Introduction to Particles AS Level Physics

Fundamental…. A fundamental particle cannot be broken down into anything simpler. The Greeks philosophised that if you take an object and break it into two pieces, then take one of the halves and halve that and so on…. you would eventually get to a point where the object would not divide any further (Greek - atom (indivisible)).

Indivisible…. It was thought that each substance had its own fundamental particle. The universe could then be thought of as composed of millions of different 'matter building bricks' …one for water one for salt, one for gold, one for grass etc….

Atoms We now know that substances such as gold are made up of atoms but substances such as water are made up of individual units called molecules that it is possible to break down into smaller components - atoms. Grass is living matter. It is made up of complex and simple molecules associated into vast biological units called cells….. but all of these could be broken down into atoms too.

Atoms The atoms, that matter is made up of, are listed in the Periodic Table. We therefore have a set of just over 90 'matter building bricks' that all of the matter in the universe could be considered to be made of!

What are atoms made of? By the end of your GCSE you were aware of the fact that atoms can be thought of as composed of protons, neutrons and electrons. The number of protons in the nucleus determines which atom we are dealing with. The number of neutrons tells us which isotope of that atom we are dealing with. The number of electrons orbiting the atom determines whether the atom was indeed a neutral atom or an ion.

A Lego Set for God… Our 'matter building brick' set is now much simpler…. from only three different types of 'brick' we could make anything - a Lego set for God! The simplicity of this has a sort of beauty… it feels 'right'…. it is an elegant picture of matter… a pleasing model. BUT we now know this is not the solution to the fundamental particle puzzle….although electrons have not yet been broken down into anything simpler, protons and neutrons can be broken down into quarks.

Simplicity

The Standard Model This is the currently held view of matter - but it is being disputed and refined and at University level you will learn of its limitations and the suggestions as to how it should be amended Remember we only have a 'model' of how things work - NOT the absolute truth - we seek to get a clearer picture, a better understanding of how the Universe works!

Antimatter Not only have we found sub atomic particles but we also now know there is 'anti-matter'. Anti-matter was predicted by Paul Dirac (English physicist - despite the name!) in 1928. He solved the equations for electron orbitals in the atom and proposed negative energy states for electrons.

Antimatter A root can produce two possible solutions +2 or -2 ! A matter and antimatter solution. In 1932 Carl Anderson found experimental evidence for one in a cloud chamber (we study those next year). Anti-matter is the complement to matter - it has the opposite characteristics.

Quantum Numbers Characteristics of particles are given by quantum numbers (for charge, baryon number, strangeness etc.) Anti-particles have the opposite sign for each one when compared to the matter particle's characteristic quantum numbers.

Anti-matter You can have anti-matter but there is no such thing as anti-energy! The rest mass of a particle is the same as the rest mass of an anti-particle. The charge, baryon number and strangeness of a particle is the opposite of the charge, baryon number and strangeness of an anti-particle.

Positron An anti-electron is a positron (b+). This is positive and has the same mass as an electron.

Positron If a positron and electron meet they annihilate one another - poof! and their combined mass (2me) is converted to electromagnetic energy (in accordance with the Einstein equation (E = mc2) - but we don't have to do the maths on that until A2).

(joule) (coulomb) (volt) Electron Volt (eV) The energies are calculated in terms of electron volts (eV) The eV is a very small unit of energy - used a lot in nuclear calculations. You need to be able to convert from one to the other. Remember at GCSE you learned that: E     =     Q         V (joule) (coulomb) (volt) So, a joule could be thought of as a 'coulomb volt'  

(electron volt) (charge on an electron) (Volt) The electron volt - eV Similarly, if we don't use a full coulomb of charge, but only the charge on an electron 'e' eV          =             e                       V (electron volt) (charge on an electron) (Volt) The charge on an electron is 1.6 x 10-19 C Therefore the eV is only 1.6 x 10-19 J

Anti-proton The anti-proton was 'made' in 1955 at the University of California. If a high-energy proton is made to collide with a stationary proton the kinetic energy it possesses can be converted into a matter/anti-matter pair. Just as on annihilation of matter and anti-matter energy is produced, so pure energy can be changed into matter and anti-matter (2mp would be needed). More recently anti-hydrogen was made (an anti-proton and a positron)

Pair Production 'Something material' comes out of 'nothing but pure energy', but it has to appear in such a way that if the process reversed we would wind up with the pure energy again. That is why particles that 'come out of thin air' are always produced in pairs. This is termed pair production. The more massive the particle to be produced the greater the energy required (we will do more on how that energy is supplied in the A2 module but you may find it interesting to look up particle accelerators at this point in the course).

