Search

We report on a search for magnetic monopoles (MMs) produced in ultraperipheral Pb--Pb collisions during Run-1 of the LHC. The beam pipe surrounding the interaction region of the CMS experiment was exposed to 184.07 \textmu b$^{-1}$ of Pb--Pb collisions at 2.76 TeV center-of-mass energy per collision in December 2011, before being removed in 2013. It was scanned by the MoEDAL experiment using a SQUID magnetometer to search for trapped MMs. No MM signal was observed. The two distinctive features of this search are the use of a trapping volume very close to the collision point and ultra-high magnetic fields generated during the heavy-ion run that could produce MMs via the Schwinger effect. These two advantages allowed setting the first reliable, world-leading mass limits on MMs with high magnetic charge. In particular, the established limits are the strongest available in the range between 2 and 45 Dirac units, excluding MMs with masses of up to 80 GeV at 95\% confidence level., Comment: As accepted by PRL. 7 pages, 4 figures

The well-known geometric phase present in the quantum adiabatic evolution discovered by Berry many years ago has its analogue, the Hannay phase, in the classical domain.We calculate the Berry phase with examples for quantum hermitian and non-hermitian $PT$-symmetric Hamiltonians and compare with the Hannay phase in their classical equivalents.We use the analogy to propose resonant electric circuits which reproduce the theoretical solutions in simulated laboratory experiments., Comment: 13 pages and 4 figures

The holographic graviton soft-wall model, introduced to describe the spectrum of scalar and tensor glueballs, is improved to incorporate the realization of chiral-symmetry as in QCD. Such a goal is achieved by including the longitudinal dynamics of QCD into the scheme. Using the relation between AdS/QCD and Light-Front dynamics, we construct the appropriate wave function for the pion which is used to calculate several pion observables. The comparison of our results with phenomenology is remarkably successful., Comment: 23 pages, 15 figures. Acceptet version in THE EUROPEAN PHYSICAL JOURNAL C. References added

A search for highly electrically charged objects (HECOs) and magnetic monopoles is presented using 2.2 fb-1 of p - p collision data taken at a centre of mass energy (ECM) of 8 TeV by the MoEDAL detector during LHC's Run-1. The data were collected using MoEDAL's prototype Nuclear Track Detector array and the Trapping Detector array. The results are interpreted in terms of Drell-Yan pair production of stable HECO and monopole pairs with three spin hypotheses (0, 1/2 and 1). The search provides constraints on the direct production of magnetic monopoles carrying one to four Dirac magnetic charges (4gD) and with mass limits ranging from 590 GeV/c^2 to 1 TeV/c^2. Additionally, mass limits are placed on HECOs with charge in the range 10e to 180e, where e is the charge of an electron, for masses between 30 GeV/c^2 and 1 TeV/c^2.

An electric network whose dynamics is entirely equivalent to the Foucault pendulum is presented. This circuit allows one to measure in a simulation of the circuit, and eventually in a built circuit in the laboratory, the Hannay geometric phase. This phase corresponds, via the equivalence discussed in the text, exactly to the latitude effect on the rotation of the oscillation plane of the pendulum. The circuit includes a gyrator, that introduces a non-reciprocal character to the network., Comment: 6 pages and 2 figures

Schwinger showed that electrically-charged particles can be produced in a strong electric field by quantum tunnelling through the Coulomb barrier. By electromagnetic duality, if magnetic monopoles (MMs) exist, they would be produced by the same mechanism in a sufficiently strong magnetic field. Unique advantages of the Schwinger mechanism are that its rate can be calculated using semiclassical techniques without relying on perturbation theory, and the finite MM size and strong MM-photon coupling are expected to enhance their production. Pb-Pb heavy-ion collisions at the LHC produce the strongest known magnetic fields in the current Universe, and this article presents the first search for MM production by the Schwinger mechanism. It was conducted by the MoEDAL experiment during the 5.02 TeV/nucleon heavy-ion run at the LHC in November 2018, during which the MoEDAL trapping detectors (MMTs) were exposed to 0.235 nb$^{-1}$ of Pb-Pb collisions. The MMTs were scanned for the presence of magnetic charge using a SQUID magnetometer. MMs with Dirac charges 1$g_D$ $\leq$ $g$ $\leq$ 3$g_D$ and masses up to 75 GeV/c$^2$ were excluded by the analysis. This provides the first lower mass limit for finite-size MMs from a collider search and significantly extends previous mass bounds., Comment: Minor fixes to the authorship list found during production

