A long time ago I read an article where one of the experiments they talked about involved whipping nuclei I think, around in an accelerator. Just from that, it sounds like something at RHIC, but I don’t know.
I can’t remember how it came up, but one of the physicists said something like ‘oh yeah, the quarks are only in the nucleus about a third of the time.’ Now maybe he said protons and neutrons, so I might be wrong on that score, but either way, do you have any idea what they could have been talking about?
I got the impression that he actually meant that they ceased to exist in this reality for that time, but that memory is very shaky. It also seemed that this was at least something of a revelation, but at the same time, no one seemed particularly unsettled by the idea.
Is this complete bullshit that I just think I remember or does this actually relate to some aspect of some theory? I know this is pretty vague, but this is one of those things I tend to recall over and over so I’d really like to know, but there’s no way, no way obvious to me, for me to research it. Thanks for any responses.
I don’t know what the physicists were referring to, but quarks do not step out for lunch. The closest I can think of is that they were talking about “parton distribution functions,” which describe how likely it is to “see” a given type of quark when you scatter off the insides of a proton. For example, a proton is made of two up-type quarks and a down-type quark. These are called the “valence” quarks, and they are always there (no lunch). At low energy when you collide with the proton, you might collide with the up-type quark roughly 2/3 of the time, or the down-type quark roughly 1/3 of the time, because 2/3 quarks are up-type and 1/3 quarks are down-type. At high energies, the insides of a quark start to look like a frothing sea of quark-anti-quark pairs, and you will often collide with the “sea quarks” (also called “partons”) instead of the valence quarks. This means that you will have some % chance of scattering off an up-type, down-type, anti-up, anti-down, charm, anti-charm quark, etc.
When you said that there is a group of multiple (frothing) quarks at high energies, can I assume that the additional mass imparted by relativistic speeds temporarily emerges as new particles? Or did I just get completely wooshed?
“Parton” just means “quark”, but is often used to distinguish between “sea quark” and “valence quark.”
That is basically right about the “multiple (frothing) quarks at high energies” (ie partons). Quarks are bound together by gluons, which are very messy. You’ve got three quarks bouncing around, stretching and pulling against a messy gluey web of gluons pulling them back and holding them together. If they have enough energy, gluons can “split” into quark-anti-quark pairs (the same way a photon can “split” into an electron-positron pair). So if a proton is going fast enough, the sticky web of gluons are constantly frothing into quark-anti-quark pairs, which means that if you scatter off the inside of a proton, you might be as likely to scatter off one of these “sea quarks” (partons) as the 3 “valence quarks” that you started out with.
OK. Apologies if I’m thick but I think I’ve got it. The extra mass is expressed by spontaneous generation of q/anti-q pairs from gluons rather than the zero point field, as I’d imagined. These are the sea quarks. Frothing is due to a continuous cycle of generation and mutual annihilation. For whatever time a sea quark may exist before annihilating, it can interact with other particles (I would imagine it’s in someway similar to Hawking radiation in that sense ). The valence quarks which had previous been bound to the gluons are now also free to interact as an independent entity.
One thing I’m curious about though. I’d heard that most of the mass of a proton is bound up in the attractive force quantized as gluons. So would it be correct to say that since each of the three gluons generates 2 quarks of the same or similar mass to the valence quarks, that therefore the gluons account for 2/3 of the mass?
Also, is there any relationship between 2/3 of the mass being gluons and the 2/3 probability of an up-quark collision?
Finally, did I understand correctly that any type of quark pair can be a sea pair? If so, shouldn’t there also be some probability of a charm or strange collision, or are they too massive even with the additional energy.
I do eventually get tired so I won’t torture you too much longer.
IANAPhysicist, but from what I understand, once you get into the range of a few trillion degrees (or sufficient density), the strong interaction weakens enough so that quarks, and their color-charged gluons become a quark-gluon “plasma”, as these building blocks of baryonic matter and gauge bosons of the strong nuclear force (the color force of QCD) become asymptotically free.
The LHC has confirmed quark-gluon plasma, although, it’s behaving a bit different than expected — like a viscous metallic liquid. I believe there’s a lot more research and experimentation to be had though. As to whether it’s similar to Hawking radiation, I can’t say. HR radiates as a blackbody, I’m not sure how a QGP would behave in that sense.
Again, IANAP, but is it correct to state that gluons are responsible for a baryon/hadron’s mass? I know that the 3 color charges of the quarks that make up any of the hadrons (mesons, and baryon/anti-baryons), need to cancel each other out (i.e. a red-up, a green-up and a blue-down combine to make “white,” or a neutral color charge).
This is where things become hazy… I’ll step aside.
That’s correct. The only thing I’m uncomfortable with is your phrase “the extra mass is expressed…”. The thing to keep in mind is that there is no increase in rest mass of a moving proton, because if you slow it down, the sea quarks go away. Sea quarks are a very reference-frame dependent quantity. Yes, when viewed in a reference frame in which it is moving quickly, a proton will have a lot of energy with the potential to be converted into rest mass (if it is scattered off something such that the center-of-mass energy is large enough), but it is misleading to “jump the gun” and talk about the masses of particles with don’t yet exist until you interact with them. I think you are thinking about relativistic mass, which is a misleading and not very useful concept, IMO. In place of your statement I would say “if you scatter off a proton with a large CM energy, some of that energy can be used up in promoting virtual fluctuations in the gluon field into real particles.”
