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Particle Physics
Marco G. Giammarchi
Istituto Nazionale di Fisica Nucleare
Via Celoria 16 – 20133 Milano (Italy)
[email protected]
http://pcgiammarchi.mi.infn.it/giammarchi/
1. Costituents of Matter
2. Fundamental Forces
3. Particle Detectors
4. Symmetries and Conservation Laws
5. Relativistic Kinematics
6. The Static Quark Model
7. The Weak Interaction
8. Introduction to the Standard Model
9. CP Violation in the Standard Model (N. Neri)
A. Y. 2013/14
I Semester
6 CFU, FIS/04
Marco Giammarchi (36 h)
Nicola Neri (12 h)
Contratto tipo A
Gratuito - Enti convenzionati
(Istituto Nazionale Fisica Nucleare)
1
Particle physics is a branch of physics
which studies the nature of particles that
are the constituents of what is usually
referred to as matter and radiation. In
current understanding, particles are
excitations of quantum fields and interact
following their dynamics. Although the
word "particle" can be used in reference
to many objects (e.g. a proton, a gas
particle, or even household dust), the
term "particle physics" usually refers to
the study of the fundamental objects of
the universe – fields that must be
defined in order to explain the observed
particles, and that cannot be defined by
a combination of other fundamental
fields.
The current set of fundamental fields and
their dynamics are summarized in a
theory called the Standard Model,
therefore particle physics is largely the
study of the Standard Model's particle
content and its possible extensions.
(Wikipedia)
2
A bacis course (primer) in Particle Physics intended to be:
• Phenomenological
• Self-consistent
PRE-REQUISITES
Basic concepts of:
• Quantum Mechanics
• Nuclear Physics
• Special Relativity
• Quantum Field Theory
• Radiation-Matter Interaction
3
General information:
• The professor is always available in principle
• However, he is quite often away
• Professor always reads e-mails
• These slides can be downloaded from the professor’s website
• The Exam Committee: Nicola Neri, Lino Miramonti
Slides in English: portability, Erasmus students, lecturing in Italian (or English)
Following the Course is warmly suggested but not mandatory (of course)
VERY IMPORTANT ANNOUNCEMENT ,
Noli iurare in verba magistri
4
Bibliography:
• Griffith – Introduction to Elementary Particles – 2008 – J. Wiley
• D. H. Perkins – Introduction to High Energy Physics – Addison Wesley – 2000
• K. Gottfried, V. Weisskopf – Concepts of Particle Physics – Oxford Univ. Press - 1984
• D. C. Cheng, G.K. O’Neill – Elementary Particle Physics – 1979 - Addison Wesley
• R. N. Cahn, G. Goldhaber – The experimental foundations of Particle Physics – 1991 –
Cambridge University Press
• E. M. Henley, A. Garcia – Subatomic Physics – 2007 – World Scientific
• I. S. Hughes – Elementary Particles – 1991 – Cambridge University Press
• B. Roe – Particle Physics at the new Millennium – Springer - 1996
• F. Halzen, A. D. Martin – Quarks and Leptons – 1984 – J. Wiley
• DrPhysicsA on youtube.com (why not?) ,
The bibliography is only indicative
5
Genesis (and other considerations)
Adam’s Creation – Michelangelo Buonarroti (1511).
(Musei Vaticani - La Cappella Sistina)
6
Parmenides (circa 500 AC), Zenon (circa 490 – 430 AC): the experience of
multiplicity can be negated.
Matter can be divided with no end. The infinite division of spatial extension gives as
a result zero, nihil. Therefore the multiplicity of bodily extension does not exist. It is
an illusory opinion.
Demokritos (circa 460 – 370 AC): the experience of multiplicity cannot be negated
Matter can be divided only down to some fundamental unit.
A-tomos, indivisible. The atom was introduced to stop the “reduction to nothing”
process of spatial extension put forward by Parmenides and Zeno.
The atom is the point where the division process stops.
This is totally different from the modern concept of science.
Particle Physics as a modern Science begins around year 1930.
Particle Physics plays the role of the theory “par excellence” in a reductionist
approach (understand everything starting from elementary building blocks).
For a critical approach to this, see :
P.W. Anderson – More is Different – Science Vol 177 (1972) pag. 393.
Particle Physics claim to be a fundamental theory: it deals of matter and energy in
extreme spacetime conditions.
7
Costituents of Matter
1. Costituents of Matter
2. Fundamental Forces
3. Particle Detectors
4. Experimental highlights
5. Symmetries and Conservation Laws
6. Relativistic Kinematics
7. The Static Quark Model
8. The Weak Interaction
9. Introduction to the Standard Model
10. CP Violation in the Standard Model (N. Neri)
8
9
Fundamental Constituents of Matter: Quarks and Leptons
Structureless building blocks down to a spatial extension of 10-18 m
Well defined spin and charge
Leptons have well defined mass as well
Lower Mass
Matter Constituents
under ordinary
conditions (low
energy/T)
Constituents of unstable
particle (produced at high
energy, in astrophysical
systems). They decay to
lower mass particles
Higher Mass
10
A reductionist example:
the Deuterium Atom
p  (u, u, d )
n  (u , d , d )
10-15 m
Quarks:
Fractional charges
Semi-integer spin
10-10 m
Quarks, electrons, and photons as fundamental Constituents of the Atom
11
Leptons: observable particles with definite mass (mass eigenstates)
Quarks: not directly observable. Not a well defined mass.
A long history of discoveries down to
smaller and smaller structures: quarks
are now considered the innermost layer
of nuclear matter.
Particles as probes to study of atomic
and nuclear structures :
- The associated wave length is
ph
h

