Searcharxiv⌕ Search

arXiv subjects

S. T. Petcov

Publications and source records attributed to S. T. Petcov.

At least 37 records · Page 2Linked to original sources
arXiv2023

$A_4$ Modular Flavour Model of Quark Mass Hierarchies close to the Fixed Point $τ= ω$

We investigate the possibility to describe the quark mass hierarchies as well as the CKM quark mixing matrix without fine-tuning in a quark flavour model with modular $A_4$ symmetry. The quark mass hierarchies are considered in the vicinity of the fixed point $τ= ω\equiv \exp({i\,2π/3})$ (the left cusp of the fundamental domain of the modular group), $τ$ being the VEV of the modulus. The model involves modular forms of level 3 and weights 6, 4 and 2, and contains eight constants, only two of which, $g_u$ and $g_d$, can be a source of CP violation in addition to the VEV of the modulus, $τ= ω+ ε$, $(ε)^* \neq ε$, $|ε|\ll 1$. We find that in the case of real (CP-conserving) $g_u$ and $g_d$ and common $τ$ ($ε$) in the down-quark and up-quark sectors, the down-type quark mass hierarchies can be reproduced without fine tuning with $|ε| \cong 0.03$, all other constants being of ${\cal O}(1)$, and correspond approximately to $1 : |ε| : |ε|^2$. The up-type quark mass hierarchies can be achieved with the same $|ε| \cong 0.03$ but allowing $g_u\sim {\cal O}(10)$ and correspond to $1 : |ε|/|g_u| : |ε|^2/|g_u|^2$. In this setting, we discuss the CKM quark mixing and CP violation.

hep-ph↗
arXiv2023

Tests of Low-Scale Leptogenesis in Charged Lepton Flavour Violation Experiments

We consider low-energy tests of low-scale leptogenesis based on the type I seesaw scenario with three right-handed singlet neutrinos $ν_{l R}$. In this scenario, successful leptogenesis is possible for quasi-degenerate in mass heavy Majorana neutrinos $N_{1,2,3}$, $M_{1,2,3}\cong M$, $|M_j - M_i|\ll M$, $i\neq j = 1,2,3$, heavy Majorana neutrino masses $M \sim (0.05 - 5\times 10^5)$ GeV, and $N_j$ charged current and neutral current weak interaction couplings as large as $\mathcal{O}(10^{-2})$. We derive the constraints on the corresponding leptogenesis parameter space from the existing data from low-energy experiments, including the limits from the experiments on $μ\rightarrow e γ$ decay and on the rate of $μ- e$ conversion in gold. We show also that the planned and upcoming experiments on charged lepton flavour violation with $μ^\pm$, MEG II on the $μ\rightarrow eγ$ decay, Mu3e on $μ\rightarrow eee$ decay, Mu2e and COMET on $μ- e$ conversion in aluminium and PRISM/PRIME on $μ- e$ conversion in titanium, can probe significant region of the viable leptogenesis parameter space, and thus have a potential for a discovery. Experiments on $τ\to eee(μμμ)$ and $τ\to e(μ)γ$ decays (e.g., BELLE II) also can probe a part of the leptogenesis parameter space, although a relatively small one.

hep-ph↗
arXiv2022

Lepton Flavour Violation Tests of Type II Seesaw Leptogenesis

Upcoming Lepton Flavour Violation experiments searching for $μ\rightarrow 3e$ and $μ$ to $ e $ conversion in nuclei processes will provide new opportunities to test the fundamental properties of the neutrino sector, and possibly the origin of matter. In recent work, it was shown that the Type II Seesaw mechanism alone can simultaneously explain the neutrino masses, Leptogenesis, and inflation. A key prediction of this model was the possibility of signals being produced in Lepton Flavour Violation decays. Searches at future experiments such as Mu3e and COMET will be integral to determining the properties of the associated triplet Higgs, and will complement other terrestrial experimental searches and cosmological measurements. In this work, we survey the detection prospects for the ingredients of the Type II Seesaw Leptogenesis scenario, and discuss the corresponding dependencies on the neutrino oscillation parameters and $\mathcal{CP}$ phases.

