重介子系统中耦合道效应的研究
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摘要
近年来,Belle的B介子工厂和Babar等实验室报道了大量含重味夸克(c,b)的新强子态,这其中引起理论和实验物理学家广泛兴趣的是所谓XYZ粒子,这些粒子的共同特点是它们的衰变末态中都含有c和c夸克。在理论上,人们试图从不同的角度对这些新强子态作出解释。量子色动力学(QCD)被公认为是强相互作用的基本理论,但在中低能区,由于微扰论不再适用,再加上非微扰QCD的复杂性,很难完全用QCD第一性原理处理低能强子问题。然而,具有QCD精神的夸克模型在描述核子-核子相互作用、重子谱、重夸克偶素能谱等强子性质方面都取得了非常大的成功。在运用组分夸克模型解释XYZ新强子态时,如果把它们当做传统的介子,虽然能够找到一些相应量子数的态,但是我们发现在阈值以上的各个态,符合得都不是很好。于是人们尝试把这些粒子解释为粲夸克偶素,四夸克态,分子态,混杂态等等,但迄今为止,没有一种解释是让人信服的。
传统的势模型只考虑介子由两夸克组成,我们认为介子组分中的高Fock态的贡献不应该被忽略,亦即整个介子体系既含有两夸克态,同时还包括四夸克成分。其中四夸克成分由真空中激发的正反夸克对与原来的价夸克形成且可看作两个介子。以粲夸克偶素为例,轻夸克对的产生会导致cc与DD介子对之间的混合,这样一个耦合道体系会显著地改变介子的质量谱,我们把这种效应称之为耦合道效应。基于组分夸克模型和3P0模型,我们计算了实验上已知的一些重夸克介子的质量平移,发现四夸克成分对能谱的影响是不能忽略的,这为考虑耦合道效应后重新计算介子谱奠定了基础。
在确定强子的量子态时,不仅需要能谱符合,强衰变、轻子衰变、辐射衰变等性质是更重要的判断标准。本文从质量谱和强衰变的角度讨论了实验上新发现的两个强子态X(4160)和X(3915)。从质量谱的角度分析,χ0(33P0),χ1(33P1),ηc2(21D2),ηc(41S0)四个态都与X(4160)实验值相符合,但我们的计算结果表明只有ηc2(21D2)的强衰变性质与X(4160)的实验结果符合较好,因此把X(4160)解释为粲夸克偶素ηc2(21D2)是合理的。虽然χ0(23P0)的质量与X(391.5)相符合,但强衰变宽度却与实验结果相差很大,所以两者不可能是同一个粒子。在考虑隐色道效应后,我们可以把X(3915)解释为I(JPG)=0(0++)的四夸克D*D*分子态。
组分夸克模型在描述强子性质方面取得了巨大的成功,但还存在一些问题。为了解释更多的新强子态,模型需要发展。以3P0模型为基础,考虑耦合道效应对介子能谱的修正是一次有意义的尝试；同时,从强衰变的角度研究新强子态,比仅仅考虑能谱更加合理。这些工作的研究,会加深我们对强子物理的和QCD的理解。
Recently, the Belle and BaBar and other collaborations in B meson factories have reported a number of new hadron states with b or c quarks. The states which aroused extensive interest for theoretical and experimental physicists are the so-called XYZ parti-cles, whose common feature is that they all contain c or c quark in the final states of their strong decay. Theoretically, people have tried to explain these new hadron states from different perspectives. The quantum chromodynamics (QCD) is generally accepted as the fundamental theory of strong interaction, but it is difficult to deal with the lower energy hadronic problems by the first principle of QCD because of the failure of perturbative theory and the complex of nonperturbative theory. However, the QCD-inspired quark models have achieved great success to describe the nucleon-nucleon interaction, spectrum of baryons, heavy quark positronium. When we apply the constituent quark model to ex-plain the new hadron states, although, we can find the states with corresponding quantum number if they are treated as traditional mesons, but the energy of most of states above the threshold of DD mesons is not in good agreement with the experimental date. So people try to explain these particles as four quark states, molecular states, hybrid states and so on except charmonium states, but no one explanation is convincing until now.
In the traditional constituent quark model, the meson is consists of two quarks. In fact that the meson system can have four-quark and more quark components besides two-quark component. The contribution from these high Fock state components should not be ignored in meson configuration. These high Fock states can be treated as hadrons which come from the combination of the quark and qnti-quark created from the vacuum with the original valence quark and antiquark. Take charmonium as an example, the light quark pair creation can result in mixing between the bare charmonium state cc and the DD meson pair. Such a coupled channel system will significantly change the meson mass spectrum, and we call it coupled channel effects. Based on constituent quark model and 3P0 model, we calculate mass shifts of some heavy quark mesons which are established well experimentally, and find that the effect on the mass spectrum can not be ignored from four-quark component, which lays the foundation for recalculating the meson mass spectrum in the unquenched quark model.
When we establish the quantum state of hadrons, not only the mass spectrum need to be in agreement with the experiment, but also strong decay, leptonic decay and ra-diative decay are important criteria. In this paper we study the properties of the new states X(4160) and X(3915) in terms of mass spectrum and strong decay of charmonium states. According to the mass spectrum of charmonium states predicted by the potential model, the statesλ0(33P0);λ1(33P1);ηc2(21D2),(41S0) all can be candidates for the X(4160). However, only the decay width of the stateηc2(21D2) in our calculation is in good agreement with the experimental date. Therefore, it is reasonable to interpret the charmonium stateηc2(21D2) as the state X(4160). For the state X(3915), although the mass ofχ0(23P0) is compatible with the experimental value, the calculated strong decay width is much larger than experimental data. Hence, the assignment of X(3915) to char-monium stateχ0(23P0) is disfavored in our calculation. X(3915) can be explained as a molecular state D*0D*+ with I(JPC)=0(0++) when the hidden color channel effects are applied.
