gluons

A Half Century Since the Birth of QCD

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This year marks a half-century since the discovery that a quantum field theory, now known as QCD (quantum chromodynamics), could be the underlying explanation for the strong nuclear force. That’s the force that holds quarks and gluons inside of protons and neutrons, and keeps protons and neutrons clumped together in atomic nuclei. This major step in theoretical physics occurred just a couple of years after it was discovered that a similar quantum field theory for the weak nuclear force (which includes W bosons, a Z boson and a Higgs boson) is mathematically consistent.

With these two breakthroughs came the sudden and unexpected triumph of quantum field theory, emerging as the basic mathematical and conceptual language for understanding the cosmos. It came after two decades in which most experts were convinced that quantum field theory was inconsistent, and only a stepping stone to something deeper.

This week I am in New York City attending two attached scientific meetings, both focused on QCD and other quantum field theories that share its key property, known as “confinement.” One meeting is hosted by New York University, and the other, the Annual Meeting of the Simons Collaboration on Confinement and QCD Strings, by the Simons Foundation. Many luminaries who have spent time on this subject are here together, ranging from David Gross, who co-invented the subject (and was a winner of the 2004 Nobel Prize), to brilliant graduate students who are hoping to reinvent it.

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The Standard Model More Deeply: Lessons on the Strong Nuclear Force from Quark Electric Charges

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For readers who want to go a bit deeper into details (though I suggest you read last week’s posts for general readers first [post 1, post 2]):

Last week, using just addition and subtraction of fractions, we saw that the ratio of production rates

  • R = Rate (e+ e— ⟶ quark anti-quark) / Rate (e+ e— ⟶ muon anti-muon)

(where e— stands for “electron” and e+ for “positron”) can be used to verify the electric charges of the quarks of nature. [In this post I’ll usually drop the word “electric” from “electric charge”.] Specifically, the ratio R, at different energies, is both sensitive to and consistent with the Standard Model of particle physics, not only confirming the quarks’ charges but also the fact that they come in three “colors”. (About colors, you can read recent posts here, here and here.)

To keep the previous posts short, I didn’t give evidence that the data agrees only with the Standard Model; I’ll start today by doing that. But I did point out that the data doesn’t quite match the simple prediction. You can see that in the figure below, repeated from last time; it shows the data (black dots) lies close to the predictions (the solid lines) but generally lies a few percent above them. Why is this? The answer: we neglected a small but noticeable effect from the strong nuclear force. Not only does accounting for this effect fix the problem, it allows us to get a rough measure of the strength of the strong nuclear force. From these considerations we can learn several immensely important facts about nature, as we’ll see today and in the next post.

Figure 1: Data (black dots) showing R as a function of the collision energy 2Ee. Horizontal colored lines show the three predictions for R in the regions where the data is simple and 3, 4 or 5 of the quarks are produced. The minor jumpiness in the data is due to measurement imperfections.

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The Standard Model More Deeply: Gluons and the Math of Quark “Color”

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For readers who want to dig deeper; this is the second post of two, so you should read the previous one if you haven’t already. (Readers who would rather avoid the math may prefer this post.)

In a recent post I described, for the general reader and without using anything more than elementary fractions, how we know that each type of quark comes in three “colors” — a name which refers not to something that you can see by eye, but rather to the three “versions” of strong nuclear charge. In the post previous to today’s, I went into more detail about how the math of “color” works; you’ll need to read that post first, and since I will sometimes refer to its figures, you may want to keep in handy in another tab.

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The Bedlam Within Protons and Neutrons

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My Structure of Matter series has been on hold for a bit, as I have been debating how to describe protons and neutrons.  These constituents of atomic nuclei, which, when combined with electrons, form atoms, are drawn in most cartoons of atoms as simple spheres.  But not only are they much, much smaller than they … Read more

The First Human-Created Higgs-Like Particle: 1988 or 89, at the Tevatron

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Yesterday’s Quiz Question: when was the first Higgs particle produced by humans? (where admittedly “Higgs” should have read “Higgs-like”) got many answers, but not the one I think is correct. Here’s what I believe is the answer.

