arXiv subjects

N. V. Nikitin

Publications and source records attributed to N. V. Nikitin.

7 recordsLinked to original sources

arXiv2026

Coulomb corrections in rare decays of neutral $B$ mesons with $\ell^+\ell^-$-pair in final state

We present a systematic analysis of Coulomb corrections for leptonic ($B^0_{d,s}\to \ell^+\ell^-$), semileptonic ($B^0_{d,s}\to h^0\,\ell^+\ell^-$, $B^0_{d,s}\to V^0\ell^+\ell^-$) and radiative leptonic ($B^0_{d,s}\to γ\ell^+\ell^-$) decays of neutral $B$-mesons. The relativization of the Coulomb factor was performed by comparing the Gamow-Sommerfeld-Sakharov factor, the exact relativistic approach of Crater-Alstine-Sazdjian applied by us to scalar systems, and well-known one-loop QED calculations. Coulomb corrections are calculated for differential, angular, and double-differential distributions, as well as for partial decay widths. We also discuss the role of the Coulomb factor among other QED corrections, in particular, the contribution of soft-photon radiation, which is effectively simulated in experiments by tools such as PHOTOS. For the $B_s^0 \to μ^+μ^-$ channel, Coulomb corrections improve the prediction of the partial width to $δ= |\mathcal{B}^{(exp)} - \mathcal{B}^{(theory)}|/\mathcal{B}^{(exp)} = 2\%$. This improvement brings the prediction closer to the LHCb/CMS experimental results within the current experimental (11\%) and theoretical (5\% lattice QCD) errors. In the decays $B^0\to K^0μ^+μ^-$ and $B^0 \to K^{0*}μ^+μ^-$, Coulomb effects also reduce the discrepancies between theoretical predictions and experimental data (from $δ= 2\%$ to less than $δ= 1\%$ and from $δ= 11\%$ to $δ= 4\%$ respectively). Finally, for the decays involving $τ$-leptons, the Coulomb correction reaches $4\%$. While currently smaller than the dominant form-factor uncertainties and experimental errors, the Coulomb correction represents a non-negligible systematic effect. It should be accounted for in the high-precision era of $B$-physics, where such effects may become significant for the interpretation of potential New Physics signals.

hep-ph↗

arXiv2023

Rare decays of $B_d$ mesons into four charged leptons in the Standard Model

We present first theoretical predictions for various observables related to the $\bar B_d \to μ^+ μ^- e^+ e^-$ decay within the framework of the Standard Model. Our study encompasses the branching ratios, differential distributions, and forward-backward leptonic asymmetries. To obtain these predictions, we account for several contributions: resonance contribution from $ρ(770)$ and significant contribution $ω(782)$; contributions from four charmonium resonances: $ψ(3770)$, $ψ(4040)$, $ψ(4160)$, and $ψ(4415)$; additional contributions from the "tails" of $J/ψ$ and $ψ(2S)$ resonances; non-resonant contributions arising from virtual photon emission by a $b$-quark of $B_d$ meson, bremsstrahlung, and weak annihilation. In our calculations we employ the model of vector meson dominance (VMD) to assess the resonance contributions accurately. We provide the predictions for the branching ratio of the $\bar B_d \to μ^+ μ^- e^+ e^-$ decay both with and without the resonant contribution from $ω(782)$.

hep-ph↗

arXiv2021

Rare decays of the $B_s-$meson into four charged leptons in the framework of the Standard Model

In the framework of the Standard Model we present new theoretical predictions for the branching ratios, double and single differential distributions and forward -- backward leptonic asymmetries for the $\bar B_s \to μ^+ μ^- e^+ e^-$ decay. In our consideration we take into account the $ϕ(1020)$ -- resonance contribution; the main contributions of four charmonium resonances : $ψ(3770)$, $ψ(4040)$, $ψ(4160)$ and $ψ(4415)$; $u\bar{u}$ -- resonant contribution from $ρ(770)$ and $ω(782)$; tails contributions from $J/ψ$ and $ψ(2S)$ resonances; non -- resonant contribution of the $b\,\bar{b}$ - pairs, bremsstrahlung and the contribution of the weak annihilation. We provide the prediction for the branching ratio of $\bar B_s \to μ^+ μ^- e^+ e^-$ decay with and without the $ϕ(1020)$ -- resonance contribution. We use the model of vector meson dominance (VMD) for calculation of resonances contributions and take into account all substantive terms in $\bar B_s \to μ^+ μ^- e^+ e^-$ amplitude that was not considered in the previously papers.

