arXiv subjects

Enrico Nardi

Publications and source records attributed to Enrico Nardi.

At least 55 records · Page 3Linked to original sources

arXiv2015

Minimal Asymmetric Dark Matter

In the early Universe, any particle carrying a conserved quantum number and in chemical equilibrium with the thermal bath will unavoidably inherit a particle-antiparticle asymmetry. A new particle of this type, if stable, would represent a candidate for asymmetric dark matter (DM) with an asymmetry directly related to the baryon asymmetry. We study this possibility for a minimal DM sector constituted by just one (generic) $SU(2)_L$ multiplet $χ$ carrying hypercharge, assuming that at temperatures above the electroweak phase transition an effective operator enforces chemical equilibrium between $χ$ and the Higgs boson. We argue that limits from DM direct detection searches severely constrain this scenario, leaving as the only possibilities scalar or fermion multiplets with hypercharge $y = 1$, preferentially quintuplets or larger $SU(2)$ representations, and with a mass in the few TeV range.

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arXiv2015

Quark Yukawa pattern from spontaneous breaking of flavour $SU(3)^3$

A $SU(3)_Q \times SU(3)_u \times SU(3)_d$ invariant scalar potential breaking spontaneously the quark flavour symmetry can explain the standard model flavour puzzle. The approximate alignment in flavour space of the vacuum expectation values of the up and down `Yukawa fields' results as a dynamical effect. The observed quark mixing angles, the weak CP violating phase, and hierarchical quark masses can be all reproduced at the cost of introducing additional (auxiliary) scalar multiplets, but without the need of introducing hierarchical parameters.

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arXiv2015

Leptogenesis in SO(10)

We consider $SO(10)$ Grand Unified Theories (GUTs) with vacuum expectation values (vevs) for fermion masses in the $\mathbf{10} + \mathbf{\overline{126}}$ representation. We show that the baryon asymmetry generated via leptogenesis is completely determined in terms of measured low energy observables and of one single high energy parameter related to the ratio of the $\mathbf{10}$ and $\mathbf{\overline{126}}$ $SU(2)$ doublet vevs. We identify new decay channels for the heavy Majorana neutrinos into $SU(2)$ singlet leptons $e^c$ which can sizeably affect the size of the resulting baryon asymmetry. We describe how to equip $SO(10)$ fits to low energy data with the additional constraint of successful leptogenesis, and we apply this procedure to the fits carried out in ref.~\cite{Dueck:2013gca}. We show that a baryon asymmetry in perfect agreement with observations is obtained.

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arXiv2014

Quark masses, mixings, and CP violation from spontaneous breaking of flavor $SU(3)^{3}$

A ${\cal G}_{\cal F}=SU(3)_{Q}\times SU(3)_{u}\times SU(3)_{d}$ invariant scalar potential breaking spontaneously the quark flavor symmetry can explain the standard model flavor puzzle. The approximate alignment in flavor space of the vacuum expectation values of the up and down ``Yukawa fields'' is explained as a dynamical effect, and the observed quark mixing angles, the weak CP violating phase, and hierarchical quark masses can be reproduced without introducing hierarchical parameters.

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arXiv2014

Cloistered Baryogenesis

The cosmological matter-antimatter asymmetry can arise from the baryon number conserving CP asymmetry in two body decays of heavy particles, when the two final states carry equal and opposite baryon number, and one couples directly or indirectly to electroweak sphalerons so that its baryon asymmetry gets partly reprocessed into a lepton asymmetry, while the other remains chemically decoupled from the thermal bath with its baryon content frozen. After sphaleron switchoff the decay of the decoupled particles inject in the thermal plasma an unbalanced baryon asymmetry, giving rise to baryogenesis. We highlight the features of this mechanism in a type-I seesaw model extended by adding a new colored scalar coupled to the heavy Majorana neutrinos. If the colored scalar has an O(TeV) mass, it would leave at the LHC a characteristic signature throughout all layers of the detectors.

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arXiv2013

New ways to TeV scale leptogenesis

We show that by adding to the standard model plus the type I seesaw different types of scalars, it is possible to construct models that satisfy the three requirements of (i) generating neutrino masses at the TeV scale, (ii) being testable at the LHC via direct production of new states, and (iii) allowing for leptogenesis at temperatures $T\sim O$(TeV).

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arXiv2013

Spontaneous breaking of the flavor symmetry avoids the strong CP problem

A promising approach to the Standard Model flavor puzzle is based on the idea that the $SU(3)^3$ quark-flavor symmetry is spontaneously broken by vacuum expectation values of `Yukawa fields' which minimize the symmetry invariant scalar potential at configurations corresponding to the observed quark masses and mixing angles. We show that this approach provides a simple and elegant explanation for CP conservation in strong interactions.

