arXiv subjects

A. Aleksejevs

Publications and source records attributed to A. Aleksejevs.

At least 19 recordsLinked to original sources

arXiv2026

Recurrence Relations and Dispersive Techniques for Precision Multi-Loop Calculations

Ab initio predictions of two-loop electroweak contributions to observables are increasingly essential for precision collider experiments, yet their evaluation remains very challenging. We connect recurrence techniques and dispersive method in order to evaluate complex multi-loop Feynman diagrams. By expressing multi-point Passarino-Veltman functions in a two-point basis and using shifted space-time dimensions with recurrence relations, we minimize the number of required dispersive integrals. This approach reduces computation time and enables a precise and efficient analysis of one- and two-loop diagrams.

hep-ph↗

arXiv2021

A Standard Model Explanation for the "ATOMKI Anomaly"

Using the $e^+e^-$ pair spectrometer at the 5 MV Van de Graaff accelerator at the Institute for Nuclear Research, Hungarian Academy of Sciences (ATOMKI), Krasznahorkay et al. have claimed a 6.8$σ$ excess at high $e^+e^-$ opening angles in the internal pair creation isoscalar transition $^8\mathrm{Be}(18.15)\to\,^8\mathrm{Be}\,e^+e^-$. A hypothetical gauge boson with the mass circa $17$ MeV, "X17", has been proposed as an explanation for the excess. We show that the observed experimental structure can be reproduced within the Standard Model by adding the full set of second-order corrections and the interference terms to the Born-level decay amplitudes considered by Krasznahorkay et al. We implement a detailed model of the ATOMKI detector, and also show how experimental selection and acceptance bias exacerbate the apparent difference between the experimental data and the Born-level prediction.

hep-ph↗

arXiv2019

Dispersive Two-Loop Calculations: Methodology and Applications

As the new-generation precision experiments such as MOLLER and P2 look for physics beyond Standard Model, it is becoming increasingly important to evaluate the higher-order electroweak radiative corrections to a sub-percent level of uncertainty. However, due to propagators with different masses and higher-order tensor Feynman integrals, the two-loop calculations involving thousands of Feynman graphs become a demanding task requiring novel computational approaches. In this paper, we describe our dispersive sub-loop insertion approach and develop two-loop integrals using two-point functions basis which is applicable to wide range of processes.

hep-th↗

arXiv2019

Dynamic Structure of Hadrons in ChPT

The Chiral Perturbation Theory (ChPT) has been very successful in describing low-energy hadronic properties in the non-perturbative regime of Quantum Chromodynamics. The results of ChPT, many of which are currently under active experimental investigation, provide stringent predictions of many fundamental properties of hadrons, including quantities such as electromagnetic polarizabilities. The paper outlines our semi-automated calculations in ChPT, the corresponding results for the electric and magnetic polarizabilities of the proton and our predictions for Compton differential cross sections.

hep-ph↗

arXiv2019

NLO radiative corrections for Forward-Backward and Left-Right Asymmetries at a B-Factory

This paper presents the first calculations of the parity-violating polarization asymmetry and forward-backward asymmetry of the $e^- e^+ \rightarrow μ^+ μ^- (γ)$ process at a center-of-mass energy of 10.58 GeV with up to one-loop electroweak radiative corrections. The calculations are relevant for future precision electroweak measurements at the Belle~II experiment, which is now collecting data at the SuperKEKB $e^- e^+$ collider with a center-of-mass energy at the mass of the $Υ(4S)$ resonance. In this paper we take under full control the bremsstrahlung process at the conditions of Belle II/SuperKEKB, and the possibilities for a soft photon approach are discussed. The scale of the obtained relative corrections to the parity-violating and forward-backward asymmetries is significant and the scattering angle dependencies of the asymmetries is non-trivial. As an additional validation cross-check using an independent formulation, the calculated asymmetries are compared to results from the KK Monte Carlo generator.

