M.Berggren, A.Favara, B.Clerbaux, G.Ganis, F.Holldorfer, G.Hughes, B.King, T.Marchant, R.McNulty, L.Pape, S.Rosier, A.Taffard, G.Wilson, T.Wyatt,  Lei Xia.

Introduction :

All four LEP experiments have searched for selectrons/smuons/staus in the data taken  at LEP. No signal has been found. Constraints have been derived in the context of the MSSM combining the data taken at energies ranging between 183 and 208 GeV. 

References to individual experiments:

• ALEPH Collaboration, "Search for scalar leptons in e+e- collisions at centre-of-mass energies up to 209 GeV Phys. Lett. B526 (2002) 206; " Absolute mass lower limit for the lightest neutralino of the MSSM from e+e- data at sqrt(s) up to 209 GeV ", Phys. Lett. B583 (2004) 247 .
• DELPHI Collaboration, " Searches for supersymmetric particles in e+e- collisions up to 208 GeV and interpretation of the results within the MSSM", Eur. Phys. J. C31 (2003) 421-479 . 
• L3 Collaboration: "Search for Scalar Quarks and Scalar Leptons at LEP" , Phys. lett  B580 (2004) 37 .
• OPAL Collaboration: "Search for anomalous production of dilepton events with missing transverse momentum in e+e- collisions at sqrt(s) = 183-GeV to 209-GeV ", Eur. Phys. J C32 (2004) 453-473 .

(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.

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/staus. The confidence level is computed using the multichannel likelihood ratio method with the FFT algorithm developped by Hu and Nielsen.
Systematic uncertainities on the efficiency are included using the Cousins and Highland receipe (Nucl. Instrum. Meth. A320, 1992, 331). Backgrounds are subtracted according to the information supplied by each group.

Combined Results from data collected at E_CM in the range 183-208 GeV:

the combination of the experiments shows no evidence for an excess of candidates compared to the estimated background; in particular the excess in the stau channel observed in the data collected at energies of 189-202 GeV has not been confirmed in 2000 data. 

The consistency between the expected background and the selected events is demonstrated in the following plots (the estimators for data-bkg compatibility are briefly defined in here):
 

Channel	Candidates / Total Bkg	CL(No Excess)/  CL(No Deficit)
Selectron
Smuon
Stau

Constraints from data collected at 183-208 GeV :

The following results have been obtained combining data collected at energies ranging from 183 GeV to 208 GeV :

• Expected and Observed Cross-Section Upper Limits (@ 95% Confidence Level); these are independent of B(R-slepton -> lepton + chi1).
• Expected and Observed Exclusion Domain in the (slepton mass, neutralino mass) plane. Braching ratio for R-slepton -> lepton + chi1 is included.

Channel	x-sect limit  Observed/Expected	Exclusion Plots
Selectron  ADLO
Smuon  ADLO
Stau  ADLO
All-In-One  ADLO

Examples of Mass Lower Limits (@ 95% Confidence Level):

Channel	Comment	Neutralino Mass	Observed Slepton  Mass Lower Limit	Expected Slepton  Mass Lower Limit
Selectron	RR coupling	0 GeV  40 GeV	99.9 GeV 99.9 GeV	99.3 GeV 99.4 GeV
Smuon	RR coupling	0 GeV  40 GeV	94.9 GeV 96.6 GeV	91.1 GeV 94.6 GeV
Stau	Z decoupled	0 GeV  40 GeV	86.6 GeV 92.6 GeV	83.8 GeV 88.5 GeV
	RR coupling	0 GeV  40 GeV	86.6 GeV 93.2 GeV	83.8 GeV 89.2 GeV

• Last update: 16 June 2004, 10:00
Gerardo.Ganis@cern.ch