Excerpt from: The Standard Model of Particle Physics by Tom W.B. Kibble
This may seem a very strange theory but it is now well established; QCD and the electroweak theory together constitute the standard model, with spin -½ leptons and quarks and spin – 1 gauge bosons. It has been tested by innumerable experiments over the last forty years and been thoroughly vindicated.
Until recently there was however a gap, the Higgs boson. Back in 1964, the existence of this extra particle was seen as a relatively minor feature; the important thing was the mechanism for giving masses to gauge bosons. But twenty years later, it began to assume a special significance as the only remaining piece of the standard – model jigsaw that had not been found.
Finding it was one of the principal goals of the large hadron collider (LHC) at CERN. This is the largest piece of scientific apparatus every constructed, a precision instrument built in a huge 27 km – long tunnel straddling the French – Swiss border near Geneva — a truly remarkable piece of engineering. Protons are sent round in both directions, accelerated close to the speed of light, and allowed to collide at four crossing points around the ring. At two of these are large detectors, Atlas and CMS, also marvels of engineering, that over a period of twenty years have been designed, built and operated by huge international teams of physicists and engineers. In 2012 this mammoth effort paid off, with the unequivocal discovery by both teams of the Higgs boson.
So is this the end of the story? Surely not. The standard model can hardly be the last word.
It is marvelously successful, but far from simple. It has something like 20 arbitrary parameters, things like ratios of masses and coupling strengths, that we cannot predict and that seem to have no obvious pattern to them. Moreover there are many features for which we have no explanation. Why for both quarks and leptons are there three generations with very similar properties but wildly varying masses? Why do quarks come in three colours?
One theory is that all these choices are random. There may have been many big bangs, each producing a universe with its own set of parameters. Most of those universes would probably be devoid of life. But that is for many a profoundly unsatisfactory answer; we certainly hoped for a more predictive theory!
On the observational side, there are still many things we cannot explain. What is the nature of the dark matter in the universe? Why does the universe contain more matter than antimatter — leptons and quarks rather than antileptons and antiquarks? Moreover there are a few points on which the standard model definitely does not agree with observation. In particular, in the standard model the neutrinos are strictly massless. But we now know that do in fact have non-zero, albeit very tiny, masses. We really have no idea why.
Finally, there is the elephant in the room: gravity, which does not appear at all in the standard model. It is in fact very difficult to reconcile our best theory of gravity, Einstein’s general theory of relativity, with quantum theory. That is a problem we have been struggling with for the best part of a century. There are hopes that string theory, or its more modern realization, M-theory, may successfully unite the two, but that effort has been going on for decades without as yet reaching a conclusion. At any rate it does appear that there is a lot more for theoretical physicists to do!
Excerpt from: The Standard Model of Particle Physics by Tom W.B. Kibble. Blackett Laboratory, Imperial College London arXiv:1412.4094 (PDF)