The Pion Electromagnetic Form Factor in a QCD-Inspired Model

We present detailed numerical results for the pion space-like electromagnetic form factor obtained within a recently proposed model of the pion electromagnetic current in a confining light-front QCD-inspired model. The model incorporates the vector meson dominance mechanism at the quark level, where the dressed photon with q⁺ > 0 decays in an interacting quark-antiquark pair, which absorbs the initial pion and produces the pion in the final state.     

格点QCD中有限体积下形状因子的内插计算

在格点QCD的框架下,讨论了计算强子的形状因子的过程中,有限体积效应对所得结果的影响,并且给出了计算连续动量空间上形状因子数值的内插算法.本文以π介子的形状因子为例,介绍了从格点QCD中得到的三点关联函数出发计算π介子的形状因子的方法,并说明有限体积效应使得计算结果只能在分立动量上给出.本文探讨了一种保持旋转对称性的内...     

Quark-Number Susceptibility at Finite Chemical Potential and Zero Temperature

We give a direct method for calculating the quark-number susceptibility at finite chemical potential and zero temperature. In this approach the quark-number susceptibility is totally determined by G[μ]（p） （the dressed quark propagator at finite chemical potential μ）. By applying the general result in our previous study [Phys. Rev. C 71 （2005） 015205, 034901, 73 （2006） 016004 ] G[μ]（p） is calculated from the model quark propagator proposed by Pagels and Stokar [Phys. Rev. D 20 （1979） 2947]. The full analytic expression of the quark-number susceptibility at finite μ and zero T is obtained.     

Calculation on U(1) Lattice Gauge Theory at Finite Temperature

The order parameter for the U(1) lattice gauge model at finite temperature with N_{τ=1 is calculated by using the variational-cumulant expansion method.The result is in agreement with the Monte Carlo simulations.The specific simplification in the case of Nτ=1 is discussed.The relationship between the variational method and the accumulation point method for determing the parameters introduced into the cumulant expansion is clarified.}     

An Improved Perturbative Expression for the Pion Form Factor with Sudakov Suppression

We present an improved perturbative expression for the pion form factor with Sudakov suppression, in which the running coupling constant is suggested to be frozen at α_s, (T>) and the intrinsic transverse momentum dependence associated with pion wavefunction is re-analysed. This expression is available for momentum transfer of the order of a few GeV as wed as for Q→∞. Our scheme can be extended to evaluate contributions to the pion form factor beyond the leading order.     

Pion Distribution Amplitude from the Lattice

Results for the second moment of the pion distribution amplitude in quenched QCD are reported, and compared with other QCD-based predictions as well as experimental determinations. A broad agreement is found, which calls for a more precise lattice computation.     

The Cavity Method at Zero Temperature

In this note we explain the use of the cavity method directly at zero temperature, in the case of the spin glass on a lattice with a local tree like structure, which is the proper generalization of the usual Bethe lattice to frustrated problems. The computation is done explicitly in the formalism equivalent to one step replica symmetry breaking; we compute the energy of the global ground state, as well as the complexity of equilibrium states at a given energy. Full results are presented for a Bethe lattice with connectivity equal to three. The main assumptions underlying the one step cavity approach, namely the existence of many local ground states, are explicitely stated and discussed: some of the main obstacles towards a rigorous study of the problem with the cavity method are outlined.     

Chaotic Temperature Dependence at Zero Temperature

We present a class of examples of nearest-neighbour, bounded-spin models, in which the low-temperature Gibbs measures do not converge as the temperature is lowered to zero, in any dimension.     

Generalized Parton Distributions of the Pion on the Transverse Lattice

The quark-generalized parton distributions of the pion are calculated from light-cone wavefunctions in transverse lattice gauge theory at large N_c. The pion effective size is found to decrease with increasing momentum transfer. An analytic ansatz, consistent with finite bound-state light-cone energy conditions, is given for the light-cone momentum dependence of the wavefunctions. This leads to simple, universal predictions for the behaviour of the distributions near the endpoints, complementing numerical DLCQ data.     

有限温度阿贝尔格点规范理论相变的解析研究

应用交分累积展开方法系统地研究了（２＋１）维和（３＋１）维的Ｚ（２）、Ｚ（３）、Ｕ（１）阿贝尔格点规范理论在有限温度下的相交．所得结果同已有的严格结果及Ｓｖｅｔｉｔｓｋｙ和Ｙａｆｆ的理论预言相符合．     

Activation-Like Processes at Zero Temperature

We examine the possibility that a metastable quantum state could experiment a phenomenon similar to thermal activation but at zero temperature. To do that we study the real-time dynamics of the reduced Wigner function in a simple open quantum system: an anharmonic oscillator with a cubic potential linearly interacting with an environment of harmonic oscillators. Our results suggest that this activation-like phenomenon exists indeed as a consequence of the fluctuations induced by the environment and that its associated decay rate is comparable to the tunneling rate as computed by the instanton method, at least for the particular potential of the system and the distribution of frequencies for the environment considered in this paper. However, we are not able to properly deal with the term which leads to tunneling in closed quantum systems, and a definite conclusion cannot be reached until tunneling and activation-like effects are considered simultaneously.     

