arXiv subjects

Q. B. Li

Publications and source records attributed to Q. B. Li.

15 recordsLinked to original sources

arXiv2007

Sea-quark effects in the pion charge form factor

It is shown that the data on the pion charge form factor admit the possibility for a substantial sea-quark components in the pion wave function. If the charge form factor is calculated with instant form kinematics in a constituent quark model that is extended to include explicit $(q\bar q)^2$ components in the pion wave function, that component will give the dominant contribution to the calculated $π^+$ charge form factor at large values of momentum transfer. The present experimental values $Q^2$ can be described well with $(q\bar q)^2$ component admixtures of up to 50%. The sensitivity of the calculated $π^+$ charge form factor to whether one of the quarks or one of the antiquarks is taken to be in the P-state is small.

hep-ph↗

arXiv2007

The Role of 5-quark Components on the Nucleon Form Factors

The covariant quark model is shown to allow a phenomenological description of the neutron electric form factor, G_E^n(Q^2), in the impulse approximation, provided that the wave function contains minor (~ 3 %) admixtures of the lowest sea-quark configurations. While that form factor is not very sensitive to whether the \bar q in the qqqq\bar q component is in the P-state or in the S-state, the calculated nucleon magnetic form factors are much closer to the empirical values in the case of the former configuration. In the case of the electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse approximation close to 9 GeV^2, when the \bar q is in the P-state. That configuration, which may be interpreted as a pion loop ("cloud") fluctuation, also leads to a clearly better description of the nucleon magnetic moments. When the amplitude of the sea-quark admixtures are set so as to describe the electric form factor of the neutron, the qqqq\bar q admixtures have the phenomenologically desirable feature, that the electric form factor of the proton falls at a more rapid rate with momentum transfer than the magnetic form factor.

nucl-th↗

arXiv2006

The $qqqq\bar q$ components and hidden flavor contributions to the baryon magnetic moments

The contributions from the $qqqq\bar q$ components to the magnetic moments of the octet as well as the $Δ^{++}$ and $Ω^-$ decuplet baryons are calculated for the configurations that are expected to have the lowest energy if the hyperfine interaction depends both on spin and flavor. The contributions from the $u\bar u$, $d\bar d$ and $s\bar s$ components are given separately. It is shown that addition of $qqqq\bar q$ admixtures to the ground state baryons can improve the overall description of the magnetic moments of the baryon octet and decuplet in the quark model without SU(3) flavor symmetry breaking, beyond that of the different constituent masses of the strange and light-flavor quarks. The explicit flavor (and spin) wave functions for all the possible configurations of the $qqqq\bar q$ components with light and strange $q\bar q$ pairs are given for the baryon and octet and decuplet. Admixtures of ~ 10% of the $qqqq\bar q$ configuration where the flavor-spin symmetry is $[4]_{FS}[22]_F[22]_S$, which is likely to have the lowest energy, in particular reduces the deviation from the empirical values of the magnetic moments of the $Σ^ -$ and the $Ξ^0$ compared with the quark model.

nucl-th↗

arXiv2006

The role of $q\bar q$ components in the N(1440) resonance

The role of 5-quark components in the pion and electromagnetic decays and transition form factors of the N(1440) is explored. The $qqqq\bar q$ components, where the 4-quark subsystem has the flavor-spin symmetries $[4]_{FS}[22]_F[22]_S$ and $[4]_{FS}[31]_F[31]_S$, which are expected to have the lowest energy of all $qqqq\bar q$ configurations, are considered in detail with a nonrelativistic quark model. The matrix elements between the 5-quark components of the N(1440) and the nucleon, $qqqq\bar q\to qqqq\bar q$, play a minor role in these decays, while the transition matrix elements $qqqq\bar q\to qqq$ and $qqq\to qqqq\bar q$ that involve quark antiquark annihilation are very significant. Both for the electromagnetic and strong decay the change from the valence quark model value is dominated by the confinement triggered $q\bar q$ annihilation transitions. In the case of pion decay the calculated decay width is enhanced substantially both by the direct $q\bar q \to π$ and also by the confinement triggered $q\bar q\to π$ transitions. Agreement with the empirical value for the pion decay width may be reached with a $\sim$ 30% $qqqq\bar q$ component in the N(1440).