p
(de Broglie)
High energies makes
us sensitive to
smaller spatial scales

p

p
in this slide
12
Particle-like quantities and Wave-like quantities. Crossing the boundary
Particles
Energy E
Frequency, Wavelength
ν,λ
2
p
,
2m

p 2  m 2 ,  mc2
h
p
 
Waves
  E /h
  c
Einstein (1905) : waves behave like particles with a given energy ,
(confirmed by the Compton effect)
De Broglie (1924) : particles also have a wavelength like waves ,
(confirmed by the Davisson & Germer experiment)
13
Particles
2
p
,
2m
Energy E
Waves
2
p  m 2 ,  mc2
  E /h
h
p
 
Frequency, Wavelength
ν,λ
  c
Is it all consistent?
For waves :
  E /h
,
  c
Using the relativistic dispersion law:

c/  E /h

E pc
.

p  h/
We can consistently attribute Energy and Momentum to the photon.
Now, what about massive particles ?
14
Particles

p2
,
2m
Energy E
For a massive particle :
2
p  m 2 ,  mc2
h

p
Frequency, Wavelength
ν,λ
  h/ p
Using the wavelike equation
Waves
  E /h
  c
  h /( mv )
E  mc2
 
h
h
h  mc2
c c
 
 c   vP  c
mv h
v 
Phase velocity. A massive particle is not just a De Broglie wave: it is a wavepacket.

p

p
in this slide
15
  E / h  E  
  h / p  p k
The particle velocity is the GROUP velocity
Given a dispersion relation:
For a non-relativistic particle :
For a relativistic particle :
E  E ( p)
   (k )
 E
vG 

k p
E   p 2  p
 
vG 
 
p p  2m  m
E 
vG 

p p


p 2c 2  m 2c 4 
pc 2
pc 2
pc 2
p



2
2 2
2 4
E
m c
m
p c m c

p

p
in this slide
16
The Uncertainty Principle in Relativistic Quantum Mechanics
In Non-Relativistic Quantum Mechanics (NRQM) one has
x  p 
Both x,p can be measured in a very short time and with arbitrary accuracy.
However, considering a momentum measurement
(v  v') t  p  
process, one can consider a duration of measurement
and the variation of the position during the
measurement :
Now, this term is limited to 2c
t  p   / c
Limited accuracy in p for a given t of measurement
An accurate measurement of momentum (momentum
eigenstate) is therefore limited, irrespective from the
accuracy on the position.
Momentum measurements are typically done using
long times, and the curvature in a magnetic field
Momentum from curvature of the charged
particle track in a magnetic field

p

p
in this slide
17
De Broglie and Compton wavelengths
Suppose we want to confine a
particle to within its λ Compton:

mc
h
p
C 


 mc  mc
x 
C 
dB  

x 
mc
p
h
p
 DB  

mc
The energy corresponding to the confinement:
p (momentum)
2
E  c p  m 2 c 2  2mc2
The energy required will be greater than the particle mass!
Creation of particles is energetically favoured with respect to
confining a particle within its Compton wavelength

in this slide p  p
18
Non relativistic composite systems : general features
C
Atomic systems
Dimensions are large
compared to electron’s
Compton wavelength
 C  D
D
A system that is large compared to the Compton wavelength
of its constituents:
• Has binding energies that are small compared to their rest masses
• Has non-relativistic internal velocities
19
A system that is large compared to the Compton wavelength
of its constituents:
• Has non-relativistic internal velocities
• Has binding energies that are small compared to their rest masses
For a confined particle
Now, if
p D

