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 w ...
(
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 behav ...
) intrinsic angular momentum (
spin
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...
). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, 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 nuclei. All common ...
s and
gluon
A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bi ...
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 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 following the discovery of high level ...
(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
The proton spin crisis (sometimes called the "proton spin puzzle") is a theoretical crisis precipitated by a 1987 experiment by the European Muon Collaboration (EMC),
which tried to determine the distribution of spin within the proton.
Physicist ...
.
Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the
COMPASS experiment 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, Ἑρμῆς) is an Olympian deity in ancient Greek religion and mythology. Hermes is considered the herald of the gods. He is also considered the protector of human heralds, travellers, thieves, merchants, and orato ...
at
DESY
The Deutsches Elektronen-Synchrotron (English ''German Electron Synchrotron''), commonly referred to by the abbreviation DESY, is a national research center in Germany. It operates particle accelerators used to investigate the structure of matt ...
, experiments at
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 (similar to MATLAB) and dynamic linking to Java class code ar ...
and
RHIC
The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy- ion colliders, and the only spin-polarized proton collider ever built. Located at Brookhaven National Laboratory (BNL) in Upton, New York, and used by ...
, 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) process ...
(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 ...
(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