Exchange Particles
E N D
Presentation Transcript
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 • 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.
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!)
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.
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.
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
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.
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!
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.
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.
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.
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.