D 
mc

v
Dm

c 
Dm
System larger than electron’s Compton λ
The kinetic energy is then roughly classical and
 v  c
Nonrelativistic
velocity
1 2
K  mv  mc 2
2
But this (Virial Theorem) is of the order of the binding energy
EB  mc 2
20
Internal transitions in nonrelativistic composite systems: general features
Let us consider an atom-like non-relativistic
bound system and write the energy in terms
of momentum and position uncertainties :
1

2
p  
E 
2m
x
same order of magnitude (Virial)
D
The two uncertainties are connected by the Heisenberg Principle :
1 2

2
2
E 



2
2
2m x 
 x mx 
mD2
If minimized with respect to Δx will give the
Borh radius
The variation of E is related
to the electron mass and the
size of the system
Atom size and electron mass
21
Internal transitions in nonrelativistic composite systems: emitted radiation
Atomic systems :
• Emitted gammas have λ’s longer with respect to the atomic dimension
 

2

In fact, since this system is large compared to
the electron’s Compton wavelength :
Radiation emitted in
atomic transition :
E  h  h
c


D 
mc
D


hc


D 2 D 2 D E

hc
c
c mD2
c




mD
1
2
D 2 D E
DE
D 

  D
Part of the Electric Dipole Approximation
22
On the Electric Dipole Approximation
A typical interaction:

A
E
t


D   ei ri

H I   D E (0, t )
The radiation field (calculated in a specific space point)
Electric dipole moment of the system of charges
i
Optical transitions in Atoms:
300 nm    D  10
Atomic transitions
Gamma transitions in Nuclei:
E  h  2
c

  2
c
E
2
10
m
Atom size
197
fm    D  1015 m
E ( MeV )
Gamma transitions
Nucleus size
23
Constituents properties:
Quarks:
• Electric charge
• Color
• Effective mass
• Spin (1/2)
Leptons:
• Electric charge
• Mass
• Spin (1/2)
IMPORTANT ,
All Contituents (Quarks, Leptons)
are Fermions.
3 families
Costituents of Matter
Force
Carriers
24
Costituents and Force Carriers: the Spin/Statistics Theorem
Half-integer Spin Particles
1
3
 ,  , ....
2 2
Fermions
Fermi-Dirac Statistics
Bose-Einstein Statistics
(W. Pauli, 1940)
0,  , 2  , ....
Bosons
Integer Spin Particles
Consequences of the Spin/Statistics Theorem:
• formal: wave functions, field operators commutation rules
• experimental: nuclear and atomic structure, Bose-Einstein condensates
25
Particles are not Apples
Why are these two apples distinguishables ?
Because we can assign coordinates to them !
X1
X2
Classical Particles (like apples) can be distinguished  Boltzmann Statistics
But we cannot assign coordinates to
Quantum Particles ! They cannot be
distinguished
Quantum Particles cannot be
distinguished
They obey a quantum statistics
26
For a single-valued many-particle wave function
The wave function must have the correct symmetry under interchange of
identical particles. If 1, 2 are identical particles :
 ( x1 , x2 )   ( x2 , x1 )
2
2
  
(1  2)
  
(1  2)
(probability must be conserved upon
exchange of identical particles)
Identical Bosons
(symmetric)
Identical Fermions
(antisymm.)
Spin-Statistics
Theorem
A consequence of the Spin/Statistics Theorem: for two identical Fermions 1,2 in
the same quantum state x:
 ( x1, x2 )   ( x2 , x1 )   ( x1 , x2 )   ( x1 , x2 )  0
Pauli Exclusion Principle!
Because identical
Spin/Statistics Theorem
27
A snapshot on Quantum Relativistic Equations
How to obtain a quantum mechanical wave equation? One simple way recipe:

p2
• Take a dispersion relation. For instance, a classical one: E 
2m
• Make the transition to operators. Energy and momentum become operators
 2
acting on a state living in a suitable Hilbert space :

