Nucleons are composed of up quarks and down quarks, and the weak force allows a quark to change its flavour by emission of a W boson leading to creation of an electron/antineutrino or positron/neutrino pair. For either electron or positron emission to be energetically possible, the energy release ( see below) or Q value must be positive.īeta decay is a consequence of the weak force, which is characterized by relatively lengthy decay times. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino or, conversely a proton is converted into a neutron by the emission of a positron with a neutrino in so-called positron emission. Proton-antineutrino collision: p + ν̄ e → n + e +Įlectrons and protons are of course attracted by the electromagnetic interaction between them, but if they collide the weak interaction can make this interaction happen.In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. Neutron-neutrino collision: n + ν e → p + e – There’s a very low probability of a neutrino interacting with matter, but here’s what happens when they do: You get an antineutrino in β – decay and a neutrino in β + decay so that lepton number is conserved. RULES FOR DRAWING FEYNMAN DIAGRAMSġ) Incoming particles start at the bottom of the diagram and move upwards.Ģ) The Baryons stay on one side of the diagram, and the leptons stay on the other side.ģ) The W bosons carry charge from one side of the diagram to the other – make sure charges balance.Ĥ) A W – particle going to the left has the same effect as a W + particle going to the right. You can draw Feynman diagrams for loads of interactions but you only need to learn the ones in this post for your exam. He worked out a really neat way of solving problems by drawing pictures rather than doing calculations.ġ) Gauge bosons are represented by wiggly lines (technical term).Ģ) Other particles are represented by straight lines. Richard Feynman was a brilliant physicist who was famous for explaining complicated ideas in a fun way that actually made sense. Creating a virtual W particle uses so much energy that it can only exist for a very short time and it can’t travel far.Ģ) On the other hand, the photon has zero mass, which gives you a force with infinite range.įeynman Diagrams Show What’s Going In and What’s Coming Out The Larger the Mass of the Gauge Boson, the Shorter the Range of the Forceġ) The W bosons have a mass of about 100 times that of a proton, which gives the weak force a very short range. The graviton may exist but there’s no evidence for it. Gravity only really matters when you’ve got big masses like stars and planets. Particle physicists never bother about gravity because it’s so incredibly feeble compared to other types of interaction. Each one has its own gauge boson and you have to learn their names: Type of Interaction Gauge bosons are virtual particles – they only exist for a very short time.Īll forces in nature are caused by four fundamental forces. The repusion between two protons is caused by the exchange of virtual photons, which are the gauge bosons of the electromagnetic force. These exchange particles are called gauge bosons. Particle exchange also explains attraction, but you need a bit more imagination.Ģ) Attraction – Each time the boomerang is thrown or caught the people get pushed together. It happens because the ball carries momentum. That’s the idea behind exchange particles.ġ) Repulsion – Each time the ball is thrown or caught the people get pushed apart. So, when two particles interact, something must happen to let one particle know that the other one’s there. You can’t have instant action at a distance (according to Einstein, anyway). To the casual observer, this might not seem entirely fair. Having learnt about hadrons (baryons and mesons) and leptons, antiparticles and quarks, you now have the esteemed privilege of learning about yet another weirdy thing called the gauge boson.
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