Understanding the origin and dynamics of hadron structure and in turn that of atomic nuclei is a central goal of nuclear physics. This challenge entails the questions of how does the roughly 1 GeV mass-scale that characterizes atomic nuclei appear; why does it have the observed value; and, enigmatically, why are the composite Nambu-Goldstone (NG) bosons in quantum chromodynamics (QCD) abnormally light in comparison? In this perspective, we provide an analysis of the mass budget of the pion and proton in QCD; discuss the special role of the kaon, which lies near the boundary between dominance of strong and Higgs mass-generation mechanisms; and explain the need for a coherent effort in QCD phenomenology and continuum calculations, in exa-scale computing as provided by lattice QCD, and in experiments to make progress in understanding the origins of hadron masses and the distribution of that mass within them. We compare the unique capabilities foreseen at the electron-ion collider (EIC) with those at the hadron-electron ring accelerator (HERA), the arXiv:1907.08218v2 [nucl-ex] Rikutaro Yoshida (ryoshida@jlab.org) INTRODUCTIONAtomic nuclei lie at the core of everything we can see; and at the first level of approximation, their atomic weights are simply the sum of the masses of all the neutrons and protons (nucleons) they contain. Each nucleon has a mass m N ∼ 1 GeV, i.e. approximately 2000-times the electron mass. The Higgs boson produces the latter, but what produces the masses of the neutron and proton? This is the crux: the vast majority of the mass of a nucleon is lodged with the energy needed to hold quarks together inside it; and that is supposed to be explained by QCD, the strong-interaction piece within the Standard Model.QCD is unique. It is a fundamental theory with the capacity to sustain massless elementary degrees-of-freedom, viz. gluons and quarks; yet gluons and quarks are predicted to acquire mass dynamically [1][2][3], and nucleons and almost all other hadrons likewise, so that the only massless systems in QCD are its composite NG bosons [4,5], e.g. pions and kaons. Responsible for binding systems as diverse as atomic nuclei and neutron stars, the energy associated with the gluons and quarks within these Nambu-Goldstone (NG) modes is not readily apparent. This is in sharp and fascinating contrast with all other "everyday" hadronic bound states, viz. systems constituted from up = u, down = d, and/or strange = s quarks, which possess nuclear-size masses far in excess of anything that can directly be tied to the Higgs boson. 1

A continuum approach to quark-antiquark bound-states is used to determine the electromagnetic form factors of pion-like mesons with masses m 0 − /GeV = 0.14, 0.47, 0.69, 0.83 on a spacelike domain that extends to Q 2 10 GeV 2 . The results enable direct comparisons with contemporary lattice-QCD calculations of heavy-pion form factors at large values of momentum transfer and aid in understanding them. They also reveal, inter alia, that the form factor of the physical pion provides the best opportunity for verification of the factorised hard-scattering formula relevant to this class of exclusive processes and that this capacity diminishes steadily as the meson mass increases. arXiv:1808.09461v1 [nucl-th]

Using a continuum approach to the hadron bound-state problem, we calculate γ * γ → η, η transition form factors on the entire domain of spacelike momenta, for comparison with existing experiments and in anticipation of new precision data from next-generation e + e − colliders. One novel feature is a model for the contribution to the Bethe-Salpeter kernel deriving from the non-Abelian anomaly, an element which is crucial for any computation of η, η properties. The study also delivers predictions for the amplitudes that describe the light-and strange-quark distributions within the η, η . Our results compare favourably with available data. Important to this at large-Q 2 is a sound understanding of QCD evolution, which has a visible impact on the η in particular. Our analysis also provides some insights into the properties of η, η mesons and associated observable manifestations of the non-Abelian anomaly. * leichang@nankai.edu.cn † cdroberts@anl.gov the bound-state's total momentum. The complete transition form factor is obtained as a sum over the various qq subcomponent contributions:where ψ q M is a flavour weighting factor originating in the meson's wave function.Notably [3-5] (τ 2 := Λ 2 QCD /Q 2 ):i.e. the DA acquires its asymptotic profile and henceConsequently, on τ 0 the γ * γ → M transition form factor exhibits simple scaling; and the anomalous dimension, characteristic of gauge field theories quantised in four dimensions, is "hidden" in the manner of approach to the τ = 0 limit. (N.B. As will become clear, owing to the non-Abelian axial anomaly in QCD, Eq. (5) is amended when M = η, η [6, 7].)An array of experiments have been performed with a view to testing Eqs. (1), (5) for the neutral pion [8][9][10][11]. Such measurements are difficult, typically involving the study of e + -e − collisions, in which one of the outgoing fermions is detected after a large-angle scattering whilst the other is scattered through a small angle and, hence, undetected. The detected fermion is assumed to have emitted a highly-virtual photon, the undetected fermion, a soft-photon; and these photons are supposed to fuse and produce the final-state pseudoscalar meson. There are many possible background processes and loss mechanisms in this passage of events, and thus ample room for systematic error, especially as Q 2 increases [12].

A flavor dependent kernel is constructed based on the rainbow-ladder truncation of the Dyson-Schwinger and Bethe-Salpeter equation approach of Quantum Chromodynamics. The quarkantiquark interaction is composed of a flavor dependent infrared part and a flavor independent ultraviolet part. Our model gives a successful and unified description of the light, heavy and heavylight ground pseudoscalar and vector mesons. For the first time, our model shows that the infrared enhanced quark-antiquark interaction is stronger and wider for the lighter quark.

We predict the masses of the lowlying Bc mesons with J P = 0 − , 1 − , 0 + , 1 + , 2 + , using a flavor dependent interaction pattern which gives an unified successful description of the light, heavy-light and heavy mesons and is also appliable to the radial excited heavy mesons. The errors are controlled carefully. With the errors from the RL approximation subduced, our predictions are consistent with the lQCD and quark model results, which supports strongly that the flavor dependent interaction pattern is reasonable. Our predictions provide significant guides to the experiment search of the Bc mesons.PACS numbers: