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A bstract We formulate three-flavor type-I leptogenesis in the μτ basis which is convenient because in the three-flavor regime, both μ and τ charged lepton Yukawa interactions are in thermal equilibrium and the thermal bath is symmetric under the exchange μ ↔ τ . We apply this formalism to models with μτ -reflection CP μτ symmetry. We confirm the previous result that leptogenesis fails in the three-flavor regime with exact CP μτ symmetry. Allowing CP μτ symmetry to be broken to various degrees, we show that leptogenesis can be successful in the three-flavor regime only in certain tuned parameter space, which could further imply additional symmetry is at play. As a bonus, we derive analytical expressions which could be utilized whenever the branching ratios for the decays to μ and τ flavors are equal or approximately so.

A bstract We present a comprehensive study of the three-active plus N sterile neutrino model as a framework for constraining leptonic unitarity violation induced at energy scales much lower than the electroweak scale. We formulate a perturbation theory with expansion in small unitarity violating matrix element W while keeping (non- W suppressed) matter effect to all orders. We show that under the same condition of sterile state masses 0.1 eV 2 ≲ m J 2 ≲ (1–10) GeV 2 as in vacuum, assuming typical accelerator based long-baseline neutrino oscillation experiment, one can derive a very simple form of the oscillation probability which consists only of zeroth-order terms with the unique exception of probability leaking term C αβ of O ( W 4 ). We argue, based on our explicit computation to fourth-order in W , that all the other terms are negligibly small after taking into account the suppression due to the mass condition for sterile states, rendering the oscillation probability sterile - sector model independent . Then, we identify a limited energy region in which this suppression is evaded and the effects of order W 2 corrections may be observable. Its detection would provide another way, in addition to detecting C αβ , to distinguish between low-scale and high-scale unitarity violation. We also solve analytically the zeroth-order system in matter with uniform density to provide a basis for numerical evaluation of non-unitary neutrino evolution.

A bstract The parameter space of freeze-in dark matter (DM) with mass m χ through light dark photon (“minimal freeze-in DM”) is currently being probed by direct detection experiments through electron and nuclear recoil. Exploring the DM production in the mass range 10 − 2 MeV < m χ < 10 3 TeV, we quantify the impact of quantum statistics and the reheating dynamics (beyond the instantaneous reheating approximation) on the DM production in the early universe, in particular, the dependence on the cosmic equation of state and the scaling of the temperature of the Standard Model bath during reheating. Special cases corresponding to matter-domination and kination are carefully studied. To fit the entire observed DM relic abundance, low-temperature reheating scenarios require an increase in the coupling between dark and visible sectors which, in turn, enhances the regions of the parameter space that are already tested and will be probed by next-generation direct detection experiments for diverse reheating scenarios.

A bstract We show that electroweak triplet scalar can significantly impact baryogenesis via leptogenesis in concrete and predictive SO(10) GUTs, even when light neutrino masses arise predominantly from the type-I seesaw mechanism. This is illustrated within a minimal renormalisable SO(10) model with 10 and 126 ¯ scalars in the Yukawa sector and a global Peccei-Quinn-like symmetry. The quark-lepton unification and the flavour structure of the fundamental Yukawa couplings enforce type-I dominance in the light neutrino masses, also suppressing the triplet-induced CP asymmetries in the right-handed neutrino decays. However, the triplet’s own decays introduce a new CP-violating source, which can enhance or suppress the total baryon asymmetry. For triplet mass near the right-handed neutrino mass scale, this contribution can dominate, making it essential in assessing the viability of SO(10) leptogenesis scenarios.

A bstract If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus N (arbitrary positive integer) sterile neutrino model we show that by restricting the active-sterile and sterile-sterile neutrino mass squared differences to ≳ 0.1 eV 2 the oscillation probability in the (3 + N ) model becomes insensitive to details of the sterile sector, providing a nearly model-independent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes low-scale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in ν e -row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the (3 + N ) model due to matter effect is discussed to first order in the matter potential.

A bstract We present a renormalizable SO(10) grand unified theory with a minimal Yukawa sector consisting of a 126 H , a real 10 H and a real 120 H , where CP violation has a spontaneous origin. We show that the Yukawa sector in this setup, which consists of only 19 real parameters, is capable of simultaneously reproducing the observed fermion masses and mixings, including neutrino oscillations, as well as the baryon asymmetry of the Universe via thermal leptogenesis. In this framework, CP is spontaneously broken when a CP -odd Higgs field 54 H , used for GUT symmetry breaking, acquires a non-zero vacuum expectation value. The Yukawa sector of the model contains four independent phases which determine the Dirac phases δ CKM and δ PMNS , the neutrino Majorana phases, as well as those responsible for leptogenesis. We show that these four phases can arise from a single complex parameter in the Higgs potential involving the CP -odd Higgs field 54 H . The proposed minimal setup predicts a normal ordering of neutrino masses, with the atmospheric mixing angle θ 23 preferred in the first octant and the δ PMNS lying in the range (–37°, +31°). The fermion fits in our scenario further yield a strongly hierarchical mass spectrum for the three right-handed neutrinos, ( M 1 , M 2 , M 3 ) ∼ (10 5 , 10 12 , 5 · 10 14 ) GeV, which is shown to result in successful N 2 -dominated leptogenesis, consistent with current cosmological data.

