Z-prime Gauge Bosons at the Tevatron

TL;DR

The paper analyzes the discovery potential for a Z' gauge boson at the Tevatron, introducing a model-bridging parameterization that connects the observable dilepton cross section to the Z' mass and its couplings to quarks and leptons. It develops a general theoretical framework, including anomaly cancellation and Z–Z' mixing constraints, and surveys realistic U(1) extensions such as B-L and E6-inspired models, with LEP II limits guiding viable regions. A hadroproduction formalism is presented that factorizes the Z' signal into a model-dependent term and a model-independent hadronic structure function, enabling clean comparisons via the c_u–c_d plane while incorporating NLO/NNLO QCD and electroweak corrections. The paper also provides projected Tevatron sensitivity, discusses potential discrimination between models through angular distributions, and highlights the interplay with LEP constraints in shaping the search landscape.

Abstract

We study the discovery potential of the Tevatron for a Z-prime gauge boson. We introduce a parametrization of the Z-prime signal which provides a convenient bridge between collider searches and specific Z-prime models. The cross section for p pbar -> Z-prime X -> l^+ l^- X depends primarily on the Z-prime mass and the Z-prime decay branching fraction into leptons times the average square coupling to up and down quarks. If the quark and lepton masses are generated as in the standard model, then the Z-prime bosons accessible at the Tevatron must couple to fermions proportionally to a linear combination of baryon and lepton numbers in order to avoid the limits on Z--Z-prime mixing. More generally, we present several families of U(1) extensions of the standard model that include as special cases many of the Z-prime models discussed in the literature. Typically, the CDF and D0 experiments are expected to probe Z-prime-fermion couplings down to 0.1 for Z-prime masses in the 500--800 GeV range, which in various models would substantially improve the limits set by the LEP experiments.

Figures (9)

• Figure 1: Lower bounds on $M_{Z^\prime} / g_z$ from the LEP-II search for $LL$, $RR$, $LR$ and $RL$ contact interactions, applied to the models of Table \ref{['TableCharge']} as a function of the continuous parameter $x$. For $U(1)_{B-xL}$, we have included the bound on vector-like $e^+ e^- \rightarrow \ell^+ \ell^-$, as is appropriate for that model.
• Figure 2: Excluded regions in the $c_d-c_u$ plane from the current 95% C.L. limit for $\sigma\cdot\hbox{Br}(Z^\prime\rightarrow l^{+}l^{-})$ given in CFD:CL, for different values of the $Z^\prime$ mass. The thick straight line corresponds to values of $c_u$ and $c_d$ in the $B-xL$ model, in which $c_u=c_d$. The area between the two thin straight lines is the region where the $q+x u$ model lies.
• Figure 3: The ratio of the ${\cal O}(\alpha_s)$ and ${\cal O}(\alpha_s^2)$ corrections to the hadronic structure function over the Born contribution, assuming SM couplings for the $Z^\prime$. The full line corresponds to the ${\cal O}(\alpha_s)$ corrections. The dashed and dotted lines, which, within the resolution of the figure, appear as a single line, correspond to the total ${\cal O}(\alpha_s^2)$ contributions and to the ${\cal O}(\alpha_s^2)$ corrections retaining only those terms that give contributions proportional to $c_u$ and $c_d$ to the cross section, respectively.
• Figure 4: NLO and NNLO $k$-factors for SM-like couplings as a function of the invariant mass of the lepton pair. Also shown is an approximation, Eq. (\ref{['eq:soft']}), for the NLO $k$-factor which only takes into account the soft corrections to the structure function in the DIS scheme.
• Figure 5: