Search for the pair production of scalar top quarks in the acoplanar charm jet final state in collisions at

Abstract

A search for the pair production of scalar top quarks,t, has been performed in 360 pb −1 of data from pp collisions at a center-of-mass energy of 1.96 TeV, collected by the DØ detector at the Fermilab Tevatron collider. Thet decay mode considered ist → cχ 0 1 , whereχ 0 1 is the lightest supersymmetric particle. The topology analyzed therefore consists of a pair of acoplanar heavy-flavor jets with missing transverse energy. The data and standard model expectation are in agreement, and a 95% C.L. exclusion domain in the (mt , mχ0 ciated with the left-and right-handed degrees of freedom of each ordinary quark. The mass eigenstates result from the diagonalization of a mass matrix, with elements determined by the specific SUSY-breaking pattern. A light SUSY partner of the top quark, or stop, is a generic prediction of models in which the scalar quark masses are equal at the grand unification scale. A first reason is that, due to the impact of the large top quark Yukawa coupling in the renormalization group equations, DØ Collaboration / Physics Letters B 645 (2007) 119-127 123 the diagonal elements of the mass matrix are driven to values smaller than those for the other scalar quarks at the electroweak scale [2]. A second reason is that the off-diagonal terms are proportional to the relevant quark mass, and hence are much larger in the case of the top quark. The mass eigenstates are therefore broadly split, with the mass of the lighter stopt thus driven to an even lower value [3]. Finally, a light stop is a necessary ingredient in the context of electroweak baryogenesis [4].In models with R-parity conservation [5], the lightest SUSY particle (LSP) is stable, and cosmological constraints imply that it should be neutral and colorless [6]. In a large class of SUSY models, the lightest of the neutralinos-the mass eigenstates resulting from the mixing of the SUSY partners of the neutral gauge and Higgs bosons-is the LSP, which furthermore appears as a viable dark matter candidate. In the following, it will be assumed that R-parity is conserved and that the LSP is the lightest neutralinoχ is considered, and it is assumed that t → cχ 0 1 , a flavor-changing loop decay [7], is the only relevant decay mode, i.e., that the tree-level four-body decays [8] t → bff χ 0 1 can be neglected. In pp collisions, stop pair production proceeds via qq annihilation and gluon-gluon fusion. The cross section has very little dependence on SUSY parameters other than the stop mass. At the center-of-mass energy of 1.96 TeV available in Run II of the Fermilab Tevatron collider, it ranges from 15 to 2.25 pb for stop masses from 100 to 140 GeV, as calculated at nextto-leading order (NLO) with PROSPINO [9], for equal renormalization and factorization scales μ rf = mt and using the CTEQ6.1M parton distribution functions (PDFs) [10]. The final state topology resulting from thet → cχ 0 1 decay is a pair of acoplanar jets, with large missing transverse energy / E T carried away by the two weakly interacting LSPs. Previous searches in this...

Light stop searches at the LHC in events with one hard photon or jet and missing energy

“…Figure 2 shows the 5σ discovery reach with the statistical significance estimated by S/ √ B. Also shown are current [9] and projected [11] 95% confidence level (C. L.) exclusion limits for light stop searches in the jets plus missing energy channel at the Tevatron collider. Observe that since Tevatron searches are more efficient for large values of ∆m, while the LHC searches explored in this article become more efficient for small values of ∆m, searches for stops at both colliders complement each other.…”

“…The charm transverse energy, in turn, is controlled by the mass difference of the stop and the neutralino. For mass differences below 40 GeV, the Tevatron has no sensitivity in stop searches [9]. 1 For larger mass differences, however, the Tevatron reach extends up to values well above the LEP limit of 100 GeV, allowing to explore stop masses up to 160 GeV [11].…”

Stop reconstruction with tagged tops

“…At the Tevatron, low-mass stop searches look for loop-induced stop decays [3][4][5] to charm quarks and the lightest neutralino [6,7]. Increasing the stop mass makes it more promising to look for decays to a bottom jet and the lightest chargino [8,9], a final state irreducible from a leptonic top decay.…”

Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks inpp¯collisions ats=1.96 TeVAaltonen1, Abulencia2, Adelman3 et al. 2007Phys. Rev. DSelf Cite29090View full textAdd to dashboardCite

“…For the search topology studied in the present analysis, LEP excludes stop (sbottom) masses smaller than 100 GeV=c 2 ( 100 GeV=c 2 ), independent of the difference between stop (sbottom) and neutralino 0 1 masses [9]. Recent results from the D0 Collaboration in the same topology [16,17], based on Run II data, have extended Tevatron's Run I reach [10,11] by excluding stop masses up to 133 GeV=c 2 , and sbottom masses up to 220 GeV=c 2 . These results are also shown in Figs.…”

“…Previous searches for stop and sbottom have been performed at CERN LEP and at the Tevatron [9][10][11][12][13][14][15][16][17]. For the search topology studied in the present analysis, LEP excludes stop (sbottom) masses smaller than 100 GeV=c 2 ( 100 GeV=c 2 ), independent of the difference between stop (sbottom) and neutralino 0 1 masses [9].…”