Results agree with Standard Model predictions

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

Prediction and discovery of neutral currents

~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

Anomalous Triple Gauge Couplings

ZZ production Zγ* production Single Z production

Four fermion processes mediated by neutral currents (Z, γ).

Existence of new massive neutral boson Z postulated

LEP2 1996 - 2000• Energy √s = 161-209 GeV.• Integrated luminosity ~670 pb-1

Threshold for the production of two massive intermediate bosons exceeded.

Four-fermion neutral currents at DELPHI

Couplings of 3 neutral bosons forbidden at the tree level.

•Oportunity to search for new physics beyond Standard Model

Double resonant ZZ production. Two massive neutral bosons. Energy threshold √s = 182.4 GeV (2 * MZ )Important - signature similar to Higgs boson production.

e+e- → ZZ → μ+μ-qq

e+e- → Z0 γ* → ννqq Z0 → νν (invisible νν) γ* → qq (low mqq monojet)

Six groups of final states.Five signatures visible in the detector.

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.

Single resonant production. (on-shell Z, off-shell photon). Test of four-fermion interactions in wide range of energy √s.

Example of sensitivity of the angular distribution on a selected anomalous coupling constant.

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 for 4 anomalous couplings h1Z, h3

Z, h1γ, h3

γ • Fit of a given coupling while all others are set to Standard Model predictions (zero). • Likelihood distributions used to determine the confidence levels.

e+e- → Z e+e-

Z → μ+μ-

Invisible

Measured/Predicted Cross-SectionR = 1.00 ± 0.12 ± 0.05

• 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.

Mariusz Witek – DELPHI Collaboration