Evidence for the $N'(1720)3/2^+$ Nucleon Resonance from Combined Studies of CLAS $π^+π^-p$ Photo- and Electroproduction Data
V. I. Mokeev, V. D. Burkert, D. S. Carman, L. Elouadrhiri, E. Golovatch, R. W. Gothe, K. Hicks, B. S. Ishkhanov, E. L. Isupov, K. Joo, N. Markov, E. Pasyuk, A. Trivedi
The analysis of the nine 1-fold differential cross sections for the $γ_{r,v} p \to π^+π^-p$ photo- and electroproduction reactions obtained with the CLAS detector at Jefferson Laboratory was carried out with the goal to establish the contributing resonances in the mass range from 1.6~GeV to 1.8~GeV. In order to describe the photo- and electroproduction data with $Q^2$-independent resonance masses and hadronic decay widths in the $Q^2$ range below 1.5~GeV$^2$, it was found that an $N'(1720)3/2^+$ state is required in addition to the already well-established nucleon resonances. This work demonstrates that the combined studies of $π^+π^-p$ photo- and electroproduction data are vital for the observation of this resonance. The contributions from the $N'(1720)3/2^+$ state and the already established $N(1720)3/2^+$ state with a mass of 1.745~GeV are well separated by their different hadronic decays to the $πΔ$ and $ρp$ final states and the different $Q^2$-evolution of their photo-/electroexcitation amplitudes. The $N'(1720)3/2^+$ state is the first recently established baryon resonance for which the results on the $Q^2$-evolution of the photo-/electrocouplings have become available. These results are important for the exploration of the nature of the ``missing'' baryon resonances.
The results on the photo- and electroexcitation amplitudes of most nucleon resonances in the mass range up to 2.0 GeV determined from the CLAS experimental data on exclusive $π^+π^-p$ photo-/electroproduction off protons in collaboration between the Jefferson Lab and Moscow State University are presented. The first and only available results on electroexcitation amplitudes from CLAS in a wide range of photon virtualities $Q^2$ $<$ 5.0 GeV$^2$ revealed the nucleon resonance structure as a complex interplay between the inner core of three dressed quarks and external meson-baryon cloud. These res…
Exclusive $π^+π^-p$ photo- and electroproduction data from CLAS have considerably extended the information on the spectrum and structure of nucleon resonances. The data from the $π^+π^-p$ and $Nπ$ channels have provided results on the electrocouplings of most resonances in the mass region up to 1.8 GeV and at photon virtualities up to 5.0 GeV$^2$. The recent CLAS data on $π^+π^-p$ photoproduction have improved knowledge on the photocouplings of nucleon resonances in the mass range of 1.6 GeV $<$ $M_{N^*}$ $<$ 2.0 GeV and on their decays to the $πΔ$ and $ρp$ final hadron states. For the first t…
Studies of the nucleon resonance electroexcitation amplitudes in a wide range of photon virtualities offer unique information on many facets of strong QCD behind the generation of all prominent excited nucleon states. Advances in the evaluation of resonance electroexcitation amplitudes from the data measured with the CLAS detector and the future extension of these studies with the CLAS12 detector at Jefferson Lab are presented. For the first time, analyses of $π^0p$, $π^+n$, $ηp$, and $π^+π^-p$ electroproduction off proton channels have provided electroexcitation amplitudes of most resonances …
Cross section and single transverse target spin asymmetry for backward pion electroproduction
核子-介子跃迁分布振幅（TDA）是核子-介子态之间非定域三夸克算符的非对角矩阵元，它是在共线因式分解描述的介子从核子反向排斥电生过程中产生的.下面我们将讨论π介子到核子时间分辨模型的问题。我们提出了一个因式分解的四重分布与软π介子定理的输入π N TDAs的。为了满足多项式性质在其完整的形式的频谱表示补充与D-项一样的贡献，从核子交换的u-通道的反应。我们提出了我们的估计后向π介子电产生非极化截面和它的横向目标单自旋不对称性在我们的复合模型为π N TDAs。
We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, $e p \to e^\prime n π^+$, above the resonance region at backward pion center-of-mass angles. The $φ^*_π$-dependent cross sections were measured, from which we extracted three combinations of structure functions of the proton. Our results are compatible with calculations based on nucleon-to-pion transition distribution amplitudes (TDAs) and shed new light on nucleon structure.
