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P. Di Bari

Publications and source records attributed to P. Di Bari.

At least 19 recordsLinked to original sources
arXiv2020

Density matrix calculation of the dark matter abundance in the Higgs induced right-handed neutrino mixing model

We present new results on the calculation of the dark matter relic abundance within the Higgs induced right-handed (RH) neutrino mixing model, solving the associated density matrix equation. For a benchmark value of the dark matter mass $M_{\rm DM} = 220\,{\rm TeV}$, we show the evolution of the abundance and how this depends on reheat temperature, dark matter lifetime and source RH neutrino mass $M_{\rm S}$, with the assumption $M_{\rm S} < M_{\rm DM}$. We compare the results with those obtained within the Landau-Zener approximation, showing that the latter largely overestimates the final abundance giving some analytical insight. However, we also notice that since in the density matrix formalism the production is non-resonant, this allows source RH neutrino masses below the W boson mass, making dark matter more stable at large mass values. This opens an allowed region for initial vanishing source RH neutrino abundance. For example, for $M_{\rm S} \gtrsim 1\,{\rm GeV}$, we find $M_{\rm DM}\gtrsim 20\,{\rm PeV}$. Otherwise, for $M_{\rm S} > M_W\sim 100\,{\rm GeV}$, one has to assume a thermalisation of the source RH neutrinos prior to the freeze-in of the dark matter abundance. This results into a large allowed range for $M_{\rm DM}$, depending on $M_{\rm S}$. For example, imposing $M_{\rm S} \gtrsim 300\,{\rm GeV}$, allowing also successful leptogenesis, we find $0.5 \lesssim M_{\rm DM}/{\rm TeV} \lesssim 500$. We also discuss in detail leptogenesis with two quasi-degenerate RH neutrinos, showing a case when observed dark matter abundance and matter-antimatter asymmetry are simultaneously reproduced. Finally, we comment on how an initial thermal source RH neutrino abundance can be justified and on how our results suggest that also the interesting case where $M_{\rm DM} < M_{\rm S}$, embeddable in usual high scale two RH neutrino seesaw models, might be viable.

hep-ph↗
arXiv2017

A White Paper on keV Sterile Neutrino Dark Matter

We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved - cosmology, astrophysics, nuclear, and particle physics - in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

hep-ph↗
arXiv2012

Leptogenesis in the two right-handed neutrino model revisited

We revisit leptogenesis in the minimal non-supersymmetric type I see-saw mechanism with two right-handed (RH) neutrinos, including flavour effects and allowing both RH neutrinos N_1 and N_2 to contribute, rather than just the lightest RH neutrino N_1 that has hitherto been considered. By performing scans over parameter space in terms of the single complex angle z of the orthogonal matrix R, for a range of PMNS parameters, we find that in regions around z \sim \pm π/2, for the case of a normal mass hierarchy, the N_2 contribution can dominate the contribution to leptogenesis, allowing the lightest RH neutrino mass to be decreased by about an order of magnitude in these regions, down to M_1 \sim 1.3*10^11 GeV for vanishing initial N_2-abundance, with the numerical results supported by analytic estimates. We show that the regions around z \sim \pm π/2 correspond to light sequential dominance, so the new results in this paper may be relevant to unified model building.

