Transcript Cosmic-Ray Physics with air-shower arrays P. Camarri University of Roma “Tor Vergata” INFN Roma Tor Vergata.

Cosmic-Ray Physics with
air-shower arrays
P. Camarri
University of Roma “Tor Vergata”
INFN Roma Tor Vergata
Outlook
• Introduction
• Methods of measurement
• Extensive Air Showers (EAS)
• EAS arrays
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Energy spectrum of primary CRs
1 particle /(m2 s)
1 particle / (m2 year)
1 particle / (km2 year)
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Energy spectrum of primary CRs
About 2 orders of magnitude are lost each decade !!
E > 1014 eV: only indirect, ground-based measurements are possible
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Indirect observations
At present, the indirect observation is the unique solution
to overcome the poor primary flux above 100 TeV.
The different approaches to investigate the chemical composition are
commonly based on the fact that the inelastic cross section of a nucleus
of mass A is proportional to A2/3, which leads to a long interaction mean
free path (m.f.p.) of protons and a short m.f.p. of nuclei.
Short m.f.p.
of nuclei
Characteristics of early development:
• Large lateral spread
• Muon rich events
• Soft secondary energy spectrum
From these observations EAS experiments
can extract information on the primary mass
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Different mean free path
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How to detect Extensive Air Showers
The classic method to detect air showers is to build an array of detectors
over an area of the order of 104-6 m2.
EAS arrays are characterized by
the number of detectors, the
distance step in the array and
the total covered area.
EAS-TOP: LNGS
KASCADE: Karlsruhe
Tibet ASγ: still operating
The detectors are chosen according to the observable to be
detected: e, γ, μ, h, Cherenkov light, fluorescence.
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Arrays of particle detectors
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Observables in EAS
Charged particles:
e.m., m, hadrons
Cerenkov light
Fluorescence light
+
Monte Carlo
simulations
E0, A
hadronic interaction
models
+ detector
response
Energy spectrum
Composition
Test of hadronic
interaction
models
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EAS – an example
EAS measured by the ARGO-YBJ experiment with high
space-time granularity and unprecedented details.
Shower core position
time (ns)
meters
Lateral distribution
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Arrival time vs position
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EAS – an example
EAS measured by the ARGO-YBJ experiment
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Extensive Air Showers - EAS
An EAS is the result of nuclear interactions with air nuclei
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EAS – Energy Flow
1. Hadrons provide energy to
muonic and e.m. components
2. “One Way” for energy into
e.m. particles
3. Details of energy transfer
processes are important
(Particle Physics)
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Discovery of Extensive Air Showers
It was Bruno Rossi in 1933 that noticed coincidences between several counters
placed in a horizontal plane, far in excess of chance coincidences.
From observation in Eritrea he noted:
“It would seem... that from time to time there arrive upon the equipment very
extensive groups of particles (‘sciami molto estesi di corpuscoli’) which
produce coincidences between counters even rather distant from each other”.
Supplemento a la Ricerca Scientifica, 1 (1934) 579.
To investigate these observations B. Rossi
sent a young student, Giuseppe (Beppo)
Occhialini, to the Cavendish Laboratory (U.K.)
to work with Blackett.
Occhialini discovered that CRs occasionally
produce complex events with many particles.
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Blackett named “showers” the groups of
particles observed in their detectors as an
english translation of the word “sciami” used
by Occhialini and Rossi in their discussions.
Occhialini and Blackett
Rossi invented the coincidence circuit. It employed
triode vacuum tubes and was capable of registering
coincident pulses from any number of counters with
a ten-fold improvement in time resolution over the
mechanical method of Bothe.
The Rossi coincidence circuit was the first effective
electronic device of particle physics.
Rossi also invented the method of anti-coincidence.
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Coincidence Rate
The most systematic investigation on these showers was undertaken by
Pierre Auger in 1938. He recorded coincidences between counters with
a horizontal separation of 75 meters.
The Observed Rate was found to be much higher than the Calculated
Chance Rate, even when the counters were placed 300 m apart.
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Particles discovered in CRs
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The shower core and the shower size
•
•
•
For the first time the existence of a single shower core was established.
For the first time the total number of charged particles (“shower size”)
was calculated by integrating the measured densities.
For the first time the absolute number of showers with a given size
(“size spectrum”) was evaluated.
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The EAS thickness: a new dimension
A new dimension was added to air-shower research when it was established by
Bassi, Clark and Rossi (1953) that the thickness of the EAS disc was quite small
and all the particles crossed the detector in less than a few nanoseconds.
This led them to suggest that, by recording the arrival times of the
shower particles at different scintillators in the array, it would be
possible to determine the primary direction with an accuracy of ~ 5º.
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The EAS analysis
The direction determination together with a particle-density measurement with
the different array detectors provided an elegant method of determining the
shower parameters (core position, shower size, direction of arrival).
Density Sampling + Fast Timing
In 1956 for the first time in the “Agassiz” experiment B. Rossi’s group
applied simoultaneously both methods in the EAS analysis.
With this approach (the so-called “MIT Standard” )
showers with more than 109 particles (E ≥ 1018 eV)
were observed.
Therefore, to detect such very high energy events
Rossi’s group built larger arrays.
The most important was the one operated at Volcano Ranch (New
Mexico). This experiment could detect an EAS induced by a 1020 eV
primary CR thus representing a milestone in the history of CRs.
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Volcano Ranch - MIT “Desert Queen”
1957-1963: exagonal array,
19 plastic scintillators
1959: John Linsley and Livio Scarsi
1800 m (3600 m)
detected an EAS induced by a CR with
energy E0 = 6·1019 eV containing
3·1010 particles.
1962: The array was enlarged up to 3.6 km and Linsley detected the first
CR with energy E0 = 1020 eV. The EAS contained 5·1010 particles !
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Volcano Ranch results
This paper described the
first event believed to be due
to a cosmic ray of energy
> 1019 eV.
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Volcano Ranch results
The first deep study of the time structure of
muons and electrons in cosmic ray showers.
“...is a seminal paper – and a required read
for all who work on surface detectors in
air-shower arrays...”
Alan Watson, 2007
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EAS longitudinal development
1.
The number of particles in the EAS
increases with the atmospheric depth,
due to the different interactions, up to a
maximum value.
2.
Multiplication stops when the individual
particle energies drop below the Critical
Energy, where collisional energy losses
exceed radiative losses.
3.
The number of secondaries gradually
decreases and the EAS can be completely
absorbed before reaching the ground.
1.
2.
3.
Shower Age s:
1. s < 1
2. s = 1
3. s > 1
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EAS – toy model for e.m. component
After traveling X0 (radiation length) the particle number doubles and the
energy is equally divided
X0 for bremsstrahlung
0.78 X0 for pair production
The shower behaviour is reproduced quite well
Reality is more complicated,…needs a MC simulation
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Basic features of e.m. showers
Despite its limitations, the Heitler model reproduces 2 basic features of the
e.m. shower development which are confirmed by accurate MC simulations
and observations:
1. The maximum size of the shower Is
proportional to the primary energy E0.
2. The depth of the maximum
increases logarithmically with the
energy, at a rate of about 85 g/cm2
per decade of the primary energy.
Elongation Rate
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E0
N max
Ecrit
Ecrit  80 MeV in air
 E0 

