Axions and axion-like particles generically appear in extensions of the Standard Model. While many searches assume only a single axion species, there may exist a whole spectrum of multiple such fields. We develop general formulas for axion-photon oscillations in the presence of multiple axions and analyze the implications for experimental searches, including light-shining-through-a-wall experiments, helioscopes and haloscopes. We demonstrate that axion multiplicity can qualitatively alter observational signatures, particularly through coherence and interference effects. Multiple axions can not only enhance signals compared to single axion scenarios, but also suppress them. We show that variations of experimental parameters and searches allow identifying contributions of multiple axions and obtaining information about their properties.

United Kingdom

Arturo Giorgi

Germany

Joerg Jaeckel

France

Sebastian Monath

Japan

Volodymyr Takhistov

Multiple axions in laboratory experiments

Highly boosted axionlike particles decaying into photon pairs are notoriously hard to detect at the LHC. The collimated decay photons cannot be individually reconstructed using only electromagnetic calorimeter information, making the signal less distinguishable from background. In this paper, we propose a search strategy to address this issue, exploiting the fact that a fraction of the decay photons convert into electron-positron pairs inside the tracking detector. The resulting tracks can be resolved with high resolution, making it possible to separate the two collimated photons and resolve a displaced decay vertex. To demonstrate the effectiveness of this approach, we apply it to ALPs in the challenging MeV–GeV range produced via vector boson fusion. We find that such a search could give sensitivity to untested parameter space.

Canada

Spain

Gonzalo Alonso-Álvarez

Brazil

Diego D. Lopes

Tracking axionlike particles at the LHC

String theory compactifications come with numerous U(1) factors, implying the presence of many hidden photons in the low-energy EFT. One may call this the “string photoverse”. We argue that, generically, these hidden photons are massless and do not couple to any light dark current such that, naively, kinetic mixing with the Standard Model is unobservable. The leading interactions of these “superhidden” photons are then dimension-6 dipole operators which couple them to quarks or leptons and the Higgs field. This induces magnetic and electric dipole moments with respect to both the superhidden photons as well as, through kinetic mixing, to the Standard Model photon. We derive these couplings by dimensionally reducing the fermionic action of 7-branes realizing the Standard Model: in the first step to 6d theories on intersection curves and then, in the presence of fluxes, to our 4d chiral EFT. We analyze how experiments and observations can employ this effect to place lower bounds on the string scale, which is relevant for compactifications with very large volumes. Finally, we briefly discuss how supersymmetry implies the presence of relatively light photinos and hence an accompanying “photinoverse”, which may be observed via renormalizable mixing effects.

Thibaut Coudarchet

Arthur Hebecker

Jonathan Steiner

The String Theory Photoverse

Axionlike particles and similar new pseudoscalar as well as vector bosons coupled to nucleons and electrons are predicted to lead to spin-dependent forces in atoms and ions. We argue that hyperfine structure measurements in hydrogenlike and lithiumlike charge states are a sensitive probe to this effect. Employing specific differences of these splittings reduces uncertainties due to nuclear effects in hyperfine structure calculations and measurements. Using this, we show that existing measurements on Be provide competitive limits in the region <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>ϕ</mi></mrow></msub><mo>≳</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>keV</mi></mrow></math>, confirming, or improving by up to a factor of 2, existing constraints for pseudoscalar couplings, depending on the nuclear model. We also find that future measurements on Cs have a further factor of 2-2.5 improved discovery potential for pseudoscalars and an order of magnitude for new vector bosons when compared with the corresponding current constraints.

Cedric Quint

Fabian Heiße

Lutz Leimenstoll

Christoph H. Keitel

Zoltán Harman

Stringent Constraints on New Pseudoscalar and Vector Bosons from Precision Hyperfine Splitting Measurements

Wavelike dark matter may feature quadratic couplings to ordinary matter. This carries profound consequences for the phenomenologies of such models. It changes the dark matter density around dense objects made from ordinary matter such as planets and stars, thereby changing the sensitivity of direct detection experiments on Earth as well as implying forces on other ordinary matter objects in the vicinity. In this paper we study the time dependence of the dark matter field around spherical objects of ordinary matter. This work indicates the timescale on which accelerating objects settle into a stationary state and delineates the applicability of stationary solutions for experimental dark matter tests. We also use this to understand (and effectively eliminate) the infinities in energies, forces, and pressures that appear when naively comparing the total energy around objects with different size but the same total number of ordinary matter particles.

