Chiral Partners Research Articles

Candidate chiral twin bands in the odd-odd nucleus132Cs:Exploring the limits of chirality in the massA≈130region

High-spin states in the doubly odd $N=77$ nucleus ${}^{132}\mathrm{Cs}$ have been studied. The known positive-parity structures have been extended. $\ensuremath{\gamma}$-ray linear-polarization and angular-correlation measurements have been performed to establish the spin and parity assignment of these structures. A new chiral partner of the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ band has been proposed. Three-dimensional tilted axis cranking model calculations have been performed and compared with the experimental results.

Excited baryons in lattice QCD

We present first results for the masses of positive and negative parity excited baryons calculated in lattice QCD using an O(a^2)-improved gluon action and a fat-link irrelevant clover (FLIC) fermion action in which only the irrelevant operators are constructed with APE-smeared links. The results are in agreement with earlier calculations of N^* resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner. An correlation matrix analysis reveals two low-lying J^P=(1/2)^- states with a small mass splitting. The study of different Lambda interpolating fields suggests a similar splitting between the lowest two Lambda1/2^- octet states. However, the empirical mass suppression of the Lambda^*(1405) is not evident in these quenched QCD simulations, suggesting a potentially important role for the meson cloud of the Lambda^*(1405) and/or a need for more exotic interpolating fields.

Vector Manifestation in Hot and/or Dense Matter

In this talk I summarize main features of the vector manifestation (VM) which was recently proposed as a novel manifestation of the Wigner realization of chiral symmetry in which the symmetry is restored at the critical point by the massless degenerate pion (and its flavor partners) and the rho meson (and its flavor partners) as the chiral partner. I show how the VM is realized in hot and dense QCD using the effective field theory of QCD based on the hidden local symmetry.

The role of the intermediate resonance a 0(980) in the decay η → πγγ

It is shown that if the a0(980) boson is the scalar chiral partner of a π meson, its contribution to the transition η → πγγ has a value comparable to that originated by vector meson exchange. Together, these two mechanisms give the decay probability consistent with the experimental observation.

REMARKS ON THE f0(400–1200) SCALAR MESON AS THE DYNAMICALLY GENERATED CHIRAL PARTNER OF THE PION

The quark-level linear σ model (LσM) is revisited, in particular concerning the identification of the f0(400–1200) (or σ(600)) scalar meson as the chiral partner of the pion. We demonstrate the predictive power of the LσM through the ππ and πNs-wave scattering lengths, as well as several electromagnetic, weak, and strong decays of pseudoscalar and vector mesons. The ease with which the data for these observables are reproduced in the LσM lends credit to the necessity to include the σ as a fundamental [Formula: see text] degree of freedom, to be contrasted with approaches like chiral perturbation theory or the confining NJL model of Shakin and Wang.

Vector manifestation in hot matter

Based on the hidden local symmetry (HLS) Lagrangian as an effective field theory of QCD, we find that the chiral symmetry restoration for hot QCD can be realized through the vector manifestation where the ρ meson becomes massless degenerate with π as the chiral partner. This is done by including, in addition to the hadronic thermal effects due to the π- and ρ-loops, the intrinsic temperature dependences of the parameters of the HLS Lagrangian through the matching of the HLS with the underlying QCD.

Mixing among light scalar mesons and L = 1 qq̄ scalar mesons

Following the re-establishment of the σ(500) and the κ(900), the light scalar mesons a0(980) and f0(980) together with the σ(500) and the κ(900) are considered as the chiral scalar partner of pseudoscalar nonet in SU(3) chiral symmetry, and the high mass scalar mesons a0(1450), K*0(1430), f0(1370) and f0(1710) turned out to be considered as the L = 1 qq̄ scalar mesons. We assume that the high mass of the L = 1 qq̄ scalar mesons is caused by the mixing with the light scalar mesons. For the structure of the light scalar mesons, we adopted the qqq̄q̄ model in order to explain the ‘scalar meson puzzle’. The inter-mixing between the light scalar nonet and the high mass L = 1 qq̄ nonet and the intra-mixing among each nonet are analysed by including the glueball into the high mass scalar nonet.

Baryon resonances from a novel fat-link fermion action

We present first results for masses of positive and negative parity excited baryons in lattice QCD using an O ( a 2) improved gluon action and a Fat Link Irrelevant Clover (FLIC) fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier calculations of N∗ resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner, even for the Wilson fermion action. The results also indicate a splitting between the lowest J P = 1 2 − states for the two standard nucleon interpolating fields.

