A Cosmic Higgs Collider

This paper examines frameworks and phenomenology of ultrarelativistic Higgs vacuum bubble collisions in a first-order phase transition associated with the Standard Model Higgs field in the early Universe. Such collisions act as a cosmic scale Higgs collider, providing access to energy scales far beyond any temperature reached in our cosmic history, potentially up to the Planck scale. This provides a unique opportunity to probe new physics that couples to the Higgs at very high scales, while also enabling novel applications for various cosmological phenomena, opening tremendous opportunities for particle physics and cosmology. As examples, we demonstrate the viability of nonthermal production of ultra-heavy Higgs portal dark matter up to $10^{16}$ GeV (with observable indirect and direct detection signals up to $m_\text{DM}=O(10)$ TeV), and leptogenesis from the production of GUT scale right-handed neutrinos.

💡 Research Summary

The paper proposes that first‑order phase transitions (FOPTs) of the Standard Model (SM) Higgs field in the early universe can generate ultrarelativistic (γ ≥ 10) Higgs‑vacuum bubbles whose collisions act as a “cosmic Higgs collider.” In such collisions the available center‑of‑mass energy can far exceed the temperature of the surrounding plasma, potentially reaching the Planck scale. The authors identify two broad physical conditions that allow the bubble walls to run away: (i) the absence of a thermal plasma that would otherwise exert a leading‑order (LO) pressure (P_{\rm LO}\sim \Delta m^{2}T^{2}) and a next‑to‑leading‑order (NLO) pressure (P_{\rm NLO}\sim g^{2}\gamma|\Delta m_{V}|T^{3}); and (ii) a symmetry‑restoring transition where the Higgs vacuum expectation value (vev) decreases across the wall, making (\Delta m^{2}<0) and turning the LO pressure negative, thereby accelerating the wall. Even when NLO pressure eventually becomes positive, the terminal Lorentz factor can remain in the ultrarelativistic regime (γ ≈ 10–100).

Four concrete scenarios are examined:

1. Supercooled electroweak transition (scEW) – a separate scalar (e.g., a dilaton or singlet) triggers a zero‑temperature electroweak FOPT. With negligible friction, the wall Lorentz factor grows linearly with bubble radius, giving typical parameters (\alpha\sim1), (\beta/H\approx100), (T_{*}\approx100) GeV, and a critical radius (R_{c}\sim m_{h}^{-1}).

2. Thermal symmetry‑restoring transition (tSR T) – the SM Higgs potential becomes unstable at large field values (≈ 4 × 10¹¹ GeV). New physics at a UV scale (\Lambda_{\rm UV}\sim10^{13}) GeV stabilizes the potential, creating a deep high‑field minimum. Thermal effects lift this minimum, causing a first‑order transition back to the electroweak vacuum. Typical parameters are (\alpha\lesssim10^{-3}), (\beta/H\sim7000), (T_{*}\approx0.009\Lambda_{\rm UV}), and a terminal γ ≲ 100.

3. Symmetry‑restoring electroweak transition (SR‑EW) – a reverse electroweak transition during reheating, where the broken‑phase vacuum is lifted above the symmetric one. This speculative scenario would have (\alpha<10^{-2}), (\beta/H\gtrsim10^{3}), and γ ≲ 100.

4. Symmetry‑restoring vacuum transition (vSR T) – the high‑field minimum sits at higher potential energy than the electroweak vacuum. With a vacuum‑energy‑dominated expansion and a secluded dark sector that does not couple to the Higgs, bubble walls again run away. Parameters can be (\alpha\sim1), (\beta/H\sim10), and (R_{c}\sim10,v_{\rm UV}^{-1}).

During collisions, the wall kinetic energy is converted into high‑energy excitations of the Higgs field and gauge bosons. The effective center‑of‑mass energy scales as (\sqrt{s}\sim\gamma\Delta v); with γ ≈ 10–100 this reaches 10¹⁴–10¹⁶ GeV, far beyond any temperature ever attained in the cosmos.

The authors explore two phenomenological applications of this “cosmic Higgs collider.”

• Ultra‑heavy Higgs‑portal dark matter – A scalar DM particle χ coupled via (\lambda_{h\chi} |H|^{2}|\chi|^{2}) can be produced non‑thermally with masses up to (10^{16}) GeV. The relic abundance is set by the bubble‑collision yield rather than thermal freeze‑out, allowing a wide range of (\lambda_{h\chi}) to match the observed density. Residual Higgs‑portal interactions give rise to direct‑detection signals for masses up to ∼10 TeV and indirect signatures (e.g., high‑energy gamma rays, neutrinos) from χ annihilation in astrophysical environments.

• Leptogenesis from GUT‑scale right‑handed neutrinos – Heavy Majorana neutrinos with masses (M_{N}\sim10^{15-16}) GeV can be produced in the collisions. Their CP‑violating decays generate a lepton asymmetry, which sphaleron processes convert into the observed baryon asymmetry. This mechanism works even when the reheating temperature is far below the RHN mass, circumventing the usual thermal leptogenesis bound.

The paper also notes that such violent bubble dynamics inevitably generate a stochastic gravitational‑wave background. The peak frequency and amplitude depend on (\alpha) and (\beta/H); for the supercooled case the signal could be within reach of future space‑based detectors (LISA, DECIGO, BBO).

Critical assessment highlights several challenges: (i) achieving truly frictionless walls requires either a completely decoupled dark sector or an extremely low temperature, which may be difficult to realize in realistic models; (ii) the stability of the Higgs potential at high scales demands new physics at (\Lambda_{\rm UV}) that must satisfy collider, flavor, and cosmological constraints; (iii) for (\alpha\sim1) the universe risks entering a vacuum‑energy‑dominated inflationary phase, raising the graceful‑exit problem; (iv) ultra‑heavy particles produced in the collisions must survive subsequent reheating and not be diluted away, implying non‑trivial reheating histories.

Nevertheless, the work provides a compelling framework that turns the Higgs field itself into a natural, Planck‑scale particle accelerator. By linking bubble‑collision dynamics to observable signatures—gravitational waves, dark‑matter searches, and leptogenesis—the authors open a novel avenue for probing physics far beyond the reach of terrestrial colliders. Future work will need to construct explicit UV completions, quantify the gravitational‑wave spectra more precisely, and explore the interplay with other early‑universe phenomena such as inflation and baryogenesis.