Search

To explain the baryon asymmetry in the early universe via leptogenesis, quantum corrections to new particles are commonly invoked to generate the necessary CP asymmetry. We demonstrate, however, that a large CP asymmetry can already arise from Standard Model leptons. The mechanism relies on resummation of soft leptons at finite temperatures. The CP asymmetry, which is kinetically forbidden in vacuum, can be resonantly enhanced from thermally resummed leptons by seven orders of magnitude. Contrary to the resonance from exotic particles, we show that the resonant enhancement from soft leptons is protected by controlled widths under finite-temperature perturbation theory. We quantify such CP asymmetries in leptogenesis with secluded flavor effects and comment on the significance and application. The mechanism exploits the maximal role of leptons themselves, featuring low-scale leptogenesis, minimal model buildings and dark matter cogenesis., Comment: 7+10 pages, 3+2 figures, comments are welcome

Even after dark matter chemically freezes out in the early universe, electromagnetic cascades from dark matter annihilation can still perturb the background photon spectrum when the universe temperature cools down to 0.5 keV. We revisit the CMB spectrum distortions caused by $s$-wave dark matter annihilation under the updated Planck data and the future CMB sensitivity, concluding that $s$-wave annihilation cannot create observable distortions under forecast sensitivities of the (Super-)PIXIE missions. We further detail the case of $p$-wave dark matter annihilation, demonstrating the observability of the primordial $\mu$-distortion. Taking current constraints from primordial light elements, structure formations, cosmic electron-positron rays, and gamma rays, we find that the $\mu$-distortion reaching the observational limit as large as $\mu\simeq 3\times 10^{-8}$ can only be realized with a dark matter mass at 10--50 MeV and a kinetic decoupling temperature around 1 keV. The upper bound of the $p$-wave annihilation cross section can be strengthened by an order of magnitude if the $\mu$-distortion is not detected., Comment: 28 pages, 3 figures, version accepted by JCAP

A singlet majoron can arise from the seesaw framework as a pseudo-Goldstone boson when the heavy Majorana neutrinos acquire masses via the spontaneous breaking of global ${\rm U}(1)_L$ symmetry. The resulting cosmological impacts are usually derived from the effective majoron-neutrino interaction, and the majoron abundance is accumulated through the freeze-in neutrino coalescence. However, a primordial majoron abundance can be predicted in a minimal setup and lead to distinctive cosmological effects. In this work, we consider such a primordial majoron abundance from relativistic freeze-out and calculate the modification to the effective neutrino number $N_{\rm eff}$. We demonstrate that the measurements of $N_{\rm eff}$ will constrain the parameter space from a primordial majoron abundance in an opposite direction to that from neutrino coalescence. When the contributions from both the primordial abundance and the freeze-in production coexist, the ${\rm U}(1)_L$-breaking scale (seesaw scale) $f$ will be pushed into a ''sandwiched window''. Remarkably, for majoron masses below 1 MeV and above the eV scale, the future CMB-S4 experiment will completely close such a low-scale seesaw window for $f\in [1,10^5]~{\rm GeV}$. We highlight that any new light particle with a primordial abundance that couples to SM particles may lead to a similar sandwiched window, and such a general phenomenon deserves careful investigation., Comment: v2: 32 pages, 6 figures, 1 table, more discussions and references added, version accepted by JCAP

A dark sector is an interesting place where a strong first-order phase transition, observable gravitational waves and/or a dark matter candidate could arise. However, the experimental tests for such a dark sector could be ambiguous due to the dark content, largely unconstrained parameter space and the connection to the visible world. We consider a minimal dark scalar-vector boson plasma to realize the three mentioned phenomena, with a unique connection to the Standard Model via the Higgs portal coupling. We discuss the important features of the Higgs portal in such a minimal dark sector, namely the dark thermalization, collider tests, and direct detection of dark matter. We perform numerical analyses of the dark phase transition associated with stochastic gravitational waves and dark matter, discussing the complementarity of collider detection, dark matter direct detection and space-based/terrestrial interferometers as a promising avenue to hear and see the minimal dark sector., Comment: 33 pages, 5 figures and 1 table. Version accepted by JCAP

