arXiv subjects

Roman N. Lee

Publications and source records attributed to Roman N. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2022

Charge asymmetry in the spectra of bremsstrahlung and pair production

We calculate the first Coulomb correction to the spectra of two processes: the electron bremsstrahlung and electron-positron photoproduction in the Coulomb field. We show that, in contrast to the results obtained in the Born approximation and in the high-energy limit, the obtained corrections for these two process are not related by the crossing symmetry substitutions. The found corrections determine the leading contribution to the charge asymmetry in these processes. We use modern multiloop methods based on the IBP reduction and on the differential equations for master integrals. The results are presented in terms of classical polylogarithms. We provide both the threshold and the high-energy asymptotics of the obtained expressions and compare them with available results.

hep-ph↗

arXiv2022

Four-loop HQET propagators from the DRA method

We use dimensional recurrence relations and analyticity to calculate four-loop propagator-type master integrals in the heavy-quark effective theory. Compared to previous applications of the DRA method, we apply a new technique of fixing homogeneous solutions from pole parts of integrals evaluated in different rational space-time dimension points. The latter were calculated from the integration-by-parts reduction of finite integrals in shifted space-time dimension and/or with increased propagators powers. We provide results for epsilon expansions of master integrals near $d = 4$ and $d = 3$ using constructed alternative sets of integrals with expansion coefficients having conjectural uniform transcendental weight.

hep-ph↗

arXiv2022

Four-loop QCD cusp anomalous dimension at small angle

We calculate the small angle expansion of the four-loop QCD cusp anomalous dimension. As a byproduct of our calculation, we also obtain the four-loop anomalous dimension of the heavy-quark field in HQET. The validity of the calculational setup is cross-checked by the independent calculation of the four-loop QCD beta-function from heavy-quark-gluon vertex renormalization in HQET. We check the obtained results for the cusp anomalous dimension and heavy-quark field anomalous dimension against available analytical and numerical results. Finally, we find that the maximal transcendentality contribution to the QCD Bremsstrahlung function coincides, up to a factor 3/2, with the Bremsstrahlung function in N = 4 supersymmetric Yang-Mills theory, at least, through 4 loops.

hep-ph↗

arXiv2022

Charge asymmetry in $e^+e^-\to π^+π^-$ process

We consider the charge asymmetry in the differential cross section of the process $e^+e^- \rightarrow π^+π^-$. Experimental data show large deviations from the results of the available theoretical calculations. Motivated by this fact, we revisit the contribution of the two photon exchange diagrams and find the origin of discrepancy in an oversimplified account of the pion internal structure in the theoretical calculations. We present a natural and simple approach that accounts for the pion structure in a more consistent way and find remarkable agreement with the experimental data.

hep-ph↗

arXiv2022

The $Hb\bar{b}$ vertex at four loops and hard matching coefficients in SCET for various currents

We compute the four-loop corrections to the Higgs-bottom vertex within massless QCD and present analytic results for all color structures. The infrared poles of the renormalized form factor agree with the predicted four-loop pattern. Furthermore, we use the results for the Higgs-bottom, photon-quark, and Higgs-gluon form factors to provide hard matching coefficients in soft-collinear effective theory up to four-loop accuracy.

hep-ph↗

arXiv2022

Quark and gluon form factors in four-loop QCD

We compute the photon-quark and Higgs-gluon form factors to four-loop order within massless perturbative Quantum Chromodynamics. Our results constitute ready-to-use building blocks for N${}^4$LO cross sections for Drell-Yan processes and gluon-fusion Higgs boson production at the LHC. We present complete analytic expressions for both form factors and show several of the most complicated master integrals.

hep-ph↗

arXiv2021

The Four-Loop $\mathcal{N}=4$ SYM Sudakov Form Factor

We present the Sudakov form factor in full color $\mathcal{N}=4$ supersymmetric Yang-Mills theory to four loop order and provide uniformly transcendental results for the relevant master integrals through to weight eight.

hep-th↗

arXiv2021

Charge asymmetry in electron/positron energy loss in nuclear Bremsstrahlung

We calculate the leading Coulomb correction to the energy loss in the electron-nucleus Bremsstrahlung for arbitrary energy of the incoming particle. This correction determines the charge asymmetry, i.e., the difference of electron and positron energy loss. The result is presented in terms of the classical polylogarithms $\mathrm{Li}_2$ and $\mathrm{Li}_3$. We use modern multiloop methods based on the IBP reduction and on the differential equations for master integrals. We provide both the threshold and the high-energy asymptotics of the found asymmetry and compare them with the available results.

hep-ph↗

arXiv2021

The Compton Scattering Total Cross Section at Next-to-Leading Order

An analytic formula is given for the total scattering cross section of an electron and a photon at order $α^3$. This includes both the double-Compton scattering real-emission contribution as well as the virtual Compton scattering part. When combined with the recent analytic result for the pair-production cross section, the complete $α^3$ cross section is now known. Both the next-to-leading order calculation as well as the pair-production cross section are computed using modern multiloop calculation techniques, where cut diagrams are decomposed into a set of master integrals that are then computed using differential equations.

hep-ph↗

arXiv2021

Fermionic corrections to quark and gluon form factors in four-loop QCD

We analytically compute all four-loop QCD corrections to the photon-quark and Higgs-gluon form factors involving a closed massless fermion loop. Our calculation of non-planar vertex integrals confirms a previous conjecture for the analytical form of the non-fermionic contributions to the collinear anomalous dimensions of quarks and gluons.

