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D. Espriu

Publications and source records attributed to D. Espriu.

At least 55 records · Page 3Linked to original sources
arXiv2004

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.

hep-ph↗
arXiv2002

Hadronic string, conformal invariance and chiral symmetry

While it is clear that in some kinematic regime QCD can be described by an effective (as opposed to fundamental) string theory, it is not at all clear how this string theory should be. The `natural' candidate, the bosonic string, leads to amplitudes with the usual problems related to the existence of the tachyon, absence of the adequate Adler zero, and massless vector particles, not to mention the conformal anomaly. The supersymmetric version does not really solve most of these problems. For a long time it has been believed that the solution of at least some of these difficulties is associated to a proper identification of the vacuum, but this program has remained elusive. We show in this work how the first three problems can be avoided, by using a sigma model approach where excitations above the correct (chirally non-invariant) QCD vacuum are identified. At the leading order in a derivative expansion we recover the non-linear sigma model of pion interactions. At the next-to-leading order the O(p^4) Lagrangian of Gasser and Leutwyler is obtained, with values for the coefficients that match the observed values. We also discuss some issues related to the conformal anomaly.

hep-th↗
arXiv2002

Hadronic string and chiral symmetry breaking

We assume that QCD can be effectively described with string-like variables. The hadronic string is built over the chirally non-invariant QCD vacuum by means of the boundary interaction with background chiral fields associated with pions. By making this interaction compatible with the conformal symmetry of the string and with the unitarity constraint on chiral fields we reconstruct the equations of motion for the latter ones and furthermore recover the Lagrangian of non-linear sigma model of pion interactions. The estimated chiral structural constants of Gasser and Leutwyler fit well the phenomenological values.

hep-th↗
arXiv2002

A study of top polarization in single-top production at the LHC

This paper complements the study of single top production at the LHC aiming to estimate the sensitivity of different observables to the magnitude of the effective couplings. In a previous paper the dominant $W$-gluon fusion mechanism was considered, while here we extend the analysis to the subdominant (10% with our set of experimental cuts) s-channel process. In order to distinguish left from right effective couplings it is required to consider polarized cross-sections and/or include $m_b$ effects. The spin of the top is accessible only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies necessarily that the top is not in a pure spin state. We discuss to what extent quantum interference terms can be neglected in the measurement and therefore simply multiply production and decay probabilities clasically. The coarsening involved in the measurement process makes this possible. We determine for each process the optimal spin basis where theoretical errors are minimized and, finally, discuss the sensitivity in the s-channel to the effective right-handed coupling. The results presented here are all analytical and include $m_b$ corrections. They are derived within the narrow width approximation for the top.

hep-ph↗
arXiv2002

Slow dynamics in the 3--D gonihedric model

We study dynamical aspects of three--dimensional gonihedric spins by using Monte--Carlo methods. The interest of this family of models (parametrized by one self-avoidance parameter $κ$) lies in their capability to show remarkably slow dynamics and seemingly glassy behaviour below a certain temperature $T_g$ without the need of introducing disorder of any kind. We consider first a hamiltonian that takes into account only a four--spin term ($κ=0$), where a first order phase transition is well established. By studying the relaxation properties at low temperatures we confirm that the model exhibits two distinct regimes. For $T_g< T < T_c$, with long lived metastability and a supercooled phase, the approach to equilibrium is well described by a stretched exponential. For $T<T_g$ the dynamics appears to be logarithmic. We provide an accurate determination of $T_g$. We also determine the evolution of particularly long lived configurations. Next, we consider the case $κ=1$, where the plaquette term is absent and the gonihedric action consists in a ferromagnetic Ising with fine-tuned next-to-nearest neighbour interactions. This model exhibits a second order phase transition. The consideration of the relaxation time for configurations in the cold phase reveals the presence of slow dynamics and glassy behaviour for any $T< T_c$. Type II aging features are exhibited by this model.

cond-mat↗
arXiv2002

Flavour Mixing, Gauge Invariance and Wave-function Renormalisation

We clarify some aspects of the LSZ formalism and wave function renormalisation for unstable particles in the presence of electroweak interactions when mixing and CP violation are considered. We also analyse the renormalisation of the CKM mixing matrix which is closely related to wave function renormalisation. We critically review earlier attempts to define a set of "on-shell" wave function renormalisation constants. With the aid of an extensive use of the Nielsen identities complemented by explicit calculations we corroborate that the counter term for the CKM mixing matrix must be explicitly gauge independent and demonstrate that the commonly used prescription for the wave function renormalisation constants leads to gauge parameter dependent amplitudes, even if the CKM counter term is gauge invariant as required. We show that a proper LSZ-compliant prescription leads to gauge independent amplitudes. The resulting wave function renormalisation constants necessarily possess absorptive parts, but we verify that they comply with the expected requirements concerning CP and CPT. The results obtained using this prescription are different (even at the level of the modulus squared of the amplitude) from the ones neglecting the absorptive parts in the case of top decay. The difference is numerically relevant.

