Developments of the quark model Physics Gets Stranger Developments of the quark model

What do we know so far? Quarks feel the strong nuclear force. So anything containing quarks will feel the strong nuclear force. Leptons don’t feel the strong nuclear force. Reactions involving this force happen on a very short timescale. Typically 10-23s (the time it would take a particle to cross the nucleus at the speed of light).

A typical reaction looks like this: P + p- K0 + L The Lambda particle is unstable and decays to a proton and a pi-meson. P + p- L But oddly this takes around 3 x 10-10 seconds. Which is a bit on the long side.

Let’s put this in perspective Imagine dropping a cup. The cup will shatter in the time it takes a shock wave to cross the cup. The speed of sound in ceramic is about 5000 m/s. The cup is perhaps 0.1m high So the cup shatters in t = s/v = 0.1/5000 s i.e. about 2 x 10-5 seconds.

Now imagine you left the cup on the floor… …and three years later one of the broken pieces fell in half. This is the sort of time difference we are talking about with lambda particles.

This happened with a range of particles. The new particles were not clearly identified by mass, charge or baryon number. So they were called strange particles. Particle Q B K +1,-1,0 S 1 X +1,-1 W

Strangeness can take any integer value from +3 to -3. Which lets us ‘explain’ their behaviour by another fundamental property called strangeness. Strangeness can take any integer value from +3 to -3. Strange particles such as L and S have negative strangeness. Their antiparticles have positive strangeness. And strangeness is conserved.

Well, mostly… Strangeness is conserved when the strong nuclear force is involved. Strange particles can decay to particles with zero strangeness via the weak force… …so long as they only change strangeness by 1 each time.. In cases involving the strong nuclear force, strangeness is conserved.

Cascade Hyperons X L0 + p- L0 p- + p S = -2 S = -1 S = 0 S = -1 In each case S changes by 1. This means the strong force is not involved.

Test the following for conservation of strangeness p- + p K+ S- K- X- S+ p+ n L Particle Strangeness K+ +1 K- -1 L S X -2 W -3

New models of the universe The threefold symmetry implied quarks. The new quark was dubbed a strange quark. It looks a bit like a down quark but it’s heavier Quark Charge Baryon number strangeness u +2/3 1/3 d -1/3 s 1

Baryons p p- n L uud udd uds

Mesons p+ p- p0 K+ K- K0 j ud uu us ds ss

Baryons are made up of 3 quarks Or 3 antiquarks Mesons are made up of a quark – antiquark pair Mesons therefore have a baryon number zero… … because the baryon contribution of their quarks always cancels out

The pattern by 1974 By 1974, 4 particles of the lepton family had been found: the electron, the muon and their neutrinos. 3 quarks had been found. This implied a fourth quark with charge +2/3 and mass of about 3GeV Late in 1974 2 laboratories simultaneously announced the discovery of a new heavy hadron.

Charm The particle was dubbed the J/Y particle and was a meson made from a new quark and its antiparticle - a charm – anticharm pair. However, at the same time a Tau lepton was found which ruined the symmetry and implied 2 more quarks and one neutrino. The bottom quark was found one year later The top quark was discovered in 1994.