N.de Filippis, K.Desch, G.Grenier, C.Hensel, A.Perrotta, S.Rosier-Lees

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. The present combination focuses on the region of small mass difference DM = M(Chargino) - M(LSP).

References to individual experiments:

• ALEPH Collaboration, "Search for charginos nearly mass degenerate with the lightest neutralino in e+e- collisions at centre-of-mass energies up to 209GeV", CERN-EP-2002-020, Subm. to: Phys. Lett. B.
• DELPHI Collaboration, "Searches for supersymmetric particles in e+e- collisions up to 208 GeV, and interpretation of the results witin the MSSM", DELPHI 2002-027-CONF-561, Summer Conferences 2002.
• L3 Collaboration, Phys. Letter B471 (999) 308-320 - L3 note 2707
• OPAL Collaboration,
  Preliminary results obtained using the analysis documented in
  [pr464] using the full OPAL year 2000 data with $\sqrt{s} \simeq 200-209$~GeV; preliminary results using these data were previously reported in [pn470], and are updated here using the final processing of the OPAL data.
  [PN464] `Searches for Charginos with Small DeltaM in e+e- Collisions at sqrt{s} = 189 - 202 GeV''
  The OPAL Collaboration, 26th October 2000
  [PN470] ``New Particle Searches in e+e- Collisions at sqrt(s) = 200-209 GeV''
  The OPAL Collaboration, 23rd February 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. Two cases were considered:

1. 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 implies that low values of DM are reached in the higgsino-like region, where µ << M₂.
2. The non-standard case where M1 and M2 are nearly degenerate and small DM is reached in the gaugino region. It is assumed that m0 is large (m(snu) = 500 GeV).

Decay length	Cross section Higgsino	Cross section Gaugino

The experiments have analyzed the chargino decays into the following topologies:

• prompt decays into leptons, leptons plus jets and jets, combined under the assumption that the charginos decay via a W*. They cover the region DM > 3 GeV.
• soft events with an Initial State Radiation photon. They cover DM values between 3 GeV and about 200 MeV.
• events with tracks displaying kinks, impact parameter offsets or heavy stable charged particles, covering the very small DM region.

Efficiencies, backgrounds, and candidates have been reported by each experiment in a standard format. Separate files were provided for the two physics cases mentioned above, as they have different efficiencies. 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 :

Luminosities accumulated by the experiments in pb^-1 are listed in the table:
 

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.

Degeneracy in the Higgsino region :

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

Channel	Candidates	Background	CL(no-excess)	CL(no-deficit)
Chargino

The following results are shown :

• Expected and obtained cross-section limits, based on the Likelihood Ratio multichannel method
• Expected (dashed) and obtained (full line) mass limits, based on the Likelihood Ratio multichannel method, as function of DM

Channel	x-sect limit expected	x-sect limit obtained	Mass limit vs DM	Mass limit vs DM (zoom)
Chargino ADLO

For any value of DM, the exclusion limits from the LEP data up to 208 GeV data are:

Degeneracy in the Gaugino region (large m0) :

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

Channel	Candidates	Background	CL(no-excess)	CL(no-deficit)
Chargino

The following results are shown :

• Expected and obtained cross-section limits, based on the Likelihood Ratio multichannel method
• Expected (dashed) and obtained (full line) mass limits, based on the Likelihood Ratio multichannel method, as function of DM for m(snu) = 500 GeV.

Channel	x-sect limit expected	x-sect limit obtained	Mass limit vs DM	Mass limit vs DM (zoom)
Chargino ADLO

For any value of DM, the exclusion limits from the LEP data up to 208 GeV data are:

• Last update 18 June 2002
  (Sylvie.Rosier@cern.ch)