arXiv subjects

Sergey Kovalenko

Publications and source records attributed to Sergey Kovalenko.

At least 55 records · Page 3Linked to original sources

arXiv2018

Deep inelastic $e-τ$ and $μ-τ$ conversion in the NA64 experiment at the CERN SPS

We study the Lepton Flavor Violating (LFV) $e(μ)-τ$ conversion in Deep Inelastic Scattering (DIS) of electron (muon) on fixed-target nuclei. Our model-independent analysis is based on the set of the low-energy effective four-fermion LFV operators composed of leptons and quarks with the corresponding mass scales $Λ_{k}$ for each operator. Using the estimated sensitivity of the search for this LFV process in events with large missing energy in the NA64 experiment at the CERN SPS, we derive lower limits for $Λ_{k}$ and compared them with the corresponding limits existing in the literature. We show that the DIS $e(μ)-τ$ conversion is able to provide a plenty of new limits as yet non-existing in the literature. We also analyzed the energy spectrum of the final-state $τ$ and discussed viability of the observation of this process in the NA64 experiment and ones akin to it. The case of polarized beams and targets is also discussed.

hep-ph↗

arXiv2018

Neutrinoless double beta decay and QCD running at low energy scales

There is a common belief that the main uncertainties in the theoretical analysis of neutrinoless double beta ($0νββ$) decay originate from the nuclear matrix elements. Here, we uncover another previously overlooked source of potentially large uncertainties stemming from non-perturbative QCD effects. Recently perturbative QCD corrections have been calculated for all dimension 6 and 9 effective operators describing $0νββ$-decay and their importance for a reliable treatment of $0νββ$-decay has been demonstrated. However, these perturbative results are valid at energy scales above $\sim 1$ GeV, while the typical $0νββ$-scale is about $\sim 100$ MeV. In view of this fact we examine the possibility of extrapolating the perturbative results towards sub-GeV non-perturbative scales on the basis of the QCD coupling constant "freezing" behavior using Background Perturbation Theory. Our analysis suggests that such an infrared extrapolation does modify the perturbative results for both short-range and long-range mechanisms of $0νββ$-decay in general only moderately. We also discuss that the tensor$\otimes$tensor effective operator can not appear alone in the low-energy limit of any renormalizable high-scale model and then demonstrate that all five linearly independent combinations of the scalar and tensor operators, that can appear in renormalizable models, are infrared stable.

hep-ph↗

arXiv2018

Fermion Masses and Mixings and Dark Matter Constraints in a Model with Radiative Seesaw Mechanism

We formulate a predictive model of fermion masses and mixings based on a $Δ(27)$ family symmetry. In the quark sector the model leads to the viable mixing inspired texture where the Cabibbo angle comes from the down quark sector and the other angles come from both up and down quark sectors. In the lepton sector the model generates a predictive structure for charged leptons and, after radiative seesaw, an effective neutrino mass matrix with only one real and one complex parameter. We carry out a detailed analysis of the predictions in the lepton sector, where the model is only viable for inverted neutrino mass hierarchy, predicting a strict correlation between $θ_{23}$ and $θ_{13}$. We show a benchmark point that leads to the best-fit values of $θ_{12}$, $θ_{13}$, predicting a specific $\sin^2θ_{23} \simeq 0.51$ (within the $3 σ$ range), a leptonic CP-violating Dirac phase $δ\simeq 281.6 ^\circ$ and for neutrinoless double-beta decay $m_{ee} \simeq 41.3$ meV. We turn then to an analysis of the dark matter candidates in the model, which are stabilized by an unbroken $\mathbb{Z}_2$ symmetry. We discuss the possibility of scalar dark matter, which can generate the observed abundance through the Higgs portal by the standard WIMP mechanism. An interesting possibility arises if the lightest heavy Majorana neutrino is the lightest $\mathbb{Z}_2$-odd particle. The model can produce a viable fermionic dark matter candidate, but only as a feebly interacting massive particle (FIMP), with the smallness of the coupling to the visible sector protected by a symmetry and directly related to the smallness of the light neutrino masses.

