arXiv subjects

Pasquale Di Bari

Publications and source records attributed to Pasquale Di Bari.

At least 19 recordsLinked to original sources

arXiv2026

Non-adiabatic transitions in the density matrix formalism

We show that a density matrix formalism provides a useful description of non-adiabatic transitions in two-state quantum systems. Compared to a traditional Hamiltonian formalism, even in the absence of decoherence when there is full equivalence between the two, the density matrix formalism provides a convenient change of variables that yields a powerful general analytical solution. This solution nicely describes a transition regime between the well known Landau-Zener-Stuckelberg-Majorana (LZSM) approximation and the extremely non-adiabatic limit. Our results have very general applications, within a large variety of problems in quantum physics, neutrino physics, cosmology.

quant-ph↗

arXiv2026

Boomerang mechanism explaining the excess radio background

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 $T$ in the range $\sim 0.1\,{\rm keV}$--$1\,{\rm MeV}$, a fraction of relic neutrinos is resonantly converted into dark neutrinos by mixing induced by a pre-existing 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.

hep-ph↗

arXiv2026

How neutrinos could help solving cosmological anomalies and tensions

In this talk I discuss how neutrinos might help solving or alleviating different anomalies and tensions in cosmology. Invisible decays of the heaviest relic neutrinos might provide a way to solve the neutrino mass tension between cosmological observations and neutrino oscillation experiments. The excess radio background mystery could be explained by radiative decays of relic neutrinos. However, the upper bound on the neutrino effective magnetic moment requires some trick to be circumvented. To this extent, I discuss a recently proposed boomerang mechanism in which the visible sector throws dark neutrinos into the dark sector at $t \sim 100\,{\rm s}$ and $T \sim 100\,{\rm keV}$, and much later (basically at the present time) the dark sector throws back photons into the visible sector. The mechanism predicts an effective neutrino magnetic moment that might be within the reach of next experiments. Some contribution to the 21 cm cosmological signal is also expected. These are exciting times for cosmological searches of BSM physics.

hep-ph↗

arXiv2026

$SO(10)$-inspired leptogenesis

In the first part of the talk, I review general properties of $SO(10)$-inspired leptogenesis. This high-scale leptogenesis scenario is based on the simple assumption that the neutrino Dirac mass matrix is not too different from the up quark mass matrix. After showing how this necessarily implies a production of the asymmetry from the next-to-lightest right handed neutrino decays, so-called $N_2$-leptogenesis, I discuss how this results into important testable constraints on low energy neutrino parameters. In particular inverted ordering is not viable if strict $SO(10)$-inspired conditions are assumed. This is an important test in view of the expected results from the JUNO experiment. I also discuss how a subset of the $SO(10)$-inspired leptogenesis solutions realises strong thermal leptogenesis, where the final asymmetry is independent of the initial conditions. In this case a signal might be discovered by next generation $0νββ$ decay experiments. In the second part, I present some new results from \cite{DiBari:2025zlv}, where the impact of flavour coupling on $SO(10)$-inspired leptogenesis has been studied in detail.

