LHC Run II与高能下的天然SUSY新边界：光Higgsinos与精细调整

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本文主要探讨了在LHC Run II（大型强子对撞机第二阶段）及其后续升级中的最新研究成果，针对具有光希格斯ino、停止（stop）和 gluino 的天然超对称（Natural Supersymmetry, SUSY）模型进行了细致且全面的重新解读。分析了基于大约15 femtobarn-1（fb-1）的13 TeV ATLAS和CMS实验数据的搜索结果，这些数据对简化模型的参数空间施加了严格的限制。研究特别关注了光希格斯ino主导的SUSY模型，如轻型MSSM（Minimal Supersymmetric Standard Model，最小超对称标准模型）中的R-parity守恒和风味对称的sfermions（超味变体粒子）。当前的实验数据表明，如果超对称理论需要保持电弱标度的10%精细调整（即理论预言与观测值之间的偏差），那么这种“最香草”版本的MSSM已经被排除。然而，模型中存在一些解耦现象，例如非味变种（decoupled valence squarks）或者缺失能量减少的情况，这些模型仍然有可能实现自然性，即无需过度调整参数来解释观测。尽管如此，文中指出，为了保持自然性，调解尺度（mediation scale）必须非常低，低于100 TeV。这意味着超对称粒子的存在可能与较低的能量层次相关联。随着高亮度LHC时代的到来，预计粒子质量的限制将进一步提升大约1 TeV，这将对天然SUSY模型的进一步发展提出更高的要求。作者Matthew Buckley、David Feld、Sebastian Macaluso、Angelo Monteux和David Shih来自美国罗格斯大学物理与天文学系的非线性高能物理中心（NHETC），他们在文中不仅阐述了现有数据的分析方法，还对未来的研究方向提出了思考，包括如何在更严格的实验条件下探索天然SUSY的可能性，以及如何设计更精细的模型来应对未来的实验挑战。这篇文章对于理解LHC Run II的实验结果在限制超对称理论中的作用以及其对自然性原理的影响具有重要意义，为未来超对称物理学的研究提供了有价值的线索。

JHEP08(2017)115

important to examine models with more exotic ﬁnal states (see [35] for a recent brief

review). Possibilities may include displaced decays (for the signatures we study, these were

already well constrained by 8 TeV data [36, 37], while displaced leptons are less tested [38]);

and collimated particles that fail isolation such as “dirty leptons” [39, 40] and “lepton

jets” [41–43]. Also, we have ignored tau leptons in this work, assuming that they will be

at least as stringently constrained by either jets+MET or lepton-based searches. It might

be worthwhile to test this assumption more rigorously, e.g. in the case of displaced taus

where there are important gaps in coverage [38].

On the model building side, there are several, well-known extensions of the MSSM

(e.g. Dirac gauginos) which provide loopholes to the tuning bounds. These are not con-

sidered here, but they are increasingly well-motivated. We will discuss them further in

section 5. Even within the more conventional context of natural SUSY with light higgsinos,

stops and gluinos, there are many interesting model building directions to pursue. As dis-

cussed above, we will show that ∆ ≤ 10 requires a very low messenger scale (Λ . 100 TeV),

and this is an important constraint of future models of natural SUSY. This is especially

true for models of Eﬀective SUSY, since these constructions generally tie SUSY breaking

to the generation of ﬂavor in the SM. This would mean that ﬂavor must also be generated

at an extremely low scale, and it is not at all obvious that this is viable. Some examples

of previous attempts include refs. [44–48]. Also, we ﬁnd that adding a HV/Stealth sector

to the MSSM can trade MET for jets and greatly reduce the bounds. It is interesting

to speculate whether this additional sector could be used for anything else, such as dark

matter or raising the Higgs mass. More generally, obtaining a 125 GeV Higgs is a major

issue for natural SUSY and requires going beyond the MSSM, and it is interesting to think

about whether extensions of the MSSM which succeed in raising the Higgs mass could also

help to hide SUSY at the LHC.

This paper is organized as follows. In section 2, we describe our methodology for rein-

terpreting LHC searches (with additional information provided in appendix A). The models

of natural SUSY we consider are described in section 3, and the resulting experimental lim-

its on these models in section 4. We conclude in section 5 with projections for the future

reach of the LHC and model-building directions suggested by the existing constraints.

2 Recasted searches and methodology

In the following sections, we will consider the status of natural SUSY after the most recent

results from the 13 TeV LHC, as mostly reported in the ICHEP 2016 conference [1]. These

results, using 12 − 18 fb

−1

of data from CMS and ATLAS, greatly extend the experimen-

tal reach of the LHC for gluinos and squarks. We concentrate on the searches listed in

table 1, each of which has many signal regions (SRs) that target speciﬁc mass spectra and

supersymmetric production modes.

We also considered the ATLAS 7-10 jets+MET search [49] with 3.2/fb and the CMS black hole

search [50] with 2.2/fb. Due to the strong possibility of control-region contamination, the latter neces-

sitated a conservative reinterpretation along the lines of [9]. Neither search set the strongest limit in any

of the simpliﬁed models considered in this paper, so they are not included here. However, an update of the

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LHC Run II与高能下的天然SUSY新边界：光Higgsinos与精细调整

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