hep-ph↗

arXiv2018

Relating CP-violating decays to the neutron EDM

We use the present upper bound on the neutron electric dipole moment to give an estimate for the upper limit of the CP-violating couplings of the $η(η')$ meson to the neutron. Using this result, we derive constraints on the CP-violating two-pion decays of the $η(η')$. Our results are relevant for the running and planned GlueX and LHCb measurements of rare meson decays.

hep-ph↗

arXiv2017

Predictive Pati-Salam theory of fermion masses and mixing

We propose a Pati-Salam extension of the standard model incorporating a flavor symmetry based on the $Δ\left( 27\right)$ group. The theory realizes a realistic Froggatt-Nielsen picture of quark mixing and a predictive pattern of neutrino oscillations. We find that, for normal neutrino mass ordering, the atmospheric angle must lie in the higher octant, CP must be violated in oscillations, and there is a lower bound for the $0νββ$ decay rate. For the case of inverted mass ordering, we find that the lower atmospheric octant is preferred, and that CP can be conserved in oscillations. Neutrino masses arise from a low-scale seesaw mechanism, whose messengers can be produced by a $Z^{\prime }$ portal at the LHC.

hep-ph↗

arXiv2017

Radiative Seesaw-type Mechanism of Fermion Masses and Non-trivial Quark Mixing

We propose a predictive inert 2 Higgs doublet model, where the Standard Model (SM) symmetry is extended by $S_{3}\otimes Z_{2}\otimes Z_{12}$ and the field content is enlarged by extra scalar fields, charged exotic fermions and two heavy right-handed Majorana neutrinos. The charged exotic fermions generate a nontrivial quark mixing and provide one-loop-level masses for the first- and second-generation charged fermions. The masses of the light active neutrinos are generated from a one loop-level radiative seesaw mechanism. Our model successfully explains the observed SM fermion mass and mixing pattern.

hep-ph↗

arXiv2017

Radiatively generated hierarchy of lepton and quark masses

We propose a model for radiatively generating the hierarchy of the Standard Model (SM) fermion masses: tree-level top quark mass; 1-loop bottom, charm, tau and muon masses; 2-loop masses for the light up, down and strange quarks as well as for the electron; and 4-loop masses for the light active neutrinos. Our model is based on a softly-broken $S_{3}\times Z_{2}$ discrete symmetry. Its scalar sector consists only of one SM Higgs doublet and three electrically neutral SM-singlet scalars. We do not need to invoke either electrically charged scalar fields, or an extra $SU_{2L}$ scalar doublet, or the spontaneous breaking of the discrete group, which are typical for other radiative models in the literature. The model features a viable scalar dark matter candidate.

hep-ph↗

arXiv2016

Nucleon tensor form factors in a relativistic confined quark model

We present results for the isotriplet and isosinglet tensor form factors of the nucleon in the relativistic confined quark model. The model allows us to calculate not only their normalizations at $Q^2=0$ and the related tensor charges, but also the full $Q^2$-dependence. Our results are compared to existing data and predictions of other theoretical approaches. We stress the importance of these form factors for the phenomenology of physics beyond the Standard Model.

hep-ph↗

arXiv2016

CP-violating decays of the pseudoscalars eta and eta' and their connection to the electric dipole moment of the neutron

Using the present upper bound on the neutron electric dipole moment, we give an estimate for the upper limit of the CP-violating couplings of the eta(eta') to the nucleon. Using this result, we then derive constraints on the CP-violating eta(eta')-pi-pi couplings, which define the two-pion CP-violating decays of the eta and eta' mesons. Our results are relevant for the running and planned measurements of rare decays of the eta and eta' mesons by the GlueX Collaboration at JLab and the LHCb Collaboration at CERN.

