Evaluation of QCD sum rules for light vector mesons at finite density and temperature

QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of mass parameters for the lightest vector mesons ρ, ω and φ in a strongly interacting medium. For conditions relevant for the starting experiments at HADES we find that the in-medium mass shifts of the ρ- and ω-mesons are governed, within the Borel QCD sum rule approach, by the density and temperature dependence of the four-quark condensate. In particular, the variation of the strength of the density dependence of the four-quark condensate reflects directly the decreasing mass of the ρ-meson and can lead to a change of the sign of the ω-meson mass shift as a function of the density. In contrast, the in-medium mass of the φ-meson is directly related to the chiral strange quark condensate which seems correspondingly accessible. Received: 15 May 2002 / Accepted: 31 July 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: Kaempfer@fz.rossendorf.de Communicated by A. Sch?fer     

Spectra and hadronic couplings of light hermaphrodite mesons

We clarify the discrepancies of previous results for the masses and decay amplitudes of hermaphrodite mesons obtained from QCD sum rules. We study the case of the strange quark within a light quark expansion formalism. We find tht the hermaphrodite masses are much higher than the ones of their ordinary meson partners. Our values of the set of masses and continuum thresholds are compared with some other sum rule results. We analyze the hadronic couplings of the isovector 1^?+ exotic hermaprodite \(\tilde \rho \) using a three-point function sum rule evaluated at the symmetric euclidean point. We find that the \(\tilde \rho \) can be very broad and prefers to decay into ρπ andK ^* K. Its most characteristic decays are the ones into πγ, ηπ and μ′π. The former and the latter are of the order of (3～8) MeV.     

Light quark masses from scalar sum rules

In this work, the mass of the strange quark is calculated from QCD sum rules for the divergence of the strangeness-changing vector current. The phenomenological scalar spectral function which enters the sum rule is determined from our previous work on strangeness-changing scalar form factors [1]. For the running strange mass in the scheme, we find . Making use of this result and the light quark mass ratios obtained from chiral perturbation theory, we are also able to extract the masses of the lighter quarks and . We then obtain and . In addition, we present an updated value for the light quark condensate. Received: 18 October 2001 / Revised version: 22 January 2002 / Published online: 12 April 2002     

Flavour symmetry breaking in quark vacuum condensates

We study SU(3) and SU(2) flavour symmetry violations in the vacuum from the viewpoint of nonperturbative quark mass generation and independently from charge symmetry-breaking considerations. The results for the ratios of quark condensates of different flavours are compatible with those of QCD sum rules. However, we find that very large SU(3) violating effects, suggested by some sum rule analyses, are barely accommodated in the present nonperturbative approach.     

The decay π → eνγ: PCAC quark model

We reexamine the PCAC sum rule for radiative pion decay, allowing for a singular short-distance behaviour of the axial divergence. It is shown that the sum rule of Das, Mathur and Okubo follows from a weak form of PCAC even if the axial divergence is allowed to have dimension three, appropriate for free (or asymptotically free) quarks. We find no quark mass corrections.     

Baryon scattering at high energies: wave function,impact factor,and gluon radiation

The scattering of a baryon consisting of three massive quarks is investigated in the high energy limit of perturbative QCD. A model of a relativistic proton-like wave function, dependent on valence quark longitudinal and transverse momenta and on quark helicities, is proposed, and we derive the baryon impact factors for two, three and four t-channel gluons. We find that the baryonic impact factor can be written as a sum of three pieces: in the first one a subsystem consisting of two of the three quarks behaves very much like the quark–antiquark pair in γ^* scattering, whereas the third quark acts as a spectator. The second term belongs to the odderon, whereas in the third (C-even) piece all three quarks participate in the scattering. This term is new and has no analogue in γ^* scattering. We also study the small x evolution of gluon radiation for each of these three terms. The first term follows the same pattern of gluon radiation as the γ^*-initiated quark–antiquark dipole, and, in particular, it contains the BFKL evolution followed by the 2→4 transition vertex (triple pomeron vertex). The odderon term is described by the standard BKP evolution, and the baryon couples to both known odderon solutions, the Janik–Wosiek solution and the BLV solution. Finally, the t-channel evolution of the third term starts with a three-reggeized gluon state, which then, via a new 3→4 transition vertex, couples to the four-gluon (two-pomeron) state. We briefly discuss a few consequences of these findings, in particular the pattern of unitarization of high energy baryon scattering amplitudes.     

Q2-Dependence of the Bjorken Sum Rule in the Resonance Region

Q²-evolution of the Bjorken sum rule in the resonance region is studied. The roles of the Roper resonance N^* (1440) estimated by the constituent quark model and by the hybrid model are stressed. Moreover, the Q²- dependence of the Gerasimov-Drell-Hearn sum rule contributed by the resonances is also discussed.     

Semileptonic decay of B-meson into D and the Bjorken sum rule

We study the semileptonic branching fraction of the B-meson into the higher resonance of the charmed meson D^** by using the Bjorken sum rule and the heavy quark effective theory (HQET). This sum rule and the current experiment of the B-meson semileptonic decay into D and D^* predict that the branching ratio D^**lν_l is about 1.7%. This predicted value is larger than the value obtained by various models.     

