Axions and weakly interacting massive particles (WIMPs), such as weakly interacting neutrinos, are both hypothetical particles that have been proposed in the field of particle physics to explain certain phenomena beyond the Standard Model. However, there are significant differences between axions and WIMPs in terms of their properties and proposed roles in physics.
Properties:
Axions: Axions are hypothetical particles proposed to solve the strong CP problem in quantum chromodynamics (QCD), which is related to the behavior of the strong nuclear force. They are predicted to have very low mass and are considered to be very light and weakly interacting particles.
WIMPs: WIMPs, including weakly interacting neutrinos, are hypothetical particles proposed as candidates for dark matter, which is the invisible and mysterious form of matter that makes up a significant portion of the universe's mass. WIMPs are expected to have larger masses compared to axions and are characterized by their weak interaction with normal matter.
Interactions:
Axions: Axions interact very weakly with ordinary matter and photons. They are primarily associated with the strong nuclear force and their interactions are mostly through the so-called "axion-photon coupling."
WIMPs: WIMPs, including weakly interacting neutrinos, interact through the weak nuclear force. They are expected to have weak interactions with both ordinary matter and other particles, which is why they are called weakly interacting.
Roles in physics:
Axions: Axions were initially proposed to address the strong CP problem, which relates to the absence of violations of charge-parity (CP) symmetry in QCD. If axions exist, they could explain why CP symmetry is preserved in strong interactions and could potentially contribute to solving the mystery of dark matter as well.
WIMPs: WIMPs, including weakly interacting neutrinos, are primarily proposed as potential candidates for dark matter. Dark matter is thought to be responsible for the observed gravitational effects in the universe that cannot be accounted for by known forms of matter. WIMPs are one of the leading candidates to explain these observations.
In summary, axions and WIMPs (including weakly interacting neutrinos) are distinct particles proposed in different contexts. Axions are associated with the strong CP problem and have very low mass, while WIMPs are primarily considered as potential dark matter candidates with weak interactions and larger masses.