I’m not sure what you mean here, so I’m not sure it is correct. If you scatter off a sea quark, the valence quarks still exist, and are still bound together. In practice, the scatter off a valence quark will transfer some momentum non-democratically among the valence quarks, perhaps leading them to separate from each other before recombining back into other hadrons.
No, although I don’t really follow your argument. There are 8 types of gluon, not 3, and there are basically an infinite number of them inside a proton, with varying energies, with energy-dependent probabilities to generate quark-anti-quark pairs. So there is no such simple relationship. Much of the mass of the proton is bound up in the gluon field, yes. Gluons do not individually have mass, but they have energy, so taken collectively they can account for a lot of the mass of the proton. Theory doesn’t currently have a clear prediction of the exact contribution, because the calculations are very difficult (QCD is non-perturbative at low energies).
At low energies there is near a 2/3 probability of an up-quark collision because there are 2 up-quarks, and 3 quarks total. But I just threw out the “2/3” number as a simple example. It’s not really that simple, because the probabilities depend crucially on the momentum transfer of the collision. For very low momentum transfers, you are more likely to interact with the sea quarks.
Exactly. There is a probability of a charm or strange collision, yes, but since those quarks are more massive, they requires more energy to be created. The probabilities are correspondingly lower, but definitely relevant at high energies.
Thank you for your response. I had thought that the Relativistic Heavy Ion Collider at Brookhaven created such a plasma, but I’m not certain. The part about color is a bit beyond me right now. I did check the wiki entry and bookmarked it though.
That’s precisely what I was doing. As a result, I have to accept the fact that I am probably a few fundamental concepts short of a passable understanding. It’s probably a bit pretentious for me to even make the attempt given the complexities involved. But from time to time I’m possessed of enough info that I get lucky. This not one of those times, but I’ve understood enough that I’m happy to have made the attempt.
It’s not really important. I was imagining that in their frothy state gluons wouldn’t be able to exert as much of an attractive force. But as I said earlier, it seems certain that I’m missing some fundamental concepts.
Again, this isn’t that important. I just meant to talk about the 3 gluons in a given proton. I should have been clearer. That last statement is quite interesting. That’s one of those ideas that I will keep tucked away. I’m not sure I really understand perturbative and non-perturbative. I think I get the idea. There are no known solutions to work backward from.
OK, I think I have this narrow concept covered – loosely speaking of course.
Oh dear. This is an example why further efforts on my behalf may be fruitless. I would have thought precisely the opposite. In fact, I would have said the sea quarks didn’t even exist until there was sufficient energy. As you observed, I was attributing all of the phenomena to relativistic energy, and now I see that it’s not. But I also don’t think I know enough for any explanation of what exactly it is to be useful. I’m still quite happy though and again, that you very much.
I think you’re doing alright. I applaud your curiosity.
I assume you mean “3 quarks in a given proton” here. There are more than 3 gluons.
Ops, you caught me in a semantical blunder (see, you’re not doing so bad). At least, I should have done a better job of explaining what I meant. Sea quarks are indeed more common at higher momentum transfer, but it is harder to achieve that higher momentum transfer by colliding off of sea quarks because, for a given momentum transfer, sea quarks have a smaller probability of carrying a substantial portion of the proton’s momentum. What this means is that if you want to collide with a sea quark, you want your proton to have a very high momentum, because you will only find a sea quark if the momentum transfer is large, and the momentum transfer is likely to be small for a sea quark. If this sounds circular, it is because it is! At the end of the day it’s complicated, which was the point I was trying to make.
Think about it this way: you have three quarks in a proton, each carrying about 1/3 of the momentum of the proton. What if, in addition to the 3 valence quarks, you have N sea quarks in the proton? Is the momentum split equally between them all? Not quite. It turns out that the valence quarks get most of the momentum, which means that large momentum transfers are most common for valence quarks. But as the proton’s momentum increases, you can get large momentum transfers even for sea quarks, even if the fraction of the proton’s momentum that is transfered is small. So to re-phrase my prior statement: for very low momentum transfers (as a fraction of the proton’s total momentum) and very high proton momentum, you are more likely to interact with the sea quarks. For low proton momentum, you can’t have high momentum transfer, and so you don’t see sea quarks. For high proton momentum, you see sea quarks, but are unlikely to see sea quarks at high momentum transfer. I like saying “see sea quarks”. It’s fun.
It’s all attributed to energy. But energy is reference-frame dependent. In one reference frame a proton is moving fast. In another it is stationary. Both frames are correct. It’s important to remember that in particle collisions, the only thing that matters is the center-of-mass energy, which is reference-frame independent.
In fact, the number of gluons isn’t even well-defined. Two observers in different reference frames could even look at the same proton at the same time and count different numbers of gluons.
). The valence quarks which had previous been bound to the gluons are now also free to interact as an independent entity.
