New physics searches at kaon and hyperon factories.

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Simplest and Most Predictive Model of Muon g-2 and Thermal Dark Matter.

The long-standing 4.2σ muon g-2 anomaly may be the result of a new particle species which could also couple to dark matter and mediate its annihilations in the early Universe. In models where both muons and dark matter carry equal charges under a U(1)_{L_{µ}-L_{τ}} gauge symmetry, the corresponding Z^{'} can both resolve the observed g-2 anomaly and yield an acceptable dark matter relic abundance, relying on annihilations which take place through the Z^{'} resonance. Once the value of (g-2)_{µ} and the dark matter abundance are each fixed, there is very little remaining freedom in this model, making it highly predictive. We provide a comprehensive analysis of this scenario, identifying a viable range of dark matter masses between approximately 10 and 100 MeV, which falls entirely within the projected sensitivity of several accelerator-based experiments, including NA62, NA64µ, M^{3}, and DUNE. Furthermore, portions of this mass range predict contributions to ΔN_{eff} which could ameliorate the tension between early and late time measurements of the Hubble constant, and which could be tested by stage 4 CMB experiments.

Dark Matter Detection with Bound Nuclear Targets: The Poisson Phonon Tail.

Dark matter (DM) scattering with nuclei in solid-state systems may produce elastic nuclear recoil at high energies and single-phonon excitation at low energies. When the DM momentum is comparable to the momentum spread of nuclei bound in a lattice, q_{0}=sqrt[2m_{N}ω_{0}] where m_{N} is the mass of the nucleus and ω_{0} is the optical phonon energy, an intermediate scattering regime characterized by multiphonon excitations emerges. We study a greatly simplified model of a single nucleus in a harmonic potential and show that, while the mean energy deposited for a given momentum transfer q is equal to the elastic value q^{2}/(2m_{N}), the phonon occupation number follows a Poisson distribution and thus the energy spread is ΔE=qsqrt[ω_{0}/(2m_{N})]. This observation suggests that low-threshold calorimetric detectors may have significantly increased sensitivity to sub-GeV DM compared to the expectation from elastic scattering, even when the energy threshold is above the single-phonon energy, by exploiting the tail of the Poisson distribution for phonons above the elastic energy. We use a simple model of electronic excitations to argue that this multiphonon signal will also accompany ionization signals induced from DM-electron scattering or the Migdal effect. In well-motivated models where DM couples to a heavy, kinetically mixed dark photon, we show that these signals can probe experimental milestones for cosmological DM production via thermal freeze-out, including the thermal target for Majorana fermion DM.

Probing Muonphilic Force Carriers and Dark Matter at Kaon Factories.

Rare kaon decays are excellent probes of light, new weakly coupled particles. If such particles X couple preferentially to muons, they can be produced in K→µνX decays. We evaluate the future sensitivity for this process at NA62 assuming X decays either invisibly or to dimuons. Our main physics target is the parameter space that resolves the (g-2)_{µ} anomaly, where X is a gauged L_{µ}-L_{τ} vector or a muonphilic scalar. The same parameter space can also accommodate dark matter freeze-out or reduce the tension between cosmological and local measurements of H_{0} if the new force decays to dark matter or neutrinos, respectively. We show that for invisible X decays, a dedicated single muon trigger analysis at NA62 could probe much of the remaining (g-2)_{µ} favored parameter space. Alternatively, if X decays to muons, NA62 can perform a dimuon resonance search in K→3µν events and greatly improve existing coverage for this process. Independently of its sensitivity to new particles, we find that NA62 is also sensitive to the standard model predicted rate for K→3µν, which has never been measured.

Search for Composite Dark Matter with Optically Levitated Sensors.