hep-ph↗
arXiv2001

Measuring effective electroweak couplings in single top production at the LHC

We study the mechanism of single top production at the LHC in the framework of an effective electroweak Lagrangian, analyzing the sensitivity of different observables to the magnitude of the effective couplings that parametrize new physics beyond the Standard Model. The observables relevant to the distinction between left and right effective couplings involve in practice the measurement of the spin of the top and this can be achieved only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies that the top is not in a pure spin state. A unique spin basis is singled out which allows one to connect top decay products angular distribution with the polarized top differential cross section. We present a complete analytical expression of the differential polarized cross section of the relevant perturbative subprocess including general effective couplings. The mass of the bottom quark, which actually turns out to be more relevant than naively expected, is retained. Finally we analyze different aspects the total cross section relevant to the measurement of new physics through the effective couplings. The above analysis also applies to anti-top production in a straightforward way.

hep-ph↗
arXiv2000

CP Violation and Family Mixing in the Effective Electroweak Lagrangian

We construct the most general effective Lagrangian of the matter sector of the Standard Model, including mixing and CP violating terms. The Lagrangian contains the effective operators that give the leading contribution in theories where the physics beyond the Standard Model shows at a scale $Λ>>M_{W}$. We perform the diagonalization and passage to the physical basis in full generality. We determine the contribution to the different observables and discuss the possible new sources of CP violation, the idea being to be able to gain some knowledge about new physics beyond the Standard Model from general considerations, without having to compute model by model. The values of the coefficients of the effective Lagrangian in some theories, including the Standard Model, are presented and we try to draw some general conclusions about the general pattern exhibited by physics beyond the Standard Model in what concerns CP violation. In the process we have had to deal with two theoretical problems which are very interesting in their own: the renormalization of the CKM matrix elements and the wave function renormalization in the on-shell scheme when mixing is present.

hep-ph↗
arXiv2000

Effective Quark Models in QCD at low and intermediate energies

The effective quark models are employed to describe the hadronization of QCD in the quark sector. They reveal a different structure depending on how the spontaneous chiral symmetry breaking (CSB) is implemented. When the generation of light pseudoscalar mesons is manifestly incorporated one deals with an extension of the chiral quark model (CQM) with the non-linear realization of chiral symmetry. If a model is built at the CSB scale by means of perturbation theory it generalizes the Nambu-Jona-Lasinio (NJL) one with chiral symmetry broken due to attractive 4-fermion forces in the scalar channel. The matching to high-energy QCD is realized at CSB scale by means of Chiral Sum Rules. Two types of models are compared in their fitting of meson physics. In particular, if the lowest scalar meson is sufficiently heavy approaching the mass of heavy π'(1300) then QCD favours an effective theory which is dominated by the simplest CQM. On the contrary, the light scalar quarkonium with a mass of order 500MeV, supports the NJL mechanism.

hep-ph↗
arXiv2000

Slow dynamics of Ising models with energy barriers

Using Monte Carlo simulations we study the dynamics of three-dimensional Ising models with nearest-, next-nearest-, and four-spin (plaquette) interactions. During coarsening, such models develop growing energy barriers, which leads to very slow dynamics at low temperature. As already reported, the model with only the plaquette interaction exhibits some of the features characteristic of ordinary glasses: strong metastability of the supercooled liquid, a weak increase of the characteristic length under cooling, stretched-exponential relaxation and aging. The addition of two-spin interactions, in general, destroys such behaviour: the liquid phase loses metastability and the slow-dynamics regime terminates well below the melting transition, which is presumably related with a certain corner-rounding transition. However, for a particular choice of interaction constants, when the ground state is strongly degenerate, our simulations suggest that the slow-dynamics regime extends up to the melting transition. The analysis of these models leads us to the conjecture that in the four-spin Ising model domain walls lose their tension at the glassy transition and that they are basically tensionless in the glassy phase.

cond-mat.stat-mech↗
arXiv1999

The Extended Chiral Quark Model confronts QCD

We discuss the truncation of low energy effective action of QCD below the chiral symmetry breaking (CSB) scale, including all operators of dimensionality less or equal to 6 which can be built with quark and chiral fields. We perform its bosonization in the scalar, pseudoscalar, vector and axial-vector channels in the large-N_c and leading-log approximation. Constraints on the coefficients of the effective lagrangian are derived from the requirement of Chiral Symmetry Restoration (CSR) at energies above the CSB scale in the scalar-pseudoscalar and vector-axial-vector channels, from matching to QCD at intermediate scales, and by fitting some hadronic observables. In this truncation two types of pseudoscalar states (massless pions and massive Pi'-mesons), as well as a scalar, vector and axial-vector one arise as a consequence of dynamical chiral symmetry breaking. Their masses and coupling constants as well as a number of chiral structural constants are derived. A reasonable fit of all parameters supports a relatively heavy scalar meson (quarkonium) with the mass \sim 1 GeV and a small value of axial pion-quark coupling constant g_A \simeq 0.55.

