The equivalence principle and the inverse-square law of gravity could be violated at short distances (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>12</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi mathvariant="normal">m</mi></mrow></math>) by scalars sporting a coupling <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mi>N</mi></msub></math> to nucleons and mass <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>eV</mi><mo>≲</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>ϕ</mi></mrow></msub><mo>≲</mo><mi>MeV</mi></mrow></math>. We show for the first time that stringent bounds on the existence of these scalars can be derived from the observed cooling of nearby isolated neutron stars (NSs). Although NSs can only be used to set limits comparable to the classic supernova (SN) 1987A cooling bound in the case of pseudoscalars such as the QCD axion, the shallow temperature dependence of the scalar emissivity results in a huge enhancement in the effect of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ϕ</mi></math> on the cooling of cold NSs. As we do not find evidence of exotic energy losses, we can exclude couplings down to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>g</mi></mrow><mrow><mi>N</mi></mrow></msub><mo>≲</mo><mn>5</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>14</mn></mrow></msup></mrow></math>. Our new bound supersedes all existing limits on scalars across 6 orders of magnitude in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>ϕ</mi></msub></math>. These conclusions also extend to Higgs-portal models, for which the bound on the scalar-Higgs mixing angle is <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>sin</mi><mi>θ</mi><mo>≲</mo><mn>6</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>11</mn></mrow></msup></mrow></math>.

Germany

Damiano F.G. Fiorillo

Italy

Alessandro Lella

Australia

Ciaran A.J. O’Hare

Edoardo Vitagliano

Leading Bounds on Micrometer to Picometer Fifth Forces from Neutron Star Cooling

We revisit the production of axionlike particles (ALPs) coupled to electrons at tree level in a relativistic plasma. We explicitly demonstrate the equivalence between pseudoscalar and derivative couplings, incorporate previously neglected processes for the first time—namely, semi-Compton production (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>γ</mi><msup><mi>e</mi><mo>−</mo></msup><mo stretchy="false">→</mo><mi>a</mi><msup><mi>e</mi><mo>−</mo></msup></math>) and pair annihilation (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo stretchy="false">→</mo><mi>a</mi><mi>γ</mi></math>)—and derive analytical expressions for the bremsstrahlung (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>−</mo></msup><mi>N</mi><mo stretchy="false">→</mo><msup><mi>e</mi><mo>−</mo></msup><mi>N</mi><mi>a</mi></math>) production rate, enabling a more computationally efficient evaluation of the ALP flux. Additionally, we assess uncertainties in the production rate arising from electron thermal mass corrections, electron-electron Coulomb interactions, and the Landau-Pomeranchuk-Migdal effect. The ALP emissivity is made available in a public repository as a function of the ALP mass, the temperature, and the electron chemical potential of the plasma. Finally, we examine the impact of ALP production and subsequent decays on astrophysical observables, deriving the leading bounds on ALPs coupling to electrons. At small couplings, the dominant constraints come from the previously neglected decay <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>a</mi><mo stretchy="false">→</mo><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mi>γ</mi></math>, except for a region of fireball formation where SN 1987A x-ray observations offer the best probe. At large couplings, bounds are dominated by the energy deposition argument, with a recently developed new prescription for the trapping regime.

United States

Tetyana Pitik

Supernova production of axionlike particles coupling to electrons, reloaded

Searches for high frequency gravitational waves using cavities based on the Gertsenshtein effect were recently proposed, building off existing axion dark matter experiments. In particular, the sensitivity of axion dark matter experiments using metamaterial plasmas (tunable plasma haloscopes) to gravitational waves has not been explored in detail. Here we perform a full analysis of gravitational wave detection in plasma haloscopes, showing that the baseline design of experiments such as ALPHA is several orders of magnitude less sensitive than previously thought. We show how simple changes to the experiment can recover that sensitivity and lead to a powerful gravitational wave detector in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mn>10</mn><mi>–</mi><mn>50</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>GHz</mi></math> frequency range.

Rodolfo Capdevilla

Graciela B. Gelmini

Jonah Hyman

Alexander J. Millar

Gravitational wave detection with plasma haloscopes

Feebly interacting particles, such as sterile neutrinos, dark photons, and axions, can be abundantly produced in the proto-neutron star (PNS) formed in core-collapse supernovae (CCSNe). These particles can decay into photons or charged leptons, depositing energy outside the PNS. Strong bounds on new particles can thus be derived from the observed luminosity of CCSNe, with even tighter bounds obtained from low-energy SNe observations. For the first time we highlight that, at sufficiently large couplings, particle production outside the PNS must also be considered. Using the prototypical case of axions coupling to two photons, we show that at large couplings the energy transfer from PNS to its surroundings is diffusive rather than ballistic, substantially reducing the deposited energy. Our findings have implications for the parameter space of particles probed in beam dump experiments and for dark matter models involving a sub-GeV mediator.

Energy Transfer by Feebly Interacting Particles in Supernovae: The Trapping Regime

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.

