Sufficiently strong first-order phase transitions (FOPTs) in the early Universe can simultaneously produce an observable stochastic gravitational wave background (SGWB) and a large-scale primordial magnetic field (PMF). The recent 3.8σ evidence for a non-zero intergalactic MF from anisotropic pair-halo searches using Fermi-LAT data further motivates a cosmological origin of this MF. We investigate an FOPT-origin of both cosmic signatures, namely, PMF and SGWB, and the correlation between them, within a minimal axion-like particle (ALP) framework in which a global U(1) symmetry is spontaneously broken through radiative corrections, with the ALP sector coupled to the Standard Model (SM) via Higgs-portal. We compute the present-day PMF amplitude and coherence length for both maximally helical and non-helical configurations, accounting for inverse cascade effects. For maximally helical configurations, we find peak field strengths up to B_0 ∼ 10^-9 G at coherence length λ_0 ∼ 10^-3-10^-1 Mpc, consistent with lower bounds on the IGMF inferred from blazar observations by MAGIC, H.E.S.S. and Fermi-LAT. We show that the ALP parameter region consistent with γ-ray blazar data (assuming maximal helicity) simultaneously produces SGWB detectable at future space-based interferometers, such as LISA, etc., over the ALP decay constant range 10^3 GeV≲ f_a ≲ 10^5 GeV. We directly map these onto effective ALP couplings to SM particles, e.g., photons, gluons, and fermions. This establishes a multi-messenger complementarity between cosmological observables and laboratory/astrophysical ALP searches, with the combined constraints preferring relatively heavy ALPs, m_a ≳ 0.1 GeV, in a regime accessible to next-generation intensity and energy-frontier experiments.

Although the sources of astrophysical neutrinos are still unknown, they are believed to be produced by a population of sources in the distant Universe. Measurements of the diffuse, all-sky astrophysical flux can thus be sensitive to flavor-and energy-dependent propagation effects, such as very long-baseline oscillations. These oscillations are present in certain neutrino mass models, such as when neutrinos are quasi-Dirac. Assuming generic models for the source flux, we find that these oscillations can still be resolved even when integrated over wide distributions in source redshift. We use two sets of IceCube all-sky flux measurements, made with muon and all-flavor neutrino samples, to set constraints at the 3 sigma level on quasi-Dirac mass splittings between (5 & times; 10-19; 8 & times; 10-19) eV2. Additionally, we find that when the mass-square splittings are different between generations, the quasi-Dirac scenario can explain tensions between the all-flavor and muon-flavor measurements. We consider systematic uncertainties on the source population and find that our results are robust under alternate spectral hypotheses or physical redshift distributions. Our analysis shows that spectral features in the all-sky neutrino measurements provide strong constraints on massive neutrino scenarios and are sensitive to uncharted parameter space.

We revisit the electric dipole moments (EDMs) of quarks and leptons in the Minimal Supersymmetric Standard Model (MSSM) with trilinear R-parity violation (RPV). In this framework, EDMs are induced at the two-loop level via RPV interactions. We perform a comprehensive recalculation of several classes of Barr-Zee type diagrams in a general R xi gauge. While we find general agreement with previous analytic results in the literature, our work provides a valuable independent cross-check of the complicated calculations. We also point out some subtleties in the intermediate steps and in the choice of the flavor basis for the numerical evaluation of the expressions. By confronting the theoretical predictions with the latest experimental limits on EDMs, we derive updated constraints on combinations of RPV couplings. We highlight an approximate, testable correlation between the proton and neutron EDM that emerges within the considered class of RPV models, offering a distinctive signature for future EDM experiments.

We show that $\sim$keV thermal solar neutrinos, arising from electroweak processes in the solar plasma, are kinematically accessible to large-volume dark matter direct detection experiments via electron ionization signatures. Using S2-only data from the XENONnT experiment, we place an upper limit on the thermal solar neutrino flux of $η\lesssim 1.2 \times 10^8$ times the standard model predicted value, while paired searches from XENONnT, LZ and PandaX give slightly weaker limits. The future XLZD experiment could improve these limits by orders of magnitude. While still far from a detection, this result establishes low-threshold direct detection experiments as a viable probe of the lowest-energy neutrino sources in astrophysics, with important implications for stellar physics and beyond.