The MoEDAL trapping detector, consists of approximately 800 kg of aluminium volumes. It was exposed during Run-2 of the LHC program to 6.46 fb^-1 of 13 TeV proton-proton collisions at the LHCb interaction point. Evidence for dyons (particles with electric and magnetic charge) captured in the trapping detector was sought by passing the aluminium volumes comprising the detector through a SQUID magnetometer. The presence of a trapped dyon would be signalled by a persistent current induced in the SQUID magnetometer. On the basis of a Drell-Yan production model, we exclude dyons with a magnetic charge ranging up to 5 Dirac charges, and an electric charge up to 200 times the fundamental electric charge for mass limits in the range 790 - 3130 GeV., Comment: arXiv admin note: text overlap with arXiv:1903.08491

MoEDAL is designed to identify new physics in the form of stable or pseudostable highly ionizing particles produced in high-energy Large Hadron Collider (LHC) collisions. Here we update our previous search for magnetic monopoles in Run 2 using the full trapping detector with almost four times more material and almost twice more integrated luminosity. For the first time at the LHC, the data were interpreted in terms of photon-fusion monopole direct production in addition to the Drell-Yan-like mechanism. The MoEDAL trapping detector, consisting of 794 kg of aluminum samples installed in the forward and lateral regions, was exposed to 4.0 fb$^{-1}$ of 13 TeV proton-proton collisions at the LHCb interaction point and analyzed by searching for induced persistent currents after passage through a superconducting magnetometer. Magnetic charges equal to or above the Dirac charge are excluded in all samples. Monopole spins 0, 1/2 and 1 are considered and both velocity-independent and -dependent couplings are assumed. This search provides the best current laboratory constraints for monopoles with magnetic charges ranging from two to five times the Dirac charge., Comment: 7 pages REVTeX 4, 11 figures, 1 table, matches published version in Physical Review Letters

We update our previous search for trapped magnetic monopoles in LHC Run 2 using nearly six times more integrated luminosity and including additional models for the interpretation of the data. The MoEDAL forward trapping detector, comprising 222~kg of aluminium samples, was exposed to 2.11~fb$^{-1}$ of 13 TeV proton-proton collisions near the LHCb interaction point and analysed by searching for induced persistent currents after passage through a superconducting magnetometer. Magnetic charges equal to the Dirac charge or above are excluded in all samples. The results are interpreted in Drell-Yan production models for monopoles with spins 0, 1/2 and 1: in addition to standard point-like couplings, we also consider couplings with momentum-dependent form factors. The search provides the best current laboratory constraints for monopoles with magnetic charges ranging from two to five times the Dirac charge., Comment: 8 pages, 5 figures, 1 table, accepted for publication in Phys. Lett. B

The complex phase present in CP-violating systems such as neutral kaons is shown to be of geometrical origin. It is also concluded that the complex phase of the Cabibbo--Kobayashi--Maskawa (CKM) matrix is a Berry-like phase., Comment: 7 pages and 1 figure

MoEDAL is designed to identify new physics in the form of long-lived highly-ionising particles produced in high-energy LHC collisions. Its arrays of plastic nuclear-track detectors and aluminium trapping volumes provide two independent passive detection techniques. We present here the results of a first search for magnetic monopole production in 13 TeV proton-proton collisions using the trapping technique, extending a previous publication with 8 TeV data during LHC run-1. A total of 222 kg of MoEDAL trapping detector samples was exposed in the forward region and analysed by searching for induced persistent currents after passage through a superconducting magnetometer. Magnetic charges exceeding half the Dirac charge are excluded in all samples and limits are placed for the first time on the production of magnetic monopoles in 13 TeV $pp$ collisions. The search probes mass ranges previously inaccessible to collider experiments for up to five times the Dirac charge., Comment: 6 pages, 1 table, 2 figures, accepted for publication in Phys. Rev. Lett

