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Electroweak phase transitions in a U(1)D extension of the standard model with dimension-six operators: Gravitational waves and LHC signatures

Arka Bhattacharyya1,*, Sanjoy Biswas1,†, and Saurabh Niyogi2,‡

  • *Contact author: arka.bhattacharyya.phy23@gm.rkmvu.ac.in
  • †Contact author: sanjoy.phy@gm.rkmvu.ac.in
  • ‡Contact author: saurabhphys@gmail.com
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Phys. Rev. D 114, 035014 – Published 10 August, 2026

DOI: https://doi.org/10.1103/6zg6-t6q8

Abstract

We investigate the possibility of realizing strong first-order electroweak phase transition (SFOEWPT) in an effective field theory framework where the Standard Model is extended with a complex scalar singlet (ϕ) charged under a local U(1)D gauge group. The tree-level scalar potential contains a dimension-six term of the form |H|2|ϕ|4. We show that this higher-dimensional operator plays a crucial role in the phase transition dynamics by weakening the correlation between the Higgs-singlet portal coupling and the scalar mixing angle that typically constrains singlet-extended models. Consequently, SFOEWPT can be achieved over a significantly extended region of parameter space. The strength of the phase transition is primarily driven by the vacuum expectation value of the singlet scalar, which plays a central role in this analysis. We analyze the phase transition in this model and identify regions of parameter space consistent with SFOEWPT. The resulting phase transition can generate stochastic gravitational-wave signals potentially observable at future interferometers. The extended scalar sector in presence of the dimension-six operator also leads to distinctive multiscalar production signatures at the LHC, intimately correlated with the singlet scalar VEV.

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Phys. Rev. D 114, 035014– Published 10 August, 2026
  • Received 12 April 2026
  • Accepted 8 July 2026
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