The Three-State Square-Lattice Potts Antiferromagnet at Zero Temperature

We study the 3-state square-lattice Potts antiferromagnet at zero temperature by a Monte Carlo simulation using the Wang–Swendsen–Kotecký cluster algorithm, on lattices up to 1024 × 1024. We confirm the critical exponents predicted by Burton and Henley based on the height representation of this model.     

Distribution of Avalanche Sizes in the Hysteretic Response of the Random-Field Ising Model on a Bethe Lattice at Zero Temperature

We consider the zero-temperature single-spin-flip dynamics of the random-field Ising model on a Bethe lattice in the presence of an external field h. We derive the exact self-consistent equations to determine the distribution Prob(s) of avalanche sizes s as the external field increases from – to . We solve these equations explicitly for a rectangular distribution of the random fields for a linear chain and the Bethe lattice of coordination number z=3, and show that in these cases, Prob(s) decreases exponentially with s for large s for all h on the hysteresis loop. We find that for z4 and for small disorder, the magnetization shows a first-order discontinuity for several continuous and unimodal distributions of the random fields. The avalanche distribution Prob(s) varies as s ^–3/2 for large s near the discontinuity.     

Transfer Matrices and Lattice Fermions at Finite Density

I discuss the connection between the Hamiltonian and path integral approaches for fermionic fields. I show how the temporal Wilson projection operators appear naturally in a lattice action. I also carefully treat the insertion of a chemical potential term.     

Vector Meson Mass in Finite Density and Temperature Lattice QCD

Vector meson mass values are studied at finite chemical potential and temperature in lattice QCD with lattice size of 24 × 122× 6 using two flavors of staggered quarks. The investigation focuses on the change of the vector meson mass in the critical region close to T c with two different types of chemical potentials switched on: the isoscalar chemical potential μS and its isovector counterpart μV. It is found that the vector meson mass increases in the QGP phase with both chemical potentials and decreases with μS in the confinement phase.     

A Model Study of Equation of State of QCD at Finite Chemical Potential and Zero Temperature

In this paper, we give a direct method for calculating the partition function, and hence the equation of state （EOS） of QCD at finite chemical potential and zero temperature. In the EOS derived in this paper the pressure density is the sum of two terms： the first term P（μ）｜μ=0 （the pressure density at μ = 0） is a μ-independent constant; the second term, which is totally determined by G[μ] （p） （the dressed quark propagator at finite μ）, contains all the nontrivial μ-dependence. By applying a general result in the rainbow-ladder approximation of the Dyson-Schwinger approach obtained in our previous study [Phys. Rev. C 71 （2005） 015205], G[μ]（p） is calculated from the meromorphic quark propagator proposed in [Phys. Rev. D 67 （2003） 054019]. From this the full analytic expression of the EOS of QCD at finite μ and zero T is obtained （apart from the constant term P（μ）｜μ=0, which can in principle be caJculated from the CJT effective action）. A comparison between our EOS and the cold, perturbative EOS of QCD of Fraga, Pisarski and Schaffner-Bielich is made. It is expected that our EOS can provide a possible new approach for the study of neutron stars.     

The Temperature Zero Limit

We prove that, for a broad class of many-fermion models, the amplitudes of renormalized Feynman diagrams converge to their temperature zero values in the limit as the temperature tends to zero.     

Quantum Lattice Models at Intermediate Temperature

We analyze the free energy and construct the Gibbs-KMS states for a class of quantum lattice systems, at low temperature and when the interactions are almost diagonal, in a suitable basis. The models we study may have continuous symmetries, our results, however, apply to intermediate temperatures where discrete symmetries are broken but continuous symmetries are not. Our results are based on quantum Pirogov–Sinai theory and a combination of high and low temperature expansions. Received: 6 December 2000 / Accepted: 18 July 2001     

Investigation of U(1) Lattice Gauge Model at Finite Temperature to Higher Order Expansion

The cumulant expansion of U(1) lattice gauge model at finite temperature (N_T=1) to the sixth order is presented. The variational parameters are determined by requiring thatthe corrections to the zeroth order approximation are the smallest. The order parameter,Polyakov line , converges well and is in good agreement with Monte Carlo simulations.Especially, the deconfinement phase transition point can be determined with accuracy.