Abstract
Naturalness arguments applied to supersymmetric theories imply a spectrum containing four light higgsinos \tz_{1,2} and \tw_1^+- with masses ~ 100-300 GeV (the closer to M_Z the more natural). The compressed mass spectrum and associated low energy release from \tw_1 and \tz_2 three-body decay makes higgsinos difficult to detect at LHC14, while the other sparticles might be heavy, and possibly even beyond LHC14 reach. In contrast, the International Linear e^+e^- Collider (ILC) with \sqrt{s}>2m(higgsino) would be a {\it higgsino factory} in addition to a Higgs boson factory and would serve as a discovery machine for natural SUSY! In this case, both chargino and neutralino production %which give rise to distinct event topologies, occur at comparable rates, and lead to observable signals above SM backgrounds. We examine two benchmark cases, one just beyond the LHC8 reach with \tw_1(\tz_2)-\tz_1 mass gap of 15 (21) GeV, and a second more difficult case beyond even the LHC14 reach, where the mass gap is just 10 GeV, close to its minimum in models with no worse than 3% fine-tuning. The signal is characterized by low visible energy events together with MET in the one or two jets +1\ell channel from chargino production, and in the opposite sign, same-flavour, acoplanar dilepton channel from \tz_1\tz_2 production. For both cases, we find that the signal is observable above backgrounds from the usual 2-> 2 SM events and from \gamma\gamma collisions with just a few fb^{-1} of integrated luminosity. We also show that with an integrated luminosity of 100~fb^{-1}, it should be possible to extract \tw_1, \tz_2 and \tz_1 masses to percent level. These measurements would point to higgsinos as the origin of new physics and strongly suggest a link to a natural origin for W, Z and h masses.
Highlights
In be a higgsino factory in addition to a Higgs boson factory and would serve as a discovery machine for natural SUSY! In this case, both chargino and neutralino production occur at comparable rates, and lead to observable signals above SM backgrounds
In this paper we examine the detectability of the light higgsinos of RNS at the Intern√ational Linear Collider (ILC), a proposed e+e− collider [45, 48] designed to operate at s ∼ 0.25 − 1 TeV, with an added capability of electron beam polarization
Within the RNS framework, mZ2 − mZ1 = 10 GeV is close to the minimum of the mass gap if we require that ∆−E1W > 3%. This can be seen from figure 1 where we show the neutralino mass gap in the m1/2 − μ plane, with the other NUHM2 parameters fixed at the same values as for the ILC2 case
Summary
In figure 2, we s√how sparticle and Higgs boson production rates for unpolarized beams at the ILC versus s for the ILC1 benchmark point. Z1Z2 production the electron beam rates for the ILC1 benchmark polarization PL(e−). The comparable rates (within an order of magnitude) for both both chargino and neutralino pair production (solid curves), together with the relatively mild polarization is characteristic of the production of higgsino-like charginos and neutralinos. The neutralino-pair production cross sections for this wino-like case are below 0.1 fb and do not show up in this frame. The polarization dependence of the chargino pair production cross section provides an independent handle that may enable us to argue the higgsino-like nature of the charginos of the ILC1 point. For a right-handed electron beam the amplitude for charged wino pair production is suppressed by a factor of MW2 /s relative to that for charged higgsino pair production, accounting for the strong drop of the dashed curve at PL(e−) = −1
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