Protons and Charm Quarks: A Lesson From Virtual Particles

There’s been a lot of chatter lately about a claim that charm quarks are found in protons. While the evidence for this claim of “intrinsic charm” (a name that goes back decades) is by no means entirely convincing yet, it might in fact be true… sort of. But the whole idea sounds very confusing. A charm quark has a larger mass than a proton: about 1.2 GeV/c2 vs. 0.938 GeV/c2. On the face of it, suggesting there are charm quarks in protons sounds as crazy as suggesting that a football could have a lead brick inside it without you noticing any difference.

What’s really going on? It’s a long story, and subtle even for experts, so it’s not surprising that most articles about it for lay readers haven’t been entirely clear. At some point I’ll write a comprehensive explanation, but that will require a longer post (or series of posts), and I don’t want to launch into that until my conceptual understanding of important details is complete.

Feynman diagram suggesting a photon is sometimes an electron-positron pair.

But in the meantime, here’s a related question: how can a particle with zero mass (zero rest mass, to be precise) spend part of its time as a combination of objects that have positive mass? For instance, a photon [a particle of light, including both visible and invisible forms of light] has zero rest mass. [Note, however, that it has non-zero gravitational mass]. Meanwhile electrons and positrons [the anti-particles of electrons] both have positive rest mass. So what do people mean when they say “A photon can be an electron-positron pair part of the time”? This statement comes with a fancy “Feynman diagram”, in which the photon is shown as the wavy line, time is running left to right, and the loop represents an electron and a positron created from the photon.

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The Amazing Feat of Quantum Tunneling

Our quantum world has many odd and counter-intuitive features.  One of these is “tunneling” — the ability of objects to pass through walls, escape from traps, and slip under mountains into the next valley.   We don’t encounter this effect in daily life; objects we’re used to are so incredibly unlikely to tunnel from one … Read more

Electrons and Their Properties

I’ve been quite busy with some physics research this week, but I have nevertheless managed to finish a new article on electrons, part of my Structure of Matter series, which aims (among other things) to introduce a non-expert to particle physics, step-by-step.  The completion of this article feels like a significant step for this website.  … Read more

Additional Atomic Articles

I’ve been adding to my series of layperson’s articles on The Structure of Matter, which eventually will serve as an introduction to particle physics for those coming to this site for the first time. You might recall that in early December I supplemented my older article on molecules with an article on atoms. I got … Read more

New Attempt at Atomic Article

It took me over six months, following my article on molecules, to write the sequel, on atoms. These are just two in a series, intended to introduce the structure of matter to novice readers who want to learn what particle physics is about.  Atoms aren’t the main focus; future articles will focus on electrons, on … Read more

New Article on Atoms

I have a number of loose ends to tie off on this site, and one of them is my set of articles for novice readers about the structure of matter, which when complete will introduce the basic particles out of which we and all of ordinary matter are made.  A while ago I wrote an … Read more

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