Properties of Chiral Scalar and Axial-Vector Mesons in Heavy-Light Quark Systems

We investigate the properties of new scalar X_B, X_D and axial-vector X_B*, X_D* mesons, which are recently predicted, in the covariant meson-classification scheme, to exist as the partners of ground-state pseudoscalar B, D and vector B*, D* mesons, respectively, realizing a linear representation of chiral symmetry for the light quark component in heavy-light-quark meson system. The mass splittings between the respective chiral partners are shown to be equal, and the decay widths of one pion emission of X_B, X_D, X_B* and X_D* become to have a common value, due to chiral and heavy quark symmetry.

Candidate chiral band in [formula omitted]La

The nucleus 134 La has been produced using the 124 Sn( 15 N, 5n) reaction. High-spin states have been observed to a maximum spin of 24 + using the CAESAR HPGe array. Bands have been observed that are suggested to be based on the πh 11/2⊗νh 11/2 , πh 11/2⊗νh 11/2 3 , πh 11/2⊗ νg 7/2h 11/2 2 , and πg 7/2⊗νg 7/2h 11/2 2 configurations. A band fragment, with features similar to the chiral band of 134 Pr, is proposed as the chiral partner of the πh 11/2⊗νh 11/2 band.

Vector manifestation of chiral symmetry.

We propose "vector manifestation" (VM) of the Wigner realization of chiral symmetry in which the symmetry is restored at the critical point by the massless degenerate pion (and its flavor partners) and the rho meson (and its flavor partners) as the chiral partner, in sharp contrast to the traditional manifestation á la the linear sigma model where the symmetry is restored by the degenerate pion and the scalar meson. The application to the chiral phase transition of large N(f) QCD is performed using the hidden local symmetry Lagrangian. Combined with the Wilsonian matching proposed recently, VM determines the critical number of massless flavors N(f) approximately equal to 5 without much ambiguity.

Application of an effective gauge-invariant model to nuclear matter inthe relativistic Hartree-Fock approximation

An effective nuclear model describing ω-, ρ- and axial-mesonsas gauge fields is applied to nuclear matter in the relativisticHartree-Fock approximation. The isoscalar two-pion exchange issimulated by a scalar field s similar to that used in the conventionalrelativistic mean-field approach. Two more scalar fields are essentialingredients of the present treatment:the σ-field, the chiral partner of the pion, and the Σ-field,the Higgs field for the ω-meson. Two versions of the model areused depending on whether the σ-field is considered as adynamical variable or `frozen', by taking its mass as infinite. The modelcontains four free parameters in the first case and three in the second onewhich are fitted to the nuclear matter saturation conditions.The nucleon and meson effective masses, compressibility modulus andsymmetry energy are calculated. The results prove the reliability of theDirac-Hartree-Fock approach within the linear realization of the chiralsymmetry.

Chiral unitary approach to the [formula omitted] , [formula omitted] couplings for the [formula omitted] resonance

Using a chiral unitary model in which the negative parity nucleon resonance N ∗≡N ∗(1535) is generated dynamically by means of the Bethe–Salpeter equation with coupled meson–baryon channels in the S=0 sector, we have obtained the π 0N ∗N ∗ and ηN ∗N ∗ couplings. The π 0N ∗N ∗ coupling has smaller strength but the same sign as the π 0NN coupling. This rules out the mirror assignment of chiral symmetry where the ground state nucleon N and the negative parity resonance N ∗ are envisaged as chiral partners in the baryon sector.

Chiral symmetry and properties of hadron correlators in matter

The constraints imposed by chiral symmetry on hadron correlation functions in nuclear medium are discussed. It is shown that these constraints imply a certain structure for the in-medium hadron correlators and lead to the cancelation of the order ρm π term in the in-medium nucleon correlator. We also consider the effect of mixing of the chiral partners correlation functions arising from the interaction of nuclear pions with corresponding interpolating currents. It reflects the phenomena of partial restoration of chiral symmetry. The different scenarios of such restoration are briefly discussed.

Comparing the broken $U3 \times U3$ linear sigma model with experiment

The linear \(\sigma\) model with broken \(U3 \times U3\) is compared with data of the lightest scalar and pseudoscalar mesons. When 5 of the 6 parameters are fixed by the pseudoscalar masses and decay constants, one finds that, already at the tree level, one has a reasonable description for the 4 scalar masses, describing mixing and up to 8 trilinear couplings of lightest scalars, taken as \(a_0(980), f_0(980), \sigma(\approx 500)\) and \(K^*_0(1430)\). This clearly indicates that these scalars are the chiral partners of the \(\pi, \eta, \eta ', K\), and this strongly suggests that they, like the latter, are (unitarized) \(q \bar q\) states.

