New data from the STAR experiment at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) add detail, if not also complexity, to an intriguing puzzle that scientists have been seeking to solve: how the building blocks that make up a proton contribute to its spin.
The results, just published as a rapid communication in the journal Physical Review D, reveal definitively for the first time that different “flavors” of antiquarks contribute differently to the proton’s overall spin — and in a way that’s opposite to those flavors’ relative abundance.
“This measurement shows that the quark piece of the proton spin puzzle is made of several pieces,” said James Drachenberg, a deputy spokesperson for STAR from Abilene Christian University who received his Ph.D. in physics from Texas A&M University in 2012. “It’s not a boring puzzle; it’s not evenly divided. There’s a more complicated picture, and this result is giving us the first glimpse of what that picture looks like.”
It’s not the first time that scientists’ view of proton spin has changed. There was a full-blown spin crisis in the 1980s when an experiment at the European Center for Nuclear Research (CERN) revealed that the sum of quark and antiquark spins within a proton could account for, at best, a quarter of the overall spin.
“The spin crisis forced us fundamentally to rethink our understanding of the proton,” said physicist Carl Gagliardi, a convener of the STAR Spin Physics working group at Texas A&M. “Before that time, we thought of protons as two ‘up’ quarks and a ‘down’ quark. We knew there are also gluons present to bind the quarks together, and the theory requires additional ephemeral quark-antiquark pairs. But we thought the gluons and antiquarks played no role in the visible proton properties.”
STAR is an international collaboration of more than 500 physicists and engineers from 60 universities and national laboratories in the U.S. and 11 other countries. Texas A&M has been a STAR institution since 2000, and at present, there are nine Texas A&M Cyclotron Institute-affiliated physicists in STAR, including three faculty members.
RHIC, a U.S. Department of Energy Office of Science user facility for nuclear physics research at Brookhaven, was built in part so scientists could measure the contributions of other components, including antiquarks and gluons. Antiquarks, which have only a fleeting existence, form as quark-antiquark pairs when gluons split.
“We call these pairs the quark sea,” Drachenberg said. “At any given instant, you have quarks, gluons and a sea of quark-antiquark pairs that contribute in some way to the description of the proton. We understand the role these sea quarks play in some respects, but not in respect to spin.”
Exploring flavor in the sea
One key consideration is whether different “flavors” of sea quarks contribute to spin differently. Quarks come in six of them — the up and down varieties that make up the protons and neutrons of ordinary visible matter, and four other more exotic species. Splitting gluons can produce up quark/antiquark pairs, down quark/antiquark pairs and sometimes even more exotic quark/antiquark pairs.
“There is no reason why a gluon would prefer to split into one or the other of these flavors,” said Ernst Sichtermann, a STAR collaborator from DOE’s Lawrence Berkeley National Laboratory (LBNL) who played a lead role in the sea quark research. “We’d expect equal numbers [of up and down pairs], but that’s not what we are seeing.”
Gagliardi notes that Texas A&M physicists played a major role in an experiment 20 years ago at DOE’s Electron-Ion Collider. This particle accelerator would use electrons to directly probe the spin structure of the internal components of a proton and should ultimately solve the proton spin puzzle.
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Contact: Shana K. Hutchins, (979) 862-1237 or email@example.com or Dr. Carl Gagliardi, (979) 845-1411 or firstname.lastname@example.org