arXiv subjects

Xuemin Jin

Publications and source records attributed to Xuemin Jin.

At least 19 recordsLinked to original sources

arXiv1999

Nucleon Sigma Term and In-medium Quark Condensate in the Modified Quark-Meson Coupling Model

We evaluate the nucleon sigma term and in-medium quark condensate in the modified quark-meson coupling model which features a density-dependent bag constant. We obtain a nucleon sigma term consistent with its empirical value, which requires a significant reduction of the bag constant in the nuclear medium similar to those found in the previous works. The resulting in-medium quark condensate at low densities agrees well with the model independent linear order result. At higher densities, the magnitude of the in-medium quark condensate tends to increase, indicating no tendency toward chiral symmetry restoration.

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arXiv1998

Interference Fragmentation Functions and Valence Quark Spin Distributions in the Nucleon

We explore further applications of the twist-two quark interference fragmentation functions introduced earlier. We show that semi-inclusive production of two pions in the current fragmentation region in deep inelastic scattering of a longitudinally polarized electron on a longitudinally polarized nucleon can provide a probe of the valence quark spin (or helicity difference) distribution in the nucleon.

hep-ph↗

arXiv1998

Interference Fragmentation Functions and the Nucleon's Transversity

We introduce twist-two quark interference fragmentation functions in helicity density matrix formalism and study their physical implications. We show how the nucleon's transversity distribution can be probed through the final state interaction between two mesons ($π^+π^-$, $K\bar K$, or $πK$) produced in the current fragmentation region in deep inelastic scattering on a transversely polarized nucleon.

hep-ph↗

arXiv1997

New QCD Sum Rules for Nucleon Tensor Charge

Two new QCD sum rules for nucleon tensor charge are derived from a mixed correlator of spin-1/2 and spin-3/2 nucleon interpolating fields. These sum rules are analyzed along with a sum rule obtained from the usual correlator of a general spin-1/2 nucleon interpolating field. The validity and reliability of the sum rules are examined by monitoring the contaminations arising from the transitions and continuum and the convergence of the operator product expansion. Valid sum rules are identified and their predictions are presented. It is found that the vacuum tensor susceptibility induced by the external field plays an important role in determining both the validity and predictions of the sum rules. The uncertainties associated with the sum-rule predictions are also discussed.

hep-ph↗

arXiv1997

Chirality and Reliability of Baryon QCD Sum Rules

The QCD sum-rule method has been widely used in studying various baryon properties. For a given problem, there are usually more than one sum rules and they do not work equally well. In this paper, we point out that chirality plays an important role in determining the reliability of a baryon sum rule. The contributions of positive- and negative-parity excited baryon states partially cancel each other in the chiral-odd sum rules, but add up in the chiral-even sum rules. As such, the chiral-odd sum rules are generally more reliable than the chiral-even sum rules. This allows one to identify the more reliable sum rules and use them in extracting the ground-state baryon property of interest. This is illustrated in an explicit example.

hep-ph↗

arXiv1996

Weak Hyperon Production in ep Scattering

We study the kinematics and cross section of the scattering process $e p \rightarrow e Σ^+$. The cross section is expressed in terms of complex form factors characterizing the hadron vertices. We estimate the cross section for small momentum transfer using known experimental information. To first order in the momentum transfer, we obtain a model independent result for the photon-exchange part of the cross section, which is completely determined by the decay width $Γ(Σ^+ \rightarrow pγ)$. For the kinematics of the parity violation experiment at MAMI, this first order result gives rise to a ratio of $(dσ/dΩ)_{ep\rightarrow eΣ^+}$ $/(dσ/dΩ)_{ep\rightarrow ep} \simeq 4.0\times 10^{-15}$. The $Z^0$-exchange and interference parts give much smaller contributions due to the suppression of the flavor changing weak neutral current in the standard model. Feasibility of the experimental measurement is briefly discussed.