Results are reported from a search for a class of composite dark matter models with feeble long-range interactions with normal matter. We search for impulses arising from passing dark matter particles by monitoring the mechanical motion of an optically levitated nanogram mass over the course of several days. Assuming such particles constitute the dominant component of dark matter, this search places upper limits on their interaction with neutrons of α_{n}≤1.2×10^{-7} at 95% confidence for dark matter masses between 1 and 10 TeV and mediator masses m_{ϕ}≤0.1 eV. Because of the large enhancement of the cross section for dark matter to coherently scatter from a nanogram mass (∼10^{29} times that for a single neutron) and the ability to detect momentum transfers as small as ∼200 MeV/c, these results provide sensitivity to certain classes of composite dark matter models that substantially exceeds existing searches, including those employing kilogram- or ton-scale targets. Extensions of these techniques can enable directionally sensitive searches for a broad class of previously inaccessible heavy dark matter candidates.

Can the Inflaton Also Be a Weakly Interacting Massive Particle?

We propose a class of models in which a stable inflaton is produced as a thermal relic in the early Universe and constitutes the dark matter. We show that inflaton annihilations can efficiently reheat the Universe, and identify several examples of inflationary potentials that can accommodate all cosmic microwave background observables and in which the inflaton dark matter candidate has a weak scale mass. As a simple example, we consider annihilations that take place through a Higgs portal interaction, leading to encouraging prospects for future direct detection experiments.

Severe Constraints on New Physics Explanations of the MiniBooNE Excess.

The MiniBooNE experiment has recently reported an anomalous 4.5σ excess of electronlike events consistent with ν_{e} appearance from a ν_{µ} beam at short baseline. Given the lack of corresponding ν_{µ} disappearance observations, required in the case of oscillations involving a sterile flavor, there is strong motivation for alternative explanations of this anomaly. We consider the possibility that the observed electronlike signal may actually be initiated by particles produced in the MiniBooNE target, without involving new sources of neutrino production or any neutrino oscillations. We find that the electronlike event energy and angular distributions in the full MiniBooNE dataset, including neutrino, antineutrino, and beam dump modes, severely limit and, in some cases, rule out new physics scenarios as an explanation for the observed neutrino and antineutrino mode excesses. Specifically, scenarios in which the particle produced in the target either decays (visibly or semivisibly) or scatters elastically in the detector are strongly disfavored. Using generic kinematic arguments, we extend the existing MiniBooNE results and interpretations to exhaustively constrain previously unconsidered new physics signatures and emphasize the power of the MiniBooNE beam dump search to further limit models for the excess.

Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension.

Large, nonstandard neutrino self-interactions have been shown to resolve the ∼4σ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force carrier with a large neutrino coupling (>10^{-4}) and mass in the keV-100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to τ-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana particles, and that a UV-complete model requires a nonminimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

Severely Constraining Dark-Matter Interpretations of the 21-cm Anomaly.

The EDGES Collaboration has recently reported the detection of a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a frequency corresponding to a redshift of z≃17. This observation has been interpreted as evidence that the gas was cooled during this era as a result of scattering with dark matter. In this Letter, we explore this possibility, applying constraints from the cosmic microwave background, light element abundances, Supernova 1987A, and a variety of laboratory experiments. After taking these constraints into account, we find that the vast majority of the parameter space capable of generating the observed 21-cm signal is ruled out. The only viable models are those in which a small fraction, ∼0.3%-2%, of the dark matter consists of particles with a mass of ∼10-80 MeV and which couple to the photon through a small electric charge, roughly 10^{-6}-10^{-4} as large as the electron charge. Furthermore, in order to avoid being overproduced in the early Universe, such models must be supplemented with an additional depletion mechanism, such as annihilations through a L_{µ}-L_{τ} gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Analyzing the Discovery Potential for Light Dark Matter.

In this Letter, we determine the present status of sub-GeV thermal dark matter annihilating through standard model mixing, with special emphasis on interactions through the vector portal. Within representative simple models, we carry out a complete and precise calculation of the dark matter abundance and of all available constraints. We also introduce a concise framework for comparing different experimental approaches, and use this comparison to identify important ranges of dark matter mass and couplings to better explore in future experiments. The requirement that dark matter be a thermal relic sets a sharp sensitivity target for terrestrial experiments, and so we highlight complementary experimental approaches that can decisively reach this milestone sensitivity over the entire sub-GeV mass range.