arXiv subjects

L. Ya. Glozman

Publications and source records attributed to L. Ya. Glozman.

At least 55 records · Page 3Linked to original sources

arXiv2015

Evidence for a new $SU(4)$ symmetry with $J=2$ mesons

Recently, a new symmetry of mesons has been found upon truncation of the quasi-zero modes of the Overlap Dirac operator in lattice simulations. Namely, the $ρ,ρ',ω,ω',a_1, b_1,h_1$ and possibly $f_1$ $J=1$ mesons get degenerate after removal of the quasi-zero modes. This emergent symmetry has been established to be $SU(4)\supset SU(2)_L \times SU(2)_R \times U(1)_A$. It is higher than the symmetry of the QCD Lagrangian and provides not only a mixing of quarks of given chirality in the isospin space, but also the mixing of left-handed and right-handed components. Here we study, with the Overlap Dirac operator, the isovector $J=2$ mesons upon the quasi-zero mode reduction and observe a similar degeneracy. This result further supports the $SU(4)$ symmetry in mesons of given spin $J \geq 1$.

hep-lat↗

arXiv2015

SU(4) symmetry of the dynamical QCD string and genesis of hadron spectra

A large degeneracy of mesons of a given spin has recently been discovered upon reduction of the quasi-zero modes of the Dirac operator in a dynamical lattice simulation. Here a symmetry group that is responsible for this degeneracy is established, which is SU(4) \supset SU(2)_L \times SU(2)_R \times U(1)_A \times C_i. It is argued that this symmetry group is a symmetry of the dynamical QCD string. Implications of this picture for a genesis of light hadron spectra are discussed.

hep-ph↗

arXiv2015

Isoscalar mesons upon unbreaking of chiral symmetry

In a dynamical lattice simulation with the overlap Dirac operator and $N_f=2$ mass degenerate quarks we study all possible $J=0$ and $J=1$ correlators upon exclusion of the low lying "quasi-zero" modes from the valence quark propagators. After subtraction of a small amount of such Dirac eigenmodes all disconnected contributions vanish and all possible point-to-point $J=0$ correlators with different quantum numbers become identical, signaling a restoration of the $SU(2)_L \times SU(2)_R \times U(1)_A$. The original ground state of the $π$ meson does not survive this truncation, however. In contrast, in the $I=0$ and $I=1$ channels for the $J=1$ correlators the ground states have a very clean exponential decay. All possible chiral multiplets for the $J=1$ mesons become degenerate, indicating a restoration of an $SU(4)$ symmetry of the dynamical QCD-like string.

hep-lat↗

arXiv2014

Mesons upon low-lying Dirac mode exclusion

We study the isoscalar and isovector $J=0,1$ mesons with the overlap operator within two flavour lattice QCD. After subtraction of the lowest-lying Dirac eigenmodes from the valence quark propagator all disconnected contributions vanish and all possible point-to-point $J=0$ correlators become identical, signaling a simultaneous restoration of both $SU(2)_L \times SU(2)_R$ and $U(1)_A$ symmetries. The ground states of the $π,σ,a_0,η$ mesons do not survive this truncation. All possible $J=1$ states have a very clean exponential decay and become degenerate, demonstrating a $SU(4)$ symmetry of a dynamical QCD-like string.

hep-lat↗

arXiv2014

Observation of a dynamical QCD string

Mesons constructed from the quark propagators without the lowest-lying eigenmodes of the Dirac operator reveal not only restored SU(2)_L * SU(2)_R chiral and U(1)_A symmetries, but actually a higher symmetry. All possible chiral and U(1)_A multiplets for the states of the same spin are degenerate, i.e., the energy of the observed quantum levels does not depend on the spin orientation of quarks in the system and their parities. The quark-spin independence of the energy levels implies absence of the magnetic interactions in the system. The ultrarelativistic quark-antiquark system with only the color-electric interactions can be interpreted (or defined) as a dynamical QCD string.

hep-lat↗

arXiv2014

Symmetries of mesons after unbreaking of chiral symmetry and their string interpretation

Using the chirally invariant overlap Dirac operator we remove its lowest-lying quasizero modes from the valence quark propagators and study evolution of isovector mesons with J=1. At the truncation level about 50 MeV SU(2)_L \times SU(2)_R and U(1)_A symmetries get restored. However, we observe a degeneracy not only within the chiral and U(1)_A multiplets, but also a degeneracy of all possible chiral multiplets, i.e., the observed quantum levels have a symmetry larger than U(2)_L \times U(2)_R and their energy does not depend on the spin orientation of quarks and their parities. We offer a possible interpretation of these energy levels as the quantum levels of the dynamical QCD string. The structure of the radial J=1 spectrum is compatible with E =(n_r +1)\hbarωwith \hbarω= 900 \pm 70 MeV.

