19GeV/c质子—铅碰撞中领头粒子∧和△～（++）产生和相对论重离子碰撞中非均匀集体纵向膨胀流的研究
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摘要
p+A作用中入射质子的碎裂一直是一个重要的物理研究课题，这是因为它与核作用中重子数输运机制密切相关。而如今还没有完全了解此机制，详细研究领头粒子的产生可以帮助我们认识核碎裂和重子数输运机制。
E941实验组专门从事领头粒子测量和研究，它的主要特点是：能够测量中子以及集中对向前较大快度区间的粒子进行测量．本文通过分析在AGS上工作的E864／E941的实验数据，给出了在19GeV／c能量下最小无偏(minimum bias)p+Pb碰撞的领头奇异超子(Λ)和共振态重子(△~(++))的不变多重数分布和dn／dy分布。本实验所给出的Λ和△~(++)的快度和横动量范围分别对应为2.2＜y＜3.4和0.2GeV／c＜PT＜1.1GeV／c。比较Λ和△~(++)的产生与质子和中子的产生，以及与其它实验进行比较将会帮助我们了解p+A作用中质子碎裂机制。
实验中发现：在质子碎裂过程中，有关重子味变化的反应截面较大，且随快度变化而急剧变化，这不同于简单的双夸克-夸克(diquark-quark)碎裂机制所预言。与非奇异领头重子比较，在向前，陕度区间的领头奇异粒子Λ存在很强的压低现象。在2.7≤y≤3.3快度区间，入射质子碎裂成△~(++)和中子的相对几率为35％。我们将讨论这些结果对重子碎裂机制以及重子数输运理论所能产生的影响。
把E941的实验结果与RQMD和双夸克-夸克碎裂机制比较，证明了目前理论对重子碎裂机制的了解还很不够，需要进一步发展除了双夸克-夸克碎裂机制外的三夸克和胶子弦结碎裂机制。下面我们对这三种碎裂机制作一个简单的介绍，质子的组份夸克可以认为被一个胶子弦结联结起来(由图.1给出)，出现这种胶子弦结图象是规范不变的结果。在这种物理图象中，对应的三种重子碎裂过程为：当一根胶子弦断裂时，对应为双夸克-夸克(diquark-quark)碎裂；当两根胶子弦同时断裂时，对应为三夸克(three-quaxk)碎裂；当三根胶子弦同时断裂时，对应为胶子弦结(gluon junction)作用。我们将利用这些定义来描述入射质子碎裂过程碎裂过程。
对于Λ和△~(++)的数据分析，我们利用混合事件方法来扣除混合的背景。这种
＠：；：：冗。。。’‘
一
一
Figure：胶子弦结构造和双夸克－夸克碎裂的示意图．
方法已经成功用在E864核－核碰撞实验中分析不稳定共振态核和 A的产生．为了
重建八的事件取样，我们取至少有一个带正电荷和一个带负电荷的径迹的事件作
为取样事件2而对于凸”＼取至少有两个正电荷的径迹的事件作为取样事件．配成
对的两径迹在描迹器的撞击点必须有两个描迹器的狭条归幻叫的间距．此外为
了建造么＋＋的背景，要求来自不同事件的带正电荷径迹的动量不能同时大于 10
GeV／c，因为这将违背能量－动量守恒定律．在这种情况下，这两个带正电荷的径迹
很可能都为质子．由此方法产生的混合事件对的能量和动量分布与相同事件对的
能量和动量分布基本上相同．
在一个事件中，我们把质子的质量赋值于具有最大动量且带正电荷的径迹，其
它的径迹赋值于7T介子的质量，这样由不同电荷径迹所组成的对和相同电荷径迹
所组成的对的不变质量分布分别对应与人和凸付 相关的分布．然后利用一种迭
代的方法来消除背景．如果来自相同事件的对的数目为儿me，来自混合事件对的
数目为几；d叩，用相同事件的对的不变质量分布减去混合事件对的不变质量分布
11
／t巴八博士学泣论文
V反臼民刃 nm”01M1。D旧L卜1／＼mw
与标度因子Ns。一／Nmb；叩的乘积·由此所得到的不变质量分布的直方图的净记录
应为零，我们记下所有的正记录为屿．然后重复上述过程，但对应的标度因子换咸
（兀。＿一M／Nt；叩．直到相邻两次仰的差值小于某一给定值时，这种循环过
程才结束．
is＿nE－－－－－－－－－－－－－－－－ldj5EYjiHI
ie”－I＝——”—’———－·一l
2 卜DI
o70o卜J D。＿＿。、D
6O o卜 厂1D
＊co卜 几 nI
阜o口卜 丛ID
扣o卜 0 回0
ZOO卜－＿h el
1＊o卜I”下．．＿出1I
o－－－－
毛刀61．OSI。11．121．141，16
nzrrss（Ge皿2）
Figure 2：一0．2 T磁场的un的不变质量分布．背景用一个指数函数来
Projectile proton fragmentation in p + A interactions has been an interesting physics topic because it is related to the dynamics of baryon number transport in nuclear collisions. At present there is no comprehensive understanding of the dynamics.
E941 is an unique experiment for the leading particle measurement and study. It has the ability of measuring neutron and has large coverage of forward rapidity region. Invariant multiplicity distributions and dn/dy of the leading hyperon (A) and resonance (A++) have been measured in minimum bias p+Pb collisions at 19 GeV/c with the E864/E941 spectrometer at the ACS. The experimental acceptance for the measurement covers a region of 2.2 < y < 3.4 and 0.2 GeV/c < pr <1.1 GeV/c for the measurement of A and A++. Comparisons of the A and A++ yields with the leading proton and neutron yields measured by the same
