arXiv subjects
Publications and source records attributed to C. Alexandrou.
We evaluate the static $qq\bar{q}\bar{q}$ and $qqqq\bar{q}$ potentials in the quenched theory at $β=5.8$ and $β=6.0$ on a lattice of size $16^3\times 32$. We compare the static potentials to the sum of two meson potentials for the tetraquark system and to the sum of the baryonic and mesonic potentials for the pentaquark state, as well as, with the confining potential obtained in the strong coupling expansion.
We present a new method to determine the momentum dependence of the N to Delta transition form factors and demonstrate its effectiveness in the quenched theory at $β=6.0$ on a $32^3 \times 64$ lattice. We address a number of technical issues such as the optimal combination of matrix elements and the simultaneous overconstrained analysis of all lattice vector momenta contributing to a given momentum transfer squared, $Q^2$.
We evaluate the static $qqqq\bar{q}$ potential in the quenched theory at $β=5.8$ and $β=6.0$ on a lattice of size $16^3\times 32$. The mass and density-density correlator for the $Θ^+$ is investigated in the quenched theory at $β=6.0$ on lattices of size $16^3\times 32$, $24^3\times 32$ and $32^3 \times 64$.
The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition γN\to Δare calculated in quenched lattice QCD at β=6.0 with Wilson fermions. Using a new method combining an optimal combination of interpolating fields for the $Δ$ and an overconstrained analysis, we obtain statistically accurate results for the dipole form factor and for the ratios of the electric and Coulomb quadrupole amplitudes to the magnetic dipole amplitude, R_{EM} and R_{SM}, up to momentum transfer squared 1.5 GeV^2. We show for the first time using lattice QCD that both R_{EM} and R_{SM} are non-zero and negative, in qualitative agreement with experiment and indicating the presence of deformation in the N- Delta system.
The matter and the pseudoscalar densities inside a hadron are calculated via gauge-invariant equal-time correlation functions. A comparison is made between the charge charge and the matter density distributions for the pion, the rho, the nucleon and the $Δ^+$ within the quenched theory, and with two flavours of dynamical quarks.
The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition $γN\to Δ$ are evaluated both in quenched lattice QCD at $β=6.0$ and using two dynamical Wilson fermions simulated at $β=5.6$. The dipole transition form factor is accurately determined at several values of momentum transfer. On the lattices studied in this work, the electric quadrupole amplitude is found to be non-zero yielding a negative value for the ratio, $ R_{EM}$, of electric quadrupole to magnetic dipole amplitudes at three values of momentum transfer.
We address the issue of hadron deformation within the framework of lattice QCD. For hadrons with spin greater than 1/2 the deformation can be determined by evaluating the charge and matter distributions. Deviation of the nucleon shape from spherical symmetry is determined by evaluating the quadrupole strength in the transition $γN \to Δ(1232)$ both in the quenched and in the unquenched theory.
The matter density distribution inside a hadron is evaluated using gauge-invariant correlation functions within the quenched and the unquenched theory. Comparison with the charge density distribution suggests that hadron deformation is a consequence of the relativistic motion of the quarks.
Calculations of the magnetic dipole, electric quadrupole and Coulomb quadrupole amplitudes for the transition $γN\to Δ$ are presented both in quenched QCD and with two flavours of degenerate dynamical quarks.
We present results on the ratio of electric quadrupole to magnetic dipole amplitudes, $R_{EM}={\cal G}_{E2}/{\cal G}_{M1}$, for the transition $γN to Δ$ from lattice QCD. We consider both the quenched and the 2-flavor theory.
Gauge-invariant spatial correlations between two and three quarks inside a hadron are measured within quenched and unquenched QCD. These correlators provide information on the shape and multipole moments of the pion, the rho, the nucleon and the $Δ$.
Using gauge invariant hadronic two- and three- density correlators we extract information on the spatial distributions of quarks in hadrons, and on hadron shape and multipole moments within quenched lattice QCD. Combined with the calculation of N to Delta transition amplitudes the issue of nucleon deformation can be addressed.
Gauge-invariant equal-time correlation functions are calculated in lattice QCD within the quenched approximation and with two dynamical quark species. These correlators provide information on the shape and multipole moments of the pion, the rho, the nucleon and the $Δ$.
We measure the static three-quark potential in SU(3) lattice gauge theory with improved accuracy, by using all available technical refinements, including Luscher-Weisz exponential variance reduction. Together with insight gained from 3-state Potts model simulations, our results allow us to sort out the merits of the Delta- and Y-ansaetze.
We examine the gluon propagator in the Laplacian gauge in quenched lattice QCD as a function of the number of colours. We observe a weak dependence on $N_c$ over the whole momentum range. This implies an almost $N_c$-independent gluon pole mass in units of the string tension.
We extend our study of the gluon propagator in quenched lattice QCD using the Laplacian gauge to a finer lattice. We verify the existence of a pole mass as we take the continuum limit and deduce a value of $\sim 600^{+150}_{-30}$ MeV for this pole mass. We find a finite value of $(454(5){\rm MeV})^{-2}$ for the renormalized zero-momentum propagator, in agreement with results on coarser lattices.
Using state of the art lattice techniques we investigate the static baryon potential. We employ the multi-hit procedure for the time links and a variational approach to determine the ground state with sufficient accuracy that, for distances up to $\sim 1.2$ fm, we can distinguish the $Y$- and $Δ$- Ansätze for the baryonic Wilson area law. Our analysis shows that the $Δ$-Ansatz is favoured. This result is also supported by the gauge-invariant nucleon wave function which we measure for the first time.
We present results for the static three- and four-quark potentials in SU(3) and SU(4) respectively. Using a variational approach, combined with multi-hit for the time-like links, we determine the ground state of the baryonic string with sufficient accuracy to test the $Y-$ and $Δ-$ ansätze for the baryonic Wilson area law. Our results favor the $Δ$ ansatz, where the potential is the sum of two-body terms.