Model Independent Properties and Cosmological Implications of the Dilaton and Moduli Sectors of 4-d Strings

TL;DR

The paper argues that in any realistic 4D string framework with softly broken SUSY and vanishing cosmological constant, the dilaton and moduli supermultiplets inevitably obtain masses of order the gravitino mass, making them relevant for low-energy physics. It analyzes their masses within 4D supergravity and demonstrates the generic O($m_{3/2}$) scale for both scalars and fermions, supported by a class of orbifold models. The cosmological implications are then explored, highlighting gravitino-like constraints for fermionic components and the Polonyi-like entropy problems for scalars, along with possible remedies such as inflationary dilution, late-time baryogenesis, and potential dark matter or baryogenesis roles. The work emphasizes the cosmological viability of these fields and their potential to provide observable signatures of string theory beyond the MSSM.

Abstract

We show that if there is a realistic 4-d string, the dilaton and moduli supermultiplets will generically acquire a small mass O(m_{3/2}), providing the only vacuum-independent evidence of low-energy physics in string theory beyond the supersymmetric standard model. The only assumptions behind this result are (i) softly broken supersymmetry at low energies with zero cosmological constant, (ii) these particles interact with gravitational strength and the scalar components have a flat potential in perturbation theory, which are well-known properties of string theories. (iii) They acquire a $vev$ of the order of the Planck scale (as required for the correct value of the gauge coupling constants and the expected compactification scale) after supersymmetry gets broken. We explore the cosmological implications of these particles. Similar to the gravitino, the fermionic states may overclose the Universe if they are stable or destroy nucleosynthesis if they decay unless their masses belong to a certain range or inflation dilutes them. For the scalar states it is known that the problem cannot be entirely solved by inflation, since oscillations around the minimum of the potential can lead to a huge entropy generation at late times. We discus some possible ways to alleviate this entropy problem, that favour low-temperature baryogenesis, and also comment on the possible role of these particles as dark matter candidates or as sources of the baryon asymmetry through their decay.