重离子碰撞中的同位旋和近阈K产生均为当今核反应研究的...

Qingfeng Li, Isospin in HICs and K production at near threshold - i - 摘要

重离子碰撞中的同位旋和近阈K 产生均为当今核反应研究的前沿内容和热点问题。我们在同位旋相关的量子分子动力学（IQMD ）模型及自洽的相对论Boltzmann-Uehling-Uhlenbeck （RBUU ）模型基础上对它们作了一些理论研究，得到了一些有意义的理论成果。

在中能重离子碰撞的同位旋研究方面，分别研究了对称势的强度（C S ）对中能重离子核反应各类碎片产物产额、相空间、及其同位旋的分布等多种实验观测量的影响，以获取对称势的信息。我们的研究表明越丰中子核系统的中子-质子差的快度分布显示出对对称势更强的依赖。用QHD-II 模型研究了介质两体截面的同位旋相关性，研究结果表明截面的介质修正也是同位旋依赖的：中子-质子介质截面对核密度的依赖很弱，而中子-中子（质子-质子）的介质截面对核密度的依赖很强。在此基础上我们研究了核阻止本领对系统大小、初始核N/Z 、平均场中对称势以及两体散射截面介质修正等的依赖情况，研究结果表明它受初始核N/Z 比以及对称势的影响很弱，而受碰撞截面介质修正的同位旋依赖效应及核态方程影响明显。我们的结果还表明测量中等大小质量的系统在中能重离子碰撞中的核阻止本领激发函数比测量重的系统能提供更为确切的有关两体截面介质修正的信息。进一步地，利用不同的同位旋相关观测量我们还研究了系统平衡程度问题，发现中子-质子差的快度分布可以探测到入射能量达到400A MeV 时反应系统的平衡程度比100A MeV 入射能量时要高，中心快度区的系统基本达到平衡，而弹、靶快度区则存在明显的入射道效应。通过比较不同能量（100和400A MeV ）时出射IMF 的N/Z ，我们发现它随着能量的上升而下降。

在中高能重离子碰撞的K 产生方面，我们首先研究了K +产生。研究表明K +集体流在相对论重离子碰撞中对核态方程和K-N 标量吸引项（∑KN ）都是敏感的，计算结果表明核子和K +介子的横向集体流都需要压缩系数在K ≈200 – 300 MeV 范围内较软的核态方程，而大约为200 - 400 MeV 的∑KN 值对解释K +流产生是合适的。较大碰撞参数下的K +集体流显示出更加灵敏地依赖于∑KN ，有可能获得∑KN 值的更为准确的信息。接着我们又研究了Ｋ－介子的吸收问题，发现虽然有强烈的Ｋ－吸收存在，其能谱在不太重的核系统的非中心碰撞中仍显示出对∑KN 的依赖，有可能提供Ｋ－在核介质中性质的知识。我们的研究表明K +/Ｋ－产额比是与核系统大小相关的，这种相关性与Ｋ－介子在核介质中所受的吸收效应有关。通过对不同核系统中的Ｋ－平均吸收路程及不同产生道的细致研究，表明Ｋ－介子产生、吸收的程度与系统的大小、Ｋ－产生元过程等有关。需要强调指出的是，在以上Ｋ－产生过程的研究中我们发现X K B -→π和B K Y -→π有重要影响。

关键词：重离子碰撞，同位旋，K产生- ii -

Qingfeng Li, Isospin in HICs and K production at near threshold - iii - Abstract （英文摘要）

The isospin effect and K production in intermediate and high energy Heavy Ion Collisions (HICs) are hot topics in the nuclear physics. Based on the Isospin-dependent Quantum Molecular Dyanmics (IQMD) model and self-consistent Relativistic Boltzmann-Uehling-Uhlenbeck (RBUU) model, we have studied them and obtained some interesting results.

