study of four-fermion neutral current processes in the delphi experiment at lep
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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