The Future of Subatomic Glue-Gazing

RHIC, the Relativistic Heavy Ion Collider at Brookhaven Lab, found it first: a “perfect” liquid of strongly interacting quarks and gluons – a quark-gluon plasma (QGP) – produced by slamming heavy ions together at close to the speed of light. The fact that the QGP produced in these particle smashups was a liquid and not the expected gas, and that it flowed like a nearly frictionless fluid, took the physics world by surprise. These findings, now confirmed by heavy-ion experiments at the Large Hadron Collider (LHC) in Europe, have raised compelling new questions about the nature of matter and the strong force that holds the visible universe together.

Similarly, searches for the source of “missing” proton spin at RHIC have opened a deeper mystery: So far, it’s nowhere to be found.

To probe these and other puzzles, nuclear physicists would like to build a new machine: an electron-ion collider (EIC) designed to shine a very bright “light” on both protons and heavy ions to reveal their inner secrets.

“An electron-ion collider would be the brightest, highest-intensity ‘femtoscope’ to shine on the structure of matter,” said Brookhaven theoretical physicist Raju Venugopalan, referring to its ability to discern structures at the scale of femtometers – that’s 10-15 meters, a millionth of a nanometer, or a millionth of a billionth of a meter!

“Snapshots” of matter at that scale over a wide range of energies would offer deeper insight into the substructure of the nucleus, its constituents, and particularly its smallest components, the quarks and gluons and how they interact.

“Increasingly, it’s looking as if gluons and their interactions may hold the keys to many of our puzzles,” Venugopalan said. An electron-ion collider would be the ideal tool for gazing at the “glue” under conditions where scientists believe that it completely dominates the structure of neutrons, protons, and nuclei.

Gluon Timeline

Evolution of physicists' understanding of proton spin: from the early view that the spins of the proton's three quarks should make up most if not all the proton's spin (left), to one in which gluons and the motion of quarks and gluons can also play significant roles (center). Current and planned investigations of the angular motion of quarks and gluons — the latter to be carried out at by an electron-ion collider (right) — may help resolve the mystery of the missing source of spin.

If an electron-ion collider becomes a reality, what the physicists learn will offer deeper insight into what holds 99 percent of the matter in the visible universe together. That’s the percentage of everything we see around us – from stars to planets to our own physical forms – that gets its mass from protons and neutrons, and thus ultimately from the quarks and gluons governed by the strong force.

“At the most fundamental level,” Venugopalan said, “we are driven by our curiosity to learn more about what we are made up of.”

Much more about the physics behind an electron-ion collider and unraveling exciting mysteries on the horizon can be found in this feature story at Brookhaven’s website. Anyone interested in relativistic time dilation, missing spin, and super-saturated color glass condensate should check it out.

-Karen McNulty Walsh, BNL Media & Communications Office

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In the links, you say, "Eventually, a balance is reached between gluons emerging from the vacuum and those recombining. The physicists call this steady state of maximal gluon concentration gluon saturation, or color glass condensate."

Should that balance "between gluons emerging from the vacuum and those recombining" happen at low as well as high energy? And are there any experiments or experimental approaches that might be able to explore low energy gluon plasmas?

What causes the gluons to so to speak emerge from the vacuum? (e.g. do colliding quarks or what) And when gluons emerge from the vacuum do they emerge in gluon/antigluon pairs or triplet of pairs of each color/anticolor)?

I'm not looking for definite answers. Rather I am interested in what people are thinking on these matters. Thanks.

Raju Venugopalan, theoretical physicist at Brookhaven Lab, responds: The question is a good one. The gluons we speak of are confined within protons and nuclei and indeed give them most of their mass. At very high energies, quarks radiate gluons, which radiate even more - a veritable ladder of gluons. Each rung of the ladder can emit more gluons. The ones furthest along the ladder "know" least about the quarks that generated them and are therefore universal to all matter. It's these gluons that form a Color Glass Condensate. Such a large gluon concentration can only be probed at high energies because fluctuations of protons and nuclei into states containing such large numbers of gluons live only for very short times.

Thank you Justin and Raju.

Let me summarize my understanding and ask another question. It's OK if you don't answer. I know you are busy.

"The gluons we speak of are confined within protons and nuclei and indeed give them most of their mass." Yes, yes.

"At very high energies, quarks radiate gluons, which radiate even more – a veritable ladder of gluons. Each rung of the ladder can emit more gluons." OK I understand this; but did not think of it as a ladder. Very nice. And I assume each rung of the ladder, the gluons are less energetic; which is why you must start with very high energies; to be able to generate many rungs i.e to get enough gluons to generate a color glass condensate. Yes, very nice.

But you say, "The ones furthest along the ladder “know” least about the quarks that generated them" OK so far, but then you say, "and are therefore universal to all matter."

Let me repeat, and paraphrase, "And the gluons of lowest energy, at the furthest rungs of the ladder 'know' least about the quarks and are therefore universal to all matter all elementary particles." But in this paraphrase, perhaps I've already speculated beyond Raju's speculation. I hope not.

And I don't want to discuss my speculations yet (which I have) because I am not a professional physicists. But I wish to understand Raju or others understanding or speculation. So before asking my next question, I went to arxiv and read Raju et all's paper, Instability induced pressure isotropization
in a longitudinally expanding system.

Proceeding into my questions.

1) bottom page 2, "we firrst considered a much simplerfieeld theory in [64, 65], a scalar field theory" . OK, this is important to me. I assume this statement means that QCD is a vector or tensor theory; but do computational complexity that a simpler "scalar field theory" is used as an approximation. Is my understanding of the paper's meaning correct.

