<p>Axion-like particles (ALPs) can be copiously produced in binary neutron star (BNS) mergers through nucleon-nucleon bremsstrahlung if the ALP-nucleon couplings <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mrow><mi>a</mi><mi>N</mi></mrow></msub></math> are sizable. The ALP-photon coupling <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi></mrow></msub></math> may trigger conversions of ultralight ALPs into photons in the magnetic fields of the merger remnant and of the Milky Way. This effect would lead to a potentially observable short gamma-ray signal, in coincidence with the gravitational-wave signal produced during the merging process. This event could be detected through multimessenger observation of BNS mergers employing the synergy between gravitational-wave detectors and gamma-ray telescopes. Here, we study the sensitivity of current and proposed MeV gamma-ray experiments to detect such a signal. As an explicit example, we consider ALP couplings related as in the Kim-Shifman-Vainshtein-Zakharov axion model, and show that in this case the proposed instruments can reach a sensitivity down to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi></mrow></msub><mo>≳</mo><mi>few</mi><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>13</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>a</mi></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>, comparable with the SN 1987A limit.</p>

Italy

Francesca Lecce

Alessandro Lella

United States

Giuseppe Lucente

Germany

Vimal Vijayan

Andreas Bauswein

Spain

Maurizio Giannotti

Alessandro Mirizzi

Probing axionlike particles with multimessenger observations of neutron star mergers

<p>Axions and other putative feebly interacting particles with a mass of tens to several hundreds of keVs can be produced in stellar cores with a Lorentz boost factor <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>E</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>/</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≲</mo><mn>10</mn></mrow></math>. Thus, starburst galaxies such as M82 are efficient factories of slow axions. Their decay <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>a</mi><mo stretchy="false">→</mo><mi>γ</mi><mi>γ</mi></math> would produce a large flux of x-ray photons, peaking around 100 keV and spread around the Galaxy by an angle that can be relatively large. We use observations of the Nuclear Spectroscopic Telescope Array mission to show that the absence of these features can constrain 30–500 keV axion masses into uncharted regions for axion-photon coupling of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>g</mi></mrow><mrow><mi>a</mi><mi>γ</mi></mrow></msub><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>10</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>12</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>. Our argument can be applied to other heavy feebly interacting particles and astrophysical sources that are hot enough to produce them, yet cold enough to avoid large boost factors which slow down the decay.</p>

Francisco R. Candón

Damiano F.G. Fiorillo

Edoardo Vitagliano

Julia K. Vogel

NuSTAR Bounds on Radiatively Decaying Particles from M82

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.

Sebastian Hoof

Joerg Jaeckel

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

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.

Sweden

Pierluca Carenza

Martina Gerbino

Massimiliano Lattanzi

Strong cosmological constraints on the neutrino magnetic moment

Sterile neutrinos with masses up to <math><mi>O</mi><mo>(</mo><mn>100</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> can be copiously produced in a supernova (SN) core through the mixing with active neutrinos. In this regard, the SN 1987A detection of neutrino events has been used to put constraints on active-sterile neutrino mixing, exploiting the well-known SN cooling argument. We refine the calculation of this limit including neutral current interactions with nucleons, which constitute the dominant channel for sterile neutrino production. We also include, for the first time, the charged current interactions between sterile neutrinos and muons, relevant for the production of sterile neutrinos mixed with muon neutrinos in the SN core. Using the recent modified luminosity criterion, we extend the bounds to the case where sterile states are trapped in the stellar core. Additionally, we study the decays of heavy sterile neutrinos, affecting the SN explosion energy and possibly producing a gamma-ray signal. We also illustrate the complementarity of our new bounds with cosmological bounds and laboratory searches.

Leonardo Mastrototaro

France

Pasquale Dario Serpico

Comprehensive constraints on heavy sterile neutrinos from core-collapse supernovae

Axionlike particles (ALPs) coupled to nucleons might be copiously emitted from a supernova (SN) core. We extend existing bounds on free-streaming ALPs to the case in which these are so strongly interacting with the nuclear matter to be trapped in the SN core. For strongly interacting ALPs, we also extend the bound from the absence of an ALP-induced signal in Kamiokande-II neutrino detector at the time of SN 1987A. We find that combining the different arguments, SNe exclude values of ALP-nucleon coupling <math><mrow><msub><mrow><mi>g</mi></mrow><mrow><mi>a</mi><mi>N</mi></mrow></msub><mo>≳</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>9</mn></mrow></msup></mrow></math> for ALP masses <math><msub><mi>m</mi><mi>a</mi></msub><mo>≲</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>. Remarkably, in the case of canonical QCD axion models, the SN bounds exclude all values of <math><msub><mi>m</mi><mi>a</mi></msub><mo>≳</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>. This result prevents the possibility for current and future cosmological surveys to detect any signatures due to hot dark matter QCD axion mass.

