Strumia, A., & Landini, G. (2025). Optical gravitational waves as signals of gravitationally-decaying particles. J. High Energy Phys., 04(4), 068–23pp.
Abstract: Long-lived heavy particles present during the big bang could have a decay channel opened by gravitons. Such decays can produce gravitational waves with large enough abundance to be detectable, and a peculiar narrow spectrum peaked today around optical frequencies. We identify which particles can decay in one or two gravitons. The maximal gravitational wave abundance arises from theories with extra hidden strong gauge dynamics, such as a confining pure-glue group. An interesting abundance also arises in theories with perturbative couplings. Future observation might shed light on early cosmology and allow some spectroscopy of sub-Planckian gravitationally-decaying particles, plausibly present in a variety of theories such as gauge unification, supersymmetry, extra dimensions, strings.
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Figueroa, D. G., & Loayza, N. (2025). Geometric reheating of the Universe. J. Cosmol. Astropart. Phys., 03(3), 073–44pp.
Abstract: We study the post-inflationary energy transfer from the inflaton (phi) into a scalar field (chi) non-minimally coupled to gravity through xi R|chi|2, considering models with inflaton potential Vinf proportional to |phi| p around phi = 0. This corresponds to the paradigm of geometric preheating, which we extend to its non-linear regime via lattice simulations. Considering alpha-attractor T-mo del potentials as a proxy, we study the viability of proper reheating for p = 2, 4, 6, determining whether radiation domination (RD) due to energetic dominance of chi over phi, can be achieved. For large inflationary scales Lambda, reheating is frustrated for p = 2, it can be partially achieved for p = 4, and it becomes very efficient for p = 6. Efficient reheating can be however blocked if chi sustains self-interactions (unless these are extremely feeble), or if Lambda is low enough, so that inflaton fragmentation brings the universe rapidly into RD. Whenever RD is achieved, either due to reheating (into chi) or to inflaton fragmentation, we characterize the energy and time scales of the problem, as a function of Lambda and xi.
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Fu, B. W., King, S. F., Marsili, L., Pascoli, S., Turner, J., & Zhou, Y. L. (2025). Non-Abelian domain walls and gravitational waves. J. High Energy Phys., 04(4), 142–29pp.
Abstract: We investigate the properties of domain walls arising from non-Abelian discrete symmetries, which we refer to as non-Abelian domain walls. We focus on S4, one of the most commonly used groups in lepton flavour mixing models. The spontaneous breaking of S4 leads to distinct vacua preserving a residual Z2 or Z3 symmetry. Five types of domain walls are found, labelled as SI, SII, TI, TII, and TIII, respectively, the former two separating Z2 vacua and the latter three separating Z3 vacua. We highlight that SI, TI and TIII may be unstable for some regions of the parameter space and decay to stable domain walls. Stable domain walls can collapse and release gravitational radiation for a suitable size of explicit symmetry breaking. A symmetry-breaking scale of order 100 TeV may explain the recent discovery of nanohertz gravitational waves by PTA experiments. For the first time, we investigate the properties of these domain walls, which we obtain numerically with semi-analytical formulas applied to compute the tension and thickness across a wide range of parameter space. We estimate the resulting gravitational wave spectrum and find that, thanks to their rich vacuum structure, non-Abelian domain walls manifest in a very interesting and complex phenomenology.
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Gao, F., Harz, J., Hati, C., Lu, Y., Oldengott, I. M., & White, G. (2025). Baryogenesis and first-order QCD transition with gravitational waves from a large lepton asymmetry. J. High Energy Phys., 06(6), 247–48pp.
Abstract: A large primordial lepton asymmetry can lead to successful baryogenesis by preventing the restoration of electroweak symmetry at high temperatures, thereby suppressing the sphaleron rate. This asymmetry can also lead to a first-order cosmic QCD transition, accompanied by detectable gravitational wave (GW) signals. By employing next-to-leading order dimensional reduction we determine that the necessary lepton asymmetry is approximately one order of magnitude smaller than previously estimated. Incorporating an updated QCD equation of state that harmonizes lattice and functional QCD outcomes, we pinpoint the range of lepton flavor asymmetries capable of inducing a first-order cosmic QCD transition. To maintain consistency with observational constraints from the Cosmic Microwave Background and Big Bang Nucleosynthesis, achieving the correct baryon asymmetry requires entropy dilution by approximately a factor of ten. However, the first-order QCD transition itself can occur independently of entropy dilution. We propose that the sphaleron freeze-in mechanism can be investigated through forthcoming GW experiments such as μAres.
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Domcke, V., Garcia-Cely, C., Lee, S. M., & Rodd, N. L. (2024). Symmetries and selection rules: optimising axion haloscopes for Gravitational Wave searches. J. High Energy Phys., 03(3), 128–51pp.
Abstract: In the presence of electromagnetic fields, both axions and gravitational waves (GWs) induce oscillating magnetic fields: a potentially detectable fingerprint of their presence. We demonstrate that the response is largely dictated by the symmetries of the instruments used to search for it. Focussing on low mass axion haloscopes, we derive selection rules that determine the parametric sensitivity of different detector geometries to axions and GWs, and which further reveal how to optimise the experimental geometry to maximise both signals. The formalism allows us to forecast the optimal sensitivity to GWs in the range of 100 kHz to 100 MHz for instruments such as ABRACADABRA, BASE, ADMX SLIC, SHAFT, WISPLC, and DMRadio.
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