Dilatons

Dilatons are hypothetical scalar fields associated with scale or conformal invariance in theoretical physics. In many contexts they are identified as the Nambu-Goldstone bosons that arise when a system’s scale symmetry is spontaneously broken. If scale invariance is only approximate, the dilaton may be a light pseudo-Goldstone boson with a small mass and a nontrivial potential.

In string theory, the dilaton is a fundamental scalar that determines the strength of the string coupling. The vacuum expectation value of the dilaton sets g_s = e^{<phi>}, and the low-energy effective action includes kinetic terms for the dilaton as well as couplings to gravity and other fields. Its dynamics influence the behavior of interactions at different energy scales and can affect cosmology and moduli stabilization.

In particle physics models with approximate conformal symmetry, the dilaton can arise as a compensating field under scale transformations. If the symmetry is spontaneously broken, the dilaton behaves like a massless mode in the exact case; explicit breaking introduces a mass and a potential, making it a pseudo-dilaton. Such a particle could couple to Standard Model fields and modify processes at high energy or in cosmological settings, though there is no experimental evidence for it to date.

In gravitational and extra-dimensional theories, dilaton-like scalars frequently appear. In Brans-Dicke–type theories, a scalar field alters effective gravitational couplings. In extra-dimensional models, a radion or dilaton describes the size of extra dimensions and may be stabilized by a potential, with phenomenological consequences.

Experimental status: no confirmed detections of dilatons. Searches constrain couplings and masses across wide parameter ranges, but dilatons remain speculative components of several beyond-Standard-Model frameworks.