Future searches for new physics beyond the Standard Model are without doubt in need of a diverse approach and experiments with complementary sensitivities to different types of classes of models. One of the directions that should be explored is feebly interacting particles (FIPs) with masses below the electroweak scale. The interest in FIPs has significantly increased in the last ten years. Searches for FIPs at colliders have intrinsic limitations in the region they may probe, significantly restricting exploration of the mass range  $$m_{\text {FIP}} &lt; 5{-}10$$  <math> <mrow> <msub> <mi>m</mi> <mtext>FIP</mtext> </msub> <mo>&lt;</mo> <mn>5</mn> <mo>-</mo> <mn>10</mn> </mrow> </math>   GeV/c $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  . Beam dump-like experiments, characterized by the possibility of extremely high luminosity at relatively high energies and the effective coverage of the production and decay acceptance, are the perfect option to generically explore the “coupling frontier” of the light FIPs. Several proposals for beam-dump detectors are currently being considered by CERN for implementation at the SPS ECN3 beam facility. In this paper, we analyse in depth how the characteristic geometric parameters of a beam dump experiment influence the signal yield. We apply an inclusive approach by considering the phenomenology of different types of FIPs. From the various production modes and kinematics, we demonstrate that the optimal layout that maximises the production and decay acceptance consists of a detector located on the beam-axis, at the shortest possible distance from the target defined by the systems required to suppress the beam-induced backgrounds.

Italy

Kyrylo Bondarenko

Netherlands

Alexey Boyarsky

CERN

Richard Jacobsson

Oleksii Mikulenko

Germany

Maksym Ovchynnikov

Towards the optimal beam dump experiment to search for feebly interacting particles

The idea that new physics could take the form of feebly interacting particles (FIPs)—particles with a mass below the electroweak scale, but which may have evaded detection due to their tiny couplings or very long lifetime—has gained a lot of traction in the last decade, and numerous experiments have been proposed to search for such particles. It is important, and now very timely, to consistently compare the potential of these experiments for exploring the parameter space of various well-motivated FIPs. The present paper addresses this pressing issue by presenting an open-source tool to estimate the sensitivity of many experiments—located at Fermilab or the CERN’s SPS, LHC, and FCC-hh—to various models of FIPs in a unified way: the Mathematica-based code senscalc.

Spain

Jean-Loup Tastet

Sensitivities to feebly interacting particles: Public and unified calculations

Axion-photon conversion is a prime mechanism to detect axion-like particles that share a coupling to the photon. We point out that in the vicinity of neutron stars with strong magnetic fields, magnetars, the effective photon mass receives comparable but opposite contributions from free electrons and the radiation field. This leads to an energy-dependent resonance condition for conversion that can be met for arbitrary light axions and leveraged when using systems with detected radio component. Using the magnetar SGR J1745-2900 as an exemplary source, we demonstrate that sensitivity to <math><mo>|</mo><msub><mrow><mi>g</mi></mrow><mrow><mi>a</mi><mi>γ</mi></mrow></msub><mo>|</mo><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>12</mn></mrow></msup><mspace width="0.2em"></mspace><mrow><mi>Ge</mi><msup><mrow><mi>V</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> or better can be gained for <math><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≲</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup><mspace width="0.2em"></mspace><mrow><mi>eV</mi></mrow></math>, with the potential to improve current constraints on the axion-photon coupling by more than one order of magnitude over a broad mass range. With growing insights into the physical conditions of magnetospheres of magnetars, the method hosts the potential to become a serious competitor to future experiments such as ALPS-II and IAXO in the search for axion-like particles.

Austria

Josef Pradler

United States

Anastasia Sokolenko

Neutron stars as photon double-lenses: Constraining resonant conversion into ALPs

We analyze the potential of the recently proposed experiment FACET (Forward-Aperture CMS ExTension) to search for new physics. As an example, we consider the models of Higgs-like scalars with cubic and quartic interactions and Heavy Neutral Leptons. We compare the sensitivity of FACET with that of other proposed “intensity frontier” experiments, including FASER2, SHiP, etc. and demonstrate that FACET could probe an interesting parameter space between the current constraints and the potential reach of the above mentioned proposals.

Ukraine

Viktor Kryshtal

Sensitivity of the FACET experiment to Heavy Neutral Leptons and Dark Scalars

The extension of the Standard Model with two gauge-singlet Majorana fermions can simultaneously explain two beyond-the-Standard-model phenomena: neutrino masses and oscillations, as well as the origin of the matter-antimatter asymmetry in the Universe. The parameters of such a model are constrained by the neutrino oscillation data, direct accelerator searches, big bang nucleosynthesis, and requirement of successful baryogenesis. We show that their combination still leaves an allowed region in the parameter space below the kaon mass. This region can be probed by the further searches of NA62, DUNE, or SHiP experiments.

