We propose a novel class of dark matter (DM) candidates in the form of a heavy composite axionlike particle (ALP) with highly suppressed electromagnetic interactions populating vast yet unexplored domains in the ALP parameter space. This is achieved for the first time in the simplest dark confining gauge theory yielding a new composite glueball ALP (GALP) DM coupling-mass relation found in terms of two distinct fundamental scales—the large dark fermion mass scale and the dynamical scale of dark confinement. The presence of a heavy fermion portal between the visible (photons) and dark (GALPs) sectors ensures a strong radiative suppression of the GALP-photon coupling naturally without any fine-tuning. The observable features of heavy GALP DM in a minimal realization are controlled by only three physical parameters. Our work paves the road for a novel research field exploring the theory and phenomenology of composite ALPs in multimessenger astrophysics and cosmology.

Sweden

Pierluca Carenza

Roman Pasechnik

China

Zhi-Wei Wang

Composite Heavy Axionlike Dark Matter

Core-collapse supernovae (SNe) provide a unique environment to study feebly interacting particles (FIPs) such as axionlike particles (ALPs), sterile neutrinos, and dark photons (DPs). This paper focuses on heavy FIPs produced in SNe, whose decay produces electrons and positrons, generating observable secondary signals during their propagation and annihilation. We focus on the in-flight annihilation of positrons, which emerge as the most significant contribution to the resulting <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>γ</mi></math>-ray spectrum. Using data from COMPTEL and EGRET, we derive the most stringent bounds on the FIP-electron couplings for heavy ALPs, sterile neutrinos, and DPs. These results strengthen existing bounds by one to two orders of magnitude, depending on the FIP model.

United Kingdom

Shyam Balaji

Spain

Pedro De la Torre Luque

Italy

Alessandro Lella

Leonardo Mastrototaro

In-flight positron annihilation as a probe of feebly interacting particles

In the presence of a magnetic field, axions can convert into photons and vice versa. The phenomenology of the conversion is captured by a system of two coupled Klein-Gordon equations, which, assuming that the axion is relativistic, is usually recast into a pair of first-order Schrödinger-like equations. In such a limit, focusing on a constant magnetic field and plasma frequency, the equations admit an exact analytic solution. The relativistic limit significantly simplifies the calculations and, therefore, it is widely used in phenomenological applications. In this work, we discuss how to evaluate the axion-photon system evolution without relying on such relativistic approximation. In particular, we give an exact analytical solution, valid for any axion energy, in the case that both the magnetic field and plasma frequency are constant. Moreover, we devise an analytic perturbative expansion that allows for tracking the conversion probability in a slightly inhomogeneous magnetic field or plasma frequency, whose characteristic scale of variation is much larger than the typical axion-photon oscillation length. Finally, we discuss a case of resonant axion-photon conversion giving useful simplified formulae that might be directly applied to dark matter axions converting in neutron star magnetospheres.

Clemente Smarra

Axion-photon conversion down to the nonrelativistic regime

The <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>γ</mi></math>-ray emission originating from in-flight annihilation (IA) of positrons is a powerful observable for constraining high-energy positron production from exotic sources. By comparing diffuse <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>γ</mi></math>-ray observations of INTEGRAL, COMPTEL, and EGRET to theoretical predictions, we set the most stringent constraints on electrophilic feebly interacting particles, thereby proving IA as a valuable probe of new physics. In particular, we extensively discuss the case of MeV-scale sterile neutrinos, where IA sets the most stringent constraints, excluding <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo stretchy="false">|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>μ</mi><mn>4</mn></mrow></msub><msup><mrow><mo stretchy="false">|</mo></mrow><mrow><mn>2</mn></mrow></msup><mo>≳</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow></msup></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo stretchy="false">|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>τ</mi><mn>4</mn></mrow></msub><msup><mrow><mo stretchy="false">|</mo></mrow><mrow><mn>2</mn></mrow></msup><mo>≳</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow></msup></mrow></math> for sterile neutrinos mixed with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi></math> neutrinos, respectively. These constraints improve existing limits by more than an order of magnitude. We briefly discuss the application of these results to a host of exotic positron sources such as dark photons, axionlike particles, primordial black holes, and sub-GeV dark matter.

