Signs of tops from highly mixed stops

Published for SISSA by Springer Received: April 20, 2015 Accepted: June 3, 2015 Published: June 17, 2015 Mihailo Backović,a Alberto Mariottib and Michael Spannowskyc a Center for Cosmology, Particle Physics and Phenomenology — CP3, Universite Catholique de Louvain, Louvain-la-neuve, Belgium b Theoretische Natuurkunde and IIHE/ELEM, Vrije Universiteit Brussel, and International Solvay Institutes, Pleinlaan 2, B-1050 Brussels, Belgium c Institute for Particle Physics Phenomenology, Department of Physics, Durham University, Durham DH1 3LE, U.K. E-mail: , , Abstract: Supersymmetric extensions of the Standard Model with highly mixed squark flavours beyond minimal flavour violation provide interesting scenarios of new physics, which have so far received limited attention. We propose a calculable realization of such scenarios in models of gauge mediation augmented with an extra interaction between the messengers and the up type squark. We compute the supersymmetric spectrum and analyze the flavour physics constraints on such models. In a simplified model approach, we show that scenarios with maximal squark flavour mixing result in interesting phenomenological signatures at the LHC. We show that the model can be probed up to masses of mũ . 950 GeV in the single-top event topology at LHC14 with as little as 300 fb−1 . The most distinctive signature of highly mixed scenarios, the same sign positive charge di-top, can be also probed to comparable squark masses at high luminosity LHC14. Keywords: Supersymmetry Phenomenology ArXiv ePrint: 1504.00927 Open Access, c The Authors. Article funded by SCOAP3 . doi:10.1007/JHEP06(2015)122 JHEP06(2015)122 Signs of tops from highly mixed stops Contents 1 2 The model formulation 2.1 Low energy constraints 2.2 Extended gauge mediation 2.3 Mass spectrum 3 3 4 11 3 Simplified model 3.1 Branching ratios 3.2 Benchmark points 11 12 14 4 Collider signatures 4.1 Production modes and cross sections 4.2 Constraints from LHC8 14 14 16 5 Top signatures at LHC14 5.1 Event generation and reconstruction 5.2 Single-top channel 5.3 Same sign top channel 19 19 19 22 6 Conclusions 24 A Soft terms from R-symmetric hidden sector 26 1 Introduction Recent results from the Large Hadron Collider (LHC) impose stringent limits on the scale of supersymmetry and considerably constrain the minimal supersymmetric extension of the Standard Model (MSSM). Yet, the spectrum of non-standard supersymmetric (SUSY) models has not been studied in full detail, while the current data constraints on non√ standard SUSY are weaker [1–6]. At the dawn of the LHC Run II at s = 14 TeV, and with the prospects for the future high luminosity LHC, it is important to revisit constraints imposed in studies of the MSSM parameter space, and to explore if some viable realizations have been overlooked and/or could be accessed through new types of collider signatures. As an example of scenarios which were previously less studied, one could consider relaxing the minimal flavour violation (MFV) assumption on SUSY models. One interesting option for non-standard SUSY beyond MFV involves considerations of mixing between squark flavours. The immediate benefit of scenarios where squark mixing is allowed is that branching ratios of squark decays into final states predicted by standard SUSY models are –1– JHEP06(2015)122 1 Introduction –2– JHEP06(2015)122 lowered, hence weakening the experimental bounds on the stop mass. Current experimental results, particularly from flavour physics, give strong constraints on mixing between first and second generation squarks, while mixing between third and first/second generations remains relatively unconstrained. Several scenarios of SUSY models with squark mixing have recently been discussed in refs. [6–10]. In this paper we investigate the possibility that the low energy part of the supersymmetric spectrum is characterized by a single (right handed) light squark, highly mixed in flavour between stop and up-squark or stop and scharm-squark. We propose an explicit realization of such scenarios in a simple extension of gauge mediated models, augmented with an extra interaction between the messengers and the right up type quark superfield. We study the constraint imposed on such models by flavour observables and by the requirement of a viable Higgs mass. A similar extension of the gauge mediation scheme has been recently studied in several different contexts [11–27], for instance with the purposes of addressing the µ − Bµ problem, of raising the Higgs mass and/or investigating their possibly unusual flavor patterns. We study in detail the reach of LHC14 for the case where the light squark is maximally mixed between top- and up-type squark. We focus on the collider signatures which are peculiar to a scenario with maximal stop-sup mixing, i.e. single top and same sign ditop production. These signatures have already been investigated in several new physics scenarios. The single top signatures have been studied for instance in [28–33]. Same sign tops have been studied in new physics models attempting to explain the forward backward asymmetry [34–40], or in composite models [41, 42]. In the context of supersymmetry, same-sign top final states can arise either in R-parity violating models [43–45] or in Rparity preserving theories through a gluino decay chain [46, 47]. However, in the latter case we expect a roughly equivalent number of same sign positive top pairs and of same sign negative top pairs. Instead, in scenarios with large sup-stop mixing, the same-sign top contribution at the LHC will be dominantly of positive charge, since it is obtained through an initial state of two up quarks. In the context of R-parity preserving MSSM, the presence of the single top signature in association with the same sign positive top signature would hence be a robust hint of large squark mixing. We define a simplified model for LHC searches that consists only of the gluino (that we fix representatively at 2 TeV), the maximally mixed sup-stop right-type squark and the neutralino (mostly Bino). Considering benchmark points not yet excluded by LHC8, we show that the single top signal can be discovered at LHC14 with 300 fb−1 up to mũ1 . 950 GeV in the aforementioned channels. The more distinctive signature of sizeable supstop mixing, (i.e. same sign tops), leading eventually to same sign positive leptons plus missing energy, can be probed at High Luminosity LHC14. Our collider analysis represents a concrete proposal to test the mixing property of the light squark in the next LHC runs. The paper is organized as follows: in section 2 we present the model setup and we explore the relevant parameter space, identifying the phenomenologically viable regions and possible benchmark points. On the basis of this analysis, in section 3 we define a simplified model for collider studies, with a light and maximally mixed sup-stop squark (MMUT). In section 4 we discuss its m (...truncated)