arXiv subjects

Q. Zhao

Publications and source records attributed to Q. Zhao.

48 records · Page 3Linked to original sources

arXiv2002

Pion photoproduction on the nucleon in the quark model

We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, $γp\to π^+ n$, $γn\to π^- p$, $γp\to π^0 p$, and $γn\to π^0 n$. Leaving the $πNΔ$ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the first resonance region. Within this effective Lagrangian approach, strongly constrainted by the quark model, we consider the issue of double-counting which may occur if additional {\it t}-channel contributions are included.

nucl-th↗

arXiv2002

Quark model approach to the $η$ meson electroproduction on the proton

A recently developed quark model approach to pseudoscalar meson photoproduction is extended to electroproduction process for the $η$ meson in the kinematics of momentum transfer $Q^2 \leq$ 4 (GeV/c)$^2$ and total center of mass energy $W \leq$ 1.6 GeV. Existing data are well reproduced and the roles of the $S_{11}(1535)$ and $D_{13}(1520)$ resonances are closely investigated. In the study of the longitudinal excitation of the $S_{11}(1535)$ resonance, a reliable constraint on the $S_{11}(1535)$ properties is obtained by cleanly removing the electromagnetic transition from the $γ_{(v)} p \to S_{11}(1535) \to ηp$ amplitude. Thus, the fitted quantities can be determined with an uncertainty of about 15%. This could be the first direct constraint on the $S_{11}(1535)$ properties in theory.

nucl-th↗

arXiv2002

Study $ω$ and $ϕ$ photoproduction in the nucleon isotopic channels

We present results for the photoproduction of $ω$ and $ϕ$ meson in the nucleon isotopic channles. A recently developed quark model with an effective Lagrangian is employed to account for the non-diffractive {\it s}- and {\it u}-channel processes; the diffractive feature arising from the {\it natural} parity exchange is accounted for by the {\it t}-channel pomeron exchange, while the {\it unnatural} parity exchange is accounted for by the {\it t}-channel pion exchange. In the $ω$ production, the isotopic effects could provide more information concerning the search of "missing resonances", while in the $ϕ$ production, the isotopic effects could highlight non-diffractive resonance excitation mechanisms at large angles.

nucl-th↗

arXiv2002

Contributions of vector meson photoproduction to the Gerasimov-Drell-Hearn sum rule

An improved version of a recently developed quark model approach to vector meson photoproduction is applied to the investigation of contributions of vector meson photoproduction to the Gerasimov-Drell-Hearn (GDH) sum rule. We find that the sum rule converges at a few GeV's. Contributions to the proton channel are found to be small while to the neutron are relatively large.

nucl-th↗

arXiv2001

Differential cross section measurement of eta photoproduction on the proton from threshold to 1100 MeV

The differential cross section for the reaction p(gamma, eta p) has been measured from threshold to 1100 MeV photon laboratory energy. For the first time, the region of the S11(1535) resonance is fully covered in a photoproduction experiment and allows a precise extraction of its parameters at the photon point. Above 1000 MeV, S-wave dominance vanishes while a P-wave contribution is observed whose nature will have to be clarified. These high precision data together with the already measured beam asymmetry data will provide stringent constraints on the extraction of new couplings of baryon resonances to the eta meson.

hep-ex↗

arXiv2001

Quark model predictions for $K^*$ photoproduction on the proton

The photoproduction of $K^*$ vector mesons is investigated in a quark model with an effective Lagrangian. Including both baryon resonance excitations and {\it t}-channel exchanges, observables for the reactions $γp\to K^{*0}Σ^+$ and $γp\to K^{*+}Σ^0$ are predicted, using the SU(3)-flavor-blind assumption of non-perturbative QCD.

nucl-th↗

arXiv2001

Vector meson photoproduction in the quark model

We present a quark model study of the $ω$ meson photoproduction near threshold. With a limited number of parameters, all the data in history are reproduced. The roles played by the {\it s}- and {\it u}-channel processes (resonance excitations and nucleon pole terms), as well as the {\it t}-channel {\it natural} (Pomeron) and {\it unnatural} parity (pion) exchanges are clarified. This approach provides a framework for systematic study of vector meson photoproduction near threshold.

