arXiv subjects

Qiang Zhao

Publications and source records attributed to Qiang Zhao.

At least 181 records · Page 10Linked to original sources

arXiv2011

$η'$ photoproduction on the nucleons in the quark model

A chiral quark-model approach is adopted to study the $γp\rightarrow η'p$ and $γn\rightarrow η' n$. Good descriptions of the recent observations from CLAS and CBELSA/TAPS are obtained. Both of the processes are governed by $S_{11}(1535)$ and $u$ channel background. Strong evidence of an $n=3$ shell resonance $D_{15}(2080)$ is found in the reactions, which accounts for the bump-like structure around $W=2.1$ GeV observed in the total cross section and excitation functions at very forward angles. The $S_{11}(1920)$ seems to be needed in the reactions, with which the total cross section near threshold for the $γp\rightarrow η' p$ is improved slightly. The polarized beam asymmetries show some sensitivities to $D_{13}(1520)$, although its effects on the differential cross sections and total cross sections are negligible. There is no obvious evidence of the $P$-, $D_{13}$-, $F$- and $G$-wave resonances with a mass around 2.0 GeV in the reactions.

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arXiv2011

$η$ photoproduction on the quasi-free nucleons in the chiral quark model

A chiral quark-model approach is adopted to study the $η$ photoproduction off the quasi-free neutron and proton from a deuteron target. Good descriptions of the differential cross sections, total cross sections and beam asymmetries for these two processes are obtained in the low energy region. For $γp\rightarrow ηp$, the dominant resonances are $S_{11}(1535)$, $S_{11}(1650)$, $D_{13}(1520)$, $D_{13}(1700)$ and $P_{13}(1720)$. While for the $γn\rightarrow ηn$ process, the dominant resonances are $S_{11}(1535)$, $S_{11}(1650)$, $D_{13}(1520)$, $D_{15}(1675)$ and $P_{13}(1720)$. Furthermore, the $u$ channel backgrounds have significant contributions to the $η$ photoproduction processes. The configuration mixings in the $S_{11}(1535,1650)$ and $D_{13}(1520,1700)$ can be extracted, i.e. $θ_S\simeq 26^\circ$ and $θ_D\simeq 21^\circ$. It shows that the narrow bump-like structure around $W= 1.68$ GeV observed in $γn\rightarrow ηn$ can be naturally explained by the constructive interferences between $S_{11}(1535)$ and $S_{11}(1650)$. In contrast, the destructive interference between $S_{11}(1535)$ and $S_{11}(1650)$ produces the shallow dip around $W= 1.67$ GeV in $γp\rightarrow ηp$. The $S$ wave interfering behaviors in the proton and neutron reactions are correlated with each other in the quark model framework, and no new exotic nucleon resonances are needed in these two reactions.

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arXiv2011

Open charm scenarios

We discuss possibilities of identifying open charm effects in direct production processes, and propose that direct evidence for the open charm effects can be found in $e^+ e^-\to J/ψπ^0$. A unique feature with this process is that the $D\bar{D^*}+c.c.$ open channel is located in a relatively isolated energy, i.e. $\sim 3.876$ GeV, which is sufficiently far away from the known charmonia $ψ(3770)$ and $ψ(4040)$. Due to the dominance of the isospin-0 component at the charmonium energy region, an enhanced model-independent cusp effect between the thresholds of $D^0\bar{D^{*0}}+c.c.$ and $D^+ D^{*-}+c.c.$ can be highlighted. An energy scan over this energy region in the $e^+e^-$ annihilation reaction can help us to understand the nature of X(3900) recently observed by Belle Collaboration in $e^+ e^-\to D\bar{D}+c.c.$, and establish the open charm effects as an important non-perturbative mechanism in the charmonium energy region.

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arXiv2011

Revisit the radiative decays of $J/ψ$ and $ψ'\to γη_c (γη_c^\prime)$

With the new measurements of $J/ψ$ and $ψ^\prime\to γη_c \ (γη_c^\prime)$ from CLEO and BES-III Collaboration, we re-investigate the intermediate meson loop (IML) contributions to these radiative decays in association with the quark model M1 transitions in an effective Lagrangian approach. It shows that the "unquenched" effects due to the intermediate hadron loops can be better quantified by the new data for $J/ψ\to γη_c$. Although the IML contributions are relatively small in $J/ψ\to γη_c$, they play a crucial role in $ψ^\prime\to γη_c \ (γη_c^\prime)$. A prediction for the IML contributions to $ψ(3770)\to γη_c \ (γη_c^\prime)$ is made. Such "unquenched" effects allow us to reach a coherent description of those three radiative transitions, and gain some insights into the underlying dynamics.