E 
• Use the appropriate form for the operators

E  i
t

p  i
p

2m

2 2
i   
 
t
2m
The non-relativistic Schroedinger equation !
The Klein-Gordon Equation
2
By analogy, taking a relativistic dispersion relation for a free particle E  p  m 2
2
and using the same recipe
 2
2
2
 2    m    0
 t

28
The Dirac Equation
Schroedinger Non Rel equation : first order in time, second order in space
Klein-Gordon equation: second order in space/time. It describes relativistic scalar
particles (in the modern interpretation of negative-energy solutions).
Dirac Equation : Relativistic equation first order in time and first order in space




i   i  k
 m
t
xk
 k

Setting :
 0 
Requiring consistency with the relativistic
dispersion relation (iterate the Dirac equation
and require the Klein-Gordon condition)
implies that α’s and β are 4x4 matrices.
 k   k
One has the covariant form of the Dirac equation :
(i      m)   0
A 4-component spinor :
 u1 
 
 u2 
  
v1
 
v 
 2
29
The Spin-Statistics Theorem in Quantum Field Theory
The requirement of MICROCAUSALITY : the requirement that two field
operators A(x), B(y) be compatible if x-y is a spacelike interval.
A,B solutions of a relativistic
equation (Klein-Gordon, Dirac)
A( x), B( y)  0
for x  y   0
2
(anti)commutator
This prescription generates the right statistics for Bosons and Fermions
The Pauli Exclusion Principle is an ansatz (ad-hoc assumption) in NonRelativistic Quantum Mechanics.
It can be demonstrated based on Microcausality in Quantum Field Theory.
30
A snapshot about our description of the world of “simple” systems
(material points in classical physics, particles in quantum physics)
Observables
Classical
Mechanics
Non Rel Quantum
Mechanics
Quantum Field
Theory
Operators

x (t )
t parameter

x ,v , t
xˆ , pˆ
 Â
A B
States
2
 ( x),  ( x), A( x)
spin 1/ 2,0,1 operators

x  ( x ,t ) parameters
 (t) in Hilbert space
t parameter
n Fock states
 Coherent states
.......
31
For a scalar (Klein-Gordon) field
 ( x),  ( y) 0
for x  y   0
2
satisfying the Klein-Gordon equation

2
t

 2  m2   0
commutator


 ikx
 ikx

 ( x)   dk a(k ) e  a (k ) e
Annihilation/Creation operators
The number operator









N (k )  a (k ) a (k ) has eigenvalues n(k )  0,1,2...
  
  

and satisfies a (k ') a (k ) 0  a (k ) a (k ') 0

 



  

a(k ), a(k ') 0 a (k ), a (k ') 0
  
a(k ), a (k ')  kk ' i
Symmetric under the
interchange of particle labels
32
For a spin 1/2 (Dirac) field
 ( x), ( y)  0
for x  y   0
2
satisfying the (4x1) Dirac equation
(i      m)   0
anticommutator
 ( x)   r  dp cr ( p) ur ( p) e ipx  d r ( p) vr ( p) eipx 

Sum over spins





Annihilation/Creation operators
 

  
  
cr ( p),cs ( p')   d r ( p), d s ( p')   rs  pp ' i
all other anticommutators 0


 
The number N r ( p)  cr ( p) cr ( p)


 
operators N ( p
)  d r ( p) d r ( p)
r
A number operator of the Dirac field has eigenvalues
and
  
  

