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 scalesTo look at smaller

things we needto use instrumentsthat can “extend”

our vision

Scattering experiments:

the resolution increases with the

energy of the probep = h/

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.

LEP 200 GeV

LHC 14 TeV

1 keV = 103 eV

1 MeV = 106 eV

1 GeV = 109 eV

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.

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.

Heisenberg

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 AntimatterThe 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 electron-positron 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 numbersSpin: 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 GeVQuarks 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!

HADRONSBaryons (qqq) composed by 3

quarks

Mesons (qq) composed by 1 quark

and 1 antiquark

q qq

q q

Baryons: p (uud), n (udd), ++ (uuu), +(uud), 0(udd), -

(ddd), +(uus), 0(uds), -(dds) etc..

Mesons: (ud), (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 L L Mass (MeV)

Electron (positron)

e- (e+) 1/2 -1 (+1) 1(-1)

1(-1)

0 0 0.511

Muon - (+) 1/2 -1 (+1) 1(-1)

0 1(-1)

0 105.7

tau - (+) 1/2 -1 (+1) 1(-1)

0 0 1(-1)

1777

Electron neutrino (antineutrino)

e (e) 1/2 0 1(-1)

1(-1)

0 0 < 0.000002

2

Muon neutrino (antineutrino)

() 1/2 0 1(-1)

0 1(-1)

0 < 0.17

Tau neutrino (antineutrino)

() 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 (,) 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:

ee

L: 1 = 1 - 1 + 1 Le:0 = 1 - 1 + 0 L:1 = 0 + 0 + 1

The generations of matterBoth 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..

InteractionsThe 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 particlesInteractions: 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: photonStrong interaction: gluonWeak interaction: bosons Z, W

Gravity: graviton

(no experimental evidence)

Electromagnetism

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 e.m. force causes like-charged particles to repel

and oppositely-charged particles to attract.

The carrier particle of the e.m. force is the photon ().

Photons have zero mass, zero electric charge and travel at “the speed of light” c in a vacuum.

Example of electron-positron annihilation

depicted by a “Feynman diagram”

Residual e.m. forceAtoms 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 interactionsThe 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 interactionsThere 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).

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.

I. Newton

Fermions and bosons

Fermions BosonsA fermion is any particle that has an odd half-integer (1/2,3/2..) spin. Quarks and leptons, as many composite particles, are fermions.

Bosons are particles which have integer spin (0,1,2..).

Fermions cannot co-exist in the same state at same location at the same time.

All the force carrier particles are bosons.

* The predicted graviton has a spin of 2.

Beyond the Standard ModelThe 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 so-called 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!

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 (i)

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