Introduction :

All four LEP experiments have searched for manifestations of R-parity  breaking couplings via processes with distinct signatures, in the data taken at centre-of-mass energies up to 208 GeV.  R-parity (R_p) is a new multiplicative quantum number defined as:

where S, B and L are the spin, baryon and lepton number of the sparticle.
R-parity discriminates between ordinary and supersymmetric particles: R_p=+1 for Standard Model particles  and  R_p=-1 for their supersymmetric partners.  The most general superpotential with  R_p violation (RPV) can be written as:

where

/\, /\' and /\"     are the Yukawa couplings
i, j, k                  are the generation indices
L,Q, E, D          are the superfields
L and Q            are the lepton and quark left-handed doublets
E,D and U        are the right-handed doublets

The first two terms, LLE  (lepton vertices) and LQD (lepton and quark vertices), violates the lepton number while the last term UDD (quark vertices) violates the baryon number.

If R-parity  is violated, sparticles can decay directly to Standard Model particles.  Two different scenarios are probed:

• In the first scenario, the  R_p-conserving decays of the sleptons via the lightest neutralino are considered.  In this case, the lightest neutralino is treated as the LSP and assumed to decay via an R-parity violating interaction. These are denoted "indirect decays" (IDC).  SUSY cascade decays via particles other than the LSP are not considered in the analyses and no experimental sensitivity to any other decay mode is conservatively assumed  when calculating the limits.

• In the second scenario, "direct decays"  (DDC) of sparticles to Standard Model particles are investigated. In this case, the sparticle considered is assumed to be the LSP, such that the R-parity conserving decay modes do not contribute.

In both scenarios, it is assumed that only one Yukawa-like couplings is non zero at the time.  This makes sure that the lepton or the baryon number is still conserved  avoiding fast proton decays. We have also assumed that the sparticle are pair-produced.  In addition, we have performed  the searches assuming prompt decays of the LSP.  This implies that the lightest neutralino should have a very short lifetime, corresponding to a mass larger than 10 GeV.  For smaller masses, the neutralino lifetime is larger and the neutralino could decay away from the primary interaction point. Topologies with secondary vertices resulting from the decay of a neutralino with a mass smaller than 10 GeV are not investigated although some existing analyses are likely to be quite efficient.

Analysis :

All four LEP experiments have searched for RPV decays.

The References to individual experiments are:

• ALEPH Collaboration, paper to appear soon.
• DELPHI Collaboration, "Search for supersymmetry with R-parity violation at sqrt{s}=192 to 208 GeV", DELPHI 2002-036-CONF-570, Summer Conferences 2002.
• L3 Collaboration, Eur. Phys. Journal C19 (2001) 397-414; CERN-EP/2001-068.
• OPAL Collaboration,
  Preliminary results obtained using the analysis documented in [pn411] using the full OPAL years 1999 and 2000 data with $\sqrt{s} \simeq 192-209$~GeV; preliminary results using these data were previously reported in [pn470], and are updated here using the final processing of the OPAL data.
  [PN411] ``Searches for R--Parity Violating Decays of Sfermions and Gauginos via lambda prime and lambda double prime Couplings at sqrt{s} = 189 GeV at LEP''
  The OPAL Collaboration, 12th July 1999
  [PN470] ``New Particle Searches in e+e- Collisions at sqrt(s) = 200-209 GeV''
  The OPAL Collaboration, 23rd February 2001

For the time being, the combination is performed only for the indirect decays  of charged and neutral  scalar leptons.  The efficiencies are derived from MC samples produced with  /\₁₃₃ not equal to 0 . This coupling  produces final states with at least 4 taus plus missing energy, yielding the worst signal selection efficiency.  The results are therefore valid for other /\-like couplings.

As the combination of the data collected by each experiment shows no evidence for an excess of candidates compared to the estimated Standard Model background, exclusion limits at 95% Confidence Level (CL) are presented. A multi-channel analysis based on the likelihood ratio method is used to combine all data.

