Fields in nonaffine bundles. III. Effective symmetries and conserved currents in strings and higher branes

International audienceThe principles of a previously developed formalism for the covariant treatment of multiscalar fields for which (as in a nonlinear sigma model) the relevant target space is not of affine type---but curved---are recapitulated. Their application is extended from ordinary harmonic models to a more general category of harmonious field models, with emphasis on cases in which the field is confined to a string or higher brane world sheet, and for which the relevant internal symmetry group is non-Abelian, so that the conditions for conservation of the corresponding charge currents become rather delicate, particularly when the symmetry is gauged. Attention is also given to the conditions for conservation of currents of a different kind---representing surface fluxes of generalized momentum or energy---associated with symmetries not of the internal target space but of the underlying space-time background structure, including the metric and any relevant gauge field. For the corresponding current to be conserved the latter need not be manifestly invariant: preservation modulo a gauge adjustment will suffice. The simplest case is that of ``strong'' symmetry, meaning invariance under the action of an effective Lie derivative (an appropriately gauge adjusted modification of an ordinary Lie derivative). When the effective symmetry is of the more general ``weak'' kind, the kinetic part of the current is not conserved by itself but only after being supplemented by a suitable contribution from the background