Speaker
Dylan Harries
(University of Adelaide)
Description
Non-minimal supersymmetric models are able to solve some of the naturalness
problems of the minimal supersymmetric standard model (MSSM), such as the need
for large radiative corrections to accommodate a 125 GeV Higgs. Models with an
additional $U(1)'$ symmetry at low energies may raise the tree level Higgs
mass through $F$- and $D$-term contributions, reducing the need for such
corrections, and also allow for the solution of the MSSM $\mu$ problem. On
the other hand, in one example of such a $U(1)$ extension, the exceptional
supersymmetric standard model (E$_6$SSM), it has been found that a new tree
level fine tuning arises due to large experimental limits on the $Z'$
mass in this model. We investigate the fine tuning associated with these
limits in a wider class of $U(1)$ extended models that are based on an
underlying $E_6$ symmetry at the grand unification (GUT) scale. We adopt a conservative approach
in which the soft parameters are set at low energies, thus removing any tuning
that comes from assumptions about how SUSY is broken. In this case, we find
that increasing the limits on the $Z'$ mass increases the fine tuning,
highlighting the importance of $Z'$ searches at run II of the LHC for
constraining naturalness in these models. In general, the severity of this
tuning depends rather strongly on the choice of $U(1)'$ charges. As a result,
models such as the $U(1)_I$ inert model are able to satisfy current $Z'$ mass
limits without having a large fine tuning, while in others the limits are
already strong enough that a moderate degree of tuning is required.
Primary author
Dylan Harries
(University of Adelaide)
Co-authors
Prof.
Anthony Williams
(University of Adelaide)
Peter Athron