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arXiv2013

Leptogenesis in the Universe

Leptogenesis is a class of scenarios in which the cosmic baryon asymmetry originates from an initial lepton asymmetry generated in the decays of heavy sterile neutrinos in the early Universe. We explain why leptogenesis is an appealing mechanism for baryogenesis. We review its motivations, the basic ingredients, and describe subclasses of effects, like those of lepton flavours, spectator processes, scatterings, finite temperature corrections, the role of the heavier sterile neutrinos and quantum corrections. We then address leptogenesis in supersymmetric scenarios, as well as some other popular variations of the basic leptogenesis framework.

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arXiv2012

Yukawa hierarchies from spontaneous breaking of the $\bma{SU(3)_L\times SU(3)_R}$ flavour symmetry?

The tree level potential for a scalar multiplet of `Yukawa fields' $Y$ for one type of quarks admits the promising vacuum configuration $ \propto {\rm diag}(0,0,1)$ that breaks spontaneously $SU(3)_L\times SU(3)_R$ flavour symmetry. We investigate whether the vanishing entries could be lifted to nonvanishing values by slightly perturbing the potential, thus providing a mechanism to generate the Yukawa hierarchies. For theories where at the lowest order the only massless states are Nambu-Goldstone bosons we find, as a general result, that the structure of the tree-level vacuum is perturbatively stable against corrections from scalar loops or higher dimensional operators. We discuss the reasons for this stability, and give an explicit illustration in the case of loop corrections by direct computation of the one-loop effective potential of Yukawa fields. Nevertheless, a hierarchical configuration $ \propto {\rm diag}(ε',ε,1)$ (with $ε', ε\ll 1$) can be generated by enlarging the scalar Yukawa sector. We present a simple model in which spontaneous breaking of the flavour symmetry can give rise to the fermion mass hierarchies.

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arXiv2012

Squeezing out predictions with leptogenesis from SO(10)

We consider the see-saw mechanism within a non-supersymmetric SO(10) model. By assuming the SO(10) quark-lepton symmetry, and after imposing suitable conditions that ensure that the right-handed (RH) neutrino masses are at most mildly hierarchical (compact RH spectrum) we obtain a surprisingly predictive scenario. The absolute neutrino mass scale, the Dirac and the two Majorana phases of the neutrino mixing matrix remain determined in terms of the set of already measured low energy observables, modulo a discrete ambiguity in the signs of two neutrino mixing angles and of the Dirac phase. The RH neutrinos mass spectrum is also predicted, as well as the size and sign of the leptogenesis CP asymmetries. We compute the cosmological baryon asymmetry generated through leptogenesis and obtain the correct sign and a size compatible with observations.

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arXiv2012

Neutrino masses in $SU(5)\times U(1)_F$ with adjoint flavons

We present a $SU(5)\times U(1)_F$ supersymmetric model for neutrino masses and mixings that implements the seesaw mechanism by means of the heavy SU(2) singlets and triplets states contained in three adjoints of SU(5). We discuss how Abelian $U(1)_F$ symmetries can naturally yield non-hierarchical light neutrinos even when the heavy states are strongly hierarchical, and how it can also ensure that $R$--parity arises as an exact accidental symmetry. By assigning two flavons that break $U(1)_F$ to the adjoint representation of SU(5) and assuming universality for all the fundamental couplings, the coefficients of the effective Yukawa and Majorana mass operators become calculable in terms of group theoretical quantities. There is a single free parameter in the model, however, at leading order the structure of the light neutrinos mass matrix is determined in a parameter independent way.

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arXiv2011

Lepton flavor violation in minimal flavor violation extensions of the seesaw

A minimal flavour violation hypothesis for leptons can be implemented essentially in two ways that are compatible with a type-I seesaw structure with three heavy singlet neutrinos $N$, and that satisfy the requirement of being predictive, in the sense that all lepton flavor violating (LFV) effects can be expressed in terms of low energy observables. The first realization is CP conserving and is based on the flavor group $SU(3)_e\times SU(3)_\ell\times O(3)_N$ (being $e$ and $\ell$ the SU(2) singlet and doublet leptons). The second realization allows for CP violation and is based on $SU(3)_e\times SU(3)_{\ell+N}$. I review the main features of the two schemes and point out their different implications for LFV observables.

