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Large Array Astrophysics Detectors (I)

2010 LNF Spring School Frascati, 13 May 2010. Large Array Astrophysics Detectors (I). Giorgio Matthiae University and Sezione INFN of Roma Tor Vergata. Cosmic ray spectrum year 2000. ~ 1 / E 3. 1 particle/km 2 /century. LHC c.m. Cosmic ray spectrum - 2009. knee.

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Large Array Astrophysics Detectors (I)

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  1. 2010 LNF Spring School Frascati, 13 May 2010 Large Array Astrophysics Detectors (I) Giorgio Matthiae University and Sezione INFN of Roma Tor Vergata

  2. Cosmic ray spectrum year 2000 ~ 1 / E3 1 particle/km2/century LHC c.m.

  3. Cosmic ray spectrum - 2009 knee

  4. Fluorescence Telescopes • N2 molecules (300-400 nm) • Longitudinal development of the shower • Calorimetric measurement of the energy • Only clear moonless nights • ~ 10% duty cycle ! The two techniques • Surface Array • Front of shower at ground • Direction and “energy” of the shower

  5. AGASA: surface array – 100 km2 (1990 – 2004) Fly’s eye - HiRes: fluorescence telescopes (Utah) Telescope Array hybrid system ~ 800 km2 (in construction) Auger South: hybrid system 3000 km2 (completed May 2008) Auger North ~ 20.000 km2 (proposed)

  6. Development of the showers • The two experimental methods • Acceleration of the primaries • Propagation in the space • Effect of the galactic magnetic field • The Auger Observatory • Results on the energy spectrum, composition and search for the sources.

  7. 1938 - PIERRE AUGER Discovery of the extensive air showers. Observation of coincidences between Geiger counters placed at different distances. 1015 eV Pierre Auger

  8. Elettromagnetic shower in Wilson chamber with Pb absorbers triggered by Geiger counters

  9. Interactions and decays in the extensive air showers

  10. The shower in the atmosphere • First interaction: cross section of protons or nuclei off nuclei of the atoms of the air. • Mesons (charged π e K ). Competition interaction-decay. Muons from decay. • Mesons π0  2 photons elettromagnetic showers. • About 90% of the energy of the primary is transferred to the electromagnetic showers (photons, electrons and positrons) • The front of the shower proceeds ad the speed of light c = 300 m/μs (the shower develops in a few tenths of microseconds)

  11. Inelastic cross section proton-air500 mb  λinel = 50 g/cm2 (500 m at sea level, 3000 m at height of 15 km) Tevatron

  12. Interaction/decay charged π mesons • Mean interaction length λint~1 km • Mean decay path λdec=γ c τ = (E/mπ) 7.8 m = (E/0.14) 7.8 m λdec ~λintfor E ~ 20 GeV

  13. Development of the electromagnetic showers • Radiation length X0=37 g/cm2, ~300 m at sea level • Critical energy Ec = 84 MeV • Molière length RM = (Es/Ec) = ¼ X0 • At very high energy Landau-Pomeranchuk-Migdal effect (LPM): effective X0 is larger.

  14. Cascade e.m. – simple model of Heitler E0 energy of the particle that Initiate the cascade After n interactions, the depth of the cascade is X=n X0 The number of particles is N(X) = 2 X/X0 The mean energy of the secondaries at this depth is E(X) = E0/N(X) --> 2 X/X0 = E0 / E(X) The multiplication will stop when the particles have energia equal to Ec ThereforeXmax= X0 log (E0/Ec) /log2. Xmax~ log E0 , Nmax = E0/Ec

  15. Hadronic cascade from simulations • General features Xmax ~ log E0 , Nmax ~ E0 • Dependence on the mass number A ( the nucleus as a collection of A nucleons, each with energy ≈ E0 / A, superposition of A showers) Xmax ~ log (E0 / A)

  16. Development of the shower

  17. Particle composition of the front of the shower 1019 eV – 1400 m a.s.l.

  18. Measurements with fluorescence telescopes(HiRes, Auger)

  19. Longitudinal profile of showers from the Auger telescopesFit with empirical formula of Gaisser-HillasCherenkov light subtracted 4 parameters 1.5x1019 eV, 550 4.5x1019 eV, 360

  20. Calorimetric measurement of the energy • Measurement of the detector sensitivity to fluorescence photons • Fit with the Gaisser-Hillas formula, Cherenkov light subtracted • Use of the fluorescence yield to correlate amount of observed light • to the number N(x) of particles of the shower and then to the • energy deposited by the shower in the atmosphere • Total visible energy of the shower from the total track length and (dE/dx) • E = ∫ (dE/dx) N(x) dx • Correction for energy loss (neutrino, muons) • Energy of the primary cosmic ray