X max  X 0  ln
 E crit 
dX max

 2.3  X 0  85g / cm2
d log10 E0
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EAS – toy model for hadronic showers
Interaction length
λπ-air ~ 120 g/cm2
λp-air ~ 85 g/cm2
Critical Energy: energy at which
the decay length < distance to
the next interaction
Eπcrit ~ 20 GeV
In each interaction: Nch π± and ½ Nch πº
Nch ~ 10
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Ntot = 3/2 Nch
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Basic features of hadronic showers
The primary energy is finally divided between Nπ pions and Ne
e.m. particles in subshowers. The number of muons is Nμ = Nπ.
e.m.

E0  Ecrit
 Ne  Ecrit
 Nm
energy conservation
The relative magnitude of the contributions from Nμ and Ne is
determined by their respective critical energies: the energy
scale at which hadronic and e.m. multiplication cease.
The importance of this relation is that E0
is simply calculable if both Ne and Nμ
are measured.
In addition, this linear relation is insensitive
both to fluctuations and to the primary mass A.
CASA-MIA
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μ and electron sizes
 E0
N m   
 Ecrit





ln(N ch )
 0.9
ln(Ntot )
Muon size grows with primary energy more slowly than proportionally.
The exponent depends on the division of energy between charged and
neutral daughter particles in each interaction.

 E0 
N e  10  

 1PeV 
6
E0  (1.5GeV )  Ne0.97
α~ 1.03

E0  Ee.m.  Eh  Ee.m.  N m  Ecrit
 E0 
Ee.m.
 1    
E0
 Ecrit 
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 1
The e.m. fraction is 72% at E0 = 1014 eV,
rising to 90% at E0 = 1017 eV.
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Depth of shower maximum
Xmax is the atmospheric depth at which the e.m. component
of the shower reaches its maximum.
p

