M.Berggren, A.Favara, D.Futyan, G.Ganis, F.Holldorfer, G.Hughes, B.King, L.Pape, S.Rosier, G.Wilson, T.Wyatt

Introduction :

All four LEP experiments have searched for sleptons in the data taken at 189 GeV. No signal has been found, and limits have been derived in the context of the MSSM. None of these limits is close to the kinematic limit, so in principle much can be gained by combining these results.

References to individual experiments:

• ALEPH Collaboration, presented at Moriond-EW 99, ALEPH 99-008 CONF 99-004
• DELPHI Collaboration, presented at Moriond-EW 99, DELPHI 99-24 CONF 223
• L3 Collaboration, presented at Moriond-EW 99, L3 note 2373
• OPAL Collaboration, "New Particle Searches in e+e- Collisions at sqrt{s}= 189 GeV", OPAL Physics Note 370

(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 is assumed, leading to the relation M1 = (5/3)tan^2(theta_W) M2, which is important in fixing the masses and field content of the neutralinos. (The neutralinos appear in the decay of the sleptons, and play a major role in the cross section for selectrons via a t-channel diagram.)

In general, the cross section for R-sleptons is smaller than for L-sleptons. As a starting point, the L-sleptons are assumed to be beyond the reach of the experiments. The limits given here apply for R-sleptons only (including staus).

Branching ratios for R-slepton -> lepton + chi1 are calculated as a function of M2 (ie, the chi1 mass) for fixed mu and tan(beta). They are typically greater than 95% except for the lightest neutralino masses at which point R-slepton -> lepton + chi2 is kinematicaly allowed (but supressed). The specific parameter choice is  mu = -200 GeV/c^2 and tan(beta) = 1.5, as this is a point at which the limits on neutralinos from chargino and neutralino searches is weakest. It is also a point of relatively small selectron cross section.

Efficiencies, backgrounds, and candidates have been reported by each group in a standard format. All four LEP collaborations provided this information for selectron, smuons and staus. Signals, backgrounds, and candidates are summed, and the confidence level is computed using the method recommended by the PDG for the actually observed number of events and for background experiments (expected limit). The results from the two calculations are compatible. The final result is obtained with a multichannel analysis based on the likelihood ratio method. It gives essentially the same result as the PDG formula, as the four sets of searches have similar sensitivities. Backgrounds are subtracted according to the information supplied by each group.

Combined Results from 189 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 189 GeV data.

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

Channel	Candidates	Total background
Selectron
Smuon
Stau

The following results are shown :

• Expected cross-section limit, based on the PDG single channel formula
• Obtained cross-section limit, based on the PDG single channel formula
• Obtained cross-section limit, based on the Likelihood Ratio multichannel method
• Obtained mass limit, based on the Likelihood Ratio multichannel method

Channel	x-sect limit expected, PDG	x-sect limit obtained, PDG	x-sect limit obtained, LR	Mass limit
Selectron ADLO
Smuon ADLO
Stau ADLO

For a mass difference M(slepton)-M(LSP) > 15 GeV, the exclusion limits from 189 GeV data are:

• Last update 04 June 1999
  Luc Pape (luc.pape@cern.ch)