The BM@N experiment is a part of the NICA accelerator complex at the Joint Institute for Nuclear Research, Dubna. The main goal of the research program on this detector is to study the interactions of relativistic heavy ion beams with fixed targets. BmnRoot software is used to process both experimental and simulated data. Not all the detectors, however, are included in the reconstruction chain. One of them is the Multi-Wire Proportional Chamber (MWPC). Here we present the description of the MWPC detector geometry and the algorithms for digitizing, hit finding and track reconstruction. The results of simulation are given.
the Joint Institute for Nuclear Research (Dubna, Russian Federation) to study properties of dense
baryonic matter. BM@N experiment (Baryonic Matter at Nuclotron) [
powerful tool for detector performance studies, event simulation and development of algorithms for
event reconstruction and physics analysis of experimental data registered by the BM@N facility. The
BmnRoot is implemented in the programming language C++ and based on the ROOT [
The detector inclusion in the simulation and reconstruction chain can be divided into several stages: creating the detector geometry to describe particle interactions with the detector; detector digitization – transformation of simulated data into detector signals; development of particle track reconstruction algorithms.
The detector is a chamber filled with a mixture of gases, in which anode wires are located between the cathode planes (Fig. 2). Upon entry, a charged particle causes ionization of the gas, and the generated electrons under the action of the electric field of the chamber trigger the wire closest to the particle’s trajectory.
wires each. Each plane is rotated by 60° relative to the previous one. The distance between the wires withinin one plane is dw = 2.5 mm, and the inter-plane distance within in a chamber is 10 mm. In a Cartesian coordinate system the z axis is perpendicular to the planes. The coordinate system of the detector is shown in (Fig. 3). In this representation, the planes have the sign "+" or "−" depending on the increasing or decreasing numbers of the wires along the coordinate axis.
location. The ROOT geometry package is used for this purpose. It is a tool to build, browse and visualize detector geometries. Previous version of MWPC geometry described only the active parts involved in the interactions, but the detector also contains frames and some additional materials where new particles may appear, and this should be taken into account (Fig. 4). Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
The main purpose of the reconstruction procedure is to process the experimental data. For further tests of track reconstruction, however, processing the simulated data is also important. It should be brought to the same form with the experimental data.
purpose the BmnMwpcDigitizer class has been developed and incorporated into BmnRoot framework. The coordinate data obtained during the simulation must be digitized, that is, presented as a discrete set of wire numbers and detector planes. Based on the geometry of the detector, this can be done by rounding to integers the following expressions for different planes: + = + 47.5, + = 47.5, − = 47.5 −
, − = 47.5 − , − = 47.5 − , + = 47. +
The next step is creating three-dimensional reconstructed points - hits, on which tracking will be built. The BmnMwpcHitMaker class has been developed and incorporated into BmnRoot framework. Tracks are built based on three hits obtained from six values of the detector wire numbers, . + one from each plane.
= ( + − 47.5) = ( + − 47.5) = 2 − √3 Hit 2 = (47.5 − −) = (47.5 − − ) = − 2 √3 Hit 3 = (47.5 − − ) = ( + − 47.5) = + = 2 − √3
classthat we have developed. At first, the data read from the BmnMwpcHits branch is sorted into three planes corresponding to different z coordinates. Further, all the hit combinations from different planes are approximated by straight lines using the least squares method and the best ones are selected from them by the chi-squared test.
∑ 2 2 → where is the difference between the hit coordinates and the approximated coordinates in the -th plane, = ⁄√12 = 0.072 cm is the standard deviation.
reconstruction is considered correct. The efficiency of the algorithm for track reconstruction for 1, 2, and 3 simulated tracks in an event as a function of the polar angle is presented in (Fig. 6).
described. It turned out that the material around the detector has a small effect on the appearance of new particles in it. Classes for digitizing the simulated data and finding reconstructed hits on which tracking is built is developed and incorporated into BmnRoot framework. Track reconstruction algorithm and an algorithm for merging tracks in a two chamber system have been developed.
We are also grateful to the Physics Educational Center of the Research Park of the Saint-Petersburg