X max
 X max
 Int  X 0  ln(3Nch )
Interaction length
Depth of γ-induced shower maximum
The elongation rate:
p   
d
Int  X 0  ln(3Nch )  58 g/cm2 per decade
d log10 E0
Λp is reduced from Λγ for e.m. showers by two effect: larger
multiplicity Nch and larger cross-section (smaller λInt)
Linsley’s elongation rate theorem (1977): Λγ for e.m. showers
represents an upper limit to the elongation rate for hadron showers.
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Superposition model
A nucleus with atomic number A and total energy E0 is taken to be A individual
single nucleons, each with energy En = E0 / A, and each acting independently.
The resulting EAS is the sum of A separate p-induced EAS all starting at the same point.
For any additive measurable quantity Q
the model predicts A times the average
value for the quantity computed in a
proton shower of energy Ep = EA / A
 E
N m   0
 Ecrit




E
N max 0
Ecrit
p
X max
 X 0  ln(E0 )
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<QA(E)> = A · <Qp(E/A)>

 E0 
1 
p

N mA  A  

A

N
m
 
 A  Ecrit 
 N mp  A0.15
 E0 
A
  N max
N max
 A  
e. m. 
 A  Ecrit 
E 
A
p
X max
 X 0  ln 0   X max
 X 0  ln A
A
 
   ln E0  ln A  
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α and β depend on the nature
of hadronic interactions.33
Nucleus-air interaction
Increasing the mass A
•
More secondary particles with less energy
→ less electrons (after max), more μ
•
Surviving hadrons have less energy
•
Larger deflection angles → flatter lateral
distributions of secondary particles
The lower energy nucleons generate
fewer interactions and so lose less
energy to e.m. components.
Showers by nuclei dissipate their energy faster than
protons, thus having shallower (smaller) Xmax .
J. Matthews, Astrop. Phys. 22 (2005) 387
J. Linsley, 15th ICRC, 12 (1977) 89.
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Energy flow in EAS
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Shower fluctuations
The main source of the shower-to-shower
fluctuations is due to the distribution of the
depth of the first interaction.
In the superposition model the resulting
EAS is the sum of A separate p-induced
EAS all starting at the same point.
 Q (E) 
A
 Q ( E / A)
p
A
In reality, showers exhibit significantly
larger fluctuations than the ones
expected in the superposition model.
J. Engel et al., PRD 46 (1992) 5013.
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Fluctuations and lateral distribution
Different first-interaction atmospheric depth
→ different lateral distribution
Therefore, the lateral distribution is
sensitive to Xmax (i.e. to particle type).
In fact, nuclei develop higher in the
atmosphere (smaller Xmax) than protons,
producing flatter lateral distributions.
Difficult to disentangle fluctuation effects from
different primary chemical composition.
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Lateral distribution: the key for EAS reconstruction
LDF = Lateral Distribution Function
•
•
•
•
LDF important since first evidence of EAS (Auger + Kolhoster 1938)
Measurement of particles at ground is a calorimetric method
Chudakhov principle: number of particles
primary energy
LDF is the basis for integration → energy and composition

Which function is the best ?
EAS-TOP
It depends on:
•
•
•
•
•
Which secondary EAS component is measured
Energy threshold of measured particles
What kind of detectors are used
At which distance from the core it is measured
Energy range of the primaries
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LDF: electron/charged particles
Nishimura – Kamata – Greisen formula:
 r
N ch
ρch(r)dr 
 C(s)  
2
2πrM
 rM



s 2

r
 1 
 rM



s  4 .5
dr
s = age parameter: describes the shape of the
particle distribution
rM = Moliere radius (79 m at s.l.)
Nch = total number of charged particles
C(s) = normalization factor
This formula (with modifications) is used in
many EAS cosmic ray experiments
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LDF: muons
KASCADE
KASCADE experiment:
ρch(r)dr 
 r
N ch


C(s)

2
2πrM
 rM



s 2

r
 1 
 rM



s  4 .5
dr
rM = 89 m for electrons
rM = 420 m for muons
Greisen function (few GeV muons):
Hillas function (Haverah Park):
 m ( r )  r   e
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r

r0
dr
α = Slope parameter
r0 = 600 m (= 74 m in Darjeeling 1990)
Valid for Eμ = 2.5 - 54 GeV, 104 < Nch < 106
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EAS - Time profile
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Basic features of time profile
Due to geometrical reasons, the arrival of the first particles
at lateral distance r from the core is expected to be delayed
with respect to an (imaginary) planar shower front.
s
1. Shower core on the detector (r = 0)
t (r  0) 
H
c
delay for a particle produced at H
moving along the shower axis
2. Detector at a distance r from the core (if axis vertical)
1
t 
c

s
r2  H 2
t (r )  
c
c

r2
r H H 
H
2
2
(r « H)
The delay increases with r.
The delay decreases with increasing height H.
EAS with the first ground particles coming from large heights
will have smaller delays at fixed distance r compared to EAS
where the measured particles originated from smaller heights.
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Time spread
Spread in time = thickness of the shower disk at r
Difference of the arrival times of particles generated
in the height interval [H1, H1 - ΔH1]:

