Scientists seeking to explore the teeming microcosm of quarks and gluons inside protons and neutrons report new data delivered by particles of light. The light particles, or photons, come directly from interactions of a quark in one proton colliding with a gluon in another at the Relativistic Heavy Ion Collider (RHIC). By tracking these “direct photons,” members of RHIC’s PHENIX Collaboration say they are getting a glimpse — albeit a blurry one — of gluons’ transverse motion within the building blocks of atomic nuclei.
“We show experimentally for the first time the potential that direct photon measurements are sensitive to the transverse motion of gluons and that we can use such measurements to start constraining things — to reduce the huge uncertainties in our knowledge of how gluons behave,” said Alexander Bazilevsky, deputy spokesperson of the PHENIX Collaboration and a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory.
The data, published in Physical Review Letters, come from collisions between beams of polarized protons at RHIC, a DOE Office of Science user facility for nuclear physics research located at Brookhaven Lab. RHIC is the only facility in the world capable of colliding protons with their spin directions aligned in a controlled way.
“RHIC’s spin polarization is a crucial requirement for this research. It gives us a way to establish which way is up so we can measure the motions of other particles relative to that reference direction,” explained Brookhaven Lab physicist Nicole Lewis, whose work on this analysis formed the basis of her Ph.D. thesis.
As Lewis explained in an invited talk at the 2021 Fall Meeting of the American Physical Society’s Division of Nuclear Physics on October 12, understanding the origin of proton spin is also one of the main research goals.
A proton’s spin, or intrinsic angular momentum, makes it act like a tiny bar magnet with two poles. This property is used every day in magnetic resonance imaging (MRI), where a powerful external magnet changes the alignment of protons’ spins in our bodies so doctors can see features inside. But where spin comes from is still a mystery.
Studies at RHIC and elsewhere show that quark spins and gluon spins both make substantial contributions to proton spin, but not enough. The motions of these fundamental particles within protons are expected to also play a role. Using direct photons to measure how gluons’ transverse motion is correlated with overall proton spin is expected to help solve this puzzle.
In addition, studying the motion of quarks and gluons within a proton will help reveal details of the interactions between these particles. Those interactions are governed by the strong nuclear force — the strongest force in nature — which is carried by gluons and binds the quarks within the protons and neutrons of atomic nuclei. So, studying gluons and the strong force is really about understanding the “glue” that binds visible matter — everything made of atoms.
The newly analyzed data from PHENIX reveal that direct photons can be used to study gluons’ motions inside a proton.
The PHENIX measurements are 50 times more precise than the only previously published direct photon data — about 30 years ago from an experiment at DOE’s Fermi National Accelerator Laboratory.
“Our results help to validate the use of this approach for future studies at RHIC — including at an upgraded sPHENIX detector currently being installed in the location of the original PHENIX detector, which ended its experimental run in 2016. sPHENIX is expected to be operational in 2023 and will have even better capabilities to detect direct photons,” Bazilevsky said.
The direct photon data from proton-proton collisions will also provide important cross-checking for experiments using electrons to probe the inner structure of protons at the future Electron-Ion Collider (EIC).
“Proton-proton and electron-proton collisions give us different, complementary ways to ‘see’ inside a proton to construct the final picture of how things look,” Bazilevsky said.
How to peer inside a proton
Proton-proton collisions can produce a range of interactions. A quark in one proton can interact with either a quark or gluon in the other. And a gluon also can interact wi