Celebrating the Standard Model: Atoms, Quarks and the Strong Nuclear Force

For the general reader:

Last week I showed you, without any technicalities, how to recognize the elementary forces of nature in the pattern of particle masses and lifetimes. This week we’ll start seeing what we can extract just from the particles’ masses alone… and what we cannot. Today we’ll focus on quarks and the strong nuclear force.

A key factor in nature, which plays an enormous role in everyday life, is the mass of a typical atom. [Note: on this website, “mass” always means “rest mass”, which does not increase with a particle’s speed.] This in turn arises mainly from the masses of protons and neutrons, which are about equal, and tiny: about 0.00000000000000000000000000167 kg (or 0.00000000000000000000000000368 pounds). Since those numbers are crazy-small, physicists use a different measure; we say the mass is about 1 GeV/c2, and more precisely, 0.938 GeV/c2. In any case, it’s tiny on human scales, but it’s some definite quantity, the same for every proton in nature. Where does this mass come from; what natural processes determine it?

You may have heard the simplistic remark that “a proton is made of three quarks” (two up quarks and a down quark), which would suggest these quarks have mass of about 1/3 of a proton, or about 0.313 GeV/c2. But something’s clearly amiss. If you look at websites and other sources about particle physics, they all agree that up and down quark masses are less than 0.01 GeV/c2; these days they usually say the up quark has mass of 0.002 GeV/c2 and the down quark has 0.005 GeV/c2. So if the proton were simply made of three quarks, it would naively have a mass of less than 1% of its actual mass.

What’s going on? A first little clue is that different sources sometimes quote different numbers for the quark masses. There are six types of quarks; from smallest mass to largest, they are up, down, strange (u,d,s, the three light quarks), charm, bottom (c,b, the two somewhat heavy quarks) and top (t, the super-heavy quark.) [Their names, by the way, are historical accidents and don’t mean anything.] But some websites say the up quark mass is 0.003 instead of 0.002 GeV/c2, a 50% discrepancy; the bottom quark’s mass is variously listed as 4.1 GeV/c2, 4.5 GeV/c2, and so forth. This is in contrast to, say, the electron’s mass; you’ll never see websites that disagree about that.

The origin of all these discrepancies is that quarks (and anti-quarks and gluons) are affected by the strong nuclear force, unlike electrons, Higgs bosons, and all the other known elementary particles. The strong forces that quarks undergo make everything about them less clear and certain. Among numerous manifestations, the most dramatic is that quarks (and anti-quarks and gluons) are never observed in isolation. Instead they’re always found in special combinations, called “hadrons“. A proton is an example, but there are many more. And the strong nuclear force can have a big effect on their masses.

The Modern Proton and the Masses of Quarks

A proton, in fact, is not simply made from three quarks, the way a hydrogen atom is simply made from a proton and an electron. As I described in this article, it’s vastly more complex; it’s made from three quarks plus lots of gluons plus lots of pairs of other quarks and anti-quarks. So the simple intuition we get from atoms does not apply to hadrons like the proton.

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Quantum Field Theory, String Theory and Predictions (Part 3)

[This is the third post in a series; here’s #1 and #2.] The quantum field theory that we use to describe the known particles and forces is called the “Standard Model”, whose structure is shown schematically in Figure 1. It involves an interweaving of three quantum field theories — one for the electromagnetic force, one … Read more

What is the “Strength” of a Force?

Particle physicists, cataloging the fundamental forces of nature, have named two of them the strong nuclear force and the weak nuclear force. [A force is simply any phenomenon that pushes or pulls on objects.] More generally they talk about strong and weak forces, speaking of electromagnetism as rather weak and gravity as extremely weak.  What do the words “strong” and “weak” mean here?  Don’t electric forces become strong at short distances? Isn’t gravity a pretty strong force, given that it makes it hard to lift a bar of gold?

Well, these words don’t mean what you think.  Yes, the electric force between two electrons becomes stronger (in absolute terms) as you bring them closer together; the force grows as one over the square of the distance between them.  Yet physicists, when speaking their own language to each other, will view this behavior as what is expected of a typical force, and so will say that “electromagnetism’s strength is unchanging with distance — and it is rather weak at all distances.

