Electromagnetic Transition Form Factors Research Articles

Odd-parity nucleon electromagnetic transitions in lattice QCD

The parity-expanded variational analysis (PEVA) technique enables the isolation of opposite-parity eigenstates at finite momentum. The approach has been used to perform the first lattice QCD calculations of excited-baryon form factors. In particular, these calculations show that the low-lying odd-parity nucleon excitations are described well by constituent quark models at moderate u and d quark masses approaching the strange quark mass. Herein, we extend the PEVA technique to establish a formalism for the determination of odd-parity nucleon electromagnetic transition form factors in lattice QCD. The formalism is implemented in the first calculation of the helicity amplitudes for transitions from the ground state nucleon to the first two odd-parity excitations. Through a comparison with constituent quark-model calculations of these amplitudes, these new results give important insight into the structure of these excitations. This work is a critical step towards confronting experimental electroproduction amplitudes for the N*(1535) and N*(1650) resonances with lattice QCD calculations. Published by the American Physical Society 2024

Electromagnetic and two-photon transition form factors of the pseudoscalar mesons: An algebraic model computation

We compute electromagnetic and two-photon transition form factors of ground-state pseudoscalar mesons: π,K,ηc,ηb. To this end, we employ an algebraic model based upon the coupled formalism of Schwinger-Dyson and Bethe-Salpeter equations. Within this approach, the dressed quark propagator and the relevant Bethe-Salpeter amplitude encode the internal structure of the corresponding meson. Electromagnetic properties of the meson are probed via the quark-photon interaction. The algebraic model employed by us unifies the treatment of all ground-state pseudoscalar mesons. Its parameters are carefully fitted performing a global analysis of existing experimental data including the knowledge of the charge radii of the mesons studied. We then compute and predict electromagnetic and two-photon transition form factors for a wide range of probing photon momentum-squared which is of direct relevance to the experimental observations carried out thus far or planned at different hadron physics facilities such as the Thomas Jefferson National Accelerator Facility (JLab) and the forthcoming Electron-Ion Collider. We also present comparisons with other theoretical models and approaches and lattice quantum chromodynamics. Published by the American Physical Society 2024

Study of η′ → π+π−l+l− decays at BESIII

With a sample of (10087 ± 44) × 106J/ψ events accumulated with the BESIII detector, we analyze the decays η′ → π+π−l+l−(l = e, μ) via the process J/ψ → γη′. The branching fractions are measured to be B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(η′ → π+π−e+e−) = (2.45 ± 0.02(stat.) ± 0.08(syst.)) × 10−3 and B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(η′ → π+π−μ+μ−) = (2.16 ± 0.12(stat.) ± 0.06(syst.)) × 10−5, and the ratio is Bη′→π+π−e+e−Bη′→π+π−μ+μ−=113.4±0.9stat.±3.7syst.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{\\mathcal{B}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{e}^{+}{e}^{-}\\right)}{\\mathcal{B}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{\\mu}^{+}{\\mu}^{-}\\right)}=113.4\\pm 0.9\\left(\ extrm{stat}.\\right)\\pm 3.7\\left(\ extrm{syst}.\\right) $$\\end{document}. In addition, by combining the η′ → π+π−e+e− and η′ → π+π−μ+μ− decays, the slope parameter of the electromagnetic transition form factor is measured to be bη′ = 1.30 ± 0.19 (GeV/c2)−2, which is consistent with previous measurements from BESIII and theoretical predictions from the VMD model. The asymmetry in the angle between the π+π− and l+l− decay planes, which has the potential to reveal the CP-violation originating from an unconventional electric dipole transition, is also investigated. The asymmetry parameters are determined to be ACPη′→π+π−e+e−=−0.21±0.73stat.±0.01syst.%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{A}}_{CP}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{e}^{+}{e}^{-}\\right)=\\left(-0.21\\pm 0.73\\left(\ extrm{stat}.\\right)\\pm 0.01\\left(\ extrm{syst}.\\right)\\right)\\% $$\\end{document} and ACPη′→π+π−μ+μ−=0.62±4.71stat.±0.08syst.%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{A}}_{CP}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{\\mu}^{+}{\\mu}^{-}\\right)=\\left(0.62\\pm 4.71\\left(\ extrm{stat}.\\right)\\pm 0.08\\left(\ extrm{syst}.\\right)\\right)\\% $$\\end{document}, implying that no evidence of CP-violation is observed at the present statistics. Finally, an axion-like particle is searched for via the decay η′ → π+π−a, a → e+e−, and upper limits of the branching fractions are presented for the mass assumptions of the axion-like particle in the range of 0 − 500 MeV/c2.

