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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

Comments

  1. Félix Desrochers-Guérin says

    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.

    Since quarks would be massless and move at the speed of light too, wouldn’t that prevent the formation of protons, neutrons and other hadrons as well?

  2. Mano Singham says

    A good question. The strong nuclear force is powerful enough to keep them bound inside protons and neutrons. In fact, the gluons are massless and they too are bound within the protons and neutrons. But the electrons are held together in atoms by the electromagnetic force which is too weak to bind them into atoms of the size we are familiar with in order to create larger molecules that make up ordinary matter.

    With quarks and gluons inside the protons and neutrons we are in a regime where classical ideas tend to break down and one has to go into the full theory mode of quantum chromodynamics (QCD) to rigorously answer these questions.

  3. Félix Desrochers-Guérin says

    So the answer is basically that the strong nuclear force is, well, strong. Also, I completely forgot about gluons being massless and still confined.

    Thank you for your prompt answer and I look forward to more of this particle physics 101 lecture.

  4. MNb says

    “it is what happens when you do that has no accurate everyday analogy”
    Maybe the magnete analogy is not accurate – no analogy ever is fully accurate – but to me it was crystal clear what you meant.

    “without the Higgs the electrons too would become massless”
    Ah, you know how to write a cliffhanger.

    “Each particle also has its corresponding anti-particle”
    Will there be a quest for the anti-higgs?

  5. Marshall says

    What a great series! I’m still a bit confused about the “force exchange” concept. In the last post in the series, commenter Somite asked about the nature of the numbers of “force particles” exchanged. I realize that even thinking of particles as little balls moving around isn’t what’s happening at all, but is there some analogous way to think about it?

    I’m imagining one particle zipping by another, and it shoots a “force particle” out that knocks the other one around a little bit, causing it to [maybe] move towards the moving particle if the charges are opposite, or something. Does the force particle become absorbed by the “receiver,” or does it bind somehow and hang around, and then shoot it back?

    I have no idea what I’m talking about.

  6. Mano Singham says

    Actually these are great questions that probe deeply into what we mean when we talk about particles in this context. In the next post in the series on particles and fields, I will go into this more, if you don’t mind waiting.

  7. Rob Grigjanis says

    Some particles are their own anti-particle. The Higgs boson is one. Others are photons, gravitons and the neutral weak vector boson Z.

  8. melvin goldstein says

    Numbers are the Supreme Court of science. However Godel proved that we may not prove everything. Physics needs numbers. There must be Physics Foibles!!

  9. Sunday Afternoon says

    Mano,

    You seem to imply that the gravitron has been discovered when you say that by 1995 all particles but the Higgs had been found. I think the gravitron is still hypothetical.

    Enjoying your series.
    SA

  10. Mano Singham says

    Yes, the graviton has still not been detected directly. I meant those particles that are relevant to the Higgs. Also, we have good reason to think its mass is zero even without detecting it.

  11. Sunday Afternoon says

    “The correct spelling of the gauge boson of quantum gravity is graviton.”
    “The correct spelling of the gauge boson of quantum gravity is graviton.”
    “The correct spelling of the gauge boson of quantum gravity is graviton.” etc…

    Regarding the expected mass of the graviton being zero: is this deduced from the result of general relativity that gravitational waves travel at c? Are there other reasons to expect a zero mass for the graviton?

  12. Mano Singham says

    There are good reasons to think the mass of the graviton is zero. One is that the range of the gravity force seems to be infinite and that corresponds to zero mass of the force particle. The other is that the quantized gravity field must couple to the stress-energy tensor in the same way that the gravity field does and this requires a massless spin-2 particle.

    Of course, these are theoretical reasons and there is always the possibility of an empirical surprise. But until shown otherwise, they provide pretty good reasons to think that the mass is zero.

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