The latest results of searches for Supersymmetry in up to 4 fb−1 of proton-antiproton collisions produced at the Tevatron and analyzed by the CDF and D0 Collaborations are reported

Canepa, A.

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DOI 10.1393/ncc/i2009-10481-9Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009Looking for SUSY at the TevatronA. CanepaTRIUMF - Vancouver, BC, Canada(ricevuto il 19 Settembre 2009; pubblicato online il 24 Novembre 2009)Summary. — The latest results of searches for Supersymmetry in up to 4 fb−1of proton-antiproton collisions produced at the Tevatron and analyzed by the CDFand D0 Collaborations are reported.PACS 12.60.Jv – Supersymmetric models.PACS 13.35.Hb – Decays of heavy neutrinos.PACS 14.80.Ly – Supersymmetric partners of known particles.1. – IntroductionSupersymmetry (SUSY) [1] is one of the most promising theories for physics beyondthe Standard Model (SM) [2] as it provides a solution to the hierarchy problem and leadsto unification of the gauge interactions; if a newly introduced parity, called R-parity anddefined as PR = (−1)3(B−L)+2s, is conserved, SUSY also provides an excellent candidatefor Dark Matter. In Supersymmetry, each SM particle acquires partners with samequantum numbers but spin differing by 12 . Several SUSY breaking scenarios are proposedto explain the non-observation of sparticles with the same mass as their SM partners.Among others, mSUGRA is a favored one. Its phenomenology depends on the sparticlemasses and on the ratio of the expectation values of the two Higgs doublets of the theory,tanβ. For large values of tanβ couplings to the third generation are enhanced.2. – Squark and gluinoLight squarks and gluinos are produced in pair if PR is conserved and subsequentelydecay as g˜ → qq˜ and q˜ → qχ˜01. The signature of the process is characterized by largetransverse missing energy due to the stable neutralino escaping detection. Depending onthe mass hierarchy, the final state will contain at least 2 jets if mq˜ < mg˜, at least 3 jetsif mq˜ ∼ mg˜ and at least 4 four jets for mq˜ > mg˜. The results of the CDF [3] and D0 [4]searches, optimized in the three scenarios and carried out in ∼ 2 fb−1 of Run II data, areconsistent with the SM expectations. Limits on the mass of the squark and gluino are setin the mSUGRA scenario with the assumption of mass degeneracy of the squarks (fig. 1).c© Societa` Italiana di Fisica 237238 A. CANEPA)2 (GeV/cg~M0 100 200 300 400 500 600)2 (GeV/cq~M0100200300400500600CDF Run II Preliminary -1L=2.0 fbno mSUGRAsolutionLEP 1 + 2UA1UA2FNAL Run Ig~ = Mq~M)2 (GeV/cq~Mobserved limit 95% C.L.expected limit <0μ=5, β=0, tan0ATheoretical uncertainties includedin the calculation of the limitFig. 1. – Exclusion plane for squark and gluino masses at 95% CL.Production of sbottom and stop is not included in the current analyses. D0 extendedthe search to final states with τ˜s to probe squark pair production with q˜ → qχ˜±1 → qτ˜1νin ∼ 1 fb−1 [5].3. – Stops and sbottomsPair productions of gluinos with g˜ → bb˜ are predicted in scenarios with large valuesof tanβ and lead to final states with 4 b-jets and large missing transverse energy. TheCDF search [6] is optmized depending on the mass difference between the gluino andthe sbottom for neutralino mass of 60GeV/c2 and light squark mass of 500GeV/c2.Dedicated Neural Networks are exploited to enhance the sensitivity. No excess withrespect to the SM expectation is observed in 2.5 fb−1 and limits on the gluino and sbottommass are set with the assumption of BR(g˜ → bb˜) = 100% and BR(b˜ → bχ˜01) = 100%(fig. 2). CDF and D0 searched for the pair production of scalar top quarks via theirdecay channel t˜1 → bχ˜±1 , in the di-lepton (2.7 fb−1 [8]) and lepton-jet (0.9 fb−1 [7])final state, respectively. A kinematic fitter is used to reconstruct the objects to a SMtt¯ hypothesis. The signal is extracted from the background using a likelihood ratio fiton the combination of the reconstructed stop mass template by CDF and exploiting amultivariate likelihood discriminant by D0. The observations agree with the predictionsand limits on the stop mass as a function of the neutralino mass are set depending onthe assumed branching ratio of the chargino (fig. 3).4. – Charginos and neutralinosCharginos and neutralinos are pair produced and subsequentally can decay as χ˜±1 →νχ˜01 and χ˜02 → χ˜01. The final state will therefore be characterized by large missingtransverse energy and three leptons. A cut-based analysis is sensitive to this low back-LOOKING FOR SUSY AT THE TEVATRON 239)2Gluino Mass (GeV/c200 250 300 350 400)2 Sbottom Mass (GeV/c100150200250300350)2 Sbottom Mass (GeV/cCDF Run I excluded kinematically forbidden1b~ b→ g~2) = 60 GeV/cχ∼M(2) = 500 GeV/cq~M(-1Run II 156 pbExcluded Limit-1DØ  Run II 310 pbSbottom Pair ProductionExcluded Limit)2 Sbottom Mass (GeV/cCDF Run II