hep-ph↗
arXiv2022

Neutrino Tomography of the Earth with ORCA Detector

Using PREM as a reference model for the Earth density distribution we investigate in the present article the sensitivity of the ORCA detector to deviations of the Earth i) outer core (OC) density, ii) inner core (IC) density, iii) total core density, and iv) mantle density, from their respective PREM densities. The analysis is performed by studying the effects of the Earth matter on the oscillations of atmospheric $ν_μ$, $ν_e$, $\barν_μ$ and $\barν_e$. We present results which, in particular, illustrate the dependence of the ORCA sensitivity to the OC, IC, core and mantle densities on the type of systematic uncertainties used in the analysis, on the value of the atmospheric neutrino mixing angle $θ_{23}$, on whether the Earth mass constraint is implemented or not, and on the way it is implemented, and on the type - with normal ordering (NO) or inverted ordering (IO) - of the light neutrino mass spectrum. We show, in particular, that in the "most favorable" NO case of implemented Earth mass constraint, "minimal" systematic errors and $\sin^2θ_{23}=0.58$, ORCA can determine, e.g., the OC (mantle) density at $3σ$ C.L. after 10 years of operation with an uncertainty of (-18\%)/+15\% (of (-6\%)/+8\%). In the "most unfavourable" NO case of "conservative" systematic errors and $\sin^2θ_{23}=0.42$, the uncertainty reads (-43\%)/+39\% ((-17\%/+20\%), while for for $\sin^2θ_{23} = 0.50$ and 0.58 it is noticeably smaller: (-37)\%/+30\% and (-30\%)/+24\% ((-13\%)/+16\% and (-11\%/+14\%)). We find also that the sensitivity of ORCA to the outer core, core and mantle densities is significantly worse for IO neutrino mass spectrum.

hep-ph↗
arXiv2022

Measurements of $μ\to 3e$ Decay with Polarised Muons as a Probe of New Physics

Working within the Standard Model effective field theory approach, we examine the possibility to test charged lepton flavour violating (cLFV) new physics (NP) by angular measurements of the outgoing electron and positrons in the polarised $μ^{+}\to e^{+}e^{+}e^{-}$ decay. This decay is planned to be studied with a sensitivity to the branching ratio of $B_{μ\to3e}\sim 10^{-15}$ by the Mu3e experiment at PSI. To illustrate the potential of these measurements, we consider a set of eight effective operators generating the decay at the tree level, including dimension-five dipole operators and dimension-six scalar and vector four-fermion operators of different chiralities. We show that if the polarised $μ^{+}\to e^{+}e^{+}e^{-}$ decay is observed and is induced by a single operator, data from three angular observables - two P-odd asymmetries and one T-odd asymmetry - can allow to discriminate between all considered operators with the exception of scalar and vector operators of opposite chirality. For these two types of operators, the P-odd asymmetry is maximal in magnitude; they can be distinguished, in principle, by measuring the helicities of the outgoing positrons. The observation of a non-zero T-odd asymmetry would indicate the presence in the decay rate of a dipole and vector operator interference term that is T (CP) violating.

hep-ph↗
arXiv2022

Modular Flavour Symmetries and Modulus Stabilisation

We study the problem of modulus stabilisation in the framework of the modular symmetry approach to the flavour problem. By analysing simple UV-motivated CP-invariant potentials for the modulus $τ$ we find that a class of these potentials has (non-fine-tuned) CP-breaking minima in the vicinity of the point of $\mathbb{Z}_3^{ST}$ residual symmetry, $τ\simeq e^{2πi/3}$. Stabilising the modulus at these novel minima breaks spontaneously the CP symmetry and can naturally explain the mass hierarchies of charged leptons and possibly of quarks.

hep-ph↗
arXiv2021

Flavoured Resonant Leptogenesis at Sub-TeV Scales

We consider sub-TeV scale flavoured resonant leptogenesis within the minimal type-I seesaw scenario with two right-handed singlet neutrinos $N_{1,2}$ forming a pseudo-Dirac pair, concentrating on the case of masses of the pseudo-Dirac pair having values $M_{1,2} \lesssim 100$ GeV. The case when the CP violating asymmetries in the individual lepton charges $L_l$, $l=e,μ,τ$, and in the total lepton charge $L$ of the Universe are generated in $1 \leftrightarrow 2$ decay processes is investigated. We show that successful leptogenesis is possible for $M_{1,2}$ lying in the interval $M_{1,2} = (0.3 - 100)$ GeV. Our results show also, in particular, that for vanishing initial $N_{1,2}$ abundance, flavour effects can play an important role in the generation of the baryon asymmetry, leading to an enhancement of the asymmetry by a factor up to $\sim 300$ with respect to the "unflavoured" leptogenesis scenario.

