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Register a new userFractional Charge and Confinement of Quarks
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In quantum chromodynamics with static quarks the confinement-deconfinement phase transition is connected to the spontaneous breaking of the global Z3 center symmetry. This symmetry is lost when one considers dynamical quarks. Owing to the fractional electric charge of quarks, we recover a global Z6 center symmetry when QCD is regarded as a part of the Standard Model. We present results from QCD-like theories extended by electromagnetic interactions and show that the weak coupling limit of the QED part of the model results in a center-like symmetry with disorder in the vacuum. This can be seen explicitly in a character expansion of the fermion determinant. Further, we show that corresponding center averages project the fermion determinant on N-ality zero and discuss whether the additional center symmetry can be used to eliminate the fermion sign problem in QCD with fundamental quarks.
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Above the pseudocritical temperature T_c of chiral symmetry restoration a chiral spin symmetry (a symmetry of the color charge and of electric confinement) emerges in QCD. This implies that QCD is in a confining mode and there are no free quarks. At the same time correlators of operators constrained by a conserved current behave as if quarks were free. This explains observed fluctuations of conserved charges and the absence of the rho-like structures seen via dileptons. An independent evidence that one is in a confining mode is very welcome. Here we suggest a new tool how to distinguish free quarks from a confining mode. If we put the system into a finite box, then if the quarks are free one necessarily obtains a remarkable diffractive pattern in the propagator of a conserved current. This pattern is clearly seen in a lattice calculation in a finite box and it vanishes in the infinite volume limit as well as in the continuum. In contrast, the full QCD calculations in a finite box show the absence of the diffractive pattern implying that the quarks are confined.
In this work, we first use Thompsons renormalization group method to treat QCD-vacuum behavior close to the regime of asymptotic freedom. QCD-vacuum behaves effectively like a paramagnetic system of a classical theory in the sense that virtual color charges (gluons) emerge in it as spin effect of a paramagnetic material when a magnetic field aligns their microscopic magnetic dipoles. Making a classical analogy with the paramagnetism of Landaus theory,we are able to introduce a kind of Landau effective action without temperature and phase transition for simply representing QCD-vacuum behavior at higher energies as magnetization of a paramagnetic material in the presence of a magnetic field H. This reasoning allows us to use Thompsons heuristic approach in order to extract an effective susceptibility ($chi>0$) of QCD-vacuum. It depends on logarithmic of energy scale u to investigate hadronic matter. Consequently,we are able to get an effective magnetic permeability ($mu>1$) of such a paramagnetic vacuum. As QCD-vacuum must obey Lorentz invariance,the attainment of $mu>1$ must simply require that the effective electrical permissivity is $epsilon<1$,in such a way that $muepsilon=1$ (c^2=1).This leads to the antiscreening effect, where the asymptotic freedom takes place. On the other hand, quarks cofinement, a subject which is not treatable by perturbative calculations, is worked by the present approach. We apply the method to study this subject in order to obtain the string constant, which is in agreement with the experiments.
A natural explanation of confinement can be given in terms of symmetry. Since color symmetry is exact, the candidate symmetry is dual and related to homotopy,i.e., in (3+1)d, to magnetic charge conservation. A set of r abelian tHooft-like tensors (r = rank of the gauge group) can be defined and the dual charge is a violation of the corresponding Bianchi identities. It is shown that this is equivalently described by non-abelian Bianchi identities.
We report a comprehensive analysis of the light and strange disconnected-sea quarks contribution to the nucleon magnetic moment, charge radius, and the electric and magnetic form factors. The lattice QCD calculation includes ensembles across several lattice volumes and lattice spacings with one of the ensembles at the physical pion mass. We adopt a model-independent extrapolation of the nucleon magnetic moment and the charge radius. We have performed a simultaneous chiral, infinite volume, and continuum extrapolation in a global fit to calculate results in the continuum limit. We find that the combined light and strange disconnected-sea quarks contribution to the nucleon magnetic moment is $mu_M,(text{DI})=-0.022(11)(09),mu_N$ and to the nucleon mean square charge radius is $langle r^2rangle_E,text{(DI)}=-0.019(05)(05)$ fm$^2$ which is about $1/3$ of the difference between the $langle r_p^2rangle_E$ of electron-proton scattering and that of muonic atom and so cannot be ignored in obtaining the proton charge radius in the lattice QCD calculation. The most important outcome of this lattice QCD calculation is that while the combined light-sea and strange quarks contribution to the nucleon magnetic moment is small at about $1%$, a negative $2.5(9)%$ contribution to the proton mean square charge radius and a relatively larger positive $16.3(6.1)%$ contribution to the neutron mean square charge radius come from the sea quarks in the nucleon. For the first time, by performing global fits, we also give predictions of the light and strange disconnected-sea quarks contributions to the nucleon electric and magnetic form factors at the physical point and in the continuum and infinite volume limits in the momentum transfer range of $0leq Q^2leq 0.5$ GeV$^2$.
We present SU(3) gluon propagators calculated on 48*48*48*N_t lattices at beta=6.8 where N_t=64 (corresponding the confinement phase) and N_t=16 (deconfinement) with the bare gauge parameter,alpha, set to be 0.1. In order to avoid Gribov copies, we employ the stochastic gauge fixing algorithm. Gluon propagators show quite different behavior from those of massless gauge fields: (1) In the confinement phase, G(t) shows massless behavior at small and large t, while around 5<t<15 it behaves as massive particle, and (2) effective mass observed in G(z) becomes larger as z increases. (3) In the deconfinement phase, G(z) shows also massive behavior but effective mass is less than in the confinement case. In all cases, slope masses are increasing functions of t or z, which can not be understood as addtional physical poles.
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