nucl-th↗

arXiv2006

Five-quark components in $Δ(1232)\to Nπ$ decay

Five-quark $qqqq\bar q$ components in the $Δ(1232)$ are shown to contribute significantly to $Δ(1232)\to Nπ$ decay through quark-antiquark annihilation transitions. These involve the overlap between the $qqq$ and $qqqq\bar q$ components and may be triggered by the confining interaction between the quarks. With a $\sim$ 10% admixture of five-quark components in the $Δ(1232)$ the decay width can be larger by factors 2 - 3 over that calculated in the quark model with 3 valence quarks, depending on the details of the confining interaction. The effect of transitions between the $qqqq\bar q$ components themselves on the calculated decay width is however small. The large contribution of the quark-antiquark annihilation transitions thus may compensate the underprediction of the width of the $Δ(1232)$ by the valence quark model, once the $Δ(1232)$ contains $qqqq\bar q$ components with $\sim$ 10% probability.

nucl-th↗

arXiv2005

The role of five-quark components in gamma decay of the $Δ(1232)$

An admixture of 10-20 % of qqqq\bar q components in the Delta(1232) resonance is shown to reduce the well known underprediction for the decay width for Delta(1232)->N gamma decay by about half and that of the corresponding helicity amplitudes from a factor ~ 1.7 to ~ 1.5. The main effect is due to the quark-antiquark annihilation transitions qqqq\bar q -> qqq gamma, the consideration of which brings the ratio A_{3/2}/A_{1/2} and consequently the E2/M1 ratio R_{EM} into agreement with the empirical value. Transitions between qqqq\bar q components in the resonance and the nucleon qqqq\bar q->qqqq\bar q gamma are shown to enhance the calculated width by only a few percent, as long as the probability of the qqqq\bar q component of the Delta(1232) and the proton is at most ~ 20 %. The transitions qqqq\bar q->qqqq\bar q gamma between the qqqq\bar q components in the Delta(1232) and the proton do not lead to a nonzero value for R_{EM}.

nucl-th↗

arXiv2005

Bound states of $Θ^+$ in nuclei

We study the binding energy and the width of the $Θ^+$ in nuclei, associated to the $K N$ and $ K πN$ components. The first one leads to negligible contributions while the second one leads to a sizeable attraction, enough to bind the $Θ^+$ in nuclei. Pauli blocking and binding effects on the $K N$ decay reduce considerably the $Θ^+$ decay width in nuclei and medium effects associated to the $ K πN$ component also lead to a very small width, as a consequence of which one finds separation between the bound levels considerably larger than the width of the states.

nucl-th↗

arXiv2005

$Θ^+$ Hypernuclei

We present results for the selfenergy of the $Θ^+$ pentaquark in nuclei associated with two sources: the $KN$ decay of the $Θ^+$ and the two meson baryon decay channels of the $Θ^+$ partners in an antidecuplet of baryons. The first source is shown to produce a small potential, unable to bind the $Θ^+$ in nuclei, while the second source gives rise to a large attractive potential. At the same time we show that the width of the $Θ^+$ in nuclei is small, such that, in light and medium nuclei, many bound $Θ^+$ states would appear with a separation between levels appreciably larger than the width of the states, thus creating an ideal scenario for pentaquark spectroscopy in nuclei.

nucl-th↗

arXiv2004

$Θ^+$ pentaquark in the nuclear medium

We study the interaction of the $Θ^+$ pentaquark with nuclear matter associated to the $KN$ decay channels and to the two meson cloud. We find that the potential is attractive and could be strong enough to lead to the existence of $Θ^+$ nuclear bound states.

nucl-th↗

arXiv2004

Interaction of the $Θ^+$ with the nuclear medium

We study the selfenergy of the $Θ^+$ pentaquark in nuclei associated with two types of interaction: the KN decay of the $Θ^+$ and two meson baryon decay channels of the $Θ^+$. Whereas the potential related to the first source is quite weak, the second kind of interaction produces a large and attractive potential that could lead to the existence of $Θ^+$ nuclear bound states.

