Glueballs Observed For The First Time

[ohwilleke]

TL;DR Summary

About fifty years after they were first predicted, scientists in China have confirmed that a resonance first observed in 2011 is a glueball (i.e. a composite particle bound by the strong force without any quarks).

One of the first particles predicted by quantum chromodynamics (QCD), which consists of the laws of physics related to the strong force, whose properties have been calculated for almost fifty years, is the glueball. The glueball is a strong force bound composite particle made up of gluons, but without any valance quarks.

Its properties were fairly easy to predict early on, because they are, at tree-level, a function of only experimentally determined Standard Model parameter, the strong force coupling constant (which is about 0.1180(9) at the Z boson mass energy scale), and because there are significant symmetries involved in glueball structure in the Standard Model.

They are hard to detect, in part, because glueballs are bosons and can blend into resonances of non-glueball particles with the same quantum numbers.

In the Standard Model (SM) of particle physics, gluons are the fundamental particles mediating the strong interaction, as photons do in electromagnetic interactions. Gluons can attract each other to form new bound states called glueballs, which are the only particles in nature entirely composed of force mediators. Finding these gluon bound states is crucial and serves as a fundamental test of the SM. No candidate has yet been unambiguously identified until the new BESIII result to be reported in this presentation.

The Beijing Electron Positron Collider (BEPCII) is a double ring e+e- collider in the 2-5 GeV energy region and the Beijing Spectrometer (BESIII) is a general purpose detector operating at the BEPCII. Decays of J/ψ particle produce a gluon-rich environment and are an ideal place to search for glueball. Discovery of the glueball has been one of the most important scientific goals of the BEPC and BEPCII for decades.

The X(2370) particle was first discovered at the BESIII experiments in 2011. To confirm its pseudoscalar glueball state nature experimentally, the most crucial step is to determine whether the spin parity quantum number of the X(2370) are indeed zero spin and negative parity.

Recently, based on ~10 billion J/ψ decays collected with BESIII, the spin-parity quantum numbers of the X(2370) were firstly measured with a complex partial wave analysis. The experimental results, including quantum numbers, mass, production and decay properties, are consistent with the features of the lightest pseudoscalar glueball. This recent study provides direct and strong experimental evidence for X(2370) being a glueball.

From a CERN Press Release (May 11, 2024).

Wikipedia notes that:

In 2024, the X(2370) particle was determined to have mass and spin parity consistent with that of a glueball. However, other exotic particle candidates such as a tetraquark could not be ruled out.

In support of those conclusions Wikipedia cites the following two papers:

1. Ablikim, M.; et al. (BESIII Collaboration) (May 2024). "Determination of Spin-Parity Quantum Numbers of X(2370) as 0−+ from J/ψ → γK0
   SK0
   η′". Physical Review Letters. 132 (18): 181901. arXiv:2312.05324. doi:10.1103/PhysRevLett.132.181901; and
2. "New particle at last! Physicists detect the first "glueball"". Big Think. 2024-05-07. Retrieved 2024-05-08.

Lattice QCD predictions have predicted the existence a pseudoscalar glueball with the following properties:

0−+	2590 ± 130 MeV/c2

At the low end of the two sigma range, this would be a mass of 2330 MeV, so a pseudoscalar particle with a mass of 2370 MeV would be consistent with this prediction. The measured mass of 2395 MeV is even closer to the predicted mass (about 1.5 sigma).

Until now, it had remained an open possibility that glueballs could not exist without valence quark sources, even though the Standard Model contained no rule requiring that rule. A new Standard Model rule imposing that requirement, however, would have been fairly easy to add to the Standard Model if observation had required it, and wouldn't have added any experimentally observed parameters to the Standard Model.

In additional to finally realizing a Standard Model prediction that is half a century old, the mass of this particle also provides a particularly clean way to reverse engineer the strong force coupling constant.

 

[Vanadium 50]

A pity that almost none of what you wrote is correct.

Let's start with your so-called "CERN Press Release". It was nothing of the sort. It was an announcement for a seminar.