hep-lat↗

arXiv2014

Chiral symmetry breaking and the generation of light hadron masses

A quantitative study of the effects of dynamical chiral symmetry breaking on the mass generation of the low-lying hadrons in a dynamical lattice QCD simulation is presented. The evolution of light hadron masses upon increasing the number of low-lying Dirac operator eigenmodes in the quark propagators is confronted with the hadron masses obtained upon removal of such low-lying eigenmodes from standard full quark propagators. The low-lying chiral symmetry breaking modes provide roughly two third of the nucleon and $ρ$ masses, while the $a_1$ mass is affected to a much smaller degree.

hep-lat↗

arXiv2013

Effects of Low vs. High Fermionic Modes on Hadron Mass Generation

A nonvanishing spectral density of the low-lying eigenmodes of the Dirac operator naturally is a signal for dynamical chiral symmetry breaking (D$χ$SB) via the Banks-Casher relation. The low-lying eigenmodes alone saturate the pseudoscalar channel and the corresponding propagator successfully reproduces the pion mass. In this paper we investigate the effects on the mass generation of hadrons other than pions. The evolution of these masses upon inclusion of an increasing number of the low-lying eigenmodes is confronted with the hadron mass spectrum upon removal of such eigenmodes.

hep-lat↗

arXiv2013

More effects of Dirac low-mode removal

In previous studies we have shown that hadrons, except for a pion, survive the removal of the lowest lying Dirac eigenmodes from the valence quark propagators. The low-modes are tied to the dynamical breaking of chiral symmetry and we found chiral symmetry to be restored by means of matching masses of chiral partners, like, e.g., the vector and axial vector currents. Here we investigate the influence of removing the lowest part of the Dirac spectrum on the locality of the Dirac operator. Moreover, we analyze the influence of low-mode truncation on the quark momenta and thereupon on the hadron spectrum and, finally, introduce a reweighting scheme to extend the truncation to the sea quark sector.

hep-lat↗

arXiv2013

Effect of Transverse Gluons on Chiral Restoration in Excited Mesons

The effect of transverse gluons on the chiral symmetry patterns of excited mesons is studied in a Coulomb gauge QCD model. The linear rising static quark-antiquark potential and the transverse gluon propagator known from lattice studies are input into the model. The non-perturbative quark propagator, which enters the meson bound state equations, is derived from the Dyson--Schwinger equations and a complete set of mesons for general spin quantum number is presented. From analyzing the bound state equations for large spins it is demonstrated, that chiral and axial symmetry are restored. In this limit a complete degeneracy of all multiplets with given spin is observed. The effect of the transverse gluon interaction is shown to vanish rapidly as the spin quantum number is increased. For vanishing dynamical quark mass the expected meson degeneracies are recovered.

hep-ph↗

arXiv2012

Confinement, chiral symmetry breaking and the mass generation of hadrons

A key question to QCD is what mechanism generates the hadron mass in the light quark sector, where both confinement and chiral symmetry breaking are in the game. Are confinement and chiral symmetry breaking in the vacuum uniquely interconnected? Can hadrons survive chiral symmetry restoration? If yes, what happens with their mass and what symmetries beyond the chiral symmetry are there? We review our recent insights. In particular, in a dynamical lattice simulation we artificially restore chiral symmetry by removing the low-lying Dirac modes of the valence quark propagators, which is a well defined procedure and keep gluodynamics intact. Hadrons survive this artificial chiral restoration and their mass is surprisingly large. All hadrons fall into chiral multiplets and some of them are degenerate, i.e. the spectrum reveals some higher symmetry, that includes the chiral symmetry as a subgroup. The U(1)_A symmetry does not get restored after removal of the chiral modes from the valence quarks.

hep-ph↗

arXiv2012

Effects of the low lying Dirac modes on excited hadrons in lattice QCD

Chiral symmetry breaking in Quantum Chromodynamics is associated with the low lying spectral modes of the Dirac operator according to the Banks-Casher relation. Here we study how removal of a variable number of low lying modes from the valence quark sector affects the masses of the ground states and first excited states of baryons and mesons in two flavor lattice QCD.