We investigate the impact of new light particles, carrying significant energy in the early universe after neutrino decoupling, on the cosmological effective relativistic neutrino species, $N_{{\rm eff}}$. If the light particles are produced from decoupled neutrinos, $N_{{\rm eff}}$ is predominantly modified through the dilution-resistant effect. This effect arises because the energy stored in the mass of new particles is less diluted than the photon and neutrino energy as the universe expands. Our study comprehensively explores this effect, deriving $N_{{\rm eff}}$ constraints on the couplings of light mediators with neutrinos, encompassing both scalar and vector mediators. We find that the dilution-resistant effect can increase $N_{{\rm eff}}$ by 0.118 and 0.242 for scalar and vector mediators, respectively. These values can be readily reached by forthcoming CMB experiments. Upon reaching these levels, future $N_{{\rm eff}}$ constraints on the couplings will be improved by many orders of magnitude., Comment: 25 pages, 6 figures and 2 tables, version accepted by JHEP

A cosmic first-order phase transition (FOPT) occurring at MeV-scale provides an attractive explanation for the nano-Hertz gravitational wave (GW) background indicated by the recent pulsar timing array data from the NANOGrav, CPTA, EPTA and PPTA collaborations. We propose this explanation can be further tested at the colliders if the hidden sector couples to the Standard Model sector via Higgs portal. Through a careful analysis of the thermal history of the hidden sector, we demonstrate that in order to successfully explain the observed GW signal, the portal coupling must be sizable that it can be probed through Higgs invisible decay at the LHC or future lepton colliders such as CEPC, ILC, and FCC-ee. Our research offers a promising avenue to uncover the physical origin of the nano-Hertz GWs through particle physics experiments., Comment: 6 pages + 2 figures + appendix + references

Light weakly interacting particles could be copiously produced in the Sun which, as a well-understood star, could provide severe constraints on such new physics. In this work, we calculate the solar production rates of light gauge bosons (e.g. dark photon) arising from various $U(1)$ extensions of the standard model. It is known that the dark photon production rate is suppressed by the dark photon mass if it is well below the plasmon mass of the medium. We show that for more general $U(1)$ gauge bosons, this suppression is absent if the couplings are not in alignment with those of the photon. We investigate a few frequently discussed $U(1)$ models including $B-L$, $L_{\mu}-L_{\tau}$, and $L_{e}-L_{\mu(\tau)}$, and derive the stellar cooling bounds for these models., Comment: 24 pages, 4 figures. Added additional bounds and astrophysical uncertainties, accepted by JCAP

The connection between a hidden nonthermal sector and a thermal plasma can be established by a light thermal fermion mediator. When the fermion mediator is much lighter than the hidden species, kinematically forbidden decay of the mediator can be opened at finite temperatures to produce the hidden species. Unlike bosons having quartic couplings, renormalizable forbidden fermion decay generically shares the same order of couplings with the scattering. We present a dedicated investigation into the freeze-in dark matter production via a thermal fermion mediator. We demonstrate that the plasma-induced decay rate differs from that calculated via the tree-level amplitude, but the former can be obtained from the latter via constant rescaling. Furthermore, we find that the relative effect of the forbidden decay and the scattering on the dark matter relic density can be simply estimated via the thermal coupling between the plasma and the mediator. Applying to different thermal interactions, we show that the forbidden decay contribution can reach the level of $4\%-45\%$ for a thermal coupling at $0.1-1$., Comment: 29 pages, 6 figures, 1 table, 1 appendix, version accepted for publication in JCAP