Next, the f0(1710) is almost pure glueball (because of mixing no particle is pure glueball) and it was discovered in 1982, possibly before. That was over 40 years ago!

 

[ohwilleke]

A pity that almost none of what you wrote is correct.

I'm sorry. Your attitude is way out of line. It isn't backed solidly by the science or the scientific history either.

The purpose of this site is not to mock people who are discussing scientific developments and concepts solidly backed by and faithfully reporting opinions from reliable sources including scientific journal articles published is well regarded peer reviewed journals and CERN. If you can't treat other people at the Physics Forum with a modicum of civility and respect, maybe you need to take a break for a while.

Next, the f0(1710) is almost pure glueball (because of mixing no particle is pure glueball) and it was discovered in 1982, possibly before. That was over 40 years ago!

This source, for example, disagrees with your assessment. So does CERN, in the language bolded in the original post, which states:

Finding these gluon bound states is crucial and serves as a fundamental test of the SM. No candidate has yet been unambiguously identified until the new BESIII result to be reported in this presentation.

The status of the f0(1710) as an almost pure glueball has been hotly debated for almost all of the last 40 years.

The resonance may have been discovered 40 years ago, but a confident interpretation of that resonance as a glueball is not 40 years old.

The glueball interpretation of the f0(1710) resonance became the leading one less than ten years ago, and then, only tentatively, in 2015 with the paper Frederic Brünner, Anton Rebhan. Nonchiral Enhancement of Scalar Glueball Decay in the Witten-Sakai-Sugimoto Model. Physical Review Letters, 2015; 115 (13) DOI: 10.1103/PhysRevLett.115.131601.

The hesitation in 2015 was due to the fact that the decay products and branching fractions were not confirmed to match theoretical expectations then, something that has now been accomplished with the X(2370).

I don't know if those defects in the 2015 assessment have been remedied since then, perhaps you are aware of more recent scientific journal articles that resolved that uncertainty and could be helpful by citing to them.

The theoretical glueball concept and calculations of their properties and possible types, however, dates to 1972.

 

[fresh_42]

Temporarily closed for moderation.

 

[PeterDonis]

Wikipedia notes that:

Among other things, it notes that (in what you quoted) "other exotic particle candidates such as a tetraquark could not be ruled out". So describing this as "confirming" that X(2370) is a glueball is an overstatement. (Note that the actual paper does not make the "confirmed" claim; it just says the mass and spin-parity quantum numbers are "consistent with" it being a glueball.)

 

[fresh_42]

We will re-open the thread and want to remind everybody to quote references for their statements. I think that even Wikipedia sheds some light on the discussion $f_0(1710)$ versus $X(2370)$:

Glueballs: https://en.wikipedia.org/wiki/Glueball
Mesons: https://en.wikipedia.org/wiki/List_of_mesons
(alternative): https://de.wikipedia.org/wiki/Liste_der_Mesonen (Google Chrome can translate instantly)

As I see the situation, it is not finally decided whether the known scalar mesons qualify as glueballs, and $X(2370)$ is currently discussed on several levels. Since it is a new publication in physics and the OP provided references, it is our central interest to allow a discussion about it, too, even if the OP might not have the full overview or insights of the matter. This is finally one goal of such threads, to clear misunderstandings.

Please refrain from ad hominem arguments.

 

[Vanadium 50]

It is important to note that the "CERN Press Release" was nothing of the sort. That is not ad hominem.

There are a number of light glueball candidates. It is unequivocal that there are more 0++ states than predicted from qqbar states. There are also more 2++ states (which are also glueball candidfates). In the quark model these are 3P0 and 3P2 states. However, the correspodning 3P1 and 1P1 states are missing. One or more of these states are glueballs, the only issue is which one.

The f0(1710) chaceks all but one of the boxes. The only contrary evidence is the branching fraction to 2 gammas which looks "too large". However, a) this number has not been measured, b) so far as I know there is no agreed-upon calculation on "how large is too large" and c) as mentioned earlier, states with the same quantum numbers mix. No state is pure glueball.

Nonetheless, the X(2370) is not the first.

