Searches for pair-produced charginos, neutralinos and scalar fermions
decaying via R-parity violating / /\' and /\'' couplings with the OPAL detector
at LEP are presented at sqrt(s)=189 GeV. Partial updates using data up to the
highest energies of LEP, sqrt(s)=209 GeV, are also given.Comment: 5 pages, 11 figures. To be published in the Proceedings of DPF2000,
  The Meeting of The Division of Particles and Fields of The American Physical
  Society, Columbus, Ohio, 9-12 August 2000, (Supplement of the International
  Journal of Modern Physics A (IJMP)

GABRIELLA PÁSZTOR

The OPAL

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arXiv

Searches for pair-produced charginos, neutralinos and scalar fermions decaying via R-parity violating / /\' and /\'' couplings with the OPAL detector at LEP are presented at sqrt(s)=189 GeV. Partial updates using data up to the highest energies of LEP, sqrt(s)=209 GeV, are also given.Searches for pair-produced charginos, neutralinos and scalar fermions decaying via R-parity violating / /\' and /\'' couplings with the OPAL detector at LEP are presented at sqrt(s)=189 GeV. Partial updates using data up to the highest energies of LEP, sqrt(s)=209 GeV, are also given

Pasztor, Gabriella

CERN Document Server

arXiv:hep-ex/0012007v1  3 Dec 2000International Journal of Modern Physics A,❢c World Scientific Publishing CompanyOPAL Conference Report CR45220 November 2000SEARCH FOR R-PARITY VIOLATING DECAYSOF SUPERSYMMETRIC PARTICLES AT LEPGABRIELLA PA´SZTOR∗CERN EPGeneva 23, CH-1211, SwitzerlandSearches for pair-produced charginos, neutralinos and scalar fermions decaying via R-parity violating λ, λ′ and λ′′ couplings with the OPAL detector at LEP are presented at√s = 189 GeV. Partial updates using data up to the highest energies of LEP,√s = 209GeV, are also given.If supersymmetry (SUSY) is the answer to the hierarchy problem of the StandardModel (SM), there should be a superpartner to each SM particle. We present here asearch for pair-produced supersymmetric particles in e+e− collisions with the OPALdetector, assuming that R-parity can be violated.R-parity violating (RPV) interactions are parametrized with a gauge-invariantsuperpotential that includes the following Yukawa coupling terms:WRPV = λijkLiLjEk + λ′ijkLiQjDk + λ′′ijkU iDjDk, (1)where i, j, k are the generation indices of the superfields L,Q,E,D and U . L andQ are lepton and quark left-handed doublets, while E, D and U are right-handedsinglet charge-conjugate superfields for the charged leptons, down- and up-typequarks, respectively. There are nine λ, 27 λ′ and nine λ′′ couplings, giving a totalof 45 λ-like R-parity violating couplings.R-parity is a discreet, multiplicative quantum number, which is +1 for SM par-ticles and −1 for their superpartners. There is no a priori law, that requires theconservation of R-parity. Moreover there is no experimental result, which excludesthe presence of WRPV under the assumption that only one λ-like coupling is signif-icantly different from zero.The model used in this work is the Constrained Minimal Supersymmetric Stan-dard Model (CMSSM). Not counting the 45 RPV Yukawa couplings, this model hasonly five free parameters: a common mass for the gauginos (m1/2) and the sfermions(m0) at the GUT scale, the mixing parameter of the two Higgs field doublets (µ),the ratio of the vacuum expectation values of the two Higgs doublets (tanβ) andthe common trilinear coupling (A).Both the theoretical results and the details of the experimental analyses relevantfor this paper are summarized in our previous publications.1,2 We assume, that there∗ Permanent address: KFKI Research Institute for Particle and Nuclear Physics of the HungarianAcademy of Sciences (KFKI