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arXiv2011

Leptogenesis from Soft Supersymmetry Breaking (Soft Leptogenesis)

Soft leptogenesis is a scenario in which the cosmic baryon asymmetry is produced from a lepton asymmetry generated in the decays of heavy sneutrinos (the partners of the singlet neutrinos of the seesaw) and where the relevant sources of CP violation are the complex phases of soft supersymmetry-breaking terms. We explain the motivations for soft leptogenesis, and review its basic ingredients: the different CP-violating contributions, the crucial role played by thermal corrections, and the enhancement of the efficiency from lepton flavour effects. We also discuss the high temperature regime $T > 10^7$ GeV in which the cosmic baryon asymmetry originates from an initial asymmetry of an anomalous $R$-charge, and soft leptogenesis reembodies in $R$-genesis.

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arXiv2011

Naturally large Yukawa hierarchies

The spontaneous breaking of the $SU(3)^5$ quark/lepton flavor symmetry by means of three multiplets of scalar `Yukawa fields' admits vacua with one O(1) and two vanishing vacuum expectation values (vevs) for each multiplet. If the number of generations is equal to three, and only in this case, the vanishing vevs are lifted to exponentially suppressed entries by the inclusion of symmetry invariant logarithmic terms. A strong hierarchy for the Yukawa couplings and a quark mixing matrix that approaches a diagonal form are obtained in a natural way from O(1) parameters. This scenario provides a concrete realization of the minimal flavor violation hypothesis.

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arXiv2011

Minimal flavour violation extensions of the seesaw

We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compatible with a type-I seesaw structure with three heavy singlet neutrinos N, and satisfying the requirement of being predictive, in the sense that all LFV effects can be expressed in terms of low energy observables. We find a new interesting realization based on the flavour group $SU(3)_e\times SU(3)_{\ell+N}$ (being $e$ and $\ell$ respectively the SU(2) singlet and doublet leptons). An intriguing feature of this realization is that, in the normal hierarchy scenario for neutrino masses, it allows for sizeable enhancements of $μ\to e$ transitions with respect to LFV processes involving the $τ$ lepton. We also discuss how the symmetries of the type-I seesaw allow for a strong suppression of the N mass scale with respect to the scale of lepton number breaking, without implying a similar suppression for possible mechanisms of N production

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arXiv2011

Early Universe effective theories: The Soft Leptogenesis and R-Genesis Cases

We discuss the effective theory appropriate for studying soft leptogenesis at temperatures T>10^7 GeV. In this regime, the main source of the B-L asymmetry is the CP asymmetry of a new anomalous R-charge that couple to generalized anomalous electroweak processes. Baryogenesis thus occurs mainly through R-genesis, and with an efficiency that can be up to two orders of magnitude larger than in usual estimates. Contrary to common belief, a sizeable baryon asymmetry is generated also when thermal corrections to the CP asymmetries in sneutrino decays are neglected which, in soft leptogenesis, implyes vanishing lepton-flavour CP asymmetries. We present general Boltzmann equations for soft leptogenesis that are valid in all temperature regimes.

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arXiv2010

Flavoured soft leptogenesis and natural values of the B term

We revisit flavour effects in soft leptogenesis relaxing the assumption of universality for the soft supersymmetry breaking terms. We find that with respect to the case in which the heavy sneutrinos decay with equal rates and equal CP asymmetries for all lepton flavours, hierarchical flavour configurations can enhance the efficiency by more than two orders of magnitude. This translates in more than three order of magnitude with respect to the one-flavour approximation. We verify that lepton flavour equilibration effects related to off-diagonal soft slepton masses are ineffective for damping these large enhancements. We show that soft leptogenesis can be successful for unusual values of the relevant parameters, allowing for $B\sim {\cal O}({\rm TeV})$ and for values of the washout parameter up to $m_{\rm eff}/m_* \sim 5\times 10^{3}$.

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arXiv2010

Supersymmetric Leptogenesis

We study leptogenesis in the supersymmetric standard model plus the seesaw. We identify important qualitative differences that characterize supersymmetric leptogenesis with respect to the non-supersymmetric case. The lepton number asymmetries in fermions and scalars do not equilibrate, and are related via a non-vanishing gaugino chemical potential. Due to the presence of new anomalous symmetries, electroweak sphalerons couple to winos and higgsinos, and QCD sphalerons couple to gluinos, thus modifying the corresponding chemical equilibrium conditions. A new constraint on particles chemical potentials corresponding to an exactly conserved $R$-charge, that also involves the number density asymmetry of the heavy sneutrinos, appears. These new ingredients determine the $3\times 4$ matrices that mix up the density asymmetries of the lepton flavours and of the heavy sneutrinos. We explain why in all temperature ranges the particle thermodynamic system is characterized by the same number of independent quantities. Numerical differences with respect to usual treatment remain at the ${\cal O}(1)$ level.

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