Quark sea structure functions of the nucleon in a statistical model

Within a statistical model of linear confined quarks we obtain the flavor asymmetry and corresponding structure function of the nucleon. The model parameters are fixed by the experimental available data. The temperature parameter is adjusted by the Gottfried sum rule violation and the chemical potentials by the corresponding up (u) and down (d) quark normalizations in the nucleon. The light antiquark and quark distributions in the proton, given by d̄/ū, d/u and d̄-ū, as well as the neutron to proton ratio of the structure functions, extracted from the experimental data, are well fitted by the model. As the quark-confining strengths should be flavor dependent, a mechanism is introduced in the model to adjust the corresponding distribution, in order to improve the comparison obtained for the sea-quark asymmetries in the nucleon with the available experimental analysis. PACS  11.30.Hv; 14.20.Dh; 12.39.Ki; 12.40.Ee; 11.55.Hx     

πNN coupling determined beyond the chiral limit

Within the conventional QCD sum rules, we calculate the πNN coupling constant, g _πN, beyond the chiral limit using two-point correlation function with a pion. For this purpose, we consider the Dirac structure, iγ₅, at m _π ² order in the expansion of the correlator in terms of the pion momentum. For a consistent treatment of the sum rule, we include the linear terms in quark mass as they constitute the same chiral order as m _π ². In this sum rule, we obtain g _πN= 13.3 ± 1.2, which is very close to the empirical πNN coupling. This demonstrates that going beyond the chiral limit is crucial in determining the coupling. Received: 8 July 1999 / Revised version: 20 August 1999     

Phenomenology of the proton spin

We analyze the various inputs that go into computing the recently measured first moment of the proton spin structure functiong ₁ ^p (x). The basic inputs are the various valence and sea quark polarisations and the gluonic contribution coming through axial anomaly. We show that the quark model predictions for valence quark polarisations, suitably modified to accommodate Bjorken sum rule, are consistent with measured value of moment of the spin structure function.     

Ground state baryon magnetic moments and nucleon axial vector coupling

Ground-state baryon magnetic moments and nucleon axial vector coupling are calculated usingqcd inspired configuration mixing and relativistic corrections. Unlike earlier attempts, we incorporate a natural mass scale for quarks, taken as one third the nucleon mass for up and down quarks, and the strange quark mass suggested by the Lipkin’s sum rule. In the parameter-free non-relativistic limit, we find a fairly good fit, which improves upon including relativistic corrections.     

Analysis of the vertices DDV and D<Superscript>*</Superscript>DV with light-cone QCD sum rules

In this article, we study the vertices DDV and D^*DV with the light-cone QCD sum rules. The strong coupling constants g_DDV and f_D*DV play an important role in understanding the final-state re-scattering effects in the hadronic B decays. They are related to the basic parameters β and λ respectively in the heavy quark effective Lagrangian; our numerical values are smaller than the existing estimations. PACS 12.38.Lg; 13.20.Fc     

A Bayesian analysis of the nucleon QCD sum rules

QCD sum rules of the nucleon channel are reanalyzed, using the maximum-entropy method (MEM). This new approach, based on the Bayesian probability theory, does not restrict the spectral function to the usual “pole + continuum” form, allowing a more flexible investigation of the nucleon spectral function. Making use of this flexibility, we are able to investigate the spectral functions of various interpolating fields, finding that the nucleon ground state mainly couples to an operator containing a scalar diquark. Moreover, we formulate the Gaussian sum rule for the nucleon channel and find that it is more suitable for the MEM analysis to extract the nucleon pole in the region of its experimental value, while the Borel sum rule does not contain enough information to clearly separate the nucleon pole from the continuum.     

Gottfried sum rule and the shape ofF 2 ⌍ -F 2 ⌍

The recent experimental determination of the Gottfried sum rule of deep inelastic scattering provides strong evidence that SU(2) flavor symmetry is broken by the sea quark distributions of the nucleon. Two mechanisms have been suggested to explain the data: Pauli blocking and non-perturbative contributions to the sea arising from the pion cloud surrounding the nucleon. We investigate the effects of these two mechanisms on several standard parametrizations of the quark distribution functions and find that each mechanism has different effects on the shapes of these distribution functions. The best agreement between the experimental data and the modified quark distributions occurs when both mechanisms are taken into account using a softπNN form factor and a small Pauli correction.     

On QCD sum rules of the Laplace transform type and light quark masses

We discuss the relation between the usual dispersion relation sum rules and the Laplace transform type sum rules in QCD. Two specific examples corresponding to the ?-coupling constant sum rule and the light quark masses sum rules are considered. An interpretation, within QCD, of Leutwyler's formula for the current algebra quark masses is also given.     

Screening of heavy quark free energies at finite temperatureand non-zero baryon chemical potential

We analyze the dependence of heavy quark free energies on the baryon chemical potential μ_b in 2-flavor QCD by performing a 6th order Taylor expansion in the chemical potential which circumvents the sign problem. The bare quark mass at corresponds to a pion mass of about 770 MeV and is thus not in the range of physical quark masses but the quark mass dependence is known to be small above T_c. At N_τ = 4 the lattices are coarse, however, we are using improved (p4 staggered) fermions. The Taylor expansion coefficients of color singlet and color averaged free energies are calculated and from this the expansion coefficients for the corresponding screening masses are determined. We find that for small μ_b the free energies of a static quark-antiquark pair decrease in a medium with a net excess of quarks and that screening is well described by a screening mass which increases with increasing μ_b. The μ_b-dependent corrections to the screening masses are well described by perturbation theory for T ≳ 2T_c. In particular, we find for all temperatures above T_c that the expansion coefficients for singlet and color averaged screening masses differ by a factor 2. PACS. 11.15.Ha, 11.10.Wx, 12.38Gc, 12.38.Mh     

由QCD求和规则计算s夸克质量

简要回顾了量子色动力学求和规则在计算s夸克质量方面的应用.结合Dominguez,Gend和Paver的工作,重新考虑渐近自由阈以下的谱函数后,计算了s夸克的质量,得到s夸克的跑动质量(-m)s(1 GeV)=219 MeV.讨论了影响计算结果精度的可能因素.