Searches for supersymmetric particles in e+e- collisions up to 208 GeV and interpretation of the results within the MSSM

TL;DR

DELPHI conducts an extensive program to search for supersymmetric particles in e+e− collisions up to 208 GeV within the MSSM framework with R-parity conservation, finding no evidence for SUSY and deriving cross-section and mass limits. The analyses cover sleptons, squarks, charginos, and neutralinos across diverse final states, employing likelihood-based and neural-network techniques, ISR photon tagging, and dedicated strategies for nearly mass-degenerate scenarios. By combining results under constrained models such as CMSSM/mSUGRA, the study yields robust lower bounds on the LSP and key sparticles, with LSP masses constrained around 39–46 GeV depending on m0 and mixings, and chargino/neutralino limits extending to ~95–103 GeV in various regimes. The findings substantially extend prior LEP limits and, together with Higgs constraints, map significant portions of the MSSM parameter space, informing future collider searches and SUSY model-building.

Abstract

DELPHI data collected at centre-of-mass energies up to 208 GeV have been analysed to search for charginos, neutralinos and sfermions in the framework of the Minimal Supersymmetric Standard Model (MSSM) with R-parity conservation. No evidence for a signal was found in any of the channels. The results of each search were used to derive limits on production cross-sections and particle masses. In addition, the combined result of all searches excludes regions in the parameter space of the constrained MSSM, leading to limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles.

Figures (44)

• Figure 1: Comparison of data and simulation in the selectron channel at preselection level. The dots with error bars show the data, shaded histograms show the simulation. Plots include data taken in the year 2000 when the DELPHI detector was fully operational. The plots show: (a) the acoplanarity, (b) the transverse momentum, (c) the opening angle, (d) the momentum of the leading charged particle. Possible signals corresponding to the mass combinations $M_{\tilde{\mathrm e}}$=90 ${\mathrm{GeV}}/c^2$,$M_{\tilde{\chi}^0_1}$=10 ${\mathrm{GeV}}/c^2$ (solid) and $M_{\tilde{\mathrm e}}$=50 ${\mathrm{GeV}}/c^2$,$M_{\tilde{\chi}^0_1}$=40 ${\mathrm{GeV}}/c^2$ (dashed) are shown by the superimposed open histograms. The signal normalisation is arbitrary.
• Figure 2: Comparison of data and simulation in the smuon channel at preselection level. The dots with error bars show the data, shaded histograms show the simulation. Plots include data taken in the year 2000 when the DELPHI detector was fully operational . The plots show: (a) the acoplanarity, (b) the transverse momentum, (c) the opening angle, (d) the momentum of the leading charged particle. Possible signals corresponding to the mass combinations $M_{\tilde{\mu}}$=90 ${\mathrm{GeV}}/c^2$,$M_{\tilde{\chi}^0_1}$=10 ${\mathrm{GeV}}/c^2$ (solid) and $M_{\tilde{\mu}}$=50 ${\mathrm{GeV}}/c^2$,$M_{\tilde{\chi}^0_1}$=40 ${\mathrm{GeV}}/c^2$ (dashed) are shown by the superimposed open histograms. The signal normalisation is arbitrary.
• Figure 3: A preselection comparison of data and simulation in the stau analysis. The plots show: (a) the number of charged particles, (b) the square of the transverse momentum with respect to the thrust axis, (c) the acoplanarity, (d) the missing transverse momentum divided by the maximum missing transverse momentum in two-photon events with no beam-remnant electrons in the detector acceptance (ie. in "no-tag" events). The dots with error bars show the data, while the simulation is shown shaded. A typical signal ($M_{\tilde{\tau}}$ = 83 ${\mathrm{GeV}}/c^2$ , $M_{\mathrm LSP}$ = 0 ${\mathrm{GeV}}/c^2$) is shown by the superimposed open histogram, with arbitrary normalisation.
• Figure 4: Comparison of data and simulation at the preselection level in the non-degenerate squark analysis. Plots include all DELPHI data from 189 to 208 GeV: (a) visible energy, (b) acoplanarity, (c) combined b-tagging variable, (d) maximal transverse momentum of a charged particle. The expected signal distributions at $\sqrt{s}=200~\hbox{${\mathrm{GeV}}$}$ are shown for one possible stop and sbottom signal ($M_{\tilde{\mathrm q}}$=90 ${\mathrm{GeV}}/c^2$, $M_{\tilde{\chi}^0_1}$ = 60 ${\mathrm{GeV}}/c^2$), with arbitrary normalisation.
• Figure 5: Number of events as a function of the signal detection efficiencies in the non-degenerate squark analysis: (a) 1998 data at 189 ${\mathrm{GeV}}$: stop analysis for $\hbox{$\Delta$M} >20~\hbox{${\mathrm{GeV}}/c^2$}$, (b) 1999 data from 192 to 202 ${\mathrm{GeV}}$: sbottom analysis for $\hbox{$\Delta$M} >20~\hbox{${\mathrm{GeV}}/c^2$}$, (c) 2000 data with TPC sector 6 off: stop analysis for $5 \leq \hbox{$\Delta$M} \leq 20~\hbox{${\mathrm{GeV}}/c^2$}$, (d) 2000 data with TPC sector 6 on: sbottom analysis for $5 \leq \hbox{$\Delta$M} \leq 20~\hbox{${\mathrm{GeV}}/c^2$}$. The efficiencies are for a given combination of squark and LSP masses, indicated in the parentheses.