And the strength of gravity between the Earth and a bar of gold isn’t relevant either; physicists are interested in the strength of forces between individual elementary (or at least small) particles, not between large objects containing enormous numbers of particles.

Clearly there is a language difference here… as is often the case with words in English and words in Physics-ese.  It requires translation.  So I have now written an article explaining the language of “strong” and “weak” forces used by particle physicists, describing how it works, why it is useful, and what it teaches us about the known forces: gravity, electromagnetism, the strong nuclear force, the weak nuclear force, and the (still unobserved but surely present) Higgs force.

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Higgs Symposium: A More Careful Summary

My rather hasty, breathless and inconsistent summaries (#1, #2 and #3) of last week’s talks at the excellent Higgs Symposium (held at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics) clearly had their limitations.  So I thought it might be useful to give a more organized overview, with more … Read more

TIME for a Little Soul-Searching

Yes, it was funny, as I hope you enjoyed in my post from Saturday; but really, when we step back and look at it, something is dreadfully wrong and quite sad.  Somehow TIME magazine, fairly reputable on the whole, in the process of reporting the nomination of a particle (the Higgs Boson; here’s my FAQ about it and here’s my layperson’s explanation of why it is important) as a Person (?) of the Year, explained the nature of this particle with a disastrous paragraph of five astoundingly erroneous sentences.   Treating this as a “teaching moment” (yes, always the professor — can’t help myself) I want to go through those sentences carefully and fix them, not to string up or further embarrass the journalist but to be useful to my readers.  So that’s coming in a moment.

But first, a lament.

Who’s at fault here, and how did this happen?  There’s plenty of blame to go around; some lies with the journalist, who would have been wise to run his prose past a science journalist buddy; some lies with the editors, who didn’t do basic fact checking, even of the non-science issues; some lies with a public that (broadly) doesn’t generally care enough about science for editors to make it a priority to have accurate reporting on the subject.  But there’s a history here.  How did it happen that we ended up a technological society, relying heavily on the discoveries of modern physics and other sciences over the last century, and yet we have a public that is at once confused by, suspicious of, bored by, and unfamiliar with science?   I think a lot of the blame also lies with scientists, who collectively over generations have failed to communicate both what we do and why it’s important — and why it’s important for journalists not to misrepresent it.

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A Real Workshop

In the field of particle physics, the word “workshop” has a rather broad usage; some workshops are just conferences with a little bit of time for discussion or some other additional feature.  But some workshops are about WORK…. typically morning-til-night work.  This includes the one I just attended at the Perimeter Institute (PI) in Waterloo, Canada, which brought particle experimentalists from the CMS experiment (one of the two general-purpose experiments at the Large Hadron Collider [LHC] — the other being ATLAS) together with some particle theorists like myself.  In fact, it was one of the most productive workshops I’ve ever participated in.

The workshop was organized by the PI’s young theoretical particle physics professors, Philip Schuster and Natalia Toro, along with CMS’s current spokesman Joseph Incandela and physics coordinator Greg Landsberg. (Incandela, professor at the University of California at Santa Barbara, is now famous for giving CMS’s talk July 4th announcing the observation of a Higgs-like particle; ATLAS’s talk was given by Fabiola Gianotti. Landsberg is a senior professor at Brown University.) Other participants included many of the current “conveners” from CMS — typically very experienced and skilled people who’ve been selected to help supervise segments of the research program — and a couple of dozen LHC theorists, mostly under the age of 40, who are experienced in communicating with LHC experimenters about their measurements. 

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Is Supersymmetry Ruled Out Yet?

[A Heads Up: I’m giving a public lecture about the LHC on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.] Is … Read more

Professor Peskin’s Four Slogans: Advice for the 2012 LHC

On Monday, during the concluding session of the SEARCH Workshop on Large Hadron Collider [LHC] physics (see also here for a second post), and at the start of the panel discussion involving a group of six theorists, Michael Peskin, professor of theoretical particle physics at the Stanford Linear Accelerator Center [and my Ph.D. advisor] opened the … Read more

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