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[UPDATE: After this post was written, but before it went live, commenter bobathon got the right answer — at 6:30 Eastern, just under the wire! Well done!]

The first human-produced Higgs particle [more precisely, the Higgs-like particle with a mass of about 125 GeV/c2 whose discovery was reported earlier this month, and which I’ll refer to as “`H”– but I’ve told you why I think it is a Higgs of some sort] was almost certainly created in the United States, at the Fermilab National Accelerator Center outside Chicago. Back in 1988 and 1989, Fermilab’s accelerator called the Tevatron created collisions within the then-new CDF experiment, during the often forgotten but very important “Run Zero”.  The energy per collision, and the total data collected, were just enough to make it nearly certain that an H particle was created during this run.

Run Zero, though short, was important because it allowed CDF to prove that precision mass measurements were possible at a proton collider.  They made a measurement of the Z particle’s mass that almost rivaled the one made simultaneously at the SLC electron-positron collider.  This surprised nearly everyone. [Unfortunately I was out of town and missed the scene of disbelief, back in 1989, when CDF dropped this bombshell during a conference at SLAC, the SLC’s host laboratory.] Nowadays we take it for granted that the best measurement of the W particle’s mass comes from the Tevatron experiments, and that the Large Hadron Collider [LHC] experiments will measure the H particle’s mass to better than half a percent — but up until Run Zero it was widely assumed to be impossible to make measurements of such quality in the messy environment of collisions that involve protons.

Anyway, it is truly astonishing that we have to go back to 1988-1989 for the first artificially produced Higgs(-like) particle!! I was a first-year graduate student, and had just learned what Higgs particles were; precision measurements of the Z particle were just getting started, and the top quark hadn’t been found yet. It took 23 years to make enough of these Higgs(-like) particles to convince ourselves that they were there, using the power of the CERN laboratory’s Large Hadron Collider [LHC]!

[Perhaps this remarkable history will help you understand why I keep saying that although the LHC experiments haven’t yet found something unexpected in their data, that absolutely doesn’t mean that nothing unexpected is there. What’s new just may be hard to see, waiting to be noticed with more sophisticated methods and/or more data.]

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CMS Finds a New (Expected, Composite) Particle

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Yes, it’s true what you’ve read; the CMS experiment at the Large Hadron Collider has found a new particle.  However, this isn’t one to get excited about.  Or rather, it’s the particle that’s excited, not the rest of us.  It’s a nice result; a neat result; but this particle is a slightly more massive version of a hadron that we already knew about, a composite object similar to a proton, built out of more fundamental particles we discovered over 30 years ago.  So in the grand scheme of things, this is minor news; no big mysteries to resolve here.  Nevertheless, congratulations to CMS! Finding such particles always involves reconstructing them from their decay products, and since this one decays in a very complicated way, the result represents a technical tour-de-force!

This is a similar story to one from last December, when ATLAS announced that it had found, with confidence, a new particle.  I explained to you then that there are particles and there are particles;

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Following Up on the Proton’s Structure

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This is a follow-up especially aimed at those non-experts who got really excited by my recent posts on the internal structure of the proton (here, here and here), in which I described the proton as being a lot more complicated than just two up quarks and a down quark, emphasizing the presence of many gluons and of many quark/anti-quark pairs in addition to those three quarks that everyone talks about.

Following those posts, I got a lot of very good questions. I’ve been absorbing them and thinking about how to answer them effectively.  I had taken you as far as I knew how to go without hitting technical barriers. You probably noticed I was very careful to address certain issues and not others — answering certain questions and avoiding others. And many of you, intelligently, asked the questions I didn’t answer. So now you get to find out why I didn’t answer them in the first place.  [You asked!]  

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How Do We KNOW a Proton Is So Complicated? (Data!)

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Among the bridges that I hope to build, as I develop this website, is one connecting what we know today about nature with how we know it. After all, you’re reading my depiction of nature, based on how I think nature works.  I can try to assure you that my depiction is the mainstream viewpoint … Read more

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