A STUDY OF COMPOUND PARTICLES IN EMULSION

An attempt is made to study the compound particles by taking black and shower particles together. Average compound particle multiplicity is found to vary linearly with heavily ionizing particle multiplicity but with black particle multiplicity it does not show a linear dependence. Dispersion of the compound multiplicity distribution has also been studied. The ratio of the mean number of compound particles to its dispersion for different target sizes has been calculated.


INTRODUCTION
When a high energy particle collides with a nucleus, a large number of particles are produced. For this purpose nuclear emulsion technique was used to detect the charged particles. Nuclear emulsion consists of elements like hydrogen (H), carbon (C), nitrogen (N), oxygen (O), silver (Ag) and bromine (Br). The particles produced in emulsion appear in the form of tracks. The tracks/particles are classified as shower, grey and black particles, their numbers in an event/interaction are written as Ns, Ng and Nb. The sum of grey and black particles are called as heavily ionizing particle (Nh=Ng+Nb). These final state particles produced in hadron-nucleus (hA) interactions have been investigated by several workers [1][2][3][4][5]. It is expected that the study of hA interactions will not only help to explain the production process but also reveal hadron structure. This phenomenon of particle production in such collisions is called the multiparticle production. Most of the studies are based on the relativistic charged particles or fast moving particles. But the studies on grey and black particle may also provide some very useful information about the production process. Jurak and Linscheid [6] studied the characteristics of proton-nucleus collisions by combining shower and grey particles together for the first time and they called this compound particle (Nc=Ns+Ng). Many [7][8][9][10][11][12][13][14][15] workers followed this method to study high energy interactions. We [16,17] also reported some results on compound particles in our earlier publications.
In this paper we have studied the characteristics of compound particles in a different way, instead of combining shower and grey particle to form a compound particle, we define a compound particle by taking shower and black particle together in an event and it is written as Nc1 (=Ns+Nb). We studied some aspects of this new compound particle in one of our papers [18]. Here, the compound particle multiplicity distribution is studied for different target sizes in pion-nucleus interactions at 340 Gec/c. The events with Nh≤1, 2≤Nh≤6 andNh≥7 are known as H, CNO and AgBr events.The variations of mean compound particle multiplicity and dispersion of its multiplicity distribution with different particle multiplicities has been studied.

EXPERIMENTAL DETAILS
The experimental data was collected using a stack of Ilford-G5 emulsion pellicles exposed to a 340 GeV/c negative pion beam. The flux was (0.5-1.50) X 10 4 particles/cm 2 . The plates were area scanned using M4000 Cooke's series microscopes with 15X eye pieces and 20X objectives. The measurement was carried out using an oil immersion objective of 100X magnification. The events were picked up after leaving 3 mm from the leading edges of the pellicles. The interactions, which were produced within 35 µm from top or the bottom surfaces of the pellicles were excluded from the data. To avoid any contamination of primary events with secondary interactions, the primaries of all the events were followed back up to the edge of the plates and only those events whose primary remained parallel to the main direction of the beam were finally picked up as genuine primary events. In each event the tracks of different particles have been categorised according to their specific ionization (g * =g/go), where gis the ionization of the track and go the ionization of the primary beam. The tracks with g * <1.4, 1.4≤g * ≤10 and g * >10 have been taken as shower (Ns), grey (Ng) and black (Nb) particles. It can be seen from the figure that compound multiplicity distribution becomes broader with increasing target size. One more thing which can be noted is that the peak of the distribution shifts towards the higher values of compound particle multiplicity. Similar results have been observed by other workers [14,15] also. The mean values of the compound multiplicities <Nc1> its dispersion defined as D(Nc1)=(<Nc1 2 >-<Nc1> 2 ) 1/2 , and the ratio <Nc1>/D(Nc1) are listed in table 1. For comparison the values of these parameters calculated for shower particles are also given in the same table. Figure 2 represents the variation of mean compound multiplicity with Nb. One may notice from the figure that <Nc> and <Nc1> increases with Nb up to Nb=13, after that some type of saturation effect is observed and the values of <Nc1> dominates over <Nc>.   Here again we observe a linear dependence and straight line fit to the data is represented by the following lines. The remarkable thing which is noted from the figure is that both the lines are parallel with equal slopes showing that the variation is similar.

CONCLUDING REMARKS
On the basis of the findings in the present investigation, we conclude the following: (i) Compound particle multiplicity distribution is observed to be target size dependent.
(ii) Mean compound multiplicity is found to vary linearly with Nh and a saturation effect is observed in the case of Nb (iii) The variation of the ratio <Nc1>/D(Nc1) with Nb and Nh is similar in nature.