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nuclear physics

Progress in QGP Search

STAR Collaboration, J. Adams et al., “Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions,” Nucl. Phys. A (submitted), nucl-ex/0501009 (2005). NP, NSF, BMBF, IN2P3, RA, RPL, EMN, EPSRC, FAPESP, RMST, MEC, NNSFC, GACR, FOM, UU, DAE, DST, CSIR, SNSF, PSCSR

The STAR Collaboration has reviewed the most important experimental results from the first three years of nucleus-nucleus collision studies at RHIC and assessed their interpretation and comparison to theory. They found that central Au+Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a quark-gluon plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages (Figure 14), but the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter.

Figure 14. STAR angular correlation results suggest that there is appreciable soft hadron emission before the attainment of local thermal equilibrium. (a) Measurements for p+p collisions at RHIC of the joint autocorrelation on the angular difference variables pseudorapidity and azimuthal angle for all but the softest pairs of charged hadrons. This plot illustrates the central role of parton fragmentation in p+p collisions, resulting in a prominent near-side jet peak and a broad away-side jet ridge. (b) The observed soft-hadron-pair correlation for central Au+Au collisions exhibits a substantially modified remnant of the jet correlation on the near side, affecting typically 10–30% of the detected hadrons. This trend suggests that while some parton fragments are not yet fully equilibrated in the soft sector, they are nonetheless rather strongly coupled to the longitudinally expanding bulk medium.

 

Understanding Superfluid Fermi Gases

S. Y. Chang, V. R. Pandharipande, J. Carlson, and K. E. Schmidt, “Quantum Monte Carlo studies of superfluid Fermi gases,” Phys. Rev. A 70, 043602 (2004). NP, NSF

Chang et al. have produced quantum Monte Carlo calculations of the ground state of dilute Fermi gases with attractive short-range two-body interactions. The strength of the interaction was varied to study different pairing regimes. They successfully calculated the ground-state energy, the pairing gap Δ, and the quasiparticle spectrum for the entire region ranging from free fermions to tightly bound Bose molecules.

Studying Pentaquarks on the Lattice

N. Mathur, F. X. Lee, A. Alexandru, C. Bennhold, Y. Chen, S. J. Dong, T. Draper, I. Horváth, K. F. Liu, S. Tamhankar, and J. B. Zhang, “Study of pentaquarks on the lattice with overlap fermions,” Phys. Rev. D 70, 074508 (2004). NP

The reported discovery two years ago of an exotic five-quark resonance has spawned intense experimental and theoretical interest. Mathur et al. have produced a quenched lattice QCD calculation of spin-1/2 five-quark states with uudds‾ quark content for both positive and negative parities. They did not observe any bound pentaquark state in these channels for either I = 0 or I = 1. The states they found are consistent with KN scattering states, which were shown to exhibit the expected volume dependence of the spectral weight. The results were based on overlap-fermion propagators on two lattices, 12³ x 28 and 16³ x 28, with the same lattice spacing of 0.2 fm, and pion mass as low as ~180 MeV.

Calculating Excited States in Nuclei

Steven C. Pieper, R. B. Wiringa, and J. Carlson, “Quantum Monte Carlo calculations of excited states in A = 6–8 nuclei,” Phys. Rev. C 70, 054325 (2004). NP

Pieper et al. have demonstrated that it is possible to use the Green’s function Monte Carlo (GFMC) method to compute the energies of multiple nuclear states with the same quantum numbers. The success of this method substantially increases the number of nuclear-level energies that can be compared to experimental values in the light p-shell region. For cases in which the physical excited state is reasonably narrow, the GFMC energy converges to a stable result. The results for many second and higher states in A = 6–8 nuclei are close to the experimental values.

Unraveling the Roper Resonance

Y. Chen, S. J. Dong, T. Draper, I. Horváth, F. X. Lee, K. F. Liu, N. Mathur, and J. B. Zhang, “Roper Resonance and S₁₁(1535) from Lattice QCD,” hep-ph/0306199 (2004). NP

Unraveling the nature of the Roper resonance, the first excited state of the nucleon, has direct bearing on our understanding of the quark structure and chiral dynamics of baryons, which is one of the primary missions at labs like Jefferson Lab. Chen et al. performed the first lattice QCD calculation to simulate the Roper resonance. They concluded that spontaneously broken chiral symmetry dictates the dynamics of light quarks in the nucleon.

Calculating Transition Form Factors

C. Alexandrou, P. de Forcrand, T. Lippert, H. Neff, J. W. Negele, K. Schilling, W. Schroers, and A. Tsapalis, “N to Δ electromagnetic transition form factors from lattice QCD,” Phys. Rev. D 69, 114506 (2004). NP, LF, AVHF, ESOP, UC

Alexandrou et al. have demonstrated the feasibility of a new method for calculating the N to Δ electromagnetic transition form factors. For the first time in lattice QCD calculations, this new method enabled measurement of the Coulomb and electric quadrupole transition form factors up to a momentum transfer of 1.5 GeV² with sufficient accuracy to determine that they are both negative and statistically distinct from zero. Rough extrapolation from the high values of the quark mass at which the exploratory calculations were carried out to the physical quark mass yielded values for these quadrupole transition form factors as well as the dominant magnetic dipole form factor in qualitative agreement with experiment.