Clare Burrage

Benjamin Elder

Australia

Yeray Garcia del Castillo

Time-dependent density of quadratically coupled dark matter around ordinary matter objects

In quantum field theories, field redefinitions are often employed to remove redundant operators in the Lagrangian, making calculations simpler and physics more evident. This technique requires some care regarding, among other things, the choice of observables, the range of applicability, and the appearance and disappearance of solutions of the equations of motion (EOM). Many of these issues can already be studied at the classical level, which is the focus of this work. We highlight the importance of selecting appropriate observables and initial/boundary conditions to ensure the physical invariance of solutions. A classical analog to the Lehmann-Symanzik-Zimmermann (LSZ) formula is presented, confirming that some observables remain independent of field variables without tracking redefinitions. Additionally, we address, with an example, the limitations of noninvertible field redefinitions, particularly with nonperturbative objects like solitons, and discuss their implications for classical and quantum field theories.

Juan Carlos Criado

Michael Spannowsky

Field redefinitions in classical field theory with some quantum perspectives

Ordinary matter coupled to light weakly interacting bosons can lead to the formation of a macroscopic bosonic field in the vicinity of large matter concentrations such as ordinary or neutron stars. When these objects are turned into black holes due to a supernova or a binary merger this “hair” could be “shorn” off. Part of the field configuration would then be released leading to an outgoing field wave. For small masses this field transient remains rather compact and can induce a transient signal in experiments, in particular those that look for wave-like dark matter. This signal can be correlated with the corresponding astrophysical signal of the event. In this note, we consider a variety of couplings and the associated signals and estimate the corresponding sensitivities.

Star shearing season: transient signals in wave-like dark matter experiments from black hole formation

Establishing the axion as the dark matter (DM) particle after a haloscope discovery typically requires follow-up experiments to break the degeneracy between the axion’s coupling to photons and its local DM abundance. Given that a discovery would justify more significant investments, we explore the prospects of ambitious light-shining-through-a-wall (LSW) setups to probe the QCD axion band. Leveraging the excellent mass determination in haloscopes, we show how to design LSW experiments with lengths on the order of 100 km and suitably aligned magnetic fields with apertures of around 1 m to reach well-motivated axion models across up to four orders of magnitude in mass. Beyond presenting a concrete plan for postdiscovery experimental efforts, we briefly discuss complementary experiments and future directions beyond LSW experiments.

Italy

Sebastian Hoof

Giuseppe Lucente

Ultimate light-shining-through-a-wall experiments to establish QCD axions as the dominant form of dark matter

Dark photons in the MeV to GeV range with kinetic mixing of the order of ≲ 10 − 10 can be produced in significant numbers at low energy colliders such as Belle II. Their decay length can be macroscopic raising the hope for a fairly clean search via displaced vertices as proposed in [1]. However, even this is not background free. Here, we calculate and discuss problematic backgrounds from displaced photon conversion and discuss their potential impact on the sensitivity. In addition we also briefly consider the dangers of prompt backgrounds.

Netherlands

Anh Phan

Searching dark photons using displaced vertices at Belle II — with backgrounds

Kinetic mixing between gauge fields of different U(1) factors is a well-studied phenomenon in 4d EFT. In string compactifications with U(1)s from sequestered D-brane sectors, kinetic mixing becomes a key target for the UV prediction of a phenomenologically important EFT operator. Surprisingly, in many cases kinetic mixing is absent due to a non-trivial cancellation. In particular, D3-D3 kinetic mixing in type-IIB vanishes while D3-anti-D3 mixing does not. This follows both from exact CFT calculations on tori as well as from a leading-order 10d supergravity analysis, where the key cancellation is between the C 2 and B 2 contribution. We take the latter approach, which is the only one available in realistic Calabi-Yau settings, to a higher level of precision by including sub-leading terms of the brane action and allowing for non-vanishing C 0. The exact cancellation persists, which we argue to be the result of  $${\text{SL}}\left(2,{\mathbb{R}}\right)$$  self-duality. We note that a B 2 C 2 term on the D3-brane, which is often missing in the recent literature, is essential to obtain the correct zero result. Finally, allowing for  $${\text{SL}}\left(2,{\mathbb{R}}\right)$$ -breaking fluxes, kinetic mixing between D3-branes arises at a volume-suppressed level. We provide basic explicit formulae, both for kinetic as well as magnetic mixing, leaving the study of phenomenologically relevant, more complex situations for the future. We also note that describing our result in 4d supergravity appears to require higher-derivative terms — an issue which deserves further study.

Ruben Kuespert

Small kinetic mixing in string theory