<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>γ</mi></math> rays from in-flight positron annihilation as a probe of new physics

High-frequency gravitational waves (<math><mrow><mi>f</mi><mo>≳</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MHz</mi></mrow></math>) may provide a unique signature for the existence of exotic physics. The lack of current and future gravitational-wave experiments sensitive at those frequencies leads to the need of employing different indirect techniques. Notably, one of the most promising ones is constituted by graviton-photon conversions in magnetic fields. In this work, we focus on conversions of a gravitational-wave background into photons inside the Milky Way magnetic field, taking into account the state-of-the-art models for both regular and turbulent components. We discuss how graviton-to-photon conversions may lead to imprints in the cosmic photon background spectrum in the range of frequencies <math><mrow><mi>f</mi><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>9</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mn>26</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>Hz</mi></mrow></math>, where the observed photon flux is widely explained by astrophysics emission models. Hence, the absence of any significant evidence for a diffuse photon flux induced by graviton-photon conversions allows us to set stringent constraints on the gravitational-wave strain <math><msub><mi>h</mi><mi>c</mi></msub></math>, strengthening current astrophysical bounds by <math><mrow><mo>∼</mo><mn>1</mn><mi>–</mi><mn>2</mn></mrow></math> orders of magnitude in the whole range of frequencies considered.

France

Francesca Calore

Alessandro Mirizzi

Constraining gravitational-wave backgrounds from conversions into photons in the Galactic magnetic field

We propose a new theory of dark matter based on axion physics and cosmological phase transitions. We show that theories in which a gauge coupling increases through a first-order phase transition naturally result in ‘axion relic pockets’: regions of relic false vacua stabilised by the pressure from a kinematically trapped, hot axion gas. Axion relic pockets provide a viable and highly economical theory of dark matter: the macroscopic properties of the pockets depend only on a single parameter (the phase transition temperature). We describe the formation, evolution and present-day properties of axion relic pockets, and outline how their phenomenology is distinct from existing dark matter paradigms. We briefly discuss how laboratory experiments and astronomical observations can be used to test the theory, and identify gamma-ray observations of magnetised, dark-matter-dense environments as particularly promising.

Joshua Eby

Oksana Iarygina

M. Marsh

Axion relic pockets — a theory of dark matter

In this paper, we explore the phenomenology of massive axionlike particles (ALPs) coupled to quarks and gluons, dubbed “QCD ALPs,” with an emphasis on the associated low-energy observables. ALPs coupled to gluons and quarks not only induce nuclear interactions at scales below the QCD scale, relevant for ALP production in supernovae (SNe), but naturally also couple to photons similarly to the QCD axion. We discuss the link between the high-energy formulation of ALP theories and their effective couplings with nucleons and photons. The induced photon coupling allows ALPs with masses <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≳</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></mrow></math> to efficiently decay into photons, and astrophysical observables severely constrain the ALP parameter space. We show that a combination of arguments related to SN events rule out ALP-nucleon couplings down to <math><msub><mi>g</mi><mrow><mi>a</mi><mi>N</mi></mrow></msub><mo>≳</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>11</mn></mrow></msup><mo>–</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>10</mn></mrow></msup></math> for <math><msub><mi>m</mi><mi>a</mi></msub><mo>≳</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>—a region of the parameter space that was hitherto unconstrained.

Denmark

Eike Ravensburg

M. C. David Marsh

Supernova limits on QCD axionlike particles

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multicomponent primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment <math><mi>μ</mi><mo>≳</mo><mn>1.6</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>11</mn></mrow></msup><msub><mi>μ</mi><mi>B</mi></msub></math>, while future cosmological experiments will be able to probe nonthermal production of RH neutrinos, becoming competitive with stellar limits.

Germany

Giuseppe Lucente

Martina Gerbino

United States

Maurizio Giannotti

Massimiliano Lattanzi

Strong cosmological constraints on the neutrino magnetic moment

During galactic supernova (SN) explosions, a large amount of feebly interacting particles (FIPs) may be produced. In this work we analyze electrophilic FIPs with masses in the MeV-range that escape from SN and decay into electron-positron pairs, causing an exotic leptonic injection. This contribution adds up to known components, leading to an unexpected excess of x-ray fluxes generated by inverse-Compton scattering of the injected particles on low-energy photon backgrounds. For the first time in the context of FIPs, we use XMM-Newton x-ray measurements to obtain the strongest and most robust bounds on electrophilic FIPs produced by SN in our Galaxy.

Robust constraints on feebly interacting particles using XMM-Newton

We study MeV-scale electrophilic feebly interacting particles (FIPs), that may be abundantly produced in supernova explosions, escape the star and decay into electrons and positrons. This exotic injection of leptons in the Milky Way leaves an imprint in both photon and cosmic ray fluxes. Specifically, positrons lose energy and annihilate almost at rest with background electrons, producing photons with 511 keV energy. In addition, electrons and positrons radiate photons through bremsstrahlung emission and upscatter the low-energy galactic photon fields via the inverse Compton process generating a broad emission from x-ray to <math><mrow><mi>γ</mi></mrow></math> ray energies. Finally, electrons and positrons are directly observable in cosmic ray experiments. In order to describe the FIP-induced lepton injection in full generality, we use a model-independent parametrization which can be applied to a host of FIPs such as axionlike particles, dark photons and sterile neutrinos. Theoretical predictions are compared to experimental data to robustly constrain FIP-electron interactions with an innovative multimessenger analysis.