hep-ph↗

arXiv2025

Impact of flavour coupling on $SO(10)$-inspired leptogenesis

We discuss the impact of flavour coupling on the predictions of low energy neutrino parameters from $SO(10)$-inspired leptogenesis (SO10INLEP). The right-handed (RH) neutrino mass spectrum is strongly hierarchical and successful leptogenesis relies on generating the asymmetry from next-to-lightest RH neutrino decays ($N_2$-leptogenesis) and circumventing the lightest RH neutrino washout. These two conditions yield distinctive predictions such as a lower bound on the lightest neutrino mass $m_1 \gtrsim 1\,{\rm meV}$. We first review the status of SO10INLEP, noticing how cosmological observations are now testing a particular neutrino mass window, $m_1 \simeq (10$--$30)\,{\rm meV}$, where only the first octant is allowed and a large range of values for the Dirac phase is excluded. Including flavour coupling, we find that the lower bound relaxes to $m_1 \gtrsim 0.65\,{\rm meV}$. Moreover, new muon-dominated solutions appear slightly relaxing the upper bound on the atmospheric mixing angle. We also study the impact on strong thermal SO10INLEP (ST-SO10INLEP) scenario where, in addition to successful leptogenesis, one can washout a large pre-existing asymmetry. Contrarily to naive expectations, for which flavour coupling could jeopardise the scenario, allowing a large pre-existing asymmetry to survive unconditionally, we show, and explain analytically, that ST-SO10INLEP is still viable within almost the same allowed region of parameters. There is even a slight relaxation of the $m_1$ viable window from (9--30)meV to (4--40)meV for a $10^{-3}$ pre-existing asymmetry. The new results from atmospheric neutrinos, mildly favouring normal ordering and first octant, are now in nice agreement with the predictions of ST-SO10INLEP. Intriguingly, the predicted $0νββ$ signal is starting to be within the reach of KamLAND-Zen.

hep-ph↗

arXiv2025

Testing an unstable cosmic neutrino background

I discuss how different cosmological observations can test the possibility that neutrinos might be unstable on cosmological times, resulting into an unstable cosmic neutrino background. I also discuss out how actually there are different independent anomalies intriguingly hint to such a possibility that would clearly point to new physics. I first focus on how the new DESI results place an upper bound on the sum of neutrino masses that starts to be in tension with the lower bound from neutrino oscillation experiments and how this tension could be easily solved assuming unstable relic neutrinos. Then I show how 21 cm cosmology allows to test radiative relic neutrino decays and how these could explain the controversial EDGES anomaly. I also discuss how the excess radio background and in particular the ARCADE 2 data can also be nicely explained by relic neutrino radiative decays. Finally, I point out the difficulties in building a model that does not clash with the upper limits on the effective magnetic moment coming from neutrino-electron scattering experiments and globular cluster stars.

hep-ph↗

arXiv2024

Leptogenesis in SO(10) with Minimal Yukawa sector

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 $\overline{126}_H$ 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 $δ_\mathrm{CP}$ indicated by global fits of neutrino oscillation data, while also satisfying the current limits from neutrinoless double beta decay experiments. In particular, the the leptonic CP-violating phase has a preference to be in the range $δ_\mathrm{CP}\simeq (230-300)^\circ$. We also show the consistency of the framework with gauge coupling unification and proton lifetime limits.

hep-ph↗

arXiv2024

The split majoron model confronts the NANOGrav signal and cosmological tensions

In the light of the evidence of a gravitational wave background from the NANOGrav 15yr data set, we reconsider the split majoron model as a new physics extension of the standard model able to generate a needed contribution to solve the current tension between the data and the standard interpretation in terms of inspiraling supermassive black hole massive binaries. In the split majoron model the seesaw right-handed neutrinos acquire Majorana masses from spontaneous symmetry breaking of global $U(1)_{B-L}$ in a strong first order phase transition of a complex scalar field occurring above the electroweak scale. The final vacuum expectation value couples to a second complex scalar field undergoing a low scale phase transition occurring after neutrino decoupling. Such a coupling enhances the strength of this second low scale first order phase transition and can generate a sizeable primordial gravitational wave background contributing to the NANOGrav 15yr signal. Some amount of extra-radiation is generated after neutron-to-proton ration freeze-out but prior to nucleosynthesis. This can be either made compatible with current upper bound from primordial deuterium measurements or even be used to solve a potential deuterium problem. Moreover, the free streaming length of light neutrinos can be suppressed by their interactions with the resulting majoron background and this mildly ameliorates existing cosmological tensions. Thus cosmological observations nicely provide independent motivations for the model.

