IL NUOVO CIMENTO VOL. 110 A, N. 11 Novembre 1997
Characteristics of the interactions of
12C,
22Ne and
28Si
with emulsion nuclei accompanied with relativistic hadron
in the backward hemisphere at Dubna energy
M. EL-NADI(1), N. ALI-MOSSA(2) and A. ABDELSALAM(1)
(1) Department of Physics, Faculty of Science, Cairo University - Cairo, Egypt
(2) Basic Science Department, Faculty of Engineering, Shoubra, Zagazig University
Cairo, Egypt
(ricevuto il 16 Maggio 1996; revisonato il 17 Marzo 1997; approvato il 18 Agosto 1997)
Summary. — A detailed study of the characteristics of the interactions accompanied by relativistic hadrons in the backward hemisphere in the collisions of12C,22Ne and 28Si projectiles with emulsion nuclei at incident momentum in the range
(4.1–4.5) A GeV/c has been carried out. For this purpose, random samples of 819, 3812 and 1209 events in case of 12C,22Ne and 28Si interactions are analyzed,
respectively. The behavior of the shower particle multiplicities and the pseudo-rapidity distributions for the different interactions is investigated in terms of the number of emitted shower particles in the backward hemisphere nb
s. The
pseudorapidity distribution of the shower particles from the interactions accompanied by the emission of backward relativistic hadrons are found to be satisfactorily fitted by a single Gaussian distribution, while it is fitted by two Gaussian distributions when the interactions are not accompanied by backward relativistic hadron. This reflects the fact that there are two different sources creating the shower particles for nb
s4 0 events. Also, the dispersion of the pseudorapidity
distributions is independent of the number of the backward relativistic hadrons nb s.
However, the average pseudorapidity decreases with the increase of the number of backward relativistic hadrons. The dependence of the average number of the shower particles produced in the backward and forward hemispheres on the projectile mass number and the impact parameter is also presented. The results yield quite interesting information regarding the mechanism of production of such backward relativistic hadrons in heavy-ions interactions.
PACS 25.75 – Relativistic heavy-ion collisions.
1. – Introduction
A principal reason for studying production of relativistic pions from nuclei in the backward direction is that, in free nucleon-nucleon collisions, such production is kinematically restricted. Emission of such pions beyond this kinematic limit may then be evidence for exotic production mechanisms, such as production from clusters [1-5].
Baldin [6] thought that simple Fermi motion could not account for such relativistic backward pion production, and stated that the dominant mechanism for producing such pions was an interaction between the incident nucleons from the projectile and multi-nucleon clusters in the target, referring to this mechanism as cumulative production. There are also other models in progress in order to explain these phenomena [1, 4, 7, 8], but until now, we do not have a good understanding about the mechanism of the relativistic backward hadron production in the high-energy heavy-ion interactions.
On the other side many experiments were conducted to study the characteristics of the relativistic backward hadron production in the interactions of hadron and heavy ions with different targets [9-12].
In the present paper an attempt was made to study various characteristics (multiplicity and pseudorapidity) of the relativistic particles emitted from the interactions accompanied by the emission of fast hadron in the backward hemisphere BHS (emission angle u D907) in cases of 12C,22Ne and 28Si-Emulsion interactions at (4.1–4.5) A GeV/c.
Another objective of this paper is to examine whether the mechanism of particle production in the backward hemisphere is significantly different from those operating in the production of the fast particles in the forward hemisphere (u G907).
2. – Experimental procedures
Three stacks of BR-2 nuclear emulsions were exposed to 12C and 28Si ions at 4.5 A GeV/c and 22Ne ions at 4.1 A GeV/c at the Dubna Synchrophasotron. The data studied in the present work consist of 819, 3812 and 1209, 12C-Em, 22Ne-Em and 28Si-Em inelastic interactions, respectively. These events were collected through along-the-track double scanning, fast in the forward and slow in the backward direction using the Russian microscope type Mbu9. The scanned primary beam tracks were further examined by measuring the delta electron density on each of them to exclude the tracks having charge less than the beam particle charge. The general characteristics of these interactions, selection rules and other details have been published before [11-13]. In the measured events the tracks of secondary particles were classified according to the following criteria:
a) Black tracks (b) having a range L in emulsion G3 mm which corresponds to a
proton kinetic energy of G26 MeV. These are the fragments of the target nucleus, and their multiplicity is denoted by nb.
