Large Array Astrophysics Detectors (I)

Giorgio Matthiae University and Sezione INFN of

Roma Tor Vergata

2010 LNF Spring School Frascati, 13 May 2010

1 particle/km2/century

LHC c.m.

~ 1 / E3

Cosmic ray spectrum year 2000

Cosmic ray spectrum - 2009

knee

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: fluorescence telescopes (Utah)

Telescope Array hybrid system ~ 800 km2 (in construction)

Auger South:hybrid system3000 km2

(completed May 2008)

Auger North~ 20.000 km2

(proposed)

• 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.

1938 - PIERRE AUGER1938 - PIERRE AUGER

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

Pierre Auger

1015 eV

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

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 microseconds)

Inelastic cross section proton-air500 mb λinel = 50 g/cm2

(500 m at sea level, 3000 m at height of 15 km)

Tevatron

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

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

E0 energy of the particle that Initiate the cascade

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)

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-HillasCherenkov light subtracted

XX

XX

XXNXN

XX

max

0max

0max exp)(

0max

4 parameters

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

Emission spectrum

~ 300 – 430 nm

Bunner 1995 spectrum

Abbasi 2008HiRes

AIRFLY

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

XmaxDepth of the maximum

Study of composition – mass of the primaries

photons

protons

iron

< 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 – FeRed - protons Note the large fluctuations !

E=1019 eV

zenith angle 35°,

One Auger event of energy 1019 eVCompared to simulations of iron nuclei,protons and photons

t(χi) = t0 + Rp· tan [(χ0 - χi)/2]

1) Shower detector plane (SDP)

Camera pixels

monocular geometry

2) Shower axis within the SDP ti

χi

≈ line but 3 freeparameters

extra free parameter

SHOWER RECONSTRUCTION with fluorescence telescopes

Resolution ~ 0.1o

Large uncertainties(few degrees)

(Rp,o)

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 eVFit of the observed particle density Determination of the energy estimator S(600)

AGASAAbsolute 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)

Lateral distribution function (LDF)

NGK

1700

700

1000)1000()(

rrSrS

size parameter

slope parameterS(1000) is energy estimator

Auger calibrates S(1000) with the fluorescence telescopes data

core

distance from the core

S(1000)

distance from the core (m)

Sign

al

(VE

M)

34 tanks

SHOWER RECONSTRUCTION from the surface array of Auger

(β) 2-2.5)

10 EeV S(1000)

Precision of S(1000) improvesas energy increases

σ(S(1000))/S(1000)

Zenith angle dependence of the energy estimator S(1000)

1.5 km

shower front

Fit of the particle arrival times with a model for the shower front (plane paraboloid)

very good time resolution (~ 12 ns)

SHOWER DIRECTION from surface array (Auger)

Vertical shower of energy 1019 eV activates 7-8 stations

Acceleration

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

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

Propagation in space

e+e–

Interaction length

Attenuation length

Interaction with CMB (2.7 0K radiation) GZK cutoffAbove E ≈ 6*1019 eV, protons loose rapidly energy via pion photoproduction. Energy loss ≈ 15 % / interaction. Interaction length = 5 – 10 Mpc

Greisen-Zatsepin-Kuz’min

p + γ CMB → n + π+ p + π0

∆+ production

{γ from π0 , ν from π+}

e+ e- pair production isless effective, energy loss

≈ 0.1% / interactionProduces a “dip” in the spectrum (Berezinsky)

protons

The interaction of protons with the photons of the CMBV.Berezinsky et al.

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

e+e-

π

1 EeV = 1018 eV

PROTONS

Protons of very high energy cannotcome from very largedistances

Survival probability of protons

z = 0.024 100 Mpc

The concept of GZK horizon

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 understood.

Effect of interaction with photonsV.Berezinsky et al.

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

protons

e+e-

π

Survival probabilityprotons 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

knee

Effect of the galactic magnetic field

Earth

~ 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 proofof extragalactic origin

LHC results on multiplicity, particle productionclose to projectile rapidity, total cross sectionvery important to tune the simulation programs

Atmosfera (standard)

Altezza

(m)

Pressione

(mbar , hPa)

Densita’ (x 10-3)

(g/cm3)

0 1013 1.22

1000 899 1.11

5000 540 0.74

10000 265 0.41

15000 121 0.19