experiment and with other experimental data will shed light on the fragmentation of the incident proton associated with the production of various baryons in p + A interactions.
The cross section for baryon flavor change is large and strongly depends on rapidity. This is very different from the expectation of simple diquark-quark fragmentation of incident proton. A suppression of leading A production in the forward rapidity region compared with the non-strange leading baryons is also observed. The relative probability of the projectile proton fragmentation into A++ and neutron is found to be about 35% in the region of 2.7collisions.
The comparison of our results with RQMD and with the diquark-quark fragmentation model demonstrated significant deficiencies in current understanding of baryon fragmentation processes. Theoretical understanding for three-quark and gluon junction fragmentation processes has to be developed. These studies will have an important impact on nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC). The measurement of baryon and anti-baryon production at RHIC showed that the net baryon density at mid-rapidity is small, but finite. The baryon numbers associated with the net baryons at mid-rapidity are transported from the beam over five units of rapidity. In order to understand the dynamics of baryon number transport, in particular to study the possible gluon junction interaction process, we should measure exclusively leading charged and neutral particles in the proton hemisphere at RHIC. A gluon junction interaction event is likely to produce a topology with exclusive leading mesons from valence quarks. Comprehensive measurement of all leading particles will provide more constraints to the dynamics of baryon number transport and energy deposition
at mid-rapidity in nuclear collisions.
In the strong coupling limit the constituent quarks of a proton may be considered as connected by a gluon junction, schematically shown in Fig 1, where the junction configuration arises due to gauge invariance. In a schematic representation for the baryon fragmentation we can categorize them into three processes: diquark-quark fragmentation where one gluon string is broken; three-quark fragmentation where two gluon strings are simultaneously broken; and gluon junction interaction where all three gluon strings are broken. We will use this notation to describe the fragmentation processes.