Dilepton and/or photon production in heavy ion collisions and the QCD phase transition

We study the electromagnetic production from highly excited hadronic matter created in heavy ion collisions. The rates include the usual lowest order processes in quark-gluon plasma plus the usual reactions in the hadronic phase, related with $\ensuremath{\rho}{,a}_{1}$ mesons. The space-time integration is done using a hydrodynamical model. Conventional $\overline{q}q$ $({\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})$ annihilation in quark-gluon plasma and hadronic phase cannot explain the observed dilepton spectrum, especially that by the CERES experiment at CERN. A decreased $\ensuremath{\rho}$ mass can account for the observed effect, provided it shifts into the region of 0.4--0.5 GeV near the phase transition. In order to test this hypothesis one should also look at the chiral partner of $\ensuremath{\rho}$, the axial ${a}_{1}$ meson: its mass must then behave similarly. Its decay ${a}_{1}\ensuremath{\rightarrow}\ensuremath{\pi}{e}^{+}{e}^{\ensuremath{-}}$ populates the low mass region seen in the same experiment. The results for direct photon production are below the current WA80 experimental bounds, for all variants considered.

Excess of pions with chiral symmetry restoration

We study the effect of the chiral phase transition on pion production in hot hadronic matter. The phase of restored chiral symmetry is characterized by the appearance of the scalar σ-meson as a chiral partner of the pion as well as by the degeneracy of the vector and axial-vector mesons. We find rapid thermal and chemical equilibration of these degrees of freedom in the symmetric phase. Provided that the chiral transition temperature is not considerably high, the presence of a chirally symmetric phase will result in ∼ 1.6 times more thermal pions in the final state.

Analysis of chiral mean-field models for nuclei

An analysis of nuclear properties based on a relativistic energy functional containing Dirac nucleons and classical scalar and vector meson fields is discussed. Density functional theory implies that this energy functional can include many-body effects that go beyond the simple Hartree approximation. Using basic ideas from effective field theory, a systematic truncation scheme is developed for the energy functional, which is based on an expansion in powers of the meson fields and their gradients. Chiral models are analyzed by considering specific lagrangians that realize the spontaneously broken chiral symmetry of QCD in different ways and by studying them at the Hartree level. Models that include a light scalar meson playing a dual role as the chiral partner of the pion and the mediator of the intermediate-range nucleon-nucleon interaction, and which include a "Mexican-hat" potential, fail to reproduce basic ground-state properties of nuclei. In contrast, chiral models with a nonlinear realization of the symmetry are shown to contain the full flexibility inherent in the general energy functional and can therefore successfully describe nuclei.

Generalized Nambu-Jona-Lasinio model in a study of the boson-exchange model of nuclear forces.

In this paper we discuss the role of chiral symmetry in the description of the nucleon-nucleon interaction. We make use of a generalized Nambu---Jona-Lasinio model which exhibits chiral symmetry at the quark level and demonstrate how the various components of the boson-exchange model of the nucleon-nucleon interaction arise in this model. In this paper we make use of our recent analysis of correlated two-pion exchange. We have found that when correlated two-pion exchange is represented by an effective low-mass sigma meson, that meson may be seen to be the chiral partner of the pion. (That result pertains if the meson momentum $q$ is spacelike such that $t={q}^{2}<0$.) We give particular attention to the vertex functions appearing in the boson-exchange model. For monopole forms of the vertex functions the cutoff parameter ${\ensuremath{\Lambda}}^{\mathrm{OBE}}$ varies from about 1.3 GeV to 2.0 GeV. In our model the nucleon form factors are characterized by a parameter $\ensuremath{\lambda}$. We show how large cutoff parameters, such as ${\ensuremath{\Lambda}}^{\mathrm{OBE}}$, can arise in our analysis even if the meson-nucleon vertex is soft, with a characteristic cutoff of about 800 MeV. We present the values of $\ensuremath{\lambda}$ that puts the NJL analysis in good agreement with the boson-exchange model of the nucleon-nucleon force.

One spin trace formalism for BK.

It has been known for some time that there are two methods to calculate $ B_K $ with staggered fermions: one is the two spin trace formalism and the other is the one spin trace formalism. Until now, the two spin trace formalism has been exclusively used for weak matrix element calculations with staggered fermions. Here, the one spin trace formalism to calculate $ B_K $ with staggered fermions is explained. It is shown that the one spin trace operators require additional chiral partner operators in order to keep the continuum chiral behavior. The renormalization of the one spin trace operators is described and compared with the two spin trace formalism.