hep-ph↗

arXiv2024

Majorana mass generation, gravitational waves and cosmological tensions

A neutrino Majorana mass generation in the early universe might have left imprints in cosmological observables. It can source the production of a detectable stochastic background of primordial gravitational waves with a spectrum that combines a contribution from a first order phase transition production and from the vibration of global cosmic strings. An intriguing possibility is given by the split seesaw model. In this case, in addition to the traditional high scale seesaw, a low scale neutrino Majorana mass generation can solve a potential primordial deuterium problem and ameliorate the cosmological tensions of the $Λ$CDM model. At the same time, it can also produce subdominant contribution to the NANOGrav signal of a stochastic background of gravitational waves (GWs), in addition to the astrophysical dominant contribution from supermassive black hole mergers.

hep-ph↗

arXiv2024

Gravitational waves from phase transitions and cosmic strings in neutrino mass models with multiple majorons

We explore the origin of Majorana masses within the majoron model and how this can lead to the generation of a distinguishable primordial stochastic background of gravitational waves. We first show how in the simplest majoron model only a contribution from cosmic string can be within the reach of planned experiments. We then consider extensions containing multiple complex scalars, demonstrating how in this case a spectrum comprising contributions from both a strong first order phase transition and cosmic strings can naturally emerge. We show that the interplay between multiple scalar fields can amplify the phase transition signal, potentially leading to double peaks over the wideband sloped spectrum from cosmic strings. We also underscore the possibility of observing such a gravitational wave background to provide insights into the reheating temperature of the universe. We conclude highlighting how the model can be naturally combined with scenarios addressing the origin of matter of the universe, where baryogenesis occurs via leptogenesis and a right-handed neutrino plays the role of dark matter.

hep-ph↗

arXiv2024

Relic neutrino decay solution to the excess radio background

The excess radio background detected by ARCADE 2 represents a puzzle within the standard cosmological model. There is no clear viable astrophysical solution, and therefore, it might indicate the presence of new physics. Radiative decays of a relic neutrino $ν_i$ (either $i=1$, or $i=2$, or $i=3$) into a sterile neutrino $ν_{\rm s}$, assumed to be quasi-degenerate, provide a solution that currently evades all constraints posed by different cosmological observations and reproduces very well the ARCADE 2 data. We find a very good fit to the ARCADE 2 data with best fit values $τ_i = 1.46 \times 10^{21}\,{\rm s}$ and $Δm_i = 4.0 \times 10^{-5}\,{\rm eV}$, where $τ_i$ is the lifetime and $Δm_i$ is the mass difference between the decaying active neutrino and the sterile neutrino. On the other hand, if relic neutrino decays do not explain ARCADE 2 data, then these place a stringent constraint $Δm_i^{3/2} τ_i \gtrsim 2 \times 10^{14}\,{\rm eV}^{3/2}\,{\rm s}$ in the range $1.4 \times 10^{-5} \, {\rm eV} < Δm_i < 2.5 \times 10^{-4}\,{\rm eV}$. The solution also predicts a stronger 21 cm absorption global signal than the predicted one from the $Λ$CDM model, with a contrast brightness temperature $T_{21} = -238^{+21}_{-20}\,{\rm mK}$ ($99\%$ C.L.) at redshift $z\simeq 17$. This is in mild tension with the even stronger signal found by the EDGES collaboration, $T_{21} = - 500^{+200}_{-500}\,{\rm mK} $, suggesting that this might have been overestimated, possibly receiving a contribution from some unidentified foreground source.

hep-ph↗

arXiv2023

Dark matter from sterile-sterile neutrino mixing

A solution to the problem of the origin of matter in the universe can be reasonably searched within extensions of the standard model that also explain neutrino masses and mixing. Models embedding the minimal seesaw mechanism can explain the observed matter-antimatter asymmetry of the universe via leptogenesis and dark matter via active-sterile neutrino mixing. In this case a keV lightest seesaw neutrino would play the role of warm dark matter particle. This traditional solution is now constrained by various cosmological observations. I will discuss the possibility that a much heavier but yet metastable (dark) right-handed neutrino with mass in the $1\,{\rm TeV}$--$1 \, {\rm PeV}$ range can play the role of (cold) dark matter particle. The right abundance would be produced by the Higgs induced mixing with a seesaw right-handed neutrino (RHINO model), i.e., by sterile-sterile neutrino mixing. Such a mixing would necessarily require a further extension of the minimal seesaw mechanism and can be described by a dimension-five effective operator. The same mixing would also necessarily induce dark neutrino instability with lifetimes that can be much longer than the age of the universe and can escape current constraints from neutrino telescopes. On the other hand, a contribution to very high energy neutrino flux produced by dark neutrino decays could explain an anomalous excess at 100 TeV energies confirmed recently by the IceCube collaboration. I will also discuss a simple UV completion where the mediator is given by a massive fermion. Intriguingly, it comes out that the favoured scale of new physics for RHINO to satisfy the dark matter requirements coincides with the grand-unified scale: a RHINO miracle.

