Introduction :

All four LEP experiments have searched for chargino in the data taken at centre-of-mass energies up to 208 GeV. No signal has been found, and limits have been derived in the context of the MSSM. Given the modest luminosity at 208 GeV, the limits from individual experiments are not close to the kinematic limit, so in principle much can be gained by combining these results. They are based on 35.2 pb^-1 collected in total.

References to individual experiments:

• ALEPH Collaboration, presented at Moriond-EW 2001,
• DELPHI Collaboration, presented at Moriond-EW 2001,
• L3 Collaboration, presented at Moriond-EW 2001,
• OPAL Collaboration, presented at Moriond-EW 2001,

(You can click on the image to get the postscript file.)

Analysis :

Cross sections and branching ratios have been calculated in the framework of the MSSM. The unification of gaugino masses at the GUT scale is assumed, leading to the relation M1 = (5/3)tan^2(theta_W) M2 at the electroweak scale, which is important in fixing the masses and field content of the charginos and neutralinos.

The experiments analyzed the chargino decays into leptons, leptons plus jets and jets. The results from these channels were then combined under the assumption that the charginos decay via a W*.

Efficiencies, backgrounds, and candidates have been reported by each experiment in a standard format. Backgrounds and candidates are summed over the four experiments and are in agreement with each other. The agreement is quantified by computing the Confidence Levels (CL) for No Excess (CL for obtaining a less background-like result than observed) and for No Deficit (CL for obtaining a more background-like result than observed). No excess is observed in the data, compared to the expectation from background. The final result is obtained with a multichannel analysis based on the likelihood ratio method. Backgrounds are subtracted according to the information supplied by each group.

Combined Results from 208 GeV :

As the combination of the experiments shows no evidence for an excess of candidates compared to the estimated background, exclusion limits at 95% Confidence Level are presented for the combination of the 208 GeV data.

The consistency between the expected background and the selected events is demonstrated in the following plots:

Channel	Candidates	Total background	Data-Bkg compatibility
Chargino

The following results are shown :

• Expected cross-section limit, based on the Likelihood Ratio multichannel method
• Obtained cross-section limit, based on the Likelihood Ratio multichannel method
• Obtained mass limit, based on the Likelihood Ratio multichannel method, as function of M(snu)

Channel	x-sect limit expected, LR	x-sect limit obtained, LR 208 GeV	Mass limit vs M(snu) 206-208 GeV
Chargino ADLO

For a mass parameter m(snu) > 300 GeV, the exclusion limits from 206-208 GeV data are:

• Last update 22 February 2001
  (Sylvie.Rosier@cern.ch)