Studies of Time-like Electromagnetic Structure of Baryons with HADES

We present results of studies of Dalitz decays of baryon resonances (R→Ne+e−) in proton-proton (pp) and pion-proton (πp) collisions performed by the HADES collaboration. They provided the first measurement of electromag netic transition form factors of baryons in the time-like region and contribute to the understanding of the photon-baryon coupling and the role of Vector Dom inance Model in a baryonic sector. We discuss also implications of the results to the understanding of the emissivity of dense and hot QCD matter created in heavy-ion collisions and the role played by the in-medium modification of the ρ meson. Further prospects for future studies with pion beams at GSI with HADES are given in the outlook.

N*(1520) transition form factors from dispersion theory

The electromagnetic transition form factors (TFFs) of the nucleon provide important information on the internal structure of hadrons. A model-independent dispersive calculation of the electro magnetic TFFs N*(1520) → N at low energies is presented. Taking pion rescattering into consideration, we derived dispersive relations for the N*(1520) → N TFFs that relate space-like and time-like regions from the first principles. Based on the space-like data from JLab and hadronic data measured by HADES, we make predictions for TFFs in the time-like region. Our predictions can be tested in future experi ments (e.g. HADES).

Hyperon production and interaction studies in protonproton scattering with HADES

Hyperons and their interactions with baryonic matter may play a key role in our understanding of the core of neutron stars. Moreover, the internal structure of hyperons and their excitations are of great interest. Their structure can be probed by exploiting their couplings to radiative probes such as by measuring electromagnetic transition form factors. HADES (High-Acceptance Di-Electron Spectrometer) at GSI collected high-statistics proton-proton scattering data in 2022 at √s = 3.48 GeV. The Λ - Λ interaction is currently being studied in this data along with production of Σ states where the Dalitz decay of hyperons might be observed for the first time. In addition, the reaction pp → pK+K+Ξ−[π−Λ[pπ−]] has been studied to probe the production mechanism of double strangeness near its production threshold. This proceeding addresses the hyperon physics case and discusses the ongoing Ξ− analysis. A preliminary upper limit for the cross section at √s = 3.48 GeV is presented taking into account only statistical uncertainties.

Light-cone sum rules for radial excitation of decuplet to octet baryons electromagnetic transition form factors

In this paper magnetic dipole moment form factor, [Formula: see text], describing the radial excitation of decuplet baryons to octet baryons electromagnetic transitions as well as the ratios, [Formula: see text] and [Formula: see text] are calculated in the framework of light-cone sum rules. We also estimate the degree of the violation of U-spin symmetry.

Electromagnetic transition form factors and Dalitz decays of hyperons

Dalitz decays of a hyperon resonance to a ground-state hyperon and an electron-positron pair can give access to some information about the composite structure of hyperons. We present expressions for the multi-differential decay rates in terms of general transition form factors for spin-parity combinations J^P = frac{1}{2}^pm , frac{3}{2}^pm of the hyperon resonance. Even if the spin of the initial hyperon resonance is not measured, the self-analyzing weak decay of the “final” ground-state hyperon contains information about the relative phase between combinations of transition form factors. This relative phase is non-vanishing because of the unstable nature of the hyperon resonance. If all form factor combinations in the differential decay formulae are replaced by their respective values at the photon point, one obtains a QED type approximation, which might be interpreted as characterizing hypothetical hyperons with point-like structure. We compare the QED type approximation to a more realistic form factor scenario for the lowest-lying singly-strange hyperon resonances. In this way we explore which accuracy in the measurements of the differential Dalitz decay rates is required in order to distinguish the composite-structure case from the pointlike case. Based on the QED type approximation we obtain as a by-product a rough prediction for the ratio between the Dalitz decay width and the corresponding photon decay width.

