The Nt=6N_t=6 equation of state for two flavor QCD

We improve the calculation of the equation of state for two flavor QCD by simulating on $N_t=6$ lattices at appropriate values of the couplings for the deconfinement/chiral symmetry restoration crossover. For $am_q=0.0125$ the energy density rises rapidly to approximately 1 ${\rm GeV/fm^3}$ just after the crossover($m_\pi/m_\rho\approx 0.4$ at this point). Comparing with our previous result for $N_t=4$~\cite{eos}, we find large finite $N_t$ corrections as expected from free field theory on finite lattices. We also provide formulae for extracting the speed of sound from the measured quantities.Comment: Contribution to Lattice 95 proceedings (combines talks presented by T. Blum and L. Karkkainen). LaTeX, 8 pages, uses espcrc2.sty, postscript figures include

Critical Behavior at the Chiral Phase Transition

Quantum chromodynamics with two zero mass flavors is expected to exhibit a phase transition with O(4) critical behavior. Fixing the universality class is important for phenomenology and for facilitating the extrapolation of simulation data to physical quark mass values. At Lattice '96 the Tsukuba and Bielefeld groups reported results from new simulations with dynamical staggered quarks at $N_t = 4$, which suggested a departure from the expected critical behavior. We report observations of similar deviations and discuss efforts in progress to understand this phenomenon.Comment: 3 pp, LaTeX with 6 encapsulated Postscript figures. Lattice '97 proceeding

Thermodynamics with 3 and 2+1 Flavors of Improved Staggered Quarks

We present preliminary results from exploring the phase diagram of finite temperature QCD with three degenerate flavors and with two light flavors and the mass of the third held approximately at the strange quark mass. We use an order $\alpha_s^2 a^2, a^4$ Symanzik improved gauge action and an order $\alpha_s a^2, a^4$ improved staggered quark action. The improved staggered action leads to a dispersion relation with diminished lattice artifacts, and hence better thermodynamic properties. It decreases the flavor symmetry breaking of staggered quarks substantially, and we estimate that at the transition temperature for an $N_t=8$ to $N_t=10$ lattice {\em all} pions will be lighter than the lightest kaon. Preliminary results on lattices with $N_t=4$, 6 and 8 are presented.Comment: 3 pages, 6 figures, contribution to Lattice2001(hightemp) August 19--24, 2001, Berlin, German

NN Core Interactions and Differential Cross Sections from One Gluon Exchange

We derive nonstrange baryon-baryon scattering amplitudes in the nonrelativistic quark model using the ``quark Born diagram" formalism. This approach describes the scattering as a single interaction, here the one-gluon-exchange (OGE) spin-spin term followed by constituent interchange, with external nonrelativistic baryon wavefunctions attached to the scattering diagrams to incorporate higher-twist wavefunction effects. The short-range repulsive core in the NN interaction has previously been attributed to this spin-spin interaction in the literature; we find that these perturbative constituent-interchange diagrams do indeed predict repulsive interactions in all I,S channels of the nucleon-nucleon system, and we compare our results for the equivalent short-range potentials to the core potentials found by other authors using nonperturbative methods. We also apply our perturbative techniques to the N$\Delta$ and $\Delta\Delta$ systems: Some $\Delta\Delta$ channels are found to have attractive core potentials and may accommodate ``molecular" bound states near threshold. Finally we use our Born formalism to calculate the NN differential cross section, which we compare with experimental results for unpolarised proton-proton elastic scattering. We find that several familiar features of the experimental differential cross section are reproduced by our Born-order result.Comment: 27 pages, figures available from the authors, revtex, CEBAF-TH-93-04, MIT-CTP-2187, ORNL-CCIP-93-0

Static hybrid quarkonium potential with improved staggered quarks

We are studying the effects of light dynamical quarks on the excitation energies of a flux tube between a static quark and antiquark. We report preliminary results of an analysis of the ground state potential and the $\Sigma^{\prime+}_g$ and $\Pi_u$ potentials. We have measured these potentials on closely matched ensembles of gauge configurations, generated in the quenched approximation and with 2+1 flavors of Asqtad improved staggered quarks.Comment: Lattice2002(heavyquark