hep-ph↗

arXiv1996

A Note on the External-Field Method in QCD Sum Rules

The external-field method has been used extensively in the QCD sum-rule approach to explore various hadron static properties. In the traditional formalism of this method, the transitions from the ground state hadron to excited states are not exponentially suppressed relative to the ground state term and thus contaminate the ground state hadron property to be extracted. In this paper, we suggest a modified formalism, in which the transition terms are exponentially suppressed relative to the ground state term. As such, the pole plus continuum spectral model, traditionally invoked in QCD sum-rule approach, can be adopted. Thus, this modified formalism has potential to improve the predictability and reliability of external-field sum-rule calculations, which is illustrated in an explicit example.

hep-ph↗

arXiv1996

New QCD Sum Rules for Nucleon Axial Vector Coupling Constants

Two new sets of QCD sum rules for the nucleon axial coupling constants are derived using the external-field technique and generalized interpolating fields. An in-depth study of the predicative ability of these sum rules is carried out using a Monte-Carlo based uncertainty analysis. The results show that the standard implementation of the QCD sum rule method has only marginal predicative power for the nucleon axial coupling constants, as the relative errors are large. The errors range from approximately 50 to 100% compared to the nucleon mass obtained from the same method, which has only 10% to 25% error. The origin of the large errors is examined. Previous analyses of these coupling constants are based on sum rules that have poor OPE convergence and large continuum contributions. Preferred sum rules are identified and their predictions are obtained. We also investigate the new sum rules with an alternative treatment of the problematic transitions which are not exponentially suppressed in the standard treatment. The new treatment provides exponential suppression of their contributions relative to the ground state. Implications for other nucleon current matrix elements are also discussed.

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arXiv1996

Background-Field Formalism in Quantum Systems

The background-field formalism is used extensively in fundamental approaches to QCD to explore hadronic matrix elements of various currents. While the lattice QCD approach is formulated in the fully-interacting Hilbert space, which includes both QCD and background field interactions, the QCD sum-rule formalism is traditionally developed in the pure-QCD Hilbert space. The latter approach encounters difficulties with excited state contaminations which are not exponentially suppressed and requires extrapolations to isolate the desired physics. Proponents of the pure-QCD Hilbert space formalism used in the QCD sum-rule approach have criticized the lattice QCD approach as neglecting important physics. In this letter, the equivalence of the two approaches is established and the flaws in the former criticisms are resolved. Finally, the application of the fully-interacting Hilbert space formalism to the study of electromagnetic polarizabilities is outlined.

hep-ph↗

arXiv1996

Rho-Omega Mixing via QCD Sum Rules with Finite Mesonic Widths

Based on the analysis of both Borel and Finite-Energy QCD sum rules, the inclusion of finite mesonic widths leads to a dramatic effect on the predictions for the momentum dependence of the rho-omega mixing matrix element. It is shown that the rho-omega mixing matrix element traditionally discussed in the literature, has the same sign and similar magnitude in the space-like region as its on-shell value. This contrasts the zero-width result where the mixing matrix element is typically of opposite sign in the space-like region.

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arXiv1996

New QCD Sum Rules for Nucleons in Nuclear Matter

Two new QCD sum rules for nucleons in nuclear matter are obtained from a mixed correlator of spin-1/2 and spin-3/2 interpolating fields. These new sum rules, which are insensitive to the poorly known four-quark condensates, provide additional information on the nucleon scalar self-energy. These new sum rules are analyzed along with previous spin-1/2 interpolator-based sum rules which are also insensitive to the poorly known four-quark condensates. The analysis indicates consistency with the expectations of relativistic nuclear phenomenology at nuclear matter saturation density. However, a weaker density dependence near saturation is suggested. Using previous estimates of in-medium condensate uncertainties, we find $M^* = 0.64^{+0.13}_{-0.09}$ GeV and $Σ_v = 0.29^{+0.06}_{-0.10}$ GeV at nuclear matter saturation density.

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arXiv1996

Change of MIT Bag Constant in Nuclear Medium and Implication for the EMC Effect

The modified quark-meson coupling model, which features a density dependent bag constant and bag radius in nuclear matter, is checked against the EMC effect within the framework of dynamical rescaling. Our emphasis is on the change in the average bag radius in nuclei, as evaluated in a local density approximation, and its implication for the rescaling parameter. We find that when the bag constant in nuclear matter is significantly reduced from its free-space value, the resulting rescaling parameter is in good agreement with that required to explain the observed depletion of the structure functions in the medium Bjorken $x$ region. Such a large reduction of the bag constant also implies large and canceling Lorentz scalar and vector potentials for the nucleon in nuclear matter which are comparable to those suggested by the relativistic nuclear phenomenology and finite-density QCD sum rules.

