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) ca ...
in physics. It is somewhat analogous to the structure factor in solid-state physics, and the
form factor (quantum field theory)
In elementary particle physics and mathematical physics, in particular in effective field theory, a form factor is a function that encapsulates the properties of a certain particle interaction without including all of the underlying physics, b ...
.
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 were ...
. (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
Deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the realit ...
.
[
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
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type o ...
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 commonly ...
s form colourless hadrons with only two stable configurations,
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
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 atomic nucleus, nuclei of atoms. Since protons and ...
? 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 In a sufficiently hard interaction between particles, the cross section can be factorized into parton distribution functions (PDFs), the hard scattering part, and fragmentation functions. The fragmentation functions, as are the PDFs, are non-per ...
*
Structure factor
In condensed matter physics and crystallography, the static structure factor (or structure factor for short) is a mathematical description of how a material scatters incident radiation. The structure factor is a critical tool in the interpretation ...
*
Photon structure function
References
Scattering
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