Role Of Chiral Symmetry Research Articles

Chiral Symmetry and Nuclear Interactions

AbstractAbout 3 decades ago, Steven Weinberg came up with an idea of using the effective chiral Lagrangian to describe nuclear interactions, which has had a long-lasting impact on nuclear physics. Here, I will reflect on what has been learned since that time about the role of chiral symmetry in this context and discuss achievements and challenges in advancing chiral EFT into a precision tool for light nuclei.

Neutral pseudoscalar and vector meson masses under strong magnetic fields in an extended NJL model: Mixing effects

Mixing effects on the mass spectrum of light neutral pseudoscalar and vector mesons in the presence of an external uniform magnetic field $\vec B$ are studied in the framework of a two-flavor NJL-like model. The model includes isoscalar and isovector couplings both in the scalar-pseudoscalar and vector sectors, and also incorporates flavor mixing through a 't Hooft-like term. Numerical results for the $B$ dependence of meson masses are compared with present lattice QCD results. In particular, it is shown that the mixing between pseudoscalar and vector meson states leads to a significant reduction of the mass of the lightest state. The role of chiral symmetry and the effect of the alignment of quark magnetic moments in the presence of the magnetic field are discussed.

Optical properties of graphene quantum dots: the role of chiral symmetry

We analyse the electronic and optical properties of graphene quantum dots (GQD) using accurate ab initio many-body GW and Bethe-Salpeter calculations. We show that most pristine GQD, including structures with irregular shapes, are characterized by dark low energy singlet excitations that quench fluorescence. We rationalize this property by exploiting the chiral symmetry of the low energy electronic states in graphene. Edge sp3 functionalization is shown to efficiently brighten these low lying excitations by distorting the sp2 backbone planar symmetry. Such findings reveal an original indirect scenario for the influence of functionalization on the photoluminescence properties.

The computational challenge of lattice chiral symmetry - Is it worth the expense?

The origin of the low-lying position of the Roper resonance in the nucleon energy spectrum has been the subject of significant interest for many years, including several investigations using lattice QCD. It has been claimed that chiral symmetry plays an important role in our understanding of this resonance. We present results from our systematic examination of the potential role of chiral symmetry in the low-lying nucleon spectrum through the direct comparison of the clover and overlap fermion actions. After a brief summary of the background motivation, we specify the computational details of the study and outline our comparison methodologies. We do not find any evidence supporting the claim that chiral symmetry plays a significant role in understanding the Roper resonance on the lattice.

Baryons and chiral symmetry

The relevance of chiral symmetry in baryons is highlighted in three examples in the nucleon spectroscopy and structure. The first one is the importance of chiral dynamics in understanding the Roper resonance. The second one is the role of chiral symmetry in the lattice calculation of [Formula: see text] term and strangeness. The third one is the role of chiral [Formula: see text] anomaly in the anomalous Ward identity in evaluating the quark spin and the quark orbital angular momentum. Finally, the chiral effective theory for baryons is discussed.

Chiral condensate with topological degeneracy in graphene and its manifestation in edge states

The role of chiral symmetry in many-body states of graphene in strong magnetic fields is theoretically studied with the honeycomb lattice model. For a spin-split Landau level where the leading electron-electron interaction is the nearest-neighbor repulsion, a chiral condensate is shown to be, within the subspace of the $n=0$ Landau level, an exact many-body ground state having a finite gap, for which a calculation of the Chern number reveals that the ground state is a Hall insulator with a topological degeneracy of two. The topological nature of the ground state is shown to manifest itself as a Kekul\'ean bond order along armchair edges, while the pattern melts in the bulk due to quantum fluctuations. These can be regarded as a realization of the bulk-edge correspondence that is peculiar to a chiral-symmetric system. We have also obtained the ground state when point defects are introduced in the honeycomb lattice to reveal how the presence or absence of the chiral symmetry affects the defect states.