Electromagnetic transitions of the singly charmed baryons with spin 3/2

We investigate the electromagnetic transitions of the singly charmed baryons with spin 3/2, based on a pion mean-field approach, also known as the chiral quark-soliton model, taking into account the rotational $1/N_c$ corrections and the effects of flavor SU(3) symmetry breaking. We examine the valence- and sea-quark contributions to the electromagnetic transition form factors and find that the quadrupole form factors of the sea-quark contributions dominate over those of the valence-quark ones in the smaller $Q^2$ region, whereas the sea quarks only provide marginal contributions to the magnetic dipole transition form factors of the baryon sextet with spin 3/2. The effects of the flavor SU(3) symmetry breaking are in general very small except for the forbidden transition $\Xi_c^0\gamma\to \Xi_c^{*0}$ by $U$-spin symmetry. We also discuss the widths of the radiative decays for the baryon sextet with spin 3/2, comparing the present results with those from other works.

Electromagnetic transition form factors, E2/M1 and C2/M1 ratios of the baryon decuplet

We investigate the electromagnetic transition form factors of the baryon decuplet to the baryon octet, based on the self-consistent SU(3) chiral quark-soliton model, taking into account the effects of explicit breaking of flavor SU(3) symmetry. We emphasize the Q^2 dependence of the electromagnetic Nrightarrow Delta transition form factors and the ratios of E2/M1 and C2/M1 in comparison with the experimental and empirical data. In order to compare the present results of the electromagnetic transition form factors of the Nrightarrow Delta with those from lattice QCD, we evaluate the form factors with the pion mass deviated from its physical value. The results of the E2/M1 and C2/M1 ratios are in good agreement with the lattice data. We also present the results of the electromagnetic transition form factors for the decuplet to the octet transitions.

Hyperons Dalitz decays @ HADES

The electromagnetic transition form factors are an ideal tool to discriminate between various models of hadron structure. However, measuring them is a great experimental challenge, especially in time-like region, that is inaccessible by annihilation experiments. The HADES detector is, thanks to ability to identify di-leptons, an ideal tool to perform measurement in the low q 2 time-like region. This gives access to transitions form factors of excited baryon (N*) states, via Dalitz decays . In this paper I will present a simulation study of a planned experiment, dedicated to measurements of Dalitz decays, produced with a proton beam of kinetic energy Ek = 4.5 GeV. In addition, the current status of the analysis of data collected at Ek = 3.5 GeV will be reported.

Analytical investigation of electromagnetic transition form factors of nucleon resonances

In this paper, we investigated the transition form factors of nucleon resonances, in an analytical approach. Employing an improved hypercentral constituent quark model, we obtained analytical eigenfunctions which have been used to calculate the transverse and longitudinal helicity amplitudes for nucleon resonances. We compared the evaluated observables with experimental data and it has been shown that the present model provides a good description of the available data.

From Transition Electromagnetic Form Factors \(\gamma ^* \gamma ^* \eta _c(1S,2S)\) to the Production of \(\eta _c(1S,2S)\) at the LHC

We review our recent results for production of $\eta_c(1S)$ and $\eta_c(2S)$ in the $\gamma^* \gamma^* \to \eta_c(1S,2S)$ fusion and in proton-proton collisions via gluon-gluon fusion. The quarkonium wave functions are calculated by solving Schr\"odinger equation for different $c \bar c$ potentials. Using Terentev prescription the light-cone wave functions are obtained. The light-cone wave functions are used then to calculate $\gamma^* \gamma^* \to \eta_c$ transition form factors. The theoretical results are compared to the Belle experimental data for $\eta_c(1S)$. In addition we discuss our results for two-photon decay width. We present also results of our calculations for proton-proton collisions obtained within $k_T$-factorization approach for different unintegrated gluon distributions. The results for hadroproduction of $\eta_c(1S)$ are compared to the LHCb experimental data.

Exploring time like tranistions in pp, πp and AA reactions with HADES

Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon converting to dielectron pair (Dalitz decays) provides important information about baryon-photon coupling at low q2 in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at q2 near vector mesons ρ/ω poles has been predicted by various calculations reflecting strong baryon-vector meson couplings. The understanding of these couplings is also of primary importance for the interpretation of the emissivity of QCD matter studied in heavy ion collisions via dilepton emission. Dedicated measurements of baryon Dalitz decays in proton-proton and pion-proton scattering with HADES detector at GSI/FAIR are presented and discussed. The relevance of these studies for the interpretation of results obtained from heavy ion reactions is elucidated on the example of the HADES results.