hep-ph↗

arXiv2019

Dimensional Regularization and Dispersive Two-Loop Calculations

The two-loop contributions are now often required by the precision experiments, yet are hard to express analytically while keeping precision. One way to approach this challenging task is via the dispersive approach, allowing to replace sub-loop diagram by effective propagator. This paper builds on our previous work, where we developed a general approach based on representation of many-point Passarino-Veltman functions in two-point function basis. In this work, we have extracted the UV-divergent poles of the Passarino-Veltman functions analytically and presented them as the dimensionally-regularized and multiply-subtracted dispersive sub-loop insertions, including self-energy, triangle, box and pentagon type.

hep-th↗

arXiv2018

Dispersion Approach in Two-Loop Calculations

The higher-order corrections become increasingly important with experiments reaching sub-percent level of uncertainty as they look for physics beyond the Standard Model. Our goal is to address the full set of two-loop electroweak corrections to Møller or electron-proton scattering. It is a demanding task which requires an application of various approaches where two-loop calculations can be automatized. We choose to employ dispersive sub-loop insertion approach and develop two-loop integrals using two-point functions basis. In that basis, we introduce a partial tensor reduction for many-point Passarino-Veltman functions, which later could be used in computer algebra packages. In this paper, we have considered self-energy, triangle and box sub-loop insertions into self-energy, vertex and box topology.

hep-th↗

arXiv2018

Crossed Topology in Two-Loop Dispersive Approach

We extend existing dispersive approach in subloop insertion to the case of crossed two-loop box type topologies. Based on the ideas of the Feynman trick, mass shift approach and dispersive representation of two-point Passarino-Veltman function we expressed two-loop scalar diagrams in the compact analytical form suitable for the automatization of the calculations. The results are expressed in a way that the numerical integration over Feynman and dispersive parameters and differentiation with respect to mass shift parameters are required in the final stage only.

hep-th↗

arXiv2017

NLO electroweak radiative corrections for four-fermionic process at Belle II

We discuss the next to the leading order (NLO) electroweak radiative corrections to the $e^- e^+ \rightarrow f^- f^+ (γ)$ cross section asymmetry, for polarized and unpolarized beam scenario. The left-right and forward-backward amplitudes, with and without radiative corrections, are evaluated and compared for various kinematics. The hard bremsstrahlung is included for arbitrary energy cuts. The radiative corrections are shown to be significant and having a non-trivial dependency on the kinematic conditions. The calculations are relevant for the ultra-precise low-energy experiment Belle~II planned at SuperKEKB.

hep-ph↗

arXiv2016

Search for Physics beyond Standard Model at the Precision Frontiers

The article outlines the recent developments in the theoretical and computational approaches to the higher-order electroweak effects needed for the accurate interpretation of MOLLER and Belle II experimental data, and shows how new-physics particles enter at the one-loop level. By analyzing the effects of $Z'$-boson on the polarization asymmetry, we show how this hypothetical interaction carrier may influence the future experimental results.

hep-ph↗

arXiv2014

Electromagnetic Polarizabilities of Mesons

The Chiral Perturbation Theory (CHPT) has been very successful in describing low-energy hadronic properties in the non-perturbative regime of Quantum Chromodynamics. The results of ChPT, many of which are currently under active experimental investigation, provide stringent predictions of many fundamental properties of hadrons, including quantities such as electromagnetic polarizabilities. Yet, even for the simplest hadronic system, a pion, we still have a broad spectrum of polarizability measurements (MARK II, VENUS, ALEPH, TPC/2g, CELLO, Belle, Crystal Ball). The meson polarizability can be accessed through Compton scattering, so we can measure it through Primakoff reaction. This paper will provide an analysis of the CHPT predictions of the SU(3) meson electromagnetic polarizabilities and outline their relationship to the Primakoff cross section at the kinematics relevant to the planned JLab experiments.

nucl-th↗

arXiv2014

Two-Loop Effects in Low-Energy Electroweak Measurements

We outline the recent results on the two-loop electroweak contributions to the electron-electron scattering cross sections and asymmetries. Although the two-loop corrections are strongly suppressed relative to the one-loop corrections, they still contribute a few percent to the polarization asymmetry, and even this small contribution cannot be ignored at for ultra-precision experiments such as MOLLER planned at JLab. The NNLO calculation techniques we developed for the electron-electron scattering can be adapted for electron-proton processes, electron-positron collisions, and other low-energy experiments involving leptons.