In summary, the following assumptions were made when computing the limits :

• Pair-production of sleptons
• Only one Yukawa coupling non zero at the time
• No experimental sensitivity to any other decay modes than the decay via the lightest neutralino which is assumed to be the LSP when the predicted MSSM BR is used
• The lightest neutralino mass > 10 GeV  to ensure prompt decays
• Delta M >  3 GeV  (where Delta M is the mass difference between the slepton and the LSP).This results from the stau decays into tau LSP kinematics

• In the first one,  upper limits on production cross-section are calculated with minimal model assumptions.  These upper limits do not depend on the details of SUSY models as  the  branching ratio (BR) for the decay of the slepton via the lightest neutralino set to 1.

• In the second approach,  limits on the slepton masses are calculated in the framework of the MSSM.  In this case, we use  the  branching ratio (BR)  for the decay of the slepton via the lightest neutralino predicted by the MSSM and assuming no experimental sensitivity to any other decay modes. The results are presented for  tanß = 1.5 and µ = -200 GeV.

For the charged sleptons (selectron, smuon and stau), we have conservatively quoted  results for right-handed selectrons, smuons and staus only since their predicted  cross-sections are  always smaller than those for left-handed charged sleptons.
 

Experimental Data :

Luminosities accumulated by the experiments in pb^-1, summed up for all energies from 189 to 208 GeV, are listed in the table:
 
 
 

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 multi-channel analysis based on the likelihood ratio method with the FFT algorithm developed by Hu and Nielsen. Backgrounds are subtracted according to the information supplied by each group.

Combined Results from 189 to 208 GeV :
(You can click on the image to get the postscript file.)
 

Cross-sections and corresponding branching ratios were calculated in the framework of the MSSM using SUSYGEN version 3.19. A scan was performed for:

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

Channel	Candidates Total background	Data-Bkg compatibility  (to come soon)
selectron
smuon
stau
sneutrino_el
sneutrino_mu

The obtained cross-section and  mass exclusion plots are summarized in the following table with the assumptions listed above.

The  various columns correspond to the  limit types:

• In the first column, the observed mass exclusion regions in the (slepton mass, neutralino mass) plane , calculated with a likelihood ratio multi-channel method, are shown. The corresponding expected excluded mass regions are calculated with a Bayesian method.
• In the second column, the observed cross-section upper limits (95 % CL) in the  (slepton mass, neutralino mass) plane, calculated with  a likelihood ratio multi-channel method, are shown.
• In the third column, the observed and expected cross-section limits in the  (slepton mass, neutralino mass) plane, calculated with  a Bayesian method, are shown.

The various rows correspond to the slepton species:

• Charged and neutral sleptons cross-sections limit plots computed with assuming BR=1 (minimal model assumptions).

• Selectron, smuon and stau mass limit plots computed with the predicted MSSM BR .  The results are presented for µ = -200 GeV and tanß = 1.5.

 
• Electron sneutrino, muon sneutrino and stau neutrino mass limit plots computed with the predicted MSSM BR .The results are presented for µ = -200 GeV and tanß = 1.5.  It can be seen that the excluded mass regions for the sneutrinos are smaller than those for the charged sleptons due to their substantially smaller predicted MSSM BR .  Therefore, in the sneutrino mass limit plots, the excluded mass regions computed with BR set to1 are superimposed;  this assumption results in an extension of the excluded mass regions towards smaller neutralino mass. In the corresponding mass exclusion plots, a region is labeled "theoretically not accessible". In this region, the  used neutralino field content (µ = -200 GeV) , corresponding to a gaugino-like neutralino,  sets a minimum value on the MSSM parameter M_1/2, yielding a lower limit on the possible sneutrino mass.  This "theoretically not accessible" region applies for the specific choice of MSSM parameters: µ = -200 GeV and tanß = 1.5. The interpretation in a general case has not yet been performed but would likely lead to a reduction of this "theoretically not accessible" region.  The validity of the limits with higher values of tanß and lower values of µ is under investigation. 
  
  
  

Channel	Mass limit  189-208 GeV	x-sect limit  obtained, LR	x-sect limits  Bayes
selectron  ADLO
smuon  ADLO
stau  ADLO
snu_el  ADLO
snu_mu  ADLO

For DeltaM > 3 GeV and the neutralino mass > 10 GeV, the exclusion limits from 189-208 GeV ADLO data are:
 
 
 

• Last update 18 June 2002
(Sylvie.Braibant@cern.ch,Luc.Pape@cern.ch)