The antineutron is the
antiparticle
In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...
of the
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 beh ...
with symbol . It differs from the neutron only in that some of its properties have
equal magnitude but opposite sign. It has the same
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
as the neutron, and no net
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
, but has opposite
baryon number
In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as
::B = \frac\left(n_\text - n_\bar\right),
where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Baryo ...
(+1 for neutron, −1 for the antineutron). This is because the antineutron is composed of
antiquarks, while neutrons are composed of
quarks. The antineutron consists of one
up antiquark
The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quark ...
and two
down antiquark
The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up q ...
s.
Background
The antineutron was discovered in
proton–antiproton collisions at the
Bevatron
The Bevatron was a particle accelerator — specifically, a weak-focusing proton synchrotron — at Lawrence Berkeley National Laboratory, U.S., which began operating in 1954. The antiproton was discovered there in 1955, resulting in ...
(
Lawrence Berkeley National Laboratory) by the team of
Bruce Cork
Bruce Cork (1916 – October 7, 1994) was a physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicis ...
, Glen Lambertson,
Oreste Piccioni
Oreste Piccioni (October 24, 1915 – April 13, 2002) was an Italian-American physicist who made important contributions to elementary particle physics during the early years of its history.
He was a graduate student of Enrico Fermi at the U ...
, and William Wenzel in 1956, one year after the
antiproton
The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.
The exis ...
was discovered.
Since the antineutron is electrically neutral, it cannot easily be observed directly. Instead, the products of its
annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy ...
with ordinary matter are observed. In theory, a free antineutron should
decay into an
antiproton
The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.
The exis ...
, a
positron and a
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
in a process analogous to the
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
of
free 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 behave ...
s. There are theoretical proposals of neutron–antineutron oscillations, a process that implies the violation of the
baryon number
In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as
::B = \frac\left(n_\text - n_\bar\right),
where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Baryo ...
conservation.
Magnetic moment
The
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagne ...
of the antineutron is the opposite of
that of the neutron.
It is for the antineutron but for the neutron (relative to the direction of the
spin). Here ''μ''
N is the
nuclear magneton
The nuclear magneton (symbol ''μ'') is a physical constant of magnetic moment, defined in SI units by:
:\mu_\text =
and in Gaussian CGS units by:
:\mu_\text =
where:
:''e'' is the elementary charge,
:''ħ'' is the reduced Planck constant ...
.
See also
*
Antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
*
Neutron magnetic moment
The nucleon magnetic moments are the intrinsic magnetic dipole moments of the proton and neutron, symbols ''μ''p and ''μ''n. Protons and neutrons, both nucleons, comprise the nucleus of atoms, and both nucleons behave as small magnets whose ...
*
List of particles
References
External links
LBL Particle Data Group: summary tablessuppression of neutron-antineutron oscillation includes information about antineutron discovery (archived link)
*
explains how the antineutron differs from the regular neutron despite having the same, that is zero, charge.
{{Particles
Antimatter
Baryons
Neutron
Nucleons