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Elementary particles and
High Energy Physics
History
Standard Model
 Accelerators
 High energy physics experiments
Introduction
Elementary particle physics studies the fundamental building
blocks of nature. But what fundamental does mean?
By fundamental we mean objects that are simple and
structureless, not made of anything smaller
Particle and high energy physics try to answer to these questions:
What is fundamental?
What is the world made of?
What holds it together?
What is fundamental ?
• During the past century the word “fundamental” was addressed firstly
to the atom. The word “atom” was introduced by Democritus (400 BC)
who described the matter as composed by small and indivisible particles
(“atom” comes from greek a-temno, which can not be divided).
•The internal structure of the atom was discovered and protons, neutrons
and electrons became the building blocks of matter:
• 1896 J.J Thompson, 1910 Millikan: the electron
• 1919 Rutherford scattering experiments: the nucleus
• scattering of alpha particles on atoms
• 1932 Chadwick experiment: the neutron (and the proton)
• scattering of alpha particles on nuclei
• After 1960, scattering experiments of high energy particles on nucleons
lead to the discovery of the quarks, which are thought now as the
fundamental consituents of matter.
Length scales
To look at smaller
things we need
to use instruments
that can “extend”
our vision
Scattering experiments:
the resolution increases
with the energy of the
probe
p = h/l
That’s why high
energy physics!
Energy units (i)
Joules are too big for particle energies, more practical units are
used:
1 eV = 1.6x10-19 J
One eV is the energy acquired by an electron
passing through a voltage drop of 1 Volt.
1 keV = 103 eV
1 MeV = 106 eV
LEP 200 GeV
1 GeV = 109 eV
LHC 14 TeV
1 TeV = 1012 eV
Energy units (ii)
Quantum Mechanics
Quantum Mechanics (QM) is the base theory explaining atomic and sub-atomic
processes. The foundations of QM were established during the first half of
the 20th century by Werner Heisenberg, Max Planck, Louis de Broglie, Niels
Bohr, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang
Pauli and others.
Heisenberg
The word “quantum” (Latin, “how much”) refers to a
discrete unit that quantum theory assigns to certain
physical quantities, such as the energy of an atom at
rest, or such as the electric charge, angular momenta
etc..The discrete values of these physical quantities
are identified by quantic numbers.
The relativistic formulation of Quantum Mechanics
was done by P.A.M. Dirac in 1928, who also predicted
the existence of the positron and antimatter.
Schrödinger
Dirac
The Standard Model (SM)
Since the sixties physicists have been looking for new particles. Up
to now about 200 particles (most of which are not fundamental) have
been discovered and categorized.
The Standard Model is a theory that explains all the
hundreds of particles and their interaction on the basis of
fundamental particles:
- 6 quarks
- 6 leptons (the best-known lepton is the electron)
- Force carrier particles (like the photon)
Experiments have verified the SM predictions with high precision
and the particles predicted by SM have been experimentally found.
BUT…gravity is not included in SM..
The Antimatter
The existence of antimatter was predicted by the theory
from P.A.M. Dirac and experimentally confirmed in 1932 by
the Anderson’s experiment, who discovered the positron (the
antiparticle of the electron).
Track of
the positron
For every type of particle, there exists a
corresponding antiparticle, which has opposite
properties (quantic numbers) w.r.t its
corresponding particle (i.e. opposite charge).
Evidence of antimatter
This photograph from a bubble
chamber shows clearly electronpositron pairs. The magnetic field in
the
chamber
makes
negative
particles curl left and positive
particles curl right.
The electron-positron pairs
appears from nowhere, but
come in fact from photons,
which don’t leave a trail
Quantic numbers
Spin:
in quantum mechanics the spin of a particle is related to an angular
momentum which has non-classical features. It can not be associated to a
rotation, but only refers to the presence of angular momentum.
Isospin:
is a quantum number related to the strong interaction, it was
introduced to explain the symmetry in particles strongly interacting and led to
the discovery and understanding of quarks (Yang-Mills theory).
