Nonperturbative Quark Research Articles

Role of glueballs in non-perturbative quark–gluon plasma

Discussed is how non-perturbative properties of quark gluon plasma, recently discovered in RHIC experiment, can be related to the change of properties of scalar and pseudoscalar glueballs. We set up a model with the Cornwall–Soni's glueball–gluon interaction, which shows that the pseudoscalar glueball becomes massless above the critical temperature of deconfinement phase transition. This change of properties gives rise to the change of sign of the gluon condensate at T>Tc. We discuss the other physical consequences resulting from the drastic change of the pseudoscalar glueball mass above the critical temperature.

Conformal symmetry and pion form factor: Soft and hard contributions

We discuss a constraint of conformal symmetry in the analysis of the pion form factor. The usual power-law behavior of the form factor obtained in the perturbative QCD analysis can also be attained by taking negligible quark masses in the nonperturbative quark model analysis, confirming the recent AdS/CFT correspondence. We analyze the transition from soft to hard contributions in the pion form factor considering a momentum-dependent dynamical quark mass from an appreciable constituent quark mass at low momentum region to a negligible current quark mass at high momentum region. We find a correlation between the shape of nonperturbative quark distribution amplitude and the amount of soft and hard contributions to the pion form factor.

Role of centre vortices in dynamical mass generation

The mass and renormalization functions of the nonperturbative quark propagator are studied in SU (3) gauge field theory with a Symanzik-improved gluon action and the AsqTad fermion action. Centre vortices in the gauge field are identified by fixing to maximal centre gauge. The role of centre vortices in dynamical mass generation is explored by removing centre vortices from the gauge fields and studying the associated changes in the quark propagator. We find that dynamical mass generation survives in the vortex-removed SU (3) gauge field theory despite the vanishing of the string tension and suppression of the gluon propagator in the infrared suggesting the possibility of decoupling dynamical mass generation from confinement.

Non-perturbative QCD Effect on K-Factor ofDrell–Yan Process

By using a non-perturbative quark propagator with thelowest-dimensionalcondensate contributions from the QCD vacuum, the non-perturbative effect toK-factor of the Drell–Yan process is numerically investigated for12 6C-12 6C collision at the center-of-mass energy(s)1/2 = 200 GeV, 630 GeV respectively. Calculated results show that thenon-perturbative QCD effect has just a weak influence on K-factor in thetwo cases.

Nonperturbative quark-antiquark production from a constant chromoelectric field via the Schwinger mechanism

We obtain an exact result for the nonperturbative quark (antiquark) production rate and its ${p}_{T}$ distribution from a constant SU(3) chromoelectric field ${E}^{a}$ with arbitrary color index $a$ by directly evaluating the path integral. Unlike the WKB tunneling result, which depends only on one gauge invariant quantity $|E|$, the strength of the chromoelectric field, we find that the exact result for the ${p}_{T}$ distribution for quark (antiquark) production rate depends on two independent Casimir (gauge) invariants, ${E}^{a}{E}^{a}$ and $[{d}_{abc}{E}^{a}{E}^{b}{E}^{c}{]}^{2}$.

MOMENTS OF THE HADRONIC MASS DISTRIBUTION IN SEMILEPTONIC B MESON DECAYS

We present a measurement of the first and second moments of the squared-mass distribution of charm hadrons in semileptonic decays of B mesons based on ~ 180 pb-1 of data taken with the CDF II detector at the Fermilab Tevatron. For a minimum lepton momentum of 0.7 GeV/c in the B meson rest frame, we measure the first two moments of the D** → D(*)π component to be [Formula: see text] and [Formula: see text]. Combining these values with known contributions from the D and D* components, we obtain the overall charm hadron moments [Formula: see text] and [Formula: see text] where [Formula: see text] is the spin-averaged D meson mass and the subscript "BR" denotes the uncertainty from the branching ratios used to combine the D, D* and D** components. From these moments we also determine the non-perturbative Heavy Quark Effective Theory parameters Λ and λ1 in the pole and 1S mass schemes.

