The structure function (also known as the proton structure function), like the fragmentation function, is a
probability density function
In probability theory, a probability density function (PDF), or density of a continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) c ...
in physics. It is somewhat analogous to the structure factor in solid-state physics, and the
form factor (quantum field theory).
The nucleon (proton and neutron) electromagnetic form factors describe the spatial distributions of electric charge and current inside the nucleon and thus are intimately related to its internal structure; these form factors are among the most basic observables of the
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 ...
. (Nucleons are the building blocks of almost all ordinary matter in the universe. The challenge of understanding the nucleon's structure and dynamics has occupied a central place in nuclear physics.)
The structure functions are important in the study of
deep inelastic scattering.
[
A. De Roeck and R.S. Thorne (2011). "Structure functions", ''Progress in Particle and Nuclear Physics'' 66 (4) pp 727–781, ]
The fundamental understanding of structure functions in terms of
QCD is one of the outstanding problems in hadron physics. Why do
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 form colourless hadrons with only two stable configurations,
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 ...
? One important step towards answering this question is to characterize the internal structure of the nucleon. High energy electron scattering provides one of the most powerful tools to investigate this structure.
See also
*
Fragmentation function
*
Structure factor
*
Photon structure function
References
Scattering
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