Quark Loops Research Articles

Higgs-boson production in the full theory at NNLO+PS

We consider the production of a Standard-Model (SM) Higgs boson in gluon fusion in hadronic collisions and compute the QCD corrections up to next-to-next-to-leading order (NNLO) and match them to parton showers (NNLO+PS). The complete dependence on the top-quark mass is taken into account without making any approximations to the top quark loops mediating the coupling between the gluons and the Higgs boson. To this end, we have included the gg→H amplitudes up to three loops and the pp→H+jet amplitudes up to two loops in the full SM theory. This is the first fully differential calculation of the top-quark mass effects up to NNLO in QCD, and we study their impact on relevant observables for the LHC.

Topological diagram analysis of [formula omitted] decays in the SU(3)F limit and beyond

Charm baryon decay plays an important role in studying non-perturbative baryonic transitions. Compared to other hadron multiplets, the flavor symmetry of baryon decuplet is more simple and attractive. In this work, we study the topological amplitudes of charmed baryon decays into decuplet baryon in the flavor symmetry and the linear SU(3)F breaking. It is found most of topological diagrams are suppressed by the Körner-Pati-Woo theorem in the SU(3)F limit. Only two independent amplitudes contributing to the Bc3‾→B10M decays, with one dominating the branching fractions. The Lee-Yang parameters of all Bc3‾→B10M modes are the same in the SU(3)F limit, and there are only four possible values for the CP asymmetries. After including the first-order SU(3)F breaking effects, the Ξc+→Σ⁎+K‾0 and Ξc+→Ξ⁎0π+ decays have non-zero branching fractions. The number of free parameter contributing to the Bc3‾→B10M decays in the linear SU(3)F breaking is smaller than the available data. The SU(3)F breaking part of the quark loop diagram can be extracted by global fitting of branching fractions, which could help us understand the CP violation in charm sector. Additionally, some new isospin equations are proposed to test the Körner-Pati-Woo theorem.

Investigation of vertex a1>>VP for hadronic tau decays

In the framework of U(3) symmetric quark model, triangular diagrams are calculated that describe the strongly interacting vertices a1>>rhopi and a1>>K*K. The divergences arising when considering quark loops are regularized by covariant cutoff parameter. Based on four-particle hadronic τ decays, the quark structure of the resonance with quantum numbers JPC=1++ was studied. Theoretical estimates of intermediate channels with the meson for the partial decay widths of tau>>pipipinu, tau>>KKpinu and tau>>KKetanu are obtained. Contributions from contact channels describing the direct production of 3pi, KKpi and KKeta mesons by τ lepton current are taken into account. The interference of contact and intermediate axial-vector channels in determining the integral decay widths is studied. The matrix elements of the processes are obtained in the leading approximation of 1/Nc expansion. It is shown that calculations on τ lepton mesonic decays confirm the quark-antiquark structure of the a1 meson and clarify the role of the intermediate non-strange axial-vector channel in determining the integral partial decay widths. The obtained results are compared with experimental data by BaBar and Belle collaborations at the BEPC II and KEK lepton colliders.

Naturalness-motivated composite Higgs model for generating the top Yukawa coupling

The large top Yukawa coupling results in the top quark contributing significantly to the quantum correction of the Higgs mass term. Traditionally, this effect is canceled by the presence of top partners in symmetry-based models. However, the absence of light top partners poses a challenge to the naturalness of these models. In this paper, we study a model based on composite Higgs with the top Yukawa coupling originating from dimension-six four-fermion operators. The low cutoff scale of the top quark loop required by the naturalness principle can be realized with a light gauge boson Eμ that connects the hyperfermions and top quarks. A scalarless dynamical model with weakly coupled extended SU(4)EC gauge group is presented. The model features an Eμ boson and a ZE′ boson both at the sub-TeV scale, which lead to a rich phenomenology, especially in the top quark physics. Published by the American Physical Society 2024

Two-loop amplitudes for tt¯H\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}H $$\\end{document} production: the quark-initiated Nf-part

We present numerical results for the two-loop virtual amplitude entering the NNLO corrections to Higgs boson production in association with a top quark pair at the LHC, focusing, as a proof of concept of our method, on the part of the quark-initiated channel containing loops of massless or massive quarks. Results for the UV renormalised and IR subtracted two-loop amplitude for each colour structure are given at selected phase-space points and visualised in terms of surfaces as a function of two-dimensional slices of the full phase space.