Preliminary -1 L dt=2.5 fb∫Observed 95% CL limitExpected 95% CL limit b (100% BR)b~ → g~ (100% BR)10χ∼ b→ b~Fig. 2. – Exclusion plane for sbottom and gluino masses at 95% CL.ground signature and carried out by the CDF [9] and D0 [10] Collaborations on 2.0 fb−1and 2.3 fb−1 of data, respectively. Agreement is found between the expectations and theobservations and limits are set in the plane m 12(unified gaugino mass at the GUT scale)and m0 (unified sfermion mass at the GUT scale) for tanβ = 3 (fig. 4). D0 excludesvalues of tanβ below 10 for a chargino mass of 130GeV/c2 (fig. 5).5. – Higgs sectorThe minimal supersymmetric SM (MSSM) has an enlarged Higgs sector including twocharged H±; one CP -odd neutral A; and two CP -even neutrals H; h0. For large tanβ2) GeV/c1t~m(120 130 140 150 160 170 1802) GeV/c0 1χ∼m(455055606570758085 2)=105.8 GeV/c±1χ∼m(b)=11±χ∼→1t~BR(Observed 95% CL)-1CDF Run II Preliminary (2.7 fbl)=0.11ν10χ∼→1±χ∼(2BRl)=0.25ν10χ∼→1±χ∼(2BRl)=0.50ν10χ∼→1±χ∼(2BRl)=1.0ν10χ∼→1±χ∼(2BRExcluded by LEPFig. 3. – Exclusion plane for neutralino and stop masses at 95% CL. We assume electrons, muonsand taus all equally contribute to the dilepton final state.240 A. CANEPA (GeV)0m0 50 100 150 200 250 (GeV)1/2m150200250300 (GeV)1/2mDØ observed limitDØ expected limitCDF observed)-1limit (2.0 fbLEP Chargino LimitLEPSleptonLimit-1DØ, 2.3 fbmSUGRA > 0μ = 0, 0 = 3, Aβtan20χ∼1±χ∼Search for )20χ∼ M(≈) l~M()1± χ∼ M(≈) ν∼M( (GeV)1/2mFig. 4. – Exclusion plane for the m 12and m0 masses at 95% CL.the lightest Higgs (h0) is degenerate with the CP -even H (CP -odd A) for low (high)mass values. Due to enhanced coupling to the third generation, h0 is produced with largecross-section and can be searched for in the di-τ final state. Both CDF [11] and D0 [12]carried out the analysis in up to ∼ 2 fb−1 of data in the final state with an electron and amuon, and with one electron/muon and a hadronically decaying τ . The observations arein agreement with the expectation and limits are calculated with a morphing techniqueand set in the plane tanβ and mA for different mixing scenarios (fig. 6).The lightest Higgs (h0) could also be produced in association with b-quarks and coulddecay into a pair of b-quarks. CDF [13] searched for an enhancement in the mass of thetwo leading jets in events with at least 3 b-tagged jets. In the MSSM scenario withmaximal mixing in the third generation, values of tanβ between 80 and 120 are excludedover a h0 mass range 90–190GeV/c2.Because the branching ratio of H± into quarks and leptons depends on tanβ, theexistence of a charged Higgs H± can be detected as a deviation in the rate of pp¯ →tt¯→ W+bH−b with respect to the SM expectations of pp¯→ tt¯→ W+bW−b. CDF [14]βtan 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 BR(3l) (pb)×) 20 χ∼ 1± χ∼ (σ00.10.20.3mSUGRA) = 1 GeV20χ∼) - M(1τ∼)= 130 GeV; M(1±χ∼M(-1DØ, 2.3 fbObserved LimitExpected Limit BR(3l) (pb)×) 20 χ∼ 1± χ∼ (σFig. 5. – Exclusion range for tanβ at 95% CL.LOOKING FOR SUSY AT THE TEVATRON 241 (GeV)AM100 120 140 160 180 200 220 240βtan 0102030405060708090100Observed limitExpected limitLEP 2-1DØ prel., 1-2.2 fb = +200 GeVμNo-mixing, Fig. 6. – Exclusion plane for mA and tanβ at 95% CL.searched 2.2 fb−1 of data for an excess of events in the invariant mass distribution of thetwo non-b-jets originating in the top decay and excluded H± masses between 120 and140GeV/c2 at 95% CL for tanβ ≤ 1. The BR(H± → cs¯) is assumed to be 100%. D0 [15]looked for deviations in the relative branching ratios in di-lepton and lepton-jet finalstates in 1.0 fb−1 of data. For leptophobic models, D0 excludes BR(t→ H+b) > 0.2 forthe charged Higgs masses between 80 and 155GeV/c2. In the tauonic model, the excludedbranching fractions range from 0.12 at low Higgs boson mass up to 0.2 at high mass.6. – Rare decaysThe flavor changing neutral current Bs → μ+μ− is suppressed in the SM to ∼ 10−9.In SUSY scenarios with large tanβ an enhancement to ∼ 10−7 is expected. CDF lookedfor the rare decay and placed an upper limit on the BR of 5.8 × 10−8 at 95% CL using2 fb−1 of data [16].7. – PR violationThird-generation neutrinos can be produced as dd¯→ ν˜τ and can decay into leptons ofdifferent flavors via PR violation interactions. D0 searched 4.1 fb−1 of data for a bump inthe high-mass region of the electron-muon invariant mass. No excess is observed and 95%CL limits on the σ × BR and RPV couplings for different sneutrino masses [17]. CDFextended the search to include final states with τs to probe ν˜τ → e(μ)τ in 1 fb−1 [18].242 A. CANEPAREFERENCES[1] Ambrosanio S., Kane G. L., Kribs G. D., Martin S. P. and Mrenna S., Phys. Rev.D, 55 (1997) 1372; Allanach B. 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Looking for SUSY at the Tevatron

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Looking for SUSY at the Tevatron

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