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arXiv1996

Recovering Relativistic Nuclear Phenomenology from the Quark-Meson Coupling Model

The quark-meson coupling (QMC) model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons is modified by the introduction of a density dependent bag constant. It is found that when the bag constant is significantly reduced in nuclear medium with respect to its free space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology. This suggests that the modification of the bag constant in the nuclear medium may play an important role in low- and medium-energy nuclear physics.

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arXiv1996

New QCD Sum Rules for Nucleon Axial Vector Coupling Constants

New QCD sum rules for the nucleon axial-vector current coupling constants are derived. One set is derived from a generalized correlator of spin-1/2 interpolating fields, and the other with a mixed correlator of spin-1/2 and spin-3/2 fields. Using a recently-developed Monte-Carlo based uncertainty analysis and techniques for determining the optimal interpolating fields, the predicative ability of QCD sum rules for axial-vector current coupling constants are determined. We find the previous studies of this matrix element are based on an invalid sum rule. A combined analysis of this sum rule together with the nucleon mass sum rules yields $g_A=1.48\pm^{1.06}_{0.65}$. The relative error of approximately 60% is large compared to a 10% error for the nucleon mass obtained from the same input parameters.

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arXiv1996

Modified Quark-Meson Coupling Model for Nuclear Matter

The quark-meson coupling model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons, is modified by introducing medium modification of the bag constant. We model the density dependence of the bag constant in two different ways: one invokes a direct coupling of the bag constant to the scalar meson field, and the other relates the bag constant to the in-medium nucleon mass. Both models feature a decreasing bag constant with increasing density. We find that when the bag constant is significantly reduced in nuclear medium with respect to its free-space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology and finite-density QCD sum rules. This suggests that the reduction of bag constant in nuclear medium may play an important role in low- and medium-energy nuclear physics.

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arXiv1996

Mesonic Width Effects on the Momentum Dependence of the Rho-Omega Mixing Matrix Element

It is shown, in a model independent way, that the large difference in rho and omega widths gives rise to a new source of momentum dependence for the rho-omega mixing matrix element. The q^2 dependence arising due to the meson widths leads to a significant alteration of the result obtained in the zero-width approximation usually discussed in the literature. The origin of this strong momentum dependence lies in the difference between the rho and omega meson widths.

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arXiv1996

Isospin Breaking in the Nucleon Isovector Axial Charge from QCD Sum Rules

The isospin breaking in the nucleon isovector axial charge, $g_A^3$, are calculated within the external field QCD sum-rule approach. The isospin violations arising from the difference in up and down current quark masses and in up and down quark condensates are included; electromagnetic effects are not considered. We find $δg^3_A/g^3_A \approx (0.5-1.0)\times 10^{-2}$, where $δg^3_A = (g^3_A)_p + (g^3_A)_n$ and $ g^3_A = [(g^3_A)_p - (g^3_A)_n]/2$. Using the Goldberger-Treiman relation, we also obtain an estimate of the isospin breaking in the pion-nucleon coupling constant, $(g_{ppπ_0}-g_{nnπ_0})/g_{NNπ} \approx (2-7) \times 10^{-3}$.

hep-ph↗

arXiv1996

Recovering Relativistic Nuclear Phenomenology from the Quark-Meson Coupling Model

The quark-meson coupling (QMC) model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons is modified by the introduction of a density dependent bag constant. The density dependence of the bag constant is related to that of the in-medium effective nucleon mass through a scaling ansatz suggested by partial chiral symmetry restoration in nuclear matter. This modification overcomes drawbacks of the QMC model and leads to the recovery of the essential features of relativistic nuclear phenomenology. This suggests that the modification of the bag constant in the nuclear medium may play an important role in low- and medium-energy nuclear physics.

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