Quarks Remember the family of ordinary matter consists of only 4 particles, (not counting their antiparticles) quark u d lepton (electron) e Lepton (electron neutrino) Of these all of matter is made only of the first three. Neutrino’s exist in plenty but are not found confined in atoms

No naked quarks The strong force holding quarks together in a hadron ( a proton or neutron) does not weaken with increasing distance. It actually becomes larger as the interquark distance increases: q q The (gamma) energy that we have to put is so high that it results in pair production before the quarks can be separated. We never see a “naked” quark.

Quark Confinement Baryon and Meson Trying to split up the baryon results in a form of pair production where a quark and antiquark pair are produced from the energy of the incident photon. A quark and antiquark pair form a meson

Strong Decay Baryon and Meson Heavy baryons produced in high energy collisions have lots of spare energy already and this is how they break up so quickly to form a more stable baryon ( eventually if not immediately a proton) and a meson. This is decay due to the strong force and takes about 10-23s e.g.

A new model for beta decay A neutron has the quark structure udd The neutron becomes a proton with the quark structure uud The only change in the baryon is that a single down quark has changed into an up quark.

The change from u to d changes the overall charge Total charge u + d + d Total charge u + u + d

A new model for beta decay u A down quark Becomes an up quark So normal (negative) beta decay can be represented by the equation below: Write a corresponding equation for positron emission

A new model for beta decay positron emission can be represented by the equation An up quark Becomes an down quark

Quarks and Forces Quarks are the only fundamental particles which get involved with all of the four fundamental forces. They have mass and are therefore affected by the gravitational force. They have electrical charge and are therefore subject to the electromagnetic force They get involved in beta decay and are therefore affected by the weak force. They make up baryons and mesons and therefore are involved in the strong force

Forces and Exchange Particles A Japanese scientist Yukawa tried to explain forces He suggested that: an exchange of particles can be responsible both for attraction and repulsion. Yukawa based his ideas on the fact that two nuclei can be held together by the exchange of an electron in a molecule

Two proton are held together by an electron in this H+ ion.

We extended this idea to show how the electromagnetic force can account for the repulsion of charged particles by the exchange of a photon. This is how all fundamental particles which respond to the electromagnetic force interact. electron electron Like charges repel each other by exchanging photons Opposite charges attract each other by exchanging photons. The photons transfer momentum from one particle to the other. These photons of exchange are known as virtual photons.

The electromagnetic force The electromagnetic force only applies to particles with charge. When charged particles interact electromagnetically, they exchange photons

Electromagnetic Attraction When a proton attracts an electron it is as a result of the exchange of photons Exchange photons of this kind are called virtual photons

The Feynman diagram for the electromagnetic force. time e- e- space The angles in the diagram mean nothing e- e-

Exchange particles of the Weak Interaction The weak interaction is slightly more complex. Photons are not the particles of exchange. The weak force is mediated by two new particles: These particles have mass and electronic charge They are the W- and W+ particles

beta decay and the weak force electron W- antineutrino down quark flips to become up quark It emits a W- particle The W- particle immediately decays to become the electron and antineutrino.

The Feynman diagram for the beta. time u νe space The angles in the diagram mean nothing W- d