Particle Physics

Why do we build particle accelerators? The surface is flat Still flat Oh no its not Big balls cannot detect small bumps

Particle accelerators One reason why physicists build particle accelerators is to investigate matter on a small scale De Broglie wavelength λ = h/mv In order to give particles a very small wavelength they need to be given a lot of momentum The size of the nucleus is about meters which is about 100 million times smaller than the wavelength of light Big λ Small λ Object not detected Object detected

E=mc 2 Another reason why physicists always want bigger and bigger accelerators is so that they can create new particles It also impresses their friends (my accelerator is bigger than yours) ΔE = Δmc 2 So if you give a particle a lot of energy and smash it into another particle it will lose this energy and some mass will be produced The more energy you give the original particle the more massive the particles you can create

Bubble Chambers Physicists analyse collisions using bubble chambers or cloud chambers The particles leave a tiny trail (of bubbles or vapour) behind them This shows where the particles went By examining the angle of these trails and using the law of conservation of momentum it is possible to work out the mass of the particles What we see What happened

What charge If the bubble chamber is in a magnetic field then the charge on each particle can be worked out An electron-positron pair being produced would look like this No charge B into page Use left hand rule This is negative positive

Conservation of charge Thousands of collisions have been photographed and analysed in bubble chambers and in every case the charge before is the same as the charge afterwards e.g. beta decay 1 0 n  1 1 p e Charge before = 0, charge after = = 0 This equation is not quite correct as you will see in a few slides time

Order in nature As Physicists studied more and more collisions things got more and more complicated It used to be simple; just protons neutrons and electrons but now there were hundreds of different particles People believed that nature was simple and has a pattern. They did not think there would be hundreds of random particles so they set out to find the pattern This led to the discovery of several conservation laws

Leptons It was noted that the number of a certain group of particles was always the same before and after reactions. This group of particles are called leptons We say that lepton number is conserved There are 6 leptons Electron electron neutrino Muon muon neutrino Tau Tau neutrino Leptons cannot be broken down. They are fundamental particles

Conservation of lepton number 1 0 n  1 1 p e Does this equation obey the law of conservation of leptons? L= 0 L = 0 L= 1 We start with no leptons and finish with one It does not, we have to add an antineutrino which has L = n  1 1 p e + ν L= 0 L = 0 L= 1 L = -1 This line indicates an antiparticle

Antiparticles It has been discovered that every particle has an antiparticle with same mass, opposite charge, lepton number and baryon number Mass (MeV/c 2 )QLB electron positron Σ+Σ Σ-Σ sigma plus

Hadrons Another group of particles was identified as hadrons They can be split into baryons (B=1) and mesons It was discovered that the baryon number is always conserved It was discovered that hadrons are not elementary. They are made of particles called quarks A baryon is made of 3 quarks A meson is made of a quark and an antiquark Isolated quarks have never been observed. +2/3e+2/3e +2/3e+2/3e -1/3e-1/3e u u d Proton = u,u,d u d d -1/3e-1/3e +2/3e+2/3e -1/3e-1/3e neutron = u,d,d π + meson = u, d ud +2/3e+2/3e +1/3e+1/3e

Can this equation happen? Ω -  Σ 0 + π - + ν Omega minus decays to sigma naught plus pi minus plus neutrino particlemassQLB Ω-Ω Σ0Σ π -π ν Almost 0010 Mass after is less than mass before so energy is released Charge before = charge after = -1 Lepton number is not conserved (0 before 1 after) Baryon number is conserved (1 before, 1 after) This breaks the law of conservation of lepton number so cannot happen

Summary particles Exchange bosons Hadrons leptons Baryons mesons These are fundamental particles: there are electrons, muons and taons. (and their neutrinos) Lepton number is conserved Made from 3 quarks. E.g. proton (u,u,d) or neutron (u,d,d). Baryon number is conserved Made from a quark and antiquark, (B=0) These are force mediators Strong force gluons Photon electromagnetic Weak nuclear W+ and Z 0 Gravity graviton

Four Forces Forces result from boson exchange The four fundamental forces have different bosons Strong force electromagnetic Weak nuclear Gravity boson gluons Photon W + boson (Z 0 ) graviton?

boson Attractive forces To picture attractive forces imagine two people (with long arms) both pulling on a boson

Feynman diagrams These exchanges can be represented in diagrams called Feynman diagrams e-e- e-e- e-e- e-e- photon Two electrons repel each other

Unified theories strong weak electromagneti c gravity in 1960 it was shown that electromagnetic and weak nuclear force are really different versions of the same thing strong electroweak gravity Grand Unified Theory gravity electroweak and strong theory of everything (4 forces unified) nobody has achieved this yet but a lot of people are trying very hard string theory ? 4 forces