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Researchers have developed a physics model exploring how the early universe may have undergone a rapid phase transition that could have produced gravitational waves still detectable today. The study examines a scale-invariant extension of the Standard Model involving a U(1)_X symmetry that breaks spontaneously, generating masses for new particles including a Z' boson and heavy neutrinos. The model predicts that future gravitational wave observatories like LISA and DECIGO could detect signals from this ancient phase transition, particularly in parameter regions where the new particles would be too heavy or weakly interacting to find at particle colliders.
Why it matters
This work demonstrates how gravitational wave astronomy and particle physics experiments can provide complementary approaches to discovering new fundamental physics. If confirmed, detection of these primordial gravitational waves would provide direct evidence of physics beyond the Standard Model and offer insights into the universe's earliest moments that cannot be accessed through traditional particle accelerators.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: We investigate the complementarity between collider searches and gravitational-wave (GW) observations in the classically scale-invariant general $U(1)_X$ extension of the Standard Model. The $U(1)_X$ scale is generated radiatively through the Coleman-Weinberg mechanism, while the electroweak scale is induced through the Higgs portal, without explicit mass terms in the scalar potential. Three right-handed neutrinos are introduced to ensure anomaly cancellation and acquire Majorana masses upon $U(1)_X$ symmetry breaking, which also generates the mass of the neutral gauge boson $Z’$. In the strongly supercooled regime, a tachyonic $U(1)_X$ phase transition can produce a stochastic GW background. Assuming three heavy Majorana neutrinos, we study their effects on the phase-transition dynamics and reheating, and estimate the resulting GW signals. We identify the regions of the $U(1)_X$ gauge coupling and $Z’$ mass accessible to future GW observatories, including LISA and DECIGO, and compare them with existing LEP and LHC constraints. We find that GWs from tachyonic phase transitions can probe regions with small gauge couplings and heavy $Z’$ bosons that are difficult to access through collider searches, demonstrating the complementarity of these probes.
Source: Implications of tachyonic phase transition in classically scale invariant general $U(1)_{X}$ models