Dmitry Gorbunov

A bstract: We study the production of hypothetical vector portal mediators, dark photons γ ^′ , with masses in the range 0.4–3.5 GeV in the negatively charged pion-proton collisions π ⁻ p → γ ^′ X via inelastic pion bremsstrahlung and QCD Drell-Yan-like process. In both cases we estimate the value of total dark photon production cross section and obtain the energy distribution for dark photons that could be produced in the NA64h experiment. We also present the mean energies of dark photons produced in the same way by the secondary pions with momenta typical for T2K, DUNE and SHiP experiments.

In recent article [PhysRevD.109.055041 (2024)] it has been argued that production of dark photons -- hypothetical massive vectors -- in nuclear reactors via mixing with the visible photons is considerably enhanced (by a factor of 10) due to Compton scattering of the latter. We revisit the production of dark photons in the reactor environment and find that although the scattering of photons indeed leads to a larger number of produced dark photons, the overall enhancement is considerably smaller than what has been obtained in Ref. [PhysRevD.109.055041 (2024)]. Our findings are validated using GEANT simulations, which take into account oscillations between ordinary and visible photons and interaction of the latter with matter. The correction to the limit on mixing parameter between dark and visible photons is expected to be below 30% for all masses. It's application must be accompanied with an update on the original spectrum of photons produced in nuclear reactions inside the nuclear reactor.

Evolution of cosmic domain walls (DWs) settles to the scaling solution, which is often assumed to be independent of initial conditions. However, lattice simulations performed in this work reveal a clear dependence of the scaling DW area on the initial configuration of the sourcing scalar field, specifically, its infrared (IR) properties. Namely, the DW area grows as one suppresses IR modes in the initial scalar field spectrum. This growth is saturated, when the area parameter ξ commonly used in the literature reaches the value ξ max ≈ 1.2 . The dependence of ξ on IR modes is argued to be of nonphysical origin: it is likely to be due to effects of the lattice boundary. Assuming that the memory of initial conditions is erased, one recognizes ξ ≈ 1.2 obtained in the situation with maximally suppressed IR modes as a genuine universal value of the area parameter in the scaling regime. We demonstrate that ignorance about initial conditions may affect predictions for the energy density of gravitational waves by the factor O ( 5 ) . The spectral shape of gravitational waves is also affected by the choice of initial conditions, most notably in the low-frequency part. Likewise, we revisit annihilation of DWs under the influence of a potential bias. It has been previously found in Babichev [.] that the annihilation happens significantly earlier compared to the estimate based on the simple balance between the potential bias and surface energy density. We further support this observation and show that the tendency toward an earlier annihilation gets even stronger upon removing IR modes in simulations.

We study the evolution of domain wall networks and their phenomenological implications in a model of a real scalar χ , where a Z ₂ -symmetry is slightly broken by a potential bias V _bias . It is demonstrated that the latter triggers domain wall annihilation considerably earlier than previously thought. Namely, we observe that the scaling relation t _ann ∝ 1/ V _bias ^2/3 for the annihilation time t _ann fits to the simulation data better than a commonly assumed t _ann ∝ 1/ V _bias . As a result, the energy density of gravitational waves produced by the network of biased domain walls, for a given tiny V _bias , is suppressed compared to naive expectations. The spectral shape of gravitational waves is similar to that resulting from unbiased domain walls, but with more power in the close-to-maximum ultraviolet part. In the far ultraviolet region, the spectrum of gravitational waves becomes nearly flat; such a plateau has been recognised earlier in the case of unbiased walls. In our investigation we mainly focus on the symmetry breaking potential V _breaking ∝ χ ³ , and argue that no significant modifications of the domain walls evolution take place if one includes higher powers of χ .

Firmly established in astrophysical observations, dark matter evades direct detection in experiments. Axions and axionlike particles are among the leading dark-matter candidates, and numerous attempts to detect them in laboratories have been performed. Here, we propose to advance these efforts substantially, extending the sensitivity for dark-matter axions in the mass range ( 38 – 54 ) μ eV down to the axion-photon couplings g a γ γ ≲ ( 10 − 14 − 10 − 15 ) GeV − 1 , motivated by generic models of quantum chromodynamics axion. Single-photon detectors operating at ultralow temperatures are key elements of the experiment. The projected sensitivity will be reached in one year of data taking with magnetic field of (1–10) T, making Cosmological Axion Sarov Haloscope (CASH) the most sensitive haloscope in this mass range.