As for the study of isospin in intermediate energy HICs, we ’ve investigated how stength (C S ) affects many measurable observables, such as the yield, phase-space, and isospin distributions of fragments. The aim is to find sensitive observables for strength of SP. Our results show the rapidity distribution of differential neutron-proton counting (N np ) from more neutron-rich reactions can be favorable to extract the information of SP. In the study of isospin-dependent nucleon-nucleon elastic cross sections in nuclear medium we find the medium correction of nucleon-nucleon scattering cross sections is also isospin

dependent, *np σdepends on the baryon density weakly and ()*

nn pp σ depends on the

baryon density significantly. And based on these findings, we further study the dependence of nuclear stopping on system size, initial N/Z , SP and the medium correction of two-body cross sections. It ’s found that the effect of initial N/Z ratio and isospin SP on stopping is weak, and the excitation function of stopping depends on the form of medium correction of two-body cross sections and the equation of state of nuclear matter strongly. For probing the isospin dependence of the medium correction of two-body cross sections, the results show that the behavior of the excitation function of stopping for medium size nuclear collisions can provide clearer information than that for heavier systems. We have further studied equilibration with respect to isospin degree of freedom and proposed that the neutron-proton differential rapidity distribution is a sensitive probe to the energy dependence of the degree of equilibration: there exists memory effect in multifragmentation process; the degree of equilibration at E =400A MeV is higher than that at E =100A MeV; the average N/Z ratio of IMF reduces largely as beam energy increases from 100A MeV to 400A MeV.

As for the K production in high energy HICs, we firstly study K + production. The results show that the kaon flow is sensitive to both the kaon-nucleon sigma term (∑KN ) and the equation of state of nuclear matter. The collective flows of both nucleons and K + mesons need a “soft ” EoS with compressibility K ≈200-300 MeV, and ∑KN =200-400 MeV seems suitable to explain the measured K + flow. We further find that anti-flow appears at certain large impact parameter, which seems to be more sensitive to ∑KN and might provide

- iv - more definite information of the magnitude of ∑KN . The absorption effect of -K mesons is also studied, we find although there exists strong -K absorption, -K spectrum can still contain the messages of -K in nuclear medium. Our investigations show the ratio of the yield of K + to Ｋ－ depends on the size of the nuclear systems, which is mainly caused by the absorption effect of the Ｋ－ in the nuclear medium. By detailed studies on both mean-absorptions path of the Ｋ－ in different nuclear systems and the inpidual effect of each of elementary production channels, we find both Ｋ－ production and Ｋ－ absorption effect depend on the size of nuclear system and the elementary production processes. It should be emphasized that the channels X K B -→π and B K Y -→π play important roles in the Ｋ－ production.

Key words : heavy ion collisions, isospin, K production

李庆峰：重离子碰撞中的同位旋及近阈K产生/ 导师：李祝霞

目录

摘要 (i)

Abstract （英文摘要） (iii)

目录 (v)

第一章序言 (1)

第二章研究基础 (5)

2.1 量子分子动力学模型（QMD） (5)

2.1.1 QMD理论 (5)

2.1.2 QMD输运过程 (13)

2.1.3 QMD模型的发展 (18)

2.2 相对论Boltzmann-Uehling-Uhlenbeck（RBUU） (20)

2.2.1 传统BUU理论的发展历史 (20)

2.2.2 自洽RBUU理论 (21)

2.2.3 RBUU输运过程及与QMD模型的比较 (28)

第三章中能重离子碰撞中的同位旋 (31)

3.1 对称势 (31)

3.2 介质中两体弹性散射截面的同位旋依赖 (37)

3.3 同位旋相关性在QMD程序中的实现 (46)

3.4 关于同位旋问题的一些模型计算及讨论 (48)

3.4.1 对称势对多重碎裂产物的同位旋分布影响 (48)

3.4.2 系统平衡程度探索 (54)

3.4.3 两体截面的同位旋效应对核阻止本领的影响 (59)

第四章中高能重离子碰撞中的K产生 (63)

4.1 K介子势 (63)

4.2 K介子产生基本截面 (67)

4.3 计算结果及讨论 (69)

4.3.1 SIS能区重离子碰撞的K+产生及集体流 (69)

4.3.2 SIS能区重离子碰撞的K－吸收对其产生的影响 (76)

4.4 K介子研究进展 (83)

第五章总结及展望 (85)

附录 (89)

附录A1 (89)

附录A2 (92)

附录A3 (94)

附录B1 (97)

参考文献 (99)

博士期间发表的论文 (105)

致谢 (107)

Qingfeng Li, Isospin in HICs and K production at near threshold - v -

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