  21. Correction for energy loss (neutrinos, muons) p / Fe : 8 – 12 % at 1019 eV (10% ± 2%) eventually important to know the composition

  22. Emission spectrum AIRFLY ~ 300 – 430 nm Bunner 1995 spectrum Abbasi 2008 HiRes

  23. Compilation F. Arqueros (NJP 2009) Dispersion of the results ≈ ± 15 % Quenching due to collisions of N2 with O2 and H2O well studied. Pressure and temperature dependence measured. 6th Air Fluorescence Workshop – LNGS , February 2009

  24. The fluorescence yield as a function of height Region of interest

  25. Study of composition – mass of the primaries Xmax Depth of the maximum

  26. < Xmax > for different primaries (photons, protons and iron nuclei) photons protons iron

  27. Results simulations <Xmax > as a function of energy

  28. Results of simulations at E = 1019 eV Blue – Fe Red - protons Note the large fluctuations !

  29. E=1019 eV

  30. One Auger event of energy 1019 eV Compared to simulations of iron nuclei, protons and photons zenith angle 35°,

  31. SHOWER RECONSTRUCTION with fluorescence telescopes 1) Shower detector plane (SDP) Resolution ~ 0.1o Camera pixels 2) Shower axis within the SDP ≈ line but 3 free parameters (Rp,co) ti monocular geometry t(χi) = t0 + Rp· tan [(χ0 - χi)/2] Large uncertainties (few degrees) extra free parameter χi

  32. A different method to study the composition of the primaries Protons: Nmuoni = E 0.85 (less than linear) A nucleus as a collection of A nucleons (A interactions with energy E/A ) Nmuons (A) = A0.15 Nmuons (p) For a given energy, a heavy primary will produce a shower with a larger number of muons. A shower from a nucleus of Fe contains a number of muons about 80% larger than a shower from a proton of the same energy.

  33. Measurements with surface arrays(AGASA, Auger)

  34. AGASA - High-energy event ~1020 eV Fit of the observed particle density Determination of the energy estimator S(600)

  35. AGASA Absolute energy calibration from simulations

  36. The best distance from the shower axis for the determination of the energy estimator is a function of the array spacing (Watson) AGASA, spacing 1 km , S(600) (Haverah Park) Auger, spacing 1.5 km , S(1000)

  37. SHOWER RECONSTRUCTION from the surface array of Auger distance from the core size parameter Lateral distribution function (LDF) NGK S(1000) is energy estimator Auger calibrates S(1000) with the fluorescence telescopes data slope parameter (β(q)= 2-2.5) core Signal (VEM) S(1000) 34 tanks distance from the core (m)

  38. σ(S(1000))/S(1000) Precision of S(1000) improves as energy increases S(1000) 10 EeV

  39. Zenith angle dependence of the energy estimator S(1000)

  40. shower front 1.5 km SHOWER DIRECTION from surface array (Auger) Fit of the particle arrival times with a model for the shower front (plane  paraboloid) Vertical shower of energy 1019 eV activates 7-8 stations very good time resolution (~ 12 ns)

  41. Acceleration

  42. Acceleration mechanism • Not well known yet • Fermi (1949) proposed a theory of stochastic acceleration • resulting from the interactions with moving magnetized • plasma. Power law comes naturally from Fermi’s theory. • Limitation of the maximum possible energy due to the size L • of the region where acceleration takes place. • {E = z B r  r = E / (z B) , where r = radius of curvature} • The particle being accelerated may be confined if • L > r = E / (z B) . • Otherwise the particle will leave the acceleration region and • no acceleration mechanism may be effective. • The maximum energy that can be reached will depend on • the product B L • E < z B L

  43. Maximum energy: ~ z B L 1 pc (parsec) 1 AU/ 1 arc sec = 3.26 anni luce It seems that in the Galaxy it is not possible to accelerate protons with energy larger than about 1019 eV

  44. Propagation in space

  45. Greisen-Zatsepin-Kuz’min Interaction with CMB (2.7 0K radiation) GZK cutoff Above E ≈ 6*1019 eV, protons loose rapidly energy via pion photoproduction. Energy loss ≈ 15 % / interaction. Interaction length = 5 – 10 Mpc p + γ CMB → n + π+ p + π0 ∆+ production {γ from π0 , ν from π+} protons e+e– e+ e- pair production is less effective, energy loss ≈0.1% / interaction Produces a “dip” in the spectrum (Berezinsky) Attenuation length  Interaction length

  46. The interaction of protons with the photons of the CMB • V.Berezinsky et al. • production of e+ e- pairs • photoproduction of pions e+e- π

  47. PROTONS Protons of very high energy cannot come from very large distances 1 EeV = 1018 eV

  48. Survival probability of protons

  49. The concept of GZK horizon z = 0.024 100 Mpc

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