Hadronic Light-by-Light Corrections to the Muon Anomalous Magnetic Moment

We review the hadronic light-by-light (HLbL) contribution to the muon anomalous magnetic moment. Upcoming measurements will reduce the experimental uncertainty of this observable by a factor of four; therefore, the theoretical precision must improve accordingly to fully harness such an experimental breakthrough. With regards to the HLbL contribution, this implies a study of the high-energy intermediate states that are neglected in dispersive estimates. We focus on the maximally symmetric high-energy regime and in-quark loop approximation of perturbation theory, following the method of the OPE with background fields proposed by Bijnens et al. in 2019 and 2020. We confirm their results regarding the contributions to the muon g−2. For this, we use an alternative computational method based on a reduction in the full quark loop amplitude, instead of projecting on a supposedly complete system of tensor structures motivated by first principles. Concerning scalar coefficients, mass corrections have been obtained by hypergeometric representations of Mellin–Barnes integrals. By our technique, the completeness of such kinematic singularity/zero-free tensor decomposition of the HLbL amplitude is explicitly checked.

The complex heavy quarkonium potential with the Gribov–Zwanziger action

Gribov–Zwanziger prescription in Yang–Mills theory improves the infrared dynamics. In this work, we study the static potential of a heavy quark-antiquark pair with the HTL resummed perturbation method within the Gribov–Zwanziger approach at finite temperature. The real and imaginary parts of the heavy quarkonium complex potential are obtained from the one-loop effective static gluon propagator. The one-loop effective gluon propagator is obtained by calculating the one-loop gluon self-energies containing the quark, gluon, and ghost loop. The gluon and ghost loops are modified in the presence of the Gribov parameter. We also calculate the decay width from the imaginary part of the potential. We also discuss the medium effect of heavy quarkonium potential with the localized action via auxiliary fields.

Interference effects in gg → H → Zγ beyond leading order

The ATLAS and CMS collaborations at the LHC have recently announced evidence for the rare Higgs boson decay into a Z boson and a photon. We analyze the interference between the process gg→H→Zγ induced by loops of heavy particles, which is by far the dominant contribution to the signal, and the continuum gg→Zγ QCD background process mediated by light quark loops. This interference modifies the event yield, the resonance line-shape and the apparent mass of the Higgs boson. We calculate the radiative corrections to this interference beyond the leading-order approximation in perturbative QCD and find that, while differing numerically from the corresponding effects on the more studied gg→γγ signal, they are generally rather small. As such, they do not impact significantly the interpretation of the present measurements of the H→Zγ decay mode.

(γ, χcJ(J = 0,1)) and (γ, ηc) production in electron-positron annihilation at [formula omitted] GeV and 4.6 GeV in the framework of Bethe-Salpeter equation

In present work we study the production of ground and excited charmonium states in e−e+→γχcJ(nP)(J=0,1), and e−e+→γηc(nS), through leading order (LO) diagrams, which proceed through exchange of a virtual photon that couples to γ and ηc/χcJ through the triangular quark loop diagram, in the framework of 4×4 Bethe-Salpeter equation (BSE), at center of mass energies s=10.6 GeV (Belle energy), and 4.6 GeV (BESIII energy). The amplitude simplifies to a general form required by Lorentz-covariance, in terms of its form factors. The cross sections for these processes with leading order diagrams alone at s=10.6 GeV provide a sizable contribution, which might be mainly due to the BSE being a fully relativistic approach that incorporates the relativistic effect of quark spins and can also describe internal motion of constituent quarks within the hadron in a relativistically covariant manner. Our results are compared with recent Belle data at 10.6 GeV, and BESIII data at 4.6 GeV, as well as other models. Plots of cross sections versus the center-of mass energy, s reveal mild fluctuations in their behaviour for all the three processes in the low energy (s=4–6 GeV) region, and are analyzed in terms of the form factors.

Master integrals for electroweak corrections to gg→γγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$gg\\rightarrow \\gamma \\gamma $$\\end{document}: light quark contributions

We present a calculation of the master integrals (MI’s) required for the calculation of the Electroweak corrections to gg→γγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$gg\\rightarrow \\gamma \\gamma $$\\end{document} production in which the process contains a light quark loop. The integrals can be broken down into five categories based on the flow of the heavy vector bosons throughout the loop. Three of the families are planar, and two are non-planar. We determine a canonical basis for each family which allows an efficient solution of the resulting differential equations via iterated integrals. We calculate the families in relevant physical kinematics and obtain an efficient numerical evaluation based on an implementation of Chen-iterated integrals.