nucl-th↗

arXiv2000

$η^\prime$ photoproduction near threshold

In this work, the $η^\prime$ meson photoproduction near threshold is studied in the quark model framework. A pseudovector effective Lagrangian is introduced for the $η^\prime NN$ coupling and the newly published data from the SAPHIR Collaboration provide good constraints to this parameter. Corrections of order $O(1/m_q^2)$ for the electromagnetic interaction vertex are taken into account, which produce corrections of order $O(1/m_q^3)$ to the transition amplitude for $γp\to η^\prime p$. Some low-lying resonances, $S_{11}(1535)$, $P_{13}(1720)$, and $P_{13}(1900)$ are found to have significant contributions. A bump structure around $E_γ\approx$ 2 GeV is found arising from the $n=3$ terms in the harmonic oscillator basis. The beam polarization asymmetries are predicted and can be tested against the forthcoming data from GRAAL.

nucl-th↗

arXiv1999

Role of nucleon resonance excitation in $ϕ$ meson photoproduction

The resonance effects are investigated in the $ϕ$ meson photoproduction near threshold through a quark model approach with an effective Lagrangian. The diffractive contribution is consistently estimated by the {\it t}-channel Pomeron exchange. Another non-diffractive process, {\it t}-channel $π^0$ exchange is also included. The numerical result shows that the Pomeron exchange plays dominant role in the $ϕ$ meson photoproduction, while the cross sections of the non-diffractive processes, i.e. {\it s}- and {\it u}-channel excitations, and {\it t}-channel $π^0$ exchange, are quite small. In the polarization observables, we find that large asymmetries are produced in the backward direction by the interferences from the {\it s}- and {\it u}-channel resonances, while in the forward direction, only very small asymmetries are generated. Meanwhile, we find that the effects from the $π^0$ exchange are generally negligible.

nucl-th↗

arXiv1999

Nucleonic resonance excitations and "missing resonances" in the $ω$ meson photoproduction

In this work, an improved quark model approach to the $ω$ meson photoproduction with an effective Lagrangian is reported. The {\it t}-channel {\it natural} parity exchange is consistently taken into account through the Pomeron exchange, while the {\it unnatural} parity exchange is described by the $π^0$ exchange. It shows that with very limited number of parameters, all the available experimental data in the low energy regime can be consistently accounted for in this framework. Effects from the intermediate nucleonic resonances are investigated in several polarization observables. The sensitivities of these observables to the resonance effects are probably the best way for identifying those so-called "missing resonances" in the baryon spectrum.

nucl-th↗

arXiv1999

Nucleonic resonance effects in the $ϕ$ meson photoproduction

The process $γp \to ϕp$ close to threshold is investigated focusing on the role played by the {\it s}- and {\it u}-channel nucleonic resonances. For this purpose, a recent quark model approach, based on the $SU(6)\otimes O(3)$ symmetry with an effective Lagrangian, is extended to the $ϕ$ meson photoproduction. Another non-diffractive process, the {\it t}-channel $π^0$ exchange, is also included. The diffractive contribution is produced by the {\it t}-channel Pomeron exchange. Contributions from non-diffractive {\it s}- and {\it u}-channel process are found small in the case of cross sections and polarization observables at forward angles. However, backward angle polarization asymmetries show high sensitivity to this non-diffractive process. Different prescriptions to keep gauge invariance for the Pomeron exchange amplitudes are investigated. Possible deviations from the exact $SU(6)\otimes O(3)$ symmetry, due to the configuration mixing, are also discussed.

nucl-th↗

arXiv1995

Using Qualitative Hypotheses to Identify Inaccurate Data

Identifying inaccurate data has long been regarded as a significant and difficult problem in AI. In this paper, we present a new method for identifying inaccurate data on the basis of qualitative correlations among related data. First, we introduce the definitions of related data and qualitative correlations among related data. Then we put forward a new concept called support coefficient function (SCF). SCF can be used to extract, represent, and calculate qualitative correlations among related data within a dataset. We propose an approach to determining dynamic shift intervals of inaccurate data, and an approach to calculating possibility of identifying inaccurate data, respectively. Both of the approaches are based on SCF. Finally we present an algorithm for identifying inaccurate data by using qualitative correlations among related data as confirmatory or disconfirmatory evidence. We have developed a practical system for interpreting infrared spectra by applying the method, and have fully tested the system against several hundred real spectra. The experimental results show that the method is significantly better than the conventional methods used in many similar systems.

cs.AI↗