What do precision Higgs measurements buy us?

TL;DR

The paper assesses how future precision Higgs measurements and electroweak observables can indirectly probe beyond-Standard-Model physics using a Standard Model EFT framework. It analyzes two benchmark scenarios: a heavy gauge-singlet coupled through the Higgs portal, which can enable a strongly first-order electroweak phase transition for baryogenesis, and light scalar top quarks in the MSSM, which alleviate fine-tuning at one loop. By integrating out the heavy states, the authors derive Wilson coefficients for bosonic dimension-six operators and map them to Higgs observables and EWPO, highlighting how Higgs universal oblique corrections and RG-induced S and T parameters constrain the models. Their results show that precision Higgs and EW measurements provide comparable sensitivity across scenarios, with future lepton colliders (including GigaZ/TeraZ) capable of testing large regions of parameter space, including the entire first-order EWPT-viable region in the singlet model and the natural MSSM stop region around $m_{\tilde t} \sim 1\,\text{TeV}$ and $X_t \sim \sqrt{6}\,m_{\tilde t}$. Overall, precision Higgs programs deliver powerful indirect probes that complement direct searches for new physics.

Abstract

We study the sensitivities of future precision Higgs measurements and electroweak observables in probing physics beyond the Standard Model. Using effective field theory--appropriate since precision measurements are indirect probes of new physics--we examine two well-motivated test cases. One is a tree-level example due to a singlet scalar field that enables the first-order electroweak phase transition for baryogenesis. The other is a one-loop example due to scalar top in the MSSM. We find both Higgs and electroweak measurements are sensitive probes of these cases.

Figures (3)

• Figure 1: $2\sigma$ contours of future precision measurements on the singlet model in Eq. \ref{['eqn:singlet_L']}. Regions below the contours will be probed. The magenta contour is the $2\sigma$ sensitivity to the universal Higgs oblique correction in Eq. \ref{['eqn:Zh']} at ILC 500up. Blue contours show the $2\sigma$ RG-induced constraints from the $S$ and $T$ parameters in Eqs. \ref{['eqn:sing_S']}-\ref{['eqn:sing_T']} from current measurements (solid) Baak:2012kk and future sensitivities at ILC GigaZ (dashed) Baak:2013fwa and TLEP TeraZ (dotted) Mishima:2013. Regions of a viable first order EW phase transition, from Eq. \ref{['eqn:first_order_PT_bounds']}, are shown in the gray, hatched regions for $k=1$ and $4\pi$.
• Figure 2: $2\sigma$ contours of precision Higgs and EW observables as a function of $m_{\tilde{t}}$ and $X_t$ in the MSSM. The contours show $2\sigma$ sensitivity of ILC 500up to the universal Higgs oblique correction (magenta) and modifications of $h\to gg$ (brown) and $h \to \gamma\gamma$ (green). Constraints from $S$ and $T$ parameters are shown in blue for current measurements (solid), ILC GigaZ (dashed), and TLEP TeraZ (dotted). The shaded red region shows contours of Higgs mass between 124-127 GeV in the MSSM Heinemeyer:1998yj*Heinemeyer:1998np*Degrassi:2002fi*Frank:2006yh. The shaded gray regions are unphysical because one of the stop mass eigenvalues becomes negative.
• Figure 3: The $1\sigma$ (darker) and $2\sigma$ (lighter) ellipses of precision EW parameters $S$ and $T$. We show current fits (solid, black) together with projected sensitivities at ILC GigaZ (dashed, blue) and TLEP TeraZ (dotted, red). The lines show the size of $S$ and $T$ parameters in our singlet model with $A = m_S^{}$ (teal) and in the MSSM with $\tan \beta = 30$ for $X_t = \sqrt{6} m_{\tilde{t}}$ (green) and $X_t = 0$ (purple). The tick marks show specific mass values in each model; $m_{\tilde{t}}$ values in $200 \text{ GeV}$ increments starting from $m_{\tilde{t}} = 200 \text{ GeV}$ for $X_t =0$ and $m_{\tilde{t}} = 400 \text{ GeV}$ for $X_t = \sqrt{6} m_{\tilde{t}}$ in the MSSM; $m_S^{}$ values in 200 GeV increments between $200$-$1000 \text{ GeV}$ and $500 \text{ GeV}$ increments between $1000$-$3000 \text{ GeV}$ in the singlet model.