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arXiv2011

Open charm effects in $e^+e^-\to J/ψη$, $J/ψπ^0$ and $ϕη_c$

We propose to study the open charm effects in $e^+ e^-\to J/ψη$, $J/ψπ^0$ and $ϕη_c$. We show that the exclusive cross section lineshapes of these processes would be strongly affected by the open charm effects. Since the final state light meson productions are through soft gluon radiations, we assume a recognition of this soft process via charmed meson loops at hadronic level. A unique feature among these three reactions is that the $D\bar{D^*}+c.c.$ open channel is located in a relatively isolated energy, i.e. $\sim 3.876$ GeV, which is sufficiently far away from the known charmonia $ψ(3770)$ and $ψ(4040)$. Therefore, the cross section lineshapes of these reactions may provide an opportunity for singling out the open charm effects with relatively well-defined charmonium contributions. In particular, we find that reaction $e^+ e^-\to J/ψπ^0$ is sensitive to the open charm $D\bar{D^*}+c.c.$ Due to the dominance of the isospin-0 component at the charmonium energy region, we predict an enhanced model-independent cusp effect between the thresholds of $D^0\bar{D}^0+c.c.$ and $D^+ D^{*-}+c.c.$ This study can also help us to understand the X(3900) enhancement recently observed by Belle Collaboration in $e^+ e^-\to D\bar{D}+c.c.$

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arXiv2011

Categorizing resonances X(1835), X(2120) and X(2370) in the pseudoscalar meson family

Inspired by the newly observed three resonances X(1835), X(2120) and X(2370), in this work we systematically study the two-body strong decays and double pion decays of $η(1295)/η(1475)$, $η(1760)/X(1835)$ and $X(2120)/X(2370)$ by categorizing $η(1295)/η(1475)$, $η(1760)/X(1835)$, X(2120) and X(2370) as the radial excitations of $η(548)/η^\prime(958)$. Our numerical results indicate the followings: (1) The obtained theoretical strong decay widths of three pseudoscalar states $η(1295)$, $η(1475)$ and $η(1760)$ are consistent with the experimental measurements; (2) X(1835) could be the second radial excitation of $η^\prime(958)$; (3) X(2120) and X(2370) can be explained as the third and fourth radial excitations of $η(548)/η^\prime(958)$, respectively.

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arXiv2011

Understanding the radiative decays of vector charmonia to light pseudoscalar mesons

We show that the newly measured branching ratios of vector charmonia ($J/ψ$, $ψ^\prime$ and $ψ(3770))$ into $γP$, where $P$ stands for light pseudoscalar mesons $π^0$, $η$, and $η^\prime$, can be well understood in the framework of vector meson dominance (VMD) in association with the $η_c$-$η(η^\prime)$ mixings due to the axial gluonic anomaly. These two mechanisms behave differently in $J/ψ$ and $ψ^\prime\to γP$. A coherent understanding of the branching ratio patterns observed in $J/ψ(ψ^\prime)\to γP$ can be achieved by self-consistently including those transition mechanisms at hadronic level. The branching ratios for $ψ(3770)\toγP$ are predicted to be rather small.

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arXiv2011

Effect of charmed meson loops on charmonium transitions

The effects of intermediate charmed mesons on charmonium transitions with the emission of one pion or eta are studied systematically. Based on a non-relativistic effective field theory we show that charmed meson loops are enhanced compared to the corresponding tree-level contributions for transitions between two S-wave charmonia as well as for transitions between two P-wave charmonia. On the contrary, for the transitions between one S-wave and one P-wave charmonium state, the loops need to be analyzed case by case and often appear to be suppressed. The relation to and possible implications for an effective Lagrangian approach are also discussed. This study at the same time provides a cross check for the numerical evaluations.

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arXiv2011

Further study of the helicity selection rule evading mechanism in $η_c$, $χ_{c0}$ and $h_c$ decaying to baryon anti-baryon pairs

We investigate the long distance contribution via charmed hadron loops in the processes $η_c$, $χ_{c0}$ and $h_c$ decaying to baryon anti-baryon pairs, which are supposed to be highly suppressed by the helicity selection rule as a consequence of the perturbative QCD framework. With an effective Lagrangian method, our estimation result indicates that such hadron loops play an important role in these hadronic decays. It is a further test of the evading mechanism for the helicity selection rule in charmonium baryon-antibaryon decays.

