arXiv subjects

Pasquale Di Bari

Publications and source records attributed to Pasquale Di Bari.

At least 37 records · Page 2Linked to original sources

arXiv2019

A full analytic solution of $SO(10)$-inspired leptogenesis

Recent encouraging experimental results on neutrino mixing parameters prompt further investigation on $SO(10)$-inspired leptogenesis and on the associated strong thermal solution that has correctly predicted a non-vanishing reactor mixing angle, it further predicts $\sinδ\lesssim 0$, now supported by recent results at $\sim 95\%$ C.L., normally ordered neutrino masses and atmospheric mixing angle in the first octant, best fit results in latest global analyses. Extending a recent analytical procedure, we account for the mismatch between the Yukawa basis and the weak basis, that in $SO(10)$-inspired models is described by a CKM-like unitary transformation $V_L$, obtaining a full analytical solution that provides useful insight and reproduces accurately all numerical results, paving the way for future inclusion of different sources of theoretical uncertainties and for a statistical analysis of the constraints. We show how muon-dominated solutions appear for large values of the lightest neutrino mass in the range $(0.01$--$1)\,{\rm eV}$ but also how they necessarily require a mild fine tuning in the seesaw relation. For the dominant (and untuned) tauon-dominated solutions we show how, turning on $V_L \simeq V_{CKM}$, some of the constraints on the low energy neutrino parameters get significantly relaxed. In particular we show how the upper bound on the atmospheric neutrino mixing angle in the strong thermal solution gets relaxed from $θ_{23} \lesssim 41^{\circ}$ to $θ_{23} \lesssim 44^{\circ}$, an important effect in the light of the most recent NO$ν$A, T2K and IceCube results.

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arXiv2019

Neutrino masses, leptogenesis and dark matter

Despite the lack of evidence of new physics at colliders, neutrino masses, dark matter and matter-antimatter asymmetry of the universe require an extension of the Standard Model. After discussing some new concepts and tools in the description of seesaw neutrino models, such as motion in flavour lepton space and the introduction of the bridging matrix that nicely complements the orthogonal matrix, I discuss recent developments in the connections between neutrino data and scenarios of leptogenesis within some well motivated extensions of the Standard Model. Finally, I briefly review a simple unified picture of neutrino masses, leptogenesis and dark matter based on an extension of the seesaw Lagrangian where a non-renormalizable effective operator coupling right-handed neutrinos to the Higgs is introduced. This operator enhances the right-handed neutrino mixing between a coupled heavy right-handed neutrino and a decoupled one playing the role of dark matter. Interference between the two coupled RH neutrinos generate usual $C\!P$ violation so that one can also achieve successful leptogenesis. Although the dark matter right-handed neutrino escapes direct and collider searches, its decays produce a detectable contribution to the very high energy neutrino flux now discovered by IceCube, so that the model is predictive and can be tested at neutrino telescopes.

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arXiv2019

Representing seesaw neutrino models and their motion in lepton flavour space

We discuss how seesaw neutrino models can be graphically represented in lepton flavour space. We examine various popular models and show how this representation helps understanding their properties and connection with experimental data showing in particular how certain texture zero models are ruled out. We also introduce a new matrix, the bridging matrix, that brings from the light to the heavy neutrino mass flavour basis, showing how this is related to the orthogonal matrix and how different quantities are easily expressed through it. We then show how one can randomly generate orthogonal and leptonic mixing matrices uniformly covering all flavour space in an unbiased way (Haar-distributed matrices). Using the isomorphism between the group of complex rotations and the Lorentz group, we also introduce the conceptof Lorentz boost in flavour space for a seesaw model and how this has an insightful physical interpretation. Finally, as a significant application, we consider $N_2$-leptogenesis. Using current experimental values of low energy neutrino parameters, we show that the probability that at least one flavoured decay parameter of the lightest right-handed neutrino is smaller than unity is about $49\%$ (to be compared with the tiny probability that the total decay parameter is smaller than unity, $P(K_{\rm I}< 1)\sim 0.1 \%$, confirming the crucial role played by flavour effects). On the other hand when $m_1 \gtrsim 0.1\,{\rm eV}$ this probability reduces to less than $5\%$, showing how also $N_2$-leptogenesis disfavours degenerate light neutrinos.

