Bulgarian national input to the European Strategy for Particle Physics

The present document summarizes the view and the vision of the Bulgarian subatomic physics scientific community for the development of the field of subatomic physics from the Bulgarian national perspective. It outlines the present activities and the strengths and weaknesses of the research field, together with the interests of the community for the future development of the technological and scientific landscape.

💡 Research Summary

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The paper “Bulgarian national input to the European Strategy for Particle Physics” presents a comprehensive overview of the current status, strengths, weaknesses, and future aspirations of the Bulgarian sub‑atomic physics community from a national perspective. It begins by recalling the post‑Higgs‑boson era, emphasizing that despite the Standard Model’s apparent completeness, fundamental questions such as the nature of dark matter, the origin of the matter‑antimatter asymmetry, the mechanism of neutrino mass generation, and the transition from perturbative QCD to hadron formation remain unresolved. The authors argue that precision measurements and low‑background experiments have become the primary avenues for new discoveries, linking particle physics to nuclear physics, astrophysics, and multimessenger astronomy.

The document then details Bulgaria’s active participation in major international facilities. In the LHC program, Bulgarian groups have been involved in CMS since 1999, contributing to the construction, operation, and upgrade of key detector subsystems such as the Hadron Calorimeter, Resistive Plate Chambers, and Gas Electron Multipliers. They hold leadership positions, develop Monte‑Carlo and machine‑learning tools, and focus on B‑physics, flavor studies, and vector‑like quark searches. In ALICE, Bulgaria became an official member in 2021 and now contributes to the Forward Calorimeter (FoCal) design, beam tests, SiPM irradiation studies, and integration of reconstruction algorithms into the O2 framework. Their physics program spans femtoscopy, collectivity in small systems, and interdisciplinary links to astrophysics, including neutron‑star equation‑of‑state studies and axion searches.

In the neutrino sector, the community supports the European Spallation Source neutrino Super Beam (ESSνSB), which aims to measure CP violation at the second oscillation maximum with a precision better than 8°, using a 5 MW proton driver and a 540 kt far detector located 360 km away. They also participate in the SND@LHC experiment, which will operate at the HL‑LHC to study neutrino cross‑sections over a broad energy range (350 GeV to TeV) and potentially observe τ‑neutrinos directly.

The paper highlights involvement in smaller but strategically important facilities. At CERN’s ISOLDE, Bulgarian researchers exploit radioactive ion beams for nuclear structure, β‑decay studies, and searches for physics beyond the Standard Model. In the PADME experiment at LNF (INFN), they have built and operated subsystems to search for the hypothesized X17 particle in the 17 MeV mass region, providing high‑precision results and training more than ten graduate students. In astroparticle physics, Bulgarian groups contribute to the MAGIC telescopes (deep‑learning based data analysis) and to the construction of the Large‑Size Telescopes for the upcoming Cherenkov Telescope Array (CTA), focusing on optical system development.

The authors assess the community’s strengths: long‑standing expertise in detector hardware, software, data analysis, and machine learning; a proven record of leadership in large collaborations; and a growing interdisciplinary portfolio linking particle, nuclear, and astrophysics. Weaknesses include limited national funding, aging infrastructure, and the risk of brain drain. Opportunities lie in the upcoming HL‑LHC era, ALICE upgrades, ESSνSB, CTA, and the continued support of small‑scale experiments that foster innovation and training.

Finally, the paper proposes a balanced strategy: sustain and deepen involvement in flagship projects (HL‑LHC, ALICE upgrades, ESSνSB) while ensuring robust support for smaller facilities (PADME, ISOLDE, local detector R&D). This approach is presented as essential for maintaining Bulgaria’s scientific impact within Europe, cultivating the next generation of physicists, and leveraging particle‑physics technologies for broader societal benefit.