The constituent quark models achieve a great success on hadron physics, there are also several unresolved problems. In order to explain the new hadron states, the development of the quark model is needed. It is a significant attempt to modify the mass spectrum of mesons based on the coupled channel effects and 3P0 model. To identify new hadron states, it is more reasonable to do according to the strong decay width of the states than to the mass spectrum only. This theoretical work is helpful to deepen our understanding of hadron physics and QCD.
传统的势模型只考虑介子由两夸克组成,我们认为介子组分中的高Fock态的贡献不应该被忽略,亦即整个介子体系既含有两夸克态,同时还包括四夸克成分。其中四夸克成分由真空中激发的正反夸克对与原来的价夸克形成且可看作两个介子。以粲夸克偶素为例,轻夸克对的产生会导致cc与DD介子对之间的混合,这样一个耦合道体系会显著地改变介子的质量谱,我们把这种效应称之为耦合道效应。基于组分夸克模型和3P0模型,我们计算了实验上已知的一些重夸克介子的质量平移,发现四夸克成分对能谱的影响是不能忽略的,这为考虑耦合道效应后重新计算介子谱奠定了基础。
在确定强子的量子态时,不仅需要能谱符合,强衰变、轻子衰变、辐射衰变等性质是更重要的判断标准。本文从质量谱和强衰变的角度讨论了实验上新发现的两个强子态X(4160)和X(3915)。从质量谱的角度分析,χ0(33P0),χ1(33P1),ηc2(21D2),ηc(41S0)四个态都与X(4160)实验值相符合,但我们的计算结果表明只有ηc2(21D2)的强衰变性质与X(4160)的实验结果符合较好,因此把X(4160)解释为粲夸克偶素ηc2(21D2)是合理的。虽然χ0(23P0)的质量与X(391.5)相符合,但强衰变宽度却与实验结果相差很大,所以两者不可能是同一个粒子。在考虑隐色道效应后,我们可以把X(3915)解释为I(JPG)=0(0++)的四夸克D*D*分子态。
组分夸克模型在描述强子性质方面取得了巨大的成功,但还存在一些问题。为了解释更多的新强子态,模型需要发展。以3P0模型为基础,考虑耦合道效应对介子能谱的修正是一次有意义的尝试；同时,从强衰变的角度研究新强子态,比仅仅考虑能谱更加合理。这些工作的研究,会加深我们对强子物理的和QCD的理解。
Recently, the Belle and BaBar and other collaborations in B meson factories have reported a number of new hadron states with b or c quarks. The states which aroused extensive interest for theoretical and experimental physicists are the so-called XYZ parti-cles, whose common feature is that they all contain c or c quark in the final states of their strong decay. Theoretically, people have tried to explain these new hadron states from different perspectives. The quantum chromodynamics (QCD) is generally accepted as the fundamental theory of strong interaction, but it is difficult to deal with the lower energy hadronic problems by the first principle of QCD because of the failure of perturbative theory and the complex of nonperturbative theory. However, the QCD-inspired quark models have achieved great success to describe the nucleon-nucleon interaction, spectrum of baryons, heavy quark positronium. When we apply the constituent quark model to ex-plain the new hadron states, although, we can find the states with corresponding quantum number if they are treated as traditional mesons, but the energy of most of states above the threshold of DD mesons is not in good agreement with the experimental date. So people try to explain these particles as four quark states, molecular states, hybrid states and so on except charmonium states, but no one explanation is convincing until now.
In the traditional constituent quark model, the meson is consists of two quarks. In fact that the meson system can have four-quark and more quark components besides two-quark component. The contribution from these high Fock state components should not be ignored in meson configuration. These high Fock states can be treated as hadrons which come from the combination of the quark and qnti-quark created from the vacuum with the original valence quark and antiquark. Take charmonium as an example, the light quark pair creation can result in mixing between the bare charmonium state cc and the DD meson pair. Such a coupled channel system will significantly change the meson mass spectrum, and we call it coupled channel effects. Based on constituent quark model and 3P0 model, we calculate mass shifts of some heavy quark mesons which are established well experimentally, and find that the effect on the mass spectrum can not be ignored from four-quark component, which lays the foundation for recalculating the meson mass spectrum in the unquenched quark model.
When we establish the quantum state of hadrons, not only the mass spectrum need to be in agreement with the experiment, but also strong decay, leptonic decay and ra-diative decay are important criteria. In this paper we study the properties of the new states X(4160) and X(3915) in terms of mass spectrum and strong decay of charmonium states. According to the mass spectrum of charmonium states predicted by the potential model, the statesλ0(33P0);λ1(33P1);ηc2(21D2),(41S0) all can be candidates for the X(4160). However, only the decay width of the stateηc2(21D2) in our calculation is in good agreement with the experimental date. Therefore, it is reasonable to interpret the charmonium stateηc2(21D2) as the state X(4160). For the state X(3915), although the mass ofχ0(23P0) is compatible with the experimental value, the calculated strong decay width is much larger than experimental data. Hence, the assignment of X(3915) to char-monium stateχ0(23P0) is disfavored in our calculation. X(3915) can be explained as a molecular state D*0D*+ with I(JPC)=0(0++) when the hidden color channel effects are applied.
The constituent quark models achieve a great success on hadron physics, there are also several unresolved problems. In order to explain the new hadron states, the development of the quark model is needed. It is a significant attempt to modify the mass spectrum of mesons based on the coupled channel effects and 3P0 model. To identify new hadron states, it is more reasonable to do according to the strong decay width of the states than to the mass spectrum only. This theoretical work is helpful to deepen our understanding of hadron physics and QCD.
引文
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