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arXiv2018

Unifying leptogenesis, dark matter and high-energy neutrinos with right-handed neutrino mixing via Higgs portal

We revisit a model in which neutrino masses and mixing are described by a two right-handed (RH) neutrino seesaw scenario, implying a strictly hierarchical light neutrino spectrum. A third decoupled RH neutrino, $N_{\rm DM}$ with mass $M_{\rm DM}$, plays the role of cold dark matter (DM) and is produced by the mixing with a source RH neutrino, $N_{\rm S}$ with mass $M_{\rm S}$, induced by Higgs portal interactions. The same interactions are also responsible for $N_{\rm DM}$ decays. We discuss in detail the constraints coming from DM abundance and stability conditions, showing that in the hierarchical case ($M_{\rm DM} \gg M_{\rm S}$) there is an allowed window on $M_{\rm DM}$, which necessarily implies a contribution from DM decays to the high-energy neutrino flux recently detected by IceCube. We also show how the model can explain the matter-antimatter asymmetry of the Universe via leptogenesis in the quasi-degenerate limit. In this case, the DM mass should be within the range 300 GeV $\lesssim M_{\rm S} < M_{\rm DM} \lesssim$ 10 PeV. We discuss the specific properties of this high-energy neutrino flux and show the predicted event spectrum for two exemplary cases. Although DM decays, with a relatively hard spectrum, cannot account for all the IceCube high-energy data, we illustrate how this extra source of high-energy neutrinos could reasonably explain some potential features in the observed spectrum. In this way, this represents a unified scenario for leptogenesis and DM that could be tested during the next years with more high-energy neutrino events.

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arXiv2018

Probing relic neutrino decays with 21 cm cosmology

We show how 21 cm cosmology can test relic neutrino radiative decays into sterile neutrinos. Using recent EDGES results, we derive constraints on the lifetime of the decaying neutrinos. If the EDGES anomaly will be confirmed, then there are two solutions, one for much longer and one for much shorter lifetimes than the age of the universe, showing how relic neutrino radiative decays can explain the anomaly in a simple way. We also show how to combine EDGES results with those from radio background observations, showing that potentially the ARCADE 2 excess can be also reproduced together with the EDGES anomaly within the proposed non-standard cosmological scenario. Our calculation of the specific intensity at the redshifts probed by EDGES can be also applied to the case of decaying dark matter and it also corrects a flawed expression appeared in previous literature.

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arXiv2018

Strong thermal $SO(10)$-inspired leptogenesis in the light of recent results from long-baseline neutrino experiments

We confront recent experimental results on neutrino mixing parameters with the requirements from strong thermal $SO(10)$-inspired leptogenesis, where the asymmetry is produced from next-to-lightest right-handed neutrinos $N_2$ independently of the initial conditions. There is a nice agreement with latest global analyses supporting $\sinδ< 0$ and normal ordering at $ \sim 95\%$ C.L. On the other hand, the more stringent experimental lower bound on the atmospheric mixing angle starts to corner strong thermal $SO(10)$-inspired leptogenesis. Prompted and encouraged by this rapid experimental advance, we obtain a precise determination of the allowed region in the plane $δ$ versus $θ_{23}$. We confirm that for the benchmark case $α_2 \equiv m_{D2} / m_{\rm charm}= 5 \, $, where $m_{D2}$ is the intermediate neutrino Dirac mass setting the $N_2$ mass, and initial pre-existing asymmetry $N_{B-L}^{\rm p, i} = 10^{-3}$, the bulk of solutions lies in the first octant. Though most of the solutions are found outside the $95\%$ C.L. experimental region, there is still a big allowed fraction that does not require a too fine-tuned choice of the Majorana phases so that the neutrinoless double beta decay effective neutrino mass allowed range is still $m_{ee}\simeq [10,30]\,{\rm meV}$. We also show how the constraints depend on $N_{B-L}^{\rm p, i}$ and $α_2$. In particular, we show that the current best fit, ($θ_{23},δ)\simeq (47^{\circ}, -130^{\circ})$, can be reproduced for $N_{B-L}^{\rm p, i} = 10^{-3}$ and $α_2 = 6$. Such large values for $α_2$ have been recently obtained in a few realistic fits within $SO(10)$-inspired models. Finally, we also obtain that current neutrino data rule out $N_{B-L}^{\rm p, i} \gtrsim 0.1$ for $α_2 \lesssim 4.7$.

