Higher-order Radiative Corrections Research Articles

Higher-order NLO radiative corrections to polarized muon decay spectrum

Higher-order NLO radiative corrections to polarized muon decay spectrum

High order radiative corrections in reactions involving electrons and protons

High order radiative corrections in reactions involving electrons and protons

Computer package DIZET v. 6.45

The new version of the DIZET electroweak library is described. Changes and additional code features concerning the previous version are explained. The software allows one to make state-of-the-art theoretical predictions for pseudo-observable quantities, including higher-order radiative corrections. The current version of the DIZET library v. 6.45 incorporates advanced recent results of theoretical calculations. Numerical comparisons with the results of the previous version are performed. Estimates of theoretical uncertainties are discussed. Program summaryProgram Title:DIZET version 6.45CPC Library link to program files:https://doi.org/10.17632/2fr58v92xc.1Code Ocean capsule:https://codeocean.com/capsule/0198682Licensing provisions: GPLv3Programming language: FORTRAN 77/90External routines: AlphaQED http://www-com.physik.hu-berlin.de/~fjeger/software.htmlJournal reference of previous version: Comput. Phys. Commun. 59 (1990) 303Does the new version supersede the previous version?: YesReasons for the new version: Implementation of new options.Summary of revisions: Implemented the complete electroweak two-loop radiative corrections to all the relevant electroweak precision pseudo-observables related to the Z-boson decay. Added support of new library for hadronic contribution to vacuum polarization.Nature of problem: The Z-resonance region is still extremely important for the experimental verification of the Standard Model. The anticipated experimental accuracy on a large variety of observables has to be matched with theory predictions of at least the same level of accuracy to achieve maximum usage of experimental data.Solution method: Theoretical calculations at NNLO in perturbation theory allow to compute higher precision pseudo-observables to verify the Standard Model at high luminosity future colliders. The package includes electroweak two-loop corrections calculated in the on-mass shell renormalization scheme. Pseudo-observables and electroweak form-factors can be calculated in a wide range of input parameters.Additional comments including restrictions and unusual features: The estimate of results at the off-resonance region should be treated with caution as two-loop electroweak corrections were calculated for the Z-resonance peak.

Coherent energy loss effects in dihadron azimuthal angular correlations in Deep Inelastic Scattering at small x

We perform an exploratory study of the role of coherent, medium-induced energy loss in azimuthal angular correlations in dihadron production in Deep Inelastic Scattering (DIS) at small x where the target proton/nucleus is modeled as a Color Glass Condensate. In this approach coherent radiative energy loss is part of the higher order corrections to the leading order dihadron production cross section. We include the effects of both gluon saturation and coherent radiative energy loss and show that radiative cold-matter energy loss has a significant effect on the so-called coincidence probability for the back to back production of dihadrons in DIS. We also define a double ratio of coincidence probabilities for a nucleus and proton targets and show that it is very robust against higher order radiative corrections.

Exploiting electron parity violation: from Standard Model tests to dark matter detection predictions

There has been recent interest in low energy, high luminosity polarized electron beams for studies of parity-violating (PV) electron scattering, such as the MESA accelerator at Mainz or an upgraded FEL facility at Jefferson Lab. Accurate measurements of the PV asymmetry in elastic electron scattering from nuclei can be used to determine Standard Model couplings, such as the weak-mixing angle or higher-order radiative corrections, as well as to extract specific information on the nuclear and nucleon structure. To this end, low uncertainties are required from modelling some confounding nuclear and nuclear structure effects, including isospin mixing, nucleon strangeness content or Coulomb distortion of electron wave functions. We estimate the sizes and theoretical uncertainties of such effects for a carbon 12 target. An experimental precision in the PV asymmetry of a few tenths of a percent may be reachable under certain kinematic conditions, that are also discussed for the same nuclear target.This high precision PV asymmetry in elastic electron scattering can also be used to relate in a very simple manner the elastic electron-nucleus scattering cross section with the elastic neutrino-nucleus cross section. This novel relationship allows us to exploit experimentally well-determined quantities to predict unknown or recently measured observables, such as coherent neutrino-nucleus cross sections. This idea can be extended to link electron scattering to an even more uncertain magnitude: the direct detection rate of hypothetical weak-interacting dark matter particles through axial and/or vector elastic interactions with nuclei.

Two-photon atomic level widths at finite temperatures

The thermal two-photon level broadening of the excited energy levels in the hydrogen and H-like helium is evaluated via the imaginary part of thermal two-loop self-energy correction for bound electron. All the derivations are presented in the framework of rigorous quantum electrodynamic theory at finite temperatures and are applicable for the H-like ions. On this basis, we found a contribution to the level broadening induced by the blackbody radiation which is fundamentally different from the usual line broadening caused by the stimulated two-photon decay and the Raman scattering of thermal photons. Numerical calculations of the two-loop thermal correction to the two-photon width for the $2s$ state in hydrogen and singly ionized helium atoms show that the effect could significantly exceed the higher-order relativistic and radiative QED corrections commonly included in the calculations. In addition, the thermal two-loop self-energy correction significantly exceeds the "ordinary" stimulated one-photon depopulation rate at the relevant laboratory temperatures. In this work, detailed analysis and the corresponding comparison of the effect with the existing laboratory measurements in H-like ions are carried out.