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arXiv2010

Puzzles in charmonium decays

The open charm effects via intermediate hadron loop transitions seem to play a crucial role in the understanding of several existing "puzzles" in charmonium exclusive decays, such as the $ψ(3770)$ non-$D\bar{D}$ decays, and "$ρπ$ puzzle" etc. In the charmonium energy region, non-perturbative mechanisms could be still sizeable, and as a consequence the intermediate hadron loop transitions also provide a mechanism for the helicity-selection-rule (HSR) violation. We report our recent progress on those existing puzzles.

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arXiv2010

Chiral quark model approach for the study of baryon resonances

We report the chiral quark model approach for the study of baryon resonances in meson photoproduction and meson-nucleon scattering processes. Focussing on the $πN$ and $K N$ scatterings, we show that the cross sections near threshold can be well accounted for by the chiral quark model with only few parameters. The $S$-wave dominance in both $π^- p\to ηn$ and $K^- p\to Σ^0π^0$ can be well understood in the quark model framework. In particular, the quark model provides a constraint on the relative signs for resonance coupling form factors, which seems to be consistent with results from isobaric models.

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arXiv2010

Study of $η_c$ and $η_c^\prime$ decays into vector meson pairs

The processes of $η_c (η_c')\to VV$ are supposed to be suppressed by the helicity selection rule (HSR) but found to be rather important decay modes in experiment for $η_c$. We try to distinguish the short-distance transitions via the singly disconnected Okubo-Zweig-Iizuka (SOZI) processes and the doubly disconnected (DOZI) processes in $η_c (η_c')\to VV$. It shows that the SOZI processes can be related to the $η_c(η_c')$ wavefunctions at the origin. Therefore, a relation of the decay branching ratio fraction between these two decay channels can be established. Such a relation is similar to that for $J/ψ$ and $ψ^\prime\to VP$, where the so-called "$ρπ$ puzzle" has been existed for a while. We also show that the intermediate charmed meson loop transitions provide an evading mechanism for the DOZI processes. This contribution would turn out to be more important in $η_c^\prime\to VV$. As a consequence, it may produce significant deviations from the SOZI-dominant scenario. Future experimental measurement of the $η_c^\prime\to VV$ by BESIII should be able to clarify the DOZI-evading mechanisms.

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arXiv2010

Novel analysis of the decays psi' -> h_c pi^0 and eta_c'-> chi_{c0} pi^0

We show that in the transitions psi' -> h_c π^0 and eta_c' -> chi_{c0} pi^0 the contributions from charmed meson loops are highly suppressed, in contrast to various other charmonium decays. We calculate the width of the psi' -> h_c pi^0, which agrees with the recent BES-III data, and predict the width of the eta_c' -> chi_{c0} pi^0, Γ(eta_c' -> chi_{c0} pi^0) = 1.5\pm0.4 keV. A confirmation of this prediction would also provide additional support for a recent analysis of psi' -> J/psi pi^0 (eta), where loops are claimed to play a prominent role.

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arXiv2010

Lineshape of $e^+ e^-\to D^* \bar D+c.c.$ and electromagnetic form factor of $D^*\to D$ transition in the time-like region

In this work, we apply the vector meson dominance (VMD) model to extract the electromagnetic time-like form factor of the $D^*\to D$ transition combining the recent Belle data for $e^+ e^-\to D^{*+} D^- + c.c.$ and data for $D^*\to Dγ$. Two solutions are obtained in the interpretation of the cross section lineshape: i) With a relatively large coupling for $ψD^*\bar{D}$ determined by experiment, destructive interferences among those charmonium components are required to bring down the overall cross sections, and then account for the cross section lineshape. ii) With a relatively small value for the $ψD^*\bar{D}$ coupling based on heavy quark theory, an apparent cross section deficit near threshold is observed, and contributions from other mechanisms are needed. It might imply the presence of an additional resonance X(3900). Meanwhile, we also point out that an enhancement like that could be produced by the $D_s^*\bar{D_s}+c.c.$ open channel effects.