Kaon-nucleon dynamics and role of chiral symmetry

The K ¯ is a flavor partner of the π meson which contains the strange quark. Because of the Λ ( 1405 ) quasi-bound state which lies below the K ¯ N threshold, the interaction of the K ¯ and the nucleon is considered to be strongly attractive, leading to the possibility of the bound K ¯ nuclei. On the other hand, the K ¯ is the Nambu–Goldstone boson of spontaneous breaking of chiral SU(3)×SU(3) symmetry in QCD. We discuss the dynamics of the K ¯ N interaction and the K ¯ -nuclear systems from the viewpoint of chiral symmetry.

Structure and reactions of pentaquark baryons

We review the current status of the exotic pentaquark baryons. After a brief look at experiments of both positive and negative results, we discuss theoretical methods to study the structure and reactions for the pentaquarks. First we introduce the quark model and the chiral soliton model, where we discuss the relation of mass spectrum and parity with some emphasis on the role of chiral symmetry. It is always useful to picture the structure of the pentaquarks in terms of quarks. As for other methods, we discuss a model independent method, and briefly mention the results from the lattice and QCD sum rule. Decay properties are then studied in some detail, which is one of the important properties of Theta+. We investigate the relation between the decay width and the quark structure having certain spin-parity quantum numbers. Through these analyses, we consider as plausible quantum numbers of Theta+, JP = 3/2-. In the last part of this note, we discuss production reactions of Theta+ which provide links between the theoretical models and experimental information. We discuss photoproductions and hadron-induced reactions which are useful to explore the nature of Theta+

CONNECTING STRUCTURE FUNCTIONS ON THE LATTICE WITH PHENOMENOLOGY

We examine the extraction of moments of parton distributions from lattice data, focusing in particular on the chiral extrapolation as a function of the quark mass. Inclusion of the correct chiral behavior of the spin-averaged isovector distribution resolves a long-standing discrepancy between the lattice moments and experiment. We extract the x-dependence of the valence u - d distribution from the lowest few lattice moments, and discuss the implications for the quark mass dependence of meson masses lying on the a2 Regge trajectory. The role of chiral symmetry in spin-dependent distributions, and in particular the lattice axial vector charge, gA, is also highlighted.

Threshold pion production in NN → dπ AND γN → Nπ

Specific threshold pion production studies are discussed. The NN→dπ results provide a concise description of the pion production amplitudes at threshold. The charged-pion γN→Nπ measurement should complement recent (and upcoming) neutral-pion experiments, providing a test of our understanding of the role of chiral symmetry in the strong interaction.

Contribution of meson exchange currents to pion double charge exchange at low energies.

The magnitude of the effect of meson exchange currents on low-energy pion nucleus double charge exchange is reexamined. In particular, the role of chiral symmetry in limiting the size of the effect is studied, using the method of effective Lagrangians. The equivalence of two commonly used versions of the theory is shown, which resolves some of the disagreements in the literature. The effect is calculated for the case of $^{42}\mathrm{Ca}$, and found to be smaller than that of ordinary multiple scattering.

Mesonic and quark degrees of freedom

On the basis of recent developments in the theory of strong interactions, it is argued that there will be a cross-over in degrees of freedom from those of nucleons and mesons to those of quarks and gluons when nuclei are probed at a density or momentum transfer higher than presently explored. The role of chiral symmetry in bridging the two regimes is particularly emphasized.

Reply to "Use of chiral symmetry in pion-condensation calculations"

We review our earlier results on the role of chiral symmetry in pion condensation and show how they are completely consistent with Sawyer's observations in the preceding comment.

Chiral symmetry and pion condensation. II. General formalism

The role of chiral symmetry in pion condensation was explored through model calculations in the preceding paper. Here we present model-independent results which justify and sometimes go beyond the previous work. Our methods are based on functional techniques used in field theory. The paper is not self-contained and is meant to be read in conjunction with the preceding one. In the course of this work we were led to look at lowenergy $\ensuremath{\pi}\ensuremath{-}N$ interactions. In a brief survey, it is pointed out that they are more complicated than is commonly assumed.