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arXiv2018

Flavor effects in leptogenesis

Flavor effects can have a significant impact on the final estimate of the lepton (and therefore baryon) asymmetry in scenarios of leptogenesis. It is therefore necessary to account fully for this flavor dynamics in the relevant transport equations that describe the production (and washout) of the asymmetry. Doing so can both open up and restrict viable regions of parameter space relative to the predictions of more approximate calculations. In this review, we identify the regimes in which flavor effects can be relevant and illustrate their impact in a number of phenomenological models. These include type I and type II seesaw embeddings, and low-scale resonant scenarios. In addition, we provide an overview of the semi-classical and field-theoretic methods that have been developed to capture flavor effects in a consistent way.

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arXiv2017

Beyond the Standard Model with leptogenesis and neutrino data

In this short review I discuss how high energy (type I) seesaw models can be nicely embedded within grand-unified models and reproduce the observed matter-antimatter asymmetry with leptogenesis. In particular, after discussing general features and results in leptogenesis, I focus on $SO(10)$-inspired leptogenesis and on a particular solution, the strong thermal $SO(10)$-inspired solution, that provides an interesting way to understand neutrino mixing parameters: the non-vanishing reactor mixing angle, the emerging negative sign of $\sinδ$ and the slight hints favouring normally ordered neutrino masses and an atmospheric mixing angle in the first octant. I also briefly discuss leptogenesis within two right-handed seesaw neutrino models. In this case a a third decoupled right-handed neutrino can provide a candidate for very heavy cold decaying dark matter produced from right-handed neutrino mixing with a mass in the TeV-EeV range and its decays would give a contribution to the IceCube high energy neutrino events in addition to an astrophysical component.

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arXiv2016

Supersymmetric $SO(10)$-inspired leptogenesis and a new $N_2$-dominated scenario

We study the supersymmetric extension of $SO(10)$-inspired thermal leptogenesis showing the constraints on neutrino parameters and on the reheat temperature $T_{\rm RH}$ that derive from the condition of successful leptogenesis from next-to-lightest right handed (RH) neutrinos ($N_2$) decays and the more stringent ones when independence of the initial conditions (strong thermal leptogenesis) is superimposed. In the latter case, the increase of the lightest right-handed neutrino ($N_1$) decay parameters helps the wash-out of a pre-existing asymmetry and constraints relax compared to the non-supersymmetric case. We find significant changes especially in the case of large $\tanβ$ values $(\gtrsim 15)$. In particular, for normal ordering, the atmospheric mixing angle can now be also maximal. The lightest (ordinary) neutrino mass is still constrained within the range $10 \lesssim m_1/{\rm meV} \lesssim 30$ (corresponding to $75\lesssim \sum_i m_i/{\rm meV} \lesssim 120$). Inverted ordering is still disfavoured, but an allowed region satisfying strong thermal leptogenesis opens up at large $\tanβ$ values. We also study in detail the lower bound on $T_{\rm RH}$ finding $T_{\rm RH}\gtrsim 1 \times 10^{10}\,{\rm GeV}$ independently of the initial $N_2$ abundance. Finally, we propose a new $N_2$-dominated scenario where the $N_1$ mass is lower than the sphaleron freeze-out temperature. In this case there is no $N_1$ wash-out and we find $T_{\rm RH} \gtrsim 1\times 10^{9}\,{\rm GeV}$. These results indicate that $SO(10)$-inspired thermal leptogenesis can be made compatible with the upper bound from the gravitino problem, an important result in light of the role often played by supersymmetry in the quest of a realistic model of fermion masses.

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arXiv2015

Successful $N_2$ leptogenesis with flavour coupling effects in realistic unified models

In realistic unified models involving an $SO(10)$-like pattern of Dirac and heavy right-handed (RH) neutrino masses, the lightest right-handed neutrino $N_1$ is too light to yield successful thermal leptogenesis, barring highly fine tuned solutions, while the second heaviest right-handed neutrino $N_2$ is typically in the correct mass range. We show that flavour coupling effects in the Boltzmann equations may be crucial to the success of such $N_2$ dominated leptogenesis, by helping to ensure that the flavour asymmetries produced at the $N_2$ scale survive $N_1$ washout. To illustrate these effects we focus on $N_2$ dominated leptogenesis in an existing model, the A to Z of flavour with Pati-Salam, where the neutrino Dirac mass matrix may be equal to an up-type quark mass matrix and has a particular constrained structure. The numerical results, supported by analytical insight, show that in order to achieve successful $N_2$ leptogenesis, consistent with neutrino phenomenology, requires a "flavour swap scenario" together with a less hierarchical pattern of RH neutrino masses than naively expected, at the expense of some mild fine-tuning. These results may be relevant for other $SO(10)$-like unified models where $N_2$ leptogenesis is necessary.

