# Tag Info

112

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 ...

34

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 ...

18

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

15

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 ...

14

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. ...

13

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 ...

12

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 ...

12

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

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 ...

10

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 ...

10

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

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, ...

9

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 ...

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.

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

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 + ...

8

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 ...

8

The proton ($uud$) turns into a neutron ($ddu$). Up and down quarks don't have equal charges; the up is $+\frac{2}{3}e$ and the down is $-\frac{1}{3}e$. By the way, such an operation has a name - isospin symmetry transformation - corresponding to an approximate SU(2) symmetry that makes the proton and neutron have almost similar masses.

7

Up and anti-up. Or down and anti-down. Funny thing is, both of those have the exact same quantum numbers - parity, spin, baryon number and the rest. So a neutral pion can be a mixture of (u + anti-u) and (d + anti-d). There actually result two types of neutral "pion" that decay differently. One is actually heavier, and we call it the eta meson. ...

7

For the light quarks, one can use chiral perturbation theory to relate the mass of the light hadrons to the mass of the light quarks. These two links give details and caveats of the procedures, as well as the most precise determinations: http://pdg.lbl.gov/2011/reviews/rpp2011-rev-quark-masses.pdf ...

7

No, quarks couldn't turn out to be non-existent anymore. The evidence that quarks exist involves more than just some playful games in which we add electric charges to construct hadrons. Quarks show up in many processes. Historically, the important experimental observation was that of the deep inelastic scattering. Much like Rutherford observed the nucleus ...

7

The answer to your question, is yes, it has indeed been considered. The bound state has even been given a name "monopolium". Here is a paper discussing prospects for detection and production. I should add the caveat that they're not strictly, in your words "confined together like quarks". You could separate them if you input enough energy, unlike the ...

7

Background (skip this if you know it all)! I too worried about this when I first learned it. Basically I think it's easiest to think of the Eightfold Way quantum mechanically first and worry about QFT later. So that's what I'll do in this answer. In quantum mechanics (at least according to Wigner) a particle is a basis vector in some representation of the ...

7

The names up and down don't refer to electric charge $Q$ but are rather references to isospin charge $I_3$.

7

When it comes to fundamental charges, the (left-handed) up-type quarks actually have either the same values of the charge as the down-type quarks, or exactly the opposite ones. It just happens that the electric charge isn't a fundamental charge in this sense. Let me be more specific. All the quarks carry a color – red, green, or blue – the charge of the ...

6

A neutron consists of three quarks $u d d$(up down down quarks). The up quark(u) carries charge $2e/3$ and the down quark(d) carries a charge $-e/3$. Thus $2e/3-e/3-e/3=0$

6

It doesn't matter whether the $b$-quark is highly energetic, it can never decay to a top quark and a $W$-boson if it is on mass shell, by which I mean, $p^2=E^2 - \vec p^2 =m_b^2$. To see this, consider energy-momentum conservation, $$b^\mu = W^\mu + t^\mu \Rightarrow m_b^2 = M_W^2 + m_t^2 + 2W\cdot t = M_W^2 + m_t^2 + 2 E_t M_W$$ However, since the energy ...

6

There may be too many questions here. I'll try to hit some of the high points of the technologies, but be aware that you could write an entire dissertation on the matter (mine was on a closely related topic). The incoming electron or proton beam is characterized by using current monitors (inductive, resonant cavity, charge cups, etc) and by measuring its ...

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