Electron-positron annihilation into hadron-antihadron pairs

The reactions e{sup +}e{sup -}{yields}{pi}{sup +}{pi}{sup -} and e{sup +}e{sup -}{yields}N{sup lowbar}N with N=p,n are studied in a nonperturbative quark model. The work suggests that the two-step process, in which the primary q{sup -}q pair first forms a vector meson which in turn decays into a hadron pair, is dominant over the one-step process in which the primary q{sup -}q pair is directly dressed by additional q{sup -}q pairs to form a hadron pair. To reproduce the experimental data of the reaction e{sup +}e{sup -}{yields}n{sup -}n and p{sup -}p, a D-wave, {omega}-like vector meson with a mass of around 2 GeV has to be introduced.

Complete angular distribution measurements of two-body deuteron photodisintegration between 0.5 and 3GeV

Nearly complete angular distributions of the two-body deuteron photodisintegration differential cross section have been measured using the CLAS detector and the tagged photon beam at JLab. The data cover photon energies between 0.5 and 3.0 GeV and center-of-mass proton scattering angles 10-160 degrees. The data show a persistent forward-backward angle asymmetry over the explored energy range, and are well-described by the non-perturbative Quark Gluon String Model.

Ground-state baryons in nonperturbative quark dynamics

We review the results obtained in an Effective Hamiltonian (EH) approach for the three-quark systems. The EH is derived starting from the Feynman--Schwinger representation for the gauge-invariant Green function of the three quarks propagating in the nonperturbative QCD vacuum and assuming the minimal area law for the asymptotic of the Wilson loop. It furnishes the QCD consistent framework within which to study baryons. The EH has the form of the nonrelativistic three-quark Hamiltonian with the perturbative Coulomb-like and nonperturbative string interactions and the specific mass term. After outlining the approach, methods of calculations of the baryon eigenenergies and some simple applications are explained in details. With only two parameters: the string tension $\sigma=0.15 GeV^2$ and the strong coupling constant $\alpha_s=0.39$ a unified quantitative description of the ground state light and heavy baryons is achieved. The prediction of masses of the doubly heavy baryons not discovered yet are also given. In particular, a mass of $3660 MeV$ for the lightest $\Xi_{cc}$ baryon is found by employing the hyperspherical formalism to the three quark confining potential with the string junction.

Calculation of vacuum properties from the global color symmetry model

A modified method for calculating the non-perturbative quark vacuum condensates from the global color symmetry model is derived. Within this approach it is shown that the vacuum condensates are free of ultraviolet divergence which is different from previous studies. As a special, the two-quark condensate and the mixed quark-gluon condensate are calculated. A comparision with the results of the other nonperturbative QCD approaches is given.

J/ψ cc̄ production in e + e − and hadronic interactions

Predictions of the nonperturbative Quark Gluon Strings model based on the 1/N-expansion in QCD and string picture of interactions for production of states containing heavy quarks are considered. Relations between fragmentation functions for different states are used to predict the fragmentation function of c-quark to J/psi-mesons. The resulting cross section for J/psi-production in e+e- annihilation is in a good agreement with recent Belle result. It is argued that associated production of c\bar{c} states with open charm should give a substantial contribution to production of these states in hadronic interactions at very high energies.

E+ e- ANNIHILATION INTO $\overline{N}N$ PAIRS

The reactions of electron-position annihilation into nucleon-antinucleon pairs have been studied in a nonperturbative quark model. The work suggests that the two-step process, in which the primary [Formula: see text] pair forms first a meson and then the meson decays into baryon pair, is dominant over the one-step process in which the primary [Formula: see text] pair is directly dressed by two additional [Formula: see text] pairs to form a baryon pair. The experimental data indicates that there exists a vector meson with quantum numbers IG(JPC) = 0-(1--) and a mass around 2 GeV.

Spectroscopy of baryons containing two heavy quarks in nonperturbative quark dynamics

We have studied the three quark systems in an Effective Hamiltonian approach in QCD. With only two parameters: the string tension sigma and the strong coupling constant alpha_s we obtain a good description of the ground state light and heavy baryons. The prediction of masses of the doubly heavy baryons not discovered yet are also given. In particular, a mass of 3620 MeV for the lightest (ccu) baryon is found by employing the hyperspherical formalism to the three quark confining potential with the string junction.