b) Grey tracks (g) having relative ionization I * (4I/I0) D1.4 and LD3 mm which corresponds to a proton kinetic energy from 26 to 400 MeV, where I is the particle track ionization and I0 is the minimum ionization. In this case I0 is the ionization of shower tracks in the forward cone of an opening angle of G37.
c) The b and/or g tracks are called the tracks of heavy ionizing particles Nh.
d) Shower particles s having I * G1.4. The tracks of such a type having an
emission angle of G37 were further subjected to rigorous multiple scattering measurement for momentum determination and consequently, for separating the produced pions from the single-charged projectile fragments. Thus projectile fragments (PFs) of Z 41 were not included in a further analysis of particles.
CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1257
Fig. 1. – Multiplicity distributions of the shower particles produced in the backward hemisphere for12C,22Ne and28Si interactions with emulsion nuclei.
e) The multicharged PFs of Z F2 are subdivided into Z42, 3, R, 14 fragments
according to the counted delta electrons density when followed until L D1 cm.
In each event, the total charge of the PFs Q 4
!
NiZi was calculated, where Niis the number of fragments having charge Zi ( 1 , 2 , R , Zp) in a given event. The number of interacting projectile nucleons in each event Nint4 Ap2 (Ap/Zp) Q was determined, where Ap and Zp are the mass and charge numbers of projectile nucleus. Accurate measurements (of the angle u) were performed using KSM1-nuclear track microscope (u is the angle between the direction of the incident beam and that given track).The shower tracks in each event which were emitted in the backward hemisphere (BHS) (i.e. 90 7GuG1807) are called backward shower particles. The multiplicity of this type of tracks in each event is denoted by nb
s. The multiplicity of the shower tracks with (u G907) which were emitted in the forward hemisphere (FHS) in each event is denoted by nf
s, where
3. – Data analysis
3.1. Multiplicity of shower particles in forward and backward hemispheres. – The multiplicity distributions of the shower particles in the backward hemisphere (BHS) in the interactions of 12C,22Ne and 28Si projectiles with emulsion nuclei at incident momentum (4.1–4.5) A GeV/c are shown in fig. 1. It is observed that the multiplicity distributions of the shower particles emitted in the BHS are nearly identical for the three projectiles within statistical errors, i.e. the creation of shower particles in the BHS is nearly independent of the projectile mass number at similar momentum per nucleon (4.1–4.5) A GeV/c. We have previously demonstrated in refs. [15-17] that the multiplicity distributions of shower particles in the forward hemisphere (FHS) get broader as the projectile mass number increases as a constant incident momentum per nucleon. This result was confirmed in table I by the average number of shower particles created in the FHS, ansfb and the corresponding values in the BHS, ansbb for the three projectile beams. In recent studies [11-14] we showed that the values of anf
sb are proportional to the average number of interacting nucleons from each projectile aNintb with emulsion nuclei, which supports the idea of considering the nucleus-nucleus collision as a superposition of nucleon-nucleus collisions, while the values of anb
sb are nearly independent of aNintb. Table II summarizes the experimental data for aNhb (average number of target associated particles which related to the impact parameter of the collision) and the average number of the interacting nucleons aNintb as a function of the number of shower particles flying in the BHS (nb
s4 0 , 1 , 2 , 3 and F 4 ) for the interactions of12C,22Ne and28Si projectiles with emulsion nuclei at incident momentum (4.1–4.5) A GeV/c. Figures 2-4 show the multiplicity distributions of the shower
TABLE I. – The mean multiplicities of shower particles in the forward anb
sb and backward ansbb
hemispheres, aNhb and the average number of interacting nucleons from aNintb12C ,22Ne and28Si
projectiles. 12C 22Ne 28Si anf sb anb sb aNhb aNintb 7 .11 60.02 0 .42 60.01 10 .74 60.37 5 .28 60.18 9 .85 60.04 0 .45 60.01 11 .44 60.19 8 .06 60.13 11 .36 60.09 0 .44 60.02 11 .67 60.35 9 .49 60.27
TABLEII. – The values of the aNhb and average number of interacting nucleons as a function of
nb
s for different projectiles.