hep-ph↗

arXiv2014

New Physics Search with Precision Experiments: Theory Input

The best way to search for new physics is by using a diverse set of probes - not just experiments at the energy and the cosmic frontiers, but also the low-energy measurements relying on high precision and high luminosity. One example of such ultra-precision experiments is the MOLLER experiment planned at JLab, which will measure the parity-violating electron-electron scattering asymmetry and allow a determination of the weak mixing angle with a factor of five improvement in precision over its predecessor, E-158. At this precision, any inconsistency with the Standard Model should signal new physics. The paper will explore how new physics particles enter at the next-to-leading order one-loop level. For MOLLER we analyze the effects of dark Z'-boson on the total calculated asymmetry, and show how this new physics interaction carriers may influence the analysis of the future experimental results.

hep-ph↗

arXiv2013

Hadron Structure in Chiral Perturbation Theory

We present our predictions for meson form factors for the SU(3) octet and investigate their impact on the pion electroproduction cross sections. The electric and magnetic polarizabilities of the SU(3) octet of mesons and baryons are analyzed in detail. These extensive calculations are made possible by the recent implementation of semi-automatized calculations in fully-relativistic chiral perturbation theory, which allows evaluation of polarizabilities from Compton scattering up to next-to-the-leading order.

hep-ph↗

arXiv2013

Dynamical Structure of Baryons

Compton scattering offers a unique opportunity to study the dynamical structure of hadrons over a wide kinematic range, with polarizabilities characterizing the hadron active internal degrees of freedom. We present calculations and detailed analysis of electric and magnetic and the spin-dependent dynamical polarizabilities for the lowest in mass SU(3) octet of baryons. These extensive calculations are made possible by the recent implementation of semi-automatized calculations in chiral perturbation theory which allows evaluating polarizabilities from Compton scattering up to next-to-the-leading order. The dependencies for the range of photon energies covering the majority of the meson photoproduction channels are analyzed.

hep-ph↗

arXiv2013

NLO and NNLO EWC for PV Moller Scattering

High-precision electroweak experiments such as parity-violating Moller scattering can provide indirect access to physics at multi-TeV scales and play an important complementary role to the LHC research program. However, before physics of interest can be extracted from experimental data, electroweak radiative corrections, which can significantly reduce the cross-section asymmetry, must be calculated with an unprecedented completeness and accuracy. Although the two-loop corrections are strongly suppressed relative to the one-loop corrections, they can no longer be dismissed for the upcoming precision experiments. We evaluate a full gauge-invariant set of one-loop and several types of two-loop radiative corrections for the parity-violating electron-electron scattering asymmetry by combining two distinct but mutually-reinforcing techniques: semiautomatic, precise, with FeynArts and FormCalc as base languages, and by hand, with some approximations. For 11 GeV relevant for the ultra-precise MOLLER experiment planned at JLab, the results obtained by two approaches are in excellent agreement, which gives us assurance that our calculations are error-free.

hep-ph↗

arXiv2012

Two Photon Exchange for Exclusive Pion Electroproduction

We perform detailed calculations of two-photon-exchange QED corrections to the cross section of pion electroproduction. The results are obtained with and without the soft-photon approximation; analytic expressions for the radiative corrections are derived. The relative importance of the two-photon correction is analyzed for the kinematics of several experiments at Jefferson Lab. A significant, over 20%, effect due to two-photon exchange is predicted for the backward angles of electron scattering at large transferred momenta.

nucl-th↗

arXiv2011

High precision calculations of electroweak radiative corrections for polarized Moller scattering at one loop and beyond

Parity-violating Moller scattering measurements are a powerful probe of new physics effects, and the upcoming high-precision experiments will require a new level of accuracy for electroweak radiative corrections (EWC). First, we perform the updated calculations of one-loop EWC for Moller scattering asymmetry using two different approaches: semi-automatic, precise, with FeynArts and FormCalc as base languages, and "by hand", with reasonable approximations. In addition, we provide a tuned comparison between the one-loop results obtained in two different renormalization schemes: on-shell and constrained differential renormalization. As the last step, we discuss the two-loop EWC induced by squaring one-loop diagrams, and show that the significant size of this partial correction indicates a need for a complete study of the two-loop EWC in order to meet the precision goals of future experiments.

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