The AXIONRUSH project has led to 17 scientific publications, out of which 13 articles in peer-reviewed journals, 1 conference proceeding and 3 preprints which at the time of writing are under peer-review process. The project was structured around four Work Packages (WPs). The first three addressing the three ROs of the Action and a forth one on training/management/outreach.
WP1 has been fully completed, with results which went well beyond expectations. In particular, axion models where the couplings to matter fields can be either enhanced or suppressed have been developed, as well as new theoretical advances in flavour-violating and CP-violating axions. Two highlights among the publications of WP1 are: [1] a Physics Reports on “QCD axion models” which has already become a reference work in the axion community (about 100 citations in 1 year); and [2] the development of a consistent framework for the calculation of CP-violating axion couplings, published on Physical Review Letters.
WP2 profited from the results developed in WP1. In particular, the physics case of the axion experimental program at DESY has been greatly enhanced by showing that the strong CP problem can be solved via an axion with an enhanced coupling to photons, potentially in the reach of ALPs II and/or IAXO. This has been obtained via two orthogonal approaches, either by enhancing the axion coupling for a fixed axion mass [3] or by suppressing the QCD axion mass for a fixed axion coupling [4].
WP3 dealt with axion properties in GUTs. In this context, I have single-authored two papers [5,6] in which I proposed new ways to embed the axion into GUTs and computed predictions for the axion mass from unification constraints. In particular, Ref. [6] provides the first successful implementations of an accidental Peccei-Quinn symmetry addressing also the axion quality problem in a GUT context.
WP4 focussed on complementary training activities, like publication of the research results, attendance of international workshops/seminars and outreach activities. In particular, the results of the Action have been disseminated via 14 seminars, out of which 11 invited seminars and 3 seminar presentations at DESY.
Moreover, the Action has led to extra contributions which were not initially foreseen. In June 2020 the XENON1T Collaboration announced an excess in low-energy electronic recoil data in their underground experiment which could be interpreted in terms of solar axions. This has led to a considerable excitement in the community and triggered a lot of theoretical speculations. In this respect, we have shown in Ref. [7] that the specific solar axion properties required by the XENON1T explanation were in strong conflict with observations of stellar evolution, thus ruling out the solar axion interpretation of the XENON1T excess. The results of Ref. [7] were published on Physical Review Letters and were further highlighted as a Physical Review Letters Editors’ Suggestion.
Highlighted publications:
[1] L. Di Luzio, M. Giannotti, E. Nardi, L. Visinelli "The landscape of QCD axion models", Physics Reports 870 (2020) 1-117;
[2] S. Bertolini, L. Di Luzio, F. Nesti , "Axion-mediated forces, CP violation and left-right interactions", Phys.Rev.Lett. 126 (2021) 8, 081801;
[3] L. Darmé, L. Di Luzio, M. Giannotti, E. Nardi, "Selective enhancement of the QCD axion couplings", Phys.Rev.D 103 (2021) 1, 015034;
[4] L. Di Luzio, B. Gavela, P. Quilez, A. Ringwald, "An even lighter QCD axion", JHEP 05 (2021) 184;
[5] L. Di Luzio, "Pati-Salam Axion", JHEP 07 (2020) 071;
[6] L. Di Luzio, "Accidental SO(10) axion from gauged flavour", JHEP 11 (2020) 074;
[7] L. Di Luzio, M. Fedele, M. Giannotti, F. Mescia, E. Nardi, "Solar axions cannot explain the XENON1T excess", Phys. Rev. Lett. 125, 131804 (2020).
