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

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

The Future Circular hadron Collider (FCC-hh) is a proposed successor of the Large Hadron Collider (LHC). FCC-hh would push both the energy and intensity frontiers of searches for new physics particles. In particular, due to higher energy and luminosity than at the LHC, at FCC-hh there would be produced around ≃ 30 times larger amount of B mesons and ≃ 120 times of W bosons, which then may decay into feebly interacting particles. In this paper we demonstrate the potential of FCC-hh by studying its sensitivity to heavy neutral leptons (HNLs) with masses m  N  &lt; m  B . We consider various locations of a displaced decay volume embedded in the planned infrastructure of FCC-hh. We demonstrate that FCC-hh may substantially improve the reach of the parameter space of HNLs as compared to the searches proposed at the LHC.

Switzerland

Lesya Shchutska

Exploring the potential of FCC-hh to search for particles from B mesons

We investigate how hypothetical particles—sterile neutrinos—can be produced in the interior of exploding supernovae via the resonant conversion of <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub></math> and <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>τ</mi></msub></math>. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account backreaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, sensitive to the temperature in the inner supernovae regions, making robust predictions of challenging. In order to understand whether this production affects supernova evolution and can therefore be constrained, detailed simulations including the effects of sterile neutrinos are needed.

Denmark

Ukraine

Vsevolod Syvolap

Oleg Ruchayskiy

Resonance production of keV sterile neutrinos in core-collapse supernovae and lepton number diffusion

SND@LHC is an approved experiment equipped to detect scatterings of neutrinos produced in the far-forward direction at the LHC, and aimed to measure their properties. In addition, the detector has a potential to search for new feebly interacting particles (FIPs) that may be produced in proton-proton collisions. In this paper, we discuss signatures of new physics at SND@LHC for two classes of particles: stable FIPs that may be detected via their scattering, and unstable FIPs that decay inside the detector. We estimate the sensitivity of SND@LHC to probe scatterings of leptophobic dark matter and decays of neutrino, scalar, and vector portal particles. Finally, we also compare and qualitatively analyze the potential of SND@LHC and FASER/FASERν experiments for these searches.

Searches for new physics at SND@LHC

Beta-spectrum of radioactive atoms was long ago predicted to bear an imprint of the cosmic neutrino background (<math><mrow><mi>C</mi><mi>ν</mi><mi>B</mi></mrow></math>) [S. Weinberg, Phys. Rev. 128, 1457 (1962)]. Over the years, it has been recognized that the best chance of achieving the signal-to-noise ratio required for the observation of this effect lies with solid-state designs [E. Baracchini et al., arXiv:1808.01892.]. Here we bring to the fore a fundamental quantum limitation on the type of beta-decayer that can be used in a such a design. We derive a simple usability criterion and show that <math><mrow><mmultiscripts><mrow><mi>H</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math>, which is the most popular choice, fails to meet it. We provide a list of potentially suitable isotopes and discuss why their use in <math><mrow><mi>C</mi><mi>ν</mi><mi>B</mi></mrow></math> detection requires further research.

Yevheniia Cheipesh

Vadim Cheianov

Navigating the pitfalls of relic neutrino detection

We reanalyze the results of the searches for Heavy Neutral Leptons (HNLs) by the CHARM experiment. We study HNL decay channel <math><mrow><mi>N</mi><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>ν</mi><mo>/</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>ν</mi></mrow></math> and show that, in addition to the constraints on the HNL’s mixings with <math><msub><mi>ν</mi><mi>e</mi></msub></math> or <math><msub><mi>ν</mi><mi>μ</mi></msub></math>, the same data also implies limits on the HNLs that mix only with <math><msub><mi>ν</mi><mi>τ</mi></msub></math> and have masses in the range <math><mrow><mn>290</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi><mo>&lt;</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>N</mi></mrow></msub><mo>≲</mo><mn>1.6</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>: the region in the parameter space that was considered in the literature as a target for HNLs searches.

Iryna Boiarska

Constraints from the CHARM experiment on heavy neutral leptons with tau mixing

The existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species <math><msub><mi>N</mi><mi>eff</mi></msub></math> in the early Universe. In this work, we investigate in detail how short-lived FIMPs that can decay into neutrinos affect <math><msub><mi>N</mi><mi>eff</mi></msub></math> and highlight the relevant effects that govern its evolution. We show that even if unstable FIMPs inject most of their energy into neutrinos, they may still decrease <math><msub><mi>N</mi><mi>eff</mi></msub></math>, and identify neutrino spectral distortions as the driving power behind this effect. As a case study, we consider heavy neutral leptons (HNLs) and indicate which regions of their parameter space increase or decrease <math><msub><mi>N</mi><mi>eff</mi></msub></math>. Moreover, we derive bounds on the HNL lifetime from the cosmic microwave background and comment on the possible role that HNLs could play in alleviating the Hubble tension.

United Kingdom

Nashwan Sabti

When feebly interacting massive particles decay into neutrinos: The <math><msub><mi>N</mi><mi>eff</mi></msub></math> story

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.

Juraj Klaric

Inar Timiryasov

An allowed window for heavy neutral leptons below the kaon mass

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.