arXiv subjects
Publications and source records attributed to L. Del Debbio.
We compute, for the four-dimensional SU(4) and SU(6) gauge theories formulated on a lattice, the string tensions sigma_k related to sources with Z_N charge k, using Monte Carlo simulations. Our results are compatible with sigma_k \propto sin (k pi/N), and show sizeable deviations from Casimir scaling.
A disorder parameter detecting dual superconductivity of the vacuum is measured across the chiral phase transition in full QCD with two flavours of dynamical staggered fermions. The observed behaviour is similar to the quenched case.
The decay constants of B, D and K mesons are computed in quenched lattice QCD at two different values of the coupling. The action and operators are O(a) improved with non-perturbative coefficients. The results are f_B=195(6)(+24/-23) MeV, f_D=206(4)(+17/-10) MeV, f_B_s=220(6)(+23/-28) MeV, f_D_s=229(3)(+23/-12) MeV and f_K= 150(3)(+12/-8) MeV. Systematic errors are discussed in detail. Results for vector decay constants, flavour symmetry breaking ratios of decay constants, the pseudoscalar-vector mass splitting and D meson masses are also presented.
We compute, in SU(3) pure gauge theory, the vacuum expectation value (vev) of the operator which creates a $Z_3$ vortex wrapping the lattice through periodic boundary conditions (dual Polyakov line). The technique used is the same already tested in the SU(2) case. The dual Polyakov line proves to be a good disorder parameter for confinement, and has a similar behaviour to the monopole condensate. The new features which characterise the construction of the disorder operator in SU(3) are emphasised.
We present the results of a lattice computation of the form factors for B^0 ->pi^- l^+nu_l decays near zero-recoil. These results will allow a determination of the CKM matrix element |Vub| when measurements of the differential decay rate become available. We also provide models for extrapolation of the form factors and rate to the full recoil range. Our computation is performed in the quenched approximation to QCD on a 24^3x48 lattice at beta=6.2, using a non-perturbatively O(a)-improved action. The masses of all light valence quarks involved are extrapolated to their physical values.
A lattice determination of the form factor and decay constants for the semileptonic decay of heavy pseudoscalar (PS) mesons at zero recoil is presented from which the soft pion relation is satisfied. Chiral extrapolation of the form factor is performed at constant $q^2$. Pole dominance is used to extrapolate the form factor in heavy quark mass. At the B mass, the form factor at zero recoil lies somewhat below the ratio of decay constants; the relation remains satisfied within error.
We present preliminary results for the second moment of the pion's distribution amplitude. The lattice formulation and the phenomenological implications are briefly reviewed, with special emphasis on some subtleties that arise when the Lorentz group is replaced by the hypercubic group. Having analysed more than half of the available configurations, the result obtained is ξ^2_L = 0.06 \pm 0.02.
The coupling $g_{B^\ast B π}$ is related to the form factor at zero momentum of the axial current between $B^\ast$ and $B$ states. Moreover it is related to the effective coupling between heavy mesons and pions that appear the heavy meson chiral Lagrangian. This coupling has been evaluated on the lattice using static heavy quarks and light quark propagators determined by a stochastic inversion of the fermionic bilinear. We found the value $g=0.42(4)(8)$. Beside its theoretical interest, this quantity has phenomenological implications in $B \to π+ \bar l l$ decays.
We discuss the influence of Dirac sheets and zero-momentum modes on the gauge variant photon correlators $Γ(τ;\vp)$ with $\vp\ne 0$ and $\vp =0$ in the pure gauge U(1) theory. A special attention has been paid to the $β$- and volume-dependence of this influence. Numerical simulations are performed on $12\times 6^3$ and $24\times 12^3$ lattices at different $yβ$'s in the Coulomb phase.