The dark matter (DM) density profile in the innermost region of the Galaxy remains an open question. In particular, while adiabatic growth of the supermassive black hole Sgr A$^\ast$ at the Galactic Center (GC) can induce a 'spike' in central DM density, the existence of such a spike is still under debate. Here we present new constraints on the spike slope $γ_{\rm sp}$ using conventional DM indirect detection searches. We first recast existing photon and neutrino line searches, which include the contribution from the GC region, into constraints on the thermally-averaged DM annihilation cross section $\langleσv\rangle$ in the presence of a DM spike. We then derive new bounds on the spike profile for a generic Weakly Interacting Massive Particle (WIMP) DM scenario, where the thermal freeze-out mechanism fixes the annihilation cross-section at $\langleσv\rangle\sim (2-3) \times 10^{-26}~{\rm cm}^3~{\rm s}^{-1}$. We find that for DM annihilation to photons, constraints from Fermi-LAT and MAGIC rule out spike profiles at the GC for a broad range of WIMP DM masses from 10 GeV to 100 TeV. Our result holds even if the photon channel constitutes only $1\%$ of the total annihilation rate. For the neutrino channel, we use the IceCube data to constrain the existence of an extremely steep spike in the $\mathscr{O}(1-10)$ TeV DM mass range. Our analysis can be easily extended to other annihilation channels.

Any observation of charged lepton flavor violation (CLFV) would be a clear signal of beyond-the-Standard-Model physics. Here, we propose a novel CLFV search using neutrino telescopes with their large cosmic-ray muon samples. Specifically, we use a recent IceCube cosmic-ray muon dataset and propose a new search for muon-to-tau conversion inside the IceCube detector. We illustrate our idea with CLFV interactions described by model-independent Effective Field Theory (EFT) operators and present the IceCube sensitivity on the relevant EFT scale. We also consider a specific realization of the EFT operator in terms of an axial-vector Z' interaction and show sensitivities in the Z' mass-coupling plane. We compare our sensitivities with those from low-energy CLFV searches, as well as from current and future collider experiments. We also show projections from next-generation neutrino telescopes, such as IceCube-Gen2 and HUNT, and demonstrate how neutrino telescopes can provide a powerful complementary probe of CLFV.

We propose a gauge-symmetry origin of a rank-two Dirac neutrino mass matrix that enforces one exactly massless neutrino, while being consistent with the oscillation data, as well as cosmological constraints. The mechanism relies on a minimal dark SU(2)_D gauge symmetry under which one right-handed-neutrino-like Weyl fermion is charged, thereby forbidding its Standard Model Yukawa coupling. Quantum consistency then fixes the minimal dark-sector completion: Cancellation of the Witten anomaly requires a second fermionic SU(2)_D doublet, while a discrete Z_4 symmetry that forbids Majorana masses allows the two dark doublets to form a vectorlike pair. This anomaly-free completion gives rise to a secluded, confining dark sector with a viable dark matter candidate, linking the protected neutrino texture to dark infrared dynamics.

We propose a boomerang mechanism for the explanation of the excess radio background detected by ARCADE 2. In an early stage of the Universe, at a temperature Tin the range similar to 0.1 keV-1 MeV, a fraction of relic neutrinos is resonantly converted into dark neutrinos by mixing induced by a preexisting lepton asymmetry. Dark neutrinos decay much later into a dark-standard photon state and a dark fermion, with a lifetime longer than the age of the Universe, as required by a solution to the excess radio background. This scenario circumvents the upper bound on the neutrino magnetic moment but still implies a testable lower bound.

We develop a general effective field theory (EFT) framework for neutrino-dark matter (DM) interactions, and apply it to systematically find all possible gauge-invariant ultraviolet (UV) completions at a given EFT operator dimension. Our goal here is to find simple UV-complete models that can realize potentially large neutrino-DM interactions, while being consistent with all existing theoretical and experimental constraints. We first construct the leading non-derivative operator basis for neutrino-DM scattering in a low-energy effective theory with neutrinos and DM (DM-LEFT), together with its gauge-invariant embedding in the Standard Model EFT (DM-SMEFT). We then construct all renormalizable tree-level UV completions that generate the relevant DM-SMEFT operators up to dimension-8 using a topology-based classification. Using this framework, we present minimal UV-complete models for different DM types that can yield effective neutrino-DM couplings up to several orders of magnitude larger than the Fermi coupling, while satisfying all constraints, most notably from neutrino mass and from the charged-lepton sector. This includes a pseudo-Dirac fermion DM realization in the scotogenic neutrino mass model and models of Majorana DM inspired by type-II and inverse seesaw-based neutrino mass models. Phenomenological implications for DM thermal relic abundance and direct detection prospects, as well as various cosmological and laboratory constraints on the model parameter space, are also analyzed.

