have added one more quote to the list, from today’s

- V. A. Petrov,
*Asymptotic Regimes of Hadron Scattering in QCD*(arXiv:1901.02628)

This is a commonplace that so far we do not have a full-fledged theory of interaction of hadrons, derived from the first principles of QCD and having a regular way of calculating of hadronic amplitudes, especially at high energies and small momentum transfers. The problem is related to a more general problem that QCD Lagrangian would yield colour confinement with massive colourless physical states (hadrons).

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]]>added one more quote in the above vein:

- Jeff Greensite,
*An Introduction to the Confinement Problem*, Lecture Notes in Physics, Volume 821, 2011 (doi:10.1007/978-3-642-14382-3)

]]>Because of the great importance of the standard model, and the central role it plays in our understanding of particle physics, it is unfortunate that, in one very important respect, we don’t really understand how it works. The problem lies in the sector dealing with the interactions of quarks and gluons, the sector known as Quantum Chromodynamics or QCD. We simply do not know for sure why quarks and gluons, which are the fundamental fields of the theory, don’t show up in the actual spectrum of the theory, as asymptotic particle states. There is wide agreement about what must be happening in high energy particle collisions: the formation of color electric flux tubes among quarks and antiquarks, and the eventual fragmentation of those flux tubes into mesons and baryons, rather than free quarks and gluons. But there is no general agreement about why this is happening, and that limitation exposes our general ignorance about the workings of non-abelian gauge theories in general, and QCD in particular, at large distance scales.

added another good citation in the above vein:

- Brambilla et al..
*QCD and strongly coupled gauge theories: challenges and perspectives*, Eur Phys J C Part Fields. 2014; 74(10): 2981 (doi:10.1140/epjc/s10052-014-2981-5)

]]>The success of the technique does not remove the challenge of understanding the non-perturbative aspects of the theory. The two aspects are deeply intertwined. The Lagrangian of QCD is written in terms of quark and gluon degrees of freedom which become apparent at large energy but remain hidden inside hadrons in the low-energy regime. This confinement property is related to the increase of $\alpha_s$ at low energy, but it has never been demonstrated analytically.

We have clear indications of the confinement of quarks into hadrons from both experiments and lattice QCD. Computations of the heavy quark–antiquark potential, for example, display a linear behavior in the quark–antiquark distance, which cannot be obtained in pure perturbation theory. Indeed the two main characteristics of QCD: confinement and the appearance of nearly massless pseudoscalar mesons, emergent from the spontaneous breaking of chiral symmetry, are non-perturbative phenomena whose precise understanding continues to be a target of research.

Even in the simpler case of gluodynamics in the absence of quarks, we do not have a precise understanding of how a gap in the spectrum is formed and the glueball spectrum is generated.

after all the recent edits to the References here, I only now noticed that the Idea-section of the entry itself was at best a stub. Now I wrote some minimum text there.

]]>Ah, no. But writing up an article on a non-perturbative physics model and trying to put things in proper perspective in the introduction.

]]>I hear a grant application in preparation :-)

]]>Thanks!! And so it’s relatively recent, too (2015). Excellent.

]]>See here for that second one.

]]>Presumably one can hope to find ways of using computers to help calculate the consequences of any kind of theory.

I thought the comparison to the Riemann hypothesis that I suggested above should be helpful to see this: No amount of checking the Riemann hypothesis case-wise on a computer will count as a proof or as providing theoretical understanding. Same for lattice QCD-tests of confinement/mass gap.

Meanwhile I found two quotes of the explicit kind that I am after. Maybe you could help me with the second, for here I only have a pdf-file, which says it is a chapter 7 of some bigger document, but I can’t find that bigger document, or just its citation data.

