Abstract
Large Hadron Collider (LHC) has provided direct evidence of Yukawa couplings between the third generation charged fermions and the 125 GeV Higgs boson. Whether the first two generation charged fermions arise from exactly the same mechanism becomes the next interesting question. Therefore, direct measurements of charm or muon Yukawa couplings will be crucial to answering this puzzle. The charm Yukawa measurement at the LHC suffers from severe QCD background and it is extremely difficult to reach the sensitivity. In this paper, we compare the potential of probing charm Yukawa coupling at the two proposed future "Higgs Factory" experiments, the Large Hadron electron Collider (LHeC) and Circular electron positron collider (CEPC). At the LHeC, Higgs bosons will be produced via weak boson fusion and the energetic forward jet may suppress the background significantly. However, due to huge $\gamma-g$ scattering background, the potential of LHeC search is still limited. With $-80\%$ polarized electron beam of 60 GeV, the signal significance can only reach $2\sigma$ for $ \kappa_{c}\simeq 1$ with a 3 ab$^{-1}$ integrated luminosity. In comparison, measurement at the CEPC can then reach $8.0\sigma$ for $\kappa_{c}\simeq 1$ with a 2 ab$^{-1}$ of data.
Highlights
The observation of Higgs boson decays in diphoton, fourlepton, dilepton channels [1,2] has provided convincing evidence that the Higgs boson is responsible for generation of the weak gauge boson masses through electroweak gauge symmetry breaking (EWSB)
By far, direct measurements of Yukawa couplings have been confirmed at the Large Hadron Collider (LHC) for only the 3rd generation charged fermions, i.e., h → bbdecay mode in the associated production (VH) [3,4] and h → τþτ− with a branching fraction at the percent level measured by ATLAS and CMS individually [5,6]; and the direct measurement of [3,7]
Given the difficulty at the LHC, we study the measurement of yc at electron-hadron colliders and positron-electron colliders, e.g., the Large Hadron Electron Collider (LHeC) at CERN and the Circular Electron-Positron Collider (CEPC)
Summary
The observation of Higgs boson decays in diphoton, fourlepton, dilepton channels [1,2] has provided convincing evidence that the Higgs boson is responsible for generation of the weak gauge boson masses through electroweak gauge symmetry breaking (EWSB). Because of the SUð2ÞL × Uð1ÞY gauge structure, the SM charged fermion masses arise from couplings between SUð2Þ doublets In many models such as two-Higgs-doublet models (2HDM) [9], Δmf is proportional to the corresponding Yukawa coupling yf and yf 1⁄4 0 corresponds to the chiral symmetry limit. In this case, precision measurement of yb is sufficient to probe the new physics scale. CEPC as a positron-electron collider could produce a large number of Higgs bosons thrpouffiffigh the eþe− → Zh process at a centerof-mass energy of s ∼ 240 GeV [41], which provides the cleanest channel for probing hp→ffiffi ccwithout other large QCD backgrounds.
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