Nucleon spin structure
Encyclopedia
Nucleon spin structure describes the partonic structure of nucleon
(proton
and neutron
) intrinsic angular momentum (spin
). The key question is how nucleon spin 1/2 is carried by its constituents, say parton
s (quark
s and gluon
s). 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
conducted experiments that suggested the spin carried by quarks is not sufficient to justify the total spin of the nucleons. This problem of where the missing spin lies is sometimes referred to as the proton spin crisis.
From a modern point of view, with the known fact that quarks only carry half the total momentum of the nucleon, it seemed not quite a surprise for us today to realized that quarks only carry a small fraction of nucleon spin. But it did astonished particle physicists at that time. Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) at CERN
, by HERMES at DESY
, by COMPASS
, by JLab
, by RHIC and others. Global analysis of data from all major experiments confirmed the original discovery by European Muon Collaboration
and showed that quark spin did contribute about 30% to the total spin of nucleons. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried by either gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,
The gluon spin compoments
is being measured by many experiments, and quark and gluon angular momentum will be measured by measuring so-called Generalized Parton Distributions (GPD) through deeply virtual compton scattering (DVCS), which is conducted mainly at JLab.
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
(proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
) intrinsic angular momentum (spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
). The key question is how nucleon spin 1/2 is carried by its constituents, say parton
Parton (particle physics)
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
s (quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s and gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s). 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
European Muon Collaboration
The European Muon Collaboration conducted high energy particle physics experiments at CERN. In 1983, it discovered that nucleons inside a nucleus have a different distribution of momentum among their component quarks...
conducted experiments that suggested the spin carried by quarks is not sufficient to justify the total spin of the nucleons. This problem of where the missing spin lies is sometimes referred to as the proton spin crisis.
From a modern point of view, with the known fact that quarks only carry half the total momentum of the nucleon, it seemed not quite a surprise for us today to realized that quarks only carry a small fraction of nucleon spin. But it did astonished particle physicists at that time. Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
, by HERMES at DESY
DESY
The DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....
, by COMPASS
COMPASS experiment
The NA58 experiment, or COMPASS is a fixed-target particle physics experiment at the Super Proton Synchrotron, a particle accelerator at the European Organization for Nuclear Research...
, by JLab
JLab
jLab is a numerical computational environment implemented in Java. The main scripting engine of jLab is GroovySci, an extension of Groovy. Additionally, the interpreted J-Scripts and dynamic linking to Java class code are supported....
, by RHIC and others. Global analysis of data from all major experiments confirmed the original discovery by European Muon Collaboration
European Muon Collaboration
The European Muon Collaboration conducted high energy particle physics experiments at CERN. In 1983, it discovered that nucleons inside a nucleus have a different distribution of momentum among their component quarks...
and showed that quark spin did contribute about 30% to the total spin of nucleons. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried by either gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,
The gluon spin compoments
is being measured by many experiments, and quark and gluon angular momentum will be measured by measuring so-called Generalized Parton Distributions (GPD) through deeply virtual compton scattering (DVCS), which is conducted mainly at JLab.