hep-ph↗
arXiv2009

Flavor symmetries, leptogenesis and the absolute neutrino mass scale

We study the interplay between flavor symmetries and leptogenesis in the case when the scale of flavor symmetry breaking is higher than the scale at which lepton number is violated. We show that when the heavy Majorana neutrinos belong to an irreducible representation of the flavor group, all the leptogenesis CP asymmetries vanish in the limit of exact symmetry. In the case of reducible representations we identify a general condition that, if satisfied, guarantees the same result. We then focus on the case of a model in which an $A_4$ flavor symmetry yields a drastic reduction in the number of free parameters, implying that at leading order several quantities are only a function of the lightest neutrino mass $m_l$, which in turn is strongly constrained. For normal ordering (NO) we find m_l\simeq (0.0044 ÷0.0056) eV while for inverted ordering (IO) m_l\gtrsim 0.017 eV. For the 0\nu2βdecay parameter this yields |m_{ee}|\simeq (0.006÷0.007) eV (NO) and |m_{ee}|\gsim 0.017 eV (IO). We show that the leptogenesis CP asymmetries only depend on m_l, on a single non-hierarchical Yukawa coupling y, and on two parameters that quantify the flavor symmetry breaking effects, and we argue that the unflavored regime for leptogenesis is strongly preferred in our model, thus realizing a rather predictive scenario. Performing a calculation of the matter-antimatter asymmetry we find that for NO the observed value is easily reproduced for natural values of the symmetry breaking parameters. For IO successful leptogenesis is possible for a limited choice of the parameters implying rather large reheating temperatures T_reh\gtrsim 5*10^13 GeV.

hep-ph↗
arXiv2007

Cosmic Microwave Background, Matter-Antimatter Asymmetry and Neutrino Masses

We study the implications of thermal leptogenesis for neutrino parameters. Assuming that decays of N_1, the lightest of the heavy Majorana neutrinos, initiate baryogenesis, we show that the final baryon asymmetry is determined by only four parameters: the CP asymmetry epsilon_1, the heavy neutrino mass M_1, the effective light neutrino mass \tilde{m}_1, and the quadratic mean \bar{m} of the light neutrino masses. Imposing the CMB measurement of the baryon asymmetry as constraint on the neutrino parameters, we show, in a model independent way, that quasi-degenerate neutrinos are incompatible with thermal leptogenesis. For maximal CP asymmetry epsilon_1, and neutrino masses in the range from (Δm^2_{sol})^{1/2} to (Δm^2_{atm})^{1/2}, the baryogenesis temperature is T_B = O(10^{10}) GeV.

hep-ph↗
arXiv2006

Quantum Zeno effect and the impact of flavor in leptogenesis

In thermal leptogenesis, the cosmic matter-antimatter asymmetry is produced by CP violation in the decays N --> l + Φof heavy right-handed Majorana neutrinos N into ordinary leptons l and Higgs particles Φ. If some charged-lepton Yukawa couplings are in equilibrium during the leptogenesis epoch, the l interactions with the background medium are flavor sensitive and the coherence of their flavor content defined by N --> l+Φis destroyed, modifying the efficiency of the inverse decays. We point out, however, that it is not enough that the flavor-sensitive processes are fast on the cosmic expansion time scale, they must be fast relative to the N <--> l +Φreactions lest the flavor amplitudes of l remain frozen by the repeated N <--> l+Φ``measurements''. Our more restrictive requirement is significant in the most interesting ``strong wash-out case'' where N <--> l +Φis fast relative to the cosmic expansion rate. We derive conditions for the unflavored treatment to be adequate and for flavor effects to be maximal. In this ``fully flavored regime'' a neutrino mass bound survives. To decide if this bound can be circumvented in the intermediate case, a full quantum kinetic treatment is required.

hep-ph↗
arXiv2004

Leptogenesis, neutrino mixing data and the absolute neutrino mass scale

Recent developments in thermal leptogenesis are reviewed. Neutrino mixing data favor a simple picture where the matter-anti matter asymmetry is generated by the decays of the heavy RH neutrinos mildly close to thermal equilibrium and, remarkably, in the full non relativistic regime. This results into predictions of the final baryon asymmetry not depending on the initial conditions and with minimized theoretical uncertainties. After a short outline of a geometrical derivation of the CP asymmetry bound, we derive analytic bounds on the lightest RH neutrino mass and on the absolute neutrino mass scale. Neutrino masses larger than 0.1 eV are not compatible with the minimal leptogenesis scenario. We discuss how the results get just slightly modified within the minimal supersymmetric standard model. In particular a conservative lower bound on the reheating temperature, T_R \gtrsim 10^9 GeV, is obtained in the relevant effective neutrino mass range tilde{m}_1 \gtrsim 3x10^{-3} eV. We also comment on the existence of a `too-short-blanket problem' in connection with the possibility of evading the leptogenesis neutrino mass bound.