nb
s 12C-Em 22Ne-Em 28Si-Em
aNhb aNintb aNhb aNintb aNhb aNintb
0 1 2 3 F 4 7 .52 60.31 15 .28 61.17 22.30 63.00 27.95 66.40 28.50 68.60 4.90 60.21 7.26 60.55 9.12 61.34 11.34 62.60 11.74 63.50 7.92 60.15 16.98 60.64 25.94 61.75 29.13 63.11 29.50 64.05 7.06 60.13 11.85 60.45 16.69 61.13 18.15 61.93 19.61 62.70 8.80 60.29 17.90 61.38 24.25 63.03 30.31 66.46 29.0 66.18 8.56 60.28 13.81 61.07 19.57 62.45 22.72 64.84 22.7 64.84
CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1259
Fig. 2. – The normalized multiplicity distributions of the shower particles emitted from interactions of12C-Em for a) nb
s4 0 and b) nsbF 1 .
particles produced in interactions for anb
sb 40 and F1. From the inspection of the
above figures and tables we notice that:
i) The backward shower particle multiplicity distributions are nearly indepen-dent of the projectile mass number at (4.1–4.5) A GeV/c.
ii) The probability of the interactions accompanied by emission of shower particles in the BHS decreases slowly with the increase of projectile mass number
Fig. 3. – The normalized multiplicity distribution of the shower particles emitted from the interactions of22Ne-Em for a) nb
s4 0 and b) nsbF 1 .
and equals 30.3, 28.0 and 23.0 in percent for the interactions of12C,22Ne and28Si with emulsion nuclei, respectively.
iii) For each value of shower particles (anb
sb 40, 1, 2, 3 and F4) emitted in the BHS, the values of aNhb are nearly equal within statistical error for the interactions of the three projectiles, in spite of the apparent increase of aNintb with the projectile beam in each group of events (table II). This means that the number of created shower particles in the BHS is a target-dependent parameter [12, 18]. The group of events accompanied by shower particles emitted in the BHS have aNhb values greater than 15 for all projectile beams (12C,22Ne and28Si) used here. The events with highest value of ansbb F3 correspond to the complete disintegration of the heavy component in nuclear emulsion (AgBr nuclei). This means that the selection of events with fast shower particles flying in the backward hemisphere leads to a violent interactions with the AgBr group of nuclei in the emulsion. It is shown clearly from table II that, for all projectile beams, the events with anb
sb F3 have values of aNhb higher than 28 and nearly all nucleons in each projectile interacted completely
(
aNintb BAp)
. Therefore,CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1261
Fig. 4. – The normalized multiplicity distribution of shower particles emitted for 28Si-Em
interactions. a) nb
s4 0 and b) nsbF 1 .
suitable criteria for selecting the central events in the heavy-ion interactions are those which exhibit the higher multiplicity of produced shower particles in the BHS (anb
sb F3) in the laboratory system, and
iv) The multiplicity distributions of the shower particles produced in the interactions of any projectile tend to become broader with increasing the number of shower particles flying in BHS.
From the above, it is clear that the backward hemisphere is intimately connected with the target fragmentation region, i.e. with that part of phase space where all shower particle characteristics are most independent of the projectile mass number. Figure 5 shows the dependence of the average number of shower particles produced in the FHS anf
sb on the number of shower particles flying in the BHS, nsb. From the figure, we can see that there is a positive linear relation between ansfb and ansbb in the region of high statistics (anb
sb G3). The data for each projectile in fig. 5 are fitted by the linear relation
Fig. 5. – The dependence of anf
sb on the number of shower particles emitted in the backward
hemisphere for12C,22Ne and28Si.
The values of the slope parameter a, the intercept b and the values of x2/DOF (DOF denotes degrees of freedom) are given in table III, for anb
sb G3.
