On the smallness of the cosmological constant in SUGRA models

by Roman Nevzorov (Glasgow)

Thursday 29 Nov 2007, 14:15 → 15:15 Europe/London

OC218

It is well known that global symmetries can protect a zero value for the cosmological constant in the $N=1$ supergravity (SUGRA). The best motivated scenario of this type is no--scale supergravity where global symmetries guarantee not only vanishing of the vacuum energy density but also preserve local supersymmetry (SUSY) in all vacua. The breakdown of these symmetries that ensures the vanishing of the vacuum energy density near the physical vacuum leads to the natural realization of the multiple point principle (MPP) assumption, i.e. results in the set of degenerate vacua with broken and unbroken local supersymmetry. We present the minimal SUGRA model where the MPP assumption is realised naturally at the tree--level. In this model vacua with broken and unbroken local supersymmetry in the hidden sector (first and second phases) have the same energy density without any extra fine-tuning. Although hidden sector does not give rise to the breakdown of supersymmetry in the second phase SUSY may be broken there dynamically in the observable sector. Then a positive value of the energy density in the second vacuum is induced which can be assigned, by virtue of MPP, to all other phases including the one in which we live. The total vacuum energy density is naturally tiny or zero in this case. In order to reproduce the observed value of the cosmological constant and preserve gauge coupling unification the low energy matter content has to involve extra matter beyond the MSSM contained in $5+\bar{5}$ representations of $SU(5)$. Extra fields have masses of the order of supersymmetry breaking scale so that they can be detected at future colliders. We also discuss the supersymmetry breakdown and possible solution of the cosmological constant problem by MPP in models with an enlarged gauge symmetry.