1
1  r 2  H
( t ( H1 , H ))  r  
 
H


H
H
H12
 1
  ( t ( H 2 , H ))
for H 2  H1
2
The spread of the arrival times of these particles at fixed
distance from the core increases for smaller production
heights.
EAS from nuclei develop higher in the atmosphere
(smaller Xmax) compared to γ–induced EAS:
smaller delays are expected.
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EAS morphology
1
t 
c


r2
r H H 
H
2
2
(r « H)
Very “young”
shower measured
by ARGO-YBJ
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EAS direction
The study of the EAS time profile is important because the EAS
direction is reconstructed by means of the “fast timing” technique
developed by Bassi, Clark and Rossi in 1953 (PR 92 (1953) 441).
t
tg   c
d
v~c
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For an EAS falling with zenith
angle θ two particles separated by
d arrive on a horizontal plane with
a time difference of d · sinθ/c
(provided that all particles in an
EAS are contained in a thin disk).
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Cosmic Rays as tool for Particle Physics
The main characteristics of hadronic
interactions that are relevant for EAS
physics are:





Cross sections (p-air, π-air, N-air)
Inelasticity of the collisions
Multiplicity/composition of secondaries
Transverse momentum distribution
Fraction of diffractive dissociation

s NN


knee
2.5
A

s NN


400
T eV
A

s pp

 14 TeV
GZK
LHC
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T eV
Elab ~ 3 · 1015 eV
Elab ~ 1020 eV
Still ~30 times lower
than the GZK energy.
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Hadronic interactions
The situation is much worse than it may
appear from energy considerations.
Measurements at colliders are limited to
an angular region that excludes the beam
pipe, and therefore a very large majority
of the high energy particles that are
emitted at small angles are unobservable.
These particles carry more than 90% of the
energy in a collision and are clearly crucial
in determining the EAS properties.
In EAS physics the fragmentation region
is more important than the central region.
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Fragmentation regions
Central region
Fragmentation
region
   ln tg

2
The particles here get
most of the momentum
of the primary CR
pLCM 2 pLCM
xF  CM 
pmax
s
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Inelasticity
The spectrum of nucleons produced in hadronic interactions
pays a fundamental role in the development of EAS.
In ND interactions the nucleons carry ~40% of the initial state energy.
The energy fraction carried by nucleons
is the “elasticity” of the interaction.
Inelasticity k = 1 - Elead / Ep
Leading energy fraction
Elead / Ep ~ 1
These high-energy nucleons feed energy
deeper into the EAS clearly playing a very
important role in the EAS development.
At the colliders most of these
nucleons are unobserved !
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proton-air cross section
λint = 40 g/cm2
λint = 96 g/cm2
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N-air cross section
The N-air cross sections are a function of the mass
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The EAS-TOP experiment
Running from 1989 till 2000 at Campo
Imperatore (2005 m a.s.l., LNGS, Italy)
rivelatori
M. Aglietta et al., U.H.E. Cosmic Ray event reconstruction
by the electromagnetic detector of EAS-TOP.
Nucl. Instrum. and Meth. A336 (1993) 310
 35 scintillators (10 m2 ) covering ~105 m2
 Hadronic calorimeter (Eh >30 GeV, Eμ >1 GeV)
 8 Cherenkov detectors
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2011
3 radio
antennae
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EAS-TOP
35 x 10 m2
active area
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KASCADE
Measurements of air showers in the energy range E0 = 100 TeV - 80 PeV
= KArlsruhe Shower Core and Array Detector
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The Energy Spectrum
The conversion from a primary with energy E0 and
mass A to a shower with size Ne is obtained by a
MC simulation of the EAS in the atmosphere
From the “size spectrum” to the energy
spectrum: we must figure out a model of
chemical composition of CRs
We “test” different models till we find the one which is compatible
with the measured size spectrum: “statistical” approach
Main hypotheses:
• cut-off depending on the rigidity: Ek(A) = Z·2·1015 eV
•γ
γ + 0.4, beyond the knee for all nuclei
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“light” knee
(p,He,CNO)
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Auger Experiment
Hybrid event
Water Cherenkov +
Fluorescent detector
3000 km2
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Surface Detector
Surface Array:
1600 Water Tanks
1.5 km spacing
3000 km2
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Event reconstruction
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Conclusion (1)


I do hope this short and necessarily limited
presentation gave you some hints about the
huge amount of information which can be
obtained from air-shower detection.
Next lecture: TeV -ray astronomy and the
ARGO-YBJ experiment.
THANK YOU!
P. Camarri
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