Is Natural SUSY Natural?

Journal of High Energy Physics (Impact Factor: 6.22). 06/2013; 2013(10). DOI: 10.1007/JHEP10(2013)133
Source: arXiv

ABSTRACT We study the fine tuning associated to a `Natural Supersymmetry' spectrum
with stops, after RG running, significantly lighter than the first two
generation sfermions and the gluino. In particular, we emphasise that this
tuning should be measured with respect to the UV parameters of the theory, and
improve the accuracy of previous approximate expressions. It is found that, if
running begins at 10^16 GeV (10^5 GeV), decreasing the UV stop mass below 0.75
(0.4) of the weak scale Majorana gluino mass does not improve the overall fine
tuning of the theory. In contrast, it is possible to raise the first two
generation sfermion masses out of LHC reach without introducing additional
tuning. After running, regions of parameter space favoured by naturalness and
consistent with LHC bounds typically have IR stop masses of order 1.5 TeV (0.75
TeV), and fine tuning of at least 400 (50) for high (low) scale mediation. We
also study the fine tuning of theories with Dirac gluinos. These allow for
substantial separation of the gluino and sfermion masses and, regardless of the
scale of mediation, lead to relatively low fine tuning of order 50. Hence
viable models can still favour light stops, but this requires extra structure
beyond the MSSM field content.

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