What are antiparticles?

Corresponding to most kinds of particles, there are associated antiparticles with the same mass and opposite charge (electric charge and/or other properties as well- depending on the particle). For example, the antiparticle of the electron is the positively charged electron, or positron, which is produced naturally in certain types of radioactive decay. Because particles and their antiparticles have properties of equal magnitude but opposite signs, they are essentially mirror images of one another. Due to this symmetry of opposing charges, when a particle and its antiparticle collide, they annihilate each other, neutralizing the charges and releasing a burst of energy in their wake.

Antiparticles

Fermions

Bosons

Quarks

Gauge Bosons

Scalar Boson

Antiup

×3 colors:
antired, antiblue, antigreen

Anticharm

×3 colors:
antired, antiblue, antigreen

Antitop

×3 colors:
antired, antiblue, antigreen

Gluon

8 Gluon Colors

There are 8 gluons in total—4 inverse pairs.

Gluons carry both color and anticolor charges between quarks, so the antiparticle of any gluon is simply another of the gluons. e.g. The antiparticle of the red-antiblue gluon is the blue-antired gluon.

Higgs Boson

(own)

Antidown

×3 colors:
antired, antiblue, antigreen

Antistrange

×3 colors:
antired, antiblue, antigreen

Antibottom

×3 colors:
antired, antiblue, antigreen

Photon

(own)

Leptons

Positron

Antimuon

Antitau

W±

(± pair)

Z0

(own)

Electron Antineutrino

Muon Antineutrino

Tau Antineutrino

Graviton*

(own*)

A Universe of (Imperfect) Symmetry

The laws of nature are very nearly symmetrical with respect to particles and antiparticles. Just as protons and neutrons and electrons come together to form the atoms that make up matter, antiprotons and antineutrons and antielectrons come together to form antiatoms that make up antimatter.

This leads to the question of why the formation of matter after the Big Bang resulted in a universe consisting almost entirely of the matter we see around us, rather than being a half-and-half mixture of matter and antimatter. The discovery of CP Violation helped to shed light on this problem by showing that this symmetry, originally thought to be perfect, was only approximate.

CP Symmetry & Violation

CP Symmetry is short for Charge Conjugation Parity Symmetry, or the combination of C-symmetry (charge conjugation symmetry) and P-symmetry (Parity Symmetry). CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle, which carries an opposite charge (“charge” or C symmetry), and inverse spatial coordinates ("mirror" or P symmetry). In other words, in a universe of perfect CP Symmetry, all particles and antiparticles would exist in equal proportion so that if the charge and spatial orientation of either were flipped, it would essentially become the other.

CP Violation refers to the violation of CP Symmetry that we experience in the universe around us. The universe is made primarily of matter, rather than consisting of equal parts of matter and antimatter as CP Symmetry would anticipate.

If CP-symmetry was preserved, The Big Bang should have produced equal amounts of matter and antimatter. In that case, there should have been a total cancellation of both— because particles and their antiparticles annihilate each other. Protons should have cancelled with antiprotons, electrons with positrons, neutrons with antineutrons, and so on. This would have resulted in a sea of radiation (energy resulting from the cancelations) in a universe with no matter. Since this is not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP-symmetry.

Particle Vocab:

CP Symmetry is short for Charge Conjugation Parity Symmetry, or the combination of C-symmetry (charge conjugation symmetry) and P-symmetry (Parity Symmetry). CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle, which carries an opposite charge (“charge” or C symmetry), and then its spatial coordinates are inverted ("mirror" or P symmetry). In other words, in a universe of perfect CP Symmetry, all particles and antiparticles would exist in equal proportion so that if the charge and spatial orientation of either were flipped, it would essentially become the other.

CP Violation refers to the violation of CP Symmetry that we expeince in the universe around us. The universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter as CP Symmetry would suggest. If CP-symmetry was preserved, The Big Bang should have produced equal amounts of matter and antimatter. If that had occurred, there should have been a total cancellation of both— because particles and their antiparticles annihilate each other. That would equate to a universe entirely devoid of matter; since this is not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP-symmetry.

C-Symmetry

C-symmetry refers to the symmetry of physical laws under a charge-conjugation transformation— basically all this means is that if each charge x were to be replaced with a charge −x, aka if you were to reverse the directions of the electric and magnetic fields, the dynamics would preserve the same form (would be symmetrical). Electromagnetism, gravity and the strong interaction all obey C-symmetry, but weak interactions violate C-symmetry.

P-Symmetry

P-symmetry refers to the symmetry of physical laws under a parity transformation. Also called a parity inversion, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is can also be described by the simultaneous flip in the sign of all three spatial coordinates; i.e. a mirror image.