hep-ph↗

arXiv2014

Three-dimensional Coherent Structures of Electrokinetic Instability

A direct numerical simulation of the three-dimensional elektrokinetic instability near a charge selective surface (electric membrane, electrode, or system of micro-/nanochannels) is carried out and analyzed. A special finite-difference method was used for the space discretization along with a semi-implicit $3\frac{1}{3}$-step Runge-Kutta scheme for the integration in time. The calculations employed parallel computing. Three characteristic patterns, which correspond to the overlimiting currents, are observed: (a) two-dimensional electroconvective rolls, (b) three-dimensional regular hexagonal structures, and (c) three-dimensional structures of spatiotemporal chaos, which are a combination of unsteady hexagons, quadrangles and triangles. The transition from (b) to (c) is accompanied by the generation of interacting two-dimensional solitary pulses.

physics.flu-dyn↗

arXiv2013

Direct Numerical Simulation of Electrokinetic Instability and Transition to Chaotic Motion

A new type of instability - electrokinetic instability - and an unusual transition to chaotic motion near a charge-selective surface was studied by numerical integration of the Nernst-Planck-Poisson-Stokes system and a weakly nonlinear analysis near the threshold of instability. Two kinds of initial conditions were considered: (a) white noise initial conditions to mimic "room disturbances" and subsequent natural evolution of the solution; (b) an artificial monochromatic ion distribution with a fixed wave number to simulate regular wave patterns. The results were studied from the viewpoint of hydrodynamic stability and bifurcation theory. The threshold of electroconvective movement was found by the linear spectral stability theory, the results of which were confirmed by numerical simulation of the entire system. The following regimes, which replace each other as the potential drop between the selective surfaces increases, were obtained: one-dimensional steady solution; two-dimensional steady electroconvective vortices (stationary point in a proper phase space); unsteady vortices aperiodically changing their parameters (homoclinic contour); periodic motion (limit cycle); and chaotic motion. The transition to chaotic motion did not include Hopf bifurcation. Numerical resolution of the thin concentration polarization layer showed spike-like charge profiles along the surface, which could be, depending on the regime, either steady or aperiodically coalescent. The numerical investigation confirmed the experimentally observed absence of regular (near-sinusoidal) oscillations for the overlimiting regimes. There is a qualitative agreement of the experimental and the theoretical values of the threshold of instability, the dominant size of the observed coherent structures, and the experimental and theoretical volt-current characteristics.

physics.flu-dyn↗

arXiv2005

Production and Decay of Charmed Baryons: Spectra of Muons and Asymmetry between mu+ and mu-

The calculation of muon spectra from the decay of Lambda_c baryons was carried out on the basis of the description of recent data on charmed-baryon production in hadronic interactions. Data are described in the framework of Quark--Gluon String Model that allowes us to consider primary proton interactions of arbitrary high energy. MC code was built for charmed-baryon semileptonic decay in order to obtain the kinematical characteristics of resulting particles. It is predicted that the charge asymmetry between energy spectra of mu+ and mu- in laboratory system is clearly seen as the consequence of asymmetry between the spectra of charmed baryons and antibaryons.This extension of QGS Model can be useful to correct the calculations of muon and neutrino spectra in astrophysics.

hep-ph↗

arXiv2003

Approximations to path integrals and spectra of quantum systems

An expression for the Green function G(E;x_1,x_2) of the Schroedinger equation is obtained through the approximations of the path integral by n-fold multiple integrals. The approximations to Re{G(E;x,x)} on the real E-axis have peaks near the values of the energy levels E_{j}. The analytic and numerical examples for one-dimensional and multi-dimensional harmonic and anharmonic oscillators, and Poeschl-Teller potential wells, show that median values of these peaks for approximate G(E;0,0) corresponds with accuracy of order 10% to the exact values of even levels already in the lowest orders of approximation n=1 and n=2, i.e. when the path integral is replaced by a line or double integral. The weights of the peaks approximate the values of the squared modulus of the wave functions at x=0 with the same accuracy.

physics.comp-ph↗