arXiv subjects

Darius Jurčiukonis

Publications and source records attributed to Darius Jurčiukonis.

16 recordsLinked to original sources

arXiv2026

Assessing boundedness from below in the $\mathbb{Z}_2 \times \mathbb{Z}_2$-symmetric three-Higgs-doublet model: algorithm and machine learning

The scalar potential of any particle-physics model must be bounded from below (BFB). We consider the extension of the Standard electroweak Model with three $SU(2)$ doublets of scalars and a symmetry under which each of those doublets changes sign. In the absence of necessary and sufficient conditions for boundedness from below (BnessFB) for this specific model, we argue that one may use increasingly stringent sets of necessary conditions. We introduce a Mathematica code, StableWein, that implements this idea. The user is allowed to choose the level of accuracy that they want in the determination of BnessFB; more precision means the use of more necessary conditions, and usually entails a longer running time for the code. Our investigation suggests that our procedure and code can be extremely precise in the determination of the potentials that are BFB. In addition, we introduce a machine-learning code that identifies, with more than 99% accuracy, which potentials are BFB.

hep-ph↗

arXiv2026

Conditions for boundedness from below of a $Δ(54)$-symmetric three-Higgs-doublet model

We investigate the orbit space of the scalar potential of a $Δ(54)$-symmetric three-Higgs-doublet model. We find that, if the potential enjoys $CP$ invariance, then its three-dimensional orbit space is a polytope; if the potential has no $CP$ symmetry, then its four-dimensional orbit space has a boundary that is sometimes slightly concave, but seems never to be convex. Consequently, we conjecture necessary and sufficient conditions for the potential to be bounded from below; brute-force minimization of a large number of potentials affirms the accuracy of our conjecture. We list all possible charge-conserving and charge-breaking minima of the potential.

hep-ph↗

arXiv2026

On the addition of an $SU(2)$ quadruplet of scalars to the Standard Model

We consider the extension of the Standard electroweak Model through an $SU(2)$ quadruplet of scalars with hypercharge either $3/2$ or $1/2$ (with an additional reflection symmetry in the latter case). We establish, through $\textit{exact analytical equations}$, the boundaries of the phase spaces of the gauge-invariant terms that appear in the (renormalizable) scalar potentials. We devise procedures for the determination of necessary and sufficient bounded-from-below conditions on those potentials; we emphasize that one mostly needs to scan the scalar potential over a few $\textit{lines}$, instead of $\textit{surfaces}$, in order to establish the boundedness-from-below; this fact allows one $\textit{to reduce by three orders of magnitude the computational time}$ devoted to that establishment.

hep-ph↗

arXiv2025

Applications of Machine Learning in Constraining Multi-Scalar Models

Machine learning techniques are used to predict theoretical constraints such as unitarity and boundedness from below in extensions of the Standard Model. This approach has proven effective for models incorporating additional SU(2) scalar multiplets, in particular the quadruplet and sixplet cases. High predictive performance is achieved through the use of suitable neural network architectures and well-prepared training datasets. Moreover, machine learning provides a substantial computational advantage, enabling significantly faster evaluations compared to scalar potential minimization.

hep-ph↗

arXiv2025

Vacuum Stability Conditions for New $SU(2)$ Multiplets

We consider the addition to the Standard Model of a scalar $SU(2)$ multiplet $Δ_n$ with dimension $n$ going from $1$ to $6$. The multiplet $Δ_n$ is assumed to have null vacuum expectation value and an arbitrary (free) hypercharge. We determine the shape of the phase space for the new terms that appear in the scalar potential (SP); we observe in particular that, in the case of a 6-plet, the phase space is slightly concave along one of its boundaries. We determine the bounded-from-below and vacuum stability conditions on the SP for each value of $n$.