hep-ph↗
arXiv2004

Some Aspects of Thermal Leptogenesis

Properties of neutrinos may be the origin of the matter-antimatter asymmetry of the universe. In the seesaw model for neutrino masses this leads to important constraints on the properties of light and heavy neutrinos. In particular, an upper bound on the light neutrino masses of 0.1 eV can be derived. We review the present status of thermal leptogenesis with emphasis on the theoretical uncertainties and discuss some implications for lepton and quark mass hierarchies, CP violation and dark matter. We also comment on the `leptogenesis conspiracy', the remarkable fact that neutrino masses may lie in the range where leptogenesis works best.

hep-ph↗
arXiv2004

Leptogenesis for Pedestrians

During the process of thermal leptogenesis temperature decreases by about one order of magnitude while the baryon asymmetry is generated. We present an analytical description of this process so that the dependence on the neutrino mass parameters becomes transparent. In the case of maximal CP asymmetry all decay and scattering rates in the plasma are determined by the mass M_1 of the decaying heavy Majorana neutrino, the effective light neutrino mass tilde{m}_1 and the absolute mass scale bar{m} of the light neutrinos. In the mass range suggested by neutrino oscillations, m_{sol} \simeq 8*10^{-3} eV \lesssim \tilde{m}_1 \lesssim m_{atm} \simeq 5*10^{-2} eV, leptogenesis is dominated just by decays and inverse decays. The effect of all other scattering processes lies within the theoretical uncertainty of present calculations. The final baryon asymmetry is dominantly produced at a temperature T_B which can be about one order of magnitude below the heavy neutrino mass M_1. We also derive an analytical expression for the upper bound on the light neutrino masses implied by successful leptogenesis.

hep-ph↗
arXiv2003

Addendum to: Update on neutrino mixing in the early Universe

In the light of the recent WMAP results we update the constraints on a class of non standard BBN models with a simultaneous combination of non standard neutrino distributions and extra effective number of neutrinos in the expansion rate. These models can be described in terms of the two parameters Delta N_nu^tot, constrained by the primordial Helium abundance Y_p measurement, and Delta N_nu^rho, constrained by a combination of CMB and primordial Deuterium data. Small deviations from standard BBN are suggested. Different non standard scenarios can be distinguished by a measurement of the difference Delta N_nu^f_nu=Delta N_nu^tot-Delta N_nu^rho. From the current data we estimate Delta N_nu^f_nu simeq -1.4^{+0.9}_{-1.4}, mildly disfavouring solutions with a low expansion rate, characterized by Delta N_nu^f_nu=0 and negative Delta N_nu^rho. Active-sterile neutrino mixing could be a viable explanation only for high values of Y_p gtrsim 0.24. The existence of large positive neutrino chemical potentials xi_i sim 0.05, implying Delta N_nu^rho simeq 0, would be a possible explanation of the data within the analyzed class of non standard BBN models. Interestingly it would also provide a way to evade the cosmological bounds for `class A 3+1' four neutrino mixing models. A scenario with a decaying sterile neutrino is also considered.

astro-ph↗
arXiv2003

The Neutrino Mass Window for Baryogenesis

Interactions of heavy Majorana neutrinos in the thermal phase of the early universe may be the origin of the cosmological matter-antimatter asymmetry. This mechanism of baryogenesis implies stringent constraints on light and heavy Majorana neutrino masses. We derive an improved upper bound on the CP asymmetry in heavy neutrino decays which, together with the kinetic equations, yields an upper bound on all light neutrino masses of 0.1 eV. Lepton number changing processes at temperatures above the temperature T_B of baryogenesis can erase other, pre-existing contributions to the baryon asymmetry. We find that these washout processes become very efficient if the effective neutrino mass \tilde{m}_1 is larger than m_* \simeq 10^{-3} eV. All memory of the initial conditions is then erased. Hence, for neutrino masses in the range from (Δm^2_sol)^{1/2} \simeq 8*10^{-3} eV to (Δm^2_atm)^{1/2} \simeq 5*10^{-2} eV, which is suggested by neutrino oscillations, leptogenesis emerges as the unique source of the cosmological matter-antimatter asymmetry.

