Is natural SUSY natural?

Journal of High Energy Physics (Impact Factor: 6.11). 06/2013; 2013(10). DOI: 10.1007/JHEP10(2013)133
Source: arXiv


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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    • ") or large visible energy present in a SUSY event. Utilising such discriminators, LHC limits on SUSY have significantly encroached into the region of parameter space consistent with natural electroweak symmetry breaking, with typical implied tuning of the order of 1% or less [1] [2] [3] [4] weakening the case for low-scale SUSY as a solution to the hierarchy problem. 1 "
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    ABSTRACT: In supersymmetric (SUSY) theories with extra dimensions the visible energy in sparticle decays can be significantly reduced and its energy distribution broadened, thus significantly weakening the present collider limits on SUSY. The mechanism applies when the lightest supersymmetric particle (LSP) is a bulk state-- e.g. a bulk modulino, axino, or gravitino-- the size of the extra dimensions larger than ~$10^{-14}$ cm, and for a broad variety of visible sparticle spectra. In such cases the lightest ordinary supersymmetric particle (LOSP), necessarily a brane-localised state, decays to the Kaluza-Klein (KK) discretuum of the LSP. This dynamically realises the compression mechanism for hiding SUSY as decays into the more numerous heavier KK LSP states are favored. We find LHC limits on right-handed slepton LOSPs evaporate, while LHC limits on stop LOSPs weaken to ~350-410 GeV compared to ~700 GeV for a stop decaying to a massless LSP. Similarly, for the searches we consider, present limits on direct production of degenerate first and second generation squarks drop to ~450 GeV compared to ~800 GeV for a squark decaying to a massless LSP. Auto-concealment typically works for a fundamental gravitational scale of $M_*$~10-100 TeV, a scale sufficiently high that traditional searches for signatures of extra dimensions are mostly avoided. If superpartners are discovered, their prompt, displaced, or stopped decays can also provide new search opportunities for extra dimensions with the potential to reach $M_*$~$10^9$ GeV. This mechanism applies more generally than just SUSY theories, pertaining to any theory where there is a discrete quantum number shared by both brane and bulk sectors.
    Preview · Article · Dec 2014 · Journal of High Energy Physics
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    • "This is a known effect [21] [22] [23] that the gluino, if large enough, can dominate the running of scalars. However, in the MSSM, obtaining a physical Higgs mass of 126 GeV requires very large stops or large stop mixing (large A-term), hence the gluino effect could be small unlike in the plus-type NMSSM models where we will show that it can be the main source of fine tuning. "
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    ABSTRACT: We present a comparative and systematic study of the fine tuning in Higgs sectors in three scale-invariant NMSSM models: the first being the standard Z3-invariant NMSSM; the second is the NMSSM plus additional matter filling 3(5+5¯) representations of SU(5) and is called the NMSSM+; while the third model comprises 4(5+5¯) and is called the NMSSM++. Naively, one would expect the fine tuning in the plus-type models to be smaller than that in the NMSSM since the presence of extra matter relaxes the perturbativity bound on λ at the low scale. This, in turn, allows larger tree-level Higgs mass and smaller loop contribution from the stops. However we find that LHC limits on the masses of sparticles, especially the gluino mass, can play an indirect, but vital, role in controlling the fine tuning. In particular, working in a semi-constrained framework at the GUT scale, we find that the masses of third generation stops are always larger in the plus-type models than in the NMSSM without extra matter. This is an RGE effect which cannot be avoided, and as a consequence the fine tuning in the NMSSM+ (Δ∼200) is significantly larger than in the NMSSM (Δ∼100), with fine tuning in the NMSSM++ (Δ∼600) being significantly larger than in the NMSSM+.
    Full-text · Article · Sep 2014 · Physical Review D
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    • "Softly-broken SUSY is an attractive solution to the HP [1] if the symmetry breaking between particle and sparticle masses is not too large. However, current collider bounds on the sparticle masses imply that the most popular SUSY theories, those based on the MSSM and its variants, must all be fine-tuned to < ∼ 1%, a level that many physicists find unpalatable [2] [3] [4] [5] [6] [7]. "
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    ABSTRACT: We study CP-conserving rare flavor violating processes in the recently proposed theory of Maximally Natural Supersymmetry (MNSUSY). MNSUSY is an unusual supersymmetric (SUSY) extension of the Standard Model (SM) which, remarkably, is untuned at present LHC limits. It employs Scherk-Schwarz breaking of SUSY by boundary conditions upon compactifying an underlying 5-dimensional (5D) theory down to 4D, and is not well-described by softly-broken \( \mathcal{N}=1 \) SUSY, with much different phenomenology than the Minimal Supersymmetric Standard Model (MSSM) and its variants. The usual CP-conserving SUSY-flavor problem is automatically solved in MNSUSY due to a residual almost exact U(1) R symmetry, naturally heavy and highly degenerate 1st- and 2nd-generation sfermions, and heavy gauginos and Higgsinos. Depending on the exact implementation of MNSUSY there exist important new sources of flavor violation involving gauge boson Kaluza-Klein (KK) excitations. The spatial localization properties of the matter multiplets, in particular the brane localization of the 3rd generation states, imply KK-parity is broken and tree-level contributions to flavor changing neutral currents are present in general. Nevertheless, we show that simple variants of the basic MNSUSY model are safe from present flavor constraints arising from kaon and B-meson oscillations, the rare decays B s,d → μ + μ −, μ → ēee and μ-e conversion in nuclei. We also briefly discuss some special features of the radiative decays μ → eγ and \( \overline{B}\to {X}_s\gamma \). Future experiments, especially those concerned with lepton flavor violation, should see deviations from SM predictions unless one of the MNSUSY variants with enhanced flavor symmetries is realized.
    Preview · Article · Sep 2014 · Journal of High Energy Physics
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