Determination of the e + e − → γγ (γ) cross-section at LEP 2
DELPHI CollaborationAbstract A test of the benchmark QED process e + e − → γγ(γ) is reported, using the data collected with the DELPHI detector at LEP 2. The data analysed were recorded at centre-of-mass energies ranging from 161 GeV to 208 GeV and correspond to a total integrated luminosity of 656.4 pb −1 . The Born cross-section for the process e + e − → γγ(γ) was determined, confirming the validity of QED at the highest energies ever attained in electron-positron collisions. Lower limits on the parameters of a number of possible deviations from QED, predicted within theoretical frameworks expressing physics beyond the Standard Model, were derived.(Accepted by Eur. Phys. J. C)
IntroductionAn analysis of the process e + e − → γγ(γ) is reported. The data analysed were collected with the DELPHI detector [1] at LEP 2, at collision energies, √ s, ranging from 161 GeV up to 208 GeV, corresponding to a total integrated luminosity of 656.4 pb −1 . The studied reaction is an almost pure QED (Quantum ElectroDynamics) process which, at orders above α 2 , is mainly affected by QED corrections, such as soft and hard bremsstrahlung and virtual corrections, compared to which the weak corrections due to the exchange of virtual massive gauge bosons are very small [2][3][4]. Therefore, any significant deviation between the measured and the predicted QED cross-sections could unambiguously be interpreted as a manifestation of non-standard physics. Moreover, the differential crosssection terms expressing interferences between QED and various non-standard physics processes, behave very differently from the QED term in their dependence on the scattering angle of the photons with respect to the incident electron/positron. Depending on the possible new physics scenario, a departure of the measured differential cross-section of e + e − → γγ from the Standard Model expectations, could then be interpreted as a measure of the energy scale of the QED breakdown [5,6], of the characteristic energy scales of e + e − γγ contact interactions [7], of the mass of excited electrons within composite models [8] or of the string mass scale [9,10] (which could be of the order of the electroweak scale in the framework of models with gravity propagating in large extra-dimensions).The data recorded by DELPHI at LEP 2 were treated according to common reconstruction procedures, selection criteria and treatment of systematic uncertainties. Previous results concerning the process e + e − → γγ(γ), using DELPHI LEP 1 data are reported in reference [11]. An analysis of the LEP 2 data collected in 1996 and 1997 can be found in reference [12], while the analysis of the 1998 and 1999 DELPHI data, from which the present analysis framework evolved is reported in [13]. The results reported in the previous publications concerning the analyses of LEP 2 data are superseded by the results hereby reported. The most recently published results from the other LEP collaborations can be found in references [14][15...