ALEPH: M. Antonelli, G. Sguazzoni, DELPHI: J. Abdallah, A. Savoy-Navarro, P. Verdier, OPAL: S. Asai, L3: I. Iashvili, S. Sushkov.

Introduction :

The four LEP experiments have searched for stop and sbottom in the data taken during the year 2000 at centre-of-mass energies ranging from 204 GeV to 208 GeV. No signal has been found. Mass limits have been derived in the context of the MSSM combining these data with those taken at lower energies during previous years.

The following results have been combined (for each channel a 100% branching ratio in the specified final state is assumed):

• ALEPH, DELPHI, L3, and OPAL results for   stop -> charm neutralino
• ALEPH, DELPHI , L3 and OPAL results for   sbottom -> bottom neutralino
• ALEPH, L3 and OPAL results for                       stop -> bottom lepton sneutrino

References to individual experiment:

• ALEPH Collaboration, "Search for scalar quarks in e+e- collisions at sqrt(s) up to 209 GeV", Phys. Lett. B537 (2002) 5-20;
  
• 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 Leptons and Scalar Quarks at LEP", Phys. Lett. B580 (2004) 37-49;
  
• OPAL Collaboration, "Search for Scalar Top and Scalar Bottom Quarks at LEP" Phys. Lett. B545 (2002) 272-284.

Combination Method :

Efficiencies, backgrounds, and candidates have been reported by each group in a standard format. The confidence level is computed using the multichannel likelihood ratio method with the FFT algorithm developped by Hu and Nielsen. This method is based on a frequentist approach. A confidence level is derived by comparing the two hypotheses: the observed events are due to signal plus background or they are due to background only. Systematics have been taken into account by following the convolution method (Cousins-Highland). The background is provided by the single experiments into two categories: subtractable and unsubtractable. The unsubtractable background is the one for which the single experiments evaluate a large systematic uncertainty. For this reason it is not used in the Confidence Level evaluation.

Combined Luminosities in pb-1:

Centre-of-Mass Energy	ALEPH	DELPHI	L3	OPAL
191.6 GeV	29.92	25.85	29.20	28.42
195.6 GeV	79.83	76.42	81.40	70.56
199.6 GeV	86.29	83.37	73.50	72.39
201.6 GeV	42.01	40.64	15.10	36.52
205 GeV	75.29	78.08	68.95	81.07
206.6 GeV	122.56	139.10	126.46	126.75
208.1 GeV	9.93	7.28	8.54	8.01

---

Combined Results from data collected up to 208 GeV :

The combination of the experiments shows no evidence for an excess of candidates compared to the estimated background. The consistency between the expected background and the selected events is demonstrated in the following plots. The estimators for data-bkg compatibility are defined:
CL(No Deficit) = probability to get Ncandidates <= than what observed.
If CL(No Deficit) is small -> Deficit -> Poor background estimate ?
CL(No Excess) = probability to get Ncandidates >= than what observed.
If CL(No Excess) is small -> Excess -> Signal ?

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

Channel	Candidates / Total Bkg	CL(No Deficit)	CL(No Excess)
stop -> c chi
sbottom -> b chi
stop -> b l snu

---

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 :

Observed cross-section upper limits (95 % CL) in pb.

Observed exclusion domain (95 % CL) in the (squark mass, LSP mass) plane, for 2 different squark mixing angles: theta = 0 (corresponding to squark_Left), and in case of "max mixing" where the mixing angle is choosen such that the cross section is minimal. Two plots with the same limits are given: the first one presents the LEP data mainly, the second one presents both LEP and Tevatron results.

Channel	x-sect limit Observed	Exclusion Plots LEP + Tevatron
stop -> c chi
sbottom -> b chi
stop -> b l snu

References to Tevatron results presented in the plots above:

Stop -> c chi: CDF Collaboration, T. Affolder et al., "Search for Scalar Top and Scalar Bottom Quarks in p anti-p Collisions at s**(1/2) = 1.8 TeV", Phys. Rev. Lett. 84, 5704 (2000).

Sbottom -> b chi: CDF Collaboration, T. Affolder et al., "Search for Scalar Top and Scalar Bottom Quarks in p anti-p Collisions at s**(1/2) = 1.8 TeV", Phys. Rev. Lett. 84, 5704 (2000).

Stop -> b l snu: D0 Collaboration, V.M.Abozov et al., "A search for the scalar top quark in pbarp collisions at sqrt-s=1.8 TeV", FERMILAB-Pub-01/233-E, Phys. Rev. Lett. 88 (2002) 171802.

---

In the plane (mass of the squark - mass of the gluino)

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

Channel	Exclusion Plots / LEP + Tevatron
squark -> q chi

References to Tevatron results presented in the plot above:
- CDF Collaboration, T. Affolder et al., "Search for Gluinos and Scalar Quarks in p anti-p Collisions at s**(1/2) = 1.8 TeV using the Missing Energy plus Multijets Signature", Phys.Rev.Lett. 88 (2002) 041801.
-D0 Collaboration, B.Abbott et al., "Search for Squarks and Gluinos in Events Containing Jets and a Large Imbalance in Transverse Momentum", Phys. Rev. Lett. 83, 4937 (1999).

---

Examples of Mass Lower Limits:

The 95 % CL mass lower limits are given below for different DM = M(squark)-M(LSP) values. The limits are given in case of no squark mixing (theta = 0) corresponding to squark_Left, and in case of "max mixing" where the mixing angle is choosen such that the cross section is minimal.

Channel	M(squark) - M(LSP)	Observed mass limit no mixing	Observed mass limit max mixing
stop -> c chi	20 GeV 40 GeV 60 GeV	100 GeV 98 GeV 98 GeV	98 GeV 95 GeV 95 GeV
sbottom -> b chi	20 GeV 40 GeV 60 GeV	99 GeV 99 GeV 99 GeV	95 GeV 95 GeV 94 GeV
stop -> b l snu	20 GeV 40 GeV	99 GeV 99 GeV	96 GeV 97 GeV

---

• Last update 15 June 2004
  (Barbara.Clerbaux@cern.ch)