2) pg 5 last paragraph of 2.1 "The interaction term... that we neglected... However this modication is negligible
at short times.. Therefore the solution that we find by neglecting
the interaction term remains valid in the immediate vicinity of the origin." OK, you through out the terms away from the point of collision (i.e. the origin) Now it would seem to me that this ladder of gluons extends in spacetime beyond the point of origin. And for you perspective maybe a very long ladder is still "in the immediate vicinity of the origin." I assume that's what you mean and that the entire color glass condensate is "in the immediate vicinity of the origin." Is this your meaning? Have I understood correctly.

I also think I understand a little bit, pg 10, sevction 3.3 ". One can see that, for the times that are common to the two
evolutions, these physical quantities do not depend on the time at which the system is initialized. One should add a word of caution here..."

pg 13 says, "If we were to pursue the time evolution much beyond this time, the system would effectively become 2-dimensional and we would get the incorrect result PL(longitudinal momentum) is much much less than PT (transverse momentum, i.e. momentum perpendicular to line of collision)" No, no, no. Novice as I am. This makes no sense. I expect PL to be much much less than PT! But I'm not to my question yet.

OK I see figur 6 pg 17, I see convergence of PT and PL. I must assume your calculation is correct. Hmm, maybe this fits into the way that I'm thinking.

Let me paraphrase what figure 6 means. Your calculations involved a longitudinal boost because the gluons (within each nuclei before collision) were moving towards each other relativistically; but the point of collision has to be boosted (so to speak to a classical standstill). So right at the origin ( t=0) the PL and PT are not equal. PL has been boosted to zero whereas PT of the accllerated gluons (which were in equilibrium in L and T directions in the accelerated gluons before collision) have retained their high energies just after collision. As time goes on after collision, the color glass (i.e. of gluon) condensate moves up the ladder (statisically so to speak) and thermolly PT and PL converge to the same value (because there no preferred direction as equilibrium after the collision is reached). That's my understanding, hopefully somewhat correct.

But important to my thinking.the energy/momentum tensor of the color gluon condensate seems to be diminishing with time. i.e. redshift in a sense. This may lead to a question. but first.

pg 17 point iii doesn't mention redshifting in general; only point i mentions early time longitudinal redshift. So perhaps I misunderstand in my fuzzy way of thinking. Continuing.

pg 19 section 6 "The main new result of our study is that the pressure tensor eventually becomes isotropic despite
the longitudinal expansion of the system." OK, despite my limitations; this makes sense.

OK, I've finished the paper, no for my amateur's question.

So this color glass condensate is like a liquid of gluons that finally equilabrates in all directions. But it is moving at the speed of light (this color glass condensate, this liquid wall of gluons). So the speed (in any reference frame does not change but the energy momentum is redshifting with time. Why? Oh yes, as we reach further and further rungs of the ladder of gluons and "The ones furthest along the ladder “know” least about the quarks that generated them" and now my question

"and are therefore universal to all matter."

As this color glass condensate reaches lower energies, do the low energy color gluons form low energy glueball specifically of four gluons?

Is it possible that such glueballs have approximately millimeter diameter rather than of fermi diameter in 4-spatial dimensions (thinking of) time as being spatial only and non temporal between the four gluons of such a glueball. time being a nonsequiture for such a glueball in its own reference frame?

OK those are my questions. General reader,my intention is not to confuse or to promote crackpot ideas. If there is No Answer to my question; assume my questions are ill formed and misinformed.

What the heck. Let me give you my net speculation.
And please break my idea tell me it disagrees with experiment here and there. That's how I learn.

The high energy gluons form a color glass condensate, which a some time along the ladder of gluons reaches a low isotropic energy momentum. This color glass condensate is like a superliquid wave traveling radially at c in 4-dimensions. These are nominally 3-space and 1-time. However from the gluon color glass condensate point of view; dimensions are tunnelling one to the other; there is no clear temporal dimension. Time doesn't emerge until there is a larger collection of matter.

Now as the gluons of color glass condensate go further up the ladder they lose energy; but of course they are moving at the speed of light. (just as when confined to a nucleus they move at c and this energy = mc^2).

Now regarding your point "The ones (gluons) furthest along the ladder “know” least about the quarks that generated them and are therefore universal to all matter."

"are universal to all matter" means to he are sensed, interact with all matter. How? These gluons escape the confinement of nuclei at some spacetime from origin. But free gluons cannot exist. Thus 4 four the 8 gluon colors form glueballs; these are the 4 gluons not the 4 antigluons.

Now the meaning of color is dimensionality. Quantum mechanically gluons cannot be in the same spacetime and they achieve this by each being in a different dimension. 3-spatial and 1-temporal.

But from a gluons point of view there is no difference between the 3-spatial dimensions and the 1-temporal dimension; they are all superimposed.

Now think of a glueball as an object made of 4 unit vectors of spin-1 gluons aligned in 4 orthogonal color-dimensions. (A color dimension is the superposition of 3-spatial dimensions and 1-temporal dimension.) Now in 4-dimension, a 4-glueball of 4-gluons each aligned orthonally (as they jump and tunnelbetween one another) is a spin-2 massless particle (assumeing this 4-gluon glueball is travelling at c).

So gluons that escape confinement of nuclei form become confined in 4-gluon spin-2 glueballs travelling at c. And such a color glass condensate of very low energy is gravity.

Now in my mind gravity is emergent from the strong force. Now back to ""The ones (gluons) furthest along the ladder “know” least about the quarks that generated them and are therefore universal to all matter." To me this means:

That spin 2 4-gluon-glueballs are color-neutral, specifically color-dimension neutral and thus somehow they interact with and are "universal to all matter" including e,g, electrons.

Thus gravity emerges from the strong force. simpatico29x

OK please tell me my thinking is wrongheaded, crackpot or break my idea with some obvious to you experimental evidence. At least thank you whoever for reading my idea.