EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-EP-2017-129 2017/10/11 CMS-SUS-16-049 Search for direct production of supersymmetric partners of the top quark in the all-jetsp final state in proton-proton collisions at s = 13 TeV The CMS Collaboration∗ Abstract A search for direct production of top squark pairs in events with jets and large trans- verse momentum imbalance is presented. The data are based on proton-proton colli- sions at a center-of-mass energy of 13 TeV, collected with the CMS detector in 2016 at the CERN LHC, and correspond to an integrated luminosity of 35.9 fb−1. The search considers a variety of R-parity conserving supersymmetric models, including ones for which the top squark and neutralino masses are nearly degenerate. Specialized jet reconstruction tools are developed to exploit the unique characteristics of the signal topologies. With no significant excess of events observed above the standard model expectations, upper limits are set on the direct top squark pair production cross sec- tion in the context of simplified supersymmetric models for various decay hypothe- ses. Models with larger differences in mass between the top squark and neutralino are probed for masses up to 1040 and 500 GeV, respectively, whereas models with a more compressed mass hierarchy are probed up to 660 and 610 GeV, respectively. The smallest mass difference probed is for masses near to 550 and 540 GeV, respectively. arXiv:1707.03316v2 [hep-ex] 10 Oct 2017 Published in the Journal of High Energy Physics as doi:10.1007/JHEP10(2017)005. c 2017 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license ∗See Appendix A for the list of collaboration members 1 1 Introduction Although the standard model (SM) of particle physics provides a remarkably accurate descrip- tion of phenomena associated with the known elementary particles and their interactions, it leaves significant problems unresolved. It cannot, for instance, explain how the Higgs boson [1–6] can evade divergent quantum corrections, without very significant fine tuning [7, 8] of SM parameters, to allow it to have its mass at the weak scale [9–14]. Moreover, an abundance of cosmological observations, including the existence of dark matter, cannot be explained within the context of the SM alone [15–17]. Supersymmetry (SUSY) provides a theoretical framework that can address these questions. At its core, SUSY is a symmetry between fermions and bosons. In SUSY, a “sparticle” (generally referred to as a superpartner) is proposed for each SM particle with the same gauge quantum numbers but differing by one half-unit of spin and potentially in mass. The superpartners of the electroweak vector W and Z bosons and scalar Higgs boson mix to produce charged and ± 0 neutral fermions referred to as charginos (ce ) and neutralinos (ce ), respectively. For a given fermion f, there are two superpartners corresponding to the fermion’s left- and right-handed states. The superpartners mix to form two mass eigenstates, ef1 and ef2, with ef1 being the lighter of the two. The quantum corrections to the value of the Higgs boson mass (mH) from sparticles could cancel the otherwise problematic SM contributions. In this way, SUSY can protect the value of mH [18–21], provided that the mass differences between the SM particles and their superpartners are not too large. This is particularly important for superpartners of third gen- eration SM particles, because they have the largest couplings to the Higgs boson, and there- fore produce the largest corrections. Furthermore, a combination of precision measurements and null search results indicate that the superpartners of the light quarks may have very large masses [22]. In view of these considerations, the superpartners of the top and bottom quarks, the et and eb squarks, respectively, are expected to be among the lightest sparticles, potentially light enough to be produced at the CERN LHC [23]. An important point to note is that SUSY models with R-parity conservation [24, 25] require sparticles to be produced in pairs, with the lightest SUSY particle (LSP) therefore stable on cosmological time scales. This means that if the 0 lightest neutralino, denoted ce1, is the LSP, then it is also a very promising dark matter candi- date [26] that would remain at the end of all R-parity conserving sparticle cascade decays. The two motivating principles above place the search for pair production of top squarks (etet) among the highest priorities of the LHC program. The most recent searches for directetet production were carried out by thep ATLAS and CMS Col- laborations in proton-proton (pp) collisions at center-of-mass energies s of 7, 8, and 13 TeV at the LHC [27–47]. The searches have provided no evidence for sparticle production in models 0 with et masses up to ∼900 GeV and ce1 masses up to ∼400 