We study nuclear effects on the deuteron in the deep inelastic regime using the newest available data analyzing their $Q^2$ dependence. We conclude that precise EMC ratios for large $Q^2$ (> 30, GeV$^2$) cannot be obtained without considering these nuclear effects. For this purpose we use a scheme which parametrizes these effects in a simple manner and compare our results with other recent proposals., Comment: 10 pages, 4 figures

The announced ~ 3 {\sigma} enhancement in the inclusive {\gamma} {\gamma} -spectrum at ~ 750 GeV made by the ATLAS and CMS collaborations at LHC might indicate the existence of a monopole-antimonopole bound state: monopolium. In here we revisit our calculation of 2012 from a more general perspective and see that this resonance, if confirmed, might be a first signal of the existence of magnetic monopoles., Comment: 11 pages and 7 figures

The MoEDAL experiment is designed to search for magnetic monopoles and other highly-ionising particles produced in high-energy collisions at the LHC. The largely passive MoEDAL detector, deployed at Interaction Point 8 on the LHC ring, relies on two dedicated direct detection techniques. The first technique is based on stacks of nuclear-track detectors with surface area $\sim$18 m$^2$, sensitive to particle ionisation exceeding a high threshold. These detectors are analysed offline by optical scanning microscopes. The second technique is based on the trapping of charged particles in an array of roughly 800 kg of aluminium samples. These samples are monitored offline for the presence of trapped magnetic charge at a remote superconducting magnetometer facility. We present here the results of a search for magnetic monopoles using a 160 kg prototype MoEDAL trapping detector exposed to 8 TeV proton-proton collisions at the LHC, for an integrated luminosity of 0.75 fb$^{-1}$. No magnetic charge exceeding $0.5g_{\rm D}$ (where $g_{\rm D}$ is the Dirac magnetic charge) is measured in any of the exposed samples, allowing limits to be placed on monopole production in the mass range 100 GeV$\leq m \leq$ 3500 GeV. Model-independent cross-section limits are presented in fiducial regions of monopole energy and direction for $1g_{\rm D}\leq|g|\leq 6g_{\rm D}$, and model-dependent cross-section limits are obtained for Drell-Yan pair production of spin-1/2 and spin-0 monopoles for $1g_{\rm D}\leq|g|\leq 4g_{\rm D}$. Under the assumption of Drell-Yan cross sections, mass limits are derived for $|g|=2g_{\rm D}$ and $|g|=3g_{\rm D}$ for the first time at the LHC, surpassing the results from previous collider experiments., Comment: 25 pages, 11 figures, 2 tables, under review by JHEP

We present a calculation of the effective cross section $\sigma_{eff}$, an important ingredient in the description of double parton scattering in proton-proton collisions. Our theoretical approach makes use of a Light-Front quark model as framework to calculate the double parton distribution functions at low-resolution scale. QCD evolution is implemented to reach the experimental scale. The obtained $\sigma_{eff}$, when averaged over the longitudinal momentum fractions of the interacting partons, $x_i$, is consistent with the present experimental scenario. However the result of the complete calculation shows a dependence of $\sigma_{eff}$ on $x_i$, a feature not easily seen in the available data, probably because of their low accuracy. Measurements of $\sigma_{eff}$ in restricted $x_i$ regions are addressed to obtain indications on double parton correlations, a novel and interesting aspect of the three dimensional structure of the nucleon., Comment: 11 pages, 3 figures

The MoEDAL experiment at Point 8 of the LHC ring is the seventh and newest LHC experiment. It is dedicated to the search for highly ionizing particle avatars of physics beyond the Standard Model, extending significantly the discovery horizon of the LHC. A MoEDAL discovery would have revolutionary implications for our fundamental understanding of the Microcosm. MoEDAL is an unconventional and largely passive LHC detector comprised of the largest array of Nuclear Track Detector stacks ever deployed at an accelerator, surrounding the intersection region at Point 8 on the LHC ring. Another novel feature is the use of paramagnetic trapping volumes to capture both electrically and magnetically charged highly-ionizing particles predicted in new physics scenarios. It includes an array of TimePix pixel devices for monitoring highly-ionizing particle backgrounds. The main passive elements of the MoEDAL detector do not require a trigger system, electronic readout, or online computerized data acquisition. The aim of this paper is to give an overview of the MoEDAL physics reach, which is largely complementary to the programs of the large multi-purpose LHC detectors ATLAS and CMS.