hep-ph↗

arXiv2016

QCD-improved limits from neutrinoless double beta decay

We analyze the impact of QCD corrections on limits derived from neutrinoless double beta decay ($0νββ$). As demonstrated previously, the effect of the color-mismatch arising from loops with gluons linking the quarks from different color-singlet currents participating in the effective operators has a dramatic impact on the predictions for some particular Wilson coefficients. Here, we consider all possible contributions from heavy particle exchange, i.e. the so-called short-range mechanism of $0νββ$ decay. All high-scale models (HSM) in this class match at some scale around a $\sim$ few TeV with the corresponding effective theory, containing a certain set of effective dimension-9 operators. Many of these HSM receive contributions from more than one of the basic operators and we calculate limits on these models using the latest experimental data. We also show with one non-trivial example, how to derive limits on more complicated models, in which many different Feynman diagrams contribute to $0νββ$ decay, using our general method.

hep-ph↗

arXiv2016

Can one measure the Cosmic Neutrino Background?

The Cosmic Microwave Background (CMB) yields information about our Universe at around 380 000 years after the Big Bang (BB). Due to the weak interaction of the neutrinos with matter the Cosmic Neutrino Background (CNB) should give information about a much earlier time of our Universe, around one second after the Big Bang. Probably the most promising method to `see' the Cosmic Neutrino Background is the capture of the electron neutrinos from the Background by Tritium, which then decays into 3He and an electron with the energy of the the Q-value = 18.562 keV plus the electron neutrino rest mass. The `KArlsruhe TRItium Neutrino' (KATRIN) experiment, which is in preparation, seems presently the most sensitive proposed method for measuring the electron antineutrino mass. At the same time KATRIN can also look by the reaction: electron neutrino (~1.95 Kelvin) + 3H --> 3He + e- (with the energy Q = 18.6 keV + electron neutrino mass). The capture of the Cosmic Background Neutrinos (CNB) should show in the electron spectrum as a peak by the electron neutrino rest mass above Q. Here the possibility to see the CNB with KATRIN is studied. A detection of the CNB by KATRIN seems not to be possible at the moment. But KATRIN should be able to determine an upper limit for the local electron neutrino density of the CNB.

nucl-th↗

arXiv2015

Adjoint $SU(5)$ GUT model with $T_{7}$ flavor symmetry

We propose an adjoint $SU(5)$ GUT model with a $T_{7}$ family symmetry and an extra $Z_{2}\otimes Z_{3}\otimes Z_{4}\otimes Z_{4}^{\prime }\otimes Z_{12}$ discrete group, that successfully describes the prevailing Standard Model fermion mass and mixing pattern. The observed hierarchy of the charged fermion masses and the quark mixing angles arises from the $Z_{3}\otimes Z_{4}\otimes Z_{12}$ symmetry breaking, which occurs near the GUT scale. The light active neutrino masses are generated by type-I and type-III seesaw mechanisms mediated by the fermionic $SU(5)$ singlet and the adjoint $\mathbf{24}$-plet. The model predicts the effective Majorana neutrino mass parameter of neutrinoless double beta decay to be $m_{ββ}=$ 4 and 50 meV for the normal and the inverted neutrino spectra, respectively. We construct several benchmark scenarios, which lead to $SU(5)$ gauge coupling unification and are compatible with the known phenomenological constraints originating from the lightness of neutrinos, proton decay, dark matter, etc. These scenarios contain TEV-scale colored fields, which could give rise to a visible signal or be stringently constrained at the LHC.

