Notice: Important information about COVID-19 here.

Subscribe by RSS Subscribe by RSS

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