Large Array Astrophysics Detectors - LNF - Infn

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					    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 / E3

           1 particle/km2/century

LHC c.m.
Cosmic ray spectrum - 2009

2 x 1020 eV = 30 J

A macroscopic energy in a
microscopic particle !

Tennis ball

Kinetic energy = ½ m v2

   m = 50 g
   v = 120 km/h
                          The two techniques
Fluorescence Telescopes
N2 molecules (300-400 nm)

• Longitudinal development of     the shower

• Calorimetric measurement of the energy
• Only clear moonless nights
     ~ 10% duty cycle !

     Surface Array
 •    Front of shower at ground
 • Direction and “energy” of the shower
AGASA: surface
array – 100 km2
(1990 – 2004)

Fly’s eye - HiRes:
telescopes (Utah)

Telescope Array
hybrid system
  ~ 800 km2
(in construction)

Auger South:
hybrid system
3000 km2
(completed May 2008)

Auger North
~ 20.000 km2
•   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
    1938 - PIERRE AUGER

   Discovery of the extensive air showers.
   Observation of coincidences between Geiger counters
   placed at different distances.

1015 eV

                                             Pierre Auger
Elettromagnetic shower in Wilson chamber with
Pb absorbers triggered by Geiger counters
Interactions and decays in the extensive air showers
       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
Inelastic cross section proton-air
500 mb  λinel = 50 g/cm2
(500 m at sea level, 3000 m at height of 15 km)

  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 ~ λint   for E ~ 20 GeV
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.
       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
 Therefore                       Xmax = X0 log (E0/Ec) /log2.

              Xmax ~ log E0            ,          Nmax = E0/Ec
   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

             Xmax ~ log (E0 / A)
Development of the shower
Particle composition of the front of the shower
            1019 eV – 1400 m a.s.l.
Measurements with
fluorescence telescopes
(HiRes, Auger)
Longitudinal profile of showers from the Auger telescopes
Fit with empirical formula of Gaisser-Hillas
Cherenkov light subtracted
                                            X max  X 0
                               X  X0         
                                                             X X 
                              X X 
               N ( X )  N max                          exp max     4 parameters
                               max   0                         

           1.5x1019 eV, 550                                       4.5x1019 eV, 360
         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
Correction for energy loss (neutrinos, muons)

     p / Fe : 8 – 12 % at 1019 eV (10% ± 2%)
     eventually important to know the composition
~ 300 – 430 nm

Bunner 1995 spectrum

                       Abbasi 2008
            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
The fluorescence yield as a function of height

        Region of interest
Study of composition – mass of the primaries
      Depth of the maximum Xmax
< Xmax > for different primaries (photons, protons and iron nuclei)



    Results simulations
<Xmax > as a function of energy
Results of simulations at E = 1019 eV
Blue – Fe
Red - protons   Note the large fluctuations !
E=1019 eV
One Auger event of energy 1019 eV
Compared to simulations of iron nuclei,
protons and photons

            zenith angle 35°,
             with fluorescence telescopes

1) Shower detector plane (SDP)

                                     ~ 0.1o

             Camera pixels

2) Shower axis within the SDP                                ≈ line but 3 free
                                (Rp,co) ti                   parameters
   monocular geometry
t(χi) = t0 + Rp· tan [(χ0 - χi)/2]                Large uncertainties
                                                  (few degrees)
                  extra free parameter
       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.
Measurements with
surface arrays
(AGASA, Auger)
AGASA - High-energy event          ~1020 eV
Fit of the observed particle density
Determination of the energy estimator S(600)
Absolute energy calibration from simulations
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)
SHOWER RECONSTRUCTION from the surface array of Auger
                               size parameter             distance from the core
 Lateral distribution
 function (LDF)                                                               
                                                r                r  700 
                              S (r )  S (1000)                          
 NGK                                            1000             1700 
S(1000) is energy estimator                                  slope parameter
                                                             (β(q) 2-2.5)
Auger calibrates S(1000) with the
fluorescence telescopes data

                                     Signal (VEM)
                        core                                  S(1000)

    34 tanks

                                                    distance from the core (m)

                                 Precision of S(1000) improves
                                 as energy increases

                     10 EeV
Zenith angle dependence of the
   energy estimator S(1000)
   SHOWER DIRECTION from surface array (Auger)


  1.5 km

Fit of the particle arrival
times with a model for the
shower front
(plane  paraboloid)
                              Vertical shower of energy
     very good                1019 eV activates 7-8 stations
time resolution (~ 12 ns)
          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
   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
Propagation in space
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+}

e+ e- pair production is
less effective, energy loss
≈ 0.1% / interaction                                     Attenuation length
Produces a “dip” in the                         
spectrum (Berezinsky)                                     Interaction length
The interaction of protons with the photons of the CMB
V.Berezinsky et al.

• production of e+ e- pairs
• photoproduction of pions



Protons of very
high energy cannot
come from very large

                       1 EeV = 1018 eV
Survival probability of protons
The concept of GZK horizon

               z = 0.024   100 Mpc
GZK Horizon - GZK sphere
maximum distance of the sources for protons
arriving at the Earth with energy above a
given value.

           Energy (EeV)
•   GZK mechanism well understood for protons
    (photoproduction cross sections well known)

•   For nuclei it is relevant the energy region of the Giant Dipole Resonance
    (20 -25 MeV in the nucleus rest frame)
    Photodissociation (γ,n) , (γ,p), etc.

•   Nuclei will suffer energy degradation but also undergo a kind of
    “stripping” with reduction of the mass number. In addition β decay of
    the nuclear fragments.

•   For nuclei there is a complex chain of events, not yet fully studied and
Effect of interaction with photons
V.Berezinsky et al.

• production of e+ e- pairs
• photoproduction of pions            π
Survival probability
protons and nuclei
Observation of the GZK suppression is indication of
extragalactic origin of the cosmic rays in the region close
to the end of the spectrum

Effect of the galactic
   magnetic field
~ 2 - 3 μGauss

Deflection in the galactic magnetic field of
extragalactic protons with energy 60 EeV
Deflection of protons in the magnetic galactic field
Galactic – extragalactic origin
    (two extreme models)
                  GZK and mass composition

Only protons and not too light nuclei are able to reach the Earth
for energies above ~ 60 EeV
Study of mass composition could help
understanding the transition from
galactic to extragalactic origin

Observation of distant sources
(within the GZK horizon) is a direct proof
of extragalactic origin

LHC results on multiplicity, particle production
close to projectile rapidity, total cross section
very important to tune the simulation programs
          Atmosfera (standard)

Altezza     Pressione     Densita’ (x 10-3)
  (m)      (mbar , hPa)      (g/cm3)
   0          1013             1.22
 1000          899              1.11
 5000          540              0.74
10000          265              0.41
15000          121              0.19

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