Could you help me understand this paper on the "European Muon Collaboration" effect?

[Ignorantsmith12]

TL;DR Summary

So I found this paper https://arxiv.org/pdf/2004.12065 on something called the EMC (European Muon Collaboration) effect which I guess is about how quarks move slower in a proton if the proton is part of a larger atom. Is it, though?

Statistical mechanics might as well be Klingon as far as I can read it, so interpreting an academic physics paper can be tricky. Still, this paper here https://arxiv.org/pdf/2004.12065 struck me as intriguing. I'm pretty sure this paper is about the innards of protons and neutrons, which I find endlessly fascinating, or at least that's how I feel from the little I understand. I have the most difficulty with the contrast between this paper and the "popular version." https://news.mit.edu/2019/quark-speed-proton-neutron-pairs-0220

The popular version just says that the authors of the paper used high-speed electrons for a momentum transfer to determine the "speed" of quarks in a small nucleus versus a larger one, but certain phrases in the original paper jump out at me and suggest something a bit more nuanced is going on. Phrases like "Quark-gluon substructure" and "Necleons are modified" make me think I'm missing something.

Sorry, I'm having trouble articulating what I wish to know, but I guess I'm asking what the phrases mentioned above mean, what this paper is about, and what you think of this work. Does it seem like a well-executed experiment? Do you agree with the conclusions? What do you think?

 

[ohwilleke]

To establish a foundation for an answer the question, the citation to the paper and its abstract are as follows:

The atomic nucleus is made of protons and neutrons (nucleons), that are themselves composed of quarks and gluons. Understanding how the quark-gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge.

Although evidence for such modification, known as the EMC effect, was first observed over 35 years ago, there is still no generally accepted explanation of its cause [1–3]. Recent observations suggest that the EMC effect is related to close-proximity Short Range Correlated (SRC) nucleon pairs in nuclei [4, 5].

Here we report the first simultaneous, high-precision, measurements of the EMC effect and SRC abundances. We show that the EMC data can be explained by a universal modification of the structure of nucleons in neutron-proton (np) SRC pairs and present the first data driven extraction of this universal modification function.

The core hypothesis of the paper is that neutrons and protons in atomic nuclei tend to pair off in SRCs that behave differently from isolated protons and neutrons. And, this hypothesis fits the data.

This implies that, in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have its quark structure distorted.

B. Schmookler, M. Duer, A. Schmidt, O. Hen, S. Gilad, E. Piasetzky, M. Strikman, L.B. Weinstein et al. (The CLAS Collaboration) , "Modified Structure of Protons and Neutrons in Correlated Pairs" arXiv:2004.12065 (April 25, 2000).

The introduction in the body text is often helpful in understanding a paper (my interlineated commentary below and above is in italics):

We study nuclear and nucleon structure by scattering high-energy electrons from nuclear targets. The energy and momentum transferred from the electron to the target determines the space-time resolution of the reaction, and thereby, which objects are probed (i.e., quarks or nucleons). To study the structure of nuclei in terms of individual nucleons, we scatter electrons in quasi-elastic (QE) kinematics where the transferred momentum typically ranges from 1 to 2 GeV/c and the transferred energy is consistent with elastic scattering from a moving nucleon. To study the structure of nucleons in terms of quarks and gluons, we use Deep Inelastic Scattering (DIS) kinematics with larger transferred energies and momenta. Atomic nuclei are broadly described by the nuclear shell model, in which protons and neutrons move in well defined quantum orbitals, under the influence of an average mean-field created by their mutual interactions. The internal quark-gluon substructure of nucleons was originally expected to be independent of the nuclear environment because quark interactions occur at shorter distance and higher-energy scales than nuclear interactions. However, DIS measurements indicate that quark momentum distributions in nucleons are modified when nucleons are bound in atomic nuclei [1, 2, 6, 7], breaking down the scale separation between nucleon structure and nuclear structure.

This sets up the fact that the study is using deep inelastic scattering to test the effect, i.e. by seeing how electrons or muons shot at nucleons act.

