Multiple astronomical observations tell us that the visible matter described by the Standard Model provides just a small part of the total density of the universe. As a result, the properties of ALPs are less theoretically constrained. We will also touch briefly on “axion-like particles” (ALPs), which do not solve the strong CP problem. This will be the main topic of this review. In this review, we reserve the term “axion” for the particle arising from the solution to the strong CP problem described below, also known as the “QCD axion” (quantum chromodynamics axion). However, this Standard Model mechanism is insufficient to explain the cosmological matter-antimatter asymmetry, and, moreover, it is a puzzle that CP violation does not appear also in the strong nuclear interaction: This is known as the “strong CP problem.” As we discuss below, this puzzle could be resolved via the hypothetical axion particle. These observations can be accommodated within the Standard Model with six quarks ( 11), albeit without a profound explanation. A form of CP violation was found in the laboratory over 50 years ago ( 10), and by now, it has been observed in many decays via the weak interactions. These are charge conjugation (denoted by C) and its combination with parity reversal (denoted by P also known as spatial inversion symmetry). This cosmological matter-antimatter asymmetry is thought to be because of differences in the interactions of elementary particles and antiparticles ( 9), which violate certain symmetries that distinguish particle from antiparticle.

However, astrophysical and cosmological observations tell us that most of the visible material in the universe is composed of the same matter particles as us on Earth and that there are no large concentrations of antimatter. The existence of every antiparticle in the Standard Model has been confirmed experimentally by observing their production in the collisions of ordinary particles. Uncharged particles such as photons may be their own antiparticles. For each matter particle in the Standard Model, special relativity and quantum mechanics require ( 8) the existence of a corresponding antimatter particle with identical mass and spin but opposite charge. One of these issues is how matter and antimatter are (and are not) distinguished at the level of fundamental particles. Following its discovery, experiments have confirmed that it has zero spin, unlike any fundamental particle known previously, and interactions with other particles that are proportional to their masses, as predicted by the Standard Model. They also confirmed the necessity of the Higgs boson and enabled its mass to be estimated numerically. Precise experimental measurements in the decades preceding 2010 verified many other predictions of the Standard Model, including the existence and mass of the top quark. The crowning success of the Standard Model was the discovery of the Higgs boson in 2012 ( 1, 2), a particle of a previously unknown type whose existence was predicted in 1964 ( 3) to solve theoretical problems associated with the masses of vector bosons. The Standard Model is a mathematically consistent quantum field theory that allows theorists to calculate accurate predictions, which have, in many cases, been verified experimentally with a precision below the per mil level at the Large Hadron Collider (LHC) and other particle accelerators. It categorizes the fundamental constituents of matter, the quarks and leptons, and the electromagnetic, weak and strong nuclear forces between them. The Standard Model of particle physics provides a successful description of the visible matter in the universe, from stars to the inner workings of atoms and nuclei.

Kate markx