arXiv subjects
Publications and source records attributed to Pengming Zhang.
A theoretical description of vortex electrons interacting with electric and magnetic fields is presented, based on Lorentz transformations. The general dynamical equations of motion of a twisted electron with intrinsic orbital angular momentum in an external field is derived. Methods for the extraction of an electron vortex beam with a given orbital polarization and for the manipulation of such a beam are developed.
We calculate the contribution of the non-perturbative Pauli couplings in the quark-photon and quark-gluon vertices to the single-spin asymmetries (SSAs) in semi-inclusive deep inelastic scattering (SIDIS). We describe the nucleon using the spectator model with scalar and axial-vector diquarks. Both of the new couplings can cause the helicity flip of the struck quark. The helicity flip and the rescattering induced by the non-perturbative gluon exchange between the struck quark and diquark lead to the SSAs. Their azimuthal dependencies are the same as that usually ascribed to the Collins and Sivers effects. Our numerical results, based on the instanton model for the QCD vacuum, show that the non-perturbative quark-gluon and quark-photon interactions have a strong influence on these asymmetries.
The lowest dimension three-gluon currents that couple to the exotic $0^{\pm-}$ glueballs have been constructed using the helicity formalism. Based on the constructed currents, we obtain new QCD SRs that have been used to extract the masses and the decay constants of the scalar exotic $0^{\pm-}$ glueballs. We estimate the masses for the scalar state and for the pseudoscalar state to be ${m_+=9.8^{+1.3}_{-1.4}}$ GeV and $m_-=6.8^{+1.1}_{-1.2}$ GeV.
The Bessel-inspired behavior of parton densities at small Bjorken $x$ values, obtained in the case of the flat initial conditions for DGLAP evolution equations, is used along with "frozen" and analytic modifications of the strong coupling constant to study the so-called EMC effect. Among other results, this approach allowed predicting small $x$ behavior of the gluon density in nuclei.
Instanton effects can give large contribution to strong interacting processes, especially at the energy scale where perturbative QCD is no longer valid. However instanton contribution to the gluon contribution in constituent quark and nucleon has never been calculated before. Based on both the constituent quark picture and the instanton model for QCD vacuum, we calculate the unpolarized and polarized gluon distributions in the constituent quark and in the nucleon for the first time. We find that the pion field plays an important role in producing both the unpolarized and the polarized gluon distributions.
We calculate the electromagnetic Pauli form factor of quark induced by the nontrivial topological fluctuations of QCD vacuum called instantons. It is shown that such contribution is significant. We discuss the possible implications of our result in the photon-hadron reactions and in the dynamics of quark-photon interactions in the dense/hot quark matter.
Recently it has been pointed out that the skyrmions carry two independent topology, the baryon topology and the monopole topology. We provide more evidence to support this. In specific, we prove that the baryon number $B$ can be decomposed to the monopole number $m$ and the shell number $n$, so that $B$ is given by $B=mn$. This tells that the skyrmions may more conveniently be classified by two integers $(m,n)$. This is because the rational map which determines the baryon number in the popular multi-skyrmion solutions actually describes the monopole topology $π_2(S^2)$ which is different from the baryon topology $π_3(S^3)$. Moreover, we show that the baby skyrmions can also be generalized to have two topology, $π_1(S^1)$ and $π_2(S^2)$, and thus should be classified by two topological numbers $(m,n)$. Furthermore, we show that the vacuum of the Skyrme theory can be classified by two topological numbers $(p,q)$, the $π_1(S^1)$ of the sigma-field and the $π_3(S^2)$ of the normalized pion-field. This means that the Skyrme theory has multiple vacua similar to the Sine-Gordon theory and QCD combined together. This puts the Skyrme theory in a totally new perspective. We discussthe physical implications of our results.
The gauge dependence in the anomalous dimension of the gauge-invariant-canonical-energy-momentum tensor for proton is studied by the background field method. The naive calculation shows the problem, the absence of the counter term in the gluonic sectors. The analysis shows that the result [Chen et al., Phys. Rev. Lett. 103, 062001 (2009)] is derived from the background field method after we introduced a trick to avoid the problem except for the gluon-to-gluon sector; it is gauge dependent. The possible reason of this gauge-dependent result comes from the nontrivial treatment of the condition $F^{μν}_{pure}=0$ at a higher order. This result shows that one needs a further improvement in treating this condition with a covariant way at a higher order by the background field method. In particular, we have to focus on two checkpoints, the gauge independence and zero eigenvalue in the anomalous-dimension matrix, in order to test the validity of the gauge-invariant-canonical-energy-momentum tensor.
We present a new calculation of the mass and width of the exotic $0^{--}$ glueball in the framework of the QCD sum rules. We next construct a new current which couples to a pure $0^{--}$ gluon state and derive consistent and stable sum rules. A previously used current in this approach was shown to be inconsistent. We obtain for this state a mass $M_G=6.3^{+0.8}_{-1.1}$ GeV and an upper limit for the total width $Γ_G\leq 235$ MeV. These values can be used as an important guide for the experimental search of this exotic state. We argue that the mixing of this glueball state with $0^{--}$ tetraquark is very small. Therefore, the exotic $0^{--}$ glueball can be considered as a pure gluon state.