3.2. Pseudorapidity distributions of the produced shower particles. – One of the fundamental experimental distribution in high-energy collisions—that is generally compared with any successful theoretical model—is the pseudorapidity (h 42ln tan (u/2) (u is the emission angle) distribution of the produced shower particles. Figures 6-8 present the pseudorapidity distributions of the shower particles produced in the inter-actions of12C,22Ne and28Si projectiles with emulsion nuclei for anb
sb 40, 1, F2 events). From these figures we can see that the pseudorapidity distributions of the emitted shower particles from the interactions with nb
CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1263 TABLEIII. – Values of the slope a intercept b and the x2
ODOF for the linear dependence of fig. 5 for different projectiles.
Reaction a b x2 ODOF 12C-Em 22Ne-Em 28Si-Em 4.83 60.67 7.78 60.45 8.79 61.02 6.50 60.27 7.89 60.15 9.27 60.29 0.09 0.50 0.38
the form and extending from the target fragmentation region to the center of the projectile fragmentation region. This result was expected because such interactions are of central type, i.e. 12C, 22Ne and 28Si beams interacted centrally with AgBr nuclei in the emulsion [19, 20]. This supports the results given in the part of multiplicity
Fig. 6. – The pseudorapidity (h 42ln tan u/2) distribution of the shower particles emitted in
12C-Em interactions. a) nb
Fig. 7. – The pseudorapidity distribution of the shower particles emitted in22Ne-Em interactions.
a) nb
s4 0 , and b) nsb4 1 and c) nsbF 2 .
distributions about the characteristics of the events which have NhD 15 . Therefore in this group of events nb
sc0 , the created shower particles acquire any pseudorapidity value between the target and projectile fragmentation regions. On the other hand we notice that the pseudorapidity distribution of the shower particles produced from the events characterized by the absence of nb
s as shown in figs. 6-8 parts (a) can be represented by two clear distinct distributions extended from the central region to the projectile fragmentation region.
The experimental pseudorapidity distributions of the shower particles emitted from the different colliding systems with nb
CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1265
Fig. 8. – The pseudorapidity distribution for the shower particles produced in the 28Si-Em
interactions. a) nb s4 0 , b) nsb4 1 and c) nsbF 2 . of the form dNOdh4 1 sk2 pexp
y
2 (h 2h–)2 2 s2z
which was previously used by van Gersdorff [19] and others [13, 20], where s is the dispersion of the GD, and h is the mean pseudorapidity value for each group of events. The smooth curves in the figures represent the GD fit of the experimental data in the pionization region in the range 0 GhGYp, where Yp is the projectile rapidity, which is nearly equal to 2.3 at 4.5 A GeV/c. The parameters characterizing the above distribu-tions are listed in table IV. The values of maximum shower particle density [20] for each distribution rmax[4 ansb /(k2 p s) ] are also listed in table IV.
TABLE. IV – The values of average pseudorapidity h–, dispersion s and the maximum pseudorapidity density rmaxas a function of nsbfor12C,22Ne and28Si interactions.
nb
s 12C-Em 22Ne-Em 28Si-Em
h
– s r
max h– s rmax h– s rmax
0 1 F 2 Total 1.85 60.88 1.44 60.11 1.28 61.03 1.10 60.02 0.902 0.960 1.530 1.000 0.55 1.00 1.70 — 1.96 60.04 1.60 60.06 1.43 60.10 1.71 60.03 0.92 1.02 0.99 1.0 0.62 1.28 3.82 — 2.01 60.07 1.64 60.13 1.55 60.20 — 0.96 1.05 1.00 1.02 0.73 1.50 2.82 —
Figure 9 presents the dependence of the average pseudorapidity of the shower particles on nb
s for the interactions of12C,22Ne and 28Si with emulsion nuclei:
The following important conclusion can be drawn from figs. 6-9 and tables III and IV: i) The pseudorapidity distribution of the emitted shower particles characterized by nb
sc0 can be fitted by only one Gaussian distribution, while that of the emitted shower particles from the interactions nb
s4 0 can be fitted by two Gaussians. This may reflect that, in case of nb
s4 0 there are two different sources creating the shower particles, one of them due to the interactions of the projectiles with the CNO group of nuclei
(
right GD in figs. 6-8 parts (a))
. The other source of emitted shower particles is attributed to peripheral interactions of the projectiles [21] with the AgBr group of nuclei(
left GD in figs. 6-8 parts (a))
, while there is only one emission source in case of events accompanied with shower particles emitted in the BHS(
nbsD 0 central events)
Fig. 9. – The dependence of h on in the interactions of12C,22Ne and28Si with emulsion nuclei at