For the A and A++ analyses the event mixing method was used for subtraction of the combinatorial background. This method has been successfully used in the E864 experiment to extract yields of unstable resonance nuclei and A's in nucleus-nucleus collisions. The events with at least one positive track and one negative track in the spectrometer are used for A reconstruction (sample I) and the events with at least two positive tracks are used for A++ reconstruction (sample II). The event mixing is done respectively in sample I for A and in sample II for A++ background. It was also required that the hit positions of two t
E941实验组专门从事领头粒子测量和研究，它的主要特点是：能够测量中子以及集中对向前较大快度区间的粒子进行测量．本文通过分析在AGS上工作的E864／E941的实验数据，给出了在19GeV／c能量下最小无偏(minimum bias)p+Pb碰撞的领头奇异超子(Λ)和共振态重子(△~(++))的不变多重数分布和dn／dy分布。本实验所给出的Λ和△~(++)的快度和横动量范围分别对应为2.2＜y＜3.4和0.2GeV／c＜PT＜1.1GeV／c。比较Λ和△~(++)的产生与质子和中子的产生，以及与其它实验进行比较将会帮助我们了解p+A作用中质子碎裂机制。
实验中发现：在质子碎裂过程中，有关重子味变化的反应截面较大，且随快度变化而急剧变化，这不同于简单的双夸克-夸克(diquark-quark)碎裂机制所预言。与非奇异领头重子比较，在向前，陕度区间的领头奇异粒子Λ存在很强的压低现象。在2.7≤y≤3.3快度区间，入射质子碎裂成△~(++)和中子的相对几率为35％。我们将讨论这些结果对重子碎裂机制以及重子数输运理论所能产生的影响。
把E941的实验结果与RQMD和双夸克-夸克碎裂机制比较，证明了目前理论对重子碎裂机制的了解还很不够，需要进一步发展除了双夸克-夸克碎裂机制外的三夸克和胶子弦结碎裂机制。下面我们对这三种碎裂机制作一个简单的介绍，质子的组份夸克可以认为被一个胶子弦结联结起来(由图.1给出)，出现这种胶子弦结图象是规范不变的结果。在这种物理图象中，对应的三种重子碎裂过程为：当一根胶子弦断裂时，对应为双夸克-夸克(diquark-quark)碎裂；当两根胶子弦同时断裂时，对应为三夸克(three-quaxk)碎裂；当三根胶子弦同时断裂时，对应为胶子弦结(gluon junction)作用。我们将利用这些定义来描述入射质子碎裂过程碎裂过程。
对于Λ和△~(++)的数据分析，我们利用混合事件方法来扣除混合的背景。这种
＠：；：：冗。。。’‘
一
一
Figure：胶子弦结构造和双夸克－夸克碎裂的示意图．
方法已经成功用在E864核－核碰撞实验中分析不稳定共振态核和 A的产生．为了
重建八的事件取样，我们取至少有一个带正电荷和一个带负电荷的径迹的事件作
为取样事件2而对于凸”＼取至少有两个正电荷的径迹的事件作为取样事件．配成
对的两径迹在描迹器的撞击点必须有两个描迹器的狭条归幻叫的间距．此外为
了建造么＋＋的背景，要求来自不同事件的带正电荷径迹的动量不能同时大于 10
GeV／c，因为这将违背能量－动量守恒定律．在这种情况下，这两个带正电荷的径迹
很可能都为质子．由此方法产生的混合事件对的能量和动量分布与相同事件对的
能量和动量分布基本上相同．
在一个事件中，我们把质子的质量赋值于具有最大动量且带正电荷的径迹，其
它的径迹赋值于7T介子的质量，这样由不同电荷径迹所组成的对和相同电荷径迹
所组成的对的不变质量分布分别对应与人和凸付 相关的分布．然后利用一种迭
代的方法来消除背景．如果来自相同事件的对的数目为儿me，来自混合事件对的
数目为几；d叩，用相同事件的对的不变质量分布减去混合事件对的不变质量分布
11
／t巴八博士学泣论文
V反臼民刃 nm”01M1。D旧L卜1／＼mw
与标度因子Ns。一／Nmb；叩的乘积·由此所得到的不变质量分布的直方图的净记录
应为零，我们记下所有的正记录为屿．然后重复上述过程，但对应的标度因子换咸
（兀。＿一M／Nt；叩．直到相邻两次仰的差值小于某一给定值时，这种循环过
程才结束．
is＿nE－－－－－－－－－－－－－－－－ldj5EYjiHI
ie”－I＝——”—’———－·一l
2 卜DI
o70o卜J D。＿＿。、D
6O o卜 厂1D
＊co卜 几 nI
阜o口卜 丛ID
扣o卜 0 回0
ZOO卜－＿h el
1＊o卜I”下．．＿出1I
o－－－－
毛刀61．OSI。11．121．141，16
nzrrss（Ge皿2）
Figure 2：一0．2 T磁场的un的不变质量分布．背景用一个指数函数来
Projectile proton fragmentation in p + A interactions has been an interesting physics topic because it is related to the dynamics of baryon number transport in nuclear collisions. At present there is no comprehensive understanding of the dynamics.
