Narrow Results

We study the production line shape of $B^{\ast }\bar{B}$ near threshold, where the $B^{\ast }\bar{B}$ pair comes from the resonance $X_b$. Our study shows that the line shape depends sensitively on the binding energy and the probability of finding an elementary state in the physical bound state. Both of the two parameters are crucial to identify the structure of $X_b$. Therefore, the line shape measurement can shed light on the structure of $X_b$.

Understanding the equation of state of dense nuclear matter is a fundamental challenge for nuclear physics. It is an especially timely and interesting challenge as we have reached a period where neutron stars, which contain the most dense nuclear matter in the Universe, are now being studied in completely new ways, from gravitational waves to satellite-based telescopes. We review the theoretical approaches to calculating this equation of state which involve a change in the structure of the baryons, along with their predictions for neutron star properties.

A critical look is taken at both positive and null evidence for the Θ⁺ pentaquark. Potential problems with experiments will be discussed and the question of what conclusion can be drawn from both the positive and the null results is examined. First the question of existence of the Θ⁺ pentaquark is considered, followed by a discussion of new experiments that are either planned or in progress to answer questions about its mass, width and isospin. Finally, indirect evidence for the parity of the Θ⁺ is examined, and suggestions for experiments to measure its parity directly are given.

Meson photoproduction was studied at the CEBAF Large Acceptance Spectrometer (CLAS) utilizing a 5.5 GeV/c photon beam incident on a hydrogen target. Here we report on the partial wave analyzes (PWA) performed on the exclusive final states π⁺π⁺π^-n and K⁺K^-π⁺n. In addition, CLAS has data on the final state π⁺π⁰π^-p which will be reported in the near future. The analysis of the π⁺π⁺π^- system shows clearly the a₂(1320), as well as evidence for the a₁(1260) and the π₂(1670). The π₁(1600), previously observed in π^-p interactions, is seen very weakly here. In the K⁺K^-π⁺n channel there is is evidence for the Θ⁺(1540) S=+1 baryon pentaquark. The PWA of this system gives an excellent description of the background which rules out mesonic reflection as a source of the signal.

The structure of the hadron resonances attracts much attention, in association with the recent observations of various exotic hadrons which do not fit well in the conventional picture. These findings urge us to consider various new configurations such as the multiquark states and the hadronic molecules. However, it is a subtle problem to define a proper classification scheme for the hadron structure, and the nonzero decay width of the hadron resonances makes the analysis complicated. In this paper, we summarize the recent developments in the studies of the structure of the hadron resonances, focusing on the notion of the compositeness in terms of the hadronic degrees of freedom.

The physics related to the form factors of the energy–momentum tensor spans a wide spectrum of problems, and includes gravitational physics, hard-exclusive reactions, hadronic decays of heavy quarkonia, and the physics of exotic hadrons described as hadroquarkonia. It also provides access to the “last global unknown property:” the D-term. We review the physics associated with the form factors of the energy–momentum tensor and the D-term, their interpretations in terms of mechanical properties, their applications, and the current experimental status.

Recent experimental observations of many unconventional hadronic states stimulate an interest in the structure of hadrons. While various internal configurations have been proposed, it is a subtle problem to identify the structure of hadron resonances in a model independent manner. Here we discuss the composite/elementary nature of hadrons using the field renormalization constant Z. In particular, we show that the magnitude of the effective range parameter r_e is related to the structure of s-wave near-threshold resonances.

Recollections of work with Ernest Henley on the structure of hadrons.

The introduction talk given at the beginning of QCD Evolution workshop held in Thomas Jefferson National Accelerator Facility (Jefferson Lab) on May 14 -17, 2012.

Starting from a definition of transverse momentum-dependent parton distributions for semi-inclusive deep inelastic scattering and the Drell-Yan process, given in terms of matrix elements of a quark bilocal operator containing a staple-shaped Wilson connection, a scheme to determine such observables in lattice QCD is developed and explored. Parametrizing the aforementioned matrix elements in terms of invariant amplitudes permits a simple transformation of the problem to a Lorentz frame suited for the lattice calculation. Results for the Sivers and Boer-Mulders transverse momentum shifts are presented, focusing in particular on their dependence on the staple extent and the Collins-Soper evolution parameter.

We present recent developments on the role of the running coupling constant at the intersection of perturbative and nonperturbative QCD. A number of experiments show a smooth transition from small to large scales given by the four-momentum transfer in the reactions. This is at variance with perturbative QCD where the running coupling constant becomes infinite when the scale equals Λ_QCD. Approaches using an effective coupling constant could help interpret the opposite trend of data as compared to standard perturbative QCD predictions. We give an overview of the role of the coupling constant in the procedure to match nonperturbative hadronic model to perturbative QCD and we propose an extraction of an effective coupling constant from inclusive electron proton scattering data at large Bjorken x.

We examine the QCD evolution of the helicity and transversity parton distribution functions when including also their dependence on transverse momentum. Using an appropriate definition of these polarized transverse momentum distributions (TMDs), we describe their dependence on the factorization scale and rapidity cutoff, which is essential for phenomenological applications.

In this talk, I demonstrate that the proposed Electron-Ion Collider (EIC) will be an ideal and unique future facility to address many overarching questions about QCD and strong interaction physics at one place. The EIC will be the world's first polarized electron-proton (and light ion), as well as the first electron-nucleus collider at flexible collision energies. With its high luminosity and beam polarization, the EIC distinguishes itself from HERA and the other fixed target electron-hadron facilities around the world. The EIC is capable of taking us to the next QCD frontier to explore the glue that binds us all.

Starting from a definition of transverse momentum-dependent parton distributions (TMDs) in terms of hadronic matrix elements of a quark bilocal operator containing a staple-shaped gauge link, selected TMD observables can be evaluated within Lattice QCD. A TMD ratio describing the Boer-Mulders effect in the pion is investigated, with a particular emphasis on its evolution as a function of a Collins-Soper-type parameter which quantifies the proximity of the staple-shaped gauge links to the light cone.

Exploring hadron structure with generalized parton distributions (GPDs) and related concepts is an essential part of the physics program of the planned Electron-Ion Collider (EIC). We discuss new topics in GPD physics (theory, experiment, interpretation) that have emerged in the last years and could be included in the EIC program. This includes: (a) D-term and QCD forces in the nucleon; (b) Transversity GPDs in pseudoscalar meson production and high-mass pair production; (c) Pion and kaon form factors and GPDs; (d) Transition distribution amplitudes and backward meson production; (e) Neutron GPDs and nuclear modifications; (f) Nuclear GPDs and quark/gluon imaging of nuclei; (g) Twist-3 GPDs and quark-gluon correlations; (h) Wigner functions and potential observables. We also comment on future theoretical developments needed for the GPD program at EIC and emerging connections with other areas of nuclear physics. [Conveners’ summary, Week 1 of INT Program INT-18-3].

Matrix elements of the energy momentum tensor (EMT) bear fundamental information like mass, spin and D-term of a particle which is the “last unknown global property.” Recent progress on EMT form factors of hadrons, their interpretations and applications as well as the experimental status is given.

Recent experimental observations of many unconventional hadronic states stimulate an interest in the structure of hadrons. While various internal configurations have been proposed, it is a subtle problem to identify the structure of hadron resonances in a model independent manner. Here we discuss the composite/elementary nature of hadrons using the field renormalization constant Z. In particular, we show that the magnitude of the effective range parameter r_e is related to the structure of s-wave near-threshold resonances.