Experiment finds gluon mass in the proton (?)

[Astronuc]

Article published at Phys.org - Experiment finds gluon mass in the proton
https://phys.org/news/2023-03-gluon-mass-proton.html
An interesting diagram accompanies the article.

Nuclear physicists may have finally pinpointed where in the proton a large fraction of its mass resides. A recent experiment carried out at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility has revealed the radius of the proton's mass that is generated by the strong force as it glues together the proton's building block quarks. The result was recently published in Nature.

One of the biggest mysteries of the proton is the origin of its mass. It turns out that the proton's measured mass doesn't just come from its physical building blocks, its three so-called valence quarks.Over the last few decades, nuclear physicists have tentatively pieced together that the proton's mass comes from several sources. First, it gets some mass from the masses of its quarks, and some more from their movements. Next, it gets mass from the strong force energy that glues those quarks together, with this force manifesting as "gluons." Lastly, it gets mass from the dynamic interactions of the proton's quarks and gluons.

This new measurement may have finally shed some light on the mass that is generated by the proton's gluons by pinpointing the location of the matter generated by these gluons. The radius of this core of matter was found to reside at the center of the proton. The result also seems to indicate that this core has a different size than the proton's well-measured charge radius, a quantity that is often used as a proxy for the proton's size.

"The radius of this mass structure is smaller than the charge radius, and so it kind of gives us a sense of the hierarchy of the mass versus the charge structure of the nucleon," said experiment co-spokesperson Mark Jones, Jefferson Lab's Halls A&C leader.

. . .

The experiment was performed in Experimental Hall C in Jefferson Lab's Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. In the experiment, energetic 10.6 GeV (billion electron-volt) electrons from the CEBAF accelerator were sent into a small block of copper. The electrons were slowed down or deflected by the block, causing them to emit bremsstrahlung radiation as photons. This beam of photons then struck the protons inside a liquid hydrogen target. Detectors measured the remnants of these interactions as electrons and positrons.

Article in Nature (requires subscription or purchase, but one can read the abstract)
https://www.nature.com/articles/s41586-023-05730-4

 

[Vanadium 50]

I'm not 100% sure how they got from that paper to that press release. Maybe "Wow" is all that can be said.

 

[artis]

I'm not 100% sure how they got from that paper to that press release. Maybe "Wow" is all that can be said.

So they made stuff up then?
But that would violate the conservation of mass/energy!

 

[topsquark]

(Apologies in advance for the vague nature of this question. I'm only going for a first level understanding of this concept, not a full blown QCD analysis, which would be much more appropriate!)

A question for clarification:

Naively speaking, the mass of the proton would be due to the quark contributions and the binding energy of the system. So, again loosely speaking, is the binding energy due to the QCD potential in the system being interpreted here to represent a gluon mass? As gluons are inherently massless(?) this is a bit bizarre to me.

Or is my oversimplified description burying too many details?

-Dan

 

[Vanadium 50]

So they made stuff up then?

Who is they? What is stuff?

 

[Vanadium 50]

Or is my oversimplified description burying too many details?

It's certainly missing some important ones:

1. You can't unbind a proton so "binding energy" is not a simple concept. Perhaps not a useful concept either.
2. This misses the energy in the gluon field, the QCD analogue of E² + B²., This energy may be more or less tightly associated"nearby" a quark.

 

[vanhees71]

One has to be very careful with any attempt to describe such experiments on the popular-science level. One should be aware that even on a fully scienctific level we do not understand in detail from first principles, i.e., QCD, how to describe the structure of the proton. What is known, however, from lattice-QCD calculations is that indeed QCD describe the hadron spectra, i.e., we can understand the masses of hadrons through QCD as the theory of the strong interaction. According to this about 98% of the mass of the proton is dynamically generated by the strong interaction, and the Higgs mechanism, which gives mass to the u and d quarks of some 10 MeV, is only a minor contribution.

From a more abstract point of view one can understand the emergence of a mass scale from by socalled trace anomaly. If you take QCD in the light-quark sector with the approximation to neglect the very small "current-quark masses" entirely, the trace of the energy-momentum tensor should be 0 as a consequence of the scale-invariance of the QCD Lagrangian, i.e., the fact that in the limit of massless quarks the QCD Lagrangian has no mass/energy/momentum scale in it. Now it turns out that this symmetry of the Lagrangian does not survive field quantization. This phenomenon is known as an anomaly, i.e., a symmetry of a classical field theory needs not necessarily by also a symmetry of the corresponding quantized theory, and indeed the scaling symmetry of the QCD Lagrangian is anomalously broken, and thus a mass scale occurs in the theory. For more on the implications for the understanding of hadron masses, see

