Measurement of the e+e−→Zγγ cross section and determination of quartic gauge boson couplings at LEP

Abstract

A first measurement of the cross section of the process e+e−→Zγγ is reported using a total integrated luminosity of 231 pby1 collected with the L3 detector at centre-of-mass energies of 182.7 GeV and 188.7 GeV. By selecting hadronic events with two isolated photons the eqeyTMZgg cross section is measured to be <math><mtext>0.49</mtext><msup><mi></mi><mn>+0.20</mn></msup><msub><mi></mi><mn>−0.17</mn></msub><mtext>±0.04</mtext><mspace sp="0.2" width="2px" linebreak="nobreak" is="true"></mspace><mtext>pb</mtext></math> at 182.7 GeV and <math><mtext>0.47±0.10±0.04</mtext><mspace sp="0.2" width="2px" linebreak="nobreak" is="true"></mspace><mtext>pb</mtext></math> at 188.7 GeV. The measurements are consistent with Standard Model expectations. Limits on Quartic Gauge Boson Couplings aa0/Λ2 and ac/Λ2 of <math><mtext>−0.009</mtext><mspace sp="0.3" width="2px" linebreak="nobreak" is="true"></mspace><mtext>GeV</mtext><msup><mi></mi><mn>−2</mn></msup><mtext>&lt;a</mtext><msub><mi></mi><mn>0</mn></msub><mtext>/Λ</mtext><msup><mi></mi><mn>2</mn></msup><mtext>&lt;0.008</mtext><mspace sp="0.3" width="2px" linebreak="nobreak" is="true"></mspace><mtext>GeV</mtext><msup><mi></mi><mn>−2</mn></msup></math> and <math><mtext>−0.007</mtext><mspace sp="0.3" width="2px" linebreak="nobreak" is="true"></mspace><mtext>GeV</mtext><msup><mi></mi><mn>−2</mn></msup><mtext>&lt;a</mtext><msub><mi></mi><mn>c</mn></msub><mtext>/Λ</mtext><msup><mi></mi><mn>2</mn></msup><mtext>&lt;0.013</mtext><mspace sp="0.3" width="2px" linebreak="nobreak" is="true"></mspace><mtext>GeV</mtext><msup><mi></mi><mn>−2</mn></msup></math> are derived at 95% confidence level.