The π and Tensor Vacuum Susceptibilities from the Global Color Symmetry Model**The project supported in part by National Natural Science Foundation of China under Grant Nos. 19975062, 10175033, and 10135030

A modified method for calculating the nonperturbative quark vacuum condensates from the global color symmetry model is derived. Within this approach it is shown that the treatment of quark vacuum condensates is different from that in the previous studies. As a special case we calculate the π and tensor vacuum susceptibilities. A comparison with the results of the other nonperturbative QCD approaches is given.

Instantons and η meson production near threshold in NN collisions

An enhancement for the η production in proton–neutron collisions as compared with that in proton–proton scattering has been recently observed. We present a calculation for the production cross section, in proton–neutron collisions near threshold, within instanton model for the QCD vacuum and show that a specific flavor dependent nonperturbative quark–gluon interaction related to instantons is able to explain the observed enhancement.

Quark correlations and deuteron electromagnetic form factors

The recent preliminary values of ${A(Q}^{2})$ and ${t}_{20}(70\ifmmode^\circ\else\textdegree\fi{})$ elastic electron-scattering data extracted from deuterium at larger values of ${Q}^{2}$ have placed more stringent constraints on models of the nucleon-nucleon system. Comparisons of the data with conventional hadronic models and with perturbative QCD have been made, and are strongly in favor of the hadronic picture provided isoscalar meson exchange currents are included. In this work a nonperturbative quark model of the deuteron with parameters determined from earlier work is used to investigate the role of quark correlations in describing the deuteron electromagnetic form factors. The results give a good description of the recent data and show that this model with many-body confinement has quark correlation contributions to the charge form factors which are remarkably similar to the meson exchange current contributions.

Nonperturbative QCD contribution to the gluon-gluon-η′vertex

We investigate the momentum dependence of the g*-g-eta' vertex form factor in the presence of the nonperturbative QCD vacuum condensate. The behaviour of this form factor is crucial for, among other things, some proposed mechanisms devised to explain the large branching ratio for fast eta' production in charmless B decays observed by CLEO. We show that the leading momentum dependence of this vertex form factor is not altered when the contribution of nonperturbative quark condensate is considered.

Nontrivial Q2-Dependence in the Gottfried Sum Induced by Nonperturbative Effect

In consideration of the lowest dimensional condensate contributions to the free quark propagator, we obtain the nonperturbative quark propagator with the nonvanishing vacuum average value for quark composite operator , Using the -corrected quark propagator and basing on the simple quark-parton model, we discuss the nonperturbative effect in the nucleon structure function. It is shown that the nonperturbative effect modifies the conventional quark-parton model formula of the nucleon structure function at finite Q2 and suggests a nontrivial Q2-dependence in the Gottfried sum.

Zero-Point Problem in Fixed-Point Gauge of Nonperturbative QCD

We discuss the essence of the zero-point in fixed-point gauge of nonperturbative QCD and describe how to set the zero-point appropriately in deriving the nonperturbative quark–gluon three-point vertex functions including gluon condensates.

Decay Z0--> pi 0 gamma reexamined.

The decay {ital Z}{sup 0}{r_arrow}{pi}{sup 0}{gamma} is treated using a relativistic nonperturbative quark model for the pion. The model assumes that the pion is made of a {ital q{bar q}} valence pair and of an effective neutral component representing the nonelementary excitations of the quark-gluonic field responsible for the confinement. The decay width is expressed in terms of the pion decay constant {ital F}{sub {pi}} and of current quark masses. Its value calculated for 2 MeV{le}{ital m}{sub {ital u}}{le}8 MeV, 5 MeV{le}{ital m}{sub {ital d}}{le}15 MeV is at least four orders of magnitude under the experimental upper bound but in agreement with other theoretical models. {copyright} {ital 1996 The American Physical Society.}