The anomalous dimension for the gauge-invariant-canonical decomposition of the energy-momentum tensor for quarks and gluons is studied by the background field method. In particular, the consistency between the background field method and the renormalization in the gluonic sectors is investigated. The analysis shows that the naive gauge-invariant-decomposition has an inconsistency between its definition and the renormalization in the background field method. Although we try to consider a trick to overcome this inconsistency in computing the anomalous dimension, the gauge-parameter dependence remains in the final result. This result should be extended to the problems on the gauge-invariant-canonical-spin decomposition.
Based on the observation that the skyrmion in Skyrme theory can be viewed as a dressed monopole, we show that the skyrmions have two independent topology, the baryon topology $π_3(S^3)$ and the monopole topology $π_2(S^2)$. With this we propose to classify the skyrmions by two topological numbers $(m,n)$, the monopole number $m$ and the shell (radial) number $n$. In this scheme the popular (non spherically symmetric) skyrmions are classified as the $(m,1)$ skyrmions but the spherically symmetric skyrmions are classified as the $(1,n)$ skyrmions, and the baryon number $B$ is given by $B=mn$. Moreover, we show that the vacuum of the Skyrme theory has the structure of the vacuum of the Sine-Gordon theory and QCD combined together, which can also be classified by two topological numbers $(p,q)$. This puts the Skyrme theory in a totally new perspective.
We consider quantum inverse scattering with singular potentials and calculate the Sine-Gordon model effective potential in the laboratory and centre-of-mass frames. The effective potentials are frame dependent but closely resemble the zero-momentum potential of the equivalent Ruijsenaars-Schneider model.
We obtain the centre-of-mass frame effective potential from the zero-momentum potential in Ruijsenaars-Schneider type 1-dimensional relativistic mechanics using classical inverse scattering methods.
Based on both the constituent quark picture and the instanton model for QCD vacuum, we calculate the unpolarized and polarized gluon distributions in the constituent quark and in the nucleon. Our approach consists of the two main steps. At the first step, we calculate the gluon distributions inside the constituent quark generated by the perturbative quark-gluon interaction, the non-perturbative quark-gluon interaction, and the non-perturbative quark-gluon-pion anomalous chromomagnetic interaction. The non-perturbative interactions are related to the existence of the instantons, strong topological fluctuations of gluon fields, in the QCD vacuum. At the second step, the convolution model is applied to derive the gluon distributions in the nucleon. A very important role of the pion field in producing the unpolarized and the polarized gluon distributions in the hadrons is discovered. We discuss a possible solution of the proton spin problem.
We suggest a new mechanism for the energy loss of fast heavy quarks in quark-gluon plasma. This mechanism is based on pion production caused by the anomalous chromomagnetic quark-gluon-pion interaction induced by strong topological fluctuations of the gluon fields represented by instantons. We found that this mechanism gives a considerable contribution to the collisional energy loss of a heavy quark in quark-gluon plasma, which shows a nontrivial role of nonperturbative phenomena in strongly interacting quark-gluon plasma.
Using DGLAP evolution equation with the corrections of parton recombinations, the flavor asymmetric sea quark distributions in the proton are first extracted from the available experimental data with the global QCD analysis method. Comparisons of our predictions to the CTEQ, MRS, and GRV parton distribution functions for sea quarks are presented. Based on the separation of the flavor symmetric and asymmetric sea, the possible relations between strange quark distribution and symmetric light quark distribution are discussed. The results are used to explain the recent HERMES result of the strange quark distribution.
A new mechanism for the pion production in the high energy reactions is suggested. It is related to the possibility for direct production of the pions induced by instantons, topologically nontrivial gluonic excitations of QCD vacuum. This mechanism does not require any fragmentation functions for production of pseudoscalar mesons in high energy reactions with hadrons. We calculate the contribution of a new mechanism to the inclusive $π^0$-meson production in high energy proton-proton collision. It is shown that it gives the dominated contribution to the inclusive cross-sections in the few GeV region for the transverse momentum of final pion. We discuss the possible application of a new mechanism to the phenomenon of the large spin effects observed in the numerous high energy reactions.
The Abelian decomposition of QCD which decomposes the gluons to the color neutral binding gluons and the colored valence gluons shows that QCD can be viewed as the restricted QCD (RCD) made of the binding gluons which has the valence gluons as colored source, and simplifies the QCD dynamics greatly. In particular, it tells that the gauge covariant valence gluons can be treated as the constituents of hadrons, and generalizes the quark model to the quark and valence gluon model. So it provides a comprehensive picture of glueballs and their mixing with quarkoniums, and predicts new hybrid hadrons made of quarks and valence gluons. We discuss how these predictions could be confirmed experimentally. In particular we present a systematic search for the ground state glueballs and their mixing with quarkoniums below 2 GeV in $0^{++}$, $2^{++}$, and $0^{-+}$ channels within the framework of QCD, and predict the relative branching ratios of the radiative decay of $ψ$ to the physical states.