Electromagnetic Transition Form Factor of the Nucleon $$\Delta $$ (1232) in The Nonrelativistic Constituent Quark Model

The study of nucleon electromagnetic form factors has long been identified as a singular source of information for conception strong interactions in the extent of quark confinement. We have performed a calculation of the helicity amplitudes and the electromagnetic transition form factors of the electromagnetic excitation in $$\Delta $$(1232) resonances. In this paper, the electromagnetic interaction for N $$(938)\rightarrow \Delta (1232)$$ transitions at four-momenta transfer $$0 \le \hbox {Q2}(\hbox {GeV}^\mathrm{{2}}) \le 8$$ in the nonrelativistic constituent quark model calculated. In comparison with present experimental, relativistic and non-relativistic data, our results are in good agreement with the experimental and the other theoretical results, in particular of the medium-high $$\hbox {Q}^{2}$$ behavior.

Structure of the Nucleon and Its First Radial Excitation

A Poincar\'e-covariant continuum approach to the three valence-quark bound-state problem in quantum field theory is used to perform a detailed analysis of the nucleon's ground and first excited states: the so-called $N(940)\frac{1}{2}^+$ and $N(1440)\frac{1}{2}^+$. Such analysis predicts the presence of nonpointlike, fully-interacting quark-quark (diquark) correlations within them, being the isoscalar-scalar and isovector-pseudovector diquarks overwhelmingly dominant with similar relative strengths in both states. Moreover, the rest-frame wave functions of both states are largely $S$-wave in nature and the first excited state in this $1/2^+$ channel has the appearance of a radial excitation of the ground state. All these features have numerous observable consequences, we show herein those related with the nucleon's elastic, Roper's elastic and nucleon-to-Roper transition electromagnetic form factors, for both charged and neutral channels.

Hyperon form factors & diquark correlations

Using e+e− annihilation data taken at the CESR collider with the CLEO-c detector, measurements of hyperon pair production cross sections and elastic and transition electromagnetic form factors have been made at the charmonium resonances: ψ(2S), , |Q2| = 13.6 GeV2, ℒ = 48 pb−1; ψ(3770), , |Q2| = 14.2 GeV2, ℒ = 805 pb−1; and ψ(4170), , |Q2| = 17.4 GeV2, ℒ = 586 pb−1. Results with good statistical precision are obtained with high efficiency particle identification. Systematics of pair production cross sections, and form factors with respect to the number of strange quarks in the hyperons are studied, and evidence is presented for effects of diquark correlations in comparative results for Λ0 and Σ0, both of which have the same uds quark content but different isospin.

Meson decay studies from MAMI A2

Decays of the light mesons π0, η, ω, and η’ provide a unique laboratory to test fundamental aspects of hadron physics. Precision studies of such diverse topics as the light quark mass ratio, π−π scattering lengths, and searches for physics beyond the Standard Model are possible. Additionally, Dalitz decays of light mesons provide a way of measuring the electromagnetic meson transition formfactors in the time-like region. The A2 tagged photon facility at the Mainz Microtron provides a high yield of light mesons produced by photo-induced reactions on protons, which makes the experiment ideal for high precision measurements of meson decays. Presented here are the contributions made by the A2 collaboration to such studies.

Electromagnetic form factors of nucleon resonances from CLAS

Exclusive single and double meson photo- and electroproduction reactions are the largest sources of information on the spectrum and structure of nucleon resonances. The excited states of the nucleon manifest as a complex interplay between the inner core of three dressed quarks and the external meson-baryon cloud. Various N* with distinctively different structure appear as unique laboratory where many features of strong QCD can be explored. With combination of nearly 4π acceptance of the CLAS detector and continuous electron beam (Jefferson Lab, USA) it is possible to obtain physics observables of the major reaction channels in the N* excitation region. The results on the electromagnetic transition form factors of N(1440)1/2+ and N(1520)3/2- are presented.

Constituent-Quark Model with Pionic Contributions: Electromagnetic {varvec{Nrightarrow varDelta }} Transition

We report on ongoing work to determine the pion-cloud contribution to the electromagnetic Nrightarrow varDelta transition form factors. The starting point is an SU(6) spin-flavor symmetric constituent-quark model with instantaneous confinement that is augmented by dynamical pions which couple directly to the quarks. This system is treated in a relativistically invariant way within the framework of point-form quantum mechanics using a multichannel formulation. The first step is to determine the electromagnetic form factors of the bare particles that consist only of three quarks. These form factors are basic ingredients for calculating the pion-cloud contributions. Already without the pion cloud, electromagnetic nucleon and Nrightarrow varDelta transition form factors compare reasonably well with the data. By inclusion of the pion-cloud contribution coming from the pi -N intermediate state the reproduction of the data is further improved.