RMKI), Budapest, P.O.Box 49, H-1525, Hungary. Supported partiallyby the Hungarian Foundation of Scientific Research under the contract number OTKA F-023259.12 Search for R-parity Violating Decays of Supersymmetric Particles at LEPis only one λ-like coupling different from zero, and that SUSY particles are heavyand decay promptly in the detector. This corresponds to a sensitivity for couplingslarger than O(10−5).The bulk of the results,3,4 presented in Section 1 to 3, are based on the combina-tion of the data sample collected at√s = 189 GeV corresponding to an integratedluminosity of approximately 180 pb−1 with previous OPAL results.1,2 Partial up-dates using data collected up to√s = 209 are presented Section 4.1. Chargino and Neutralino SearchesThe direct decays of neutralinos result in three SM fermions: νℓ+ℓ− via λ, νqqor ℓqq via λ′ and qqq via λ′′ couplings. Thus, the final state topologies of theirpair-production vary from four leptons with missing energy to six jets.Charginos can decay directly to SM fermions, or indirectly through a neutralinoand a (virtual) W boson. In direct decays, three SM fermions are produced: ννℓ orℓℓℓ via λ, νqq or ℓqq via λ′ and qqq via λ′′ couplings. When decaying indirectly, fiveSM fermions are created, their type depending on the decay modes of the neutralinoand the W boson. Therefore, pair-production of charginos may be observed inwidely different final states, ranging from two leptons with missing energy to tenjets.When interpreting the results of all these topological searches, first cross-sectionlimits are derived for the individual channels with the minimal model assumptionsgiven above. We combine the individual results to get cross-section limits for thedirect and the indirect modes. It is assumed that the indirect decays proceedthrough the production of a (virtual) W boson and the lightest neutralino, which isassumed to be the lightest SUSY particle, and that the mass difference between thechargino and the neutralino is greater than 5 GeV. Different channels are combinedaccording to the measured W branching ratios.Decay mode independent results are calculated by varying the relative branchingratio of the direct and indirect decays between 0 and 1, and taking the worst case.For any λ coupling, the upper limit on the neutralino pair-production cross-section is smaller than 0.16 pb in the full mass range from 45 GeV to the kinematiclimit. The cross-section limits for charginos are summarized in Table 1.Table 1. Upper limits on the chargino pair-production cross-section for direct decays, indirectdecays and independently of the decay mode for any λ, λ′ and λ′′ couplings. The first / secondnumber corresponds to a chargino mass of 45 / 90 GeV.coupling direct indirect mode indep.λ 1.1 / 0.55 pb 0.42 / 0.16 pb 2.0 / 0.9 pbλ′ 2.0 / 0.35 pb 4.5 / 0.7 pbλ′′ 1.4 / 0.50 pb 1.1 / 0.50 pb 1.4 /0.55 pbFinally, the results are interpreted in CMSSM. The excluded topological cross-sections, and the measured Z boson width are used to constrain the CMSSM pa-rameter space. The results for any λ and λ′′ couplings are shown in Figure 1.Search for R-parity Violating Decays of Supersymmetric Particles at LEP 3OPAL Preliminary OPAL Preliminary= 200 GeVµ  (GeV) M2  (GeV)020406080100120140160180200-200 -150 -100 -50 0 50 100 150 200tanβ = 1.5m0200 GeVtanβ= 35m0 =µ  (GeV) M2  (GeV)020406080100120140160180200-200 -150 -100 -50 0 50 100 150 200µ  (GeV) M2  (GeV)020406080100120140160180200-200 -150 -100 -50 0 50 100 150 200tanβ = 1.5m0= 200 GeVtanβ= 35m0 =µ  (GeV) M2  (GeV)020406080100120140160180200-200 -150 -100 -50 0 50 100 150 200200 GeV(a) (b) (c) (d)λ λ λ′′ λ′′Fig. 1. Excluded regions in CMSSM in the M2−µ planea form0 = 200 GeV and (a,c) tan β = 