matt@oddjob.UChicago.UUCP (Matt Crawford) (06/20/84)
Well... nobody stepped up with an explanation of the axion, so I will take a swing at it. The axion is a particle which results from a proposed solution to a field theory problem. The problem is that the QCD (Quantum ChromoCynamic) vacuum adds a CP-nonconserving contribution to the lagrangian [1]. This piece involves an arbitrary angle, theta, and CP is conserved only if theta is zero or pi. Now we know that CP, or charge-conjugation time parity, symmetry is in fact NOT conserved, but it is not broken with the strength that it would be if QCD forces were involved. When weak interactions are considered [2] the puzzle is worse: If the quark mass matrix is "M", the CP-violation parameter becomes theta' = theta + Arg(Det(M)). Measurements of the neutron electric dipole moment [3] place a limit of about 10^-8 on theta'. The solution proposed by Peccei & Quinn [4] involves a U(1) symmetry of the total lagrangian which removes the CP-violating angle but leaves behind a Goldstone boson -- the axion -- when the U(1) is broken by the vacuum expectation values of the Higgs scalars. The axion would actually have a mass Ma = 25*N*(x + 1/x) keV where x is a free parameter, and a lifetime for decay to two photons of t = 0.8*( 100keV / Ma )^5 sec -------summary-------- Existence of axions is neither established nor ruled out. They are postulated to remedy a hole in theory, but then so was the charmed quark. References: [1] G. 't Hooft, Phys Rev Lett 37(1976)8, Phys Rev D14(1976)3432 [2] R.D. Peccei, Proc. 1981 Int'l Conf on Neutrino Physics an Astrophysics [3] V. Baluni, Phys Rev D19(1979)2227 [4] R.D. Peccei & H.R. Quinn, Phys Rev D16(1977)1791 ___________________________________________________________ Matt University ARPA: crawford@anl-mcs.arpa Crawford of Chicago UUCP: ihnp4!oddjob!matt