hep-ph↗
arXiv2021

Aspects of High Scale Leptogenesis with Low-Energy Leptonic CP Violation

Using the density matrix equations (DME) for high scale leptogenesis based on the type I seesaw mechanism, in which the CP violation (CPV) is provided by the low-energy Dirac or/and Majorana phases of the neutrino mixing (PMNS) matrix, we investigate the 1-to-2 and the 2-to-3 flavour regime transitions, where the 1, 2 and 3 leptogenesis flavour regimes in the generation of the baryon asymmetry of the Universe $η_B$ are described by the Boltzmann equations. Concentrating on the 1-to-2 flavour transition we determine the general conditions under which $η_B$ goes through zero and changes sign in the transition. Analysing in detail the behaviour of $η_B$ in the transition in the case of two heavy Majorana neutrinos $N_{1,2}$ with hierarchical masses, $M_1 \ll M_2$, we find, in particular, that i) the Boltzmann equations in many cases fail to describe correctly the generation of $η_B$ in the 1, 2 and 3 flavour regimes, ii) the 2-flavour regime can persist above (below) $\sim 10^{12}$ GeV ($\sim 10^9$ GeV), iii) the flavour effects in leptogenesis persist beyond the typically considered maximal for these effects leptogenesis scale of $10^{12}$ GeV. We further determine the minimal scale $M_{1\text{min}}$ at which we can have successful leptogenesis when the CPV is provided only by the Dirac or Majorana phases of the PMNS matrix as well as the ranges of scales and values of the phases for having successful leptogenesis. We show, in particular, that when the CPV is due to the Dirac phase $δ$, there is a direct relation between the sign of $\sin δ$ and the sign of $η_B$ in the regions of viable leptogenesis in the case of normal hierarchical light neutrino mass spectrum; for the inverted hierarchical spectrum the same result holds for $M_1 \lesssim 10^{13}$ GeV. The considered scenarios of leptogenesis are testable and falsifiable in low-energy neutrino experiments.

hep-ph↗
arXiv2021

Double Cover of Modular $S_4$ for Flavour Model Building

We develop the formalism of the finite modular group $Γ'_4 \equiv S'_4$, a double cover of the modular permutation group $Γ_4 \simeq S_4$, for theories of flavour. The integer weight $k>0$ of the level 4 modular forms indispensable for the formalism can be even or odd. We explicitly construct the lowest-weight ($k=1$) modular forms in terms of two Jacobi theta constants, denoted as $\varepsilon(τ)$ and $θ(τ)$, $τ$ being the modulus. We show that these forms furnish a 3D representation of $S'_4$ not present for $S_4$. Having derived the $S'_4$ multiplication rules and Clebsch-Gordan coefficients, we construct multiplets of modular forms of weights up to $k=10$. These are expressed as polynomials in $\varepsilon$ and $θ$, bypassing the need to search for non-linear constraints. We further show that within $S'_4$ there are two options to define the (generalised) CP transformation and we discuss the possible residual symmetries in theories based on modular and CP invariance. Finally, we provide two examples of application of our results, constructing phenomenologically viable lepton flavour models.

hep-ph↗
arXiv2021

Theia: An advanced optical neutrino detector

New developments in liquid scintillators, high-efficiency, fast photon detectors, and chromatic photon sorting have opened up the possibility for building a large-scale detector that can discriminate between Cherenkov and scintillation signals. Such a detector could exploit these two distinct signals to observe particle direction and species using Cherenkov light while also having the excellent energy resolution and low threshold of a scintillator detector. Situated in a deep underground laboratory, and utilizing new techniques in computing and reconstruction techniques, such a detector could achieve unprecedented levels of background rejection, thus enabling a rich physics program that would span topics in nuclear, high-energy, and astrophysics, and across a dynamic range from hundreds of keV to many GeV. The scientific program would include observations of low- and high-energy solar neutrinos, determination of neutrino mass ordering and measurement of the neutrino CP violating phase, observations of diffuse supernova neutrinos and neutrinos from a supernova burst, sensitive searches for nucleon decay and, ultimately, a search for NeutrinoLess Double Beta Decay (NLDBD) with sensitivity reaching the normal ordering regime of neutrino mass phase space. This paper describes Theia, a detector design that incorporates these new technologies in a practical and affordable way to accomplish the science goals described above. We consider two scenarios, one in which Theia would reside in a cavern the size and shape of the caverns intended to be excavated for the Deep Underground Neutrino Experiment (DUNE) which we call Theia 25, and a larger 100 ktonne version (Theia 100) that could achieve an even broader and more sensitive scientific program.

physics.ins-det↗
arXiv2019

Leptogenesis in the Neutrino Option

We examine the compatibility between the Neutrino Option, in which the electroweak scale is generated by PeV mass type I seesaw Majorana neutrinos, and leptogenesis. We find the Neutrino Option is consistent with resonant leptogenesis. Working within the minimal seesaw scenario with two heavy Majorana neutrinos $N_{1,2}$, which form a pseudo-Dirac pair, we explore the viable parameter space. We find that the Neutrino Option and successful leptogenesis are compatible in the cases of a neutrino mass spectrum with normal (inverted) ordering for $1.2 \times 10^6 < M \text{ (GeV)} < 8.8 \times 10^6$ ($2.4 \times 10^6 < M \text{ (GeV)} < 7.4 \times 10^6$), with $M = (M_1 + M_2)/2$ and $M_{1,2}$ the masses of $N_{1,2}$. Successful leptogenesis requires that $ΔM/M \equiv (M_2 - M_1)/M \sim 10^{-8}$. We further show that leptogenesis can produce the baryon asymmetry of the Universe within the Neutrino Option scenario when the requisite CP violation in leptogenesis is provided exclusively by the Dirac or Majorana low energy CP violation phases of the PMNS matrix.