Spain

Francisco R. Candón

Giuseppe Lucente

Julia K. Vogel

NuSTAR Bounds on Radiatively Decaying Particles from M82

We revisit supernova (SN) bounds on a hidden sector consisting of millicharged particles <math><mi>χ</mi></math> and a massless dark photon. Unless the self-coupling is fine-tuned to be small, rather than exiting the SN core as a gas, the particles form a relativistic fluid and subsequent dark QED fireball, streaming out against the drag due to the interaction with matter. Novel bounds due to excessive energy deposition in the mantle of low-energy supernovae can be obtained. The cooling bounds from SN 1987A are unexpectedly not affected in the free-streaming regime. The inclusion of <math><mrow><mi>χ</mi><mover><mrow><mi>χ</mi></mrow><mrow><mo>¯</mo></mrow></mover><mo>→</mo><mtext>standard</mtext></mrow></math> model substantially modifies the constraints in the trapping regime, which can only be treated hydrodynamically. Our results can be adapted to generic sub-GeV self-interacting dark sectors.

Denmark

Damiano F. G. Fiorillo

Self-Interacting Dark Sectors in Supernovae Can Behave as a Relativistic Fluid

Domain walls (DWs) are topological defects originating from phase transitions in the early Universe. In the presence of an energy imbalance between distinct vacua, enclosed DW cavities shrink until the entire network disappears. By studying the dynamics of thin-shell bubbles in general relativity, we demonstrate that closed DWs with sizes exceeding the cosmic horizon tend to annihilate later than the average. This delayed annihilation allows for the formation of large overdensities, which, upon entering the Hubble horizon, eventually collapse to form primordial black holes (PBHs). We rely on 3D percolation theory to calculate the number density of these late-annihilating DWs, enabling us to infer the abundance of PBHs. A key insight from our study is that DW networks with the potential to emit observable gravitational waves are also likely to yield detectable PBHs. Additionally, we study the production of wormholes connected to baby universes and conclude on the possibility to generate a multiverse.

Israel

Yann Gouttenoire

Primordial black holes and wormholes from domain wall networks

When hypothetical neutrino secret interactions (<math><mi>ν</mi><mi>SI</mi></math>) are large, they form a fluid in a supernova (SN) core, flow out with sonic speed, and stream away as a fireball. For the first time, we tackle the complete dynamical problem and solve all steps, systematically using relativistic hydrodynamics. The impact on SN physics and the neutrino signal is remarkably small. For complete thermalization within the fireball, the observable spectrum changes in a way that is independent of the coupling strength. One potentially large effect beyond our study is quick deleptonization if <math><mi>ν</mi><mi>SI</mi></math> violate lepton number. By present evidence, however, SN physics leaves open a large region in parameter space, where laboratory searches and future high-energy neutrino telescopes will probe <math><mi>ν</mi><mi>SI</mi></math>.

Georg G. Raffelt

Large Neutrino Secret Interactions Have a Small Impact on Supernovae

Neutrino-neutrino scattering could have a large secret component that would turn neutrinos within a supernova (SN) core into a self-coupled fluid. Neutrino transport within the SN core, emission from its surface, expansion into space, and the flux spectrum and time structure at Earth might all be affected. We examine these questions from first principles. First, diffusive transport differs only by a modified spectral average of the interaction rate. We next study the fluid energy transfer between a hot and a cold blackbody surface in plane-parallel and spherical geometry. The key element is the decoupling process within the radiating bodies, which themselves are taken to be isothermal. For a zero-temperature cold plate, mimicking radiation into free space by the hot plate, the energy flux is 3%–4% smaller than the usual Stefan-Boltzmann law. The fluid energy density just outside the hot plate is numerically 0.70 of the standard case, the outflow velocity is the speed of sound <math><mrow><msub><mrow><mi>v</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>=</mo><mi>c</mi><mo>/</mo><msqrt><mrow><mn>3</mn></mrow></msqrt></mrow></math>, conspiring to a nearly unchanged energy flux. Our results provide the crucial boundary condition for the expansion of the self-interacting fluid into space, assuming an isothermal neutrino sphere. We also derive a dynamical solution, assuming the emission suddenly begins at some instant. A neutrino front expands in space with luminal speed, whereas the outflow velocity at the radiating surface asymptotically approaches <math><msub><mi>v</mi><mi>s</mi></msub></math> from above. Asymptotically, one thus recovers the steady-state emission found in the two-plate model. A sudden end to neutrino emission leads to a fireball with constant thickness equal to the duration of neutrino emission.

Supernova emission of secretly interacting neutrino fluid: Theoretical foundations

Majoron-like bosons would emerge from a supernova (SN) core by neutrino coalescence of the form <math><mi>ν</mi><mi>ν</mi><mo>→</mo><mi>ϕ</mi></math> and <math><mover><mi>ν</mi><mo>¯</mo></mover><mover><mi>ν</mi><mo>¯</mo></mover><mo>→</mo><mi>ϕ</mi></math> with 100-MeV-range energies. Subsequent decays to (anti)neutrinos of all flavors provide a flux component with energies much larger than the usual flux from the “neutrino sphere.” The absence of 100-MeV-range events in the Kamiokande-II and Irvine-Michigan-Brookhaven signal of SN 1987A implies that less than 1% of the total energy was thus emitted and provides the strongest constraint on the Majoron-neutrino coupling of <math><mi>g</mi><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>MeV</mi><mo>/</mo><msub><mi>m</mi><mi>ϕ</mi></msub></math> for <math><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>≲</mo><msub><mi>m</mi><mi>ϕ</mi></msub><mo>≲</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>. It is straightforward to extend our new argument to other hypothetical feebly interacting particles.