High temperature QCD with three flavors of improved staggered quarks

We present an update of our study of high temperature QCD with three flavors of quarks, using a Symanzik improved gauge action and the Asqtad staggered quark action. Simulations are being carried out on lattices with Nt=4, 6 and 8 for the case of three degenerate quarks with masses less than or equal to the strange quark mass, $m_s$, and on lattices with Nt=6 and 8 for degenerate up and down quarks with masses in the range 0.2 m_s \leq m_{u,d} \leq 0.6 m_s, and the strange quark fixed near its physical value. We also report on first computations of quark number susceptibilities with the Asqtad action. These susceptibilities are of interest because they can be related to event-by-event fluctuations in heavy ion collision experiments. Use of the improved quark action leads to a substantial reduction in lattice artifacts. This can be seen already for free fermions and carries over into our results for QCD.Comment: Lattice2002(Non-zero temperature and density

Thermodynamics for two flavor QCD

We conclude our analysis of the N_t=6 equation of state for two flavor QCD, first described at last year's conference. We have obtained new runs at am_q=0.025 and improved runs at am_q=0.0125. The results are extrapolated to m_q=0, and we extract the speed of sound as well. We also present evidence for a restoration of the SU(2) X SU(2) chiral symmetry just above the crossover, but not of the axial U(1) chiral symmetry.Comment: Poster presented at LATTICE96(finite temperature). 4 pages, LaTeX plus 5 encapsulated Postscript figure

I=3/2 KπK \pi Scattering in the Nonrelativisitic Quark Potential Model

We study $I=3/2$ elastic $K\pi$ scattering to Born order using nonrelativistic quark wavefunctions in a constituent-exchange model. This channel is ideal for the study of nonresonant meson-meson scattering amplitudes since s-channel resonances do not contribute significantly. Standard quark model parameters yield good agreement with the measured S- and P-wave phase shifts and with PCAC calculations of the scattering length. The P-wave phase shift is especially interesting because it is nonzero solely due to $SU(3)_f$ symmetry breaking effects, and is found to be in good agreement with experiment given conventional values for the strange and nonstrange constituent quark masses.Comment: 12 pages + 2 postscript figures, Revtex, MIT-CTP-210

Kaon-Nucleon Scattering Amplitudes and Z∗^*-Enhancements from Quark Born Diagrams

We derive closed form kaon-nucleon scattering amplitudes using the ``quark Born diagram" formalism, which describes the scattering as a single interaction (here the OGE spin-spin term) followed by quark line rearrangement. The low energy I=0 and I=1 S-wave KN phase shifts are in reasonably good agreement with experiment given conventional quark model parameters. For $k_{lab}> 0.7$ Gev however the I=1 elastic phase shift is larger than predicted by Gaussian wavefunctions, and we suggest possible reasons for this discrepancy. Equivalent low energy KN potentials for S-wave scattering are also derived. Finally we consider OGE forces in the related channels K$\Delta$, K$^*$N and K$^*\Delta$, and determine which have attractive interactions and might therefore exhibit strong threshold enhancements or ``Z$^*$-molecule" meson-baryon bound states. We find that the minimum-spin, minimum-isospin channels and two additional K$^*\Delta$ channels are most conducive to the formation of bound states. Related interesting topics for future experimental and theoretical studies of KN interactions are also discussed.Comment: 34 pages, figures available from the authors, revte

Quark Loop Effects with an Improved Staggered Action

We have been studying effects of dynamical quarks on various hadronic observables, using our recently formulated improvement for staggered fermions. To illustrate improvement, we show that the light hadron spectrum in the quenched approximation gives remarkably good scaling. We highlight three new results: (1) We find no apparent quark loop effects in the Edinburgh plot with 2+1 flavors of dynamical quarks at a = 0.14 fm. (2) We show that dynamical quarks modify the shape of the heavy quark potential. (3) We present results hinting at meson decay effects in light hadron spectroscopy.Comment: 5 pages, 7 figures, Lattice 2000 (Spectrum