hep-ph↗
arXiv2019

The Nature of the Neutrino (Dirac/Majorana) and Double Beta Decay with or without Neutrinos

The history of research associated with the fundamental problem of the nature of massive neutrinos, which can be Dirac particles or Majorana fermions, and the related processes of neutrinoless double beta decay, $(A,Z)\rightarrow (A,Z+2) + e^- + e^-$, and two neutrino double beta decay, $(A,Z)\rightarrow (A,Z+2) + e^- + e^- + \bar{ν_e} + \bar{ν_e}$, is reviewed.

hep-ph↗
arXiv2019

Generalised CP Symmetry in Modular-Invariant Models of Flavour

The formalism of combined finite modular and generalised CP (gCP) symmetries for theories of flavour is developed. The corresponding consistency conditions for the two symmetry transformations acting on the modulus $τ$ and on the matter fields are derived. The implications of gCP symmetry in theories of flavour based on modular invariance described by finite modular groups are illustrated with the example of a modular $S_4$ model of lepton flavour. Due to the addition of the gCP symmetry, viable modular models turn out to be more constrained, with the modulus $τ$ being the only source of CP violation.

hep-ph↗
arXiv2019

Modular $A_5$ Symmetry for Flavour Model Building

In the framework of the modular symmetry approach to lepton flavour, we consider a class of theories where matter superfields transform in representations of the finite modular group $Γ_5 \simeq A_5$. We explicitly construct a basis for the 11 modular forms of weight 2 and level 5. We show how these forms arrange themselves into two triplets and a quintet of $A_5$. We also present multiplets of modular forms of higher weight. Finally, we provide an example of application of our results, constructing two models of neutrino masses and mixing based on the supersymmetric Weinberg operator.

hep-ph↗
arXiv2019

Trimaximal Neutrino Mixing from Modular A4 Invariance with Residual Symmetries

We construct phenomenologically viable models of lepton masses and mixing based on modular $A_4$ invariance broken to residual symmetries $\mathbb{Z}^{T}_3$ or $\mathbb{Z}^{ST}_3$ and $\mathbb{Z}^S_2$ respectively in the charged lepton and neutrino sectors. In these models the neutrino mixing matrix is of trimaximal mixing form. In addition to successfully describing the charged lepton masses, neutrino mass-squared differences and the atmospheric and reactor neutrino mixing angles $θ_{23}$ and $θ_{13}$, these models predict the values of the lightest neutrino mass (i.e., the absolute neutrino mass scale), of the Dirac and Majorana CP violation(CPV) phases, as well as the existence of specific correlations between i) the values of the solar neutrino mixing angle $θ_{12}$ and the angle $θ_{13}$ (which determines $θ_{12}$), ii) the values of the Dirac CPV phase $δ$ and of the angles $θ_{23}$ and $θ_{13}$, iii) the sum of the neutrino masses and $θ_{23}$, and iv) between the two Majorana phases.

hep-ph↗
arXiv2019

Modular $S_4$ Models of Lepton Masses and Mixing

We investigate models of charged lepton and neutrino masses and lepton mixing based on broken modular symmetry. The matter fields in these models are assumed to transform in irreducible representations of the finite modular group $Γ_4 \simeq S_4$. We analyse the minimal scenario in which the only source of symmetry breaking is the vacuum expectation value of the modulus field. In this scenario there is no need to introduce flavon fields. Using the basis for the lowest weight modular forms found earlier, we build minimal phenomenologically viable models in which the neutrino masses are generated via the type I seesaw mechanism. While successfully accommodating charged lepton masses, neutrino mixing angles and mass-squared differences, these models predict the values of the lightest neutrino mass (i.e., the absolute neutrino mass scale), of the Dirac and Majorana CP violation (CPV) phases, as well as specific correlations between the values of the atmospheric neutrino mixing parameter $\sin^2θ_{23}$ and i) the Dirac CPV phase $δ$, ii) the sum of the neutrino masses, and iii) the effective Majorana mass in neutrinoless double beta decay. We consider also the case of residual symmetries $\mathbb{Z}^{ST}_3$ and $\mathbb{Z}^S_2$ respectively in the charged lepton and neutrino sectors, corresponding to specific vacuum expectation values of the modulus.

hep-ph↗