Despite being mostly secluded, dark sector particles may feebly interact with photons via a small mass-dimension 4 millicharge, a mass-dimension 5 magnetic and electric dipole moment, or a mass-dimension 6 anapole moment and charge radius. If sufficiently light, the LHC may produce an intense and collimated beam of these particles in the far forward direction. We study the prospects of searching for such dark sector particles with electromagnetic form factors via their electron scattering signature in the Forward Liquid Argon Experiment (FLArE) at the Forward Physics Facility (FPF). We find that FLArE can provide new probes of sub-GeV dark particles with dipole moments and strong sensitivities for millicharged particles in the 100 MeV to 100 GeV region. This complements other search strategies using scintillation signatures or dark matter direct detection and allows for probing strongly interacting dark matter motivated by the EDGES anomaly. Along with the FORMOSA detector, this leads to a very versatile and leading experimental program in the search for millicharged particles in the FPF.

Germany

Felix Kling

United States

Jui-Lin Kuo

Poland

Sebastian Trojanowski

Yu-Dai Tsai

FLArE up dark sectors with EM form factors at the LHC forward physics facility

We consider electrically neutral complex-vector particles <math><mi>V</mi></math> below the GeV mass scale that, from a low-energy perspective, couple to the photon via higher-dimensional form factor interactions. We derive ensuing astrophysical constraints by considering the anomalous energy loss from the Sun, Horizontal Branch, and Red Giant stars as well as from SN1987A that arise from vector pair production in these environments. Under the assumption that the dark states <math><mi>V</mi></math> constitute dark matter, the bounds are then complemented by direct and indirect detection as well as cosmological limits. The relic density from freeze-out and freeze-in mechanisms is also computed. On the basis of a UV-complete model that realizes the considered effective couplings, we also discuss the naturalness of the constrained parameter space, and provide an analysis of the zero mass limit of <math><mi>V</mi></math>.

Austria

Xiaoyong Chu

Japan

Junji Hisano

Alejandro Ibarra

CERN

Josef Pradler

Multipole vector dark matter below the GeV scale

While most searches for cosmic axions so far focused on their cold relics as (a component of) dark matter, various well-motivated cosmological sources can produce “boosted” axions that remain relativistic today. We demonstrate that existing/upcoming neutrino experiments such as Super-Kamiokande, Hyper-Kamiokande, DUNE, JUNO, and IceCube can probe such energetic axion relics. The characteristic signature is the monoenergetic single photon signal from axion absorption induced by the axion-photon coupling. This proposal offers to cover parameter ranges complementary to existing axion searches and provides new opportunities for discovery with neutrino facilities.

Yanou Cui

Shining light on cosmogenic axions with neutrino experiments

We consider a muonphilic axionlike particle (ALP), denoted as <math><mi>a</mi></math>, lighter than twice the muon mass. ALPs of this mass range dominantly decay into a pair of photons, induced by a triangular muon loop. Such light muonphilic ALPs are naturally long lived. At the atmosphere, the ALPs are copiously produced from charged-meson decays in air showers, such as <math><msup><mi>π</mi><mo>±</mo></msup><mo>→</mo><msup><mi>μ</mi><mo>±</mo></msup><mi>ν</mi><mi>a</mi></math>, via the ALP-muon coupling <math><msub><mi>g</mi><mrow><mi>a</mi><mi>μ</mi><mi>μ</mi></mrow></msub></math>. After propagating tens of kilometers, the ALPs decay with <math><mi>a</mi><mo>→</mo><mi>γ</mi><mi>γ</mi></math> inside large-volume Cherenkov detectors near Earth’s surface, such as Super-Kamiokande (SK). We find the present SK observation constrains on muonphilic ALPs of mass range [1 MeV, 30 MeV] and ALP-muon coupling <math><mrow><mo>[</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup><mo>,</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>2</mn></mrow></msup><mo>]</mo></mrow></math>, assuming the proper decay length <math><mi>c</mi><msub><mi>τ</mi><mi>a</mi></msub></math> in [<math><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>km</mi></math>, <math><msup><mn>10</mn><mn>6</mn></msup><mtext> </mtext><mtext> </mtext><mi>km</mi></math>] either dependent on or independent of <math><msub><mi>g</mi><mrow><mi>a</mi><mi>μ</mi><mi>μ</mi></mrow></msub></math>. We conclude that atmospheric searches of such exotic states can be complementary to collider and beam-dump experiments as well as astrophysical probes.

South Korea

Taiwan

Kingman Cheung

Po-Yan Tseng

Zeren Simon Wang

Atmospheric axionlike particles at Super-Kamiokande

Thermal dark matter at the MeV mass scale has its abundance set during the highly nontrivial epochs of neutrino decoupling and electron annihilation. The technical obstacles attached to solving Boltzmann equations of multiple interacting sectors being both relativistic and nonrelativistic have to-date prevented the full treatment of this problem. Here, for the first time, we calculate the freeze-out of light dark matter, taking into account the energy transfer between the dark sector, neutrinos, and the electromagnetically interacting plasma from annihilation and elastic scattering processes alike. We develop a numerically feasible treatment that allows to track photon and neutrino temperatures across freeze-out and to arrive at a precision prediction of <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> for arbitrary branching ratios of the dark matter annihilation channels. In addition, our treatment resolves for the first time the dark matter temperature evolution across freeze-out involving three sectors. It enters in the efficiency of velocity-dependent annihilation channels and for a flavor-blind <math><mi>p</mi></math>-wave annihilation into electron- and neutrino-pairs of all generations, we find the present Planck data exclude a complex scalar dark matter particle of mass of <math><msub><mi>m</mi><mi>ϕ</mi></msub><mo>≤</mo><mn>7</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>.

