15148506
Baryon Spectroscopy at Beijing Electron Positron Collider
Baryons are the basic building blocks of our world. If we cut any piece of object smaller and smaller,we will ﬁnally reach the nucleons, i.e., the lightest baryons, and we cannot cut them smaller anyfurther. So without mention any theory, we know that the study of baryon structure is at theforefront of exploring microscopic structure of matter. From theoretical point of view, since baryonsrepresent the simplest system in which the three colors of QCD neutralize into colorless objectsand the essential non-Abelian character of QCD is manifest, understanding the baryon structure isabsolutely necessary before we claim that we really understand QCD.Spectroscopy has long proved to be a powerful tool for exploring internal structures and basicinteractions of microscopic world. Ninety years ago detailed studies of atomic spectroscopy resultedin the great discovery of Niels Bohr’s atomic quantum theory [1]. Forty to sixty years later, stilldetailed studies of nuclear spectroscopy resulted in Nobel Prize winning discoveries of nuclear shellmodel [2] and collective motion model [3] by Aage Bohr et al. Comparing with the atomic andnuclear spectroscopy at those times, our present baryon spectroscopy is still in its infancy [4].Many fundamental issues in baryon spectroscopy are still not well understood [5]. The possibilityof new, as yet unappreciated, symmetries could be addressed with accumulation of more data.The new symmetries may not have obvious relation with QCD, just like nuclear shell model andcollective motion model.Joining the new eﬀort on studying the excited nucleons, N
118107909
Status of Excited baryons at BES II
The πN system in J/ψ → NNπ is limited to be pure isospin 1/2 due to isospin conservation. This is a big advantage in studying N* → πN from the J/ψ decays, compared with πN and γN experiments which mix isospin 1/2 and 3/2 for πN system. Based on the 58 million J/ψ data collected at the Beijing Electron Positron Collider,J/ψ → pπ−n and other decay channels are analyzed. In the decay J/ψ → pπ−n, three peaks show up very clearly at 1400 MeV/c2, 1500 MeV/c2 and 1670 MeV/c2 in the πN invariant mass spectrum with additional peak around 2030 MeV/c2 which may be caused by a new N* resonance.
120957793
New hadronic states observed at BES II
Abstract.The report includes the new observation of X(1835) in J/ψ→γη′π+π-, the ωφ threshold enhancement in J/ψ→γωφ, the ωω structure in J/ψ→γωω and the broad 1- structure at the low K+K- invariant-mass spectrum in J/ψ→K+K-π0.
112524918
High-Energy QCD Spin Physics
13534760
Constructing parton convolution in effective field theory.
Parton convolution models have been used extensively in describing the sea quarks in the nucleon and explaining quark distributions in nuclei (the EMC effect). From the effective field theory point of view, we construct the parton convolution formalism which has been the underlying conception of all convolution models. We explain the significance of scheme and scale dependence of the auxiliary quantities such as the pion distributions in a nucleon. As an application, we calculate the complete leading nonanalytic chiral contribution to the isovector component of the nucleon sea.
117193137
Delta -> N gamma* Coulomb quadrupole amplitude in PQCD
We present a leading-order PQCD factorization formula for the helicity-flip Delta-->Ngamma(*) matrix element G(0) (Coulomb quadrupole amplitude C2), taking into account the transverse momenta of the quarks and the contribution from the gluons. In the large Q(2) limit, its scaling behavior acquires a double-logarithmic correction log(2)(Q(2)/Lambda(2)) compared with the standard scaling analysis, due to the contribution from the orbital motion of the small-x partons. Based on this and on the latest JLab experimental results of the C2-M1 ratio R-SM at Q(2)=3-4 GeV2, we make a phenomenological prediction for the latter at higher values of Q(2).
11996358
Generalized parton distributions and the spin structure of the nucleon
Abstract Generalized parton distributions are a type of hadronic observables which has recently stimulated great interest among theorists and experimentalists alike. Introduced to delineate the spin structure of the nucleon, the orbital angular momentum of quarks in particular, the new distributions contain vast information about the internal structure of the nucleon, with the usual electromagnetic form factors and Feynman parton distributions as their special limits. While new perturbative QCD processes, such as deeply virtual Compton scattering and exclusive meson production, have been found to measure the distributions directly in experiments, lattice QCD offers a great promise to provide the first-principle calculations of these interesting observables.
15840748
Implications of color gauge symmetry for nucleon spin structure
We study the chromodynamical gauge symmetry in relation to the internal spin structure of the nucleon. We show that ~1! even in the helicity eigenstates the gauge-dependent spin and orbital angular momentum operators do not have gauge-independent matrix element, ~2! the evolution equations for the gluon spin take very different forms in the Feynman and axial gauges, but yield the same leading behavior in the asymptotic limit, and ~3! the complete evolution of the gauge-dependent orbital angular momenta appears intractable in the light-cone gauge. We define a new gluon orbital angular momentum distribution Lg(x) which is an experimental observable and has a simple scale evolution. However, its physical interpretation makes sense only in the light-cone gauge just like the gluon helicity distribution Dg(x). @S0556-2821~99!02207-9# PACS number~s!: 12.38.Aw, 14.20.Dh The spin structure of the nucleon has been a subject of intense debate for about ten years. Much progress has been made on both experimental and theoretical frontiers @1#. However, some of the fundamental theoretical issues remain unsettled, as exemplified by a number of recent works in the literature. In particular, in analyzing the spin structure of the nucleon, color gauge invariance is still the cause of some confusion. In this paper, we intend to explore several important and relevant issues in detail. To set the stage, let us briefly recall the forms of angular momentum operator in quantum chromodynamics ~QCD! .I n Ref. @2#, a natural gauge-invariant expression is introduced