Flavour quantic numbers:
specific numbers for different particles
species, as the leptonic and barionic number, or charm, strangeness,
bottomness, topness.
Electric charge
Conservation laws:
the occurrence or not of the different decays and
interactions is governed by conservation laws of the quantic numbers.
Quarks (i)
The quarks are the fundamental constituents of strongly interacting particles,
called HADRONS (from Greek adros, strong) (as p,n).
There are 6 “flavours” of quarks and for each of them the
corresponding antiquark
Quark
Symbol Spin
Charge
Baryon number
S
C B
T
Mass
up
u
1/2
+2/3
1/3
0
0 0
0
360 MeV
down
d
1/2
-1/3
1/3
0
0 0
0
360 MeV
charm
c
1/2
+2/3
1/3
0
1 0
0
1500MeV
strange
s
1/2
-1/3
1/3
-1 0 0
0
540 MeV
top
t
1/2
+2/3
1/3
0
0 0
1
174 GeV
bottom
b
1/2
-1/2
1/3
0
0 1
0
5 GeV
Quarks have the unusual characteristic of having a fractional electric charge.
They have also a “color charge”, typical of strong interaction (QCD).
The top quark was found in 1995.
Quarks (ii)
The name “quark” was given by M. Gell-Mann, who suggested their
existence together with G. Zweig, inspired by the nonsense word
used by J. Joyce in the novel Finnegan’s Wake:
Three quarks for Muster Mark!
HADRONS
Baryons (qqq) composed by 3 quarks
qq
q
Mesons (qq) composed by 1 quark and 1 q q
antiquark
Baryons: p (uud), n (udd), ++ (uuu), +(uud), 0(udd), -(ddd),
S+(uus), S0(uds), S-(dds) etc..
Mesons: p+ (ud), K+ (us) etc..
Leptons (i)
The other type of matter particles are leptons (from Greek leptos,
light, small). There are 6 leptons, three of which have electric charge
and three of which do not. They appear to be point-like particles
without an internal structure.
Lepton (antilepton) Symbol
Spin
Charge
L
Le
Lm
Lt
Mass
(MeV)
Electron (positron)
e- (e+)
1/2
-1 (+1)
1(-1) 1(-1)
0
0
0.511
Muon
m- (m+)
1/2
-1 (+1)
1(-1)
0
1(-1)
0
105.7
tau
t- (t+)
1/2
-1 (+1)
1(-1)
0
0
1(-1)
1777
Electron neutrino
(antineutrino)
ne (ne)
1/2
0
1(-1) 1(-1)
0
0
<
0.0000022
Muon neutrino
(antineutrino)
nm (nm)
1/2
0
1(-1)
0
1(-1)
0
< 0.17
Tau neutrino
nt (nt)
1/2
0
1(-1)
0
0
1(-1)
< 15.5
Leptons (ii)
Neutrino masses are known to be non-zero because of neutrino
oscillations (neutrinos can change their flavour) but their masses
are sufficiently light that they have not been measured directly.
The havier leptons (m,t) are not found in ordinary matter. This is
because when they are produced they decay into lighter leptons.
The decays are governed by the conservation of the lepton
family number:
m -> e- + ne + nm
L: 1 = 1 - 1 + 1
Le:0 = 1 - 1 + 0
Lm:1 = 0 + 0 + 1
The generations of matter
Both quarks and leptons exist in 3 distinct sets.
Each set of quark and lepton charge is called
“generation”.
All visible matter is made from the first
generation of matter particles. All second and
third generation particles are unstable and
decay into stable first generation particles.
Why do the 2nd and 3rd generation exist?
We don’t know..Perhaps the answer is that
quarks and leptons are not fundamental, but
are made up of even more elementary
particles..
Interactions
The Universe exists because of interactions of the
fundamental particles. These interactions include attractive
and repulsive forces, decays and annihilation. All the forces
can be attributed to 4 interactions:
gravity, electromagnetic, strong, weak
Force: effect on a particle due to the presence of other particles
Interactions: include all the forces and also decays, annihilations
There exist particles which carry the interactions, called force
carrier particles (as the photon for the electromagnetic
interaction).
