I Does this hint of entanglement within protons suggest new physics?

  • Thread starter Tghu Verd
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Noting that this study does not yet appear to be peer reviewed - and that the LHC data only provides a "strong direct indication" - does quantum entanglement at sub-nucleonic scales suggest new physics or is it entirely expected within the Standard Model?
I'm happy to admit that I am struggling to digest this recent arXiv paper - https://arxiv.org/abs/1904.11974 - but I am interested in whether its reasoning and provisional conclusion seems insightful or merely interesting.

My summary of their work is that the researchers consider colour confinement as an example of the EPR paradox at the sub-nucleonic scale, and so used CMS data to study the entropy resulting from entanglement within the proton to test for this. Their results suggest that quarks and gluons are entangled, with an acknowledgement that verification via electron-proton and electron-ion collisions at small x is needed, as well as studies of real-time evolution of quantum entanglement in high-energy processes.

The underlying driver for their study is the unknown dynamics of QCD and in particular, that the hard processes used to probe the parton structure lead to the view of partons as independent constituents that carry different fractions of the nucleon's momentum, which has consequences for entropy within their configuration space.

Given that the researchers were looking for an effect, and the basis for their comparison is Monte Carlo simulation, I am mindful that their approach might be skewed toward the outcome.

Still, a strong direct indication does warrant further investigation, but my question really is whether this represents hints of things beyond the Standard Model, or merely confirmation of it?
There is nothing beyond the standard model here, but there is a new perspective on QCD.

Suppose we think of a proton as a superposition of classical field configurations. If we then focus on a particular spatial region within the proton, there will be a corresponding "density matrix" which is a truncation of the proton-wide superposition-of-fields to just that region.

Now consider two such subregions of the proton. Each will have its own density matrix. If we start with just the density matrices of the subregions, and try to reassemble the overall wavefunction of the proton, we are lacking some information. We also need to know how the subregions were entangled with each other, in order to know how they fit together into the larger quantum state.

(I will note that I am proceeding here, by analogy with how this works for finite-dimensional entangled quantum systems. Generalizing to quantum fields will introduce new technicalities, but the principle that to reassemble a quantum whole from its parts, you also need to know how the parts were entangled, must still be valid.)

In effect, these authors are saying that partons have been studied without regard for the way they are entangled within the proton, and that the experimental data actually confirms their enhanced analysis which does take into account inter-parton entanglement.

You can see some of this described in recent slides by coauthor Kharzeev. For example, from slide 15 forwards, he talks about quantum states of subregions within the proton.

For me the most intriguing part is slide 27 forwards, "Possible relation to CFT", because I think it could relate to the description of the proton in the "infinite momentum frame", where it looks like a two-dimensional pancake, and to some of the mysteries of "pomerons", "odderons", and "Regge theory" pointed out in a series of posts by PF user @Anashim.

I will also point out that the study of entanglement, and entanglement entropy, in QFT is not a new topic. The Reeh-Schlieder theorem revealed long ago that the vacuum state of a QFT exhibits entanglement (because of zero-point energy / vacuum fluctuations), and it has also been a big topic in AdS/CFT holography (e.g. Ryu-Takayanagi formula), which is probably connected to the pomeron stuff I previously mentioned.
Appreciate your detailed but accessible reply, @mitchell porter, and esp. the references, I will work through them as best I can. I find QM in general challenging to conceptualize, and QCD more so, so each incremental improvement in my understanding is a win!

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