Right-handed neutrinos (RHNs) provide a natural portal to a dark sector accommodating dark matter (DM). In this work, we consider that the dark sector is connected to the standard model only via RHNs and ask how DM can be produced from RHNs. Our framework concentrates on a rather simple and generic interaction that couples RHNs to a pair of dark particles. Depending on whether RHNs are light or heavy in comparison to the dark sector and also on whether one or both of them are in the freeze-in/out regime, there are many distinct scenarios resulting in rather different results. We conduct a comprehensive and systematic study of all possible scenarios in this paper. For illustration, we apply our generic results to the type-I seesaw model with the dark sector extension, addressing whether and when DM in this model can be in the freeze-in or freeze-out regime. Some observational consequences in this framework are also discussed., Comment: 33 pages, 5 figures; calculations on non-thermal spectra added; matching the JCAP published version

It is generically believed that the two-body scattering is suppressed by higher-order weak couplings with respect to the two-body decay. We show that this does not always hold when a heavy particle is produced by forbidden decay in a thermal plasma, where the scattering shares the same order of couplings with the decay. We find that there is a simple and close relation between the forbidden decay and the same-order scattering. To illustrate this point, we consider freeze-in production of heavy dark matter via a light scalar mediator. We point out that, when the Boltzmann (quantum) statistics is used, the forbidden decay can contribute to the dark matter relic density at 5$\%$-24$\%$ (10$\%$-$39\%$) with a weak thermal coupling, while the contribution from the scattering channel can be several orders of magnitude larger than from the forbidden decay if the thermal coupling is much smaller. Such a relative effect between the scattering and the forbidden decay could also exist in other plasma-induced processes, such as the purely thermal generation of the right-handed neutrino dark matter, or of the lepton asymmetry in leptogenesis., Comment: 8 pages, 3 figures, version accepted for publication in PRD

In the early universe, Dirac neutrino magnetic moments due to their chirality-flipping nature could lead to thermal production of right-handed neutrinos, which would make a significant contribution to the effective neutrino number, $N_{\rm eff}$. We present in this paper a dedicated computation of the neutrino chirality-flipping rate in the thermal plasma. With a careful and consistent treatment of soft scattering and the plasmon effect in finite temperature field theories, we find that neutrino magnetic moments above $2.7\times 10^{-12}\mu_B$ have been excluded by current CMB and BBN measurements of $N_{\rm eff}$, assuming flavor-universal and diagonal magnetic moments for all three generation of neutrinos. This limit is stronger than the latest bounds from XENONnT and LUX-ZEPLIN experiments, and comparable with those from stellar cooling considerations., Comment: 23 pages, 6 figures, 2 tables. An appendix added to demonstrate the consistency with early results

If the leptoquarks proposed to account for the intriguing anomalies observed in the $B$-meson decays, $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$, as well as in the anomalous magnetic moment of the muon, $(g-2)_\mu$, can be embedded into the scotogenic Dirac neutrino model, all these flavor anomalies, together with the origin of neutrino masses and the nature of dark matter, would be potentially addressed in a unified picture. Among the minimal seesaw, one-loop, and two-loop realizations of the dimension-4 effective operator $\mathcal{L}_{4}$ for the Dirac neutrino masses, we show that plenty of diagrams associated with the two-loop realizations of $\mathcal{L}_{4}$ can support the coexistence of leptoquarks and dark matter candidates. After a simple match of these leptoquarks to those that can accommodate all the flavor anomalies, we establish the scotogenic Dirac neutrino models embedded with leptoquarks, which could address all the problems mentioned above., Comment: 15 pages, 16 tables, 9 figures

