Search

As a consequence of the LSND anomaly and other hints of an eV scale sterile neutrino from particle physics and cosmology, the neutrino sector of the standard model of particle physics has to be extended and the smallest extension is the (3 + 1) model, i.e., three active neutrinos plus one sterile one. In this work we study the neutrino mass matrix Mv with three texture zeros in the (3+1) model, assuming all the neutrino states are of Majorana type. With the mass hierarchy between the active and sterile neutrinos as well as the smallness of the reactor neutrino mixing and active-sterile mixing, the analytical expressions can be greatly simplified. We systematically examine all the 120 texture zeros, via both analytical and numerical analysis, and find that the 100 textures with zeros in the fourth column and row of Mv can hardly be compatible with experiments; whereas, 19 of the other 20 are feasible. We can foresee that the active neutrino masses are required to be around 0.01 eV and the next generation neutrinoless double beta decay experiments are promising to test these textures. Many other phenomenologically meaningful predictions are also obtained. All the CP conserving (3+1) textures with three zeros are excluded by experiments. [ABSTRACT FROM AUTHOR]

The DAMPE e+e- excess at around 1.4 TeV could be explained in the type-II seesaw model with a scalar dark mater D which is stabilized by a discrete Z2 symmetry. The simplest scenario is the annihilation DD→H++H-- followed by the subsequent decay H±±→e±e±, with both the DM and triplet scalars roughly 3 TeV with a small mass splitting. In addition to the Drell-Yan process at future 100 TeV hadron colliders, the doubly charged components could also be produced at lepton colliders like ILC and CLIC in the off shell mode and mediate lepton flavor violating processes e+e-→ℓ±iℓ∓j (with i≠j). A wide range of parameter space of the type-II seesaw could be probed, which are well below the current stringent lepton flavor constraints. [ABSTRACT FROM AUTHOR]

Recently, the KOTO experiment at J-PARC has observed three anomalous events in the flavor-changing rare decay KL→π0ν̅ν, which indicates that the corresponding branching ratio is almost 2 orders of magnitude larger than the Standard Model (SM) prediction. Taking this intriguing result at face value, we explore model implications of its viable explanation by a long-lived light SM-singlet scalar (S) emission, i.e., KL→π0S, with S decaying outside the KOTO detector. We derive constraints on the parameter space of such a light scalar in the context of three simple models: (i) a real singlet scalar extension of the SM; (ii) a B-L extension where neutrino masses arise via type-I seesaw mechanism from B-L breaking; and (iii) a TeV-scale left-right symmetric model. The flavor-changing couplings needed to explain the KOTO excess in models (i) and (ii) originate from tree-level mixing of the scalar with SM Higgs field (h), and in model (iii), from the mixing of S and h with the neutral component of the heavy bidoublet Higgs field. After taking into account the stringent constraints from high-precision searches for flavor-changing charged and neutral kaon decays at NA62, E949, KOTO and CHARM experiments, as well as the astrophysical and cosmological constraints on a light scalar, such as those from supernova energy loss, big bang nucleosynthesis and relativistic degrees of freedom, we find that the light scalar interpretation of the KOTO excess is allowed in all these models. Parts of the parameter range can be tested in future NA62 and DUNE experiments. [ABSTRACT FROM AUTHOR]

We point out how future lepton colliders can provide unique insight into the scalar sector of TeV scale models for neutrino masses with local B-L symmetry. Our specific focus is on the TeV scale left-right model, which naturally embeds this B-L symmetry. In particular, we make a detailed study of the lepton collider implications of the neutral (H3) and doubly-charged (H±±) scalars from the right-handed triplet Higgs that is responsible for the spontaneous breaking of the B-L symmetry and implementing the seesaw mechanism. Due to mixing with other scalars, the neutral scalar H3 could acquire sizable flavor violating couplings to the charged leptons. Produced on-shell or off-shell at the planned e+e- colliders, it would induce distinct lepton flavor violating signals like e+e-→μ±τ∓(+H3), with the couplings probed up to ∼10-4 for a wide range of neutral scalar mass, which is well beyond the reach of current searches for charged lepton flavor violation. The Yukawa couplings of the doubly-charged scalar H±± to the charged leptons might also be flavor violating, which is correlated to the heavy right-handed neutrino masses and mixings. With a combination of the pair, single and off-shell production of H±± like e+e-→H++H--, H±±e∓μ∓, μ±τ∓, the Yukawa couplings can be probed up to 10-3 at future lepton colliders, which is allowed by current lepton flavor data in a large region of parameter space. For both the neutral and doubly-charged cases, the scalar masses could be probed up to the few-TeV range in the off-shell channel. [ABSTRACT FROM AUTHOR]

We analyze the sensitivity of low-energy fundamental symmetry tests to interactions mediated by doubly-charged scalars that arise in type II seesaw models of neutrino mass and their left-right symmetric extensions. We focus on the next generation measurement of the parity-violating asymmetry in Møller scattering planned by the MOLLER collaboration at Jefferson Laboratory. We compare the MOLLER sensitivity to that of searches for charged lepton flavor violation (CLFV) and neutrinoless double beta-decay (0νββ-decay) as well as present and possible future high-energy collider probes. We show that for the simplest type-II seesaw scenario, CLFV searches have the greatest sensitivity. However, in a left-right symmetric extension where the scale of parity-breaking is decoupled from the SU(2)R-breaking scale, the MOLLER experiment will provide a unique probe of scalar triplet interactions in the right-handed sector for a doubly-charged scalar mass up to ~10 TeV and help elucidate the mechanism of 0νββ-decay. [ABSTRACT FROM AUTHOR]

