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Solving the proton spin crisis: a look into recent lattice QCD calculations of the glue spin in the proton
Fri., Jan. 26, 2018 12:00 p.m. - Fri., Jan. 26, 2018 1:00 p.m.
Location: LB 239
Description: Spin, a quantum mechanical property carried by elementary and
composite particles as well as atomic nuclei, is a form of intrinsic
angular momentum. Electrons are elementary particles and carry spin of
1/2. Unlike electrons, protons are composite particles that are made up
of three valence quarks, a sea of short-lived quark-antiquark pairs, and
the gluons holding them together (where quarks carry spin of 1/2 and
gluons, spin of 1). Proton's total spin contains contributions from
valence quarks, sea quarks, and gluon spins, as well as contributions
from quarks and gluons orbital angular momenta. And yet remarkably, the
total spin of the proton turns out to be 1/2, the same as that of an
electron.
Naively, one expects the three valence quarks inside proton (uud)
to carry all of its spins (i.e. two quarks with 1/2 spins and the third
with -1/2). Surprisingly, in the late 1980s, EMC (CERN) and SLAC
experimentally measured the quark contribution to be rather small, (12
+/- 9 +/- 14)% of the total proton spin, marking the beginning of the
so-called "proton spin crisis". Although, subsequent measurements at
SLAC and HERMES (DESY) in the 1990s as well as COMPASS (CERN),
RHIC-Spin, and JLab in the 2000s constrained the total quark
contribution at around 30% (including contributions from quark-antiquark
sea), we still do not have a thorough understanding of how these
contributions combine to yield total spin 1/2 of the proton. Early
QCD-inspired models, space-time lattice calculations, and p-p collisions
data at RHIC suggested a gluon spin contribution close to zero for
moderate momenta values. However, the analysis of recent RHIC data in
2014 found evidence of non-zero gluon polarization, indicating much
strong contributions from gluon spins than previously expected. This has
sparked wide interest from the lattice QCD community and I will be
discussing some of these recent lattice results.
Presenter: Samip Basnet, Department of Physics, University of Regina
Reading Materials
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Viewpoint Article : https://physics.aps.org/articles/v10/23
Phys.org Article : https://phys.org/news/2017-10-proton-puzzle.html
Recent RHIC Results :
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.012001
Lattice Results (#1) :
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.102001
Lattice Results (#2) :
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.142002