We investigate the phenomenology of 2 Higgs doublet models (2HDMs) with a new U(1) gauge symmetry, U(1)X, by which flavor changing neutral currents are forbidden at tree level. As an important consequence of the spontaneous breaking of both the U(1)X and electroweak symmetries by electroweak vacuum expectation values, upper limits appear on masses of an additional gauge boson Z′ and extra Higgs bosons which are less than the TeV scale. In addition, the standard model (SM) like Higgs boson h and a heavier Higgs boson H mainly decay into a pair of Z′ which induces four lepton final states. These new decay modes cannot be suppressed by taking no Z-Z′ mixing and/or the Higgs alignment limit. We find that the minimum setup of these 2HDMs has been excluded by current data for four lepton searches at LHC. Such severe constraints can, however, be avoided by introducing a pair of vector-like fermions χ which are singlet under the SM symmetry but charged under U(1)X, and can be a candidate of dark matter. Thanks to the existence of χ, Z′ can mainly decay into <InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math display="inline"> <mi>χ</mi> <mover accent="true"> <mi>χ</mi> <mo stretchy="true">¯</mo> </mover> </math></EquationSource> <EquationSource Format="TEX">$ \chi \overline{\chi} $</EquationSource> </InlineEquation> instead of SM leptons. As benchmark models, we consider the U(1)H and U(1)R models realized by fixing specific U(1)X charges, and find regions of parameter space allowed by theoretical and current experimental constraints. We clarify that mH ∈ [160, 220] GeV and tan β ∈ [3, 4.4] are allowed in the U(1)H model, while mH ∈ [160, 380] GeV and tan β ∈ [1.6, 4.4] are allowed in the U(1)R model. In both the models, the Z′ mass is constrained to be 100 GeV ≲ <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math display="inline"> <msub> <mi>m</mi> <msup> <mi>Z</mi> <mo>′</mo> </msup> </msub> </math></EquationSource> <EquationSource Format="TEX">$ {m}_{Z^{\prime }} $</EquationSource> </InlineEquation> ≲ 110 GeV. Such a quite limited parameter space can further be explored at future collider experiments, e.g., High-Luminosity LHC and lepton colliders.

Are 2HDMs with a gauged U(1) symmetry alive?

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2026年6月16日

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Yuanchao Lou,
Takaaki Nomura,
Xinran Xu,
Kei Yagyu

摘要

We investigate the phenomenology of 2 Higgs doublet models (2HDMs) with a new U(1) gauge symmetry, U(1)_X, by which flavor changing neutral currents are forbidden at tree level. As an important consequence of the spontaneous breaking of both the U(1)_X and electroweak symmetries by electroweak vacuum expectation values, upper limits appear on masses of an additional gauge boson Z′ and extra Higgs bosons which are less than the TeV scale. In addition, the standard model (SM) like Higgs boson h and a heavier Higgs boson H mainly decay into a pair of Z′ which induces four lepton final states. These new decay modes cannot be suppressed by taking no Z-Z′ mixing and/or the Higgs alignment limit. We find that the minimum setup of these 2HDMs has been excluded by current data for four lepton searches at LHC. Such severe constraints can, however, be avoided by introducing a pair of vector-like fermions χ which are singlet under the SM symmetry but charged under U(1)_X, and can be a candidate of dark matter. Thanks to the existence of χ, Z′ can mainly decay into $χ \bar{χ}$ $ \chi \overline{\chi} $ instead of SM leptons. As benchmark models, we consider the U(1)_H and U(1)_R models realized by fixing specific U(1)_X charges, and find regions of parameter space allowed by theoretical and current experimental constraints. We clarify that m_H ∈ [160, 220] GeV and tan β ∈ [3, 4.4] are allowed in the U(1)_H model, while m_H ∈ [160, 380] GeV and tan β ∈ [1.6, 4.4] are allowed in the U(1)_R model. In both the models, the Z′ mass is constrained to be 100 GeV ≲ $m_{Z^{'}}$ $ {m}_{Z^{\prime }} $ ≲ 110 GeV. Such a quite limited parameter space can further be explored at future collider experiments, e.g., High-Luminosity LHC and lepton colliders.