hep-ph↗

arXiv2025

On the extension of the SM through a scalar quadruplet

We consider the extension of the Standard Model (SM) through a scalar quadruplet with hypercharge either $1/2$ or $3/2$; in the first case, we assume $CP$ invariance of the scalar potential (SP). We write down the unitarity conditions on the SP. We use a partly numerical method to find the bounded-from-below conditions on the SP. We determine the masses of the new scalars of the model. We compute the three- and four-Higgs couplings and compare them to the ones of the SM.

hep-ph↗

arXiv2024

Constraints on large scalar multiplets added to the Standard Model

We study the extension of the Standard Model (SM) by introducing a scalar multiplet with arbitrary isospin $J$ and hypercharge $Y$. We explicitly consider various possible values of the weak isospin $J$, up to and including $J=7/2$. The mass differences among the components of the multiplet originate from its coupling to the Higgs doublet of the SM, as present in the scalar potential (SP). We derive exact bounded-from-below (BFB) and unitarity (UNI) conditions for this model, even when the SP includes the most general quartic terms involving the multiplet components. We find that the upper bound on the mass differences depends not only on the UNI conditions but also on the BFB ones, thus imposing constraints on the mass differences. We compare these constraints to those derived from the oblique parameters (OPs) and from solutions of the renormalization-group equations (RGEs).

hep-ph↗

arXiv2024

On the addition of a large scalar multiplet to the Standard Model

We consider the addition of a single $SU(2)$ multiplet of complex scalar fields to the Standard Model (SM). We explicitly consider the various possible values of the weak isospin $J$ of that multiplet, up to and including $J = 7/2$. We allow the multiplet to have arbitrary weak hypercharge. The scalar fields of the multiplet are assumed to have no vacuum expectation value; the mass differences among the components of the multiplet originate in its coupling, present in the scalar potential (SP), to the Higgs doublet of the SM. We derive exact bounded-from-below and unitarity conditions on the SP, thereby constraining those mass differences. We compare those constraints to the ones that may be derived from the oblique parameters.

hep-ph↗

arXiv2024

The oblique parameters from arbitrary new fermions

We compute the six oblique parameters $S, T, U, V, W, X$ in a New Physics Model with an arbitrary number of new fermions, in arbitrary representations of $SU(2) \times U(1)$, and mixing arbitrarily among themselves. We show that $S$ and $U$ are automatically finite, but $T$ is finite only if there is a specific relation between the masses of the new fermions and the representations of $SU(2) \times U(1)$ that they sit in. We apply our general computation to two illustrative cases.

hep-ph↗

arXiv2024

Machine Learning for Prediction of Unitarity and Bounded from Below Constraints

The machine learning (ML) techniques to predict unitarity (UNI) and bounded from below (BFB) constraints in multi-scalar models is employed. The effectiveness of this approach is demonstrated by applying it to the two and three Higgs doublet models, as well as the left-right model. By employing suitable neural network architectures, learning algorithms, and carefully curated training datasets, a significantly high level of predictivity is achieved. Machine learning offers a distinct advantage by enabling faster calculations compared to alternative numerical methods, such as scalar potential minimization. This research investigates the feasibility of utilizing machine learning techniques as an alternative for predicting these constraints, offering potential improvements over traditional numerical calculations.

hep-ph↗

arXiv2023

The $Z b \bar b$ vertex in a left-right model

We consider the one-loop corrections to the $Z b \bar b$ vertex in a $CP$-conserving left--right model (LRM), $viz$. a model with gauge group $SU(2)_L \times SU(2)_R \times U(1)$. We allow the gauge coupling constants of $SU(2)_L$ and $SU(2)_R$ to be different. The spontaneous symmetry breaking is accomplished only by doublets and/or singlets of $SU(2)_L$ and $SU(2)_R$. The lightest massive neutral gauge boson of our LRM is assumed to have the same Yukawa couplings to bottom-quark pairs as the $Z$ of the Standard Model (SM); this assumption has the advantage that, then, the infrared divergences automatically cancel down in the subtraction of the $Z b \bar b$ vertex in the SM from the same vertex in the LRM. We effect a proper renormalization of the $Z b \bar b$ vertex and check explicitly both its gauge invariance and the cancellation of all the ultraviolet divergences. We find out that a LRM with the above assumptions cannot achieve a better fit to the $Z b \bar b$ vertex than a multi-Higgs extension of the SM, $viz$. both models can only achieve a decent fit when one admits scalar particles with very low masses $\lesssim 50$ GeV. This is true even when we allow for markedly different gauge coupling constants of $SU(2)_L$ and $SU(2)_R$.