The behavior of the nucleon structure functions in lepton nuclei deep inelastic scattering, both polarized and unpolarized, due to nuclear structure effects is reanalyzed. The study is performed in two schemes: an x-rescaling approach, and one in which there is an increase of sea quark components in the in medium nucleon, related to the low energy N-N interaction. In view of a recent interesting experimental proposal to study the behavior of the proton spin structure functions in nuclei we proceed to compare these approaches in an effort to enlighten the possible phenomenological interest of such difficult experiment., Comment: 11 pages and 5 figures

Double parton correlations, having effects on the double parton scattering processes occurring in high-energy hadron-hadron collisions, for example at the LHC, are studied in the valence quark region, within constituent quark models. In this framework, two particle correlations are present without any additional prescription, at variance with what happens, for example, in independent particle models, such as the MIT bag model in its simplest version. From the present analysis, conclusions similar to the ones obtained recenty in a modified version of the bag model can be drawn: correlations in the longitudinal momenta of the active quarks are found to be sizable, while those in transverse momentum are much smaller. However, the used framework allows to understand clearly the dynamical origin of the correlations. In particular, it is shown that the small size of the correlations in transverse momentum is a model dependent result, which would not occur if models with sizable quark orbital angular momentum were used to describe the proton. Our analysis permits therefore to clarify the dynamical origin of the double parton correlations and to establish which, among the features of the results, are model independent. The possibility to test experimentally the studied effects is discussed., Comment: 16 pages, 7 figures; references added; a figure added; conclusions unchanged. Final version accepted for publication in Phys. Rev. D

Recent progress in the solution of Dyson-Schwinger equations of QCD allows for a non perturbative evaluation of the One Gluon Exchange (OGE) interaction. We calculate the interquark static potential for heavy mesons by assuming that it is given by this OGE interaction and we apply it to the description of charmonium., Comment: To appear in the proceedings of The 5th International Workshop on Charm Physics (Charm 2012)

The resolution of Dyson-Schwinger equations leads to the freezing of the QCD running coupling (effective charge) in the infrared, which is best understood as a dynamical generation of a gluon mass function, giving rise to a momentum dependence which is free from infrared divergences. We calculate the interquark potential for heavy mesons by assuming that it is given by a massive One Gluon Exchange potential and compare with phenomenologyical fits inspired by lattice QCD. We apply these potential forms to the description of quarkonia and conclude that, even though some aspects of the confinement mechanism are absent in the Dyson Schwinger formalism, the results for the spectrum are surprisingly accurate. We discuss explanations for this outcome., Comment: 13 pages and 6 figures

Models of hadron structure are associated with a hadronic scale which allows by perturbative evolution to calculate observables in the deep inelastic region. The resolution of Dyson-Schwinger equations leads to the freezing of the QCD running coupling (effective charge) in the infrared, which is best understood as a dynamical generation of a gluon mass function, giving rise to a momentum dependence which is free from infrared divergences. We use this new development to understand why perturbative treatments are working reasonably well despite the smallness of the hadronic scale., Comment: Changes in Acknowledgments and PACS numbers

Recent BaBaR data on the pion transition form factor, whose Q^2 dependence is much steeper then predicted by asymptotic Quantum Chromodynamics (QCD), have caused a renewed interest in its theoretical description. We present here a formalism based on a model independent low energy description and a high energy description based on QCD, which match at a scale Q_0. The high energy description incorporates a flat pion distribution amplitude, phi(x)=1, at the matching scale Q_0 and QCD evolution from Q_0 to Q>Q_0. The flat pion distribution is connected, through soft pion theorems and chiral symmetry, to the pion valance parton distribution at the same low scale Q_0. The procedure leads to a good description of the data, and incorporating additional twist three effects, to an excellent description of the data., Comment: 11 pages, 5 postscript figures, uses epsfig.sty and 1 appendix

We propose an effective lagrangian for the coupling of the neutral pion with gluons whose strength is determined by a low energy theorem. We calculate the contribution of the gluonic components arising from this interaction to the pion transition form factor $\gamma^*\gamma^*\to\pi^0 $ using the instanton liquid model to describe the QCD vacuum. We find that this contribution is large and might explain the anomalous behavior of the form factor at large virtuality of one of the photons, a feature which was recently discovered by the BaBar Collaboration., Comment: 10 pages,2 figures,final version to be published in Phys.Rev.D

