Discovery and Identification of Extra Gauge Bosons

TL;DR

The paper analyzes the discovery potential for heavy Z' and W' gauge bosons across hadron and e+e- colliders, and develops a framework to diagnose their underlying gauge structure by measuring quark and lepton couplings. It evaluates several representative models (Effective Rank-5, LR-based, ALRM, SSM, UNSM) and derives how well future machines can extract Z' couplings via observables like forward-backward asymmetries, rapidity ratios, rare decays, and associated productions. The LHC offers robust, direct mass reach and coupling magnitudes, while the NLC provides complementary, often more precise coupling information including signs, enabling near-complete determination of Z' couplings for MZ' up to 1–2 TeV when combined. The study concludes that LHC and NLC are complementary and jointly capable of identifying the nature of the extended gauge symmetry (e.g., E6 parameterization) or, if no signal is seen at DI-TeVATRON, that Z' would lie beyond their diagnostic reach.

Abstract

The discovery potential and diagnostic abilities of proposed future colliders for new heavy neutral ($Z'$) and charged ($W'$) gauge bosons are summarized. Typical bounds achievable on $M_{Z',W'}$ at the TEVATRON, DI-TEVATRON, LHC, 500 GeV NLC, and 1 TeV NLC are $\sim$1~TeV, $\sim$2~TeV, $\sim$4~TeV, 1--3~TeV, and 2--6~TeV, respectively. For $M_{Z'} \sim$1 TeV the LHC will have the capability to determine the magnitude of normalized $Z'$ quark and lepton couplings to around $10-20\%$, while the NLC would allow for determination of the couplings (including their signs) with a factor of 2 larger error-bars, provided heavy flavor tagging and longitudinal polarization of the electron beam is available.

Figures (6)

• Figure 1: The cross section for the process $pp \rightarrow Z' \rightarrow \ell^+\ell^-$ as a function of $M_{Z'}$ for (a) $p\bar{p}$ with $\sqrt{s}=4$ TeV and (b) $pp$ with $\sqrt{s}=14$ TeV. In both cases the solid line is for $Z_\chi$, the dashed line for $Z_\psi$, the dotted line for $Z_{LR}$ and the dot-dashed line for $Z_{ALR}$.
• Figure 2: $\sigma(e^+e^- \rightarrow \mu^+\mu^-)$, $R_{had}$, $A_{LR}$, and $A_{LR}^{had}$ for $\sqrt{s}=500$ GeV as a function of $M_{Z'}$. The error bars are statistical errors based on 50 fb$^{-1}$ integrated luminosity. In all cases the solid line is the standard model prediction, the dot-dash line is for model-$\chi$, the dot-dot-dash model is for model-LR, the dashed line is for model-ALR and the dotted line is for model-SSM.
• Figure 3: Discovery limits for extra neutral gauge bosons ($Z'$) for the models described in the text. The discovery limits at hadron colliders are based on 10 events in the $e^+e^-\ +\ \mu^+\mu^-$ channels while those for $e^+e^-$ colliders are 99% C.L. obtained from a $\chi^2$ based on $\sigma (e^+e^- \rightarrow \mu^+\mu^-)$, $R^{had}=\sigma (e^+e^- \rightarrow hadrons)/\sigma_0$, $A_{LR}^{\mu^+\mu^-}$, and $A^{had}_{LR}$. The integrated luminosities are based on a $10^7$ sec year of running.
• Figure 4: 90% confidence level ($\Delta \chi^2=6.3$) contours for the $\chi$, $\psi$ and $\eta$ models are plotted for $\tilde{U}$, versus $\tilde{D}$, versus $\gamma_L^\ell$. The input data are for $M_{Z'}=1$ TeV at the LHC ($\sqrt s = 14$ TeV and ${\cal L}_{int}=100\ \hbox{fb}^{-1}$) and include statistical errors only.
• Figure 5: 90% confidence level ($\Delta \chi ^2 = 6.3)$ regions for the $\chi ,\psi$ and $\eta$ models with $M_{Z'}=1$ TeV are plotted on $P_R^u$ versus $P_R^d$ versus $P_V^\ell$ at the $NLC$.