Revision Notes - Unit 1

Revision Notes - Unit 1Particles

INTRODUCTION toElementary Particle PhysicsFundamental building blocks of which all matter is composed: Elementary Particleselectronprotonneutronphoton

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 quarks1932 positron or anti-electron discovered, followed by many other particles (muon, pion etc)CLASSIFICATON OF PARTICLESWith the advent of particle acceleratorin the 1950s 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

Year of discovery of the particlesFUNDAMENTAL INTERACTIONS AND THE CLASSIFICATION OF PARTICLES Fundamental interactionsParticipating particles gravitation electromagnetic strong nuclear force weak nuclear force

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

HADRONSHadrons 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 quarksLEPTONSLeptons 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 m-, and tau t -, and 3 neutrinos ne, nm, nt

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 massRead more in Tipler p. 1336Note that each distinct neutrino is associated with one of the other leptons Matter & AntimatterEvery particle has an antiparticle partnere- - electron e+ - positronp - proton p - antiprotonHere are some examplesn - neutronn - neutrinon- antineutronn- antineutrino

AntimatterFor each particle there is an associated antiparticle

Anti-particles always created in particle-anti particle pairsssg -> e- + e+e-e+E2 x 511 keVElectron Pair Production* 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 annihilationSome Fundamental ParticlesParticleSymbolRest energy MeVChargeSpinAntiparticleMass less boson001photonLeptonsNeutrinoElectronMuonMesonPionBaryonsProtonneutrone00.511105.70-1-11/21/21/2o140135+1000o

p+no938.3939.6+101/21/2p-en-The Conservation LawsCan a conceivable reaction or decay occur? Conservation of StrangenessThere 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, Lm and Lt to the particlese.g. for e and ne, Le=+1, for their anti-particles Le=-1, and for all other leptons and other particles Le=0Answer: a) Baryons = 1+1 on left hand side Baryons = 2 on right hand side too! Allowed reaction! b) Baryons = 2 on left hand side Baryons = -1 on right hand side Forbidden reactionChecking Baryon Numbers a) p+ + np+ + n

2p+ + p + n p+ + p + p ___

Answer: a) Before decay Le = 0 and Lm = +1 After decay Le = 0 and Lm = +1 Allowed reaction! b) Before decay Lm = 0 and Le = 0 After decay Lm = 0 and Le = 1 Forbidden reaction!Checking Lepton Numbers a) -b) +

e- + ne + n+ + n + ne_Is Strangeness Conserved?a) + + nb) - + p

K+ + -+ Answer: a) Initial state has S = 0 Final state has S = +1 - 1 = 0 Allowed reaction! b) Initial state has S = 0 Final state has S = -1 Forbidden reaction!Conservation Laws

Test the following decays for violation of the conservation of electric charge, baryon number and lepton number. (a) n -> p+ + p- + m+ + m-(b) p0 -> e+ + e- + g Conservation LawsSolutionMethod: Use the table from the formula sheet and the conservation laws for Baryon number and Lepton number(a) n -> p+ + p- + m+ + m-Total charge on both sides = 0 : conservedBaryon number changes from +1 to 0: violatedLm = 0 on both sides : conservedProcess not allowed(b) p0 -> e+ + e- + g Total charge on both sides = 0 : conservedBaryon number on both sides = 0 : conservedLe = 0 on both sides: conservedProcess is allowed

Three Different Types of QUARKSThere are three elementary quarks (flavors) That make up the fundamental particles:

Up u Down d Strange sName Spin Charge Baryon StrangenessUp u 1/2 +2/3 1/3 0Down d 1/2 -1/3 1/3 0Strange s 1/2 -1/3 1/3 -1

Anti-quarks maintain spin, but change sign of S and B!ud+MesonuudpBaryonDifferent types of quarks contd.Mesons quark + anti-quark q q Baryons three quarks q q q Anti-baryons three anti-quarks

By 1967 it was realised that new kinds of quarks were required to explain discrepancies between the model and experiment

Charm (c)Bottom (b) discovered 1977Top (t) discovered 1995

Quark combinations

Find the baryon number, charge & strangeness of the following quark combinations and identify the hadron:(a) uud(b) udd(c) uus(d) ddsQuark combinations

