# Tag Info

148

Ah, I know this one! What's in a proton? A proton is really made of quantum fields. 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 this because quantum fields behave in very ...

39

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 ...

24

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.

22

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 ...

20

I cannot resist this mother goose quote: What are little boys made of? What are little boys made of? Frogs and snails, And puppy-dogs' tails; That's what little boys are made of. What are little girls made of? What are little girls made of? Sugar and spice, And all that's nice; That's what little girls are ...

19

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: $$Q = 2/3 + 2 \times (-1/3) = 0$$

18

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 ...

18

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. ...

18

Dear qftme, I agree that your question deserves a more expansive answer. The answer, "pions" or "gluons", depends on the accuracy with which you want to describe the strong force. Historically, people didn't know about quarks and gluons in the 1930s when they began to study the forces in the nuclei for the first time. In 1935, Hideki Yukawa made the most ...

16

No, the elements of the periodic table don't form any representation of a group or, more precisely, any irreducible representation. Even more precisely, the real insights by Mendeleev – that the reactivity etc. is a repeating function of the atomic number – doesn't follow from any property of a representation that could be derived by group theory. The ...

15

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 ...

13

I think it's bizarre that a particle doesn't have a definite composition. Yeah, it is. As qftme said, that's quantum mechanics for you. It really doesn't make sense until you immerse yourself in the subject for long enough (and even then, only somewhat). But it does appear to be the way the universe works. Anyway, just so everyone is on the same page, ...

12

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 ...

12

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 ...

11

The centripetal acceleration that the protons feel as they circulate in the LHC is roughly: $$a = \gamma^2 \frac{v^2}{r}$$ This is the usual equation for centripetal acceleration but multiplied by a factor of $\gamma^2$ to allow for the time dilation the protons experience. The speed $v$ is approximately $c$. The radius of the LHC is about 4.3km but the ...

11

The real problem here is that when things get really, really small, they don't behave like the world we see around us. That can make a lot of what goes on in that weird world quite hard to grasp. The diagram is misleading. Protons aren't really round, grey blobs, and quarks aren't really little spheres that sit inside them. Down at the subatomic level, ...

11

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 ...

11

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 + ...

11

The first conclusive evidence for structure within the proton/neutron was from deep inelastic scattering. This shows there is structure within the particles that matches what we expect from quarks. As Pranav says, it's possible that something else may be going on that just looks like quarks at the energies we can generate, but this seems unnecessarily ...

10

There are three flavours of quarks in the fundamental $3$ representation of $SU(3)$, the QCD gauge group. Their antiparticles are in the conjugate representation $\bar3$ or $3^\star$. QCD is confining; the quarks form bound, colorless states, which are singlets in $SU(3)$. Mesons are $q\bar q$. The general tensor $3\times\bar 3$ can be decomposed into ...

10

Briefly, a hadron has to be color singlet ${\bf 1}$ under the $SU(3)_C$ color gauge group, due to color confinement. Examples: A single quark $q$ transforms in the fundamental representation ${\bf 3}$ of $SU(3)_C$, and is hence not allowed. See also related Phys.SE post here. A diquark $qq$ belongs to the tensor representation ${\bf 3}^{\otimes 2}:={\bf ... 9 That is a good question but I think you might be a bit confused. The quark charges are quantised as they are fractional values of the electron charges, so when you refer to 2/3 and -1/3 these mean 2/3 of the electron charge and -1/3 of the electron charge respectively. As such, a Hydrogen atom with a proton in the nucleus and an electron in the shell, is ... 9 Color charge in the sense of "being blue, red, green" is not a quantum mechanical observable because the$\mathrm{SU}(3)$gauge transformations mix the colors. This means it is meaningless to say "We have a blue particle", because we can perform a gauge transformation and then we "have a red particle". Since physical descriptions related by gauge ... 9 The "resources" linked in the post are bad. But there was a time when serious people were interested in the possibility that quarks have integer charges. Han and Nambu introduced the idea, Pati and Salam made a gauge theory of it, Witten suggested how to test it, and this was done at CERN in the 1980s (see page 11). There would be several ways in which the ... 9 Although there is no known group representation which encapsulates all the properties of the periodic table, there are, however, attempts to gain a representation theoretical understanding of the periodic table at least qualitatively and there are recent works mainly by M. Kibler in this direction, please see the following two articles ... 9 You didn't understand any of these questions right. Antiquarks and their bound states, including the antineutrons, are produced and observed as easily as bread and butter. Lots of details experiments with e.g. antineutrons have been performed, e.g. Scattering of antineutrons with hydrogen ... 8 What do you think a$\Delta^+$or$\Delta^0$is, if not an excited nucleon? (To be sure the$^{++}$and$^{-}\$ states do not correspond directly to a nucleon, because there is no allowed lower spin state with that valence content.) The thing I am not sure about is how closely these excitation match the ones you are envisioning. They match nuclear excitation ...

8

That would be an anti-Xi baryon. Production of Xi / anti-Xi pairs in discussed in Branching Fraction Measurements of psi(2S) Decay to Baryon-Antibaryon Final States. So yes it does form something and it is a fault of the applet.

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