Two-fermion and Two-photon Final States at LEP2 and Search for Extra Dimensions

TL;DR

This paper reports on LEP2 experiments testing the Standard Model through fermion and photon pair production, setting new limits on contact interactions, extra dimensions, and string models, with no significant deviations observed.

Contribution

First comprehensive LEP2 results combining four experiments to constrain new physics phenomena at unprecedented energy scales.

Findings

Contact interaction scales above 10-20 TeV

QED cut-offs Lambda_+ > 0.44 TeV, Lambda_- > 0.37 TeV

Lower limits on low gravity scale M_s^+ > 1.13 TeV, M_s^- > 1.39 TeV

Abstract

For the first time the experiments ALEPH, DELPHI, L3 and OPAL have presented preliminary results for fermion-pair and photon-pair production in e+e- collisions on the full LEP2 data set. The details of the experimental measurements and results from their LEP-wide combination are presented. No statistically significant deviations from the Standard Model expectations are observed and lower limits, some obtained in dedicated analyses, for new physics phenomena at 95 % confidence level are derived. The scales of contact interactions are constrained to lie above 10-20 TeV, depending on the helicity structure. The Standard Model has thus been tested at LEP2 down to distances 10^{-19} - 10^{-20} m. This has many implications, one example being the interpretation of the new result on the anomalous magnetic moment of the muon as coming from muon substructure. For gamma-gamma final states the QED cut-offs are Lambda_+ > 0.44 TeV and Lambda_- > 0.37 TeV. In a combined analysis, using e+e- and gamma-gamma final states, the most stringent lower limits to date, M_s^+ > 1.13 TeV (lambda = +1) and M_s^- > 1.39 TeV (lambda = -1), on the low gravity effective Planck scale are set. Constrains on the scales of string models like TeV strings and D-branes are derived. In the last case the lower limit is 1.5 - 4 TeV, depending on the coupling strength.