Weak nuclear processes in the quest for elusive particles

Nuclear processes involving the weak interaction can be used to extract information on some elusive properties of particles. Remarkably useful to this goal is parity-violating elastic electron scattering off nuclei, which can be used to determine accurately the distribution of neutrons within the nucleus, including information on the neutron skin that can be related to the structure of neutron stars. It can also be used to determine the content of strange quark-antiquark virtual pairs in nucleons and can help in evaluating accurately Standard Model parameters or higher-order radiative corrections. To achieve these goals it is essential keeping under control the theoretical uncertainties that arise in modelling some confounding nuclear effects, such as isospin mixing or Coulomb distortion of electron wave functions.The paradigm of an evasive particle in current physics is dark matter. Sterile neutrinos are hypothetical dark matter candidates that could be produced in nuclear beta decays leaving a signal in the energy spectrum of the emitted charged lepton. They can also be coherently scattered by nuclei through an indirect weak neutral interaction, whose cross section can be written in terms of elastic electron scattering observables. We study the probability of these production and detection mechanisms using experimental and cosmological constraints on the sterile neutrino properties.The coherent scattering cross section off nuclei has also been analyzed for the Standard Model neutrinos, being a notably elusive process that has been recently measured for the first time and that can be used for Standard Model tests or for nuclear structure studies in ways analogous to parity-violating electron scattering.

Two-loop beta function for complex scalar electroweak multiplets

We present the general form of the renormalizable four-point interactions of a complex scalar field furnishing an irreducible representation of SU(2), and derive a set of algebraic identities that facilitates the calculation of higher-order radiative corrections. As an application, we calculate the two-loop beta function for the SM extended by a scalar multiplet, and provide the result explicitly in terms of the group invariants. Our results include the evolution of the Higgs-portal couplings, as well as scalar “minimal dark matter”. We present numerical results for the two-loop evolution of the various couplings.

Polarized NLO EW [formula omitted] cross section calculations with ReneSANCe-v1.0.0

In this paper we present a new Monte Carlo event generator ReneSANCe for simulation of processes at electron–positron colliders. In the current release of the generator the Bhabha scattering (e+e−→e−e+) and Higgs-strahlung (e+e−→ZH) process are implemented. Based on the SANC (Support for Analytic and Numeric Calculations for experiments at colliders) modules, the new generator takes into account complete one-loop and some higher-order electroweak radiative corrections with finite particle masses and polarizations. The new generator effectively operates in the collinear region and at the ZH production threshold. It is constructed in such a way that new processes can be easily added. The paper contains a theoretical description of the SANC approach, numerical validations and manual. Program summaryProgram Title: ReneSANCe-v1.0.0Program Files doi:http://dx.doi.org/10.17632/wp36f7t3ft.1Licensing provisions: GPLv3Programming language: Fortran, C, C++Supplementary material: Looptools [1], FOAM [2]Nature of problem: Theoretical calculations at next-to-leading order in perturbation theory allow to compute higher precision amplitudes for Standard Model processes and decays, provided proper treatments of UV divergences and IR singularities are performed.Solution method: Numerical integration of the precomputed differential expressions for cross sections of certain processes implemented as SANC modules [3,4].Restrictions: The list of processes is limited to e+e−→e−e+ and e+e−→ZH.

Probing non-perturbative QED with electron-laser collisions

The vast majority of QED results are obtained in relatively weak fields and so in the framework of perturbation theory. However, forthcoming laser facilities providing extremely high fields can be used to enter not-yet-studied regimes. Here, a scheme is proposed that might be used to reach a supercritical regime of radiation reaction or even the fully non-perturbative regime of quantum electrodynamics. The scheme considers the collision of a 100 GeV-class electron beam with a counterpropagating ultraintense electromagnetic pulse. To reach these supercritical regimes, it is unavoidable to use a pulse with ultrashort duration. Using two-dimensional particle-in-cell simulations, it is therefore shown how one can convert a next-generation optical laser to an ultraintense (I ≈ 2.9 × 1024 Wcm−2) attosecond (duration ≈ 150 as) pulse. It is shown that if the perturbation theory persists in extreme fields, the spectrum of secondary particles can be found semi-analytically. In contrast, a comparison with experimental data may allow differentiating the contribution of high-order radiative corrections if the perturbation theory breaks.