We present Monte Carlo simulation results for the three dimensional Thirring model for numbers of fermion flavors N_f=4 and 6. For N_f=4 we find a second order chiral symmetry breaking transition at strong coupling, corresponding to an ultra-violet fixed point of the renormalisation group defining a non-trivial continuum limit. The critical exponents extracted from a fit to a model equation of state are distinct from those found for N_f=2. For N_f=6, in contrast, we present evidence for tunnelling between chirally symmetric and broken vacua at strong coupling, implying that the phase transition is first order and no continuum limit exists. The implications for the phase diagram of the model in the plane of coupling strength and N_f are briefly discussed.
The coupling $g_{B^\ast B π}$ is related to the form factor at zero momentum of the axial current between $B^\ast$- and $B$-states. This form factor is evaluated on the lattice using static heavy quarks and light quark propagators determined by a stochastic inversion of the fermionic bilinear. The $\gBBP$ coupling is related to the coupling $g$ between heavy mesons and low-momentum pions in the effective heavy meson chiral lagrangian. The coupling of the effective theory can therefore be computed by numerical simulations. We find the value $g = 0.42(4)(8)$. Besides its theoretical interest, the phenomenological implications of such a determination are discussed.
We present numerical evidence that supports the theory of quark confinement based on center vortex condensation. We introduce a special gauge ("maximal center gauge") and center projection, suitable for identification of center vortices. Main focus is then put on the connection of vortices in center projection to "confiners" in full, unprojected gauge-field configurations. Topics briefly discussed include: the relation between vortices and monopoles, first results for SU(3), and the problem of Casimir scaling.
We discuss the implementation of the ``direct'' maximal center gauge (a gauge which maximizes the lattice average of the squared-modulus of the trace of link variables), and its use in identifying Z(2) center vortices in Yang-Mills vacuum configurations generated by lattice Monte Carlo. We find that center vortices identified in the vacuum state account for the full asymptotic string tension. Scaling of vortex densities with lattice coupling, change in vortex size with cooling, and sensitivity to Gribov copies is discussed. Preliminary evidence is presented, on small lattices, for center dominance in SU(3) lattice gauge theory.
We study the dynamical breaking of chiral symmetry in the 3-d Thirring model for a small number of fermion species. The critical point is identified by fitting lattice data to an equation of state. The spectrum of the theory is studied to confirm the phase structure of the model.
We present a method for locating center vortices (``fluxons'') in thermalized lattice gauge field configurations. We find evidence, in lattice Monte Carlo simulations, that the asymptotic string tension of fundamental-representation Wilson loops is due to fluctuations in the number of center vortices linking those loops.
We present a variety of numerical data supporting the Center Vortex theory of confinement. A method is introduced for identifying the location of center vortices, in thermalized gauge-field configurations generated by lattice Monte Carlo. Fluctuations in the number of such vortices, linked to fundamental representation Wilson loops, appear to be responsible for the asymptotic string tension extracted from those loops.
We formulate the three dimensional Thirring model on a spacetime lattice and study it for various even numbers of fermion flavors N_f by Monte Carlo simulation. We find clear evidence for spontaneous chiral symmetry breaking at strong coupling, contradicting the predictions of the 1/N_f expansion. The critical point appears to correspond to an ultra-violet fixed point of the renormalisation group; a fit to a RG-inspired equation of state in the vicinity of the fixed point yields distinct critical exponents for N_f=2 and N_f=4, while no fit is found for N_f=6, suggesting there is a critical number N_fc<6 beyond which no chiral symmetry breaking occurs. The spectrum of the N_f=2 theory is studied; the states examined vary sharply but continuously across the transition.
We find, in close analogy to abelian dominance in maximal abelian gauge, the phenomenon of center dominance in maximal center gauge for $SU(2)$ lattice gauge theory. Maximal center gauge is a gauge-fixing condition that preserves a residual $Z_2$ gauge symmetry; ``center projection'' is the projection of $SU(2)$ link variables onto $Z_2$ center elements, and ``center dominance'' is the fact that the center-projected link elements carry most of the information about the string tension of the full theory. We present numerical evidence that the thin $Z_2$ vortices of the projected configurations are associated with ``thick'' $Z_2$ vortices in the unprojected configurations. The evidence also suggests that the thick $Z_2$ vortices may play a significant role in the confinement process.