https://arxiv.org/abs/1606.03909

All we know about the structure of the proton comes from some scattering experiments and also experiments like hydrogen atoms or muonic hydrogen-like atoms in a trap. Scattering experiments with electrons, deep inelastic scattering, revealed in the 1960ies that protons are indeed not elementary Dirac fermions but bound states of "three partons", which since then have been identified with (constituent quarks), and even more accurate experiments with high-energetic electrons have also revealed something of the "inner structure" of the proton. According to this, deep inelastic scattering can be understood on the one hand as a hard scattering between an electron with a quark inside the proton and on the other hand the socalled parton-distribution functions, describing how these quarks are distributed within the proton before the scattering and which have to be found by fitting to the measured quantities characterizing these scattering events, the socalled form factors.

Here an experiment is described where the proton was probed with photons, producing a $J/\psi$, which is mediated dominantly by a two-gluon exchange process, producing the $c\bar{c}$ pair making up the $J/\psi$, and from these measurements the formfactors related to the energy-momentum tensor of the proton and particularly the gluonic contribution.

 

[topsquark]

I apologize. I have covered the "basic" concepts of QCD, but not something like bound quarks at any level of detail, so I was trying to "dumb the problem down" at bit. I see that this was a mistake.

I think I have it now, thank you, both!

-Dan

 

[Vanadium 50]

I don't see any need to apologize, It's the people who ask questions and then argue that the answers are wrong who need to apologize.

 

[artis]

Who is they? What is stuff?

I just had the impression that you were saying that what the paper describes VS what is written in the press release is overblown or otherwise not accurate?

 

[vanhees71]

Popular-science articles are much more likely to be bad than scientific papers, because it's utmost more difficult to write a good popular-science article than to write a good scientific paper! That's because in popular-science writing you cannot use the only adequate language to express the content of scientific knowledge, which is math. Often I've the problem to even understand what they talk about in popular-science articles. If it's a topic I've some knowledge about and I read the scientific paper the popular-science writing is based on, I understand what has been really done and what they are talking about.

 

[ohwilleke]

The press release isn't fundamentally wrong but treats as a new discovery the old, long well-established fact that most of a proton's mass comes from gluons as the paper's introduction explains, which is necessary but not sufficient to understand the balance of the paper. As the introduction to the paper explains in the body text:

The triumphant discovery of the Higgs boson offered a crucial explanation for the origin of quark masses. However, the quarks are almost massless (few MeV) and account only for a tiny fraction of the total proton mass of about 1 GeV, even when accounting for their relativistic nature. Thus the question arises: How do the massless gluons provide the sizeable remaining mass of the proton, and how is this mass distributed across the confinement size of the proton?

While Einstein’s original definition of the mass of a body, m= E/c² starts to answer this question, we can gain real insight through the measurement of the proton’s gravitational form factors (GFFs) and the determination of the trace anomaly. GFFs are the matrix elements of the proton’s energy-momentum tensor (EMT) and encode its mechanical properties, while the trace anomaly of the EMT is a key component of the origin of mass according to Quantum Chromodynamics (QCD).

Moreover, with the advent of lattice QCD, we can challenge and benchmark our understanding of the proton’s internal structure with ab-initio calculations.

In the past 40 years, we have extensively investigated the electric charge and spin of the proton. For example, we learned how the proton charge and magnetization, carried by the electrically charged moving quarks, are distributed and determined the proton electric charge radius through elastic electron scattering.

In contrast, the description of the mass distribution of the proton, carried mainly by gluons and their color interactions, is a subject in its infancy: Gluons carry no charge and thus are not amenable to direct study using an electromagnetic probe.

But the press release only tangentially and vaguely describes what the paper is actually doing to move the scientific ball forward by describing where within the proton the gluonic mass is located (let alone the very impressive and difficult means by which they manage to reach this conclusion).

This paper actually just assumes as a known fact, rather than discovering for the first time, the press release's statement that most of the proton mass comes from gluons, which has been known since long before the Higgs boson was discovered in 2012. Then, the paper describes, roughly speaking, the shape of the gravitational mass distribution within a proton based upon their experiments that its authors conducted at the GlueX experiment in Virginia.

The actual distribution of gluons in a proton differs from many popular textbook artist's illustrations, in that it is more concentrated towards the center of gravity of the proton, in something like a Tinkertoy kind of distribution with a gluonic hub in the middle, rather than, for example, being distributed evenly throughout the proton, or being located mostly on straight flux lines from one valence quark to another.

