New data from the STAR experiment a the
Relativistic Heavy Ion Collider (RHIC) add detail -- and 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. "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. RHIC, a U.S. Department of
Energy Office of Science user facility for nuclear physics research at
Brookhaven National Laboratory, was built in part so scientists could
measure the contributions of other components, including antiquarks and
gluons (which "glue" together, or bind, the quarks and antiquarks to
form particles such as protons and neutrons).
https://www.sciencedaily.com/releases/2019/03/190314192651.htm
