On the smallness of the cosmological constant in SUGRA models
by
Roman Nevzorov(Glasgow)
→
Europe/London
OC218
OC218
Description
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.