Nuclear Physics
Quantum Monte Carlo studies of Fermi gases
Determining the properties of Fermi gases is an intriguing topic for many-body physics, with applications to phenomena such as neutron stars. Carlson et al. conducted quantum Monte Carlo calculations of superfluid Fermi gases with short-range two-body attractive interactions with infinite scattering length. The energy of such gases is estimated to be (0.44 ± 0.01) times that of the noninteracting gas, and their pairing gap is approximately twice the energy per particle.
J. Carlson, S.-Y. Chang, V. R. Pandharipande, and K. E. Schmidt, “Superfluid Fermi gases with large scattering length,” Phys. Rev. Lett. (in press), arXiv:physics/0303094 (2003). NP, NSF
Data analysis and simulations for KamLAND
The primary goal of the Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND) is to search for the oscillation of antineutrinos emitted from distant power reactors. KamLAND’s first results indicate that it is measuring only 61% of the reactor anti-neutrino flux expected in the no-oscillation hypothesis. This result (using CPT invariance) excludes all solutions to the solar-neutrino problem except for the “large mixing angle” (LMA) region (Figure 11).
K. Eguchi et al. (KamLAND Collaboration), “First results from KamLAND: Evidence for reactor antineutrino disappearance,” Phys. Rev. Lett. 90, 021802 (2003). NP, MEXT
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Figure 11 |
Excited baryon physics from lattice QCD
Unraveling the nature of Roper resonance (the first excited state of the nucleon with the same quantum number) has a direct bearing on understanding the quark structure and chiral dynamics of baryons, one of the primary missions at laboratories such as Jefferson Lab. Dong et al. have calculated the correct level-ordering between the Roper (1440) resonance and the S11 (1535) resonance for the first time on the lattice. This result verifies that the Roper state is a radial excitation of the nucleon with three valence quarks, and indicates that spontaneously broken chiral symmetry dictates the dynamics of light quarks in the nucleon.
S. J. Dong, T. Draper, I. Horvath, F. X. Lee, K. F. Liu, N. Mathur, and J. B. Zhang, “Roper resonance and S11 (1535) in lattice QCD," Phys. Rev. Lett. (submitted, 2003), hep-ph/0306199. NP
Electronic structure of superheavy elements
Malli investigated the effects of relativity on the electronic structure and bonding in the tetroxides of the superheavy element hassium and its lighter congener osmium, with two major conclusions: (1) Relativistic effects lead to a dramatic increase of ~225% and 185% in the predicted atomization energy for HsO4 and OsO4 , respectively. (2) Mulliken population analysis of the Dirac-Fock self-consistent field wave functions, in contrast to the nonrelativistic Hartree-Fock wave function, predicts the HsO4 to be less volatile than the lighter congener OsO4 , and this prediction is in accord with the first experimental work on hassium.
G. L. Malli, “Dramatic relativistic effects in atomization energy and volatility of the superheavy hassium tetroxide and OsO4,” J. Chem. Phys. 117, 10441 (2002). NP
Exploring nucleon structure using lattice QCD
An important question about the nucleon and its excited state, Δ, is whether they are spherical or deformed. Recent experiments have accurately measured the electric and Coulomb quadrupole and magnetic dipole multipoles of the nucleon-to-Δ transition form factor, which directly reflect the presence of deformation. Alexandrou et al. subsequently calculated this form factor using lattice QCD. The calculated magnetic dipole form factor and electric quadrupole amplitude were consistent with experimental results, but systematic errors due to lattice artifacts prevented a determination of the Coulomb quadrupole form factor. Further study of these lattice artifacts is needed for better control of systematic errors.
C. Alexandrou, P. de Forcrand, T. Lippert, H. Neff, J. W. Negele, K. Schilling, and A. Tsapalis, "N to Δ electromagnetic transition form factors from lattice QCD," hep-lat/0307018 (2003). NP, ESOP, UC
HFB continuum problem solved for deformed nuclei
One of the fundamental questions of nuclear structure physics is, What are the limits of nuclear stability? Terán et al. have solved, for the first time, the Hartree-Fock-Bogoliubov (HFB) continuum problem in coordinate space for deformed nuclei in two spatial dimensions without any approximations. The novel feature of the new Vanderbilt HFB code is that it takes into account high-energy continuum states with an equivalent single-particle energy of 60 MeV or more. In the past, this has only been possible in 1D calculations for spherical nuclei.
E. Terán, V. E. Oberacker, and A. S. Umar, “Axially symmetric Hartree-Fock-Bogoliubov calculations for nuclei near the drip-lines,” Phys. Rev. C 67, 064314 (2003). NP
Measuring the fate of parton fragment jets
In collisions of heavy nuclei at high energies, a new state of matter consisting of deconfined quarks and gluons at high density is expected. Large transverse momentum (pT) partons in the high-density system result from the initial hard scattering of nucleon constituents. After a hard scattering, the parton fragments to create a high-energy cluster (jet) of particles. The STAR Collaboration measured two-hadron angular correlations at large transverse momentum for p + p and Au + Au collisions. These measurements provided the most direct evidence for production of jets in high-energy nucleus-nucleus collisions and allowed the first measurements of the fate of back-to-back jets in the dense medium as a function of the size of the overlapping system. The back-to-back correlations were reduced considerably in the most central Au + Au collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traversed the medium.
C. Adler et al. (STAR Collaboration), “Disappearance of back-to-back high pT hadron correlations in central Au + Au collisions at √sNN = 200 GeV,” Phys. Rev. Lett. 90, 082302 (2003). NP, HEP, NSF, BMBF, IN2P3, EPSRC, FAPESP, RMST, MEC, NNSFC
Calculating the energy spectra of light nuclei
The absence of stable five- or eight-body nuclei is crucial to both primordial and stellar nucleosynthesis, enabling stars such as our sun to burn steadily for billions of years. Wiringa and Pieper demonstrated that the binding energies, excitation structure, and relative stability of light nuclei, including the opening of the A = 5 and 8 mass gaps, are crucially dependent on the complicated structure of the nuclear force. They calculated the energy spectra of light nuclei using a variety of nuclear-force models ranging from very simple to fully realistic, and they observed how features of the experimental spectrum evolve with the sophistication of the force. They found that the absence of stable five- and eight-body nuclei depends crucially on the spin, isospin, and tensor components of the nuclear force.
R. B. Wiringa and S. C. Pieper, “Evolution of nuclear spectra with nuclear forces,” Phys. Rev. Lett. 89, 182501 (2002). NP