Quarks Within Protons Are Entangled

A proton is a composite particle made up of three valence quarks (two up quarks and one down quark) and myriad "sea quarks" which are "confined" by gluons in a coherent and discrete unit. 

The internal structure of a proton is a matter of ongoing investigation and one insight reached recently based upon experimental data is that quarks within protons, as previously suspected, appear to be "entangled" which is to say that their behavior is correlated, rather than being independent of each other.

The investigators think that the quantum entanglement may help to explain why quarks and gluons are always confined in composite particles.

[D]ata from the Large Hadron Collider hint that protons’ constituents don’t behave independently. Instead, they are tethered by quantum links known as entanglement, three physicists report in a paper published April 26 at arXiv.org.  

Quantum entanglement has previously been probed on scales much larger than a proton. In experiments, entangled particles seem to instantaneously influence one another, sometimes even when separated by distances as large as thousands of kilometers (SN: 8/5/17, p. 14). Although scientists suspected that entanglement occurs within a proton, signs of that phenomenon hadn’t been experimentally demonstrated inside the particle, which is about a trillionth of a millimeter across. 

“The idea is, this is a quantum mechanical particle which, if you look inside it, … it’s itself entangled,” says theoretical physicist Piet Mulders of Vrije Universiteit Amsterdam, who was not involved with the research. 

In the new study, the team analyzed collisions of protons, which had been accelerated to high speeds and slammed together at the Large Hadron Collider in Geneva. Using data from the CMS experiment there, the researchers studied the entropy resulting from entanglement within the proton. Entropy is a property that depends on the number of possible states a system can take on, on a microscopic level. An analogy is a deck of cards: A shuffled deck has multiple ways that it could be ordered, whereas an ordered deck has only one, so the scrambled cards have higher entropy. 

If entanglement exists within a proton, there will be additional entropy as a result of those linkages. That entropy can be teased out by counting the number of particles produced in each collision. The amount of entropy the researchers found agreed with that expected assuming the quarks and gluons were entangled, the physicists report in their paper[.]

From Science News. The paper is:

Z. Tu, D. Kharzeev and T. Ullrich. The EPR paradox and quantum entanglement at sub-nucleonic scales. arXiv:1904.11974 (April 26, 2019).