hep-ph↗

arXiv2023

Completing RHINO

The right-handed (RH) Higgs-induced neutrino mixing (RHINO) model explains neutrino masses and origin of matter in the universe within a unified picture. The mixing, effectively described by a dimension five operator, is responsible both for the production of dark neutrinos, converting a small fraction of seesaw neutrinos acting as source, and for their decays. We show that including the production of source neutrinos from Higgs portal interactions, their abundance can thermalise prior to the onset of source-dark neutrino oscillations, resulting into an enhanced production of dark neutrinos that thus can play the role of decaying dark matter (DM) for a much higher seesaw scale. This can be above the sphaleron freeze-out temperature and as high as $\sim 100\,{\rm TeV}$, so that strong thermal resonant leptogenesis for the generation of the matter-antimatter asymmetry is viable. We obtain a $\sim 1\,{\rm TeV}$--$1\,{\rm PeV}$ allowed dark neutrino mass range. Intriguingly, their decays can also explain a neutrino flux excess at ${\cal O}(100\,{\rm TeV})$ energies recently confirmed by the IceCube collaboration analysing 7.5yr HESE data. Our results also point to an effective scale for Higgs portal interactions nicely identifiable with the grandunified scale and many orders of magnitude below the effective scale for the mixing. We explain this hierarchy in a UV-complete model with a very heavy fermion as mediator: the first scale corresponds to the fundamental scale of new physics, while the second is much higher because of a very small coupling identifiable with a symmetry breaking parameter. Therefore, RHINO realises a simple unified model of neutrino masses and origin of matter in the universe currently under scrutiny at neutrino telescopes and potentially embeddable within a grandunified model.

hep-ph↗

arXiv2022

Gravitational waves from neutrino mass genesis

The discovery of gravitational waves opens new opportunities to test BSM physics. In particular, the production of a stochastic background of primordial gravitational waves could provide a signature of the generation of the right-right Majorana neutrino mass term necessary, within type-I seesaw mechanism, to explain lightness of neutrinos and their mixing parameters. I will discuss the possibility that such a generation occurs during a strong first order phase transition within Majoron models [1]. As well known, this can indeed produce a stochastic background of gravitational waves. The scale of the phase transition can or cannot coincide with the seesaw scale. In the latter case a low scale phase transition, occurring in the pre-recombination era, might be tested at very low frequencies ($10^{-9}$--$10^{-6}\,{\rm Hz}$). Even though the signal can hardly reproduce the NANOGrac putative signal such new physics at low scale might help ameliorating the tensions in the $Λ$CDM cosmological model (e.g., the Hubble tension). I will also discuss how a phase transition might be responsible for the generation of dark matter in the form of dark neutrinos coupling to the seesaw neutrinos via Higgs induced right handed-right handed neutrino mixing [2].

hep-ph↗

arXiv2021

Gravitational waves from first-order phase transitions in Majoron models of neutrino mass

