We briefly review a non-standard version of the MSSM whose phenomenology leads naturally to the study of multitop final states. Then we discuss a promising strategy to detect such final states in the early running of the LHC

D'Agnolo, R. T.

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DOI 10.1393/ncc/i2011-10761-9Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Multitop final states at the LHCR. T. D’AgnoloScuola Normale Superiore - Piazza dei Cavalieri 7, 56126 Pisa, ItalyINFN, Sezione di Pisa - Largo Bruno Pontecorvo 3, 56126 Pisa, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 28 Gennaio 2011)Summary. — We briefly review a non-standard version of the MSSM whose phe-nomenology leads naturally to the study of multitop final states. Then we discuss apromising strategy to detect such final states in the early running of the LHC.PACS 12.60.Jv – Supersymmetric models.PACS 14.65.Ha – Top quarks.PACS 13.85.Qk – Inclusive production with identified leptons, photons, or othernonhadronic particles.PACS 13.85.Ni – Inclusive production with identified hadrons.1. – A slightly uncommon SUSY spectrumThe large Yukawa couplings in the third generation of quarks may hide new physicsthat will manifest itself in top-rich signatures at the LHC.It is not hard to introduce a specific model to back up this statement. If we startwith a generic MSSM scenario, we can end up with a striking signature containing upto four top quarks rather easily, in particular if we invoke naturalness to constrain thes-particles spectrum.Following [1] and [2] we take the μ parameter in the MSSM Lagrangian between100 and 200GeV, enough to evade the LEP bound on the chargino mass. The thirdgeneration squarks are constrained by the LEP2 bound on the Higgs boson mass and bynaturalness. Their preferred mass range is around a few hundreds of GeVs. If we turnto the gluino, on the one hand the running of the squark masses favours a light gluino,because of its two-loop contribution that could drive them negative. On the other,FCNC constraints (mainly gluino-sbottom exchanges in ΔF = 2 transitions) point to aheavier range for its mass. So we find it convenient to assume a gluino mass between400GeV and 1TeV. The squarks of the first two generations can be as heavy as 2–3TeVwithout endangering naturalness. Furthermore the lightest sfermions are higgsino-like,with masses in the neighbour of μ.A simplified phenomenology of this model involves a single production mechanismpp → g˜g˜ and a decay chain with two possible final states: g˜ → tt¯χ0 and g˜ → tb¯χ−.The two branching ratios will depend in general on the stop and sbottom masses, forc© Societa` Italiana di Fisica 249250 R. T. D’AGNOLOFig. 1. – Expected events, with L = 100 pb−1, in HT bins, for the signal g˜g˜ → tt¯tt¯χ0χ0 (in black)and the backgrounds: QCD, W+jets, Z+jets and t¯t+jets, after the selection criteria indicatedin the legend. Note that the first two pT cuts pertain to the leptons while the other eight to thejets.a detailed discussion we refer to [1]. We have neglected g˜ → bb¯χ0, because it is highlysuppressed with respect to the other two decay modes, since b˜ → bχ0 ∼ (mb/mt) tanβ.We are now ready to attack the experimental problem directly and we will do so bystudying the case in which both gluinos decay to two tops all the time.2. – A preliminary analysisThe signal, that we have generated with PYTHIA8 [3], is characterized bymg˜ = 450GeV, mt˜1 = 600GeV and mχ0 = 100GeV. The production cross sectionfor this value of the gluino mass is σ ∼ 2.2 pb.The big enemy is, as usual, generic QCD with a substantial help from tt¯+jets events.We have generated the background with MADGRAPH [4], using for the QCD samplesix hard partons in the initial state and including also heavy flavours.The strategy is to get rid of the dominant dijet background with cascade cuts on upto eight of the jets pT (see fig. 1). We also take advantage of the large HT (the scalar sumof all the ET in the event) of the signal and of the semileptonic top decays, requiring thepresence of two same sign leptons. In this way, as can be seen in fig. 1, we are alreadysensitive to the signal with 100 pb−1.The main drawback of this kind of analysis is the impossibility to estimate systematicuncertainties at this stage.In this paper we have just pointed out a direction which we find interesting to exploreand which we will further study in a future paper.∗ ∗ ∗I would like to thank P. Azzuri for very helpful discussions and for the careful readingof the paper and L. Rolandi and R. Barbieri for having inspired this analysis.REFERENCES[1] Barbieri R. and Pappadopulo D., JHEP, 0910 (2009) 061.[2] Barbieri R., Bertuzzo E., Farina M., Lodone P. and Pappadopulo D., JHEP, 1008(2010) 024, arXiv: 1004.2256[hep-ph].[3] http://home.thep.lu.se/ torbjorn/Pythia.html.[4] http://madgraph.hep.uiuc.edu/.

Multitop final states at the LHC

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Multitop final states at the LHC

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