Updated Status of the Global Electroweak Fit and Constraints on New Physics

TL;DR

This work revisits the global electroweak fit of the Standard Model using updated precision data and a new hadronic vacuum polarization input, delivering a consistent framework (Gfitter) to translate EW observables into constraints on oblique parameters $S$, $T$, $U$ and on a broad class of new physics models. The analysis yields a light Higgs preference in the SM ($M_H o 91^{+30}_{-23}$ GeV) that shifts upward when direct Higgs searches are included ($M_H o 120^{+12}_{-5}$ GeV) and provides a precise indirect determination of $M_W$ and $ ext{sin}^2 heta^{ m eff}_{ m lep}$, while constraining the strong coupling $\a_S(M_Z^2) o 0.1194\pm0.0028$. The oblique fits show compatibility with many BSM scenarios—such as a sequential fourth generation, two-Higgs-doublet models, inert-Higgs models, little-Higgs with T-parity, and various extra-dimension constructions—often allowing heavier Higgs masses when new weak-isospin breaking effects compensate the SM contributions. These results illustrate how EW precision data continue to shape the viable parameter spaces of proposed new physics, and demonstrate the continuing relevance of EW fits in the LHC era for testing SM consistency and guiding model-building.

Abstract

We present an update of the Standard Model fit to electroweak precision data. We include newest experimental results on the top quark mass, the W mass and width, and the Higgs boson mass bounds from LEP, Tevatron and the LHC. We also include a new determination of the electromagnetic coupling strength at the Z pole. We find for the Higgs boson mass (91 +30 -23) GeV and (120 +12 -5) GeV when not including and including the direct Higgs searches, respectively. From the latter fit we indirectly determine the W mass to be (80.360 +0.014 -0.013) GeV. We exploit the data to determine experimental constraints on the oblique vacuum polarisation parameters, and confront these with predictions from the Standard Model (SM) and selected SM extensions. By fitting the oblique parameters to the electroweak data we derive allowed regions in the BSM parameter spaces. We revisit and consistently update these constraints for a fourth fourth fermion generation, two Higgs doublet, inert Higgs and littlest Higgs models, models with large, universal or warped extra dimensions and technicolour. In most of the models studied a heavy Higgs boson can be made compatible with the electroweak precision data.

Figures (22)

• Figure 1: Contribution to the $\chi^2$ test statistic versus $M_H$ derived from the experimental information on direct Higgs boson searches made available by the LEP Higgs Boson and the Tevatron New Phenomena and Higgs Boson Working Groups Barate:2003sz:2010arCDF:1336706 and the ATLAS Collaboration:2011qi and CMS Collaborations Chatrchyan:2011tz. The solid (black) and dashed (dark red) lines show the contribution from LEP and Tevatron, while the dotted (light red) and dashed-dotted (blue) lines indicate the constraints obtained from the 2010 data by ATLAS and CMS, respectively. Following the original figures they have been interpolated by straight lines for the purpose of presentation and in the fit. The light green area gives the combination of these measurements. Correlations due to common systematic errors have been neglected in this combination. See text for a description of the method applied.
• Figure 2: Comparing fit results with direct measurements: pull values for the complete fit (left), and results for $M_H$ from the standard fit excluding the respective measurements from the fit (right).
• Figure 4: Indirect determination of the Higgs boson mass: $\Delta\chi^2$ as a function of $M_H$ for the standard fit (top) and the complete fit (bottom). The solid (dashed) lines give the results when including (ignoring) theoretical errors. Note that we have modified the presentation of the theoretical uncertainties here with respect to our earlier results Flacher:2008zq. Before, the minimum $\chi^2_{{\rm min}}$ of the fit including theoretical errors was used for both curves to obtain the offset-corrected $\Delta\chi^2$. We now individually subtract each case so that both $\Delta\chi^2$ curves touch zero. In spite of the different appearance, the theoretical errors used in the fit are unchanged and the numerical results, which always include theoretical uncertainties, are unaffected.
• Figure 5: P-value versus $M_H$ of the standard electroweak fit as obtained from pseudo-MC simulation. The error band represents the statistical error from the MC sampling size.
• Figure 6: Indirect determination of the $W$ boson mass: profile of $\Delta\chi^2\xspace$ versus $M_W$ for the complete fit (blue shaded curve) and the standard fit (green shaded curve). In both fits the direct $M_W$ measurement, indicated by the dot with $1\sigma$ error bar, is not included. The widths of the bands indicate the size of the cumulative theoretical uncertainty in the fit. The grey shaded curve shows the constraint one would obtain for a hypothetical Higgs discovery at 120 $\mathrm{Ge V}$ (with negligible error on $M_H$).