Supersymmetry, Naturalness, and Light Higgsinos

Abstract

We compare and contrast three different sensitivity measures, $Δ_{EW}^{-1}$, $Δ_{HS}^{-1}$ and $Δ_{BG}^{-1}$ that have been used in discussions of fine-tuning. We argue that though not a fine-tuning measure, $Δ_{EW}$, which is essentially determined by the particle spectrum, is important because $Δ_{EW}^{-1}$ quantifies the minimum fine-tuning present in any theory with a specified spectrum. We emphasize the critical role of incorporating correlations between various model parameters in discussions of fine-tuning. We provide toy examples to show that if we can find high scale theories with specific correlations amongst parameters, the value of the traditional fine-tuning measure $Δ_{BG}^{-1}$ (which differs significantly from $Δ_{HS}^{-1}$ only when these correlations are important) would be close to $Δ_{EW}^{-1}$. We then set up the radiatively driven natural SUSY framework that we advocate for phenomenological analyses of natural models of supersymmetry, and review the implications of naturalness for LHC and ILC searches for SUSY as well as for searches for SUSY dark matter.

23 pages, 4 figures, Invited contribution to the Volume Commemorating C.V.Raman's 125th Birth Anniversary; v2: added footnote 7 and updated references, published version; v3: The cross sections for the pair production of electroweak-inos shown in Fig.2 were too high by a factor 2. The error is confined to just this figure, and does not affect any other results in the paper