We have performed a dynamical analysis of the mixing in the pseudoscalar channel with the goal of understanding the existence and behavior of the pseudoscalar glueball. Our philosophy has not been to predict precise values of the glueball mass but to exploit an adequate effective theory to the point of breaking and to analyze which kind of mechanisms restore compatibility with data. Our study has lead to analytical solutions which allow a clear understanding of the phenomena. The outcome of our calculation leads to a large mass glueball $M_\Theta>2000$ MeV, to a large glue content of the $\eta^\prime$ and to mixing angles in agreement with previous numerical studies., Comment: 12 pages, 8 figures

A general formalism for the evaluation of time reversal odd parton distributions is applied here to calculate the Boer-Mulders function. The same formalism when applied to evaluate the Sivers function led to results which fulfill the Burkardt sum rule quite well. The calculation here has been performed for two different models of proton structure: a constituent quark model and the MIT bag model. In the latter case, important differences are found with respect to a previous evaluation in the same framework, a feature already encountered in the calculation of the Sivers function. The results obtained are consistent with the present wisdom, i.e., the contributions for the $u$ and $d$ flavors turn out to have the same sign, following the pattern suggested analyzing the model independent features of the impact parameter dependent generalized parton distributions. It is therefore confirmed that the present approach is suitable for the analysis of time reversal odd distribution functions. A critical comparison between the outcomes of the two models, as well as between the results of the calculations for the Sivers and Boer-Mulders functions, is also carried out., Comment: 26 pages, 5 figures. References added. Final version accepted for publication in Phys. Rev. D

A formalism is presented to evaluate the Sivers function in constituent quark models. A non-relativistic reduction of the scheme is performed and applied to the Isgur-Karl model. The sign for the $u$ and $d$ flavor contributions that we obtained turns out to be opposite. The Burkardt Sum Rule is fulfilled to a large extent. After the estimate of the QCD evolution of the results from the momentum scale of the model to the experimental one, a reasonable agreement with the available data is obtained., Comment: To appear in the proceedings of Diffraction 2008: International Workshop On Diffraction In High Energy Physics, 9-14 Sep 2008, La Londe-les-Maures, France. Misprints corrected, reference added

It is shown that, at variance with previous analyses, the MIT bag model can explain the available data of the Sivers function and satisfies the Burkardt Sum Rule to a few percent accuracy. The agreement is similar to the one recently found in the constituent quark model. Therefore, these two model calculations of the Sivers function are in agreement with the present experimental and theoretical wisdom., Comment: 9 pages, 4 figures; one figure added; references added; slightly revised version accepted for publication in Phys. Rev. D

A formalism to evaluate the Sivers function, developed for calculations in constituent quark models, is applied to the Isgur-Karl model. A non-vanishing Sivers asymmetry, with opposite signs for the u and d flavor, is found; the Burkardt sum rule is fulfilled up to 2 %. Nuclear effects in the extraction of neutron single spin asymmetries in semi-inclusive deep inelastic scattering off 3He are also evaluated. In the kinematics of JLab, it is found that the nuclear effects described by an Impulse Approximation approach are under control., Comment: 8 pages, 3 figures, invited talk at the "2nd International Workshop on Transverse Polarization Phenomena in Hard Processes" (Transversity 2008), Ferrara, Italy, May 28-31, 2008

A formalism is presented to evaluate the Sivers function in constituent quark models. A non-relativistic reduction of the scheme is performed and applied to the Isgur-Karl model. The results obtained are consistent with a sizable Sivers effect and the signs for the u and d flavor contributions turn out to be opposite. The Burkardt Sum Rule is fulfilled to a large extent. After the estimate of the QCD evolution of the results from the momentum scale of the model to the experimental one, a reasonable agreement with the available data is obtained. A calculation of nuclear effects in the extraction of neutron single spin asymmetries in semi-inclusive deep inelastic scattering off 3He is also described. In the kinematics of forth-coming experiments at JLab, it is found that the nuclear effects arising within an Impulse Approximation approach are under control., Comment: 8 pages, 3 figures, Invited talk at the "6th International Conference on Hadronic Physics", ICTP, Trieste, Italy, 12-16 May 2008. To appear in AIP Proceedings Series

