Updated Status of the Global Electroweak Fit and Constraints on New Physics (1107.0975v2)

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.

• The paper presents a rigorous statistical fit of Standard Model parameters using likelihood functions and high-energy collider data.
• It refines the key oblique parameters S, T, and U, highlighting constraints that may indicate the presence of new physics.
• The analysis reinforces the validity of the SM while identifying parameter ranges that guide future experimental tests for beyond-SM theories.

Evaluation of the Standard Model via Parameter Constraints

Research in fundamental physics often involves the rigorous examination and testing of theoretical frameworks by comparing predictions with experimental data. This paper concerns itself with the Standard Model (SM) of particle physics, specifically focusing on the fit of SM parameters and oblique parameters, which are crucial for understanding electroweak precision tests. The authors present a structured analysis of various constraints imposed by empirical data on these parameters, with the aim of refining SM predictions.

Introduction and Methodology

The paper begins with a comprehensive introduction to the applicability of SM in describing the fundamental interactions among elementary particles. Recognizing the model's well-documented successes, the authors delve into areas where precision testing can lead to either validation or the necessity for new physics. The discussion soon transitions to the methodology employed in obtaining a robust fit for SM parameters, emphasizing the role of theoretical constraints and the integration of various experimental results.

SM Parameter Fit and Oblique Parameters

The main body of work is dedicated to the statistical fitting process for SM parameters. This involves using likelihood functions constrained by established physical laws and current data from high-energy experiments. The result is a refined set of parameter estimates that align with empirical observations. The exploration of oblique parameters—S, T, and U—receives special attention as these quantities are sensitive to new physics beyond the SM.

Constraints and Implications

A rigorous examination of constraints stemming from existing particle accelerators and other experimental setups provides insights into the viability of the current SM formulations. The authors delve into constraints imposed by collider data, which limit possible extensions to the SM. These findings reinforce certain parameter ranges while suggesting areas where deviations may imply physics beyond the current theory, such as in the presence of heavy new particles or unknown interactions.

Conclusions and Future Directions

The paper concludes by synthesizing the fit results and their implications. The analysis suggests that while the SM remains a robust framework, the potential for new physics indicated by the challenging constraints on oblique parameters is significant. The authors advocate for continued precision tests and experimental validations, which could illuminate any departures from SM expectations.

Future research directions speculated upon include the refinement of models involving electroweak symmetry breaking and potential discoveries at upcoming collider experiments. The findings serve as a call for both theoretical advancements and experimental precision to limit or expand the existing parameter space of the Standard Model, potentially leading to the development of new theories in particle physics.