The diffractive electro- or photo-production of two mesons separated by a large rapidity gap gives access to generalized parton distributions (GPDs) in a very specific way. First, these reactions allow to easily access the chiral-odd transversity quark GPDs by selecting one of the produced vector meson to be transversely polarized. Second, they are only sensitive to the so-called ERBL region where GPDs are not much constrained by forward quark distributions. Third, the skewness parameter $ξ$ is not related to the Bjorken $x_\text{Bj}$ variable, but to the size of the rapidity gap. We analyze d…
Impressive progress achieved in the past decade in experimental studies of exclusive meson photoproduction off protons and global multi-channel amplitude analyses has resulted in the discovery of several long-awaited new nucleon resonances, with a decisive impact from the results of $KΛ$ and $KΣ$ photoproduction measured with the CLAS detector at Jefferson Lab. Further extension of these efforts towards combined studies of exclusive meson photo- and electroproduction data off protons will be presented. A new excited state of the nucleon, the $N'(1720)3/2^+$, discovered from combined analyses o…
A theory of two-pion photo- and electroproduction off the nucleon is derived considering all explicit three-body mechanisms of the interacting $ππN$ system. The full three-body dynamics of the interacting $ππN$ system is accounted for by the Faddeev-type ordering structure of the Alt-Grassberger-Sandhas equations. The formulation is valid for hadronic two-point and three-point functions dressed by arbitrary internal mechanisms provided all associated electromagnetic currents are constructed to satisfy their respective (generalized) Ward-Takahashi identities. It is shown that coupling the photo…
Studying the structure of excited nucleon states employing the electroproduction of exclusive reactions is an important avenue for exploring the nature of the non-perturbative strong interaction. The electrocouplings of $N^*$ states in the mass range below 1.8~GeV have been determined from analyses of CLAS $πN$, $ηN$, and $ππN$ data. This work has made it clear that consistent results from independent analyses of several exclusive channels with different couplings and non-resonant backgrounds but the same $N^*$ electro-excitation amplitudes, is essential to have confidence in the extracted res…
A consistent model for πN transition distribution amplitudes and backward pion electroproduction
推广广义部分子分布的概念，引入了重子-介子跃迁分布振幅（TDAs），即核子-介子态间非定域三夸克算符的非对角矩阵元。我们提出了一个通用的框架，模拟核子到π介子（$πN$）TDAs。我们的主要工具是π N TDAs的四重分布的谱表示。我们提出了一个因式分解的四重分布与输入的软π介子定理的π N TDAs。光谱表示的补充与D-项一样的贡献，从核子交换中的交叉通道。然后，我们研究了后向π介子电产生的QCD共线因式分解方法，其中振幅的非微扰部分涉及π N TDAs。在我们的π N TDAs的两个组件模型中，我们更新了以前的非极化截面的领先扭转估计。最后，我们计算了横向目标单自旋不对称性作为偏斜度的函数。我们发现它是相当大的价区域和敏感的唯象输入我们的π N TDA模型。
One of the greatest challenges within the Standard Model is to discover the source of visible mass. Indeed, this is the focus of a "Millennium Problem", posed by the Clay Mathematics Institute. The answer is hidden within quantum chromodynamics (QCD); and it is probable that revealing the origin of mass will also explain the nature of confinement. In connection with these issues, this perspective describes insights that have recently been drawn using contemporary methods for solving the continuum bound-state problem in relativistic quantum field theory and how they have been informed and enabl…