Force carrier particles
Particles interact without touching: how it is possible?
At a fundamental level a force in not something which happens
to the particles. It is a think which is passed between two
particles.
All the interactions which affect matter particles are due to an
exchange of force carrier particles.
A particular force carrier particle can only be produced or
absorbed by a matter particle which is affected by that
particular force.
Electromagnetic interaction: photon
Strong interaction: gluon
Weak interaction: bosons Z, W
Gravity: graviton
(no experimental evidence)
Electromagnetism
The e.m. force causes like-charged particles to repel
and oppositely-charged particles to attract.
It holds atoms together and it is much stronger than gravity and than
the weak force. The theory of e.m. interaction is the Quantum
Electrodynamics (QED).
The carrier particle of the e.m. force is the photon (g).
Photons have zero mass, zero electric charge
and travel at “the speed of light” c in a
vacuum.
Example of electronpositron annihilation
depicted
by
a
“Feynman diagram”
Residual e.m. force
Atoms usually have the same number of electrons and protons,
i.e. they are neutral. What causes them to stick together to
form stable molecules?
The responsible is a residual e.m.
force: the charged parts of one atom
can interact with charged parts of
another atom.
Strong interactions
The strong force holds the nuclei together to form hadrons.
The theory of strong interactions is called Quantum
Chromodynamics (QCD). This name is due to the fact that
quarks, besides the electric charge, have a different kind of
charge called “color charge”, which is responsible of the strong
force.
The force carrier particles are
called “gluons”, since they so
tightly “glue” quarks together
Gluons have color charge, quarks have color charge but hadrons
have no net color charge (“color neutral”). For this reason, the
strong force only takes place on the small level of quark
interactions.
Color charge
Color charged particle interact by exchanging gluons. Quarks
constantly change their color charges as they exchange gluons
with other quarks.
There are 3 color charges and 3
corresponding anti-color charges.
Each quark has one of the color
charges and each antiquark has one
of the anticolor charges.
In a baryon a combination of red, green and blu is color neutral. Mesons are
color neutral because they carry combinations as red and antired.
Because gluon emission and absorption always changes color, gluons can be
thought of as carrying a color and an anticolor charge. QCD calculations
predict 8 different kinds of gluons.
Quark confinement
Color-charged particles cannot be found individually.
They are confined in hadrons.
Quarks can combine only in 3-quarks objects
(baryons) and quark-antiquark objects (mesons) which
are color-neutral, particle as ud or uddd cannot exist.
If one of the quarks in a given hadron is pulled
away from its neighbours, the color force field
stretches between that quark and its
neighbours. More and more energy is added to
the color-force field as the quark are pulled
apart.
At some point it’s energetically cheaper to snap into a new quark-antiquark
pair. In so doing energy is conserved because the energy of the color-force
field is converted in the mass of the new quarks.
Residual strong force
Strong force binds quark together, but what holds the nucleus
together?
The strong force between the quarks in
one proton and the quarks in another
proton is strong enough to win the
repulsive electromagnetic force.
Weak interactions
There are 6 kinds of leptons, but all the stable matter appears to be made
of just the 2 least-massive quarks, the least-massive charged lepton and
the neutrinos.
Weak interactions are responsible for the decay of massive quarks and
leptons into lighter quarks and leptons.
The carrier particles of weak interactions are the W and Z particles.
Feynman diagram of the
muon decay
ElectroWeak interactions
In the Standard Model the weak and the electromagnetic
interactions have been combined into a unified electroweak theory.
At very short distances (10-18 m) the strength of the weak
interaction is comparable to that of the electromagnetic. On
the other hand, at thirty times that distance (3x10-17 m) the
strength of the weak interaction is 1/10000th than that of
the electromagnetic interaction. At distances typical for
quarks in a proton (10-15 m) the force is even tinier.