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arXiv2010

The lineshape of $ψ(3770)$ and low-lying vector charmonium resonance parameters in $e^+ e^-\to D\bar D$

We investigate the $D\bar{D}$ production in $e^+e^-$ annihilations near threshold in an effective Lagrangian approach. This shows that the lineshape of the cross section near threshold is sensitive to the contributions from $ψ^\prime$, though it is below the $D\bar{D}$ threshold. The recent experimental data from the BES and Belle collaborations allow us to determine the $ψ^\prime D\bar{D}$ coupling constant which appears to be consistent with other theoretical studies. As a consequence, the $ψ^\prime$-$ψ(3770)$ mixing parameter can be extracted around the $ψ(3770)$ mass region. Resonance parameters for $ψ(3770)$, X(3900), $ψ(4040)$, and $ψ(4160)$ are also investigated. The X(3900) appears as an enhancement at around 3.9 GeV in the Belle data. In addition to treating it as a resonance, we also study the mechanism that the enhancement is produced by the $D\bar{D^*}+c.c.$ open channel effects. Our result shows that such a possibility cannot be eliminated.

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arXiv2010

The evasion of helicity selection rule in $χ_{c1}\to VV$ and $χ_{c2}\to VP$ via intermediate charmed meson loops

The hadronic decays of $χ_{c1}\to VV$ and $χ_{c2}\to VP$ are supposed to be suppressed by the helicity selection rule in the pQCD framework. With an effective Lagrangian method, we show that the intermediate charmed meson loops can provide a mechanism for the evasion of the helicity selection rule, and result in sizeable decay branching ratios in some of those channels. The theoretical predictions can be examined by the forthcoming BES-III data in the near future.

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arXiv2010

Strong decays of newly observed $D_{sJ}$ states in a constituent quark model with effective Lagrangians

The strong decay properties of the newly observed states $D_{sJ}(3040)$, $D_{sJ}(2860)$ and $D_{sJ}(2710)$ are studied in a constituent quark model with quark-meson effective Lagrangians. We find that the $D_{sJ}(3040)$ could be identified as the low mass physical state $|2{P_1}>_L$ ($J^P=1^+$) from the $D_{s}(2^1P_1)$-$D_{s}(2^3P_1)$ mixing. The $D_{sJ}(2710)$ is likely to be the low-mass mixed state $|(SD)>_L$ via the $1^3D_1$-$2^3S_1$ mixing. In our model, the $D_{sJ}(2860)$ cannot be assigned to any single state with a narrow width and compatible partial widths to $DK$ and $D^*K$. Thus, we investigate a two-state scenario as proposed in the literature. In our model, one resonance is likely to be the $1^3D_3$ ($J^P=3^-$), which mainly decays into $DK$. The other resonance seems to be the $|1{D_2}'>_H$, i.e. the high-mass state in the $1^1D_2$-$1^3D_2$ mixing with $J^P=2^-$, of which the $D^*K$ channel is its key decay mode. We also discuss implications arising from these assignments and give predictions for their partner states such as $|(SD)'>_H$, $|2{P'_1}>_H$, $2^3P_0$ and $2^3P_2$, which could be helpful for the search for these new states in future experiment.

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arXiv2009

A possible mechanism for producing the threshold enhancement in $J/ψ\toγp\bar{p}$

In the $J/ψ$ radiative decay channels $J/ψ\to γV\bar{V}$, the result of partial wave analysis indicates that the $V\bar{V}$ systems are predominately pseudoscalar component, and most of these channels have relatively large branching ratios at an order of $10^{-3}$. Meanwhile, vector mesons, such as $ρ$, $ω$ and $K^*$, have strong couplings with nucleons and/or hyperons. This suggests a dynamical mechanism describing the $ηp\bar{p}$ form factors for higher $η$ mesons, such as $η(1405/1475)$ and $η(1760)$. It is thus natural to expect that rescatterings of these vector meson pairs into $p\bar{p}$ of $0^-$ partial wave could be an important source contributing to $J/ψ\toγp\bar{p}$ of which the branching ratio is at the order of $10^{-4}$. Our calculation justifies this point. In particular, we find that interferences between different rescattering amplitudes can produce a significant threshold enhancement in the invariant mass spectrum of $p\bar{p}$. Without introducing dramatic ingredients, our model provides a natural explanation for the peculiar threshold enhancement observed by BES-II in $J/ψ\to γp\bar{p}$. Additional experimental constraints on the $V\bar{V}\to p\bar{p}$ transitions are examined. This mechanism in $J/ψ\to ωp\bar{p}$ is also discussed.

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