E941 is an unique experiment for the leading particle measurement and study. It has the ability of measuring neutron and has large coverage of forward rapidity region. Invariant multiplicity distributions and dn/dy of the leading hyperon (A) and resonance (A++) have been measured in minimum bias p+Pb collisions at 19 GeV/c with the E864/E941 spectrometer at the ACS. The experimental acceptance for the measurement covers a region of 2.2 < y < 3.4 and 0.2 GeV/c < pr <1.1 GeV/c for the measurement of A and A++. Comparisons of the A and A++ yields with the leading proton and neutron yields measured by the same
experiment and with other experimental data will shed light on the fragmentation of the incident proton associated with the production of various baryons in p + A interactions.
The cross section for baryon flavor change is large and strongly depends on rapidity. This is very different from the expectation of simple diquark-quark fragmentation of incident proton. A suppression of leading A production in the forward rapidity region compared with the non-strange leading baryons is also observed. The relative probability of the projectile proton fragmentation into A++ and neutron is found to be about 35% in the region of 2.7
The comparison of our results with RQMD and with the diquark-quark fragmentation model demonstrated significant deficiencies in current understanding of baryon fragmentation processes. Theoretical understanding for three-quark and gluon junction fragmentation processes has to be developed. These studies will have an important impact on nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC). The measurement of baryon and anti-baryon production at RHIC showed that the net baryon density at mid-rapidity is small, but finite. The baryon numbers associated with the net baryons at mid-rapidity are transported from the beam over five units of rapidity. In order to understand the dynamics of baryon number transport, in particular to study the possible gluon junction interaction process, we should measure exclusively leading charged and neutral particles in the proton hemisphere at RHIC. A gluon junction interaction event is likely to produce a topology with exclusive leading mesons from valence quarks. Comprehensive measurement of all leading particles will provide more constraints to the dynamics of baryon number transport and energy deposition
at mid-rapidity in nuclear collisions.
In the strong coupling limit the constituent quarks of a proton may be considered as connected by a gluon junction, schematically shown in Fig 1, where the junction configuration arises due to gauge invariance. In a schematic representation for the baryon fragmentation we can categorize them into three processes: diquark-quark fragmentation where one gluon string is broken; three-quark fragmentation where two gluon strings are simultaneously broken; and gluon junction interaction where all three gluon strings are broken. We will use this notation to describe the fragmentation processes.
For the A and A++ analyses the event mixing method was used for subtraction of the combinatorial background. This method has been successfully used in the E864 experiment to extract yields of unstable resonance nuclei and A's in nucleus-nucleus collisions. The events with at least one positive track and one negative track in the spectrometer are used for A reconstruction (sample I) and the events with at least two positive tracks are used for A++ reconstruction (sample II). The event mixing is done respectively in sample I for A and in sample II for A++ background. It was also required that the hit positions of two t
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