c (k ' ) cs (k ) 0   cs (k ) cs (k ' ) 0

s
 u1 
 
 u2 
  
v
 1
v 
 2

nr ( p)  0,1
Antiymmetric under the
interchange of particle labels
33
Particles and Antiparticles: the “birth” of Particle Physics
1928: Dirac Equation, merging Special Relativity and Quantum Mechanics.
A relativistic invariant Equation for spin ½ particles. E.g. the electron
(i      m)   0
• E>0, s=+1/2
• E>0, s=-1/2
• E<0, s= +1/2
• E<0, s= -1/2
Electron, s=+1/2
Electron, s=-1/2
Positron, s=1/2
Positron, s=-1/2
Rest frame solutions: 4 independent states:
Upon reinterpretation of
negative-energy states as
antiparticles of the electron:
The positron, a particle identical to
the electron e- but with a positive
charge: e+. The first prediction of the
relativistic quantum theory.
34
Beginning of the story of Particle Physics: the discovery of the Positron
Positrons were discovered in cosmic rays
interaction observed in a cloud chamber place
in a magnetic field (Anderson,1932)
Existence of Antiparticles: a general (albeit non
unversal) property of fermions and bosons
Antiparticle: same mass of the particle but
oppiste charge and magnetic moment
All fundamental
constituents have their
antiparticle
35
A few more details about the discovery of the Positron :
36
Discovery of the first «Elementary» Particles
Known particles at the end of the 30’s
Faraday, Goldstein, Crookes,
J. J Thomson (1896)
• Electron
Avogadro, Prout (1815)
• Proton
Einstein (1905), Compton (1915)
• Photon
Chadwick (1932)
• Neutron
• Positron
• Muon
Conventional birth date
of Nuclear Physics
Anderson (1932)
• Pion
Neutrino: a particle whose existence
was hypotesized without a discovery!
Cosmic rays interaction
studies. Pion/Muon separation
37
The discovery of Muon and Pion
A little preview about the fundamental interactions:
• Gravitational Interactions: known since forever. Classical theory (A. Einstein)
in 1915. Responsible of macroscopic-scale matter stability.
• Electromagnetic Interactions: Classical theory (Faraday, Maxwell) completed
in 1861. Responsible of the interaction between charged (and therefore of the
stability of atomic structures). Important also at the nuclear scale.
• Strong Nuclear Interactions: responsible of forces at the nuclear level (and of
nuclear stability). It is a very short range interaction: 10-15 m (1 fermi, fm).
• Weak Nuclear Interactions: responsible of some relevant nuclear processes
(weak fusion, weak radioactivity). Also, a short (subnuclear) range interaction.
Search for the Pion was motivated (Yukawa) by the research on the Carrier of
the Strong Nuclear Force. And by the observation of a new particle.
38
Yukawa Hypotesis: the existence of the Pion as Mediator of the Strong Nuclear
Force
Nucleon
The first cosmic rays researchers found (in the
30’s) a particle with a mass that was intermediate
between the Electron and the Proton
(“MESON”)
Nucleon
Meson
It was thought that this was the carrier of the
force between two nucleons (Yukawa, 1935):

E 2  p 2c 2  m2c 4

E   i t
p   i
Relativistic relation between energy, momentum, mass
The quantization recipe
2 2
2
m
c
1


 2  2   2 2  0

c t
The static solution:
g r / R
  e
r
The Klein-Gordon Equation

R
mc
Interaction range
39
Using the Uncertaintly Principle
Interaction length of a force  Compton wavelength of the Carrier
Source
Source
The creation of the carrier
requires ∆E = mc2
Carrier
The event is restricted to take place on a time scale:
 t   / mc2
During this time the carrier can travel:

Rct 
mc
Interaction Range
In the case of the Strong Interaction, since the range is known to be 10-15 m
c 197 MeV fm
mc  
 200 MeV
R
1 fm
2
…the expected «meson» mass
40
A few remarks:
 c  197
MeV  fm
The expected «meson» mass was of about 200 MeV
The «meson» was postulated to be the Strong Nuclear Force carrier
Today we know that the force is a residual force of the «true» Strong Interaction
(Van der Waals)
Electromagnetic residual force
Strong residual force
This is not the fundamental Strong Nuclear Interaction !
41
Cosmic Rays: the Universe as a Particle Accelerator
Hess (1912) first discovered that counting rates of radiation
detectors would increase when the detectors were above
the sea level (on the Tour Eiffel, on a balloon… )
Sources of Cosmic Rays
(in increasing order of energy):
• The Sun
• The Galaxy
• The Universe
Increase of ionization with altitude as measured by
Hess in 1912 (left) and by Kolhörster (right)
42
The Cosmic Ray all-energies spectrum
~102/m2/sec
Direct
measurements
~1/m2/year
Knee
Indirect
measurements
~1/km2/yr
Ankle
~1/km2/century
43
High-Energy Cosmic Rays and their interactions in the Atmosphere: an unvaluable
source of particles at High Energy
The first particle accelerator available
At high energy, particles
interact with other particles by
means of the fundamental
interactions
In Cosmic Rays, the Primary
Particle (e.g. a proton coming
from outside our Galaxy) hits a
nucleus of an air molecule (e.g.
by means of the Strong
Interaction) and produces
particles that in turn experience
Strong, Weak and
Electromagnetic Interaction.
An high-energy cosmic ray
shower can generate perhaps
as many as 108 particles at sea
level !
44
The discovery of Pions and Muons
Study of particles in cosmic radiation
Penetrating
particle
Absorbed particle
A first phenomenological
distinction was made between
penetrating and not penetrating
radiation
Muon
Pion
Say, 1 meter of lead !
Secondaries
Absorber
In 1934 Bethe and Heitler published their theory of radiation
losses by electrons and positrons. Based on this theory,
it was possible to exclude that both new particles seen
(Pion and Muon) were electrons or positrons
WOW!
Key concept: energy loss by radiation inversely proportial to
mass squared ~ 1/m2
45
The protonic hypotesis : why the
two particles (Pion and Muon)
cannot be just protons ?
Penetrating
particle
Absorbed particle
Among other things : they come in
Two charge states, while we do not
know (1935) of a negative proton
Muon
Pion
Say, 1 meter of lead !
Secondaries
Anderson and Neddermeyer expt.
Street and Stevenson expt.
Analysis of momentum and of
momentum loss. Cloud chambers.
Magnetic fields. Heavy plates.
Geiger-Muller counters. Photos.
Absorber
Conclusively showed that there were
new particles of intermediate mass
between those of the proton and the
electron (“mesons”)
46
The instability of subnuclear particles (in particular Pion and Muon)
Experiments on
• the variation of intensity of comic rays with altitude (balloons) and
• zenith angle (directionality)
showed some behaviour that was difficult to interpret in terms of energy loss in
the atmosphere. Hence, the “mesons” probably do decay in flight.
In 1938 Kulenkampff put forward the hypotesis that cosmic particles could decay
in other particles  instability of elementary particles.
B. Rossi and others confirmed the instability hypotesis and estimated the
«Meson» lifetimes to be in the microsecond range.
…and in 1940 the first cloud-chamber picture of «meson» decay was made by
Williams and Roberts. Electrons and positrons where found to be present quite
often among the decay products.
In 1941 Rasetti observed the delayed emission of charged particles from
absorbers where «mesons» have come to rest. A lifetime effect.
In 1943 Rossi and Nereson measured the lifetime to be ~ 2 μs.
47
Lifetime of a particle
Decay of an unstable particle: a quantum mechanical process, analog to
radioactive decay. For many particles, the number will change as :
dN
1
 N
dt

Lifetime in the rest frame

Lifetime in the laboratory frame
Path travelled by the particle in
the laboratory frame


dN
x 

 exp 



dx

c



48
Conversi, Pancini, Piccioni (1947) experiment: the general idea
The carrier of the Strong Interaction would be absorbed quickly in matter
In fact, let us suppose that the “meson” is the “Yukawa” carrier
What does it mean that a negative
“meson” gets absorbed ?
Absorbed particle
Muon
It means that it will fall in the Bohr
first orbit (that has a significant
overlap with the nucleus)
r0 
137 
Z mc
Penetrating
particle
Pion
Bohr 1S orbit for the
meson with mass m
Secondaries
Absorber
Decay
And will have a radial part of the
wave function of the type
 (r )  (0)exp( r / r0 )
On the other hand, if the “meson” is the carrier of the
nuclear force, the nuclear radius will be (mass m):
The fraction of time the “meson” would spend
inside the nucleus would then be
R  R0 A1/ 3 
R3
AZ 3