Sabine Hossenfelder at Backreaction shows what this looks like in an artist's conception form that is more accurate than many common textbook illustrations of the proton-gluon structure within a proton (which the cited paper, unfortunately, does not attempt). The middle illustration is the most accurate of the three.

The mass radius of a proton is smaller than the charge radius of the proton, because the mass of a proton is concentrated in central gluons (on average over time, shown in white).

In contrast, the electromagnetic charge of a proton comes mostly from its electromagnetically charged valence quarks (shown in red, green, and blue to represent their respective QCD color charges). Since, the valence quarks of a proton are concentrated closer to the outer area of a proton (on average over time) than the proton's gluons, the proton's electric charge radius is larger than it gravitational mass radius.

The cited paper and its abstract are as follows, FWIW:

The proton is one of the main building blocks of all visible matter in the Universe. Among its intrinsic properties are its electric charge, mass and spin. These properties emerge from the complex dynamics of its fundamental constituents—quarks and gluons—described by the theory of quantum chromodynamics. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering. An example is the highly precise measurement of the electric charge radius of the proton. By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge.

Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/ψ particle. We determined the gluonic gravitational form factors of the proton from our measurement. We used a variety of models and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics. This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.

B. Duran, et al., "Determining the gluonic gravitational form factors of the proton" 615 Nature 813-816 (March 29, 2023) (open access copy available here).

The money chart from the body text is as follows:

By way of comparison, the proton charge radius is 0.842 fm.

The radius we care about and understand as non-nuclear scientists is the mass radius r_m in the second to most far right column, and not the scalar radius r_s in the furthest right column, which can safely be ignored by a reader at less than the advanced PhD holder level.

A statistical Chi-squared divided by degrees of freedom test (in the first column ) does not significantly prefer the Holographic model over the GPD (for "generalized parton distribution") model used to estimate the proton gravitational mass radius from the experimental data.

But the lattice QCD ab initio calculation of the proton gravitational mass radius, made using only raw physical constants rather than experimental measurements of protons themselves, closely matches a fit to the data using the Holographic model over the GPD model, so the data fit using the Holographic model is probably closer to the truth.

The paper doesn't meaningfully or carefully discuss what it is about the differences between the Holographic model and the GPD model might make the Holographic model a better fit to the Lattice QCD calculations than the GPD model. This lack of explanation is not unusual. Picking the right QCD model to use to fit experimental results into quantities that we care about is more art than science.

 

[ohwilleke]

Naively speaking, the mass of the proton would be due to the quark contributions and the binding energy of the system. So, again loosely speaking, is the binding energy due to the QCD potential in the system being interpreted here to represent a gluon mass? As gluons are inherently massless(?) this is a bit bizarre to me.

Or is my oversimplified description burying too many details?

-Dan

Most of the mass in the proton comes from the mass-energy of the gluon field in the proton rather than from the three valence quarks in the proton which have a combined mass of about 8.99 - 0.84 + 0.41 MeV. But the total rest mass of the proton is 938.272 088 16(29) MeV (according to the Particle Data Group).

Gluons (in Standard Model QCD theory, at least) have zero rest mass, but because they carry substantial energy, they have a substantial mass-energy which gives rise to a substantial gravitational pull, and because they remain confined within the proton, their mass-energy acts like rest mass at the scale of the composite proton particle as a whole. Gluons are the source of most (ca. 99%) of the proton's rest mass.

Many operational approximations of QCD treat gluons as dynamically massive bosonic particles for purposes of operationalizing pure Standard Model QCD in a way that is easier to calculate with. See, for example, this paper:

The interpretation of the Landau gauge lattice gluon propagator as a massive-type bosonic propagator is investigated. Three different scenarios are discussed: (i) an infrared constant gluon mass; (ii) an ultraviolet constant gluon mass; (iii) a momentum-dependent mass. We find that the infrared data can be associated with a massive propagator up to momenta ~500 MeV, with a constant gluon mass of 723(11) MeV, if one excludes the zero momentum gluon propagator from the analysis, or 648(7) MeV, if the zero momentum gluon propagator is included in the data sets. The ultraviolet lattice data are not compatible with a massive-type propagator with a constant mass.

The scenario of a momentum-dependent gluon mass gives a decreasing mass with the momentum, which vanishes in the deep ultraviolet region. Furthermore, we show that the functional forms used to describe the decoupling-like solution of the Dyson–Schwinger equations are compatible with the lattice data with similar mass scales.

From O Oliveira and P Bicudo, Running gluon mass from a Landau gauge lattice QCD propagator (2011) J. Phys. G: Nucl. Part. Phys. 38 045003 doi:10.1088/0954-3899/38/4/045003.