Using an electric field to provide energy Energy for a collision can be provided by an electric field: a 2000V field would accelerate an electron (or a proton - they both have the same charge!) at rest to give it 2keV of kinetic energy; a 6MV field would accelerate an electron at rest to give it 6MeV of kinetic energy; a 9GV field would accelerate an electron at rest to give it 9GeV of kinetic energy etc.

Pair Production If a very high energy proton (enough excess to be converted into a proton and an anti-proton) 'bumps into' (better expressed as 'interacts with') an anti-proton of equal energy, then as well as the two original particles there would be another proton/anti-proton pair!.... the kinetic energy would produce matter instead!

Pair Production Rest mass energy (the energy needed to produce a proton or anti-proton particle at rest!) of a proton is 940 MeV. So, to produce a pair we would need 2 x 940 MeV = 1900 MeV = 1.9 GeV (NB - 2 sig figs here!) If we used a 3 GeV accelerator we would have 1.1 GeV left over.... this would be seen as kinetic energy among the particles.

Hadrons and Leptons We now have two sets of sub-atomic particles: hadrons (from the Greek meaning 'massive' or 'bulky' - heavy ones) that are made up of quarks and leptons (light ones) that are 'fundamental'. Particle physics involves the study of these and their interactions. We have to look at some of these in this course.

Leptons Lepton is Greek for 'small'. Leptons do NOT experience the strong nuclear force, rather any nuclear interactions they are involved in are via the weak nuclear force. The leptons (we have to know about!) are: the electron (e- or b- ) the positron (e+ or b+ ) the muon (m-) and the antimuon (m+) - basically a 'heavy electron' (about 200 times the mass) the neutrino (n) and antineutrino (n) ( both with virtually no mass and zero charge)

Leptons Electron leptons have a lepton number Le of +1 and their antileptons a lepton number -1. Muon leptons have a lepton number Lm of +1 and their antileptons a lepton number -1. Leptons have zero baryon number and zero strangeness.  

Hadrons Hadrons are made of quarks There are two types of hadron: Baryon - 3 quarks - baryon number of 1 (or -1 for an anti-baryon) - proton and neutron Meson - 2 quarks (a quark and an anti-quark) - baryon number 0 ('cos its not a baryon!) - kaon, pion

Quarks You only need to learn about three quarks - the up 'u', down 'd' and strange 's' but there are others… you don't need to worry about them for this course, but it would be a good idea to look them up and give yourself a more complete picture. Each quark has characteristics that are listed on your data sheet - you do NOT have to learn these, you should be able to use them to work things out!

Quark Properties Quark Charge Baryon Number Strangeness up 'u' 2/3 1/3 down 'd' -1/3 strange 's' -1 u -2/3 d s 1

The only baryons you need to consider are the proton and neutron.

Baryons They are made up of only up and down quarks. They have three quarks. Use the data sheet to work out the quark composition of a proton.

Proton Use the data sheet to work out the quark composition of a proton. Charge is +1 therefore it is uud

Neutron Use the data sheet to work out the quark composition of a neutron. Charge is 0 therefore it is udd

Meson The only two types of meson you need to know about are the pion p and the kaon k.

Meson BOTH have a quark and an antiquark. You only have to worry about up, down and strange quarks.

Meson - Kaon The kaon k involves 'strangeness'. It is the only strange particle you have to deal with in this course.

The pion p has no strange quark. Meson - pion The pion p has no strange quark. It is just up and down.

Meson - pion After the symbol comes a charge sign – plus, minus or zero. This you can work out from the data sheet.

Quark Properties Quark Charge Baryon Number Strangeness up 'u' 2/3 1/3 down 'd' -1/3 strange 's' -1 u -2/3 d s 1

Meson Try making up some mesons Take a quark and add an anti-quark. Work out if it is a pion or a kaon (is there a strange in there???) Work out the charge… Write out the symbol.. k or p with the sign you worked out as being its net charge after it…. p+ k+ po k + p- ko

Quark Antiquark Meson u po d p+ s ko k o k+ k- p-

How much do you know? New words: Hadron Lepton Meson Baryon Quark Antimatter Pair production Annihilation