This scale separation breakdown in nuclei was first observed thirty-five years ago in DIS measurements performed by the European Muon Collaboration (EMC) at CERN [8]. These showed a decrease of the DIS cross-section ratio of iron to deuterium in a kinematical region corresponding to moderate- to high-momentum quarks in the bound nucleons. The EMC effect has been confirmed by subsequent measurements on a wide variety of nuclei, using both muons and electrons [9, 10], and over a large range of transferred momenta, see reviews in [1, 2, 6, 7]. The maximum reduction in the DIS cross-section ratio of a nucleus relative to deuterium increases from about 10% for 4He to about 20% for Au.

The frequency with which an electron shot at a nucleus declines as the nucleus gets bigger. The effect is significant (especially for the jump from heavy hydrogen to helium), but it is not a huge effect.

The EMC effect is now largely accepted as evidence that quark momentum distributions are different in bound nucleons relative to free nucleons [1, 2, 7]. However, there is still no consensus as to the underlying nuclear dynamics driving it.

The observed data are real and not just flukes or systemic errors from a single experiment.

Currently, there are two leading approaches for describing the EMC effect, which are both consistent with data: (A) all nucleons are slightly modified when bound in nuclei, or (B) nucleons are unmodified most of the time, but are modified significantly when they fluctuate into SRC pairs. See Ref. [1] for a recent review. SRC pairs are temporal fluctuations of two strongly interacting nucleons in close proximity, see e.g. [1, 11]. Electron scattering experiments in QE kinematics have shown that SRC pairing shifts nucleons from low momentum nuclear shell-model states to high-momentum states with momenta greater than the nuclear Fermi momentum. This “high-momentum tail” has a similar shape for all nuclei. The relative abundance of SRC pairs in a nucleus relative to deuterium approximately equals the ratio of their inclusive (e,e′) electron scattering cross-sections in selected QE kinematics [12–15].

They are trying to determine if all nucleons in a nucleus exhibit this effect, or only on neutrons and protons in SRC pairs. There is evidence that nucleons in SRC pairs have higher momentum.

Recent studies of nuclei from 4He to Pb [16–22], showed that SRC nucleons are “isophobic”; i.e., similar nucleons are much less likely to pair than dissimilar nucleons, leading to many more np SRC pairs than neutron-neutron (nn) and proton-proton (pp) pairs. The probability for a neutron to be part of an np-SRC pair is observed to be approximately constant for all nuclei, while that for a proton increases approximately as N/Z, the relative number of neutrons to protons [22].

SRC pairs generally form between neutrons and protons, not between pairs of neutrons or pairs of protons, which should create a simple pattern based upon the number or protons and number of neutrons in an atomic nucleus.

The first experimental evidence supporting the SRC modification hypothesis as an explanation for the EMC effect came from comparing the abundances of SRC pairs in different nuclei with the size of the EMC effect. Not only do both increase from light to heavy nuclei, but there is a robust linear correlation between them [4, 5]. This suggests that the EMC effect might be related to the high momentum nucleons in nuclei.

The remainder of the paper is devoted to showing that the effect is due to interactions of adjacent nucleons when they form proton-neutron pairs.

Does it seem like a well-executed experiment? Do you agree with the conclusions?

The preprint (it is actually a post-print) was published in the Journal Nature, volume 566, page 354 in 2019, which is one of the top scientific journals. So, the paper at least met the "smell-test" in peer review.

The claim is plausible and I have no reason as an educated layman to see it is obviously flawed. I'm in no position to sit in judgment on it.

The most familiar evidence supporting the idea that nucleons behave differently when bound in atomic nuclei than they do when they are not bound in atoms is the neutron itself. In isolation, a "free" neutron decays in about fifteen minutes, on average. But, bound neutrons are stable. It is surely plausible that this is not the only effect arising from being part of a bound atomic nucleus, and there is evidence to back up this claim from multiple independent sources cited in this article and from its own experimental results.

According to arXiv, there have been 138 citations to this paper in five years. At least nine have been supportive, and none of them alleged that it is flawed. This is very impressive for a five year old journal article.

So, the position taken by the article is very credible, mainstream science. Maybe this conclusion will be modified in the future, but there is no particular reason to expect that this will happen.