Hot answers tagged

207

Ah, I know this one! What's in a proton? A proton is really made of excitations in quantum fields (kind of like localized waves). Remember that. Any time you hear any other description of the composition of a proton, it's just some approximation of the behavior of quantum fields in terms of something people are likely to be more familiar with. We need to do ...


44

What is the experimental evidence that the nucleons are made up of three quarks? Some strong pieces of evidence for the quark model of the proton and the neutron, not stated in another answer, are the magnetic moment of the proton and the magnetic moment of the neutron, which are consistent with the quark model and are inconsistent with the magnetic ...


43

You can't consider a proton just as three quarks (called valence quarks, because they determine the quantum numbers) because virtual quarks and antiquarks are constantly being created and anhilated via strong force. So a proton is more like a quark sea. In fact, this process gives most part of the proton's mass (the valence quarks are just the 2% of the mass)...


37

A neutron is not "a proton and an electron". A neutron is not composed of a proton and an electron inside of the neutron. In quantum mechanics, particles can appear and disappear or change into other particles. With the neutron, one of the down quarks can decay change into an up quark by emitting a W boson, turning into a proton. The W boson quickly decays ...


35

The idea that baryons contain three quarks is a significant oversimplification wrong. It works for some purposes, but in this case it causes way more confusion than it's worth. So you should stop thinking of baryons as groups of three quarks and start thinking of them as excitations in quantum fields - and in particular, excitations in all the quantum fields ...


33

A free quark is like the free end of a rubber band. If you want to make the ends of a rubber band free you have to pull them apart, however the farther apart you pull them the more energy you have to put in. If you wanted to make the ends of the rubber band truly free you'd have to make the separation between them infinite, and that would require infinite ...


33

Note that the original SU(3) quark model was entirely mathematical (The Eightfold Way) and was a brilliant way to explain the observed spectra of baryons and meson. The whimsically named quarks were not intended to represent real objects. Per @Geoffrey's answer, it was Deep Inelastic Scattering: $$ e^-+p \rightarrow e^-+X $$ often written: $$ e(p, X)e' $$...


26

This is covered by a few existing answers (see for example About free quarks and confinement) though surprisingly it doesn't appear that anyone has asked this exact question before. Anyhow, the answer is that the colour force is mediated by particles called gluons just as the electromagnetic force is mediated by photons. The difference is that while photons ...


25

Quarks do not violate quantization of charge, it's simply that $\frac{1}{3}e$ instead of the electron charge $e$ is the smallest unit of electric charge.


25

Mesons are not elementary, they are composed of quarks. So take a look at their quark content. The charmed eta meson consists of a charm and an anti-charm quark, denoted $c\overline{c}$. An anti charmed eta meson would therefore be an anti-charm and an anti-anti-charm (which is just a charm) quark, i.e. $\overline{c}c$, which is obviously the same as $c\...


24

The question you are asking has been answered in terms of popularized description. The real physics picture is not simple and depends a lot on a number of experimental measurements by many experiments. If you look at figure 9.18 of the link you will see that the composition of the proton changes according to the momentum transfer from the probing particle. ...


21

You say: Now, when we talk about energetically favourably bound systems, they have a total mass-energy less than the sum of the mass-energies of the constituent entities. and this is perfectly true. For example if we consider a hydrogen atom then its mass is 13.6ev less than the mass of a proton and electron separated to infinity - 13.6eV is the binding ...


20

The up quark has a charge of $+2/3$, the down has a charge of $-1/3$. If you have a bound state of charged particles, the total charge is just the charge of the elementary constituents. The neutron consists of one up quark and two down quarks, so the total charge $Q$ is: $$Q = 2/3 + 2 \times (-1/3) = 0$$


20

A neutron isn't a proton and an electron. The reaction involved in beta decay is $$n \to p + e^- + \bar{\nu}_e$$ where $\bar{\nu}_e$ is an electron anti-neutrino. But even that doesn't mean a neutron is a proton plus an electron plus a anti-neutrino. It means that a neutron's quantum numbers are the same as a state consisting of a proton an electron and ...


20

Thanks for finding this amazing historical video. He's talking about the deep inelastic scattering electron proton experiment at SLAC. This showed evidence that high energy electrons scattered off pointlike charged particles within the proton, which Feynman named 'partons'. It took some time to establish that these partons are the same as quarks, which had ...


18

John Rennie's answer is good, just a few words to add on 'single top quarks'. The mental image of rubber bands works fine at low energies. Maybe you've heard about mesons or baryons, or simply particles like the pion. These are 'bound states of quarks', i.e. what happens when you break the rubber band and create a new pair of quarks. Then a quark + anti-...


16

This is indeed a problem but not for the reason you think. If the electron obeyed classical mechanics and it was only subject to electrostatic attraction to the nucleus, it would never fall into the nucleus despite the fact that it would be constantly attracted to it. This is exactly analogous to why the Earth doesn't fall into the Sun: it has too much ...


16

Quarks as we know them are fundamental particles, which means that they do not have smaller constituents. This however does not imply that they cannot decay. A particle in quantum field theory does not need to have constituents to decay into, it can in principle decay into any particle its corresponding field couples to (interacts with), as long as it obeys ...


16

You say: a zillion gluons and quarks and anti-quarks self annihilating and popping into existence and while this is a very common way to describe the interior of a hadron like a proton it is actually rather misleading. Nothing is popping into existence then disappearing again. But explaining what is actually happening is a little involved. Our current best ...


15

The model you are thinking about is really rudimentary and cannot explain the dynamics of Quantum ChromoDynamics, QCD . In this link there is a better exposition of what a proton is, within QCD. You may have heard that a proton is made from three quarks. Indeed here are several pages that say so. This is a lie — a white lie, but a big one. In fact ...


15

No, muons can't decay into quarks because quarks are confined; the final product cannot be quarks, but rather composite particles made of quarks, such as mesons and baryons. The lightest mesons are the pions, which are already heavier than the muon, so any such decay is forbidden by energy conservation. On the other hand, the extremely heavy tau can and ...


14

Color charge in the sense of "being blue, red, green" is not a quantum mechanical observable because the $\mathrm{SU}(3)$ gauge transformations mix the colors. This means it is meaningless to say "We have a blue particle", because we can perform a gauge transformation and then we "have a red particle". Since physical descriptions related by gauge ...


14

Yes, there are the quantum numbers Charm, Strangeness, Topness and Bottomness, which are conserved by strong and electromagnetic interactions, but not by weak interactions. Upness and Downness are simply the Isospin, which is also preserved for strong interactions, when the quark masses can be neglected, which is usually a very good approximation as $m_u,m_d\...


14

In the first stages of the Universe Quarks and Gluons were asymptotically free. This state of matter is called Quark-Gluon Plasma. Then, as the temperature of the Universe kept decreasing, the so-called hadronization (quarks combine to form hadrons) took place. The coupling constant of the QCD (which, to make it simple, sort of represents the intensity of ...


13

The "resources" linked in the post are bad. But there was a time when serious people were interested in the possibility that quarks have integer charges. Han and Nambu introduced the idea, Pati and Salam made a gauge theory of it, Witten suggested how to test it, and this was done at CERN in the 1980s (see page 11). There would be several ways in which the ...


13

Yes, the 6 antiquarks are antiparticles of the 6 quarks – in other words, they're particles of "antimatter". The word "antimatter" sometimes represents just a relative label – antimatter of something (antimatter of antimatter is matter again), sometimes it means the antimatter of the particles we routinely see in the world around us. Because the 6 antiquark ...


Only top voted, non community-wiki answers of a minimum length are eligible