hep-ph↗

arXiv2020

Libra: a package for transformation of differential systems for multiloop integrals

We present a new package for Mathematica system, called Libra. Its purpose is to provide convenient tools for the transformation of the first-order differential systems $\partial_i \boldsymbol j = M_i \boldsymbol j$ for one or several variables. In particular, Libra is designed for the reduction to $ε$-form of the differential systems which appear in multiloop calculations. The package also contains some tools for the construction of general solution: both via perturbative expansion of path-ordered exponent and via generalized power series expansion near regular singular points.Libra also has tools to determine the minimal list of coefficients in the asymptotics of the original master integrals, sufficient for fixing the boundary conditions.

hep-ph↗

arXiv2020

Total cross sections of $eγ\to e X\bar{X}$ processes with $X=μ,γ, e$ via multiloop methods

Using modern multiloop calculation methods, we derive the analytical expressions for the total cross sections of the processes $e^-γ\to e^-X\bar{X}$ with $X=μ,\,γ$ or $e$ at arbitrary energies. For the first two processes our results are expressed via classical polylogarithms. The cross section of $e^-γ\to e^-e^-e^+$ is represented as a one-fold integral of complete elliptic integral $\mathrm{K}$ and logarithms. Using our results, we calculate the threshold and high-energy asymptotics and compare them with available results.

hep-ph↗

arXiv2020

Electron-positron annihilation to photons at $O(α^3)$ revisited

We apply the modern multiloop methods to the calculation of the total cross sections of electron-positron annihilation to 2 and 3 photons exactly in $s/m^2$ with the accuracy $O(α^3)$. Examining the asymptotics of our results, we find agreement with Ref. [Andreassi_et_al_1962] and discover mistakes in the results of Refs. [Eidelman&Kuraev_1978,Berends&Kleiss_1981]. This mistake is due to the terms, omitted in differential cross section in Refs. \[Eidelman&Kuraev_1978,Berends&Kleiss_1981], which are peaked in the kinematic region with all three photons being quasi-parallel to the collision axis. After restoring these terms, we find an agreement of the corrected result of Ref. [Berends&Kleiss_1981] with our result.

hep-ph↗

arXiv2019

Differential equations, recurrence relations, and quadratic constraints for $L$-loop two-point massive tadpoles and propagators

We consider $L$-loop two-point tadpole (watermelon) integral with arbitrary masses, regularized both dimensionally and analytically. We derive differential equation system and recurrence relations (shifts of dimension and denominator powers). Since the $L$-loop sunrise integral corresponds to the $(L+1)$-loop watermelon integral with one cut line, our results are equally applicable to the former. The obtained differential system has a Pfaffian form and is linear in dimension and analytic regularization parameters. In general case, the solutions of this system can be expressed in terms of the Lauricella functions $F_C^{(L)}$ with generic parameters. Therefore, as a by-product, we obtain, to our knowledge for the first time, the Pfaffian system for $F_C^{(L)}$ for arbitrary $L$. The obtained system has no apparent singularities. Near odd dimension and integer denominator powers the system can be easily transformed into canonical form. Using the symmetry properties of the matrix in the right-hand side of the differential system, we obtain quadratic constraints for the expansion of solutions near integer dimension and denominator powers. In particular, we obtain quadratic constraints for Bessel moments similar to those discovered by Broadhurst and Roberts.

hep-ph↗

arXiv2019

Master integrals for two-loop $C$-odd contribution to $e^+e^-\to \ell^+\ell^-$ process

We calculate two-loop master integrals for the process of heavy lepton pair production in $e^+e^-$ collisions. We consider the $C$-odd diagrams with three photons in the intermediate state and evaluate the corresponding families of the master integrals in the limit of zero electron mass. Our results for the master integrals are directly applicable in the whole physical region corresponding to the annihilation channel.

hep-ph↗

arXiv2019

Four-loop quark form factor with quartic fundamental colour factor

We analytically compute the four-loop QCD corrections for the colour structure $(d_F^{abcd})^2$ to the massless non-singlet quark form factor. The computation involves non-trivial non-planar integral families which have master integrals in the top sector. We compute the master integrals by introducing a second mass scale and solving differential equations with respect to the ratio of the two scales. We present details of our calculational procedure. Analytical results for the cusp and collinear anomalous dimensions, and the finite part of the form factor are presented. We also provide analytic results for all master integrals expanded up to weight eight.

hep-ph↗

arXiv2018

Massive quark form factors

In this contribution we provide further details to the recent calculation of QCD corrections to massive three-loop form factors presented in Ref. [1].

hep-ph↗

arXiv2018

Symmetric $ε$- and $(ε+1/2)$-forms and quadratic constraints in "elliptic" sectors

Within the differential equation method for multiloop calculations, we examine the systems irreducible to $ε$-form. We argue that for many cases of such systems it is possible to obtain nontrivial quadratic constraints on the coefficients of $ε$-expansion of their homogeneous solutions. These constraints are the direct consequence of the existence of symmetric $(ε+1/2)$-form of the homogeneous differential system, i.e., the form where the matrix in the right-hand side is symmetric and its $ε$-dependence is localized in the overall factor $(ε+1/2)$. The existence of such a form can be constructively checked by available methods and seems to be common to many irreducible systems, which we demonstrate on several examples. The obtained constraints provide a nontrivial insight on the structure of general solution in the case of the systems irreducible to $ε$-form. For the systems reducible to $ε$-form we also observe the existence of symmetric form and derive the corresponding quadratic constraints.

hep-ph↗