Patrick Janot edited EWSB parameters.tex  over 10 years ago

Commit id: 38594e69e725cdaddecaea38fb031cf5bbe0de4f

deletions | additions      

       

\section{\ref{sec:EWSB} Precise Measurements of the EWSB parameters}  \label{sec:EWSB}  Electroweak loops have the remarkable property of being sensitive to the existence of weakly-coupled particles, even if they cannot be directly produced or observed in current experiments. For example, the measurements of the Z resonance line-shape parameters, undertaken at LEP during a dedicated scan in 1993, led to a prediction of the top quark mass $m_{\rm top}$ of $172\pm 20$ GeV/$c^2$ by the time of the Moriond conference in March 1994~\cite{cite:Pietrzyk}. The uncertainty on $m_{\rm top}$ was dominated by the range of assumptions for the Higgs boson mass, varied from 60 to 1000 GeV/$c^2$. When the top quark was discovered at the Tevatron in 1995, and its mass measured with precision of a few GeV/$c^2$ within one standard deviation of the prediction, the Electroweak fits of the LEP data became sensistive sensitive  to the only remaining unknown quantity in the Standard Model, the Higgs boson mass $m_{\rm H}$, predicted to be $m_{\rm H} = 99^{+28}_{-23}$ GeV/$c^2$~\cite{_Lys_Murayama_Wohl_et_al__2012}. It is remarkable that the observation of the H(126) particle at the LHC falls, once again, within one standard deviation of this prediction. These two historical examples are specific of the Standard Model, with its specific particle content -- and nothing else. Now that the Higgs boson mass is measured with a precision of a fraction of a GeV/$c^2$, and barring accidental or structural cancellations, these fits rule out the existence of any additional particle that would have contributed to the Electroweak loop corrections in a measurable way. As emphasized in Ref.~\cite{_Lys_Murayama_Wohl_et_al__2012}, the corrections to the W and Z masses do not necessarily decouple when the mass of new additional particles increase (contrary to the corrections to, e.g., $(g-2)_{\mu}$). For example, the top-quark loop correction scales like $(m^2_{\rm top} - m^2_{\rm b}) / m^2_{\rm W} $. The Electroweak loop corrections are also delicately sensitive to the details of the Electroweak Symmetry Breaking Mechanism.