If the Dirac neutrino masses are generated by a new scalar doublet with a vacuum at keV order, there would be rare hope to probe the framework in low-energy flavor physics, such as the lepton-flavor violating processes. However, the predicted neutrino Yukawa couplings being around the order of electronic Yukawa can realize a purely thermal Dirac leptogenesis, and thus render a more direct link between the high- and the low-energy CP violation. Despite its inert property in the low-energy flavor processes, the keV-vacuum induced Dirac neutrino model can generate a significant shift of the effective neutrino number, which can in turn be probed by the big-bang nucleosynthesis and cosmic microwave background epochs. We show that such a keV-vacuum induced Dirac neutrino model is already constrained by the current observations and will be probed with forecast sensitivity, serving therefore as a complementary avenue to test the Dirac leptogenesis., Comment: 8 pages, and 3 figures

Four next-to-tribimaximal (NTBM) mixing patterns are widely considered as the feasible candidates for the observed leptonic mixing structure. With the recent measurements from T2K, as well as intensive global fits, the interval of $\sin\delta_{ CP} > 0$ for the Dirac CP-violating phase is persistently small in the normal ordering while $\sin\delta_{ CP} < 0$ is successively obtained up to $3\sigma$ level in the inverted ordering. In this paper, we advocate the fitting results of Dirac CP-violating phase as a constraint, and show that, it can basically rule out half of the regions allowed by the constraint from three mixing angles only. In addition, given the small $\sin\delta_{ CP} > 0$ interval, we find that a unique NTBM pattern can be selected out of the four candidates within $3\sigma$ uncertainty of the latest NuFIT 5.1, when the patterns are exposed to a Yukawa texture-independent leptogenesis for the baryon asymmetry of the Universe. For the surviving pattern, we find an interesting correlation between the Dirac CP-violating phase and the octant of atmospheric angle $\theta_{23}$ , where $\theta_{23}>45^\circ$ can be predicted once $5\pi/6 < \delta_{CP} <\pi$ in the normal ordering can be confirmed in the future measurements., Comment: 8 pages, 4 figures, Published version in PRD

The updated measurements from the LHCb and SH0ES collaborations have respectively strengthened the deviations of the ratio $R_{K}$ in rare semi-leptonic $B$-meson decays and the present-day Hubble parameter $H_0$ in the Universe, implying tantalizing hints of new physics beyond the Standard Model. In this paper, we consider a simple flavor-specific two-Higgs-doublet model, where the long-standing $R_{K^{(*)}}$ anomalies can be addressed by a one-flavor right-handed neutrino. One of the intriguing predictions resulting from the parameter space for the $R_{K^{(*)}}$ resolution under flavor- and collider-physics constraints points toward a shift of the effective neutrino number, $\Delta N_{\rm eff}=N_{\rm eff}-N_{\rm eff}^{\rm SM}$, as favored to ease the $H_0$ tension. Depending on whether the neutrino is of Dirac or of Majorana type, we show that the resulting shift is $\Delta N_{\rm eff}\simeq 1.0$ for the former and $\Delta N_{\rm eff}\simeq 0.5$ for the latter case, respectively. While the Dirac case is disfavored by the CMB polarization measurements, the Majorana solution is consistent with the recent studies via a combined data set from various sources. Consequently, such a simple flavor-specific two-Higgs-doublet model provides a link between the $R_{K^{(*)}}$ anomalies and the $H_0$ tension, which in turn can be readily verified or falsified by the upcoming measurements., Comment: 31 pages, 6 figures; more discussions and references added, final version published in the journal

We provide a simple analysis of the big-bang nucleosynthesis (BBN) sensitivity to the light dark matter (DM) generated by the thermal freeze-in mechanism. It is shown that the ratio of the effective neutrino number shift $\Delta N_{\nu}$ over the DM relic density $\omega\equiv \Omega h^2$, denoted by $R_\chi\equiv\Delta N_\nu/\omega$, cancels the decaying particle mass and the feeble coupling, rendering therefore a simple visualization of $\Delta N_{\nu}$ at the BBN epoch in terms of the DM mass. This property drives one to conclude that the shift with a sensitivity of $\Delta N_{\nu}\simeq \mathcal{O}(0.1)$ cannot originate from a single warm DM under the Lyman-$\alpha$ forest constraints. For the cold-plus-warm DM scenarios where the Lyman-$\alpha$ constraints are diluted, the ratio $R_\chi$ can be potentially used to test the thermal freeze-in mechanism in generating a small warm component of DM and a possible excess at the level of $\Delta N_{\nu}\simeq \mathcal{O}(0.01)$., Comment: 7 pages, 1 figure; more discussions and references added, final version published in the journal

