In
particle physics
Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
, 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.
Until the 1960s, nucleons were thought to be ele ...
(
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
and
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
) intrinsic angular momentum (
spin
Spin or spinning most often refers to:
* Spin (physics) or particle spin, a fundamental property of elementary particles
* Spin quantum number, a number which defines the value of a particle's spin
* Spinning (textiles), the creation of yarn or thr ...
). The key question is how the nucleon's spin, whose magnitude is 1/2
ħ, is carried by its constituent
partons (
quark
A quark () is a type of 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 nucleus, atomic nuclei ...
s and
gluon
A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...
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
The European Muon Collaboration (EMC) was a consortium of particle physicists formed in 1973 to study the interactions of high energy muons at CERN. These experiments were motivated by the interest in determining the quark structure of the nucleon ...
(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 Nort ...
at
CERN
The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...
, experiments E142, E143, E154 and E155 at
SLAC
SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center,
is a federally funded research and development center in Menlo Park, California, United States. Founded in 1962, the laboratory is now sponsored ...
,
HERMES
Hermes (; ) is an Olympian deity in ancient Greek religion and mythology considered the herald of the gods. He is also widely considered the protector of human heralds, travelers, thieves, merchants, and orators. He is able to move quic ...
at
DESY
DESY, short for Deutsches Elektronen-Synchrotron (English: ''German Electron Synchrotron''), is a national research centre for fundamental science located in Hamburg and Zeuthen near Berlin in Germany. It operates particle accelerators used to ...
, experiments at
JLab
GroovyLab, formerly jLab, is a numerical computational environment implemented in Java. The main scripting engine of GroovyLab is GroovySci, an extension of Groovy. Additionally, the interpreted Groovy Scripts (similar to MATLAB) and dynamic l ...
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
In particle physics, the parton model is a model of hadrons, such as protons and neutrons, proposed by Richard Feynman. It is useful for interpreting the cascades of radiation (a parton shower) produced from quantum chromodynamics (QCD) processes ...
(GPD) through
deeply virtual compton scattering
''Deeply'' is a 2000 film directed by Sheri Elwood, starring Julia Brendler, Lynn Redgrave and Kirsten Dunst.
Synopsis
Claire McKay ( Julia Brendler)—having suffered the death of her boyfriend—is brought by her mother to Ironbound Island i ...
(DVCS) experiments, conducted at
CERN
The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...
COMPASS and at
Jefferson Lab
Thomas Jefferson National Accelerator Facility (TJNAF), commonly called Jefferson Lab or JLab, is a US Department of Energy National Laboratory located in Newport News, Virginia.
Since June 1, 2006, it has been operated by Jefferson Scienc ...
, 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