A bstract Gravitational production of massive particles due to cosmic expansion can be significant during the inflationary and reheating period of the Universe. If the particle also has non-gravitational interactions that do not significantly affect its production, numerous observational probes open up, including cosmological probes. In this work, we focus on the gravitational production of light vector bosons that couple feebly to the Standard Model (SM) particles. Due to the very feeble coupling, the light vector bosons never reach thermal equilibrium, and if the Hubble scale at the end of inflation is above 10 8 GeV, the gravitational production can overwhelm the thermal production via the freeze-in mechanism by many orders of magnitude. As a result, much stronger constraints from the Big Bang Nucleosynthesis (BBN) can be placed on the lifetime and mass of the vector bosons compared to the scenario where only thermal production is considered. As an example, we study the sub-GeV scale dark photons, which couple to the SM only through kinetic mixing, and derive constraints on the mass and kinetic mixing parameter of the dark photon from the photodisintegration effects on the light element abundances relevant at the end of the BBN when the cosmic age was around 10 4 s.

A bstract In prior studies, a very minimal Yukawa sector within the SO(10) Grand Unified Theory framework has been identified, comprising of Higgs fields belonging to a real 10 H , a real 120 H , and a 126 ¯ H dimensional representations. In this work, within this minimal framework, we have obtained fits to fermion masses and mixings while successfully reproducing the cosmological baryon asymmetry via leptogenesis. The right-handed neutrino ( N i ) mass spectrum obtained from the fit is strongly hierarchical, suggesting that B − L asymmetry is dominantly produced from N 2 dynamics while N 1 is responsible for erasing the excess asymmetry. With this rather constrained Yukawa sector, fits are obtained both for normal and inverted ordered neutrino mass spectra, consistent with leptonic CP-violating phase δ CP indicated by global fits of neutrino oscillation data, while also satisfying the current limits from neutrinoless double beta decay experiments. In particular, the leptonic CP-violating phase has a preference to be in the range δ CP ≃ (230 – 300)°. We also show the consistency of the framework with gauge coupling unification and proton lifetime limits.

A bstract We present the possibility that the seesaw mechanism and nonthermal leptogenesis can be investigated via primordial non-Gaussianities in the context of a majoron curvaton model. Originating as a massless Nambu-Goldstone boson from the spontaneous breaking of the global baryon ( B ) minus lepton ( L ) number symmetry at a scale v B−L , majoron becomes massive when it couples to a new confining sector through anomaly. Acting as a curvaton, majoron produces the observed red-tilted curvature power spectrum without relying on any inflaton contribution, and its decay in the post-inflationary era gives rise to a nonthermal population of right-handed neutrinos that participate in leptogenesis. A distinctive feature of the mechanism is the generation of observable non-Gaussianity, in the parameter space where the red-tilted power spectrum and sufficient baryon asymmetry are produced. We find that the non-Gaussianity parameter f NL ≳ O (0 . 1) is produced for high-scale seesaw ( v B−L at O (10 14 − 17 ) GeV) and leptogenesis ( M 1 ≳ O (10 6 ) GeV) where the latter represents the lightest right-handed neutrino mass. While the current bounds on local non-Gaussianity excludes some part of parameter space, the rest can be fully probed by future experiments like CMB-S4, LSST, and 21 cm tomography.

A bstract We study the generation of the baryon asymmetry in Composite Higgs models with partial compositeness of the Standard Model (SM) fermions and heavy right-handed neutrinos, developing for the first time a complete picture of leptogenesis in that setup. The asymmetry is induced by the out of equilibrium decays of the heavy right-handed neutrinos into a plasma of the nearly conformal field theory (CFT), i.e. the deconfined phase of the Composite Higgs dynamics. This exotic mechanism, which we call Conformal Leptogenesis , admits a reliable description in terms of a set of “Boltzmann equations” whose coefficients can be expressed in terms of correlation functions of the CFT. The asymmetry thus generated is subsequently affected by the supercooling resulting from the confining phase transition of the strong Higgs sector as well as by the washout induced by the resonances formed after the transition. Nevertheless, a qualitative description of the latter effects suggests that conformal leptogenesis can successfully reproduce the observed baryon asymmetry in a wide region of parameter space. A distinctive signature of our scenarios is a sizable compositeness for all the generations of SM neutrinos, which is currently consistent with all constraints but may be within reach of future colliders.