SolutionMethod: for each quark combination determine the baryon number B, the charge q and the strangeness S; then use Table from formula sheet to find a match.(a) uudB = 1/3 + 1/3 + 1/3 = 1q = 2/3 + 2/3 1/3 = 1S = 0 It is a proton(b) uddB = 1/3 + 1/3 + 1/3 = 1q = 2/3 1 /3 1/ 3 = 0S = 0It is a neutron(c) uusDitto, B=1, q=1, S= -1 and it is a S+(d) ddsDitto, B=1, q=-1, S= -1 and it is a S-True or false?(a) Leptons consist of three quarks(b) Mesons consist of a quark and an anti-quark(c) The six flavors of quark are up, down, charmed, strange, left and right (d) Neutrons have no charm

(a) False: leptons are fundamental particles e.g e-(b) True(c) False: there is no left and right quark, but there are top and bottom quarks(d) True: neutrons are made of udd quarks Quark confinementNo isolated quark has ever been observedBelieved impossible to obtain an isolated quarkIf the PE between quarks increases with separation distance, an infinite amount of energy may be required to separate themWhen a large amount of energy is added to a quark system, like a nucleon, a quark-antiquark pair is createdOriginal quarks remain confined in the original systemBecause quarks always confined, their mass cannot be accurately knownCrib sheet(or what you need to know to pass the exam)The zoo of particles and their propertiesLeptons (e-, m-, p-, ne , nm, np)Hadrons (baryons and mesons)Their anti-particlesThe conservation laws and how to apply them (energy, momentum, baryon number, lepton numbers, strangeness)Quarks and their propertiesFlavors: up, down, strange, charm, top ,bottomHow to combine quarks to form baryons and mesonsQuark spin and colorThe eight-fold way patterns Fundamental forces and field particlesThe standard model

The Photoelectric Effect

What you need to knowRelationship between the energy of a photon, and the frequency of the photonThe electronvoltThe work functionPhotoelectric equationWhat is the photoelectric effect?Provides evidence that electromagnetic waves (eg light) have particle like behaviour.When a metallic surface is exposed to electromagnetic radiation (light) above a certain frequency (called the threshold frequency) the photons from the light are absorbed and current is produced.An electron in the metal can absorb the energy from the photon, and if there is enough energy the electron can escape the metal!

From experimentationThere is no emission of electrons below the threshold frequency.This frequency is different for different metals.Above the threshold frequency, electrons are emitted.The kinetic energy of the elcectons can vary.Their kinetic energy is given as K.E. = 1/2 mv2

ContinuedIncreasing the frequency of the radiation, increases the maximum kinetic energy of the electrons.This however has no effect on the photoelectric current which is the rate of emission of electrons.If you increase the intensity of the radiation (for example by shining more light on the metal), will have no effect if the frequency is still below the threshold.If the intensity is increased, and the frequency is above the threshold, then you will increase the photo electric current. (more light in = more electrons out)

What this meansFrequency BELOW ThresholdFrequency ABOVE ThresholdIncrease frequencyIf frequency still below threshold then nothing. If frequency is above threshold then electrons are emittedIncreased kinetic energy of electronsIncrease intensityNOTHINGGreater photoelectric currentExplanation of Photoelectric EffectRelies on the idea of a photon being a quantum of enegy.What does this mean?Quantum is another term for packet.Therefore the photoelectric affect relies on the idea that light is not made up of waves, but that it is made up of particles called photons, that have packets of energy.The relationship between the energy (E) of a photon and its frequency (f) is:

E = hf

Where h is Planks constant which is equal to 6.63x10-34 JsThe ElectronvoltThis is something that always scares people when they first see it! DONT PANIC! It is simply a unit used to describe energy (like Joules).I electronvolt (eV) is the amount of energy needed to move 1 electron across a potential difference of 1 volt

1 eV = 1.60x10-19 JEinsteins Explanation of Photoelectric EmissionAn electron needs to absorb a minimum amount of energy to escape from a metal.This minimum amount is a property of a metal and is called the work function ()If the photons hitting the metal have energy (hf) which is less than then no electrons are emitted.Electrons can be emitted just when hf = .The Work Function cont.For photons with an energy larger than , the electrons emitted from the metal have a ranges of energies.The electrons with the largest (or maximum) energy needed the minimum energy to escape.Increasing the intensity of the radiation increases the number of photons emitted, but does NOT affect the electrons kinetic energy.Einsteins Photoelectric EquationThis relates the maximum kinetic energy of the emitted electrons to the work function and the energy of each photon:hf = + EkEk = (1/2 mv2) which is the maximum kinetic energy.At the threshold frequency, Ek equals zero so hf =