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Rethinking Ultraviolet Scenarios for Hunting the AXION

Periodic Reporting for period 1 - AXIONRUSH (Rethinking Ultraviolet Scenarios for Hunting the AXION)

Reporting period: 2019-11-01 to 2021-10-31

The AXIONRUSH project "Rethinking Ultraviolet Scenarios for Hunting the AXION" deals with a broad program for the investigation of theoretical and phenomenological aspects of axion physics. The axion is a hypothetical particle that has been sought for decades and whose discovery would shed light on several unanswered question of the Standard Model of particle physics, such as the origin of the dark matter of the universe and provide a solution to the so-called strong CP problem. The axion experimental program has been developing extremely fast in the recent years, with several new experimental searches and detection concepts promising for the exploration of regions of the axion parameter space that were thought to be unreachable until a decade ago. In particular, some of those foreseen experimental projects (ALPS II, IAXO and MADMAX) will be located at the host institution of the present Action (DESY, Hamburg).

The main goal of AXIONRUSH is to bridge theoretical aspects of axion physics with experiments and to provide a global comparison of general axion models with experimental sensitivities and astrophysical bounds. The development of these new theoretical tools will enlarge the physics scopes of the axion experimental program at DESY, Hamburg, by further addressing specific research goals related to the experiments ALPS-II, IAXO, MADMAX. A complementary objective of AXIONRUSH is to investigate scenarios where the Peccei Quinn mechanism is embedded into Grand Unified Theories (GUTs). The latter provide a predictive framework for narrowing down the axion mass range, with a potential impact on the scanning strategy of the axion dark matter experiments.

More specifically, the research objectives (ROs) of this Marie Skłodowska Curie Action (MSCA) are:

RO1. Redefinition of the Quantum Chromodynamics (QCD) axion parameter space, by revisiting axion couplings from a UV perspective (including axion couplings to photons, nucleons, electrons, as well as flavour and CP-violating axion couplings);

RO2. Impact on the axion experimental program at DESY, by addressing several sub-projects related to local axion experiments (e.g. ALPs II, IAXO, MADMAX);

RO3. Axion properties in GUTs. In particular, provide axion mass predictions and develop new approaches to the so-called axion quality problem.
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).
The work of AXIONRUSH has impacted the physics case of the axion experimental program at DESY, showing how axions can hide in regions of parameter space that lie well beyond the boundaries of canonical windows and that will be probed by future axion experiments. These results go beyond the state of the art and what was the common lore about the QCD axion parameter space. Potential users of the research outputs are theoretical researchers in the field of axion physics as well as experimentalists (also beyond DESY) which can use the results of the project in order to reinforce the physics scopes of their axion experiments. One of the major aims of the project was indeed to motivate experimental colleagues to explore all the accessible regions in the axion parameter space.

Lat, but not least, the MSCA had a fundamental role for establishing my professional maturity and independence to become an expert in axion physics. The obtained results have greatly enhanced my visibility within the scientific community and played a crucial role also for a successful step forward in my career.
L. Di Luzio, M. Giannotti, E. Nardi, L. Visinelli "The landscape of QCD axion models", Physics Repor