Fernando Arias-Aragón

Neutron stars are very dense media in which axions may be produced. This has been used to set a limit on the QCD axion mass, usually under the assumption that only neutrons, protons, electrons, and muons appear in the star core. Given the extreme conditions reached within neutron stars, it is reasonable to consider that other particles, such as hyperons and Δ resonances, may appear in the neutron star core. In this work, we study how the limit on the mass of QCD axions, namely Kim-Shifman-Vainshtein-Zakharov and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) invisible axions, is altered when different equations of state are used, allowing for heavier particles to appear in the neutron star core. We find that this dependence is in general mild and thus reinforces the reliability of the known limit. Additionally, in the DFSZ scenario, it may drive the limit within the sensitivity window for IAXO. This would allow this experiment to discern the composition of neutron star cores if an axion were to be observed within that window.

Dark sector models present a rich phenomenology that requires high-intensity beams and precision detectors for thorough exploration. The NA62 experiment has already published several constraints on dark sector models, leveraging proton beam dump and meson decay techniques. This proposal aims to demonstrate the NA62 detector discovery potential for dark sector candidates by using the positron-on-target technique. High-intensity secondary positron beams, reaching up to ∼ 150 GeV energy, have already been produced at the North Area extracted beam lines. If a positron beam with an intensity in the range of 2 × 10 14 positrons on target per year is delivered, the NA62 detector would be ideal for searches of dark sector particles in both visible and invisible decay channels. Additionally, positron on target collisions would enable precision measurements of key standard model observables, including a detailed scan of σ ( e + e − → π + π − ) and σ ( e + e − → μ + μ − ) at the di-pion and di-muon production threshold, with discovery potential for the True Muonium ( μ + μ − ) bound state.

A bstract: Atomic electron motion is responsible for well-studied effects observed in low-energy (MeV-scale) processes. Recently, interest in this phenomenon has also emerged within the high-energy physics community, due to its potential to increase significantly the center-of-mass energy in fixed-target experiments. However, direct experimental evidence of this effect in high energy collisions has yet to be observed. We argue that a striking manifestation of atomic electron momenta could be revealed by the NA64 experiment at CERN during the proposed run with a 40 GeV positron beam. At this energy, μ ⁺ μ ⁻ production via positron annihilation on electrons at rest is kinematically forbidden. The detection of μ ⁺ μ ⁻ pairs from the annihilation channel would thus constitute direct evidence of an increase in the center-of-mass energy resulting from atomic electron motion. We also investigate the expected signatures for the proposed 60 GeV run, as well as for the data already collected at 70 GeV. Intriguingly, in both these cases, the predicted number of μ ⁺ μ ⁻ pairs from positron annihilation is reduced compared to the electron-at-rest approximation.

Due to Heisenberg’s uncertainty principle, atomic electrons localized around the nucleus exhibit a characteristic momentum distribution that, in elements with high atomic number, remains significant up to relativistic values. Consequently, in fixed-target experiments, atoms can effectively act as electron accelerators, increasing the centre-of-mass energy in collisions with beam particles. In this work, we leverage this effect to explore its potential for new physics searches. We consider positrons from beams of various energies annihilating with atomic electrons in a ⁷⁴W fixed target. We compute the production rates of new vector bosons and pseudoscalar particles as functions of their couplings and masses. We show that the electron-at-rest approximation significantly underestimates the mass reach for producing these new states compared to the results obtained by properly accounting for atomic electron momenta. In particular, we estimate the sensitivity for detecting these new particles using the positron beam at the Beam Test Facility linac at the Laboratori Nazionali di Frascati, the H4 beamline in the CERN North Area, and the proposed Continuous Electron Beam Accelerator Facility of Jefferson Laboratory.

The PADME experiment at the Frascati DAΦNE LINAC has performed a search for the hypothetical X17 particle, with a mass of around 17 MeV, by scanning the energy of a positron beam striking a fixed target. The X17 should be produced from the resulting e ⁺ e ⁻ annihilation. Since the expected mass of this particle is only roughly known, data sidebands cannot be clearly defined. Furthermore, the need to keep the analysis blind to potentially sizable signal contributions prevents a clear assessment even of the quality of the data sample in this search. In light of these challenges, this paper presents an innovative strategy adopted by the PADME Collaboration to perform data quality checks without disclosing the X17 sample. Moreover, the procedure designed to eventually unblind the data is described, together with the statistical approach adopted to extract the limits on the coupling between the X17 and the Standard Model.

The hadronic vacuum polarization contribution to ( g − 2 ) μ can be determined via dispersive methods from e + e − → hadrons data. We propose a novel approach to measure the hadronic cross section σ had ( s ) as an alternative to the initial-state radiation and energy scan techniques, which relies on positron annihilation off atomic electrons of a high Z target ( U 238 , Z = 92 ). We show that by leveraging the relativistic electron velocities of the inner atomic shells, a high-intensity 12 GeV positron beam, such as the one foreseen at JLab, can allow measuring σ had ( s ) with high statistical accuracy from the two-pion threshold up to above s ∼ 1 GeV .

We revisit and update the axion-induced pair production process in a nuclear electric field mediated by the axion-electron coupling, a + X Z A → X Z A + e + + e − . This process emerges as one of the most efficient channels for detecting axions with energies above a few MeV in large underground detectors. It is particularly relevant for detecting axions produced in nuclear reactions, such as the p + d → He 3 + a ( 5.5 MeV ) reaction in the solar pp-chain, and for axions originating in supernovae. Despite recent interest in detecting high-energy axions, the pair production process has received limited attention, even in scenarios where it is the dominant detection channel. This study fills this gap by demonstrating that pair production is a highly effective detection mechanism for high-energy axions. We apply our results to axions from supernovae and the solar 5.5 MeV line, recasting the current bounds of Borexino and comparing the detection capabilities of the JUNO and Hyper-Kamiokande detectors. Published by the American Physical Society 2025