Tests of the Standard Model and Constraints on New Physics from Measurements of Fermion-pair Production at 189 GeV at LEP

TL;DR

OPAL measurements of fermion-pair production at $\sqrt{s}\approx 189$ GeV test the Standard Model and constrain new physics. By combining hadronic and leptonic cross-sections and angular distributions, the study extracts the running of the electromagnetic coupling $\alpha_{\mathrm{em}}(\sqrt{s})$ and sets limits on four-fermion contact interactions, $R$-parity-violating sneutrinos, and gravity in extra dimensions via an effective scale $M_s$. The results largely agree with SM predictions, but yield significant new limits: $\Lambda \sim 3$–$13$ TeV for contact interactions, $\lambda$-couplings at the $10^{-2}$–$10^{-1}$ level for sneutrinos with masses near LEP energies, and $M_s$ bounds around $0.5$–$0.7$ TeV. These findings tighten the allowed parameter space for several beyond-Standard-Model scenarios and demonstrate LEP2’s sensitivity to high-scale phenomena.

Abstract

Cross-sections and angular distributions for hadronic and lepton pair final states in e+e- collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant alpha_em, and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the mu+mu- and tau+tau- final states is also presented.

Figures (14)

• Figure 1: The distributions of reconstructed $\sqrt{s'}$ for (a) hadronic events, (b) electron pair events with $\hbox{$|\cos \theta_{{\rm e}^+} |$} < 0.9$, $\hbox{$|\cos \theta_{{\rm e}^-} |$} < 0.9$ and $\theta_{\mathrm{acol}} < 170^$^∘$$, (c) muon pair and (d) tau pair events at 188.63 GeV. In each case, the points show the data and the histogram the Monte Carlo prediction, normalized to the integrated luminosity of the data, with the contribution from events with true $s'/s > 0.7225$ shaded in (a), (c) and (d), and the contribution from events with $\theta_{\mathrm{acol}} < 10^$^∘$$ shaded in (b). The arrows in (a), (c) and (d) show the position of the cut used to select 'non-radiative' events.
• Figure 2: Measured total cross-sections ($s'/s>0.01$) for hadronic events at lower energies bib:OPAL-LS90bib:OPAL-LS91bib:OPAL-LS92bib:OPAL-SM172bib:OPAL-SM183, and this analysis. Cross-section measurements for $s'/s>0.7225$ from this analysis and from bib:OPAL-SM172bib:OPAL-SM183 are also shown; where necessary, the latter have been corrected from $s'/s > 0.8$ to $s'/s > 0.7225$ by adding the prediction of ZFITTER for this difference before plotting. The curves show the predictions of ZFITTER. The insets show the percentage differences between the measured values and the ZFITTER predictions for the high energy points for (a) $s'/s>0.01$ and (b) $s'/s>0.7225$.
• Figure 3: Measured cross-sections for electron pair events at lower energies bib:OPAL-LS90bib:OPAL-LS91bib:OPAL-LS92bib:OPAL-SM172bib:OPAL-SM183, and this analysis. The curves show the predictions of ALIBABA. The insets show the percentage differences between the measured values and the ALIBABA predictions for the high energy points for (a) $\hbox{$|\cos \theta |$} < 0.96$, $\theta_{\mathrm{acol}} < 10^$^∘$$, (b) $\hbox{$|\cos \theta |$} < 0.9$, $\theta_{\mathrm{acol}} < 170^$^∘$$ and (c) $\hbox{$|\cos \theta_{{\rm e}^-} |$} < 0.7$, $\theta_{\mathrm{acol}} < 10^$^∘$$.
• Figure 4: Measured total cross-sections ($s'/s>0.01$) for muon pair events at lower energies bib:OPAL-LS90bib:OPAL-LS91bib:OPAL-LS92bib:OPAL-SM172bib:OPAL-SM183, and this analysis. Cross-section measurements for $s'/s>0.7225$ from this analysis and from bib:OPAL-SM172bib:OPAL-SM183 are also shown; where necessary, the latter have been corrected from $s'/s > 0.8$ to $s'/s > 0.7225$ by adding the prediction of ZFITTER for this difference before plotting. The curves show the predictions of ZFITTER. The insets show the percentage differences between the measured values and the ZFITTER predictions for the high energy points for (a) $s'/s>0.01$ and (b) $s'/s>0.7225$.
• Figure 5: Measured total cross-sections ($s'/s>0.01$) for tau pair events at lower energies bib:OPAL-LS90bib:OPAL-LS91bib:OPAL-LS92bib:OPAL-SM172bib:OPAL-SM183, and this analysis. Cross-section measurements for $s'/s>0.7225$ from this analysis and from bib:OPAL-SM172bib:OPAL-SM183 are also shown; where necessary, the latter have been corrected from $s'/s > 0.8$ to $s'/s > 0.7225$ by adding the prediction of ZFITTER for this difference before plotting. The curves show the predictions of ZFITTER. The insets show the percentage differences between the measured values and the ZFITTER predictions for the high energy points for (a) $s'/s>0.01$ and (b) $s'/s>0.7225$.