Stop searches in flavourful supersymmetry

Published for SISSA by Springer Received: April 14, 2016 Revised: August 17, 2016 Accepted: August 18, 2016 Published: September 14, 2016 Andreas Crivellin,a,d Ulrich Haischb,d and Lewis C. Tunstallc a Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, United Kingdom c Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland d CERN, Theory Division, CH-1211 Geneva 23, Switzerland b E-mail: , , Abstract: Natural realisations of supersymmetry require light stops t̃1 , making them a prime target of LHC searches for physics beyond the Standard Model. Depending on the kinematic region, the main search channels are t̃1 → tχ̃01 , t̃1 → W bχ̃01 and t̃1 → cχ̃01 . We first examine the interplay of these decay modes with c̃1 → cχ̃01 in a model-independent fashion, revealing that a large parameter space region with stop mass values mt̃1 up to 530 GeV is excluded for any t̃1 → cχ̃01 branching ratio by LHC Run I data. The impact of c̃1 → cχ̃01 decays is further illustrated for scenarios with stop-scharm mixing in the right-handed sector, where it has previously been observed that the stop mass limits can be significantly weakened for large mixing. Our analysis shows that once the c̃1 → cχ̃01 bounds are taken into account, non-zero stop-scharm mixing can lead to an increase in the allowed parameter space by at most 35%, with large areas excluded for arbitrary mixing. Keywords: Supersymmetry Phenomenology ArXiv ePrint: 1604.00440 Open Access, c The Authors. Article funded by SCOAP3 . doi:10.1007/JHEP09(2016)080 JHEP09(2016)080 Stop searches in flavourful supersymmetry Contents 1 2 Stop search combination for mt̃1 − mχ̃01 > mt 3 3 Stop search combination for mW + mb < mt̃1 − mχ̃01 < mt 8 4 Exclusion limits for purely right-handed up-squark mixing 11 5 Conclusions and outlook 14 A Event generation 15 1 Introduction A key feature of supersymmetric extensions of the Standard Model (SM) is the fact that radiative corrections to the Higgs potential can induce electroweak symmetry breaking in a technically natural fashion. Since top quarks and top squarks dominate the radiative corrections, naturalness requires their masses to be of similar magnitudes to ensure a sufficient cancellation of quadratic divergences. Apart from the gluino, Higgsinos and the left-handed bottom squark, the rest of the superpartners are less important for naturalness, and may well have masses above the reach of the LHC [1–7]. A spectrum with the above hierarchy is a typical starting point for phenomenological analyses in supersymmetry (SUSY). Although light stops are required for naturalness, they can reintroduce fine-tuning in minimal SUSY because the Higgs is typically predicted to be light. For instance, to accommodate a Higgs mass of 125 GeV in the Minimal Supersymmetric SM (MSSM), the stop masses must be around 1 TeV, at the cost of tuning at the percent level or worse. Reconciling these two features, light stops for naturalness and heavier stops for the Higgs mass, constitutes the “little hierarchy problem”. However, in contrast to naturalness, the little hierarchy problem is model dependent and tightly bound to the MSSM. SUSY models that can generate a sufficiently heavy Higgs with improved naturalness include scenarios with non-decoupling D-terms [8] and the next-to-minimal supersymmetric SM with special parameter choices [9]. Naturalness considerations aside, there are additional reasons to expect light stops if SUSY is realised in nature. For instance, the renormalisation group evolution from a high scale with universal squark masses typically drives the masses of the third generation squarks to small values [10]. Light stops also help accommodate the observed dark matter relic density [11, 12] and are an essential ingredient in realising baryogenesis [13–15]. Experimentally, the bounds on the lightest stop mass mt̃1 are much weaker than the limits on the other coloured superpartners, i.e. the squarks of the first two generations and –1– JHEP09(2016)080 1 Introduction the gluino [16, 17]. There are three main kinematic regions where different channels are used to search for stops, namely R1) mt̃1 − mχ̃01 > mt : t̃1 → tχ̃01 , R2) mW + mb < mt̃1 − mχ̃01 < mt : t̃1 → W bχ̃01 , R3) mc < mt̃1 − mχ̃01 < mW + mb : t̃1 → cχ̃01 and t̃1 → bf f 0 χ̃01 . –2– JHEP09(2016)080 Here mχ̃01 denotes the mass of the lightest neutralino, constituting the lightest superpartner (LSP), while mW , mb and mc are the mass of the W boson, the bottom quark and the charm quark, respectively. In each region, the results from the ATLAS and CMS searches are interpreted in the context of simplified models, where the branching ratio for each decay mode is fixed to 100% and flavour violation is assumed to be absent. Under these assumptions, the resulting limits on mt̃1 in the region R1 are strong, reaching up to stop masses of 715 GeV in the case of ATLAS [18–23] and of almost 800 GeV in the case of CMS [24–28]. It has been observed [29–34], however, that these limits can be weakened if non-minimal sources of flavour violation are present. This occurs because flavour-violating effects enhance the decay width for t̃1 → cχ̃01 , and thereby reduce the branching ratio for t̃1 → tχ̃01 from unity. On the other hand, if the decay width of t̃1 → cχ̃01 becomes large, the limits from direct c̃1 pair production and subsequent scharm decay c̃1 → cχ̃01 [35] become relevant, which apply to t̃1 → cχ̃01 as well once the branching ratio is large. In the second region R2, the situation is similar. The limits on mt̃1 reach only up to around 300 GeV [18, 20, 24] and the three-body decay t̃1 → W bχ̃01 is suppressed by phase space, so that t̃1 → cχ̃01 can compete for relatively small off-diagonal elements in the squark mass matrix [36]. Again, once stop-scharm mixing and therefore the decay width for t̃1 → cχ̃01 is sizeable, searches for charm signatures [37] can become relevant. Finally, in the third region R3, t̃1 → cχ̃01 is typically the dominant decay mode and the four-body decay t̃1 → bf f 0 χ̃01 [20, 37] can only compete for scenarios resembling Minimal Flavour Violation (MFV) [38]. The purpose of this article is to examine the complementarity of c̃1 → cχ̃01 searches with the standard channels t̃1 → W bχ̃01 and t̃1 → cχ̃01 in the presence of non-minimal sources of flavour violation. In section 2, we introduce the basic ideas behind our combination procedure and apply it to set model-independent limits on mt̃1 , mχ̃01 and the branching ratio of t̃1 → cχ̃01 in the kinematic region R1 using ATLAS Run I data. The very same exercise is performed in section 3 for the region R2. Focusing on flavour mixing in the righthanded up-squark sector, which is largely unconstrained by quark flavour observables, we then quantify in section 4 the inte (...truncated)