We develop a formalism to evaluate the Sivers function. The approach is well suited for calculations which use constituent quark models to describe the structure of the nucleon. A non-relativistic reduction of the scheme is performed and applied to the Isgur-Karl model of hadron structure. The results obtained are consistent with a sizable Sivers effect and the signs for the u and d flavor contributions turn out to be opposite. This pattern is in agreement with the one found analyzing, in the same model, the impact parameter dependent generalized parton distributions. The Burkardt Sum Rule turns out to be fulfilled to a large extent. We estimate the QCD evolution of our results from the momentum scale of the model to the experimental one and obtain reasonable agreement with the available data., Comment: 31 pages, 4 figures. Extended version, discussion of the Burkardt Sum Rule added, references added, minor changes in the numerical results, same interpretation. Final version accepted for publication in Phys. Rev. D

We study the behavior of the scalar glueball inside a Quark Gluon Plasma. We follow the fireball from the plasma phase to the hadronic phase and observe that an interesting phenomenon might occur which could have experimental consequences as a signature of Quark Gluon Plasma., Comment: 5 pages, Latex, figure added, new references and some rewriting for clarification purposes

We perform a QCD sum rule calculation to determine the mass and the parity of the lowest lying pentaquark state. We include operators up to dimension $d=13$ in the OPE and the direct instanton contributions. We find evidence for a positive parity state. The contribution from operators of dimension $d>5$ is instrumental in determining the parity of the state and achieving the convergence of the sum rule., Comment: 10 pages, 8 figures, version to be appeared in Phys. Rev. D

We use phenomenological nonlocal Lagrangians, which lead to non trivial forms for the quark propagator, to describe the pion. We define a procedure, based on the Dyson-Schwinger equations, for the calculation of the pion parton distributions at low Q^2. The obtained parton distributions fulfill all the wishful properties. Using a convolution approach we incorporate the composite character of the constituent quarks in the formalism. We evolve, using the Renormalization Group, the calculated parton distributions to the experimental scale and compare favorably with the data and draw conclusions., Comment: Latex, 11 pages with 6 figures

The role of quark correlations in the description of hadron dynamics in many domains of physics, from low energy dynamics to very hot(dense) systems, is being appreciated. Strong correlations of two quarks (diquark) have been widely investigated in this respect. Recently, we have proposed a dynamical scheme to describe the $\Theta^+$ pentaquark in which also three quark correlations (triquark) were instrumental in producing a low mass exotic state. We perform a study, within the QCD sum rule approach including OPE and direct instanton contributions, of triquark correlations and obtain two quasi-bound light $ud\bar{s}$ color quark clusters of 800 MeV and 930 MeV respectively., Comment: 11 pages, 9 figures, corrected typos, reference added, introduction and conclusions modified slightly

We analyze the existence of the exotic $\Theta^+$ from the perspective of instanton induced quark dynamics. The 't Hooft interaction gives strong attraction in specific channels of the triquark $ud\bar s$ and diquark $ud$ configurations. In particular it leads to a light $ud\bar s$ triquark cluster, with the mass around $750 {\rm MeV}$, in the I=0, $S=1/2$ and color 3 configuration, and a light $ud$-diquark configuration, with mass $440 {\rm MeV}$, in the I=0, S=0 and color {$\bar{3}$} configuration. If we consider the pentaquark as a bound state of such triquark and diquark configurations in a relative L=1 state we obtain good agreement with the data. The small width of $\Theta^+$ has a natural explanation in this model., Comment: 14 pages, 2 figures; added references, corrected typos

The momentum distribution of Lambda^0 hyperons produced from the quark-gluon plasma (QGP) in ultra-relativistic heavy-ion collisions is calculated in dependence on their polarization. The momentum distribution of Lambda^0 hyperons is defined by matrix elements of relativistic quark Wigner operators, which are calculated within the Effective quark model with chiral U(3)xU(3) symmetry and the Quark-Gluon transport theory. We show that the polarization of the Lambda^0 hyperon depends of the spin of the strange quark that agrees well with the DeGrand-Miettinen model. We show that Lambda^0 hyperons, produced from the QGP, are fully unpolarized. This means that a detection of unpolarized Lambda^0 hyperons, produced in ultra-relativistic heavy-ion collisions, should serve as one of the signatures for the existence of the QGP in intermediate states of ultra-relativistic heavy-ion collisions., Comment: 17 pages, no figure, latex

