The String Dilaton and a Least Coupling Principle

TL;DR

This paper proposes a mechanism where string-loop effects fix the dilaton's value, making it compatible with experiments, and predicts small deviations from general relativity that motivate more precise tests of fundamental physics.

Contribution

It introduces a cosmological evolution model for the dilaton that naturally leads to its decoupling from matter, under a universality assumption, and estimates residual couplings today.

Findings

Dilaton can be driven to decouple from matter through cosmological evolution.

Residual dilaton-matter coupling is weak but potentially observable.

Results motivate improved experimental tests of Einstein's Equivalence Principle.

Abstract

It is pointed out that string-loop modifications of the low-energy matter couplings of the dilaton may provide a mechanism for fixing the vacuum expectation value of a massless dilaton in a way which is naturally compatible with existing experimental data. Under a certain assumption of universality of the dilaton coupling functions , the cosmological evolution of the graviton-dilaton-matter system is shown to drive the dilaton towards values where it decouples from matter (``Least Coupling Principle"). Quantitative estimates are given of the residual strength, at the present cosmological epoch, of the coupling to matter of the dilaton. The existence of a weakly coupled massless dilaton entails a large spectrum of small, but non-zero, observable deviations from general relativity. In particular, our results provide a new motivation for trying to improve by several orders of magnitude the various experimental tests of Einstein's Equivalence Principle (universality of free fall, constancy of the constants,\dots).