Multi-Photon Production in e+e- collisions at sqrt(s) = 181 - 209 GeV

TL;DR

This study tests QED in e^+e^- annihilations at LEP energies 181–209 GeV using a large OPAL data set, measuring total and differential γγ(γ) cross-sections and comparing them with QED predictions. Radiative corrections are applied to obtain Born-level results, and the analysis places competitive limits on cut-off scales, eeγγ contact interactions, excited electrons, and extra-dimensional gravity. A search for a photonically decaying resonance X in three-photon events finds no signal, setting upper limits on σ(Xγ) × BR(X → γγ) of ~0.02 pb for certain masses. Overall, results strongly support QED and significantly constrain a range of beyond-Standard-Model scenarios in the photon sector.

Abstract

The process e+e- -> gamma gamma (gamma) is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross- sections are determined and found to be good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type e+e-gamma gamma, gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance X-> gamma gamma is found and limits are placed on the product of the Xgamma production cross-section and branching ratio.

Figures (8)

• Figure 1: Last OPAL $\hbox{e}^+\hbox{e}^-\to\gamma\gamma(\gamma)$ event, taken three minutes before the final shut-down of LEP. Two high-energy clusters are detected in the electromagnetic calorimeter. This is a class $I\!I$ event with acollinearity $\xi_{\rm acol}=17^\circ$. One photon has converted between CV and CJ; the two corresponding tracks are visible in the tracking chambers.
• Figure 2: Distributions used in the kinematic event selection. The energy sum for events of all classes is shown for barrel and endcap regions separately in plots a) $\cos{\theta^{\ast}} < 0.81$ and b) $\cos{\theta^{\ast}}\geq 0.81$. For events that are not in class $I$ plot c) shows the transverse momentum and d) the longitudinal momentum. The points are the OPAL data after application of all cuts except that on the quantity which is plotted. The histograms show the Monte Carlo expectation. The arrows in a), b) and c) show the positions of the cuts. For the longitudinal momentum there is no cut at a specific value, events are selected if $p_{\,\rm l} < E_{1,2}$ as indicated by the Monte Carlo distribution shown as the shaded histogram. Background comes mainly from cosmic-ray events. In plot d) some Bhabha background is visible.
• Figure 3: Acollinearity $\xi_{\rm acol}$ for all selected events. Because of the non-uniform bin size the entries are normalised to events/1$^\circ$. The measured distribution is compared to the ${\mathcal{O}}(\alpha^3)$ Monte Carlo prediction including full detector simulation. The Monte Carlo distribution at generator level is also shown. The resolution is about $0.35^\circ$ and the possible systematic bias on the angle is around 0.1$^\circ$.
• Figure 4: The measured differential cross-section at the Born level for the process $\hbox{e}^+\hbox{e}^-\to\gamma\gamma(\gamma)$ for six ranges of centre-of-mass energy. The points show the number of observed events corrected for efficiency and radiative effects. The solid curve corresponds to the Born-level QED prediction. The dashed lines represent the 95% confidence level interval from the fit to the function given in Equation \ref{['lambda']}.
• Figure 5: Total cross-section at the Born level for the process $\hbox{e}^+\hbox{e}^-\to\gamma\gamma(\gamma)$ with $\cos{\theta^{\ast}} < 0.93$. The curve corresponds to the Born-level QED expectation. The data are corrected for efficiency loss and higher-order effects. The errors shown for the measurements are statistical only. The systematic errors are correlated between energies and are plotted as a band around the QED expectation. The inner band represents the experimental error and the outer band includes the theoretical error.