Applicability of Perturbative QCD to Pion Virtual Compton Scattering

We study explicitly the applicability of perturbative QCD (pQCD) to the pion virtual Compton scattering. It is found that there are central-region singularities introduced by the QCD running coupling constant, in addition to the end-point singularities generally existed in other exclusive processes such as the pion form factor. We introduce a simple technique to evaluate the contributions from these singularities, so that we can arrive at a judgement that these contributions will be unharmful to the applicability of pQCD at certain energy scale, i.e., the ``work point'' which is defined to determine when pQCD is applicable to exclusive processes. The applicability of pQCD for different pion distribution amplitudes are explored in detail. We show that pQCD begins to work at 10 ${GeV}^2$. If we relax our constraint to a weak sense, the work point may be as low as 4 ${GeV}^2$.Comment: 13 Latex pages, 10 figures, to appear in PL

Bound States of Heavy Flavor Hyperons

Several realistic phenomenological nucleon-nucleon interaction models are employed to investigate the possibility of bound deuteron-like states of such heavy flavor hyperons and nucleons, for which the interaction between the light flavor quark components is expected to be the most significant interaction. The results indicate that deuteron-like bound states are likely to form between nucleons and the $\Xi_c^{'}$ and $\Xi_{cc}$ charm hyperons as well as between $\Xi$ hyperons and double-charm hyperons. Bound states between two $\Sigma_c$ hyperons are also likely. In the case of beauty hyperons the corresponding states are likely to be deeply bound.Comment: 25 pages, 3 figures. Accepted Nucl. Phys.

Heavy-heavy-light quark potential in SU(3) lattice QCD

We perform the first study for the heavy-heavy-light quark (QQq) potential in SU(3) lattice QCD. The calculations are done with the standard gauge and $O(a)$-improved Wilson fermion action on the $16^4$ lattice at $\beta =6.0$ at the quenched level. We calculate the energy of QQq systems as the function of the distance between the two heavy quarks, and find that the QQq potential is well described with a Coulomb plus linear potential form. Compared to the static three-quark case, the Coulomb term does not change but the effective string tension between the heavy quarks is significantly reduced by the light-quark effect. We also investigate the light-quark mass dependence of the QQq potential using the four hopping parameters, $\kappa =0.1200,0.1300,0.1340$, and 0.1380. The reduction of the effective string tension is considered to be a general property for baryons and multi-quark hadrons.Comment: 4 pages, 3 figure

Doubly heavy spin--1/2 baryon spectrum in QCD

We calculate the mass and residue of the heavy spin--1/2 baryons containing two heavy b or c quarks in the framework of QCD sum rules. We use the most general form of the interpolating current in its symmetric and anti-symmetric forms with respect to the exchange of heavy quarks, to calculate the two-point correlation functions describing the baryons under consideration. A comparison of the obtained results with existing predictions from various approaches is also made.Comment: 17 Pages, 2 Figures and 2 Table

SELEX RICH Performance and Physics Results

SELEX took data in the 1996/7 Fixed Target Run at Fermilab. The excellent performance parameters of the SELEX RICH Detector had direct influence on the quality of the obtained physics results.Comment: Contributed talk at the Fourth Workshop on RICH Detectors, June 5-10, 2002, Pylos, Greece. Accepted for publication in NIM

Effective field theories for baryons with two- and three-heavy quarks

Baryons made of two or three heavy quarks can be described in the modern language of non-relativistic effective field theories. These, besides allowing a rigorous treatment of the systems, provide new insight in the nature of the three-body interaction in QCD.Comment: 7 pages, 1 figure; published versio

Isospin splittings of doubly heavy baryons

The SELEX Collaboration has reported a very large isospin splitting of doubly charmed baryons. We show that this effect would imply that the doubly charmed baryons are very compact. One intriguing possibility is that such baryons have a linear geometry Q-q-Q where the light quark q oscillates between the two heavy quarks Q, analogous to a linear molecule such as carbon dioxide. However, using conventional arguments, the size of a heavy-light hadron is expected to be around 0.5 fm, much larger than the size needed to explain the observed large isospin splitting. Assuming the distance between two heavy quarks is much smaller than that between the light quark and a heavy one, the doubly heavy baryons are related to the heavy mesons via heavy quark-diquark symmetry. Based on this symmetry, we predict the isospin splittings for doubly heavy baryons including Xi_{cc}, Xi_{bb} and Xi_{bc}. The prediction for the Xi_{cc} is much smaller than the SELEX value. On the other hand, the Xi_{bb} baryons are predicted to have an isospin splitting as large as (6.3\pm1.7) MeV. An experimental study of doubly bottomed baryons is therefore very important to better understand the structure of baryons with heavy quarks.Comment: 11 page

Dynamic versus Static Hadronic Structure Functions

"Static" structure functions are the probabilistic distributions computed from the square of the light-front wavefunctions of the target hadron. In contrast, the "dynamic" structure functions measured in deep inelastic lepton-hadron scattering include the effects of rescattering associated with the Wilson line. Initial- and final-state rescattering, neglected in the parton model, can have a profound effect in QCD hard-scattering reactions, producing single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing, and non-universal nuclear antishadowing--novel leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also review how "direct" higher-twist processes -- where a proton is produced in the hard subprocess itself -- can explain the anomalous proton-to-pion ratio seen in high centrality heavy ion collisions.Comment: Invited talk presented at the International Conference on Particles and Nuclei (PANIC08), Eilat, Israel, November 9-14, 200

Charm and Charmonium Spectroscopy

Recent experimental results in charm and charmonium spectroscopy are reviewed.Comment: 4 pages, contributed to the Proceedings of BEACH 2006, Lancater, Englan