# How do we know the electrical charge of individual quarks?

If it’s impossible to isolate a quark, how can we measure it’s charge?

Thanks,
Rob

We define a proton to have charge 1 and a neutron to have charge 0. Since a proton contains two up quarks and one down quark, and a neutron contains one up quark and two down quarks, you can algebraically calculate that down quarks should have charge -1/3 and up quarks should have charge 2/3.

How to we know that quark charge is constant over time?

If it isn’t, that affects everything we know in physics. Literally world-changing.

People occasionally make claims that values change over long periods of time or distance (one of the explanations for dark matter is a change in gravity, e.g. - see MOND) but none of these has been confirmed and they are considered quite unlikely. No mechanism for such variability is known either, making it exponentially harder to prove a claim that charge varies.

Does the Standard Model mathematically require that quarks have to have the charges they do, or is that an empirical observation based on the charges of hadrons?

You can measure a lot about a thing without “isolating” it. When you smack a proton or neutron hard enough, you see distinct signatures of the pieces inside. A not-that-terrible analogy might be trying to figure out what’s inside a burlap bag without taking anything out. To make the analogy even more like particle physics, say your options are “it’s a bag of bowling balls” or “it’s a bag of melons”. You could fire an appropriate caliber weapon at it and study the behavior of the bullets to readily distinguish between these two cases.

Since we define quark charge to proton charge, and apparently we calculate it algebraically from proton charge (+1) and neutron charge (0), the numbers have to stay the same. Whether or not proton charge stays the same over time, it’s pretty fundamental that it remain equal and opposite to electron charge (or else large bodies in the solar system and the universe as a whole) will accumulate positive or negative charge).

Given that there’s no such thing as a naked quark it might help if you try not to think of quarks as “particles”.

My understanding is that there is some basis for the idea that quarks actually exist as physical things, but they were originally proposed as simply a way of assigning properties to particles.

So…quarks could be considered particles, but they’re not particles the same way protons and neutrons and electrons are particles. They’re a different sort of thing.

Oh, they exist. Their confinement at low energy is a minor inconvenience in establishing their reality. Of note, the property of asymptotic freedom means quarks are largely freed from their hadronic shackles in high energy collisions. Also, the production of quarks via the decays of heavy particles or in high energy collisions provides copious information about individual quarks. The situation is far from “we can never see these things”.

It is correct that they were thought perhaps to be a bookkeeping device when first proposed a half-century ago, but that can be said of countless theoretical leaps in physics.

There was an article on Of Particular Significance with plots that only make sense if individual quarks/gluons within a proton interact in a proton-proton collision. Which is neat for proving the actual existence of these things.

Is the quark charge just a matter of book keeping? Also, charges on two quark particles (are they called mesons?) are all integer charges, correct?

Would it violate known physics if the charge inside a proton or whatever were fluid, moving from quark to quark as long as the total charge was +1 (or whatever)? Or is it a matter of applying Occam’s Razor?

Thanks,
Rob

Yes, mesons are two-quark particles, or more precisely, one quark and one antiquark.

Quarks all have either +2/3 or -1/3, and antiquarks all have either -2/3 or +1/3. Any possible arrangement of three quarks or three antiquarks, or any possible arrangement of one of each, will therefore always have an integer charge. And the requirement of three of a kind or one of each is enforced by color neutrality: Any particle which can be isolated must have equal amounts of red, green, and blue.

So the “pentaquark” is possible because 5 = 3+2?

Edited- Which means it can have color neutrality. But still no fractional charge.

Can we tell the color charge of the individual quarks? In a meson, can we tell if it’s blue-antiblue or red-antired?

One of the quirks of the colour force is that the mediators of the force, the gluons, themselves have colour charge. So inside composite particles, the quarks are changing colours all the time by emitting/absorbing gluons.

Heck, the quarks composing a particle are sometimes even changing flavor. For instance, a neutral pion can be either an up and an anti-up, or a down and an anti-down, and it’s not typically well-defined which one it even is.

Other than the math working out, have we confirmed the charges on quarks, or is it just a matter of applying Occam’s Razor? Could the charge on various quark “molecules” be an emergent property rather than the net charge of the individual quarks, or is this ruled out by known physics?

Thanks,
Rob

Have quarks ever been blasted out of a nucleon in a high-energy accelerator collision? If we’d ever seen one’s trajectory in a magnetic field, using modern detectors the way we used to use bubble chambers, we’d know its charge by the deflection.

Are they bound so tightly, they’ve never been knocked away on flights of their own?

This, more or less. You can put enough energy into a proton to knock one of its quarks out, but in doing so, you’re also putting in enough energy to make a new quark-antiquark pair, and so what you end up with is a pion flying one way and a new baryon flying the other. You could knock a quark a small distance with less energy, but that distance is so small it’d never show up as a separate track in any detector we can build.

This manifests in a detector as a hadron jet. A collision that knocks a quark out of a proton tends to produce a whole slew of hadrons, not just one.