1.5,(b,d) tan β = 35 for any (a,b) λ and (c,d) λ′′ couplings. The kinematic limit is shown by dashedlines.2. Squark SearchesWe have studied the direct decays of scalar top quarks to SM fermions via λ′ and λ′′couplings. The cross-section upper limits are shown for both the qℓqℓ and the qqqqfinal states on Figure 2. In CMSSM the production cross-section depends on thestop mixing angle, θt˜, defined as t˜1 = cos θt˜t˜L+sin θt˜t˜R. The maximal cross-sectionis predicted for θt˜ = 0 rad, while the minimal for θt˜ = 0.98 rad. The limit on thestop mass varies between 79 and 90 GeV depending on which λ-like coupling isdifferent from zero and the value of the mixing angle.OPAL Preliminary00.10.20.365 70 75 80 85 90 95θ = 0 rad~tθ = 0.98 rad~tm(t) (GeV)σ (pb)~λ33k’λ23k’λ13k’OPAL Preliminary00.10.20.30.40.545 50 55 60 65 70 75 80 85 90 95λ" θ = 0 rad~tθ = 0.98 rad~tm(t) (GeV)σ (pb)~(a) (b)Fig. 2. Upper limits on the pair-production cross-section of (a) scalar top quarks decaying via λ′couplings and (b) a scalar quark of any flavor decaying via λ′′ couplings. The predicted minimaland maximal cross-sections of stop pair-production in CMSSM are also shown.3. Slepton SearchesScalar leptons, charged or neutral, can decay directly to two SM fermions or indi-rectly, through the production of a SM lepton and a neutralino, to four SM fermions.Thus, when produced in pairs, the possible final states range from two leptons andmissing energy to four leptons and four hadronic jets.Similarly to the gaugino searches, first cross-section limits are computed withminimal model assumptions. Then a lower limit on the slepton mass is derivedin CMSSM. In the case of direct decays, it is assumed that the branching ratioBR(f˜i → fjfk) = 1. For indirect decays the branching ratio BR(f˜ → fχ˜0) istaken from CMSSM. Since the neutralino decay modes via λ′ couplings depend onCMSSM parameters, we vary the branching ratio of the decay involving a chargedlepton and the branching ratio of the decay involving a neutrino simultaneouslya M2, the SU(2) soft SUSY breaking gaugino mass, is related to m1/2.4 Search for R-parity Violating Decays of Supersymmetric Particles at LEPbetween 0 and 1, and take the worst case result.As previously, only the weakest limits are shown. All CMSSM exclusion plots aregiven for tanβ = 1.5 and µ = −200 GeV. This choice of parameters is conservative,since the theoretical cross-section usually increases for larger tanβ and |µ|.The upper limit on the sneutrino pair-production cross-section is better than0.22 pb in the mass range of interest for any λ coupling. The cross-section limitsare generally weaker for λ′ couplings. For example, for indirect decays of sneutrinos,the limit is 1.2−1.3 pb for a mass of 45 GeV and 0.2 pb for a mass of 90 GeV. TheCMSSM mass limits are shown in Figure 3(a) and (b) for any λ and λ′ couplings.OPAL preliminary OPAL preliminary)010203040506070809050 60 70 80 90m(νµ) (GeV)m(χ0 ) (GeV)Indirect(a)Directtg β = 1.5µ = -200 GeVλ~~ν~eν~µν~µ010203040506070809050 60 70 80 90m(νe ) (GeV)m(χ0 ) (GeV)Indirect~~010203040506070809050 60 70 80 90m(eR) (GeV)m(χ) (GeV)Indirect(c)λ’1jkλ’2jkλ’3jk010203040506070809050 60 70 80 90m(µR) (GeV)m(χ0 ) (GeV)Indirect(d)λ’1jkλ’2jkλ’3jk~ ~~(b)Fig. 3. Excluded mass regions in CMSSM (a) from direct and indirect decays of sneutrinos forany λ coupling, and from indirect decays of (b) electron sneutrinos, (b) right-handed selectronsand (c) right-handed smuons for any λ′ coupling. No CMSSM exclusion is possible for muon andtau sneutrinos decaying via λ′ couplings due to the small expected theoretical cross-section. Thekinematic limit is shown by dashed lines.The charged slepton limits for λ couplings have been updated using data col-lected