Second-order leptonic radiative corrections for lepton-proton scattering

The interpretation of high-precision lepton-nucleon scattering experiments requires the knowledge of higher-order radiative corrections. We present a calculation of the cross section for unpolarized lepton-proton scattering including leptonic radiative corrections up to second order, including one- and two-loop corrections, radiation of one and two photons and one-loop corrections for one-photon radiation. Numerical results are given for the planned P2 experiment at the MESA facility in Mainz, and some results are also discussed for Qweak and the suggested MUSE experiment.

Radiative corrections to elastic-electron proton scattering and uncertainty in proton charge radius

Higher-order QED radiative corrections to elastic electron-proton scattering are discussed. It is shown that they are relevant for high-precision experiments on proton form factor measurements. Analytic result are obtained for next-to-leading second order corrections to the electron line. Light pair corrections are taken into account. The role of the hadronic contribution to vacuum polarization is discussed. Numerical results are given for the conditions of the experiment on proton form factors performed by A1 Collaboration. Preliminary results are also shown for the set-up with reconstruction of momentum transfer from the recoil proton momentum.

Direct numerical computation and its application to the higher-order radiative corrections

The direct computation method(DCM) is developed to calculate the multi-loop amplitude for general masses and external momenta. The ultraviolet divergence is under control in dimensional regularization. In this paper we report on the progress of DCM to several scalar multi-loop integrals after the presentation in ACAT2016. Also the discussion is given on the application of DCM to physical 2-loop processes including numerator functions.

Higher-order radiative corrections for bb¯→H−W+

I present higher-order radiative corrections from collinear and soft gluon emission for the associated production of a charged Higgs boson with a $W$ boson. The calculation uses expressions from resummation at next-to-leading-logarithm accuracy. From the resummed cross section I derive analytical formulas at approximate NNLO and N$^3$LO. Total cross sections are presented for the process $b{\bar b} \rightarrow H^- W^+$ at various LHC energies. The transverse-momentum and rapidity distributions of the charged Higgs boson are also calculated.

Renormalization scheme dependence and renormalization group summation

We consider logarithmic contributions to the free energy, instanton effective action, and Laplace sum rules in QCD that are a consequence of radiative corrections. Upon summing these contributions by using the renormalization group, all dependence on the renormalization scale parameter μ cancels. The renormalization scheme dependence in these processes is examined, and a renormalization scheme is found in which the effect of higher-order radiative corrections is absorbed by the behaviour of the running coupling.

On Higgs Boson Production via Gluon Fusion

A simple phenomenological formula for the gluon distribution function in a proton is proposed. The reported results of calculations of the cross section of Higgs boson production via gluon fusion with higher-order QCD radiative corrections are reproduced using this formula and the one-loop amplitude.

High Performance and Increased Precision Techniques for Feynman Loop Integrals

For the investigation of physics within and beyond the Standard Model, a precise evaluation of higher order corrections in perturbative quantum field theory is required. We have worked on the development of a computational method for Feynman loop integrals with a fully numerical approach. It is based on numerical integration and extrapolation techniques. In this paper, we describe the status and new developments in our techniques for the numerical computation of Feynman loop integrals.Separation of ultra-violet divergences is important for the renormalization procedure. In our analyses, the separation can be done numerically. For 2-loop integrals we have performed the calculations for up to 4-point functions, and for 2-point functions we can handle up to 4- loop integrals. We report the status and accuracy of the computations with detailed numerical comparisons to results in the literature, in order to demonstrate that our method will evolve into an important component of automated systems for the study of higher-order radiative corrections.

Understanding electroweak physics in the Standard Model and beyond

This contribution discusses the current status of tests of the electroweak theory and some of their essential theoretical ingredients. At medium and low energies, electroweak precision measurements are sensitive to high-scale physics through virtual loop corrections. At the energy frontier, on the other hand, the electroweak theory is tested through anomalous gauge boson couplings. It is discussed how effective field theories can be used to describe these couplings in a model-independent and theoretically well-defined way, and to correlate them to other observables. Finally, focusing on the example of W+W− production, it is demonstrated that the precise measurement of anomalous gauge boson couplings at the LHC requires good control of higher-order radiative corrections.

Bottomonium hyperfine splitting on the lattice and in the continuum

We revise the analysis of the bottomonium hyperfine splitting within the lattice nonrelativistic QCD. The Wilson coefficients of the radiatively improved lattice action are evaluated by a semianalytic approach based on the asymptotic expansion about the continuum limit. The nonrelativistic renormalization group is used to estimate the high-order radiative corrections. Our result for the $1S$ hyperfine splitting is $M_{\Upsilon(1S)}-M_{\eta_b(1S)}=52.9\pm 5.5~{\rm MeV}$. It reconciles the predictions of the continuum and lattice QCD and is in very good agreement with the most accurate experimental measurement by Belle collaboration.

A new generator for the Drell-Yan process

We present the Monte Carlo event generator LePaProGen for lepton pair production at hadron colliders. LePaProGen focuses on the description of higher-order electroweak radiative corrections. The generator is implementing a new algorithm for the selection of the optimal variables for phase space parametrization.