3
3
r0 (137)
 1/ 3
A
mc
49
Now, our “meson” will have a linear velocity on the Bohr orbit :
Zc
vBohr 
137
Penetrating
particle
Absorbed particle
Muon
Pion
The length traversed in nuclear
matter will be given by the formula :
Zc
AZ 4
l  vBohr 
  c
137
137 4
As an example, for Carbon
(testing the muon hypotesis):
Secondaries
Absorber
Decay
cm
l  310
210 6 s 410 5  3 cm
s
10
Is it possible for the Yakawa “meson” to traverse 3 cm of nuclear matter without
being absorbed ? NO
The Yukawa “meson” (the Pion) will interact before decaying (most of the time).
The Muon will instead have enough time to decay !
50
The Pion was identified in 1947 by Lattes, Powell and Occhialini
Pion and Muon decay sequence: a cascade of decays
The charged pion actually lives 2.6 x 10-8 s. And this makes it easy to see the
decay in spite of its strong interaction with matter.
       (  2.6 108 s)
Muon decay
m (  )  139.6 MeV / c 2
Nuclear
Emulsion
   e  e (  2.2 106 s)
Muon decay scheme
m(  ) 105.7 MeV / c 2
In all these decays, neutrinos are emitted !
51
Pion – Muon – Electron sequences observed in emulsions
  e
Experimental strategy:
Exposure of Emulsions to Cosmic Rays
(Ballons, Mountains)
The pion in term of quarks
52
The early era of cosmic rays particle physics experiments :
1950 : AGS di Brookhaven
AGS Synchrotron at Brookhaven
The first big particle accelerator
33 GeV reached in 1960
53
Particle Physics Laboratories in the World
Hadron (proton) accelerators
CERN (LHC. Large Hadron Collider)
Electron-Positron machines
Electron-proton accelerators
Secondary Beams
Small scale synchrotron (Orsay)
54
Cosmic Rays Laboratories in the World (yes, today!)
The Pierre Auger Observatory
The HESS array (Namibia)
55
The Neutrino case: a particle first hypotesized and then discovered
Understanding beta decays (energy, angular momentum)
A( Z , N )  A( Z  1, N  1)  e   e
Example:
14
6
C  147 N     v
The spectrum of the recoiling
electron (non monoenergetic)
was indicating the presence of
invisible energy
Neutrino mass effects on the
spectrum endpoint
Pauli hypotesis (1932): the presence of a new particle could save the
energy conservation of:
• Energy
Neutrino hypotesis!
• Momentum
• Angular momentum
Experimental confirmation in 1956 (Reines & Cowan experiment)
56
Why is the Neutrino a typical case ?
• Beta particles (electrons)
Experimental problems
Two possibilities:
Abbandonment of well
consolidated physical laws
Introduction of new particles/fields
are emitted with a continumm
spectrum in beta decay. This is incompatibie with a twobody decay (since energy levels in the nucleus are known
to be discrete).
• The electron trajectory is not collinear with the
trajectory of the recoiling nucleus.
• Nuclear spin variation is not compatible with the
emission of a single electron (∆=0,±1).
• E conservation
• P conservation
• M conservation
Neutrino hypotesis!
57
Why is the Neutrino a typical case ?
Experimental problems
Two possibilities:
Abandonment of well
consolidated physical laws
Introduction of new particles/fields
• Star rotation curves in Galaxies show excessive
peripheral velocities
• The motion of Galaxies Galaxy Clusters features
excessive velocities
• Theory of Gravitation
Dark Matter hypotesis !
58
Why is the Neutrino a typical case ?
Experimental problems
Two possibilities:
Abandonment of well
consolidated physical laws
Introduction of new particles/fields
• The Universe has identical properties in causally
disconnected domains (Horizon problem)
• The Universe at large scale is flat to an extremely good
accuracy (Flatness problem)
• CMB perturbations (structure formation)
• Incompleteness of the
Big Bang Model ?
Inflation hypotesis !
59
Neutrino discovery:
Principle of the experiment
In a nuclear power reactor, antineutrinos come from  decay of radioactive nuclei
produced by 235U and 238U fission. And their flux is very high.
1. The antineutrino reacts
with a proton and forms n
and e+
 e p  n e
Inverse Beta Decay
2. The e+ annihilates
immediately in gammas
3. The n gets slowed down
and captured by a Cd
nucleus with the emission of
gammas (after several
microseconds delay)
Water and
cadmium
Liquid
scintillator
4. Gammas are detected by the scintillator:
the signature of the event is the delayed
gamma signal
 ( e p  ne  ) 10 43 cm 2
1956: Reines and Cowan at the Savannah nuclear power reactor
60
A first classification of known Elementary Particles (the «low energy world»)

e ,e

, 

p  (u, u, d )
n  (u , d , d )
The photon
   (u, d ) ,    (d , u )
Leptons (heavier copies of the electron)
The neutrino, postulated to explain beta decay
and observed in inverse beta decay, is always
associated to a charged lepton.