hep-lat↗

arXiv2012

Symmetries of hadrons after unbreaking the chiral symmetry

We study hadron correlators upon artificial restoration of the spontaneously broken chiral symmetry. In a dynamical lattice simulation we remove the lowest lying eigenmodes of the Dirac operator from the valence quark propagators and study evolution of the hadron masses obtained. All mesons and baryons in our study, except for a pion, survive unbreaking the chiral symmetry and their exponential decay signals become essentially better. From the analysis of the observed spectroscopic patterns we conclude that confinement still persists while the chiral symmetry is restored. All hadrons fall into different chiral multiplets. The broken U(1)_A symmetry does not get restored upon unbreaking the chiral symmetry. We also observe signals of some higher symmetry that includes chiral symmetry as a subgroup. Finally, from comparison of the Δ- N splitting before and after unbreaking of the chiral symmetry we conclude that both the color-magnetic and the flavor-spin quark-quark interactions are of equal importance.

hep-lat↗

arXiv2012

How Chiral Symmetry Breaking Affects the Spectrum of the Light-Heavy Mesons in the 't Hooft Model

We find the spectrum and wave functions of the heavy-light mesons in $(1 + 1)$-dimensional QCD in the 't Hooft limit, both in the rest frame, using the Coulomb (axial) gauge, and on the light cone. Our emphasis is on the effects of chiral symmetry breaking on the spectrum. While dynamical equations in both cases look different, the results for the spectrum are identical. The chiral symmetry breaking is clearly seen from the gap and Bethe--Salpeter equations in the laboratory frame. At the same time, while vacuum is trivial on the light cone (no chiral condensate), the effects of the spontaneous breaking of the chiral symmetry manifest themselves in the same way, as it follows from the coincidence of the spectra obtained from the laboratory-frame Bethe--Salpeter equation on the one hand, and the light-cone 't Hooft-type equation on the other.

hep-th↗

arXiv2012

Axial charges of excited nucleons from CI-fermions

We report lattice QCD results on the axial charges of ground and excited nucleon states of both parities. This is the first study of these quantities with approximately chiral (CI) fermions. Two energy levels in the range of the negative parity resonances N*(1535) and N*(1650) are observed and we determine the axial charge for both. We obtain a small axial charge for one of them, which is consistent with the chiral symmetry restoration in this state as well as with the small axial charge of the N*(1535) predicted within the quark model. This result agrees with the findings of Takahashi et al. obtained with Wilson quarks which violate chiral symmetry for finite lattice spacing. At the same time for the other observed negative parity state we obtain a large axial charge, that is close to the axial charge of the nucleon. This is in disagreement both with the quark model prediction as well as with the chiral restoration but allows for an interpretation as an s-wave π N state.

hep-lat↗

arXiv2011

Chiral symmetry breaking and the spin content of hadrons

From the parton distributions in the infinite momentum frame one finds that only about 30% of the nucleon spin is carried by spins of the valence quarks, which gave rise to the term "spin crisis". Similar results hold for the lowest mesons, as it follows from the lattice simulations. We define the spin content of a meson in the rest frame and use a complete and orthogonal $\bar q q$ chiral basis and a unitary transformation from the chiral basis to the (2S+1)LJ basis. Then, given a mixture of different allowed chiral representations in the meson wave function at a given resolution scale, one can obtain its spin content at this scale. To obtain the mixture of the chiral representations in the meson we measure in dynamical lattice simulations a ratio of couplings of interpolarors with different chiral structure. For the rho meson we obtain practically the 3S1 state with no trace of the spin crisis. Then a natural question arises: which definition does reflect the spin content of a hadron?

hep-ph↗

arXiv2011

Chiral restoration phase transition within the quarkyonic matter

We overview a possible mechanism for confining but chirally symmetric matter at low temperatures and large densities. As a new development we employ a diffused quark Fermi surface and show that such diffusion does not destroy possible existence of a confining but chirally symmetric matter at low temperatures and large density.

hep-ph↗

arXiv2011

Chirally symmetric and confining dense matter with a diffused quark Fermi surface

It is possible that at low temperatures and large density there exists a confining matter with restored chiral symmetry, just after the dense nuclear matter with broken chiral symmetry. Such a phase has sofar been studied within a confining and chirally symmetric model assuming a rigid quark Fermi surface. In the confining quarkyonic matter, however, near the Fermi surface quarks group into color-singlet baryons. Interaction between quarks leads to a diffusion of the quark Fermi surface. Here we study effects of such diffusion and verify that it does not destroy a possible existence of a confining but chirally symmetric matter at low temperatures.

hep-ph↗