Sunday, May 21, 2017

Particle Physics versus Nuclear Physics


The difference between particle physics and nuclear physics lies in energy scales..
The energy scales relevant to nuclear physics are much lower than those relevant to particle physics and this is the reason why particle physics is also called high energy physics...
The nuclear force in particle physics is a fundamental force of nature based on the gauge principle and is called the colour force...It is called so because it acts on the nuclear charges carried by the quarks and the gluons which are known as colours..
There exists three types of colour charges and their anti-colours in nature in the same way that there exists one type of electric charge and its anti-charge..We call them for example red, green and blue and anti-red, anti-green and anti-blue which are the analogue of the plus charge + and the minus charge - of the electromagnetic force..
Thus, the nuclear force in particle physics acts on fundamental particles: the quarks (6 of them), the anti-quarks (6 of them) and the gluons (8 of them)...The gluons are the gauge bosons, i.e. the force carriers or mediators, of the nuclear colour force in the same way that the photon is the the gauge boson or force carrier of electromagnetism...
In contrast, the nuclear force in nuclear physics is an effective residual force which does not see neither the quarks nor the gluons ..It acts directly between the protons and neutrons and the other hadrons (baryons and mesons)...These hadrons carry no colour charges and we say that they are singlets under the gauge group of the nuclear colour force which is by the way given by the Lie group SU(3)...
Although the hadrons are singlets, i.e. have zero colour charge, they feel a residual nuclear force which is precisely the force studied in nuclear physics...
This residual force is the equivalent of the Van der Waals force between polarized molecules or atoms..Although the molecules are neutral they act on each other via the Van der Waals force which is a force derived from the more fundamental electromagnetic interaction which is described approximately by Maxwell's equations and exactly by quantum electrodynamics...
In exactly this same way, the nuclear force between protons and neutrons and other hadrons can in principle be derived from quantum chromodynamics, which is the fundamental theory of the nuclear colour force between quarks and gluons...