Particle theory

Abstract:

We calculate the light "glueball" mass spectrum in N f=2 lattice QCD using a fermion action that is nonperturbatively O(a) improved. We work at lattice spacings a∼0.1 fm and with quark masses that range down to about one-half the strange quark mass. We find the statistical errors to be moderate and under control on relatively small ensembles. We compare our mass spectrum to that of quenched QCD at the same value of a. While the tensor mass is the same (within errors), the scalar mass is significantly smaller in the dynamical lattice theory, by a factor of ∼0.84±0.03. We discuss what the observed mq dependence of this suppression tells us about the dynamics of glueballs in QCD. We also calculate the masses of flux tubes that wind around the spatial torus, and extract the string tension from these. As we decrease the quark mass we see a small but growing vacuum expectation value for the corresponding flux tube operators. This provides clear evidence for "string breaking" and for the (expected) breaking of the associated gauge center symmetry by sea quarks. © 2002 The American Physical Society.

Abstract:

We find quasinormal modes of near extremal black branes by solving a singular boundary value problem for the Heun equation. The corresponding eigenvalues determine the dispersion law for the collective excitations in the dual strongly coupled (Formula presented) supersymmetric Yang-Mills theory at finite temperature. © 2002 The American Physical Society.

Abstract:

We calculate the string tension and part of the mass spectrum of SU(4) and SU(6) gauge theories in 2+1 dimensions using lattice techniques. We combine these new results with older results for Nc=2, . . . ,5 so as to obtain more accurate extrapolations to Nc=∞. The qualitative conclusions of the earlier work are unchanged: SU(Nc∞) theories in 2+1 dimensions are linearly confining as N c→∞ the limit is achieved by keeping g2Nc fixed; SU(3), and even SU(2), are "close" to SU(∞). We obtain more convincing evidence than before that the leading large-Nc correction is O(1/N c2). We look for the multiplication of states that one expects in simple flux loop models of glueballs, but find no evidence for this. © 2002 The American Physical Society.

Abstract:

For many event-shape observables, the most difficult part of a resummation in the Born limit is the analytical treatment of the observable's dependence on multiple emissions, which is required at single logarithmic accuracy. We present a general numerical method, suitable for a large class of event shapes, which allows the resummation specifically of these single logarithms. It is applied to the case of the thrust major and the oblateness, which have so far defied analytical resummation and to the two-jet rate in the Durham algorithm, for which only a subset of the single logs had up to now been calculated. © SISSA/ISAS 2002.

Abstract:

The spectrum of glueballs in 2 +1 dimensions is calculated within an extended class of Isgur-Paton flux tube models and compared to lattice calculations of the low-lying SU(N≥2) glueball mass spectrum. Our modifications of the model include a string curvature term and a new way of dealing with the short-distance cutoff. We find that the generic model is remarkably successful at reproducing the positive charge conjugation, C = +, sector of the spectrum. The only large (and robust) discrepancy involves the 0-+ state, raising the interesting possibility that the lattice spin identification is mistaken and that this state is in fact 4-+. Additionally, the Isgur-Paton model does not incorporate any mechanism for splitting C= - from C=+ (in contrast with the case in 3 + 1 dimensions), while the "observed" spectrum does show a substantial splitting. We explore several modifications of the model in an attempt to incorporate this physics in a natural way. At the qualitative level we find that this constrains our choice to the picture in which the C= ± splitting is driven by mixing with new states built on closed loops of adjoint flux. However, a detailed numerical comparison suggests that a model incorporating an additional direct mixing between loops of opposite orientation is likely to work better, and that, in any case, a nonzero curvature term will be required. We also point out that a characteristic of any string model of glueballs is that the SU(N →∞) mass spectrum will consist of multiple towers of states that are scaled up copies of each other. To test this will require a lattice mass spectrum that extends to somewhat larger masses than currently available. © 2002 The American Physical Society.