/45.8 • R = 1.17 ± 0.10 ± 0.01 • NO SIGNIFICANT EXCESS • upper limit on the flux of neutrons coming from GC: • Galactic Plane:NO SIGNIFICANT EXCESS AGASA 5°, top-hat SUGAR G.P. astro-ph/0607382 (Astropart. Phys., 2007) (check proceedings ICRC 07 for an update) Φs < 0.08 ξ km-2 yr-1 at 95% C.L.

  46. Overdensity search (galactic center) 1 EeV < E <10 EeV 0.1 EeV < E < 1 EeV significance Li, Ma ApJ 272, 317-324 (1983) All distributions consistent with isotropy

  47. anisotropy searches • All-sky blind searches for sources: NO EXCESS FOUND • Right-ascension (RA) distribution of the events is remarkably isotropic! • Upper limit of 1.4% on the first harmonic amplitude (dipole in the RA modulation) • Angular coincidences between Auger events and BL Lac objects (as possibly seen by HiRes) was not confirmed; • Search for clustering (as seen by AGASA), no strong excess was observed • Scan in angle and energy: hints of clustering at larger energies and intermediate angular scales • Large scale distribution of nearby sources? • Chance probability of such a signal from an isotropic flux ~ 2% (marginally significant)

  48. Anisotropy studies For each target: specify a priory probability levels and angular scales avoids uncertainties from “penalty factors” due to a posteriori probability estimation • Targets: • low energy: Galactic center and AGASA-SUGAR location • high energy: nearby violent extragalactic objects • (ICRC 05) New results are coming out! Stay tuned!

  49. other physics topics to be explored • Neutrinos • Gamma ray burst detection • Measurement of the primary cosmic ray cross section; • and many others ...

  50. Conclusion e perspectives • More events > 10 EeV than from AGASA or HiRes • and close to more than their total • AND with superior angular and energy resolution • Auger South: about 90% complete • Detector working very well ( SD: 97% uptime) • First rate physics results: spectrum, composition, anisotropy and many others • Auger statistics will totally dominate after another year !!

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