hep-ph↗
arXiv1999

Vector mesons in the Extended Chiral Quark Model

We extend our previous formulation of low-energy QCD in terms of an effective lagrangian containing operators of dimensionality $d\le 6$ constructed with pseudoscalars and quark fields, describing physics below the scale of chiral symmetry breaking. We include in this paper the vector and axial-vector channels. We follow closely the Extended Chiral Quark Model approach and consistently work in the large-$N_c$ and leading log approximation and take into account the constraints from chiral symmetry and chiral symmetry restoration. The optimal fit of all parameters gives further support to a heavy scalar meson with a mass $\sim 1$ GeV and a value of the axial pion-quark coupling constant $g_A \simeq 0.55$ to 0.66, depending on some assumptions concerning the Weinberg sum rules.

hep-ph↗
arXiv1999

Chiral lagrangians and the QCD string

We propose a method to derive the low-energy efective action of QCD assuming that the long-distance properties of strong interactions can be described by a string theory. We bypass the usual problems related to the existence of the tachyon and absence of the adequate Adler zero by using a sigma model approach where excitations above the correct (chirally non-invariant) QCD vacuum are included. Two-dimensional conformal invariance then implies the vanishing of the O(p^4) effective lagrangian coefficients. We discuss ways to go beyond this limit.

hep-th↗
arXiv1999

QCD

This is the written version of the lecture on deep inelastic scattering and related topics in QCD, delivered in the course of the XXVI International Meeting on Fundamental Physics to an audience of young experimentalists. The aim is fundamentally pedagogical. I review the theoretical setting of the Altarelli-Parisi equations, discuss recent determinations of $α_s$ from deep-inelastic scattering and then move to the kinematical region explored by HERA. In the way I mention some unsolved theoretical problems. I discuss low-x physics and to what extent $\log{1/x}$ resummations are called for.

hep-ph↗
arXiv1998

The Extended Chiral Quark Model and QCD

We consider the low energy effective action of QCD below the chiral symmetry breaking scale, including, in Wilson's spirit, all operators of dimensionality less or equal to 6 which can be built with quark and chiral fields. The effect of the residual gluon interactions is contained in a number of coupling constants, whose running is studied. The resulting model is an extension of both the chiral quark model and the Nambu-Jona-Lasinio one. Constraints on the coefficients of the effective lagrangian are derived from the requirement of chiral symmetry restoration at energies above the chiral symmetry breaking scale, from matching to QCD at intermediate scales, and by fitting some hadronic observables. In this model two types of pseudoscalar states (massless pions and massive Π-mesons), as well as one scalar one arise as a consequence of dynamical chiral symmetry breaking. Their masses and coupling constants are studied. We also predict a number of low energy structural constants. We find out that QCD favours a low-energy effective theory which is largely dominated by the simplest chiral quark model, whereas higher dimensional operators (such as those of the Nambu-Jona-Lasinio type) can be assumed to be small.

hep-ph↗
arXiv1997

The Phase Diagram of the $U(2)\times U(2)$ Sigma Model

We study the phase diagram of the $U(2) \times U(2)$ scalar model in $d=4$ dimensions. We find that the phase transition is of first order in most of the parameter space. The theory can still be relevant to continuum physics (as an effective theory) provided the transition is sufficiently weakly first order. This places restrictions on the allowed coupling constants.

hep-lat↗
arXiv1997

The phase diagram of the $U(2) \times U(2)$ Sigma Model and its Implications for Chiral Hierarchies

Motivated by the issue of whether it is possible to construct phenomenologically viable models where the electroweak symmetry breaking is triggered by new physics at a scale $Λ\gg 4πv$, where $v$ is the order parameter of the transition ($v\sim 250$ GeV) and $Λ$ is the scale of new physics, we have studied the phase diagram of the $U(2) \times U(2)$ model. This is the relevant low energy effective theory for a class of models which will be discussed below. We find that the phase transition in these models is first order in most of parameter space. The order parameter can not be made much smaller than the cut-off and, consequently a large hierarchy does not appear sustainable. In the relatively small region in the space of parameters where the phase transition is very weakly first order or second order the model effectively reduces to the O(8) theory for which the triviality considerations should apply.

hep-lat↗