Study of four-fermion neutral current processes in the DELPHI experiment at LEP

1. Study of four-fermion neutral current processes in the DELPHI experiment at LEP Six groups of final states. Five signatures visible in the detector. Four fermion processes mediated by neutral currents (Z, γ). Invisible Four-fermion neutral currents at DELPHI Prediction and discovery of neutral currents • LEP2 1996 - 2000 • Energy √s = 161-209 GeV. • Integrated luminosity ~670 pb-1 ~1965 – prediction of neutral current processes ~1968 – Glashow-Weinberg-Salam theory formulated 1973 – discovery of neutral current processes by Gargamelle bubble chamber at CERN SPS Threshold for the production of two massive intermediate bosons exceeded. Existence of new massive neutral boson Z postulated Zγ* production Single Z production ZZ production Electroweak analogy of Compton scattering. Dominated by the scattering of a quasi-real photon emitted by an electron on a positron of the other beam. Forward peaked production. Double resonant ZZ production. Two massive neutral bosons. Energy threshold √s = 182.4 GeV (2 * MZ ) Important - signature similar to Higgs boson production. Single resonant production. (on-shell Z, off-shell photon). Test of four-fermion interactions in wide range of energy √s. e+e-→ Z0 γ*→ ννqq Z0 → νν (invisible νν) γ*→ qq (low mqq monojet) e+e-→ ZZ→ μ+μ-qq e+e-→ Ze+e- Z→ μ+μ- Measured/Predicted Cross-Section R = 1.00 ± 0.12 ± 0.05 • Results for 4 anomalous couplings h1Z, h3Z, h1γ, h3γ • Fit of a given coupling while all others are set to Standard Model predictions (zero). • Likelihood distributions used to determine the confidence levels. • Most general Lagrangian contains 12 anomalous couplings describing different (forbidden in Standard Model) 3 neutral boson vertices. • Anomalous Triple Gauge Couplings would modify both total cross-sections and differential cross-sections (in particular angular distributions). • The measurements for ZZ , Zγ* and single Z production were used to determine the confidence limits of anomalous coupling constants. • Different couplings contribute to different processes. Anomalous Triple Gauge Couplings Example of sensitivity of the angular distribution on a selected anomalous coupling constant. • Couplings of 3 neutral bosons forbidden at the tree level. • Oportunity to search for new physics beyond Standard Model Distribution for Zγ events. α* is an angle between quark or anti-quark in the rest frame of Z with respect to the direction of Z in the e+e- center of mass system. Results agree with Standard Model predictions Mariusz Witek – DELPHI Collaboration