hep-ph↗

arXiv2015

Precision measurements constraints on the number of Higgs doublets

We consider an extension of the Standard Model with an arbitrary number $N$ of Higgs doublets (NHDM), and calculate their contribution to the oblique parameters $S$ and $T$. We examine the possible limitations on $N$ from precision measurements of these parameters. In view of the complexity of the general case of NHDM, we analyze several benchmark scenarios for the Higgs mass spectrum, identifying the lightest CP-even Higgs with the Higgs-like particle recently observed at the LHC with the mass of $\sim 125$ GeV. The rest of the Higgses are put above the mass scale of $\sim 600$ GeV, below which the LHC experiments do not detect any Higgs-like signals except for the former famous one. We show that, in a scenario, with all the heavy Higgses degenerate at any scale, there are no limitations on the number $N$ of the Higgs doublets. However, upper limits appear for certain not completely degenerate configurations of the heavy Higgses.

hep-ph↗

arXiv2015

A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (Search for Hidden Particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $τ\to 3μ$ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the Standard Model and describe interactions between new particles and four different portals - scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the Standard Model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation

hep-ph↗

arXiv2015

Axions in gravity with torsion

We study a scenario allowing a solution of the strong charge parity problem via the Peccei-Quinn mechanism, implemented in gravity with torsion. In this framework there appears a torsion-related pseudoscalar field known as Kalb-Ramond axion. We compare it with the so-called Barbero-Immirzi axion recently proposed in the literature also in the context of the gravity with torsion. We show that they are equivalent from the viewpoint of the effective theory. The phenomenology of these torsion-descended axions is completely determined by the Planck scale without any additional model parameters. These axions are very light and very weakly interacting with ordinary matter. We briefly comment on their astrophysical and cosmological implications in view of the recent BICEP2 and Planck data.

hep-ph↗

arXiv2014

An SU(5) grand unified model with discrete flavour symmetries

We propose a model based on the $SU(5)$ grand unification with an extra $Z_{2}\otimes Z_{2}^{\prime}\otimes Z_{2}^{\prime \prime}\otimes Z_{4}\otimes Z_{12}$ flavor symmetry, which successfully describes the observed SM fermion mass and mixing pattern. The observed quark mass and mixing pattern is caused by the $Z_{4}$ and $Z_{12}$ symmetries, which are broken at very high scale by the $SU(5)$ scalar singlets $σ$ and $χ$, charged respectively under these symmetries and which acquire VEVs at the GUT scale. The light neutrino masses are generated via a type I seesaw mechanism with three heavy Majorana neutrinos. The model has in total 17 effective free parameters, from which 2 are fixed and 15 are fitted to reproduce the experimental values of the 18 physical parameters in the quark and lepton sectors. The model predictions for both quark and lepton sectors are in excellent agreement with the experimental data.

hep-ph↗

arXiv2014

Arbitrary mass Majorana neutrinos in neutrinoless double beta decay

We revisit the mechanism of neutrinoless double beta (NLDBD) decay mediated by the exchange with the heavy Majorana neutrino N of arbitrary mass mN, slightly mixed with the electron neutrino. By assuming the dominance of this mechanism, we update the well-known NLDBD-decay exclusion plot in the mass-mixing angle plane taking into account recent progress in the calculation of nuclear matrix elements within quasiparticle random phase approximation and improved experimental bounds on the NLDBD-decay half-life of Ge-76 and Xe-136. We also consider the known formula approximating the mN dependence of the NLDBD-decay nuclear matrix element in a simple explicit form. We analyze its accuracy and specify the corresponding parameters, allowing one to easily calculate the NLDBD-decay half-life for arbitrary mN for all the experimentally interesting isotopes without resorting to real nuclear structure calculations.

hep-ph↗

arXiv2014

Torsion in Extra Dimensions and One-Loop Observables

We study gravity with torsion in extra dimensions and derive an effective four-dimensional theory containing four-fermion contact operators at the fundamental scale of quantum gravity in the TeV range. These operators may have an impact on the low-energy observables and can manifest themselves or can be constrained in precision measurements. We calculate possible contributions of these operators to some observables at the one-loop level. We show that the existing precision data on the lepton decay mode of Z boson set limits on the fundamental scale of the gravity and compactification radius, which are more stringent than the limits previously derived in the literature.

hep-ph↗