In a class of new physics models, an extended Higgs sector and new CP-violating sources are simultaneously present in order to explain the baryon asymmetry in the Universe. The aim of this work is to study the implications of beyond the Standard Model (SM) CP violation for the searches of heavy scalars at the LHC. In particular, we focus on the diphoton channel searches in the CP -violating two-Higgs-doublet model (CPV 2HDM). To have a sizable CPV in the scalar sector, the two heavy neutral scalars in 2HDM tend to be nearly degenerate. The theoretical constraints of unitarity, perturbativity and vacuum stability are considered, which requires that the heavy scalars MH ≲ 1 TeV in a large region of the parameter space. The experimental limits are also taken into account, including the direct searches for heavy neutral scalars in the final state of the SM h, W and Z bosons; the differential tt data; those from the charged scalar sector which is implied by the oblique T parameter; and the precise measurements of the electric dipole moments of electrons and mercury. The quantum interference effects between the resonances and the SM background are crucially important for the diphoton signals, and the CPV mixing of the quasidegenerate heavy scalars could enhance significantly the resonance peak. With an integrated luminosity of 3000 fb-1 at the LHC, almost the whole parameter space of CPV 2HDM could be probed in the diphoton channel, and the CPV could also be directly detected via the diphoton spectrum. [ABSTRACT FROM AUTHOR]

We point out that in the minimal left-right realization of TeV scale seesaw for neutrino masses, the neutral scalar from the right-handed S U (2) R breaking sector could be much lighter than the right-handed scale. We discuss for the first time the constraints on this particle from low-energy flavor observables and find that the light scalar is necessarily long-lived. We show that it can be searched for at the LHC via displaced signals of a collimated photon jet and can also be tested in current and future high-intensity experiments. In contrast to the unique diphoton signal (and associated jets) in the left-right case, a generic beyond-standard-model light scalar decays mostly to leptons or jets. Thus, the diphoton channel proposed here provides a new avenue to test the left-right framework and reveal the underlying neutrino mass generation mechanism. [ABSTRACT FROM AUTHOR]

We point out that light gauge boson mediators could induce new interference effects in neutrino-electron scattering that can be used to enhance the sensitivity of neutrino-flavor-selective high-intensity neutrino experiments, such as Deep Underground Neutrino Experiment (DUNE). We particularly emphasize a destructive interference effect, leading to a deficit between the Standard Model expectation and the experimental measurement of the differential cross sections, which is prominent only in either the neutrino or the antineutrino mode, depending on the mediator couplings. Therefore, the individual neutrino (or antineutrino) mode could allow for sensitivity reaches superior to the combined analysis and, moreover, could distinguish between different types of gauge boson mediators. [ABSTRACT FROM AUTHOR]

There are broadly three channels to probe axionlike particles (ALPs) produced in the laboratory: through their subsequent decay to Standard Model (SM) particles, their scattering with SM particles, or their subsequent conversion to photons. Decay and scattering are the most commonly explored channels in beam-dump type experiments, while conversion has typically been utilized by light-shining-through-wall (LSW) experiments. A new class of experiments, dubbed PASSAT (particle accelerator helioscopes for slim axionlike-particle detection), has been proposed to make use of the ALP-to-photon conversion in a novel way: ALPs, after being produced in a beam-dump setup, turn into photons in a magnetic field placed near the source. It has been shown that such hybrid beam-dump-helioscope experiments can probe regions of parameter space that have not been investigated by other laboratory-based experiments, hence providing complementary information; in particular, they probe a fundamentally different region than decay or LSW experiments. We propose the implementation of PASSAT in future neutrino experiments, taking a DUNE-like experiment as an example. We demonstrate that the magnetic field in the planned DUNE multipurpose detector is already capable of probing the ALP-photon coupling down to gaγγ∼few×10-5  GeV-1 for ALP masses ma≲10  eV. The implementation of a CAST or BabyIAXO-like magnet would improve the sensitivity down to gaγγ∼10-6  GeV-1. [ABSTRACT FROM AUTHOR]

We propose a new mechanism for generating matter-antimatter asymmetry via the interference of tree-level diagrams only, where the imaginary part of the Breit-Wigner propagator for an unstable mediator plays a crucial role. We first derive a general result that a nonzero CP-asymmetry can be generated via at least two sets of interfering tree-level diagrams involving either 2→2 or 1→n (with n≥3) processes. We illustrate this point in a simple TeV-scale extension of the Standard Model with an inert Higgs doublet and right-handed neutrinos, along with an electroweak-triplet scalar field, where small Majorana neutrino masses are generated via a combination of radiative type-I and tree-level type-II seesaw mechanisms. The imaginary part needed for the required CP-asymmetry comes from the trilinear coupling of the inert doublet with the triplet scalar, along with the width of the triplet scalar mediator. The real part of the neutral component of the inert doublet serves as a cold dark matter candidate. The evolutions of the dark matter relic density and the baryon asymmetry are intimately related in this scenario. [ABSTRACT FROM AUTHOR]