The Higgs Story-Part 3: The basic elements of the Standard Model

All the stuff of everyday matter is composed of atoms that are made up of protons and neutrons and electrons. The three quarks in the protons and neutrons consist of just the up and down varieties and make up only about 1% of their masses, if we use the current quark mass values (see part 2 in this series). There are also gluons that hold the quarks within the proton and neutron so that they never become isolated free particles the way that (say) electrons do .

To understand this business of what is called quark confinement, we need another analogy. Think of the gluons as creating spring-like forces that connect pairs of quarks in (say) a proton. If you try to separate a pair of quarks, the force of the spring trying to prevent their separation gets larger with the distance between the quarks, just like a regular spring would. But regular springs can be pulled so far that they break. Can you also break the gluon spring? Yes you can, but it is what happens when you do that has no accurate everyday analog. The closest is what happens when you break a bar magnet (which has a north and south pole) in two. You do not end up with one half containing just a north pole and the other just a south pole. Instead you end up with two magnets each having a north and south pole. It seems like breaking the original bar magnet resulted in the sudden creation of two new poles that attached themselves to the original poles is such a way that we have now two separate bar magnets where there had been just one before.

Similarly, when the gluon spring breaks, a new quark-antiquark pair is created out of the vacuum, with each one of them attaching to the two ends of the spring where the break occurred. So you now end up with two quarks connected by a spring (that then rejoin the other quark that was within the original proton) and a quark-antiquark pair (which is called a meson) connected by a spring. This meson is no longer connected by gluon springs to the other quarks in the proton and so is free to go its own way. But you still don’t have a free isolated quark, just as you did not have an isolated magnetic pole. You have one bound set of three quarks and another bound set of a quark-antiquark pair.

So if protons and neutrons each consist of three quarks that make up about 1% of their masses and also gluons that are massless, where does the other 99% of their mass come from? It comes from the energy of interaction of the gluons with the quarks. In other words, from the energy stored in the spring-like forces.

As I will discuss later, the Higgs phenomenon, via the Higgs field, gives rise to the masses of just the elementary particles, which in this case are the quarks and leptons and the W+, W-, and Z particles. So if the Higgs field were not there, the quarks would become massless but the masses of the protons and neutrons would be practically unchanged. If we omit for the moment the as-yet-undetected dark matter whose composition we do not know, almost all the mass of known matter in the universe comes from protons and neutrons. (To learn more about dark matter and dark energy, see parts 8 and 9 of the 16-part series of posts titled Big Bang for Beginners that I wrote in 2010.)

Since the proton and neutron masses would be almost unchanged by the absence of the Higgs, the mass of the almost everything in the universe would remain pretty much the same as it is now. So could life as we know it still exist? The catch is with the electrons. Even though their mass is so small and thus makes a negligible contribution to the mass of everyday objects, without the Higgs they too would become massless and would then travel at the speed of light (like all massless particles do) and thus would not become bound to form electrically neutral atoms. Since matter as we know it is made up of those neutral atoms bound together in a wide variety of ways, we could not exist.

So to summarize: The current Standard Model of elementary particle physics consists of six quarks (up, down, strange, charm, bottom, and top); six leptons (electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino); and the so-called gauge bosons that act as the agents that carry the four kinds of forces that exist in nature: the graviton (for gravity), the photon (for electromagnetism), the gluon (for the strong nuclear force), and the W+, W-, and the uncharged Z bosons (for the weak nuclear force), making 18 particles in all.

And then we have the recently discovered Higgs particle of mass 126.5 GeV, standing alone. The Higgs particle has many properties in common with the other force particles so that it is often lumped together with them for that reason, but unlike them is not the carrier of any known force.

All the particles except the Higgs were detected by 1995, the last of them being the top quark. The masses of the quarks are hard to determine precisely since they are never found in an isolated state but always in combination with other quarks and gluons inside non-elementary particles. The masses of the neutrinos are hard to determine because their interactions with matter is so weak. (How we detect and measure the properties of something microscopic is by seeing how it disturbs the things that we can see and measure. But that requires the particle to interact with matter in the first place and neutrinos are notoriously reluctant to do so.)

It is these 19 particles that form the elementary particle spectrum from which the Standard Model is built. (Each particle also has its corresponding anti-particle but since the properties of each antiparticle are completely determined by the properties of the corresponding particle, they are not listed separately.) Of these 19 particles, the masses of the graviton, photon, and gluon are predicted to be identically zero, and the masses of W+ and W- are predicted by theory to be identical. That leaves us with 15 masses whose values have to be determined by experiment.

In addition, we have to include as additional parameters the strength of the interaction of each of the four kinds of forces, giving 19 parameters in all to be determined by experiment.

Next: Particle and waves