How massless are massless fields in AdSd

Massless fields of generic Young symmetry type in AdS(d) space are analyzed. It is demonstrated that in contrast to massless fields in Minkowski space whose physical degrees of freedom transform in irreps of o(d - 2) algebra, AdS massless mixed symmetry fields reduce to a number of irreps of o(d - 2) algebra. From the field theory perspective this means that not every massless field in flat space admits a deformation to AdS(d) with the same number of degrees of freedom, because it is impossible to keep all of the flat space gauge symmetries unbroken in the AdS space. An equivalent statement is that, generic irreducible AdS massless fields reduce to certain reducible sets of massless fields in the flat limit. A conjecture on the general pattern of the flat space limit of a general AdS(d) massless field is made. The example of the three-cell "hook" Young diagram is discussed in detail. In particular, it is shown that only a combination of the three-cell fiat-space field with a graviton-like field admits a smooth deformation to AdS(d). (C) 2000 Elsevier Science B.V. All rights reserved.