Here is the first:

- J J Cobos-Martínez,
*Non-perturbative QCD and hadron physics*2016 J. Phys.: Conf. Ser. 761 012036 (doi:10.1088/1742-6596/761/1/012036)

However, the QCD Lagrangian does not by itself explain the data on strongly interacting matter, and it is not clear how the observed bound states, the hadrons, and their properties arise from QCD. Neither confinement nor dynamical chiral symmetry breaking (DCSB) is apparent in QCD’s lagrangian, yet they play a dominant role in determining the observable characteristics of QCD. The physics of strongly interacting matter is governed by emergent phenomena such as these, which can only be elucidated through the use of non-perturbative methods in QCD [4, 5, 6, 7]

And here is the second, of which I am lacking the citation data:

]]>There is overwhelming theoretical and experimental evidence that QCD is the theory of strong interactions. Yet, QCD is to a large extent unsolved. In particular, standard perturbation theory becomes completely unreliable in the infrared regime, where QCD is strongly coupled. Experimentally, there is a large number of facts that lack a detailed qualitative and quantitative explanation. The most spectacular manifestation of our lack of theoretical understanding of QCD is the failure to observe the elementary degrees of freedom, quarks and gluons, as free asymptotic states (color con- finement) and the occurrence, instead, of families of massive mesons and baryons (hadrons) that form approximately linear Regge trajectories in the mass squared. The internal, partonic structure of hadrons, and nucleons in particular, is still largely mysterious. Since protons and neutrons form almost all the visible matter of the Universe, it is of basic importance to explore their static and dynamical properties in terms of quarks and gluons interacting according to QCD dynamics.

That seems quite complicated how terms – perturbative/non-perturbative, effective, phenomenology, computational model – intertwine here and in general.

actual theory as opposed to computer simulation

Presumably one can hope to find ways of using computers to help calculate the consequences of any kind of theory.

Is the issue here more that the right kind of theory is not yet in place,

]]>the explicit non-perturbative formulation of Yang-Mills theories such as QCD is presently wide open

Have read it now. Besides reporting that neat computation, this article is an exceptionally good account of lattice QCD in general.

(While this is nice, let me recall that the point in #1 is really about actual theory as opposed to computer simulation. Might be compared to the situation with the Riemann hypothesis: computer experiment checks the hypothesis already to fantastic “accuracy”, but none of this has any effect on the hypothesis as a theoretical open problem. Same here with confinement/mass gap and its implications.)

]]>Thanks! That’s good.

]]>Maybe this much longer review article by two of the authors of that Science paper should be added:

- Zoltan Fodor, Christian Hoelbling,
*Light Hadron Masses from Lattice QCD*, Rev. Mod. Phys. 84, 449, (arXiv:1203.4789)

So I’ll add it.

]]>Thanks for the pointer to the arXiv article! I’ll include pointer to that into relevant entries.

I hadn’t seen that blog post you point to, but it’s one among many other informal such discussions that I have seen. Am hoping to collect more citeable references.

]]>Ethan Siegel was live blogging a talk on this topic here, but perhaps you saw that. Maybe the kind of authoritative references you are after can be be found there. He writes

So long as you have enough computing power, you can recover the predictions of QCD to whatever precision you like, simply by making the lattice spacing smaller, which costs more in terms of computational power but improves your calculational accuracy. Over the past three decades, this technique has led to an explosion of solid predictions, including the masses of light nuclei and the reaction rates of fusion under specific temperature and energy conditions. The mass of the proton, from first principles, can now be theoretically predicted to within 2%.

and references Ab-initio Determination of Light Hadron Masses, published in Science.

]]>I am trying to collect citable/authorative references that amplify the analog of the mass gap problem in particle phenomenology, where it tramslates into the open problem of computing hadron masses and spins from first principles (due to the open problem of showing existence of hadrons in the first place!).

This is all well and widely known, but there is no culture as in mathematics of succinctly highlighting open problems such that one could refer to them easily.

I have now created a section *References – Phenomenology* to eventually collect references that come at least close to making this nicely explicit. (Also checked with the PF community here)