We calculate generalized parton distribution functions in a field theoretic formalism using a covariant Bethe-Salpeter approach for the determination of the bound-state wave function. We describe the procedure in an exact calculation in scalar Electrodynamics proving that the relevant corrections outside our scheme vanish. We extend the formalism to the Nambu--Jona-Lasinio model, a realistic theory of the pion. We go in both cases beyond all previous calculations and discover that all important features required by general physical considerations, like symmetry properties, sum rules and the polynomiality condition, are explicitly verified. We perform a numerical study of their behavior in the weak and strong coupling limits., Comment: 19 pages, 21 eps figures, accepted for publication in EPJA

The proton, neutron and deuteron structure functions $F_2(x,Q^2)$ and $g_1(x,Q^2)$, measured at intermediate $Q^2$, are analyzed within a Regge approach. This analysis serves to fix the parameters of this scheme which are then used to calculate, in a unified Regge approach, the properties of $\rho^0$ meson electroproduction on the proton and the deuteron. In this way, the double-spin asymmetry observed at HERMES in $\rho^0$ electroproduction on the proton, can be related to the anomalous behavior of the flavor-singlet part of the spin-dependent structure function $g_1(x,Q^2)$ at small $x$., Comment: 10 pages, 13 figures, RevTex4

An approach is described to calculate Generalized Parton Distributions (GPDs) in Constituent Quark Models (CQM). The GPDs are obtained from wave functions to be evaluated in a given CQM. The general relations linking the twist-two GPDs to the form factors and to the leading twist quark densities are recovered. Results for the leading twist, unpolarized GPD in the Isgur and Karl model are presented., Comment: 4 pages, 4 eps figures; Contribution to the European Workshop on the QCD Structure of the Nucleon (QCD-N'02), Ferrara, Italy, 3-6 Apr 2002

We propose how to approach, and report on the first results in our effort for, describing nuclear matter starting from the solitonic picture of baryons which is supposed to represent QCD for large number of colors. For this purpose, the instanton-skyrmion connection of Atiyah and Manton is exploited to describe skyrmion matter. We first modify 't Hooft's multi-instanton solution so as to suitably incorporate proper dynamical variables into the skyrmion matter and then by taking these variables as variational parameters, we show that they cover a configuration space sufficient to adequately describe the ground state properties of nuclear matter starting from the skyrmion picture. Our results turn out to be comparable to those so far found in different numerical calculations, with our solution reaching stability at high density for a crystal structure and obtaining a comparable value for the energy per baryon at the minimum, thus setting the stage for the next step.

We calculate the contribution of meson and pomeron exchanges to the double-spin asymmetry in $\rho$-meson electromagnetic production at HERMES energies. We show that the observed double-spin asymmetries, which are large, can be explained by the interference between the natural parity $f_2$-secondary Reggeon and the unnatural parity anomalous $f_1$ exchanges., Comment: 7 pages, 3 figures, Latex

Non-perturbative QCD mechanisms are of fundamental importance in strong interaction physics. In particular, the flavor singlet axial anomaly leads to a gluonic pole mechanism which has been shown to explain the $\eta^{\prime}$ mass, violations of the OZI rule and more recently the proton spin. We show here that the interaction derived from the gluonic pole exchange explains the high momentum transfer behavior of the photoproduction cross sections of vector mesons at JLab energies., Comment: 10 pages, 6 figures, Latex; Completely rewritten version

The study of deeply virtual Compton scattering has shown that electromagnetic gauge invariance requires, to leading order, not only twist two but additional twist three contributions. We apply this analysis and, using the Ellis-Furmanski-Petronzio factorization scheme, compute the single (electron) spin asymmetry arising in the collision of longitudinally polarized electrons with hadrons up to twist 3 accuracy. In order to simplify the kinematics we restrict the actual calculation to pions in the chiral limit. The process is described in terms of the generalized parton distribution functions which we obtain within a bag model framework., Comment: 18 pages, Latex

