2. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Exchange Particles

3. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. The Four Forces that govern the behaviour of everything in the Universe! Strong Nuclear Force Electromagnetic Force Weak Nuclear Force Gravitational Force

4. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Search for the Grand Unified Theory Scientists are currently trying to find a theory (and a set of equations that incorporate everything!) that will bind together the four forces into one - the Grand Unified Theory.

5. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. 1. Strong Nuclear Force Acts only within the nucleus - acts between nucleons over a very short range Exchange Particles: Pions (mesons) Relative Strength: 1 (others are compared to it!)

6. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. 2. Electromagnetic Force Acts between charged particles - binds atoms and molecules together Exchange Particles: photons (quanta of electromagnetic radiation E = hf) Relative strength: 10 -2 times the Strong Nuclear Force.

7. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. 3. Weak Nuclear Force Acts within the nucleus - governs radioactive beta decay - involves leptons Exchange Particles: bosons ~ w-bosons and z-particles (not on the syllabus) Relative Strength: 10 -6 times the Strong Nuclear Force.

8. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. 4. Gravitational Force Acts between all masses - very important for large masses in space such as planets and stars – insignificant in intermolecular or inter nuclear interactions. Exchange Particles: gravitons (experimental evidence of their existance has not yet been found) Relative Strength: 10 -38 times the Strong Nuclear Force

9. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Why Exchange Particles? When interaction between particles occurs there is a change in the energy state of each particle For example one particle gains energy the other loses it. We know that mass and energy are interchangeable as E = mc 2 We can therefore consider the exchange of energy to be an 'exchange particle' going from one particle to the other.

10. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Never been detected – therefore virtual! When we detect a particle it interacts with our detector! Exchange particles are not an end state – rather intermediary Therefore, if we detected the exchange particle it would not be acting as an exchange particle any more but have come out of the system That is why we can call exchange particles virtual……if they did more than exchange energy/mass they would not be exchange particles at all!

11. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. The Boson The W and Z particles are the exchange particles which are involved in the nuclear weak interaction between electrons and neutrinos. They are called Intermediate Vector Bosons. The W and Z particles are massive. They act via the weak force.

12. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Bosons They were predicted by Weinberg, Salam, and Glashow in 1979 and measured at CERN in 1982. The exchange particle involved in weak nuclear reactions that relate to beta decay is the W-boson. Remember them as 'w' for 'weak‘ Nuclear because the decay takes place in the nucleus.

13. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Bosons They carry charge as well as energy and so they also need a sign, there are two varieties the W- and the W+. Look carefully at the sign and direction the boson travels in a Feynman diagram to see how its sign relates to the direction charge moves in the interaction.

14. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Example

15. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Example

16. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Bosons Z-bosons are neutral and therefore are not involved in the proton - neutron interchanges that you have to know. Those that involve charge movement (which is what happens in proton neutron interchange) are W-boson interactions.

17. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Pion Exchange

18. Garfield Graphics included with kind permission from PAWS Inc. All Rights Reserved. Consider each vertex – do the quarks balance?