One could say "but this is the first pseudoscalar glueball". That would be fair, but it does not support the claim "Until now, it had remained an open possibility that glueballs could not exist". That is just not true.

 

[ohwilleke]

Re the f0(1710), note also the paper below from January 2023 indicating that its status as a glueball candidate is still in question:

Our theoretical prediction based on this picture is in good agreement with the latest BESIII data, which further supports the molecular state picture of a0(1710)[a0(1817)]. If it is indeed the isospin partner state of f0(1710), this would rule out f0(1710) as a glueball candidate.

E. Oset, L. R. Dai, L. S. Geng, "Repercussion of the a0(1710) [a0(1817)] resonance and future developments" arXiv:2301.08532 (Jan 25, 2023) (published as a "Perspective" in Science Bulletin).

 

[Vanadium 50]

That's actually not in the paper itself.

What the paper says is that IF there is an a(1810) AND it is the isopartner of the f(1710), that supports the notion that it is the f(1500) as opposed to the f(1710) that is primarily glueball. I have no objection to that. Note that in neither case is the lightest or earliest glueball the X(2370), which I guess we should now call the η(2370).

A discussion about whether the f(1500)) or f(1710) is the state that is more gluey than the other does not mean the first glueball is the η(2370). A discussion on whether calculus was invented by Newton or Leibnitz does not mean it was invented by Taylor Swift.

 

[ohwilleke]

A discussion about whether the f(1500)) or f(1710) is the state that is more gluey than the other does not mean the first glueball is the η(2370). A discussion on whether calculus was invented by Newton or Leibnitz does not mean it was invented by Taylor Swift.

Actually, it does, because it reflects the uncertainty in the interpretation of those resonances.

Certainly, all of the glueball candidates were "observed" as unclassified resonances long before interpretations of their character as glueball candidates was advanced, or came to be considered seriously on a wide spread basis. The fact that a resonance is observed, even at the five sigma level, however, does not imply that the interpretation of the structure of the resonance has been discovered with that level of confidence.

There were serious hints of a significant glueball component in the f(1500)) or f(1710) scalar mesons before this, but the strength of those hints has varied and the classification has always been marked by some controversy. There was a question about the X(2370) as late as 2022, before the most recent result.

But the support for X(2370) as the lightest pseudoscalar glueball now is stronger than the support for the f0(1710) as a scalar glueball was at late as 2015, because the X(2370) has branching fractions and decay properties better established than the f0(1710) which has had mass, and J^PC numbers supporting it did, but with a less definitive match from other data.

My search of the literature didn't find any papers after 2015 that strengthened the support for the f0(1710) as a scalar glueball hypothesis from new data, although I'm not omniscient and could have missed it.

Also, the X(2370) appears to be a particularly pure glueball, matching the theoretically calculated mass prediction for a pure pseudoscalar glueball exactly (indeed to spurious accuracy given the precision of the estimated mass and measured mass), while both of the light scalar glueball candidates are assumed to be mixed resonances in most analyses, with glueball percentages that are high, but not right on 100%.

So, while X(2370) may not be the first hint of a possible glueball component, it is the most definitive and pure glueball candidate resonance to date, which justifies the claims made by others than it is the first resonances to be truly discovered to be a glueball.

Another source describing it that way, although not independent of the other ones previously cited, is here. The independent Chinese Academy of Science announcement hedges a little calling it "glueball-like". A Science Times report is roughly in line with the Chinese Academy of Science announcement.

Nomenclature

The the PDG review article regarding nomeclature for hadrons does support the notation change from X(2370) to η(2370) suggested by Vanadium 50, now that its PC values are better established (whether or not it is a glueball):

Still, η(2370) is arguably insufficiently distinct for such a special resonance, and it may really deserves a new and different base symbol of its own to reflect that in some future version of the nomeclature for hadrons.

Also while mass number changes are discouraged for back continuity in searching for journal articles about a resonance, if you are going to relabel it anyway, maybe η(2395) would be a better name for the lightest pseudoscalar glueball.

 

[Vanadium 50]

Actually, it does, because it reflects the uncertainty in the interpretation of those resonances.

Newton or Leibniz? Must be Taylor Swift.