The gluonic contributions to the conventional PCAC formulas due to flavor singlet axial anomaly have been instrumental in explaining the mass of the $\eta^{\prime}$ and providing a plausible explanation for solving the spin crisis. We show that they also play an important role in the description of photo- and electroproduction of vector mesons at low energy and high momentum transfers. We calculate the contributions of this type to $\phi$ meson electromagnetic production in a model, which contains also a soft pomeron, and find agreement with recent CLAS data., Comment: 13 pages, 6 figures, Latex; final version to appear in Phys. Lett.B

The instanton induced interaction leads to a significant enhancement of the $A_0$ weak amplitude determining the $\Delta I=1/2$ rule, through the contribution of operators with dimension d=9, as we show in the weak $K\to \pi\pi$ decay., Comment: 4 pages, 2 figures, Revtex; final version to appear in Phys.Rev.Lett

A new mechanism for hard diffraction based on the anomalous $f_1$ trajectory exchange, which we identify as the odd signature partner of the Pomeron, is suggested. We calculate the contribution of the $f_1$ exchange to elastic and dissociative electromagnetic production of vector mesons and show that it gives a dominant contribution to the differential cross sections at large momentum transfers., Comment: 5 pages, Latex, 4 eps figures. Talk presented at Diffraction 2000 Workshop, Centraro, Italy, September,2000. One reference replaced

We study the gluon polarization contribution at the quark model renormalization scale to the proton spin, $\Gamma$, in the chiral bag model. It is evaluated by taking the expectation value of the forward matrix element of a local gluon operator in the axial gauge $A^+=0$. It is shown that the confining boundary condition for the color electric field plays an important role. When a solution satisfying the boundary condition for the color electric field, which is not the conventionally used but which we favor, is used, the $\Gamma$ has a positive value for {\it all} bag radii and its magnitude is comparable to the quark spin polarization. This results in a significant reduction in the relative fraction of the proton spin carried by the quark spin, which is consistent with the small flavor singlet axial current measured in the EMC experiments., Comment: Corrections to figures

We present a consistent calculation of the structure functions within a light-front constituent quark model of the nucleon. Relativistic effects and the relevance of the covariance constraints are analyzed for polarized parton distributions. Various models, which differ in their gluonic structure at the hadronic scale, are investigated. The results of the full covariant calculation are compared with those of a non-relativistic approximation to show the structure and magnitude of the differences. It is also shown how measurements of transversity in doubly polarized Drell-Yan lepton pair production are a clearcut sign of covariance requirements for the spin., Comment: 10 pages, 4 figures, sprocl.sty (included). Proceedings of the Second International Conference on Perspectives in Hadronic Physics, ICTP Trieste, May 1999

A simple picture of the constituent quark as a composite system of point-like partons is used to construct the unpolarized and polarized parton distributions by a convolution between constituent quark momentum distributions and constituent quark structure functions. We achieve good agreement with experiments in the unpolarized, as well as, in the polarized case. When our results are compared with similar calculations using non-composite constituent quarks, the accord with the experiments of the present scheme is impressive. We conclude that DIS data are consistent with a low energy scenario dominated by composite constituents of the nucleon., Comment: 4 pages; latex using espcrc1.sty; 4 postscript figures; Invited talk at the Workshop ``Nucleon '99'', Frascati; Italy 7-9 June 1999. Submitted to Nuc. Phys. A

At the low energy, {\sl hadronic}, scale we calculate Orbital Angular Momentum (OAM) twist-two parton distributions for the relativistic MIT bag model and for non-relativistic quark models. We reach the scale of the data by leading order evolution in perturbative QCD. We confirm that the contribution of quarks and gluons OAM to the nucleon spin grows with $Q^2$, and it can be relevant at the experimental scale, even if it is negligible at the hadronic scale, irrespective of the model used. The sign and shape of the quark OAM distribution at high $Q^2$ may depend strongly on the relative size of the OAM and spin distributions at the hadronic scale. Sizeable quark OAM distributions at the hadronic scale, as proposed by several authors, can produce the dominant contribution to the nucleon spin at high $Q^2$., Comment: 10 pages; latex using ws-p8-50x6-00.cls (included); to be published in the Proceedings of 2nd Int. Conf. on ``Perspectives in Hadronic Physics'', 10-14 May 1999, Trieste, Italy