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arXiv2015

Decrypting $SO(10)$-inspired leptogenesis

Encouraged by the recent results from neutrino oscillation experiments, we perform an analytical study of $SO(10)$-inspired models and leptogenesis with hierarchical right-handed (RH) neutrino spectrum. Under the approximation of negligible misalignment between the neutrino Yukawa basis and the charged lepton basis, we find an analytical expression for the final asymmetry directly in terms of the low energy neutrino parameters that fully reproduces previous numerical results. This expression also shows that is possible to identify an effective leptogenesis phase for these models. When we also impose the wash-out of a large pre-existing asymmetry $N^{\rm p,i}_{B-L}$, the strong thermal (ST) condition, we derive analytically all those constraints on the low energy neutrino parameters that characterise the {\rm ST}-$SO(10)$-inspired leptogenesis solution, confirming previous numerical results. In particular we show why, though neutrino masses have to be necessarily normally ordered, the solution implies an analytical lower bound on the effective neutrino-less double beta decay neutrino mass, $m_{ee} \gtrsim 8\,{\rm meV}$, for $N^{\rm p,i}_{B-L}=10^{-3}$, testable with next generation experiments. This, in combination with an upper bound on the atmospheric mixing angle, necessarily in the first octant, forces the lightest neutrino mass within a narrow range $m_1 \simeq (10$-$30)\,{\rm meV}$ (corresponding to $\sum_i \, m_i \simeq (75$-$125)\,{\rm meV}$). We also show why the solution could correctly predict a non-vanishing reactor neutrino mixing angle and requires the Dirac phase to be in the fourth quadrant, implying $\sinδ$ (and $J_{CP}$) negative as hinted by current global analyses. Many of the analytical results presented (expressions for the orthogonal matrix, RH neutrino mixing matrix, masses and phases) can have applications beyond leptogenesis.

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arXiv2014

Radiative Inflation and Dark Energy RIDEs Again after BICEP2

Following the ground-breaking measurement of the tensor-to-scalar ratio $r = 0.20^{+0.07}_{- 0.05}$ by the BICEP2 collaboration, we perform a statistical analysis of a model that combines Radiative Inflation with Dark Energy (RIDE) based on the $M^2 |Φ|^2 \ln \left(|Φ|^2/Λ^2 \right)$ potential and compare its predictions to those based on the traditional chaotic inflation $M^2|Φ|^2$ potential. We find a best-fit value in the RIDE model of $r=0.18$ as compared to $r=0.17$ in the chaotic model, with the spectral index being $n_S=0.96$ in both models.

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arXiv2014

Strong thermal leptogenesis and the absolute neutrino mass scale

We show that successful strong thermal leptogenesis, where the final asymmetry is independent of the initial conditions and in particular a large pre-existing asymmetry is efficiently washed-out, favours values of the lightest neutrino mass $m_1 \gtrsim 10\,{\rm meV}$ for normal ordering (NO) and $m_1 \gtrsim 3\,{\rm meV}$ for inverted ordering (IO) for models with orthogonal matrix entries respecting $|Ω_{ij}^2| \lesssim 2$. . We show analytically why lower values of $m_1$ require a high level of fine tuning in the seesaw formula and/or in the flavoured decay parameters (in the electronic for NO, in the muonic for IO). We also show how this constraint exists thanks to the measured values of the neutrino mixing angles and can be tighten by a future determination of the Dirac phase. Our analysis also allows to place more stringent constraint for a specific model or class of models, such as $SO(10)$-inspired models, and shows that some models cannot realise strong thermal leptogenesis for any value of $m_1$. A scatter plot analysis fully supports the analytical results. We also briefly discuss the interplay with absolute neutrino mass scale experiments concluding that they will be able in the coming years to either corner strong thermal leptogenesis or find positive signals pointing to a non-vanishing $m_1$. Since the constraint is much stronger for NO than for IO, it is very important that new data from planned neutrino oscillation experiments will be able to solve the ambiguity.