hep-ph↗
arXiv2002

News on Leptogenesis

The possibility to explain the CMB measurement of the baryon asymmetry with leptogenesis results in a stringent bound on the neutrino masses such that [(m_1)^2+(m_2)^2+(m_3)^2]^(1/2) < 0.30 eV. We discuss the implications of such a bound for future experiments on the absolute neutrino mass scale.

hep-ph↗
arXiv2002

A bound on neutrino masses from baryogenesis

Properties of neutrinos, the lightest of all elementary particles, may be the origin of the entire matter-antimatter asymmetry of the universe. This requires that neutrinos are Majorana particles, which are equal to their antiparticles, and that their masses are sufficiently small. Leptogenesis, the theory explaining the cosmic matter-antimatter asymmetry, predicts that all neutrino masses are smaller than 0.2 eV, which will be tested by forthcoming laboratory experiments and by cosmology.

hep-ph↗
arXiv2001

Active-sterile neutrino oscillations in the early Universe: asymmetry generation at low |delta m^2| and the Landau-Zener approximation

It is well established that active-sterile neutrino oscillations generate large neutrino asymmetries for very small mixing angles ($\sin^2 2θ_0\lesssim 10^{-4}$), negative values of $δm^2$ and provided that $|δm^2|\gtrsim 10^{-4} {\rm eV^2}$. By numerically solving the quantum kinetic equations, we show that the generation still occurs at much lower values of $|δm^2|$. We also describe the borders of the generation at small mixing angles and show how our numerical results can be analytically understood within the framework of the Landau-Zener approximation thereby extending previous work based on the adiabatic limit. This approximate approach leads to a fair description of the MSW dominated regime of the neutrino asymmetry evolution and is also able to correctly reproduce its final value. We also briefly discuss the impact that neutrino asymmetry generation could have on big bang nucleosynthesis, CMBR and relic neutrinos.

hep-ph↗
arXiv2001

The cosmological information on neutrino mixing

Cosmology provides interesting information on neutrino mixing models with sterile neutrinos. In this case non standard BBN effects can be relevant. We show how the recent measurement of the baryon content from the observations of CMB anisotropies together with the primordial nuclear abundances measurements can be used to constrain them. In particular four neutrino mixing models are potentially at variance with the cosmological observations. We also discuss the possible scenarios from future experiments.

hep-ph↗
arXiv2001

Update on neutrino mixing in the early Universe

From the current cosmological observations of CMB and nuclear abundances we show, with an analytic procedure, that the total effective number of extra neutrino species $ΔN_ν^{\rm tot}< 0.3$. We also describe the possible signatures of non standard effects that could be revealed in future CMB observations. This cosmological information is then applied to neutrino mixing models. Taking into account the recent results from the SNO and SuperKamiokande experiments, disfavouring pure active to sterile neutrino oscillations, we show that all 4 neutrino mixing models, both of 2+2 and 3+1 type, lead to a full thermalization of the sterile neutrino flavor. Moreover such a sterile neutrino production excludes the possibility of an electron neutrino asymmetry generation and we conclude that $ΔN_ν^{\rm tot}\simeq 1$, in disagreement with the cosmological bound. This result is valid under the assumption that the initial neutrino asymmetries are small. We suggest the existence of a second sterile neutrino flavor, with mixing properties such to generate a large electron neutrino asymmetry, as a possible way out.

hep-ph↗
arXiv2000

Active-Sterile neutrino oscillations and BBN+CMBR constraints

We show how active-sterile neutrino oscillations in the early Universe can play an interesting role in explaining the current observations of CMBR anisotropies and light element abundances. We describe different possible phenomenological scenarios in the interpretation of present data and how active-sterile neutrino oscillations can provide a viable theoretical framework.

hep-ph↗