lpetrich
Contributor
So we've had a long history of solving physical riddles being connected with proposing new particles. This tradition continues with several particles that are proposed but not (yet) detected.
A curious result of QCD is possible CP violation. That would give electric dipole moments to neutrons, protons, and other hadrons with spin. But such EDM's are not observed - Particle Data Group - 2020 Review - Neutron electric dipole moment - Electron electric dipole moment
Upper limits: electron: 1.1*10^(-29) e-cm, muon: 1.8*10^(-19) e-cm, tau lepton: 4.5*10^(-16) e-cm, neutron: 1.8*10^(-26) e-cm, proton: 2.1*10^(-25) e-cm, lambda baryon: 1.5*10^(-16) e-cm
The Standard Model predicts very small values: electron 10^(-38) e-cm, neutron 10^(-31) e-cm. Scaling by (mass)^2 suggests that the Standard-Model muon value is 10^(-33) e-cm, proton and lambda and tau values close to the neutron one.
However, supersymmetric extensions of the Standard Model predict much higher values -- values extending up to experimental upper limits for the neutron.
The neutron upper limit is roughly 10^(-10) what one would expect for QCD, suggesting some very fine tuning -- or else some particle that forces it to zero. A particle named the axion. This particle is expected to interact with all Standard-Model gauge particles, not only the gluon (the QCD one), but also with the electroweak ones, notably the photon. This has inspired some lab searches for axions.
So far, this particle has not been detected, but its mass can be made arbitrarily small, making it *very* weakly interacting.
A curious result of QCD is possible CP violation. That would give electric dipole moments to neutrons, protons, and other hadrons with spin. But such EDM's are not observed - Particle Data Group - 2020 Review - Neutron electric dipole moment - Electron electric dipole moment
Upper limits: electron: 1.1*10^(-29) e-cm, muon: 1.8*10^(-19) e-cm, tau lepton: 4.5*10^(-16) e-cm, neutron: 1.8*10^(-26) e-cm, proton: 2.1*10^(-25) e-cm, lambda baryon: 1.5*10^(-16) e-cm
The Standard Model predicts very small values: electron 10^(-38) e-cm, neutron 10^(-31) e-cm. Scaling by (mass)^2 suggests that the Standard-Model muon value is 10^(-33) e-cm, proton and lambda and tau values close to the neutron one.
However, supersymmetric extensions of the Standard Model predict much higher values -- values extending up to experimental upper limits for the neutron.
The neutron upper limit is roughly 10^(-10) what one would expect for QCD, suggesting some very fine tuning -- or else some particle that forces it to zero. A particle named the axion. This particle is expected to interact with all Standard-Model gauge particles, not only the gluon (the QCD one), but also with the electroweak ones, notably the photon. This has inspired some lab searches for axions.
So far, this particle has not been detected, but its mass can be made arbitrarily small, making it *very* weakly interacting.