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arXiv2013

SO(10)-inspired solution to the problem of the initial conditions in leptogenesis

We show that, within SO(10)-inspired leptogenesis, there exists a solution, with definite constraints on low energy neutrino parameters, able simultaneously to reproduce the observed baryon asymmetry and to satisfy the conditions for the independence of the final asymmetry of the initial conditions (strong thermal leptogenesis). We find that the wash-out of a pre-existing asymmetry as large as O(0.1) requires: i) reactor mixing angle in the range θ_13 = (2 - 20) degrees, in agreement with the experimental result θ_13 = (8 - 10) degrees; ii) atmospheric mixing angle in the range θ_23 = (16 - 41) degrees, compatible only with current lowest experimentally allowed values; iii) Dirac phase in the range δ\simeq -π/2 - π/5, with the bulk of the solutions around δ\simeq -π/5 and such that sign(J_CP)= - sign(η_B); iv) neutrino masses m_i normally ordered; v) lightest neutrino mass in the range m_1 \simeq (15 - 25) meV, corresponding to \sum_i m_i \simeq (85 - 105) meV; vi) neutrinoless double beta decay (0νββ) effective neutrino mass m_ee \simeq 0.8 m_1. All together this set of predictive constraints characterises the solution quite distinctively, representing a difficultly forgeable, fully testable, signature. In particular, the condition m_ee \simeq 0.8 m_1 \simeq 15 meV can be tested by cosmological observations and (ultimately) by 0νββexperiments. We also discuss different aspects such as theoretical uncertainties, stability under variation of the involved parameters, form of the orthogonal and RH neutrino mixing matrices.

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arXiv2013

Dark Radiation or Warm Dark Matter from long lived particle decays in the light of Planck

Although Planck data supports the standard ΛCDM model, it still allows for the presence of Dark Radiation corresponding up to about half an extra standard neutrino species. We propose a scenario for obtaining a fractional "effective neutrino species" from a thermally produced particle which decays into a much lighter stable relic plus standard fermions. At lifetimes much longer than 1 sec, both the relic particles and the non-thermal neutrino component contribute to Dark Radiation. By increasing the stable-to-unstable particle mass ratio, the relic particle no longer acts as Dark Radiation but instead becomes a candidate for Warm Dark Matter with mass O(1keV - 100GeV). In both cases it is possible to address the lithium problem.

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arXiv2013

Leptogenesis with heavy neutrino flavours: from density matrix to Boltzmann equations

Leptogenesis with heavy neutrino flavours is discussed within a density matrix formalism. We write the density matrix equation, describing the generation of the matter-antimatter asymmetry, for an arbitrary choice of the right-handed (RH) neutrino masses. For hierarchical RH neutrino masses lying in the fully flavoured regimes, this reduces to multiple-stage Boltzmann equations. In this case we recover and extend results previously derived within a quantum state collapse description. We confirm the generic existence of phantom terms. However, taking into account the effect of gauge interactions, we show that they are washed out at the production with a wash-out rate that is halved compared to that one acting on the total asymmetry. In the N_1-dominated scenario they cancel without contributing to the final baryon asymmetry. In other scenarios they do not in general and they have to be taken into account. We also confirm that there is a (orthogonal) component in the asymmetry produced by the heavier RH neutrinos which completely escapes the washout from the lighter RH neutrinos and show that phantom terms additionally contribute to it. The other (parallel) component is washed out with the usual exponential factor, even for weak washout. Finally, as an illustration, we study the two RH neutrino model in the light of the above findings, showing that phantom terms can contribute to the final asymmetry also in this case.

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arXiv2012

The minimal scenario of leptogenesis

We review the main features and results of thermal leptogenesis within the type I seesaw mechanism, the minimal extension of the Standard Model explaining neutrino masses and mixing. After presenting the simplest approach, the vanilla scenario, we discuss various important developments in recent years, such as the inclusion of lepton and heavy neutrino flavour effects, a description beyond a hierarchical heavy neutrino mass spectrum and an improved kinetic description within the density matrix and the closed-time-path formalisms. We also discuss how leptogenesis can ultimately represent an important phenomenological tool to test the seesaw mechanism and the underlying model of new physics.

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arXiv2012

An introduction to leptogenesis and neutrino properties

This is an introductory review of the main features of leptogenesis, one of the most attractive models of baryogenesis for the explanation of the matter-antimatter asymmetry of the Universe. The calculation of the asymmetry in leptogenesis is intimately related to neutrino properties so that leptogenesis is also an important phenomenological tool to test the see-saw mechanism for the generation of neutrino masses and mixing and the underlying theory beyond the Standard Model.

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