Scalar non-standard interaction (SNSI) of neutrinos contributes as modifications to the neutrino mass matrix in the oscillation Hamiltonian and can induce a small active-sterile mass splitting due to the matter effect induced by the relic neutrino background via a Majorana-type interaction. This framework leads to pseudo-Dirac behavior of neutrinos, introducing rich phenomenology in neutrino oscillations, particularly for high-energy astrophysical neutrinos. We show that these hyperfine active-sterile splittings imprint themselves in two complementary ways on high-energy astrophysical neutrino flux, namely, in modifying the flavor composition and energy distribution. In this work, we perform both flavor and spectral analyses of the high-energy astrophysical neutrino flux to probe SNSI. We confront the predicted flavor ratios with current IceCube measurements and with the projected reach of next-generation detectors such as IceCube-Gen2. For the spectral analysis, we use the diffuse-flux ESTES (tracks) and cascade data sets, together with point-source spectral shape analysis based on a recent catalog of neutrino-bright sources. The regions excluded by the combined flavor and spectral analyses are translated into limits on the underlying SNSI parameters, namely, Yukawa couplings and scalar mass, providing new sensitivities on the SNSI parameter space for ultra-light mediators.

We discuss the role of heavy scalar fields in mediating neutrinoless double beta decay (0νββ) within the SU(5) Grand Unified Theory framework, extended suitably to include neutrino mass. In such a minimal realistic SU(5) setup for fermion masses, the scalar contributions to 0νββ are extremely suppressed as a consequence of the proton decay bound. We circumvent this problem by imposing a discrete 𝒵_3 symmetry. However, the scalar contributions to 0νββ remain suppressed in this SU(5)×𝒵_3 model due to the neutrino mass constraint. We find that the 0νββ contribution can be enhanced by extending the scalar sector with an additional 15-dimensional scalar representation with suitable 𝒵_3 charge. Such an extension not only yields realistic fermion mass spectra but also leads to experimentally testable predictions in upcoming ton-scale 0νββ searches, which can be used as a sensitive probe of the new scalars across a broad range, from LHC-accessible scales up to ∼ 1010 GeV.

We propose heavy decaying dark matter (DM) as a new probe of the cosmic neutrino background (CνB). Heavy DM, with mass ≳ 10^9 GeV, decaying into neutrinos can be a new source of ultrahigh-energy (UHE) neutrinos. Including this contribution along with the measured astrophysical and predicted cosmogenic neutrino fluxes, we study the scattering of UHE neutrinos with the CνB via standard weak interactions mediated by the Z boson. We solve the complete neutrino transport equation, taking into account both absorption and reinjection effects, to calculate the expected spectrum of UHE neutrino flux at future neutrino telescopes, such as the IceCube-Gen2 radio. We argue that such observations can be used to probe the CνB properties and, in particular, local CνB clustering. We find that, depending on the absolute neutrino mass and the DM mass and lifetime, a local CνB overdensity ≳ 10^6 can be probed at the IceCube-Gen2 radio within ten years of data taking.

High-energy neutrinos arise from processes at large center-of-mass energies, offering a window to test physics at comparable scales or beyond those accessible in collider experiments on Earth. Here, we present a recipe for extracting two-sided bounds on the inelastic pp and pγ cross sections from neutrino point-source data, by independently constraining every astrophysical input (cosmic-ray luminosities and target densities) through electromagnetic observations or theoretical arguments. The cross section is then the only remaining free parameter. Applying this framework to the IceCube associations with TXS 0506+056, NGC 1068, and the Galactic Plane, to a stacked population of eleven X-ray bright Seyfert galaxies, to the ultra-high-energy KM3NeT event KM3-230213A, and to projected observations of ultra-high-energy neutrinos, we obtain constraints that span center-of-mass energies from √(s)∼ 1 GeV to ∼ 10^5 GeV, some of which are well beyond the reach of the LHC and, for the pγ channel, beyond HERA. Several of these bounds are more stringent than unitarity limits.

