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

Protoneutron stars, highly compact objects formed in the core of exploding supernovae (SNe), are powerful sources of axionlike particles (ALPs). In the SN core, ALPs are dominantly produced via nucleon-nucleon bremsstrahlung and pion conversion, resulting in an energetic ALP spectrum peaked at energies <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mn>100</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>. In this work, we revisit the diffuse ALP background, produced from all past core-collapse supernovae, and update the constraints derived from Fermi-LAT observations. Assuming the maximum ALP-nucleon coupling allowed by the SN 1987A cooling, we set the upper limit <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi><mi>γ</mi></mrow></msub><mo>≲</mo><mn>2</mn><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 ALP mass <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>10</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>, which is approximately a factor of two improvement with respect to the existing bounds. On the other hand, 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>10</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>, we find that including pion conversion strengthens the bound on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi><mi>γ</mi></mrow></msub></math>, approximately by a factor of two compared to the constraint obtained from bremsstrahlung alone. Additionally, we present a sensitivity study for future experiments such as AMEGO-X, e-ASTROGAM, GRAMS-balloon, GRAMS-satellite, and MAST. We find that the expected constraint from MAST would be comparable to Fermi-LAT bound. However, SN 1987A constraint remains one order of magnitude stronger as compared to the bound derived from the current and future gamma-ray telescopes.

Spain

Francisco R. Candón

South Korea

Sougata Ganguly

United States

Maurizio Giannotti

India

Tanmoy Kumar

Federico Mescia

Fresh look at the diffuse ALP background from supernovae

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.

Francesca Lecce

Giuseppe Lucente

Vimal Vijayan

Andreas Bauswein

Alessandro Mirizzi

Probing axionlike particles with multimessenger observations of neutron star mergers

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

Sweden

Pierluca Carenza

Pedro De la Torre Luque

Leonardo Mastrototaro

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

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

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

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

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

Thomas Rauscher

Getting the most on supernova axions

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