The hadrons, particles made up of quarks and obeying mainly
to strong nuclear interaction
61
Particles with Strangeness
Presence of unknown particles in experiments with cloud chambers or emulsions
on atmospheric balloons (1947, Rochester and Butler).
They turned out to be secondary particles with a characteristic “V” shape decay
 p  K  p   




These particles were produced and were
decaying in two different modes:
• Strong Interaction production (cross section)
• Weak Interaction decays (lifetimes)


S=0
Strong Interaction
p
S = +1

Weak Interacrtion


p
S = -1
  mb   1010 s

Associated production
of particles with a new
(strange) property:
Strangeness s
(A. Pais, 1952)
Strong Interaction conserves s
Weak Interaction violates s
62
Particles with Strangeness s : a new quark (different from u,d) !
  p   , n 0 (  2.6 1010 s)
K s0     ,  0 0 (  0.895 10 10 s)
Branching Ratios
BR = 64.1%
BR = 35.7%
BR = 69.2%
BR = 30.7%
m ()  1115.6 MeV / c 2
m (  )  139.6 MeV / c 2
  (u, d , s)
m ( 0 )  135.0 MeV / c 2
m( K )  495 MeV / c 2
Particle similar to a proton.
The long lifetime was explained by the
disappearance of a «strange» quark
K particles (Kaons), similar to
Pions but with the s Quark
63
The Neutral Pion
Kemmer and Bahba proposed that the Yukawa’s
“meson” had both charged and neutral states, to
account for p-p forces (p-p, n-p, n-n)
m ( 0 )  135.0 MeV / c 2
Decay mode
 0   ,  e e
Electromagnetic decay !
BR =98.8%
BR =1.2%
  0.84 1016 s
Decay lifetime
Experimental evidence. Cosmic ray studies of «star-like» events in high
resolution nuclear emulsions (1950, Carlson, Hooper, King).
High energy experiments at the Berkeley Cyclotron (Bjorklund et al.)
e
e

p
e
“Primary” vertex


e
The detachment from the
“primary vertex” of the
interaction is caused by the
tiny neutral pion lifetime
64
The Pi-zero was found to be produced with a cross section similar to the one of
charged pions, in reactions like :
 p n0
The two gammas in the final state can be studied exploiting
the pair production mechanism
0 
  e e
The first mass determination of the pi-zero was made by Panofsky et al. (1951)
 0  
  ee
Nowadays the same reconstruction
technique is used to produce
“invariant mass plots” for pi-zero.
• Electron reconstruction
• Positron reconstruction
• Two gammas reconstruction
• Pi-zero distribution
65
The discovery of the Antiproton
The existence of the positron (as antimatter counterpart
of the electron) begged the question of an antimatter
counterpart of the proton. The antiproton – however –
was not seen in Cosmic Rays (because of its paucity in
the Universe with respect to protons : ≈ 10-5 )
One of the very first particle accelerator – the Berkeley Bevatron – was built in
1954 that featured 6.5 GeV proton energy. That was sufficient energy to produce
antiprotons through nuclear reactions on free protons
p p p pp p
Problem : many competing reactions producing Pions (the most commonplace
particle in nuclear reactions)
66
Pions / AntiProtons Ratio ≈ 10-4
A severe background problem !
The “Berkeley” spectrometer
1. Momentum selection by the magnetic
spectrometer arrangement M1,Q1 and
M2,Q2
2. C1 and C2 Cerenkov counters (measure v)
3. S1 and S2 measure Time of Flight
A paper titled "Observation of antiprotons," by Owen
Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas
Ypsilantis, members of what was then known as the
Radiation Laboratory of the University of California at
Berkeley, appeared in the November 1, 1955 issue of
Physical Review Letters. It announced the discovery of a new
subatomic particle, identical in every way to the proton —
except that its electrical charge was negative instead of
positive.
S1 - Scintillator 1
S2 - Scintillator 2
C1, C2 Cerenkov Counters
S3 - Scintillator 3
Existence of antileptons and antiquarks
67
A first classification (updated)

e ,e

, 
  (u, d , s)

Photon
p  (u, u, d ) p(u ,u ,d )
n  (u , d , d )
Proton-like particles (baryons)
   (u, d ) ,    (d , u )
0 ,K
Mesons
Leptons (heavier copies of the electron)
The neutrino, postulated to explain beta decay
and observed in inverse beta decay, is always
associated to a charged lepton.

The hadrons, particles made up of quarks and obeying mainly
to strong nuclear interaction
68
A surprise test !
Particle
ee+
K
Mass
Lifetime
0.511 MeV

0.511 MeV

 495 MeV
108 / 1010 s
A decay mode?
   , 
μ-
106 MeV
γ
0

p
938 MeV

π+ πo
140 / 135 MeV
108 / 1016 s
 , 
Λ
1116 MeV
10 10 s
p  , n
n
940 MeV
980 s
p e
106 s
e 
69