nucl-th↗

arXiv2000

Possible S-wave Dibaryons in SU(3) Chiral Quark Model

In the framework of the SU(3) chiral quark model, the $S-$wave baryon-baryon bound states are investigated. It is found that according to the symmetry character of the system and the contributions from chiral fields, there are three types of bound states. The states of the first type, such as $[ΩΩ]_{(0,0)}$ and $[Ξ^{*}Ω]_{(0,1/2)}$ are deeply bound dibaryon with narrow widths. The second type states, $[Σ^{*} Δ]_{(0,5/2)}$,$[Σ^{*} Δ]_{(3,1/2)}$, $[ΔΔ]_{(0,3)}$ and $[ΔΔ]_{(3,0)}$ are also bound states, but with broad widths. $[ΞΩ- Ξ^{*}Ω]_{(1,1/2)}$, $[ΞΞ]_{(0,1)}$, and $[N Ω]_{(2,1/2)}$ are third type states. They, like {\em d}, are weakly bound only if the chiral fields can provide attraction between baryons.

nucl-th↗

arXiv2000

Dibaryon Systems In SU(3) Chira Quark Model

The possible candidates of $S-$wave dibaryons with various strange numbers are studied under the chiral SU(3) quark model. It is shown that there are three types of baryon-baryon bound states. The states of the first type are called deuteron-like states. If chiral fields can provide enough attraction between interacting baryons, these systems, such as $[ΞΩ- Ξ^{*}Ω]_{(1,1/2)}$, $[ΞΞ]_{(0,1)}$, $[N Ω]_{(2,1/2)}$ would be weakly bound. The states of the second type such as $[Σ^{*} Δ]_{(0,5/2)}$, $[Σ^{*} Δ]_{(3,1/2)}$, $[ΔΔ]_{(0,3)}$ and $[ΔΔ]_{(3,0)}$ are named as $ΔΔ$-like states. Due to the highly symmetric character in orbital space, these systems could be relatively deeply bound, but the strong decay modes of composed baryons cause the widths of the states much broader. The states of the third type are entitled as $ΩΩ$-like states. Due to the same symmetry character shown in the systems of the second type and the only weak decay mode of composed baryons, for instance in $[ΩΩ]_{(0,0)}$, or at most one strong decay mode of composed baryons, for example in $[Ξ^{*}Ω]_{(0,1/2)}$, these states are deeply bound states with narrow widths. The states of latter two types are most interesting new dibaryon states and should be carefully investigated both theoretically and experimentally.

nucl-th↗

arXiv1999

Possible $ΔΔ$ dibaryons in the quark cluster model

In the framework of RGM, the binding energy of one channel $ΔΔ_{(3,0)}$($d^*$) and $ΔΔ_{(0,3)}$ are studied in the chiral SU(3) quark cluster model. It is shown that the binding energies of the systems are a few tens of MeV. The behavior of the chiral field is also investigated by comparing the results with those in the SU(2) and the extended SU(2) chiral quark models. It is found that the symmetry property of the $ΔΔ$ system makes the contribution of the relative kinetic energy operator between two clusters attractive. This is very beneficial for forming the bound dibaryon. Meanwhile the chiral-quark field coupling also plays a very important role on binding. The S-wave phase shifts and the corresponding scattering lengths of the systems are also given.

nucl-th↗

arXiv1999

$NΩ$ and $ΔΩ$ dibaryons in SU(3) chiral quark model

The binding energy of the six quark system with strangeness s=-3 is investigated under the chiral SU(3) constituent quark model in the framework of $RGM$. The calculations of the single $NΩ$ channel with spin S=2 and the single $ΔΩ$ channel with spin S=3 are performed. The results show that both systems could be dibaryons and the interaction induced by the chiral field plays a very important role on forming bound states in the systems considered. The phase shifts and scattering lengths in corresponding channels are also given.

nucl-th↗

arXiv1999

Possible Dibaryons with Strangeness s=-5

In the framework of $RGM$, the binding energy of the six quark system with strangeness s=-5 is systematically investigated under the SU(3) chiral constituent quark model. The single $Ξ^*Ω$ channel calculation with spins S=0 and 3 and the coupled $ΞΩ$ and $Ξ^*Ω$ channel calculation with spins S=1 and 2 are considered, respectively. The results show following observations: In the spin=0 case, $Ξ^* Ω$ is a bound dibaryon with the binding energy being $80.0 \sim 92.4 MeV$. In the S=1 case, $ΞΩ$ is also a bound dibaryon. Its binding energy is ranged from $26.2 MeV$ to $32.9 MeV$. In the S=2 and S=3 cases, no evidence of bound dibaryons are found. The phase shifts and scattering lengths in the S=0 and S=1 cases are also given.

nucl-th↗