Photons as a Signal of Deconfinement in Hadronic Matter under Extreme Conditions

The photon production by conversion of gluons gg→γ via quark loop in the framework of the mean-field approach to the QCD (quantunm chromodynamics) vacuum is studied here. According to the domain model of QCD vacuum, the confinement phase is dominated by Abelian (anti-)self-dual gluon fields, while the deconfinement phase is characterized by a strong chromomagnetic field. In the confinement phase, photon production is impossible due to the random spacial orientation of the statistical ensemble of vacuum fields. However, the conditions of Furry theorem are not satisfied in the deconfinement phase, the conversion of gluons is nonzero and, in addition, photon distribution has a strong angular anisotropy. Thus, the photon production in the discussed process acts as one of the important features of transition in quark-gluon plasma to the deconfinement phase.

Composite scalar boson mass dependence on the constituent mass anomalous dimension

We perform a Bethe–Salpeter equation (BSE) evaluation of composite scalar boson masses in order to verify how these masses can be smaller than the composition scale. The calculation is developed with a constituent self-energy dependent on its mass anomalous dimension ([Formula: see text]), and we obtain a relation showing how the scalar mass decreases as [Formula: see text] is increased. We also discuss how fermionic corrections to the BSE kernel shall decrease the scalar mass, whose effect can be as important as the one of a large [Formula: see text]. An estimate of the top quark loop effect that must appear in the BSE calculation gives a lower bound on the composite scalar mass.

Radiative Decay Width of J/ψ→γη(2) from Nf=2 Lattice QCD

The large radiative production rate for pseudoscalar mesons in the J/ψ radiative decay remains elusive. We present the first lattice QCD calculation of partial decay width of J/ψ radiatively decaying into η_{(2)}, the SU(2) flavor singlet pseudoscalar meson, which confirms QCD U_{A}(1) anomaly enhancement to the coupling of gluons with flavor singlet pseudoscalar mesons. The lattice simulation is carried out using N_{f}=2 lattice QCD gauge configurations at the pion mass m_{π}≈350 MeV. In particular, the distillation method has been utilized to calculate light quark loops. The results are reported here with the mass m_{η_{(2)}}=718(8) MeV and the decay width Γ(J/ψ→γη_{(2)})=0.385(45) keV. By assuming the dominance of U_{A}(1) anomaly and flavor singlet-octet mixing angle θ=-24.5°, the production rates for the physical η and η^{'} in J/ψ radiative decay are predicted to be 1.15(14)×10^{-3} and 4.49(53)×10^{-3}, respectively, which agree well with the experimental measurement data. Our study manifests the potential of lattice QCD studies on the light hadron production in J/ψ radiative decays.

Amplitude evaluation with pySecDec: A Higgs + three gluons example

We present an application of the major new features of pySecDec, a program designed to numerically calculate multi-loop integrals. One important new feature is the ability to integrate weighted sums of integrals in a way which is optimised to reach a given accuracy goal on the sums rather than on the individual integrals. Another feature is the option to perform asymptotic expansions. These new assets of the program represent an important step towards the efficient evaluation of multi-loop amplitudes in a largely automated way. We will illustrate them in a pedagogical example, through the calculation of the one-loop amplitude for Higgs plus jet production in the gluon channel, mediated by a top quark loop. Numerical results in the heavy top limit, compared to their analytic counterparts, are shown as well.

Simulating rare kaon decays K+→π+ℓ+ℓ− using domain wall lattice QCD with physical light quark masses

We report the first calculation using physical light-quark masses of the electromagnetic form factor $V(z)$ describing the long-distance contributions to the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decay amplitude. The calculation is performed on a $2+1$ flavor domain wall fermion ensemble with inverse lattice spacing ${a}^{\ensuremath{-}1}=1.730(4)\text{ }\text{ }\mathrm{GeV}$. We implement a Glashow-Iliopoulos-Maiani cancellation by extrapolating to the physical charm-quark mass from three below-charm masses. We obtain $V(z=0.013(2))=\ensuremath{-}0.87(4.44)$, achieving a bound for the value. The large statistical error arises from stochastically estimated quark loops.