GeV. This paper presents a searchp for direct etet production in R-parity conserving SUSY using data collected in pp collisions at s = 13 TeV by the CMS experiment at the LHC in 2016, and cor- responding to an integrated luminosity of 35.9 fb−1. The search is based on methods presented in Ref. [44], and represents an extension of that search to larger sparticle masses by means of a significantly larger dataset and the development of more sensitive search tools. This search focuses on all-hadronic final states, defined as those events whose visible content is made up solely of hadronic jets, as would be expected for signal processes in which all W bosons de- cay to quarks. These final states have the largest accessible branching fraction. In many SUSY models, the favored et decay modes depend strongly on the mass hierarchy of the sparticles. In 0 particular, different ranges of mass difference Dm between the et and ce1 correspond to very dif- 2 1 Introduction the2 basis for our searches are displayed in Fig. 1. 1 Introduction t(∗) b b + e e W W+∗ p t1 e0 p t e χ1 1 p t1 e+ 0 + χ1 χe χe e0 1 1 χ1 − e0 0 e0 χe χ1 χe p e χ1 p 1 p 1 t1 et et 1 W− 1 (∗) t b t (a) (b) (c) b c b f f ′ +∗ p e W ′ e t1 f p e f p t1 χe0 0 t1 1 χe + e0 1 χe χ1 1− χe0 e 0 1 χ1 χe p ′ 1 e0 e p e¯ ′ p e χ1 t1 f t1 −∗ t W f 1 f f b b¯ c (d) (e) (f) FigureFigure 1: 1: Feynman Diagrams diagrams for the decay for pairmodes production of pair-produced of top squarks top squarks with studied the decay in this modes analysis. of the simplifiedThe decay models cascades that are are denoted: studied (a) in T2tt, this (b) analysis. T2bW, (c) An T2tb, asterisk (d) T2ttC, indicates (e) T2bWC, the particle and (f) mayT2cc. be producedAn asterisk off-shell. indicates that the particle may be produced off-shell. Theferent search final-state regions signatures. (SR) are optimized Only the lightest for differentet massmodels eigenstate, andet1, ranges is assumed of Dm to. be The involved simplest in the models considered in this paper,(∗) 0 although the results are expected± to be equivalent± 0 for decays that we consider are et1 ! t ce1, denoted “T2tt”, and et1 ! bce1 ! bW ce1, denoted the heavier eigenstate. The et1 decay modes± of the simplified models [48–50] that are used0 as “T2bW”, under the assumption that the ce1 mass lies halfway between the et1 and ce1 masses. the basis for our searches± are displayed in Fig. 1. The choice of moderate ce1 mass in the latter model permits high momentum objects in the ± 0 finalThe state. search The regionsce1 represents (SR) are optimized the lightest for different chargino, models and ce and1 is ranges the stable of Dm LSP,. In which models escapes with detectionDm larger to than produce the W a bosonlarge transverse mass mW (“high momentumDm models”), imbalance the simplest in the event. decays Another that we model, con- (∗) 0 ± ± 0 denotedsider are “T2tb”,et1 ! t is consideredce1, denoted under “T2tt”, the and assumptionet1 ! bce1 ! ofbW equalce1 branching, denoted “T2bW”, fractions under of the the two ± 0 aforementionedassumption that decay the ce modes.1 mass lies This halfway model, between however, the assumeset1 and ce a1 compressedmasses. The mass choice spectrum of mod- in ± ± 0 ± whicherate thece1 mass inof the latterce1 is model only 5 permits GeV greater high momentum than that of objects the ce1. in As the a finalresult, state. the W The bosonsce1 0 fromrepresents chargino the decays lightest are chargino, produced and farce1 off-shell.is the stable LSP, which escapes detection to produce a large transverse momentum imbalance in the event. Another model, denoted “T2tb”, is con- Insidered models under with theDm assumptionless than the of W equal boson branching mass m fractionsW, the et1 ofcan the decay two aforementioned through the T2tt decay decay modemodes. with This off-shell model, t however, and W, throughassumes athe compressed same decay mass chain spectrum as in thein which T2bW the model, mass of via the off- 0 shell± W bosons, or decay through a flavor changing0 neutral current process (et1 ! cc , where ce1 is only 5 GeV greater than that of the ce1. As a result, the W bosons from charginoe decays1 c isare the produced charm quark). far off-shell. These will be referred to as the “T2ttC”, “T2bWC”, and “T2cc” models, respectively, where C denotes the hypothesis of a compressed mass spectrum in the first two cases.In models Observations with Dm less in such than lowmW D(“lowm modelsDm models”), are experimentally the et1 can decay challenging through sincethe T2tt the decay visible mode with off-shell t and W, through the same decay chain as in the T2bW model, via off- decay products are typically very soft (low-momentum), and therefore often evade identifi- shell W bosons, or decay through a flavor changing neutral current process (t ! cc0, where cation.