COMPARATIVE STUDY OF P+P, P+PB AND PB+PB REACTIONS AT 158 A·GEV PROJECTILE ENERGY.

ANDRZEJ RYBICKI Institute of Nuclear Physics, Cracow, Poland

representing the NA49 Collaboration

The NA49 detector provides a possibility of exploring a wide range of hadronic interactions, going from the elementary hadron-nucleon processes via hadron-nucleus reactions with con­trolled centrality, up to heavy ion collisions. Preliminary results of a comparative study are presented, concentrating on three main aspects: longitudinal spectra, transverse momentum behaviour and strangeness production. Experimental data are compared to model predictions.

1 Introduction

A comparative study of elementary and nuclear collisions made within the framework of a single experiment offers a number of advantages, the most important being the possibility of isolating the phenomena common to all types of hadronic interactions and the principal differences be­tween the various types. This becomes especially interesting if the comparison includes on one hand the nucleus-nucleus collisions, and on the other hand the more elementary hadron-nucleon and hadron-nucleus reactions.

The study of h+n and h+A interactions with the NA49 detector started in 1996 as com­plementary to the investigation of A+A reactions. The statistics of the data available so far is around 400000 p+p and 480000 h+A events, the latter being divided into various samples with different running conditions. Both data sets are to be increased in the coming years.

Due to the large acceptance of the detector, and particle identification over most of this acceptance (see [1] for more details) , a detailed analysis based on identified particles in a wide range of phase space (-0.1 :::; XF :::; 0.5) is possible. Furthermore, as it will be discussed in Sec. 2 , due to the addition of the Centrality Detector to the NA49 experimental setup, a control of centrality of investigated hadron-nucleus collisions is available.

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collisions are scaled down by 181 , which is the estimated number of participant pairs.

2 Centrality Detector

The aim of the NA49 Centrality Detector is to measure and trigger on the number of intermedi­ate energy ( "grey" ) particles ejected from the target by the projectile, and thus get a handle on the centrality of a iiven h+A collision. The detector (Fig. la) has a shape of a vertical cylinder surrounding a thin foil target. It consists of 32 proportional tubes read out on 256 cathode elements. It subtends lab polar angles from of 45 to 315 degrees, leaving free the tracking accep­tance wedge of the NA49 spectrometer (see [1] for more details) . Fig. lb shows the distribution of the number of cathode pads hit in a given p+Pb collision, together with a model prediction.

Detection of grey particles is performed not exclusively with the Centrality Detector. Parti­cles emitted forward falling out of the detector acceptance are registered in the TPC's and can be identified via dE/dx (Fig. le) , thus increasing the overall efficiency of the detection.

3 Results

3. 1 Longitudinal Spectra

Fig. 2 shows XF distributions of mean charged pions ((11'+ + 71'-)/2) and net protons (p - p), for three types of hadronic collisions (p+p, central p+Pb, central Pb+Pb) . For a large XF range (0-0.4) and over two orders of magnitude in cross section, the behaviour of the pion yield for p+p interactions shows a remarkable similarity with that of. the central Pb+ Pb collisions. A steepening of the spectrum is however observed for the central p+Pb sample. For net protons, the XF distribution steepens from p+p via central Pb+ Pb up to central p+Pb interactions.

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Pb+ Pb collisions (right).

3. 2 Transverse Momentum Behaviour

Fig. 3 shows the mean transverse momentum for n+ and protons as function of XF for p+p, central p+Pb, and central Pb+Pb collisions. A dependence of the transverse momentum dis­tribution on reaction type is evident. For pions, the well-known "seagull" structure in p+p reactions steepens up when going to central p+Pb collisions, whereas the central Pb+Pb sam­ple shows an intermediate behaviour. For protons, a completely different pattern is observed: the XF-dependence is rather fiat, with a clear increase of the mean PT up to central Pb+Pb reactions.

A comparison of some of the obtained results to model predictions can be seen in Fig. 4. If the central Pb+Pb interactions seem to be relatively well described by the model, this is not the case for the more elementary central p+Pb reactions.

3. 3 Strangeness

Fig. 5 shows the x F-dependence of the charged ( K / 7r) ratios for central p+ Pb and central Pb+ Pb collisions, referred to the same ratios for p+p interactions. For p+Pb central, both positive and negative particles, the ratios stay visibly above 1 in all the investigated XF range, indicating clearly a strangeness enhancement in the projectile hemisphere. A dependence on XF is observed, at least for the case of positives. There is a visible disagreement between experimental results and model predictions. For central Pb+Pb reactions, an XF-dependence is visible for both positive and negative particles, which again is not described by the model. The overall size of the effect is comparable to that of the central p+Pb collisions.

A complementary view on strangeness production in hadronic interactions is given by the

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compared to VENUS simulation.

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when going from p+p to central Pb+ Pb reactions.

4 Summary

As it has been shown on the example of p+ p, p+ Pb and Pb+ Pb reactions, various characteristics of a wide range of hadronic interactions can be compared in great detail (i.e. for identified

particles, as function of kinematic variables, and with control on centrality of h+A and A+A collisions) . This provides a way of testing the available models in a differential fashion.

A number of preliminary results has been presented for transverse and longitudinal spectra,

revealing a characteristic pattern emerging as function of particle and reaction type.

Some preliminary data have been shown in the strangeness production domain. A compari­

son of various reaction types leads to a conclusion that strangeness enhancement is present not only for central Pb+Pb, but also for central p+Pb collisions, at least as far as the projectile

hemisphere (xF = 0 or larger) is concerned. its dependence on XF can be studied in detail.

References

c; i\ fanc1Biev et 0/. [NA49 Collaboration] , CERN-EP-99-001 .

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