Giampaolo Co’

Switzerland

United Kingdom

Thomas Rauscher

Getting the most on supernova axions

Solar nuclear reactions can occasionally produce sub-MeV elusive beyond the Standard Model particles that escape the solar interior without further interactions. This study focuses on massive spin-one particles. We construct the general theoretical framework and identify two crucial mixing sources involving the photon, which facilitate communication between the hidden and visible sectors: kinetic mixing with the photon, and plasma-induced mixing due to thermal electron loops. For both cases, we focus on the second stage of the solar proton-proton chain and evaluate the fluxes of monochromatic 5.49 MeV hidden vectors produced by the p(d,3 He)γ′ nuclear reaction. We then investigate their terrestrial detection via Compton-like scatterings. The incoming fluxes are polarized, and we evaluate the cross sections for Compton-like scatterings for transverse and longitudinal vectors. Finally, we apply this framework to a concrete case by investigating the sensitivity of the forthcoming Jiangmen Underground Neutrino Observatory (JUNO) experiment and identifying parameter space where current terrestrial bounds will be improved.

Francesco D’Eramo

Newton Nath

South Korea

Seokhoon Yun

Terrestrial detection of hidden vectors produced by solar nuclear reactions

The parameter space of massive axionlike particles (ALPs) with <math><msub><mi>m</mi><mi>a</mi></msub><mo>∼</mo><mi>O</mi><mo>(</mo><mn>100</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> and coupled with nucleons is largely unexplored. Here, we present new constraints in this parameter region. In doing so, we characterize the supernova emissivity of heavy ALPs from a protoneutron star, including for the first time mass effects in both nucleon-nucleon Bremsstrahlung and pionic Compton processes. In addition, we highlight novel possibilities to probe the couplings with photons and leptons from supernova ALP decays.

Protoneutron stars as cosmic factories for massive axionlike particles

A radiative decaying big bang relic with a mass <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≃</mo><mn>5</mn><mi>–</mi><mn>25</mn><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>, which we dub “blue axion,” can be probed with direct and indirect observations of the cosmic optical background (COB). The strongest bounds on blue-axion cold dark matter come from the Hubble Space Telescope (HST) measurements of COB anisotropies at 606 nm. We suggest that new HST measurements at higher frequencies (336 nm and 438 nm) can improve current constraints on the lifetime up to one order of magnitude, and we show that also thermally produced and hot relic blue axions can be competitively probed by COB anisotropies. We exclude the simple interpretation of the excess in the diffuse COB detected by the Long Range Reconnaissance Imager (LORRI) as photons produced by a decaying hot relic. Finally, we comment on the reach of upcoming line intensity mapping experiments, that could detect blue axions with a lifetime as large as <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>26</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>s</mi></mrow></math> or <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>27</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>s</mi></mrow></math> for the cold dark matter and the hot relic case, respectively.

Israel

Probing the blue axion with cosmic optical background anisotropies

We investigate the 5.49 MeV solar axions flux produced in the p(d,He3)a reaction and analyze the potential to detect it with the forthcoming large underground neutrino oscillation experiment Jiangmen Underground Neutrino Observatory (JUNO). The JUNO detector could reveal axions through various processes such as Compton and inverse Primakoff conversion, as well as through their decay into two photons or electron-positron pairs inside the detector. We perform a detailed numerical analysis in order to forecast the sensitivity on different combinations of the axion-electron (<math><msub><mi>g</mi><mrow><mi>a</mi><mi>e</mi></mrow></msub></math>), axion-photon (<math><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi></mrow></msub></math>), and isovector axion-nucleon (<math><msub><mi>g</mi><mrow><mn>3</mn><mi>a</mi><mi>N</mi></mrow></msub></math>) couplings, using the expected JUNO data for different benchmark values of axion mass in a model-independent way. We find that JUNO would improve by approximately one order of magnitude current bounds by Borexino and it has the best sensitivity among neutrino experiments.

Francesco Capozzi

Probing high-energy solar axion flux with a large scintillation neutrino detector