Nucleon spin structure describes the
partonic structure of
nucleon
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number).
Until the 1960s, nucleons were ...
(
proton and
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
) intrinsic angular momentum (
spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its constituent
partons (
quarks and
gluons). It was originally expected before the 1980s that quarks carry all of the nucleon spin, but later experiments contradict this expectation. In the late 1980s, the
European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of the nucleons. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the
proton spin crisis.
Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the
COMPASS experiment
The NA58 experiment, or COMPASS (standing for "Common Muon and Proton Apparatus for Structure and Spectroscopy") is a 60-metre-long fixed-target experiment at the M2 beam line of the SPS at CERN. The experimental hall is located at the CERN No ...
at
CERN, experiments E142, E143, E154 and E155 at
SLAC
SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center,
is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...
,
HERMES
Hermes (; grc-gre, wikt:Ἑρμῆς, Ἑρμῆς) is an Olympian deity in ancient Greek religion and Greek mythology, mythology. Hermes is considered the herald of the gods. He is also considered the protector of human heralds, travelle ...
at
DESY, experiments at
JLab and
RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin did contribute about 30% to the total spin of the nucleon. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried either by gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,
:
The gluon spin components
are being measured by many experiments. Quark and gluon angular momenta will be studied by measuring so-called
generalized parton distributions (GPD) through
deeply virtual compton scattering (DVCS) experiments, conducted at
CERNCOMPASS and at
Jefferson Lab
Thomas Jefferson National Accelerator Facility (TJNAF), commonly called Jefferson Lab or JLab, is a US National Laboratory located in Newport News, Virginia. Its stated mission is "to provide forefront scientific facilities, opportunities and ...
, among other laboratories.
External links
Polarized colliders may prove to be the key in mapping out proton spin structureDr. Deshpande research webpageSpin Muon CollaborationHERMESCOMPASSThe Spin Structure of the Nucleon - Status and Recent Results
Standard Model