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Exchange Particles

Exchange Particles. The Four Forces that govern the behaviour of everything in the Universe!. Strong Nuclear Force Electromagnetic Force Weak Nuclear Force Gravitational Force. Search for the Grand Unified Theory.

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Exchange Particles

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  1. Exchange Particles

  2. The Four Forces that govern the behaviour of everything in the Universe! • Strong Nuclear Force • Electromagnetic Force • Weak Nuclear Force • Gravitational Force

  3. 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.

  4. 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!)

  5. 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.

  6. 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.

  7. 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

  8. 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 = mc2 • We can therefore consider the exchange of energy to be an 'exchange particle' going from one particle to the other.

  9. 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!

  10. 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.

  11. 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.

  12. 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.

  13. Example

  14. Example

  15. 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.

  16. Pion Exchange

  17. Consider each vertex – do the quarks balance?

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