We present here a novel and testable mechanism for leptogenesis, which is characterized by a purely thermal generation of lepton asymmetry via scalar decay. Guided by the thermal mass matrix diagonalization in the finite-temperature regime, we propose a scenario in which the baryon asymmetry is formulated in terms of the masses and mixing from hierarchical Dirac neutrinos, as well as a vacuum scale accountable for the sub-eV neutrino masses. This allows a natural and direct link between the high-scale \textit{CP} asymmetry and the low-energy Dirac \textit{CP}-violating phase without involved model structures, and thus circumvents the haunting problem encountered in general leptogenesis scenarios. The mechanism can also be applied as a prototype for solving the baryon asymmetry problem to a broad class of well-motivated model buildings., Comment: 6 pages, 2 figures; more discussions and references added, final version published in the journal

A classically scale-invariant scalar singlet can be a MeV-scale dark matter, with a feeble Higgs portal coupling at $\mathcal{O}(10^{-10})$. Besides, an $\mathcal{O}(0.1)$ self-interaction coupling could further serve to alleviate the small-scale problems in the Universe. We show that, such a dark matter candidate can naturally arise in the warped extra dimension, with the huge span of parameter space predicted well within $\mathcal{O}(1)$ fundamental parameters., Comment: 6 pages, 2 figures, v3:added explanations, matches version published in PLB

With the hypothesis of minimal flavor violation, we find that there exists a power-aligned relation between the Yukawa couplings of the two scalar doublets in the two-Higgs-doublet model with Hermitian Yukawa matrices. Within such a power-aligned framework, it is found that a simultaneous explanation of the anomalies observed in the electron and muon anomalous magnetic moments can be reached with TeV-scale quasi-degenerate Higgs masses, and the resulting parameter space is also phenomenologically safer under the B-physics, $Z$ and $\tau$ decay data, as well as the current LHC bounds. Furthermore, the flavor-universal power that enhances the charged-lepton Yukawa couplings prompts an interesting correlation between the two anomalies, which makes the model distinguishable from the (generalized) linearly aligned and the lepton-specific two-Higgs-doublet models that address the same anomalies but in a non-correlative manner, and hence testable by future precise measurements., Comment: v1: 23 pages, 2 figures; v2: more discussions and references added, final version published in the journal; v3: figure 2 updated by including the latest Fermilab Muon g-2 result, and the lower bound on the charged-Higgs mass changed from 1.4TeV to 1.7TeV, conclusion unchanged

We present a freeze-in realization of the Dirac neutrinogenesis in which the decaying particle that generates the lepton-number asymmetry is in thermal equilibrium. As the right-handed Dirac neutrinos are produced non-thermally, the lepton-number asymmetry is accumulated and partially converted to the baryon-number asymmetry via the rapid sphaleron transitions. The necessary CP-violating condition can be fulfilled by a purely thermal kinetic phase from the wavefunction correction in the lepton-doublet sector, which has been neglected in most leptogenesis-based setup. Furthermore, this condition necessitates a preferred flavor basis in which both the charged-lepton and neutrino Yukawa matrices are non-diagonal. To protect such a proper Yukawa structure from the basis transformations in flavor space prior to the electroweak gauge symmetry breaking, we can resort to a plethora of model buildings aimed at deciphering the non-trivial Yukawa structures. Interestingly, based on the well-known tri-bimaximal mixing with a minimal correction from the charged-lepton or neutrino sector, we find that a simultaneous explanation of the baryon-number asymmetry in the Universe and the low-energy neutrino oscillation observables can be attributed to the mixing angle and the CP-violating phase introduced in the minimal correction., Comment: 28 pages and 7 figures; more discussions and one figure added, final version published in the journal

