Hypercharge-universal topcolor

We propose and discuss the phenomenology of a topcolor-assisted technicolor model with a flavor-universal hypercharge sector. After discussing the symmetry-breaking pattern and low-energy effective Lagrangian, we examine various experimental and theoretical constraints, finding that precision electroweak measurements yield the strongest bounds on the model. We perform a combined fit to all available $Z$-pole and LEP2 data and find that the goodness of fit for hypercharge-universal topcolor is comparable to that of the standard model. In contrast, topcolor-assisted technicolor models with a flavor nonuniversal hypercharge sector are markedly disfavored by the data.

Figure 1

The gap triangle corresponding to Eqs. (2.24, 2.25, 2.26) lies between the solid lines (i) the lower bound for $⟨\overline{b}b⟩=0$, (ii) the upper bound for $⟨\overline{\tau }\tau ⟩=0$ and (iii) the lower bound for $⟨\overline{t}t⟩\ne 0$. The values ${\alpha }_Y(M_Z)=0.010$ and ${\alpha }_S(M_Z)=0.118$ from Ref. 29 were used in the calculations. The dashed line represents a solution of the gap equation for the scale $u=500\mathrm{GeV}$ and $p=1$ for a dynamical top mass of $m_t\approx 170\mathrm{GeV}$. Solutions for higher scales lie to the left of this line.

Figure 2

Lower bound for $M_C$ due to $K\overline{K}$ (solid line) and $B\overline{B}$ (dashed line) mixing as a function of ${\kappa }_3$.

Figure 3

The solid (dashed) curve shows the restriction on model parameter space obtained from a fit of all $Z$ pole and off-pole data listed in the first two tables of the Appendix for an 800 GeV (1500 GeV) reference Higgs mass. The region outside the parabola is excluded at 95% C.L.

Figure 4

The solid (dashed) curve shows limits on the $Z^{\prime }$ mass as a function of ${\kappa }_1$ resulting from a fit of all $Z$ pole and off-pole data listed in the first two tables of the Appendix for an 800 GeV (1500 GeV) reference Higgs mass. Values outside the boomerang shaped region are excluded at 95% C.L.

Figure 5

Limits on the $Z^{\prime }$ mass as a function of ${\kappa }_1$ resulting from a fit of all $Z$ pole and off-pole data listed in the first two tables of the Appendix for an 800 GeV reference Higgs mass. Values outside the appropriate boomerang shaped region are excluded at 95% C.L. The overlapping pair of solid curves corresponds to $f_t=75\mathrm{GeV}$; the upper one includes exchange of a top-pion of mass 1200 GeV and lower one does not include top-pion exchange. The overlapping pair of dashed curves corresponds to $f_t=150\mathrm{GeV}$; the upper one includes exchange of a top-pion of mass 250 GeV and the lower one does not include top-pion exchange. Note that the main effect on the shape and position of the allowed region comes from altering $f_t$.

Figure 6

Fits to $Z$-pole observables in the hypercharge-universal TC2 model introduced in this paper. From left to right, the columns show the experimental values, the 1-loop SM values ${\mathcal{O}}_i^{1\mathrm{\text{−}}\mathrm{loop}}$ with $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$, and the predictions for the TC2 model with their pulls. The TC2 model fit assumed $f_t=75\mathrm{GeV}$ and $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$.

Figure 7

Fits to LEP II observables at several values of $\sqrt{s}$ in the hypercharge-universal TC2 model introduced in this paper. From left to right, the columns show the experimental values, the 1-loop SM values ${\mathcal{O}}_i^{1\mathrm{\text{−}}\mathrm{loop}}$ with $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$, and the predictions for the TC2 model with their pulls. The TC2 model fit assumed $f_t=75\mathrm{GeV}$ and $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$.

Figure 8

One-loop fits to $Z$-pole observables in the standard model with $M_{\mathrm{H},\mathrm{ref}}=115\mathrm{GeV}$, showing the experimental values, the predicted values, and the pulls.

Figure 9

One-loop fits to LEP II observables at several values of $\sqrt{s}$ in the standard model with $M_{\mathrm{H},\mathrm{ref}}=115\mathrm{GeV}$, showing the experimental values, the predicted values, and the pulls.

Figure 10

Fits to $Z$-pole observables in the classic [11] or flavor-universal TC2 [16, 21] model. From left to right, the columns show the experimental values, the 1-loop SM values ${\mathcal{O}}_i^{1\mathrm{\text{−}}\mathrm{loop}}$ with $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$, and the predictions for the TC2 model with their pulls. The TC2 model fit assumed $f_t=75\mathrm{GeV}$ and $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$.

Figure 11

Fits to LEP II observables at several values of $\sqrt{s}$ in the classic [11] or flavor-universal TC2 [16, 21] model. From left to right, the columns show the experimental values, the 1-loop SM values ${\mathcal{O}}_i^{1\mathrm{\text{−}}\mathrm{loop}}$ with $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$, and the predictions for the TC2 model with their pulls. The TC2 model fit assumed $f_t=75\mathrm{GeV}$ and $M_{\mathrm{H},\mathrm{ref}}=800\mathrm{GeV}$.