D. Espriu

The QCD Effective Action at Long Distances

We describe the derivation, under a well-defined set of approximations, of a low-energy effective chiral lagrangian of the strong interactions from QCD. The effective lagrangian includes only the Nambu-Goldstone modes (i.e. the octet of pseudoscalar particles) in the presence of external sources, with inclusion of the explicit chiral symmetry breaking due to the light quark masses M. The coupling constants of the O(p4) terms of the chiral lagrangian are calculated first to leading order in the 1/Nc expansion but with O(α) terms and higher orders neglected. Next, the corrections O(α) to the couplings which remain in the chiral limit M → 0 are also taken into account. In view of our results, an improved chiral power counting for higher-order terms in the effective chiral lagrangian is proposed.

Chiral Lagrangians as a tool to probe the symmetry breaking sector of the SM at LEP

Abstract An effective SU (2) L ×U(1) Y invariant chiral lagrangian can accomodate any symmetry breaking scheme in the standard model. It is shown that all process-independent quantities relevant to LEP precision measurements ( Δπ , Δκ and Δr ) depend only on one coefficient of the weak chiral lagrangian, taking different values depending on the precise symmetry breaking scheme. We remove any ambiguity due to the use of an effective theory by a careful choice of observables and make definite predictions that can be tested in the foreseeable future. Our analysis complements nicely that of longitudinal W and Z scattering.

Rare decay modes of the K-mesons in the chiral lagrangian

Abstract The decays KS → γγ, KL → γγ and KL → μ+μ− are studied within the framework of chiral lagrangians. The one loop contribution to KS → γγ turns out to be finite, giving an unambiguous prediction. The results also indicate that the long-distance dispersive part of KL → μ+μ− is the dominant one. As for KL → γγ, a possible difficulty is pointed out with the recently published value ϑ ⋍ 20° for the η − η′ mixing angle.

BARYON MASSES AND CHIRAL SYMMETRY BREAKING

Abstract We study the 1 2 + baryon octet within the ITEP sum rule approach. With only the input of one mixing angle for the interpolating baryon operator, the quark vacuum condensates and the strange quark mass, the best values for the masses are obtained for 〈 s s〉 1 3 ≅ 0.93〈 u u〉 1 3 and ms ≅ 115 MeV. We also give results for the 3 2 + decuplet.

Matching Regge theory to the OPE

The spectra of masses and decay constants for non-strange meson resonances in the energy range 0–2.5 GeV is analyzed. It is known from meson phenomenology that for given quantum numbers these spectra approximately follow linear trajectories with a universal slope. These facts can be understood in terms of an effective string description for QCD. For light meson states the trajectories deviate noticeably from the linear behavior. We investigate the possible corrections to the linear trajectories by matching two-point correlators of quark currents to the Operator Product Expansion (OPE). We find that the allowed modifications to the linear Regge behavior must decrease rapidly with the principal quantum number. After fitting the lightest states in each channel and certain low-energy constants the whole spectrum for meson masses and residues is obtained in a satisfactory agreement with phenomenology. We briefly speculate on possible implications for the QCD effective string.

On the Fine Structure of Strings

Abstract We investigate the consequences of adding a term proportional to the extrinsic curvature to the Nambu-Goto action, as suggested by Polyakov. A classical analysis of the equations of motion, constraint structure and hamiltonian of this system is performed. Quantum corrections are calculated in the mean field approximation for a large number of dimensions of the embedding space. The exact beta function of the new coupling constant in this limit is obtained; it possesses only one zero at the origin. Effects induced by adding a term that weights string configurations according to the number of self-intersections are discussed. We briefly comment on a similar proposal for free paths made recently.

On the possibility of P-violation at finite baryon-number densities

We show how the introduction of a finite baryon density may trigger spontaneous parity violation in the hadronic phase of QCD. Since this involves strong interaction physics in an intermediate energy range we approximate QCD by a σ model that retains the two lowest scalar and pseudoscalar multiplets. We propose a novel mechanism based on interplay between lightest and heavy meson states which cannot be realized solely in the Goldstone boson (pion) sector and thereby is unrelated to the one advocated by Migdal some time ago. Our approach is relevant for dense matter in an intermediate regime of few nuclear densities where quark percolation does not yet play a significant role.

Phase transitions in random surfaces

Abstract We investigate the statistical properties of triangulated random surfaces of fixed connectivity embedded in d-dimensional space and weighted with an action that contains the extrinsic curvature of the surface as well as the usual Nambu-Goto term. Numerically, we find no second-order phase transition for finite values of the rigidity coupling, in contrast to results obtained by Kantor and Nelson using a different action. Rather, there is a third order “crumpling” transition which, however, is not associated with an infinite correlation length between the normals to the surface. We compare the Monte Carlo results with several approximations, particularly with the mean field solution of the model. Our results indicate that there are no fixed points other than those already found in perturbation theory. We comment on several other aspects of random surfaces.

Anomalous positron excess from Lorentz-violating QED

We entertain the idea that a suitable background of cold (very low momentum) pseudoscalar particles or condensate, may trigger a background that effectively generates Lorentz-invariance violation. This ae ther-like background induces a Chern-Simons modification of QED. Physics is different in different frames and, in the rest frame of the pseudoscalar background, high momentum photons can decay into pairs. The threshold for such decay depends quadratically on the rest mass of the particles. This mechanism could explain in a natural way why antiprotons are absent in recent cosmic ray measurements. A similar signal could be used as a probe of pseudoscalar condensation in heavy ion collisions.

Triangulated random surfaces

Abstract A model of discretized random surfaces that contains the extrinsic curvature as well as the usual area term in the action is considered. The renormalization group predicts that at large distances the model is indistinguishable from previous proposals of triangulated surfaces that contained only the area term, but, unlike them, does not grow spikes. The partition function and all its moments are finite and well defined. The model is solved for large d in the vicinity of the IR fixed point. The Hausdorff dimension is ∞ and the entropy exponent agrees with the one obtained by Zamolodchikov and others for the Polyakov action in the continuum.