Classification of Particles [Secs 2.1, 14.1, 14.2 Dunlap]

The basic structure of matter Fortunate that the electron is not effected by the strong force – if it did it would be “sucked in” to the nucleus and we would have not atoms – no chemistry and no life!

Leptons, Mesons, Baryons, Hadrons, Bosons, Fermions – what are they? It is important when coming at particle physics to realize that much of the classification of particles (i.e. Leptons, Mesons, Baryons, Hadrons, Bosons, and Fermions) have their roots in history. If we had to classify these particles today with what we now know about them we would probably choose different names. Looking at the history is however highly instructive

Going back in history 1913 1933 Life in 1913 was simple – only three known particles – the proton, electron and photon. By 1933 life had become more complicated. The neutron, and positron had been discovered. The neutrino has a strong evidence

Hideki Yukawa (1907 – 1981) Nobel laureate (1949) The Yukawa Particle In the 1930s Yukawa tackled the problem of what keeps the nucleus together despite the repulsive force between protons. He knew the interaction was extremely strong and short range but what caused it? In 1936 Yukawa published his theory that there was a particle (the Yukawa particle) that had to have a mass of around 100MeV that interacted with the nucleons to produce this ultra strong force. Photon (EM) field obeys: Particle of mass m obeys: Put Quantum operators Hideki Yukawa (1907 – 1981) Nobel laureate (1949) EM wave equation with static solutions: Klein-Gordon equation with state solutions: Thus short range explanation for the strong force was that it was mediated by a particle with mass

How can a particle mediate a force? Attractive and repulsive forces arise from particle exchange In the ball players analogy – the players throw the ball and get a momentum “kick” backwards. If on the other had they exchange the balls as shown below then there is an attractive force. The analogy breaks though – classically the ball exists – in the quantum world where does the mediating particle come from? Answer: it comes through the borrowing of energy (to make the particle E=mc2 for a short time) as allowed by the uncertainty principle

What is the Feynman diagram for the nuclear strong force? Well it looks like this p n p n Latter we will see what this looks like at the quark level. Note that the n and p do not have to change – there is also a Lets assume the π particle travels close to the speed of light then it is out for time: To do this though it has to borrow amount of energy ΔE from the bank So that: or

Discovery of the muon Those were exciting days the 1930s – for Carl Anderson in particular – who discovered the positron in 1932 and in 1936 the muon. How did he do this – using the cloud chamber to study Cosmic Rays. Originally Anderson thought this must be the Yukawa particle since it had 207 electron masses. Yukawa had predicted his particle should have ~200 electron masses. Soon it became clear though that this could not be Yukawa’s particle since it did not interact with the nucleus of atoms. It behaved like a heavy electron! Physicist Rabbi philosophically said “Who ordered it”? Carl Anderson (1905 – 1991) 1939 Nobel Prize (140MeV) 3x 10-8 sec (106 MeV) 2x 10-6 sec (0.5 MeV)

The discovery of the pion There were 3 major players in the discovery of the pion. The discovery took place in 1947 in two laboratories Marietta Blau (1894 – 1970) Discovered the technique of nuclear emulsions for looking at short lived particle tracks Donald Perkins (1925) Was the first see a pion event - i.e. a particle of the right mass interacting with a nucleus Cecil Powell (1903 – 1969) Nobel laureate 1950 And his research group in Bristol saw the 2nd and 3rd events – and saw that the pion decayed to a muon

Going back in history to 1947 To begin with particles were classified according to their weight (mass). The diagram shows what physicists knew about particles in 1947. There do seem to be three groupings by weight (mass). Light particles (which they called LEPTONS) = (e-, e+, e, ) [from Greek; leptos = small] (ii) Middle weight particles (which they called MESONS) ( ) [from Greek, mesos=middle] (iii) Heavy weight particles (which they called BARYONS) = p, n, [from Greek, barus=heavy). Because the MESONS and BARYON groups seemed so much heavier than the leptons, they were collectively known as “bulky” particles or HADRONS (Gk: hadros = bulky)_ i.e. HADRONS = MESONS + BARYONS 1947 BARYONS MESONS LEPTONS

Modern classification - leptons Leptons are FERMIONS (spin half particles) that do not participate in the strong interaction. Leptons interact only through the electro-weak force (i.e. electric force plus weak force) and thus we may think of the leptos as now meaning “light – as in delicate - interaction” Leptons appear to be point like particles having no internal structure (size <10-19m). Electrons must have size greater than 10-50m otherwise they would be black-holes. String theories put their size around 10-30m Leptons are seen in the left hand column of mass in the Figure in previous slide. NB. The muon is not referred to now as a mu-meson, because it not a meson – ( a meson is strongly interacting and it is a boson)

Modern classification - mesons Mesons are BOSONS of either spin 0 (usual) or spin 1 (rare) that DO interact via the strong interaction (also electric and weak) Mesons tend to have masses between the electron mass and the p, n masses. The reason is as we shall see latter – that the mesons comprise of 2 quarks bound together. Mesons have internal structure and measurable size (~1F) Mesons cause the nucleon-nucleon force – they are “go between” or “in the middle” particles – a new meaning of “mesos” NB. Not all “go between” – mediating particles are mesons. The photon “sticks” the electron to the atomic nucleus in the same kind of way – as a pion. The photon is not a meson however because it does not interact via the strong interaction, Moreover the photon is massless.

Modern classification – Baryons and Hadrons BARYONS are FERMIONS (spin ½ or 3/2) that interact via the strong interaction (also electric and weak). Baryons have masses equal to or greater than the proton (because as we shall see they comprise of three bound quarks) Baryons have internal structure and measurable size (~1fm) HADRONS: is a collective term for strongly interacting particles. i.e the hadron family contains MESONS + BARYONS

Classification of Fermions and Bosons HADRONS LEPTONS

Perhaps an easier classifier ?

The fundamental force carriers GLUONS - mediate the STRONG interaction between quarks (not pions) PHOTONS – mediate the ELECTRIC interaction between quarks and charged leptons W and Zs – mediate the WEAK interaction between quarks and leptons GRAVITONS – mediate the GRAVITATIONAL interaction (questionably)

The particle situation 1983 Notable additions are the TAON – super heavy electron discovered 1974 K- MESONS D- MESONS HYPERONS such as