Switzerland

Juraj Klaric

Denmark

Oleg Ruchayskiy

Vsevolod Syvolap

Inar Timiryasov

An allowed window for heavy neutral leptons below the kaon mass

In the inhomogeneous Universe, the cosmological conversion of dark photons into ordinary photons (and vice versa) may happen at a great number of resonance redshifts. This alters the CMB observed energy spectrum and degree of small-scale anisotropies. We utilize results from the EAGLE simulation to obtain the conversion probability along random line-of-sights to quantify these effects. We then apply our results to the case where dark photons are sourced by dark matter decay and their high-redshift conversion into ordinary photons modify the global 21 cm signal expected from the cosmic dawn era. Concretely, we show that a significant portion of the parameter space for which a converted population of photons in the Rayleigh-Jeans tail of the CMB explains the absorption strength observed by EDGES, is ruled out from the brightness temperature measurements of COBE/FIRAS and the CMB anisotropy measurements of Planck and SPT.

Constraining dark photons and their connection to 21 cm cosmology with CMB data

The most general renormalizable interaction between the Higgs sector and a new gauge-singlet scalar is governed by two interaction terms: cubic and quartic. The quartic term is only loosely constrained by invisible Higgs decays and given current experimental limits about 10% of all Higgs bosons at the LHC can be converted to new scalars with masses up to 2. By including this production channel, one significantly extends the reach of the LHC-based Intensity Frontier experiments. We analyze the sensitivity of the FASER experiment to this model and discuss modest changes in the FASER 2 design that would allow exploring an order-of-magnitude wider part of the Higgs portal’s parameter space.

Iryna Boiarska

Norway

Light scalar production from Higgs bosons and FASER 2

Traditional direct searches for dark matter, looking for nuclear recoils in deep underground detectors, are challenged by an almost complete loss of sensitivity for light dark matter particles. Consequently, there is a significant effort in the community to devise new methods and experiments to overcome these difficulties, constantly pushing the limits of the lowest dark matter mass that can be probed this way. From a model-building perspective, the scattering of sub-GeV dark matter on nucleons essentially must proceed via new light mediator particles, given that collider searches place extremely stringent bounds on contact-type interactions. Here we present an updated compilation of relevant limits for the case of a scalar mediator, including a new estimate of the near-future sensitivity of the NA62 experiment as well as a detailed evaluation of the model-specific limits from Big Bang nucleosynthesis. We also derive updated and more general limits on DM particles upscattered by cosmic rays, applicable to arbitrary energy- and momentum dependences of the scattering cross section. Finally we stress that dark matter self-interactions, when evaluated beyond the common s-wave approximation, place stringent limits independently of the dark matter production mechanism. These are, for the relevant parameter space, generically comparable to those that apply in the commonly studied freeze-out case. We conclude that the combination of existing (or expected) constraints from accelerators and astrophysics, combined with cosmological requirements, puts robust limits on the maximally possible nuclear scattering rate. In most regions of parameter space these are at least competitive with the best projected limits from currently planned direct detection experiments.

Torsten Bringmann

Marco Hufnagel

Kai Schmidt-Hoberg

Direct detection and complementary constraints for sub-GeV dark matter

We review and revise the phenomenology of the scalar portal — a new scalar particle with the mass in GeV range that mixes with the Higgs boson. In particular, we consider production channels B → SK 1(1270) and   <math> <mi>B</mi> <mo>→</mo> <mi>S</mi> <msubsup> <mi>K</mi> <mn>0</mn> <mo>∗</mo> </msubsup> <mfenced> <mn>700</mn> </mfenced> </math>  $$ B\to S{K}_0^{\ast }(700) $$  and show that their contribution is significant. We extend the previous analysis by comparing the production of scalars from decays of mesons, of the Higgs bosons and direct production via proton bremsstrahlung, deep inelastic scattering and coherent scattering on nuclei. Relative efficiency of the production channels depends on the energy of the beam and we consider the energies of DUNE, SHiP and LHC-based experiments. We present our results in the form directly suitable for calculations of experimental sensitivities.

Volodymyr Gorkavenko

Phenomenology of GeV-scale scalar portal

In recent years, a number of intensity frontier experiments have been proposed to search for feebly interacting particles with masses in the GeV range. We discuss how the characteristic shape of the experimental sensitivity regions — upper and lower boundaries of the probed region, the maximal mass reach — depends on the parameters of the experiments. We use the SHiP and the MATHUSLA experiments as examples. We find a good agreement of our estimates with the results of the Monte Carlo simulations. This simple approach allows to cross-check and debug Monte Carlo results, to scan quickly over the parameter space of feebly interacting particle models, and to explore how sensitivity depends on the geometry of experiments.