This is because the strength of the interaction depend
strongly on both the mass of the force carrier and and the
distance of the interaction. The difference observed between
the strength of the two interactions is due to the huge
difference in mass between the W and Z particles (very
massive) and the photon (zero mass).
I. Newton
Gravity
Gravity is one of the fundamental interactions, but the
Standard Model cannot satisfactorily explain it.
This is one of the major unanswered problems in physics today
The particle force carrier for gravity, the graviton, has not
been found
Fortunately, the effects of gravity are extremely tiny in most
particle physics situations compared to the other three
interactions, so theory and experiment can be compared without
including gravity in the calculations. Thus, the Standard Model
works without explaining gravity.
Fermions and bosons
Fermions
Bosons
A fermion is any particle that has an
odd half-integer (1/2,3/2..) spin.
Quarks and leptons, as many
composite particles, are fermions.
Fermions cannot co-exist in the
same state at same location at the
same time.
*
Bosons are particles which have
integer spin (0,1,2..).
All the force carrier particles are
bosons.
The predicted graviton has a spin of 2.
Beyond the Standard Model
The SM explains the structure and stability of matter, but there
are many unanswered questions:
Why do we observe matter and almost no antimatter?
Why can’t the SM predict a particle’s mass?
Are quarks and leptons actually fundamental?
Why are there 3 generations of quqrks and leptons?
How does gravity fit into all of this?
Is the SM wrong?
No, we need to extend the SM with something totally new in
order to explain mass, gravity and other phenomena.
Higgs boson
The Standard Model cannot explain why a particle has
a certain mass.
Physicists have theorized the existence of the socalled Higgs field, which in theory interacts with
other particles to give them mass. The Higgs field
requires a particle, the Higgs boson. The Higgs boson
has not been observed, but we are looking for it!
Higgs mechanism (i)
The Higgs mechanism was postulated by British physicist
Peter Higgs in the 1960s. The theory hypothesizes that a sort
of lattice, referred to as the Higgs field, fills the universe.
This is something like an e.m. field, which affects particles
moving in it. It is known that when an electron passes through
a positively charged crystal lattice, its mass can increase as
much as 40 times. The same may be true in the Higgs field: a
particle moving through it creates a little bit of distortion and
lends mass to the particle.
Higgs mechanism (ii)
To understand the
Higgs
mechanism,
imagine that a room
full
of
physicists
chattering quietly is
like space filled with
the Higgs field ...
... a well-known
scientist walks in,
creating a disturbance
as he moves across the
room and attracting a
cluster of admirers
with each step ...
... this increases his
resistance to
movement, in other
words, he acquires
mass, just like a
particle moving
through the Higgs
field...
Higgs mechanism (iii)
... if a rumor crosses
the room, ...
... it creates the same
kind of clustering, but
this time among the
scientists themselves.
In this analogy, these
clusters are the Higgs
particles.
Grand Unified Theory (ii)
Physicists hope that a Grand Unified Theory will unify the strong, weak,
and electromagnetic interactions. If a Grand Unification of all the
interactions is possible, then all the interactions we observe are all
different aspects of the same, unified interaction.
However, how can this be the case if strong and weak and
electromagnetic interactions are so different in strength and effect?
Current data and theory suggests that these varied forces merge into
one force when the particles being affected are at a high enough energy.
Grand Unified Theory (ii)
Supersymmetry
Some physicists attempting to unify gravity with the
other fundamental forces have come to a startling
prediction: every fundamental matter particle should
have a massive "shadow" force carrier particle, and
every force carrier should have a massive "shadow"
matter particle.
This relationship between matter particles and force
carriers is called supersymmetry. For example, for
every type of quark there may be a type of particle
called a "squark."
No supersymmetric particle has yet been found, but
experiments are underway at CERN and Fermilab to
detect supersymmetric partner particles.
Cosmic Rays had revealed STRANGE particles
1955 CERN accelerators replicate cosmic rays on Earth…
..record the images and reveal the real heart of matter….
…..the beginnings of modern high energy particle physics