Session D7 - Hadron Physics.
ORAL session, Saturday afternoon, May 01
Governor's Square 16, Adam's Mark Hotel

[D7.001] SUPER-RADIANT STATES AND NARROW HADRONIC RESONANCES

The recent experimental discovery of the exotic ``pentaquark" resonance \Theta^+(1540) with strangeness S=+1 and very narrow width (earlier predicted in the topological soliton model) triggered an avalanche of alternative explanations in terms of special features of quark structure, isospin selection rules, quasimolecular kaon-nucleon correlation etc. Here we suggest that the narrow width can result from a generic mechanism of the redistribution of the widths of overlapping resonances due to their coupling through few common decay channels. The underlying universal dynamics appear when N unstable intrinsic states with identical quantum numbers and ratio \gamma/D\geq 1 of their bare widths to energy spacings can decay into k

[D7.002] A systematic study of the binding energies of eta-mesic nuclei

The binding energy \epsilon_\eta of eta-mesic nucleus has been studied for a wide range of nuclei with mass number A ranging from 12 to 208. In particular, the empirical dependence of \epsilon_\eta on A has been determined. It is found that \epsilon_\eta depends logarithmically on A, irrespective of the formalism (microscopic, factorization approximation, or on-shell potential) used in the calculations. The dependence of \epsilon_\eta on nuclear form factors has also been examined in this study.

[D7.003] Pion-nucleon charge exhange cross sections in the momentum interval from 110 to 540 MeV/c

Absolute differential crossections for pion-nucleon charge exchange (CEX), \pi^-+p\to\pi^0+n, are extracted from experiments E913 and E958 at Brookhaven National Laboratory. The Crystal Ball multi-photon spectrometer was used to measure the energy and trajectory of individual photons from \pi^0 decay. The Crystal Ball covers \sim93% of 4\pi sr for single photons, allowing the measurement of a complete CEX angular distribution except for the most forward angles at the higher momenta. Particular attention has been devoted to beam normalizations and momentum calibrations. Data in the momentum interval of the \Delta resonance (148 to 323 MeV/c) have been submitted for publication. This presentation will concentrate on the analysis of the data at both lower and higher momenta, which extend to the Roper resonance region. Comparison will be made to recent partial-wave analyses.

[D7.004] The Generalized GDH sum rule: Measuring the Spin Structure of 3He and the Neutron using Nearly Real Photons

The Gerasimov-Drell-Hearn (GDH) sum rule is one of the most important tools available to study the nucleon spin structure. The GDH sum rule was initially derived for real photon absorption (corresponding to Q^2=0) and has been extended to finite Q^2. The goals of Jefferson Lab experiment E97-110 are to study the Q^2 dependence of the extended GDH integral between 0.02 and 0.3 (GeV/c)^2, the slope of the GDH integral at Q^2\sim0, and the extrapolation to the real photon point for 3He and the neutron. In this domain, the measurement of the extended GDH integral tests Chiral Perturbation Theory and checks its limits of applicability at low Q^2. The low Q^2 measurements completed in this experiment will provide new constraints on understanding the 3He and neutron spin structure and a better understanding of the effect of nucleon resonances. The data collection was completed in August of 2003, using the Jefferson Lab high polarization continuous electron beam and a polarized 3He target. The status and perspectives of the data analysis will be discussed.

[D7.005] Isospin Breaking in Baryon Chiral Perturbation Theory

We present a study of isospin breaking in QCD within the consistent framework of Chiral Perturbation Theory including the effects induced by considering the presence of virtual photons on the effective theory. First, the results for the pion decay constants are reviewed. Second, we present the electromagnetic and strong isospin breaking contributions to the baryon masses and the \sigma-terms to fourth order in Lorentz covariant baryon chiral perturbation theory where both Lorentz invariance and chiral power counting are preserved using the infrared regularization prescription for evaluating loop integrals. Finally, we compare our results to those obtained earlier within the heavy baryon formalism of the effective field theory.

[D7.006] Measurement of the Nuclear Transparency by Double Scattering in D(e,e'p)

We have measured the Q ^2 -dependence (from 1 to 6 GeV ^2 ) of final-state interactions (FSIs) in quasi-elastic electron scattering on deuterium using the CEBAF Large Acceptance Spectrometer (CLAS). The significant reduction of FSIs with Q ^2 compared to predictions based on the eikonal approach provides evidence for color transparency (CT). The large acceptance of CLAS allows us to detect high-momentum protons in the direction of the virtual photon (from which neutron recoil momenta can be deduced) or low-momentum (250-800 MeV/c) recoil protons at large angles. FSIs enhance the recoil momentum perpendicular to the virtual photon as compared to the usual Fermi motion. The experimental ratio of the number of events with recoil momentum near 450 MeV/c compared to near 200 MeV/c decreases with Q^2 as predicted in models of CT. Preliminary results will be shown.

[D7.007] Measurement of Target Spin Asymmetry of DVCS Using CLAS

Preliminary results will be presented for the target spin asymmetry in exclusive Deeply Virtual Compton Scattering (DVCS). The asymmetry is the results of interference between the DVCS and the Bethe-Heitler amplitudes. The data were taken with the CLAS detector in Jefferson Lab's Hall B.A 5.7 GeV electron beam was directed onto an NH3 target, which was dynamically polarized parallel to the beam direction. The target polarization was on average 70%, and was frequently reversed for control of systematics. Data were also taken with C-12 targets. These data were used to subtract the unpolarized nitrogen background in the NH3 target sample. After background subtraction, the total event sample contained approximately 3000 single photon events in the hadronic mass range W = 1.8 - 3.0 GeV, and for photon virtualities from Q^2 = 1.5 to 4.5 GeV^2.

