Dirac plus Nambu monopoles in the Standard Model.

We show how in the standard electroweak model three SU(2)L Nambu monopoles, each carrying electromagnetic (EM) and Z magnetic fluxes, can merge (through Z strings) with a single U(1)Y Dirac monopole to yield a composite monopole that only carries EM magnetic flux. Compatibility with the Dirac quantization condition requires this composite monopole to carry six quanta (12π/e) of magnetic charge, independent of the electroweak mixing angle θw. The Dirac monopole is not regular at the origin and the energy of the composite monopole is therefore divergent. We discuss how this problem is cured by embedding U(1)Y in a grand unified group such as SU(5). A second composite configuration with only one Nambu monopole and a colored U(1)Y Dirac monopole that has minimal EM charge of 4π/e is also described. Finally, there exists a configuration with an EM charge of 8π/e as well as screened color magnetic charge. [ABSTRACT FROM AUTHOR]