CHARACTERISTICS OF THE INTERACTIONS OF12C,22NeAND28SiETC. 1267
(
violent sources due to central collisions of the projectiles with AgBr nuclei)
. To confirm this result, we summed the number of shower particles under the right side GD in the case of the group of events having nbs 4 0
(
parts a) in fig. 6-8)
. This was done because we believed that such particles may be emitted from the interactions of the different projectiles with the CNO group of nuclei in the emulsion. Using the values of the average number of shower particles emitted in the interactions of12C,22Ne and28Si projectiles with the CNO group at (4.1–4.5) A GeV which are given in ref. [17], we found that the probability of the interactions of12C,22Ne and28Si group of nuclei are equal to 33.8, 38.3 and 39.0%, respectively. These values agree with those calculated from the equations of the cross-section [20, 22] and using the concentration of different component in the emulsion(
A 34.3, 38.6 and 40.1 in percent for the three projectiles respectively)
. This result is consistent with those given in refs. [13, 20] in which they found that the pseudorapidity distribution of the shower particles emitted from central interactions of different projectiles with AgBr target at different energies were fitted by one Gaussian distribution.ii) The dispersions s of the pseudorapidity distributions are nearly independent of the number of shower particles flying in the BHS and of the projectile mass number at (4.1–4.5) A GeV/c. The same result was obtained by the EMUO1 collaboration [20] in the interactions of 16O and 28Si with the AgBr nuclei in the emulsion at energies (3.6–200) A GeV.
iii) The values of the mean pseudorapidity h are seen to be slowly increasing with the mass number of the projectile beams. They decrease with increasing the number of shower particles flying in the BHS. This result can be interpreted if we recall the cascading mechanism. Decreasing the impact parameter of the interactions, the number of participating nucleons from both colliding nuclei increases. Therefore the average number of collisions done by the projectile nucleons increases leading to the decrease of the values of the pseudorapidity.
iv) The values of the maximum shower particle density increase as the projectile mass number and nb
s increase. On other words rmaxincreases with increasing the size of the two colliding systems [20].
5. – Conclusions
We have studied the multiplicity and pseudorapidity distributions of the shower particles produced in the interactions of12C,22Ne and 28Si ions with emulsion nuclei in the momentum range (4.1–4.5) A GeV/c as a function of the number of emitted shower particles in the BHS in the laboratory system (uLD 90 7). From the exhaustive analysis of the data we conclude the following:
1) The multiplicity distributions of shower particles emitted in the BHS are nearly independent of the projectile mass number with maximum range of nb
sA 6 . 2) The values of anb
sb are nearly independent of the projectile mass number and of order 0.4 for all the projectiles used12C,22Ne and 28Si at (4.1–4.5) A GeV/c.
3) The nsdistributions for the events nsb4 0 are narrower than the corresponding distributions from the events having nb
sF 1 . The first distribution peaks at lower ns values while the latter is nearly flat over all range of ns.
4) The interactions accompanied by relativistic hadrons emitted in the BHS have aNhb values greater than 15 over all projectiles,12C,22Ne and28Si. This means that such
events are mainly due to the central collisions with AgBr target and nearly half of each projectile nucleons participated in these interactions.
5) A new criteria to select central events in heavy-ion collisions can be drawn by considering those events having nb
sF 2 .
6) The pseudorapidity distributions of the shower particles from events having relativistic hadron in the BHS can be fitted by one Gaussian distribution with dispersion independent of the size of the two colliding nuclei. Such events are due to interactions with AgBr target nuclei.
7) The pseudorapidity distribution of the shower particles emitted from events having nb
s4 0 is fitted by two Gaussian distributions, one of them representing the peripheral interactions with AgBr target nuclei while the other giving the distribution due to the interactions with the CNO group of nuclei.
* * *
The authors would like to thank Profs. M. K. HEGAB and M. EL-NAGDY for their
numerable helpful discussion. Thanks are also to Prof. A. M. BALDIN and Prof. A. D. KOVALENKOfrom the JINR in Dubna for their cooperation in supplying us with nuclear
emulsion plates.
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