hep-ph↗

arXiv2023

The centers of discrete groups as stabilizers of Dark Matter

The most usual option to stabilize Dark Matter (DM) is a $Z_2$ symmetry. In general, though, DM may be stabilized by any $Z_N$ with $N \ge 2$. We consider the way $Z_N$ is a subgroup of the internal-symmetry group $G$ of a model; we entertain the possibility that $Z_N$ is the center of $G$, yet $G$ is not of the form $Z_N \times G^\prime$, where $G^\prime$ is a group smaller (i.e. of lower order) than $G$. We examine all the discrete groups of order smaller than 2001 and we find that many of them cannot be written as the direct product of a cyclic group and some other group, yet they have a non-trivial center that might be used in Model Building to stabilize DM.

hep-ph↗

arXiv2022

Two-body lepton-flavour-violating decays in a 2HDM with soft family-lepton-number breaking

We evaluate the decays $\ell_1^\pm \to \ell_2^\pm γ$, $Z \to \ell_1^+ \ell_2^-$, and $h \to \ell_1^+ \ell_2^-$, where $\ell_1$ and $\ell_2$ are charged leptons with different flavours and $h$ is the scalar particle with mass 125.25 GeV, in a two-Higgs-doublet model where all the Yukawa-coupling matrices conserve the lepton flavours but the Majorana mass terms of the right-handed neutrinos break the flavour lepton numbers. We find that (1) the decays $\ell_1^\pm \to \ell_2^\pm γ$ require large Yukawa couplings and very light right-handed neutrinos in order to be visible, (2) the decays $Z \to \ell_1^+ \ell_2^-$ will be invisible in all the planned experiments, except in a very restricted range of circumstances, but (3) the decays $h \to \ell_1^+ \ell_2^-$ may be detected in future experiments for rather relaxed sets of input parameters.

hep-ph↗

arXiv2021

Fitting the $Z b \bar b$ vertex in the two-Higgs-doublet model and in the three-Higgs-doublet model

We investigate the new contributions to the parameters $g_L$ and $g_R$ of the $Z b \bar b$ vertex in a multi-Higgs-doublet model (MHDM). We emphasize that those contributions generally worsen the fit of those parameters to the experimental data. We propose a solution to this problem, wherein $g_R$ has the opposite sign from the one predicted by the Standard Model; this solution, though, necessitates light scalars and large Yukawa couplings in the MHDM.

hep-ph↗

arXiv2019

Future Opportunities in Accelerator-based Neutrino Physics

This document summarizes the conclusions of the Neutrino Town Meeting held at CERN in October 2018 to review the neutrino field at large with the aim of defining a strategy for accelerator-based neutrino physics in Europe. The importance of the field across its many complementary components is stressed. Recommendations are presented regarding the accelerator based neutrino physics, pertinent to the European Strategy for Particle Physics. We address in particular i) the role of CERN and its neutrino platform, ii) the importance of ancillary neutrino cross-section experiments, and iii) the capability of fixed target experiments as well as present and future high energy colliders to search for the possible manifestations of neutrino mass generation mechanisms.

hep-ex↗

arXiv2014

Progress in the parametrisation of the Neutrino sector

Adding gauge singlets to the original Standard Model allows an explanation for the observed smallness of the neutrino masses using the seesaw mechanism. Following our plans presented in the last conference of this series we present the results for the non-standard setting, when the number of the singlets is smaller than the number of the SM generations.

hep-ph↗