Гипотеза о контракции калибровочной группы Стандартной модели и экспериментальные данные БАК

Within the framework of the contraction hypothesis of the
 gauge group of the Standard Model, the behavior of the amplitude
 of the dominant Higgs boson production process in the
 four-lepton decay with an increasing temperature T is analyzed.
 It is shown that the modified process breaks down into
 a number of channels depending on the contribution of the
 color components in the loop of virtual quarks, leading to the
 creation of the Higgs boson. The dependence on T of the cross
 section of each channel is found. Comparison with LHC data
 on Higgs boson creation cross sections at energies (temperatures)
 of 7, 8, 13, and 14 TeV showed that the hypothesis about
 the contraction of the gauge group of the Standard Model does
 not contradict these data.

Collective modes of gluons in an anisotropic thermomagnetic medium

We study the collective modes of gluons in an anisotropic thermal medium in the presence of a constant background magnetic field using the hard-thermal-loop perturbation theory. The momentum space anisotropy of the medium has been incorporated through the generalized Romatschke-Strickland form of the distribution function, whereas the magnetic modification arising from the quark loop contribution has been taken into account in the lowest Landau level approximation. We consider two special cases: (i) a spheroidal anisotropy with the anisotropy vector orthogonal to the external magnetic field and (ii) an ellipsoidal anisotropy with two mutually orthogonal vectors describing anisotropies along and orthogonal to the field direction. The general structure of the polarization tensor in both cases is equivalent and consists of six independent basis tensors. We find that the introduction of momentum anisotropy ingrains azimuthal angular dependence in the thermomagnetic collective modes. Our study suggests that the presence of a strong background magnetic field can significantly reduce the growth rate of the unstable modes, which may have important implications in the equilibration of magnetized quark-gluon plasma.

Semileptonic Decays of $$\rho$$, $$\omega$$, $$\phi$$, $$\eta$$ and $$\eta ^{\prime }$$ Mesons in Ground and First Radially Excited States in the $$U(3)\times U(3)$$ NJL Model

In the extended $U(3)\times U(3)$ NJL model, semileptonic decays of $\rho$, $\omega$, $\phi$, $\eta$ and $\eta'$ mesons in both the ground and first radially excited states are described. Quark loops of the anomalous type are considered, which are based on the vertex $\eta \to VV$. The obtained results are in satisfactory agreement with the existing experimental data. In addition, a number of predictions for experimentally unmeasured decays of radially excited mesons are presented.

Singlet and nonsinglet three-loop massive form factors

We consider Quantum Chromodynamics with external vector, axial-vector, scalar and pseudo-scalar currents and compute three-loop corrections to the corresponding vertex function taking into account massive quarks. We consider all non-singlet contributions as well as those singlet contributions where the external current couples to a massive quark loop. We apply a semi-numerical method which is based on expansions around singular and regular kinematical points. They are matched at intermediate values of the squared partonic center-of-mass energy $s$ which allows to cover the whole kinematic range for negative and positive values of $s$. Our method permits a systematic increase of the precision by varying the expansion depth and the choice of the intermediate matching points. In our current set-up we have at least seven significant digits for the finite contribution of all form factors. We present our results as a combination of series expansions and interpolation functions which allows for a straightforward use in practical applications.

Vector boson pair production at one loop: analytic results for the process mathrm{q}overline{mathrm{q}}ell overline{ell}{ell}^{prime }{overline{ell}}^{prime}mathrm{g}

We present compact analytic results for the one-loop amplitude for the process 0 → qoverline{q}ell overline{ell}{ell}^{prime }{overline{ell}}^{prime }g , relevant for both the production of a pair of Z and W-bosons in association with a jet. We focus on the gauge-invariant contribution mediated by a loop of quarks. We explicitly include all effects of the loop-quark mass m, appropriate for the production of a pair of Z-bosons. In the limit m → 0, our results are also applicable to the production of W-boson pairs, mediated by a loop of massless quarks. Implemented in a numerical code, the results are fast. The calculation uses novel advancements in spinor-helicity simplification techniques, for the first time applied beyond five-point massless kinematics. We make use of primary decompositions from algebraic-geometry, which now involve non-radical ideals, and p-adic numbers from number theory. We show how to infer whether numerator polynomials belong to symbolic powers of non-radical ideals through numerical evaluations.