NSUSY fits

TL;DR

This work assesses Natural SUSY with light stops by exploiting the predictive impact of stop loops on Higgs couplings to gluons and photons, encapsulated in the Wilson coefficients $c_g$ and $c_\gamma$ with the relation $c_g = 3 \left(1 + \frac{3 \alpha_s}{2\pi}\right) \frac{c_\gamma}{8}$. A comprehensive global fit combines Higgs signal strengths (48 channels), electroweak precision data, flavor constraints, and collider bounds, mapped through an EFT framework and explicit loop calculations, to constrain NSUSY parameter space. The analysis finds that while NSUSY can mildly improve fits relative to the SM, reproducing the observed $m_h \approx 125$ GeV with sub-TeV stops is difficult unless sizable mixing or additional new physics is present; a narrow “funnel” region with near-degenerate stops can reconcile Higgs and EW data but requires tuning. Indirect probes via Higgs data, $m_W$, and ${\rm Br}(\bar B \rightarrow X_s \gamma)$ thus offer powerful constraints complementary to direct searches, with projections suggesting meaningful exclusion potential by the end of the 8 TeV run, especially for certain mixing scenarios and stop spectra.

Abstract

We perform a global fit to Higgs signal-strength data in the context of light stops in Natural SUSY. In this case, the Wilson coefficients of the higher dimensional operators mediating g g -> h and h -> γγ, given by c_g, c_γ, are related by c_g = 3 (1 + 3 α_s/(2 π)) c_γ/8. We examine this predictive scenario in detail, combining Higgs signal-strength constraints with recent precision measurements of m_W, b-> s γconstraints and direct collider bounds on weak scale SUSY, finding regions of parameter space that are consistent with all of these constraints. However it is challenging for the allowed parameter space to reproduce the observed Higgs mass value with sub-TeV stops. We discuss some of the direct stop discovery prospects and show how global Higgs fits can be used to exclude light stop parameter space difficult to probe by direct collider searches. We determine the current status of such indirect exclusions and estimate their reach by the end of the 8 TeV LHC run.

Figures (12)

• Figure 1: The one dimensional fit to $F_g$ in the NSUSY scenario to global Higgs data (left), and the approximate projection of the relationship between the Wilson coefficients into the higher dimensional operator space (right).The green, yellow and gray regions correspond to the $1,2,3 \, \sigma$ allowed regions in the 1D or 2D fit space (defined with the CDF appropriate to each case. This difference accounts for the mismatch in the $\Delta \chi^2$'s that define the best-fit regions). Also shown as solid (brown) contours is the enhancement of the $\mu_{\gamma \, \gamma}$ signal strength and how such a condition projects into the best fit space.
• Figure 2: The projection into stop parameter space of the best-fit regions from a global fit to Higgs signal strength data. Here $\delta m = (m_{\tilde{t}_2}^2 - m_{\tilde{t}_1}^2)^{1/2}$. Colour convention is the same as in previous plots. The inset zooms into the low mass best-fit region (ruled out by LHC monophoton searches). We have varied $\tan \beta$ in the range $(2,20)$ and taken the overlap of the best fit spaces, slightly increasing the allowed parameter space. The three plots show the cases of no mixing ($\theta_{\tilde{t}} = 0$), intermediate mixing ($\theta_{\tilde{t}} = \pi/12$) and maximal mixing ($\theta_{\tilde{t}} = \pi/4$). The dashed line corresponds to the second minimum in the one parameter $\chi^2$ shown in the previous figure.
• Figure 3: Overlay of the best-fit regions of stop parameter space from global fit to Higgs signal strength data with the allowed space due to ${\rm Br}(\bar{B} \rightarrow X_s \, \gamma)$ at the level of $1,2,3 \,\sigma$. The $3 \, \sigma$ allowed region is overlaid with a vertical mesh, the $2 \, \sigma$ has a horizontal and vertical mesh, while the $1 \, \sigma$ allowed region is the green region with the further addition of the diagonal mesh. In V1 an V2 of this paper a sign error in the Appendix loop functions affected these results allowing a spike region for large $\mu$ to be consistent with ${\rm Br}(\bar{B} \rightarrow X_s \, \gamma)$ constraints.
• Figure 4: Constraints from precision measurements of $m_W$, shown as $1,2,3$ sigma shaded regions, overlaid on the allowed stop parameter space consistent with the global fit to Higgs signal strength data. Plot colour/display convention is the same as in previous figures.
• Figure 5: Best-fit regions in stop parameter space from the global $\chi^2$. Colour conventions the same as in previous figures. We show the three mixing cases $\theta_{\tilde{t}} = 0, \pi/12, \pi/4$ for two values of $\mu$ that set the Higgsino mass scale in these minimal scenarios, $\mu = 100,200 \, {\rm GeV}$. The $\tan \beta$ dependence of the result is primarily driven by the ${\rm Br}(\bar{B} \rightarrow X_s \, \gamma)$ constraint. Larger values of $\tan \beta$ select for a more degenerate stop spectrum when $\theta_{\tilde{t}} \neq 0$. In determining $\chi^2_{min}$, we have assumed a prior $150 \, {\rm GeV} \leq m_{\tilde{t}_1}$ , due to monophoton constraints. In each figure the best fit point is marked with a dot if present in the shown space. Note that for the plots where $\theta_{\tilde{t}} \neq 0$, the best fit point is greater than 1 $\rm{TeV}$, and this is why the point is absent. However we note again that the $\chi^2$ is a shallow function in the degenerate stop mass best fit region.