Toward a full description of MeV dark matter decoupling: A self-consistent determination of relic abundance and <math><msub><mi>N</mi><mi>eff</mi></msub></math>

We study the possibility that dark radiation, sourced through the decay of dark matter in the late Universe, carries electromagnetic interactions. The relativistic flux of particles induces recoil signals in direct detection and neutrino experiments through its interaction with millicharge, electric/magnetic dipole moments, or anapole moment/charge radius. Taking the DM lifetime as 35 times the age of the Universe, as currently cosmologically allowed, we show that direct detection (neutrino) experiments have complementary sensitivity down to <math><mi>ε</mi><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>11</mn></mrow></msup></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>12</mn></mrow></msup><mo>)</mo></math>, <math><msub><mi>d</mi><mi>χ</mi></msub><mo>/</mo><msub><mi>μ</mi><mi>χ</mi></msub><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup><msub><mi>μ</mi><mi>B</mi></msub></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>13</mn></mrow></msup><msub><mi>μ</mi><mi>B</mi></msub><mo>)</mo></math>, and <math><msub><mi>a</mi><mi>χ</mi></msub><mo>/</mo><msub><mi>b</mi><mi>χ</mi></msub><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>8</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo>)</mo></math> on the respective couplings. Finally, we show that such dark radiation can lead to a satisfactory explanation of the recently observed XENON1T excess in the electron recoil signal without being in conflict with other bounds.

Maxim Pospelov

Terrestrial probes of electromagnetically interacting dark radiation

We revisit the possibility of light-scalar dark matter, in the MeV to GeV mass bracket and coupled to electrons through fermion or vector mediators, in light of significant experimental and observational advances that probe new physics below the GeV scale. We establish new limits from electron colliders and fixed-target beams and derive the strength of loop-induced processes that are probed by precision physics, among other laboratory probes. In addition, we compute the cooling bound from SN1987A, consider self-scattering, structure formation, and cosmological constraints as well as the limits from dark matter–electron scattering in direct detection experiments. We then show that the combination of constraints largely excludes the possibility that the galactic annihilation of these particles may explain the long-standing International Gamma-Ray Astrophysics Laboratory excess of 511 keV photons as observed in the Galactic bulge. As a caveat to these conclusions, we identify the resonant annihilation regime where the vector mediator goes nearly on shell.

Australia

Céline Bœhm

Scalar dark matter candidates revisited

We consider electromagnetically neutral dark states that couple to the photon through higher dimensional effective operators, such as electric and magnetic dipole moment, anapole moment and charge radius operators. We investigate the possibility of probing the existence of such dark states, taking a Dirac fermion <math><mi>χ</mi></math> as an example, at several representative proton-beam experiments. As no positive signal has been reported, we obtain upper limits (or projected sensitivities) on the corresponding electromagnetic form factors for dark states lighter than several GeV. We demonstrate that while the current limits from proton-beam experiments are at most comparable with those from high-energy electron colliders, future experiments, such as DUNE and SHiP, will be able to improve the sensitivities to electric and magnetic dipole moment interactions, owing to their high intensity.

Dark sector-photon interactions in proton-beam experiments

Electromagnetically neutral dark sector particles may directly couple to the photon through higher dimensional effective operators. Considering the electric and magnetic dipole moment, anapole moment, and charge radius interactions, we derive constraints from the stellar energy loss in the Sun, horizontal branch and red giant stars, as well as from cooling of the proto-neutron star of SN1987A. We provide the exact formula for in-medium photon-mediated pair production to leading order in the dark coupling and compute the energy loss rates explicitly for the most important processes, including a careful discussion on resonances and potential double counting between the processes. Stringent limits for dark states with masses below 3 keV (40 MeV) arise from red giant stars (SN1987A), implying an effective lower mass-scale of approximately <math><mrow><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mn>9</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> (<math><mrow><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mn>7</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>) for mass-dimension five and 100 GeV (2.5 TeV) for mass-dimension six operators as long as dark states stream freely; for the proto-neutron star, the trapping of dark states is also evaluated. Together with direct limits previously derived by us in Chu et al. (2018), this provides the first comprehensive overview of the viability of effective electromagnetic dark-state interactions below the GeV mass-scale.

Lukas Semmelrock

Stellar probes of dark sector-photon interactions

Relativistic millicharged particles (<math><msub><mi>χ</mi><mi>q</mi></msub></math>) have been proposed in various extensions to the standard model of particle physics. We consider the scenarios where they are produced at nuclear reactor core and via interactions of cosmic rays with the Earth’s atmosphere. Millicharged particles could also be candidates for dark matter and become relativistic through acceleration by supernova explosion shock waves. The atomic ionization cross section of <math><msub><mi>χ</mi><mi>q</mi></msub></math> with matter are derived with the equivalent photon approximation. Smoking-gun signatures with significant enhancement in the differential cross section are identified. New limits on the mass and charge of <math><msub><mi>χ</mi><mi>q</mi></msub></math> are derived, using data taken with a point-contact germanium detector with 500 g mass functioning at an energy threshold of 300 eV at the Kuo-Sheng Reactor Neutrino Laboratory.

India

L. Singh

J. W. Chen

H. C. Chi

C.P. Liu

M. K. Pandey

H. T. Wong

C. P. Wu

Türkiye

M. Agartioglu

M. Deniz

H. B. Li

China

S. T. Lin

V. Sharma

M. K. Singh

V. Singh

Q. Yue

Constraints on millicharged particles with low-threshold germanium detectors at Kuo-Sheng Reactor Neutrino Laboratory