#### 1. Introduction

In relativistic heavy ions collisions, strongly-interacting matter is formed at high temperatures and densities. The initial geometry of two nuclei in the collision affects its evolution. The region of the nuclei overlapping after the collision depends on the impact parameter, which is the length of the vector connecting the centers of the two colliding nuclei. Event classes are used to study the geometry of collisions. A more central collision corresponds to a higher multiplicity of the produced particles and a lower energy in spectators. The Glauber model is used to map the event classes to a range of model parameters, i.e. impact parameter, number of participant nucleons and number of nucleon-nucleon collisions which can not be measured in the experiment. In the NA49 experiment, the event classes can be determined using the multiplicity of the produced particles and/or the energy in spectators.

#### 2. NA49 experimental setup

Main components of the NA49 experimental setup are four large-volume time projection chambers (TPC). The vertex TPCs VTPC-1 and VTPC-2, are placed in the magnetic field of two superconducting dipole magnets VTX-1 and VTX-2. The other two TPCs (MTPC-L and MTPC-R) are positioned downstream of the magnets. The TPCs serve to measure multiplicity of produced charged particles. A Veto Calorimeter placed about 20 m downstream of the target behind a collimator measures the energy carried by spectators [1].

The data on Pb+Pb collisions at 40

#### 3. Centrality determination procedure

In current analysis Modified Wounded Nucleon model (MWN) [2] was used to map particle yield of event to geometrical quantities of the initial state. MWN originates from Glauber MC model and describes procedure of mapping between MC results and experimental data. According to MWN emitted particles are produced by a set of ancestors with negative binomial distribution. MWN suggests the following form of multiplicity distribution

(1)
where

Similarly one can write probability density of spectators energy as follows

(2)

Assuming

Determination of event centrality classes involves the following steps [3]:

#### 4. Results and Discussion

The fit ranges were

On the fig. 2a profile histogram is built horizontally to the correlation and is shown as black triangles, then this correlation was fitted with a polynomial function (black line). Profile the correlation along the polynomial fit line is indicated with red triangles and refit with red line.

After mapping procedure one can use MWN model to estimate geometrical parameters of initial state of event. On fig. 2b are shown impact parameter distributions for events in specific centrality class. For each centrality class mean value and width was obtained. The resulting dependency of

#### 5. Summary

The implementation of the procedure within the software framework of the future CBM experiment was adopted for event classification in the NA49 experiment. Event classes in the experiment and parameters of the MWN model in event classes are determined. Impact parameter resolution is similar for centrality determination via spectator energy and track multiplicity. In future, the procedure will be used for analysis of the new Pb-Pb data collected by the NA61/SHINE experiment and for comparison with the results previously obtained by STAR at RHIC and NA49 at the CERN SPS.

#### Acknowledgements

This work was partially supported by the Ministry of Science and Education of the Russian Federation, grant N 3.3380.2017/4.6, and by the National Research Nuclear University MEPhI in the framework of the Russian Academic Excellence Project (contract No. 02.a03.21.0005, 27.08.2013).