Search for Chargino and Neutralino Production at sqrt(s) = 192-209 GeV at LEP

TL;DR

The OPAL collaboration conducted a comprehensive search for chargino and neutralino production in e+e− collisions at sqrt(s) = 192–209 GeV using about 438 pb−1 of data. The analysis employs three channels (hadronic and semileptonic chargino decays, and hadronic neutralino associated production) with an extended likelihood framework and a mass-grid approach to set 95% CL limits, including a Bayesian treatment across multiple energies. No evidence for SUSY is observed, and the study provides both model-independent cross-section limits and CMSSM-excluded regions, significantly constraining chargino and neutralino masses up to near the LEP kinematic reach. These results tighten constraints on MSSM parameter space with heavy sfermions and illustrate the power of combining multiple final states and energies in LEP analyses.

Abstract

Approximately 438 pb-1 of e+e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 Gev, are analyzed to search for evidence of chargino pair production, e+e- -> tilde chi^+_1 tilde chi^-_1, or neutralino associated production, e+e- -> tilde chi^0_2 tilde chi^0_1. Limits are set at the 95% confidence level on the product of the cross-section for the process e+e- -> tilde chi^+_1 tilde chi^-_1 and its branching ratios to topologies containing jets and missing energy, of jest with a lepton and missing energy, and on the product of the cross-section for e+e- -> tilde chi^0_2 tilde chi^0_1 and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the tilde chi^+-_1, tilde chi^0_1 and tilde chi^0_2, and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on sigma(e+e- -> tilde chi^+_1 tilde chi^-_1) with the assumption that each chargino decays via a W boson, and on sigma(e+e- -> tilde chi^0_2 tilde chi^0_1) with the tilde chi^0_2 assumed to decay via a Z^0.

Figures (13)

• Figure 1: Comparison of data and Monte Carlo distributions of selected likelihood variables for the hadronic chargino analysis, shown at preselection level for $\sqrt{s}=206$ GeV. The shaded histograms show the various expected backgrounds (the darkest shades are two-fermion processes, predominantly $\hbox{$\mathrm{q}$} {\bar{\hbox{$\mathrm{q}$}}}$, the intermediate grey represents the two-photon contribution and the light grey indicates the dominant four-fermion background) and the points with error bars are the OPAL data. The dashed empty histograms show a sample signal with $(M,\Delta M)=$(100 GeV, 76.9 GeV), the empty histograms with the solid line show a sample signal with $(M,\Delta M)=$(100 GeV, 49.2 GeV), and the dotted empty histograms show a sample signal with $(M,\Delta M)=$(100 GeV, 12.2 GeV), all with arbitrary normalization.
• Figure 2: Sample distributions of $L_R$ for Standard Model background in the hadronic channel (solid histogram), chargino signal with arbitrary normalization (dashed histogram) and OPAL data at $\sqrt{s}=207$ GeV (points with error bars) at $(M,\Delta M)=$ (100 GeV, 49 GeV) and (100 GeV, 77 GeV).
• Figure 3: Comparison of data and Monte Carlo distributions of selected likelihood variables for the semileptonic chargino analysis, shown at preselection level for $\sqrt{s}=207$ GeV. The solid histograms represent the expected background, with dark grey for two-fermion events, cross-hatching for four-fermion events and single-hatching for two-photon events. The points with error bars are the OPAL data taken in the year 2000 at an average centre-of-mass energy of 206.1 GeV. Dotted histograms show a "$\hbox{$\mathrm{W}$}^+\hbox{$\mathrm{W}$}^-$-like" signal with $(M,\Delta M)=$(80 GeV, 76.3 GeV) and dashed histograms show a "two-photon--like" signal with $(M,\Delta M)=$(85 GeV, 6.9 GeV), both with arbitrary normalization.
• Figure 4: Sample distributions of $L_R$ for Standard Model background in the semileptonic channel (solid histogram), chargino signal with arbitrary normalization (dashed histogram) and OPAL data at $\sqrt{s}=207$ GeV (points with error bars) at $(M,\Delta M)=$ (76 GeV, 70 GeV) and (100 GeV, 48 GeV).
• Figure 5: The expected and observed 95% confidence level limits on $\sigma({\hbox{$\mathrm{e}$}}^+{\hbox{$\mathrm{e}$}}^-\to\tilde{\chi}^+_1\tilde{\chi}^-_1) BR^2(\tilde{\chi}^\pm\to q{\bar{q}}\tilde{\chi}^0_1)$, using data taken at 192-209 GeV. White areas indicate that the limit is greater than 2 pb or that no limit could be set.