at√s = 192− 209 and are summarized in the next section. For λ′ couplings,the selectron and smuon pair-production cross-section upper limits for indirect de-cays are better than 0.52 pb. The CMSSM mass limits are given in Figure 3 (c,d).4. Updates at√s = 192− 209 GeVPreliminary updates5 using data collected in 1999 and 2000, corresponding to anintegrated luminosity of 400 pb−1, are performed in the multi-lepton and in thejets+leptons final states. There is a nice agreement between data and the SM ex-pectation, with the exception of events with multiple jets and identified tau leptonsat the highest energy,√s > 206.5 GeV. The biggest excess, corresponding to aPoisson probability of 2× 10−3, is found in the final state with two tau leptons andat least four jets, in the search for indirect decays of gauginos via λ′ coupling, with8 events observed and 2.2 expected from SM processes.Including around 285 pb−1 of√s = 192− 209 GeV data collected by July 2000,new upper limits, shown in Figure 4(a), are set on the production cross-section ofcharged sleptons decaying via λ couplings.6 Limits for direct decays are weaker, thecross-section limit, based on 200 pb−1 data collected at√s = 192 − 202 GeV andcombined with previous results, is shown in Figure 5(a).7The CMSSM interpretation of the results is given in Figure 4(b,c) for smuonsand staus.6 For selectrons, due to the higher expected CMSSM cross-sections, theSearch for R-parity Violating Decays of Supersymmetric Particles at LEP 5OPAL Preliminary00.050.10.150.250 60 70 80 90 100λIndirect decaysm(l∼±)σ (pb)e∼µ∼ τ∼OPAL Preliminary02040608010050 60 70 80 90 100m(µ∼ R)m(χ∼ 10) (GeV)Indirect(b)Directtg β = 1.5µ = -200 GeVλ02040608010050 60 70 80 90 100m(τ∼R)m(χ∼ 10) (GeV)Indirect(c)Directtg β = 1.5µ = -200 GeVλ(a)Fig. 4. (a) Upper limit on the pair-production cross-sections for indirect decays of ℓ˜R. CMSSMexclusion regions for (b) smuon and (c) stau pair-production. The kinematic limit is shown bydashed lines.OPAL Preliminary01234550 60 70 80 90 100λDirect decaysm(l∼±)σ (pb)02040608010050 60 70 80 90 100m(e∼R)m(χ∼10 ) (GeV)IndirectDirect (b)tg β = 1.5µ = -200 GeVλm(χ∼0 ) (GeV)100)(a)Fig. 5. (a) Upper limit on the pair-production cross-sections for direct decays. (b) CMSSMexclusion regions for selectron pair-production. The kinematic limit is shown by dashed lines.excluded region is considerably larger. The mass limits based on the data collectedup to 202 GeV are plotted in Figure 5(b).75. SummarySearches for charginos, neutralinos and scalar fermions are performed by the OPALdetector. Since no evidence for supersymmetry with R-parity violation is found,limits are placed on production cross-sections, sparticle masses and the CMSSMparameter space.References1. The OPAL Collaboration, G. Abbiendi et al., Eur. Phys. J. C11 (1999) 619.2. The OPAL Collaboration, G. Abbiendi et al., Eur. Phys. J. C12 (2000) 1.3. The OPAL Collaboration, Searches for R-Parity Violating Decays of SupersymmetricParticles with λ Couplings at 189 GeV at LEP, OPAL Physics Note 394, 2 July 1999.4. The OPAL Collaboration, Searches for R-Parity Violating Decays of SupersymmetricParticles with λ′ and λ′′ Couplings at 189 GeV at LEP, OPAL Physics Note 411, 12July 1999.5. The OPAL Collaboration, New Particle Searches in e+e− Collisions at√s = 200−209GeV, OPAL Physics Note 466, 1 November 2000.6. The OPAL Collaboration, New Particle Searches in e+e− Collisions at√s = 200−209GeV, OPAL Physics Note 435, 18 July 2000.7. The OPAL Collaboration, New Particle Searches in e+e− Collisions at√s = 192−202GeV, OPAL Physics Note 418, 5 November 1999.

Search for R-parity Violating Decays of Supersymmetric Particles at LEP

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