We show how the generation of right-handed neutrino masses in Majoron models may be associated with a first-order phase transition and accompanied by the production of a stochastic background of gravitational waves (GWs). We explore different energy scales with only renormalizable operators in the effective potential. If the phase transition occurs above the electroweak scale, the signal can be tested by future interferometers. We consider two possible energy scales for phase transitions below the electroweak scale. If the phase transition occurs at a GeV, the signal can be tested at LISA and provide a complementary cosmological probe to right-handed neutrino searches at the FASER detector. If the phase transition occurs below 100 keV, we find that the peak of the GW spectrum is two or more orders of magnitude below the putative NANOGrav GW signal at low frequencies, but well within reach of the SKA and THEIA experiments. We show how searches of very low frequency GWs are motivated by solutions to the Hubble tension in which ordinary neutrinos interact with the dark sector. We also present general calculations of the phase transition temperature and Euclidean action that apply beyond Majoron models.

hep-ph↗

arXiv2021

On the origin of matter in the Universe

The understanding of the physical processes that lead to the origin of matter in the early Universe, creating both an excess of matter over anti-matter that survived until the present and a dark matter component, is one of the most fascinating challenges in modern science. The problem cannot be addressed within our current description of fundamental physics and, therefore, it currently provides a very strong evidence of new physics. Solutions can either reside in a modification of the standard model of elementary particle physics or in a modification of the way we describe gravity, based on general relativity, or at the interface of both. We will mainly discuss the first class of solutions. Traditionally, models that separately explain either the matter-antimatter asymmetry of the Universe or dark matter have been proposed. However, in the last years there has also been an accreted interest and intense activity on scenarios able to provide a unified picture of the origin of matter in the early universe. In this review we discuss some of the main ideas emphasising primarily those models that have more chances to be experimentally tested during next years. Moreover, after a general discussion, we will focus on extensions of the standard model that can also address neutrino masses and mixing, since this is currently the only evidence of physics beyond the standard model coming directly from particle physics and it is, therefore, reasonable they might also provide a solution to the problem of the origin of matter in the universe.

hep-ph↗

arXiv2020

The $SO(10)$-inspired leptogenesis timely opportunity

We study the connection between absolute neutrino mass and neutrino mixing parameters within $SO(10)$-inspired leptogenesis. We show that current favoured values of the unknown neutrino mixing parameters point toward values of the absolute neutrino mass scale that will be fully tested by cosmological observations and neutrinoless double beta decay experiments during next years. In particular, for $m_{D2}/m_{\rm charm} \leq 5$, where $m_{D2}$ is the intermediate Dirac neutrino mass, and for current best fit values of the Dirac phase $δ$ and the atmospheric neutrino mixing angle $θ_{23}$, we derive a lower bound on the neutrinoless double beta decay effective neutrino mass $m_{ee} \gtrsim 31\,{\rm meV}$ and on the sum of the neutrino masses $\sum_i m_i \gtrsim 125\,{\rm meV}$. These lower bounds hold for normally ordered neutrino masses, as currently favoured by global analyses and approximately for $δ\in [155^\circ,240^\circ]$ and $θ_{23}$ in the second octant. If values in this region will be confirmed by future planned long baseline experiments, then a signal at next generation neutrinoless double beta decay experiments is expected, despite neutrino masses being normally ordered. Outside the region, the lower bounds strongly relax but a great fraction of the allowed range of values still allows a measurement of the lightest neutrino mass. Therefore, in the the next years low energy neutrino experiments will provide a very stringent test of $SO(10)$-inspired leptogenesis, resulting either in severe constraints or in a strong evidence.

hep-ph↗

arXiv2020

Gravitational waves from neutrino mass and dark matter genesis

We introduce a model in which the genesis of dark matter (DM) and neutrino masses is associated with a first order phase transition of a scalar singlet field. During the phase transition a source right-handed neutrino (RHN) acquires a spacetime-dependent mass dynamically, a small fraction of which is converted via resonant oscillations into a very weakly mixed dark RHN which decays to a dark matter RHN with the observed relic abundance. Neutrino masses are generated via a traditional two RHN type-I seesaw between a fourth RHN and the source neutrino. The gravitational waves produced during the phase transition have a peak frequency that increases with the DM mass, and are detectable at future gravitational wave interferometers for DM masses above ~ 1 MeV. Since the source RHNs are heavier than the electroweak scale, successful leptogenesis is also attainable.

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