Precision Constraints on Extra Fermion Generations

TL;DR

This work summarizes the various motivations for extra families and analyzes them in view of the latest electroweak precision data.

Abstract

There has been renewed interest in the possibility of additional fermion generations. At the same time there have been significant changes in the relevant electroweak precision constraints, in particular, in the interpretation of several of the low energy experiments. We summarize the various motivations for extra families and analyze them in view of the latest electroweak precision data.

Figures (2)

• Figure 1: Individual 1 $\sigma$ constraints (39.35%) on $S$ and $T$. The contours assume $U = 0$ and $M_H = 117$ GeV except for the central and upper 90% C.L. filled contours ($\Delta \chi^2 = 4.605$) allowed by all data, which are for the indicated values. $\alpha_s$ is additionally constrained by the $\tau$ lifetime. Since the theory has changed, the strongly $\alpha_s$-dependent solid (dark green) contour from $Z$ line shape and cross section measurements LEPEWWG:2005ema has moved significantly towards negative $S$ and $T$ compared to our previous analysis Amsler:2008zzb. The long-dashed (magenta) contour from $\nu$ scattering has moved closer towards the global averages. The long-dash-dotted (indigo) contour from polarized $e$ scattering Anthony:2005pmYoung:2007zs is near the upper tip of an elongated ellipse centered at $(S,T) = (-15,-21)$. The dash-dotted (black) contour from APV now agrees perfectly with the SM after the completion of a state-of-the-art atomic theory calculation Porsev:2009pr. The shaded (light green) 1 $\sigma$ ellipse shows the combined low energy data (APV and lepton scattering).
• Figure 2: $S$ and $T$ for various mass splittings. The 90% CL ellipse is for $M_H =112$ GeV, while the solid line is the envelope for $M_H \geq 112$ GeV with the kink arising from the Tevatron exclusion window, 131 GeV $< M_H < 204$ GeV Aaltonen:2010sv, in the presence of a fourth generation. Each symbol refers to a choice of $m_{t'}^2 - m_{b'}^2$ (with $m_{b'} = 338$ GeV) which is increased in steps of 7,000 GeV$^2$ starting with degeneracy (black circles). Likewise, moving from right to left increases $m_{\l'}^2 - m_{\nu'}^2$ (with $m_{\nu'} = 101$ GeV) by the same increments, where the third entries correspond to the choice of Ref. Kribs:2007nz.