Physicists have found a new way to use data from high-energy particle smashups to peer inside protons. Their approach uses quantum information science to map out how particle tracks streaming from electron-proton collisions are influenced by quantum entanglement inside the proton. The results reveal that quarks and gluons, the fundamental building blocks that make up a proton’s structure, are subject to so-called quantum entanglement.
Data from past proton-electron collisions provide strong evidence of entanglement among the proton’s sea of quarks (spheres) and gluons (squiggles), which may play an important role in their strong-force interactions. Image credit: Valerie Lentz / Brookhaven National Laboratory.
“Before we did this work, no one had looked at entanglement inside of a proton in experimental high-energy collision data,” said Brookhaven Lab physicist Zhoudunming (Kong) Tu.
“For decades, we’ve had a traditional view of the proton as a collection of quarks and gluons and we’ve been focused on understanding so-called single-particle properties, including how quarks and gluons are distributed inside the proton.”
“Now, with evidence that quarks and gluons are entangled, this picture has changed. We have a much more complicated, dynamic system.”
“This latest paper refines our understanding of how entanglement impacts proton structure.”
“Mapping out the entanglement among quarks and gluons inside protons could offer insight into other complex questions in nuclear physics, including how being part of a larger nucleus affects proton properties.”
“This will be one focus of future experiments at the Electron-Ion Collider (EIC), a nuclear physics research facility expected to open at Brookhaven Lab in the 2030s.”
For the study, Dr. Tu and colleagues used the language and equations of quantum information science to predict how entanglement should impact particles streaming from electron-proton collisions.
Such collisions are a common approach for probing proton structure, most recently at the Hadron-Electron Ring Accelerator (HERA) particle collider in Hamburg, Germany, from 1992 to 2007, and are planned for future EIC experiments.
The equations predict that if the quarks and gluons are entangled, that can be revealed from the collision’s entropy, or disorder.
“Think of a kid’s messy bedroom, with laundry and other things all over the place. In that disorganized room, the entropy is very high,” Dr. Tu said.
According to the calculations, protons with maximally entangled quarks and gluons — a high degree of ‘entanglement entropy’ — should produce a lot of particles with a ‘messy’ distribution — a high degree of entropy.
“For a maximally entangled state of quarks and gluons, there is a simple relation that allows us to predict the entropy of particles produced in a high energy collision,” said Dr. Dmitri Kharzeev, a theorist affiliated with both Brookhaven Lab and Stony Brook University.
“In our paper, we tested this relation using experimental data.”
The scientists started by analyzing data from proton-proton collisions at CERN’s Large Hadron Collider, but they also wanted to look at the ‘cleaner’ data produced by electron-proton collisions.
The physicists cataloged detailed information from data recorded in 2006-2007, including how particle production and distributions varied and a wide range of other information about the collisions that produced these distributions.
When they compared the HERA data with the entropy calculations, the results matched the predictions perfectly.
These analyses, including the latest results on how particle distributions change at various angles from the collision point, provide strong evidence that quarks and gluons inside protons are maximally entangled.
“The revelation of entanglement among quarks and gluons sheds light on the nature of their strong-force interactions,” Dr. Kharzeev said.
“It may offer additional insight into what keeps quarks and gluons confined within protons, which is one of the central questions in nuclear physics that will be explored at the EIC.”
“Maximal entanglement inside the proton emerges as a consequence of strong interactions that produce a large number of quark-antiquark pairs and gluons.”
The team’s work appears in the journal Reports on Progress in Physics.
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Martin Hentschinski et al. 2024. QCD evolution of entanglement entropy. Rep. Prog. Phys 87, 120501; doi: 10.1088/1361-6633/ad910b
This article is based on a press-release provided by Brookhaven National Laboratory.