[D7.008] Single-Spin Transverse Asymmetry in Neutral Pion Production at PHENIX

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) began operation as the first polarized proton collider in 2001. From data collected in the 2001-2002 run, the PHENIX experiment has measured the single-spin transverse asymmetry (A_N) for neutral pion production at x_F\sim0 over a transverse momentum range of~1.0 to 5.0~GeV/c from polarized proton-proton interactions at a center of mass energy (\sqrts) of 200~GeV. Interest in these measurements arises from the observation of large (\sim10-30%) single-spin transverse asymmetries in pp_\uparrow\!\!\rightarrow\!\!\pi X at forward angles by the E704 collaboration at Fermilab (\sqrts = 19.4GeV) and the STAR collaboration at RHIC (\sqrt(s) = 200GeV), as well as single-spin, azimuthal asymmetries in semi-inclusive deep-inelastic scattering by the HERMES collaboration at DESY. Such large asymmetries were initially surprising because, at leading-order, pQCD predicts only small effects. Recently it has been argued that these large asymmetries may be produced by initial-state effects (e.g. the Sivers effect), final-state effects (e.g. transversity with the Collins effect), higher-twist contributions, or a combination of the three. In this talk, we will report on the results ofthis measurement.

[D7.009] Pentaquark Search with STAR at RHIC

Several observations of a five-quark bound system, pentaquarks, from various experiments in photon-nucleus, kaon-nucleus, and proton-proton reactions have been reported*. The presence of these states was predicted by Diakonov at al. using chiral soliton models of baryons in 1997. **

The high energies and particle densities resulting from collisions at RHIC are expected to favor pentaquark production. The large acceptance of STAR's Time Projection Chamber is ideal for such rare particle searches. The short lifetimes predicted for pentaquarks require that a mixing technique be used to reconstruct the pentaquarks via their decay products. This technique has already been used successfully by STAR to reconstruct and study short-lived resonances.

We report on the progress of the pentaquark search by the STAR collaboration in pp, dAu, and AuAu collisions through one of the decay modes, \Theta^+\rightarrow p+K^0.

*T.Nakano et al. (LEPS Collaboration) Phys. Rev. Lett. 91, 0122002(2003) *S.Stepanyan et al. (CLAS Collaboration) hep-exp/0307018 *V.V.Barmin at al. (DIANA Collaboration) hep-exp/0304040

**D. Diakonov, V. Petrov and M. Polakov Z.Phys. A359 (1997) 305-314

[D7.010] Search for the \Theta^- with PHENIX

There is mounting evidence for the existance of exotic hadrons consisting of 5 quarks. Resonance states have been observed in K^+ and pK^0_s invariant mass spectra near 1.540 GeV/c^2 (Q^+) and in \Xi-\pi invariant mass spectra around 1.86 GeV/c^2 (\Xi^--).

PHENIX has in principle the ability to reconstruct the Q^- in the øverlinenK^- channel. In addition to identifying charged particles, PHENIX can detect anti-neutrons with its highly segmented electromagnetic calorimeter. Using these capabilities we can perform a search for the Q^-. The current state of this search will be presented.

[D7.011] Multiquark-cluster formation in nuclei

The wavefucntions of nucleons bound in a nucleus may sufficiently overlap to creat color singlets made of more than 3 valence quarks. A quasi-classical network-based algorithm has been developed to calculate the probabilities for multi-quark cluster formation. The spatial (one-body) nucleon distribution used is either standard Woods-Saxon or a Fourier-Bessel series fitted to scattering data. The resulting cummulative probabilities are in agreement with those needed to describe the EMC-effect and J/\Psi suppresion in hadron-nucleus collisions.

[D7.012] Results from Polarized Proton-Proton Collisions at STAR

RHIC has delivered transversely and longitudinally polarized proton-proton collisions at an energy of sqrt(s)= 200 GeV to the STAR detector in FY2003. An integrated luminosity of approximately 200 nb(-1) was recorded at an average polarization per beam near 25Using single tower and jet patch energy thresholds, electromagnetic calorimeters were used to trigger on events containing high transverse momentum particles. We present the status of jet reconstruction from the recent proton-proton run and compare the results of inclusive jet asymmetries to NLO perturbative QCD calculations. Inclusive jet asymmetries from longitudinally polarized proton-proton collisions are related to partonic processes which are predicted to be sensitive to the polarized gluon distribution function of the proton.

[D7.013] Search for Exotic Particles with the STAR detector at RHIC

Exotic particles such as pentaquark states and di-baryons are allowed to exist within the framework of Quantum Chromo Dynamics (QCD). However the physics of these exotic particles explores a new domain of multi-quark dynamics beyond conventional mesons and baryons. Recently observations of a pentaquark state, \Theta^+(uudd\bars), have been reported by several experiments. The NA49 experiment also reported evidence for a narrow \Xi^-\pi^- baryon resonance. We report the status of the search for exotic particles at mid-rapidity from the STAR experiment. High-energy nuclear collisions at RHIC provide a unique environment for exotic particle production. The STAR detector, with large acceptance and good particle identification capability, is suitable for searches of rarely produced particles at RHIC. The current searches cover channels: \Theta^++ \rightarrow pK^+, \Xi^-- \rightarrow \Xi^-\pi^- and others in p+p, d+Au and Au+Au collisions. Statistical significance of our searches and future plans will also be discussed.

[D7.014] Boson-Baryon and Boson-Neutrino Reactions

In the paper "The Space of 4-Operators and the Unification of the Fundamental Interactions" (see APS paper with log number 10016) we discussed the Fundamental Particle Scheme (not the Standard Model). As a further application of the theory, we discuss in this paper formal theory of Meson Production as a special case of Boson-Baryon and Boson-Neutrino Reactions.

Part D of program listing