Fundamental FOrce MediatingparticlesAbstractThe currently known fundamental forces ofnature are the electromagnetic, weak, strong, and gravitational forces. Thispaper will give a basic overview of the current standard model that is used todescribe these forces. It will also explore some of the history behind thedevelopments.IntroductionAs quantum mechanics was developing, itbecame clear that the interactions between particles could be described as the exchangeof particles. The particles that mediate the fundamental interactions are knownas gauge bosons. Gauge bosons often behave as virtual particles duringinteractions; this is where they mediate a force but cannot be directly observed1.Feynman Diagrams will be used throughoutas they are a useful visual conveyor of information. They will have time on thex-axis and space on the y-axis.
For those unfamiliar with these diagrams, a fewthings are important to note: The arrows on lines for electrically chargedparticles represents the direction of flow of negative electric charge; and someparticles may be illustrated to seemingly travel in space but not in time, theseare virtual particles.The standard model may be presented ashaving two parts. These are the Electroweak Model, which includes both the weakand electromagnetic interactions, and Quantum Chromodynamics, which include thestrong interaction. The Electroweak Model is the one that will be discussed first.ElectroweakInteractionsQuantum ElectrodynamicsPhotons are the mediators of the electromagneticforce.
The theories around these interactions between light and matter aresometimes referred to by the mouthful of a name, quantum electrodynamics. Photonshave an infinite range, but if they have a high enough energy they can undergopair production creating a particle-antiparticle pair. Often described aspackets of energy, they carry only energy with no mass and no quantum numbers.What is specifically notable is that they don’t carry electrical charge, yetthey mediate the electromagnetic force – this is not the case in other gaugebosons which will be discussed later.Here is an illustrationshowing an example electromagnetic interaction: ? e- e- e- e- Figure X. A Feynman diagram showingelectron scattering.The photon behaves as a virtual particle,and mediates electromagnetic repulsion between the two electrons.
It is becausethe photon is behaving as a virtual particle that it can be drawn as a verticalline where no time passes. A similar diagram may be drawn to represent anattraction between opposite charge.A similar interaction could be illustratedfor any charged particle Electroweak UnificationBetween the years 1961 and 1967 the firstpart of the standard model, the Electroweak theory, was developed. This wasdone mainly by Abdus Salam, Sheldon Glashow, and Steven Weinberg. This theoryunified the electromagnetic and weak interactions into a single theory. Itshowed that although at low energies there is a big distinction between theweak and the electromagnetic interactions; at very high energies thedistinction wasn’t present.
It predicted the weak force mediators, the W+,W-, and Z0 bosons. These particles were later discoveredin 1983 in CERN with the masses predicted. 6 1These bosons differ greatly form thephoton in that they are massive. This is explained by the Higgs boson which Iwill not cover here, but further reading can be found here: 6.Some, namely W+ and W-, carry electriccharge therefore they themselves can interact by the electromagnetic interaction.
They also have what is known as a weak charge, as do all particles that interactby the weak force. Interestingly enough, this means that they can also interactwith themselves by the weak force. 3Here is an illustration showing a weakinteraction: u d e- ?? e W- Figure X. A Feynman diagram showing a weakinteraction, specifically ?- decay of a down quark into an up quark.
Here you can see it’s also conventional to use wiggly lines to represent theweak force bosons. This fits nicely due to the unification of the two forces.In this example the boson produces a pairof particles. The anti-electron neutrinomust be produced to conserve the electron lepton quantum number. A similarinteraction might happen but in reverse, where a up quark decays into a downquark. This would instead produce a W+ boson to conserve charge, andthat would proceed to decay into a positron electron-neutrino pair.
StrongInteractionsThe Yukawa InteractionThe first suggestion of a force mediatorfor the strong interaction came from Hideki Yukawa. In his paper “On the Theoryof Elementary Particles. I” 4, he suggested that a theoretical quantised particlecould act as a mediator for the strong interaction. At this time the quarkmodel was yet to be developed, so his theory was developed around hadrons beingfundamental particles.
Nevertheless, he managed to predict the later discoveredpi-meson (or pion), which does indeed mediate the interaction between hadronsas expected. This type of interaction is now known as a Yukawa interaction. 15As time progressed it could be seen that thelepton family formed compact, nicely organised group, but the hadrons not somuch. There were many reasons to suspect an underlying structure for hadrons,but one main obvious one was that newly discovered hadrons continued to pileup. In 1964, George Zweig and Murray Gell-Mann independently developed a theorywhich in effect defined the substructure of hadrons. Known as the quark model,this model in a more developed form is incorporated in what is now the secondhalf of the standard model. 5Quantum ChromodynamicsQuantum chromodynamics describes thestrong interaction as the exchange of gluons.
Gluons, like photons, are massless. One of the main parts of QuantumChromodynamics (QCD) revolves around the quantum number ‘colour charge’ which willbe referred to as colour. Like electric charge, colour is always conserved in interactions.Both quarks and gluons have colour.
Thereare three types of colour, each with a corresponding ‘anticolour’. These are:red, antired, green, antigreen, blue, and antiblue. While quarks carry unit colour (or anticolourfor antiquarks), gluons consist of a colour-anticolour combination. The allowedcombinations give a total of 8 possible gluons – these are two colour neutralgluons, and three with corresponding antiparticles.
A baryon will have a quarkof each colour, while a meson will have a colour and its correspondinganticolour – they are therefore both colour neutral overall. 1 g qR qB qB qR Figure X – A Feynman Diagram Showing theinteraction between a red quark and a blue quark mediated by a gluon.The colour of the gluon could be either RB?or BR?. It may be helpful to think of it as one particle travelling in onedirection being identical to its antiparticle travelling in the oppositedirection. g qG? qB? qG qB Figure X –A Feynman Diagram showing the interaction between a green quark and anantigreen quark.
The colourof the gluon could be either GB? or BG ConclusionThis piece has only been a brief dip ofthe toes into the basics of particle physics with a focus on the gauge bosons. Thestandard model is good, but far from complete. Currently there is no theory notbased on conjecture that unifies QCD with the electroweak theory, but this issomething that many people hope to achieve.
Such theories are known as ‘grandunified theories’ – If I succeeded in developing one I would definitely embracethe pretentious name. Some of these include estimations for proton decaywithout conserving baryon number, but this is of course near impossible toverify experimentally because the predicted half life of the proton is muchlonger than the estimated life of the universe. The number of protons that would need to be observedat once in conditions free of interfering particles is likely not achievable. 3One step to have a more complete particlemodel of the fundamental interactions would be to verify the existence of the theoreticalparticle the graviton.
It is theorised to be the force mediating particle ofthe gravitational force. Successfully doing this would involve developing a theoryto take over from the large-scale approximation in Einstein’s theory of generalrelativity. But things relating to gravity on small scales are very difficult todo due to the tiny impact that gravity has at these scales. 2What the reader should take away from thisis: Whoever you are, and whatever your knowledge level, there is sure to bemore for you to discover.