The particle discovered in the Higgs boson searches at the LHC with a mass of about 125 GeV is compatible within the present uncertainties with the Higgs boson predicted in the Standard Model (SM), but it could also be identified with one of the neutral Higgs bosons in a variety of beyond the SM (BSM) theories with an extended Higgs sector. The possibility that an additional Higgs boson (or even more than one) could be lighter than the state that has been detected at 125 GeV occurs generically in many BSM models and has some support from slight excesses that were observed above the background expectations in Higgs searches at LEP and at the LHC. The couplings between additional Higgs fields and the electroweak gauge bosons in BSM theories could be probed by model-independent Higgs searches at lepton colliders. We present a generator-level extrapolation of the limits obtained at LEP to the case of a future \(e^+e^-\) collider, both for the search where the light Higgs boson decays into a pair of bottom quarks and for the decay-mode-independent search utilising the recoil method. We find that at the ILC with a centre-of-mass energy of 250 GeV, an integrated luminosity of 500 fb\(^{-1}\) and polarised beams, the sensitivity to a light Higgs boson with reduced couplings to gauge bosons is improved by more than an order of magnitude compared to the LEP limits and goes much beyond the projected indirect sensitivity of the HL-LHC with 3000 fb\(^{-1}\) from the rate measurements of the detected state at 125 GeV.

在LHC上希格斯玻色子搜索中发现的质量约为125 GeV的粒子在当前不确定性范围内与标准模型（SM）中预测的希格斯玻色子兼容，但也可以用中性希格斯玻色子之一来识别在SM（BSM）理论之外的各种理论中，Higgs领域得到了扩展。在许多BSM模型中，通常会出现一个额外的希格斯玻色子（或什至一个以上）可能比在125 GeV处检测到的状态更轻的可能性，并且这种现象得到了某些支持，这是由于希格斯搜索中超出背景期望值而略有超出在LEP和LHC。BSM理论中其他希格斯场与弱电玻色子之间的耦合可以通过轻子对撞机上与模型无关的希格斯搜索来探究。\（e ^ + e ^-\）对撞机，既用于希格斯玻色子衰变成一对底夸克的搜索，又用于利用反冲方法进行与衰减模式无关的搜索。我们发现，在质心能量为250 GeV，积分光度为500 fb \（^ {-1} \）和偏振光束的ILC上，对希格斯玻色子的敏感度降低了与规范玻色子的耦合与LEP限制相比，改善了一个数量级以上，并且远远超过了从125 GeV的检测状态的速率测量得出的3000 fb \（^ {-1} \）的HL-LHC预计的间接灵敏度。