Option 212: UNIT 2Elementary Particles

Department of Physics and Astronomy

SCHEDULE

26-Jan-15 13.00pm LRB Intro lecture 28-Jan-15 12.00pm LRB Problem solving

(2-Feb-15 10.00am E Problem Workshop) 4-Feb-15 12.00pm LRB Follow-up

UNIT 2: OUTLINE SYLLABUS:

1st Lecture IntroductionHadrons and LeptonsSpin & Anti-ParticlesThe conservation laws: Lepton Number

Baryon number Strangeness

2nd Lecture

3rd Lecture Follow-upFundamental forces and field particlesThe standard model

Problem solving Check a decay for violation of conservation laws Quarks Properties of a particle given quark combination

Recommended Books Chapter 41, PA Tipler

Quarks Leptons and The Big Bang, J Allday

The Cosmic Onion, F Close

Web Sites

Brief introduction to Particle Physicshttp://superstringtheory.com/experm/index.html CERN web sitehttp://home.web.cern.ch/

212 Option - Lecture notes in MS Powerpoint & PDFhttp://www.star.le.ac.uk/mrb1/lectures.html

INTRODUCTION toElementary Particle Physics

* Fundamental building blocks of which all matter is composed: Elementary Particles

electronproton

neutronphoton

Cosmic Rays

* Pre-1930s it was thought there were just four elementary particles

We will discover that the electron and photon are indeed fundamental, elementary particles, but protons and neutrons are made of even smaller elementary particles called quarks

1932 positron or anti-electron discovered, followed by many other particles (muon, pion etc)

CLASSIFICATON OF PARTICLESCLASSIFICATON OF PARTICLES

With the advent of particle acceleratorin the 1950’s many new elementary particles were discovered.

The question arose whether perhaps there were too many to all be elementary.

This has led to the need for classification ofparticles.

An elementary particle is a point particle without structure that is not constructed from more elementary entities

FUNDAMENTAL INTERACTIONS AND THE CLASSIFICATION OF PARTICLES

Fundamental interactions Participating particleso gravitationo electromagnetico strong nuclear forceo weak nuclear force

• all particles with mass• those carrying charge• Hadrons (and quarks)• Leptons (and quarks)

HADRONSHADRONS

Hadrons interact through strong forces.There are two classes, mesons and baryons.

Mesons have zero or integral spin (0 or 1) with masses that lie between the electron and the proton.

Baryons have half integral spin (1/2 or 3/2) and have masses that are always greater than or equal to that of theproton.

Hadrons are not elementary particles. As we will see later, they are made of quarks

LEPTONSLEPTONS

Leptons interact through weak inter-actions, but not via the strong force.

All leptons have spin of 1/2. There are six kinds of lepton: electron e-, muon and tau t -, and 3 neutrinos e

Leptons were originally named because they were “Light-particles”, but we now know the Tau is twice as heavy as a proton

Neutrinos were originally thought to be massless, but they probably have a small mass

Read more in Tipler p. 1336Note that each distinct neutrino is associated with one of the other leptons

Beta Decay and the discovery of the neutrino (Tipler p.1314)

31H

32He + e- ×

• In Beta decay a neutron decays into a proton plus an electron • If decay energy shared by proton and emitted electron, energy of electron would be unique• But observed electrons have a range of energies – must be a third particle involved: the neutrino• Third particle must have no charge or mass, as they are accounted for

by the He nucleus and electron.

31H

32He + e- + e

-

√

SpinSpin

A particle has an intrinsic spin angular momentum

Spin ½ particles:

Electrons, protons, neutrons and neutrinos all have an intrinsic spin characterised by the quantum number s = 1/2

Particles with half-integer spin (1/2, 3/2, 5/2, …) are called FermionsFermions

They obey the Pauli exclusion principle

Particles with integer spin (s = 0, 1, 2, …. ), e.g. mesons, are called BosonsBosons

They do not need to obey the Pauli exclusion principle, and any number can occupy the same quantum state

Matter & Antimatter

Every particle has an antiparticle partner

e- - electron e+ - positronp - proton p - antiproton

Here are some examples

n - neutron

- neutrino

n - antineutron

- antineutrino

Read Tipler P.1339 to find out how Dirac predicted the existence of anti-particles in 1927

Antimatter

For each particle there is an associated antiparticle

Anti-particles always created in particle-anti particle pairs

s

s -> e- + e+

e-

e+

E2 x 511 keV

Electron Pair Production

Antimatter

* Antiparticle has the same mass and magnitude of spin as the particle

* Antiparticle has the opposite charge to the particle

* The positron is stable but has a short-term existence because our Universe has a large supply of electrons

* The fate of a positron is annihilation

Electron Pair Annihilation

s

e- + e+ ->2

s s

moc2

s = 1/2

e- e+Each photon getsEach photon getse = mec2

p= mecmoc2

s = 1/2

Some Fundamental Particles

Particle Symbol Rest energy MeV Charge Spin Antiparticle

Mass less boson 0 0 1

photon

LeptonsNeutrinoElectronMuon

MesonPion

BaryonsProtonneutron

e

00.511105.7

0-1-1

1/21/21/2

o

140135

+10

00

o

p+

no938.3939.6

+10

1/21/2

p-

e

n

-

The Conservation LawsCan a conceivable reaction or decay occur?

• Conservation of energy

The total rest mass of the decay products must be less than the initial rest mass of the particle before decay

• Conservation of linear momentum

When an electron and a positron at rest annihilate, two photons must be emitted

• Angular momentum must be conserved in a decay or reaction

• Net electric charge before must equal net charge after a decay or reaction

The Conservation LawsCan a conceivable reaction or decay occur?

• Conservation of Baryon number

We assign Baryon Number B=+1 to all Baryons, B=-1 to all anti-Baryons, and B=0 to all other particles

Baryon number must be conserved in a reaction

• Conservation of Lepton number

Lepton number must be conserved in a reaction

BUT…..

The Conservation LawsCan a conceivable reaction or decay occur?

• Conservation of Strangeness

There are other conservation laws which are not universal, e.g. strange particles have a property called strangeness which must be conserved in a decay or reaction

• Conservation of Lepton number contd:

…..because the neutrino associated with an electron is different to a neutrino associated with a muon, we assign separate Lepton numbers Le, L and Lto the particles

e.g. for e and e, Le=+1, for their anti-particles Le=-1, and for all other leptons and other particles Le=0

Some Fundamental Particles

Particle Symbol Rest energy MeV B Le Antiparticle

NeutrinoElectronMuonTau

Pion

Kaon

ProtonNeutronLambdaSigma

e

00.511105.71784

o

140135

o

p+

no

938.3 939.61115.61189.41192.51197.3

e

L L S

0 0 photon 0 0 0 0

Leptons

Photon

Hadrons Mesons

K+

Ko

493.7497.7

K-

Ko

Baryons p-

n

Category

See also Tipler Table 41-1 Page 1337For strangeness, examine Figure 41-3 Page 1344

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