It has been suggested that the density of dark matter (DM) halo can be highly enhanced around supermassive black holes at the centers of massive galaxies. If real, these DM spikes would offer new opportunities to probe the properties of DM. In this work, we point out that DM spikes can significantly impact the composition and survivability of ultrahigh-energy cosmic rays accelerated near supermassive black holes. A large DM-nucleon cross section would fragment heavy nuclei into lighter elements and prevent them from attaining the energies observed at Earth. While the origin of cosmic rays remains a mystery, we show that if the highest-energy cosmic rays on Earth come from sources like NGC 1068, then cross sections of size from setting robust limits, our study highlights an important connection between DM spikes and cosmic ray physics that is complementary to existing cosmological and direct detection constraints.

Heavy neutral leptons (HNLs) are well-motivated new physics candidates. The mixing of sub-GeV HNLs with active neutrinos is severely constrained by cosmology. In particular, the success of big bang nucleosynthesis (BBN) requires the HNL lifetime to be shorter than about 0.02 sec if they were in thermal equilibrium, thus excluding a wide range of mixing angles accessible to terrestrial experiments. In order to justify the laboratory searches in this cosmologically forbidden region, it is often argued that adding new dark sector decay modes of HNLs can evade the stringent BBN constraint. Here we rule out this possibility and show that, contrary to the naive expectation, HNLs with significant dark decay modes actually lead to stronger cosmological bounds. This is mainly because of the increase in the extra radiation energy density in the Universe around the BBN epoch, which causes observable effects in the primordial helium fraction and Delta Neff. Our result has major implications for laboratory searches of HNLs.

We reassess existing limits and derive new constraints on heavy axion-like particle (ALP) coupling to photons using gamma-ray bursts (GRBs). ALPs can be produced in the hot dense fireball plasma during the initial stage of GRB outflow, thus potentially disrupting the primary fireball and altering the GRB luminosity. We consider the ALP production rate for various GRB parameters in two different energy injection scenarios of GRB fireball formation, and point out that ALP production is less efficient than previously assumed unless a GRB event is exceptionally energetic. We update the existing energy loss bounds using more realistic GRB parameters. We also point out that in the region of parameter space previously constrained by GRB luminosity criterion, ALP production turns out to be still efficient enough to form a secondary fireball via ALP decay to two photons and their subsequent annihilation to electron-positron pair. This secondary fireball reprocesses the gamma-rays from heavy ALP decay into X-rays, emitted isotropically from its surface, thus allowing us to probe 𝒪(100 MeV)-scale ALPs indirectly using X-ray (or future MeV gamma-ray) telescopes, not necessarily directed toward the GRB jet itself. We show that the future point-source sensitivity of X-ray and MeV gamma-ray telescopes may allow us to constrain new ALP parameter space.

The neutrino sector is the least known in the Standard Model. We briefly review the open questions in neutrino physics and the future experimental prospects of answering them. We argue that a synergistic approach at multiple frontiers is needed.

New heavy gauge bosons appear in many beyond Standard Model (BSM) extensions. For a new SU(2) symmetry, the corresponding charged vector boson W^' can be either left handed or right handed. Therefore, it is crucial to devise some observables that can distinguish the pure left handed versus pure right handed case provided the heavy W^' discovered at the LHC in order to pinpoint the underlying gauge structure of the BSM. Here, we investigate the role of τ polarization in determining the chirality of W^' boson in the context of left right symmetric model. We analyze the purely leptonic mode of W^' where W^' decays to a final state τ lepton and neutrino. Assuming the existence of right handed heavy neutrino, we further analyze this process on the basis of Majorana neutrino decays. Here, we construct various observables and kinematical distributions to discriminate the chirality of W^' bosons as well as to understand the underlying gauge structure of BSM physics.

We derive new constraints on light CP-even scalars using old gamma-ray observations in the direction of SN1987A by the Solar Maximum Mission (SMM) satellite. Light scalars can be abundantly produced in the supernova core via the nucleon bremsstrahlung process, can stream out of the supernova-environment and decay into photons – either primary photons or secondary photons from lepton-antilepton pairs – thus leading to a gamma-ray signal. From the non-observation of excess photon flux by SMM after the detection of the neutrino burst from SN1987A, we set new constraints on the mixing angle of the CP-even scalar with the Standard Model Higgs boson.

The burgeoning field of multi-messenger astronomy is poised to revolutionize our understanding of the most enigmatic astrophysical phenomena in the Universe. At the same time, it has opened a new window of opportunity to probe various particle physics phenomena. This is illustrated here with a few example new physics scenarios, namely, decaying heavy dark matter, pseudo-Dirac neutrinos and light dark sector physics, for which new constraints are derived using recent multi-messenger observations.