We implement a symmetry violation guideline into a two-Higgs-doublet model embedded with three right-handed neutrinos, and exploit the generic Yukawa structures of the model via a hypothetical symmetry restoration of a global $U_Q(1)^3\times U_L(1)^3$ symmetry. We then apply a mass-powered parametrization to construct the phenomenologically motivated Yukawa interactions, which enables us to incorporate correlatively the neutrino mass, dark matter, as well as the lepton-flavor universality violations in $R_{K^{(*)}}$ and $R_{D^{(*)}}$. Specifically, two atmospheric-scale neutrino masses are generated by a low-scale seesaw mechanism, while the much lighter one, being of $\mathcal{O}(10^{-6})~{\rm eV}$, is fixed by a $7.1~{\rm keV}$ sterile neutrino dark matter produced primordially by the freeze-in mechanism. On the other hand, the neutrino and the charged-lepton mass hierarchies encoded in the mass-powered textures can naturally account for the $R_{K^{(*)}}$ and the $R_{D^{(*)}}$ anomalies, respectively. As a further application, a milder discrepancy of the muon $g-2$ is observed, which has also been implied by the recently refined lattice results., Comment: 9 pages, 3 figures; formulation of the model refined, and several references added, final version published in the journal

We show in this paper that, in a $U(1)$-symmetric two-Higgs-doublet model (2HDM), the two additional neutral Higgs bosons would become nearly degenerate in the large $\tan\beta$ regime, under the combined constraints from both theoretical arguments and experimental measurements. As a consequence, the excess observed in the anomalous magnetic moment of the muon could not be addressed in the considered framework, following the usual argument where these two neutral scalars are required to manifest a large mass hierarchy. On the other hand, we find that, with an $\mathcal{O}(1)$ top-Yukawa coupling and a relatively light charged Higgs boson, large contributions from the two-loop Barr-Zee type diagrams can account for the muon $g-2$ anomaly at the 1$\sigma$ level, in spite of a large cancellation between the scalar and pseudoscalar contributions. Furthermore, the same scenario can survive the tight constraints from the $B$-physics observables., Comment: v1:10 pages, 4 figures; v2: more discussions and references added, some references updated, final version published in PRD; v3:plots and numerical analyses updated by including the latest Fermilab Muon g-2 result as well the updated oblique parameters and the B-meson mixing observable, conclusion unchanged

Given that the two-Higgs-doublet model of type III (2HDM-III) has the potential to address the $R_{D^{(\ast)}}$ anomalies while the resolution to the $R_{K^{(\ast)}}$ deficits requires new degrees of freedom within this framework, we consider in this paper a unified scenario where the low-scale type-I seesaw mechanism is embedded into the 2HDM-III, so as to accommodate the $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$ anomalies as well as the neutrino mass. We first revisit the $R_{D^{(\ast)}}$ anomalies and find that the current world-averaged results can be addressed at $2\sigma$ level without violating the bound from the branching ratio $\mathcal{B}(B_c^-\rightarrow \tau^- \bar{\nu})\leqslant 30\%$. The scenario predicts two sub-eV neutrino masses based on a decoupled heavy Majorana neutrino and two nearly degenerate Majorana neutrinos with mass around the electroweak scale. For the $R_{K^{(\ast)}}$ anomalies, the same scenario can generate the required Wilson coefficients in the direction $ C_{9 \mu}^{\rm NP}=-C_{10 \mu}^{\rm NP}<0$, with $\mathcal{O}(1)$ Yukawa couplings for the muon and the top quark., Comment: 32 pages, 4 figures; More discussions and references added, final version to be published in JHEP