hxBrowser - Introduction

A "helix" is the mathematical description of the path of a charged particle in a homogenous magnetic field. Several parameterization of a helix exist. In STAR we have chosen a pretty common one that uses the origin, curvature, dip angle, phase, and helicity. Click here to learn about it in detail. Depending on the field and the actual path the kinematical parameters (e.g. momentum vector) of the particle can be extracted from the mathematical parameters. hxBrowser is a program that allows you to define and display a helix using either the mathematical or the physical parameterization. The underlying "code" are two classes from STAR's class library called StHelix and StPhysicalHelix. Both do not only store the parameters and allow you to transform a mathematical description into a physical meaningful one but they also have several algorithms build in to calculate the distance of closest approach to space point, the intersection with a cylinder, and much more. hxBrowser allows you to test and display some of these algorithm.

When you launch hxBrowser the first thing you see on the screen is the main window. A screenshot is shown below.

The main window consist of a big square display area and two sets of input fields on the upper and lower right. One is to define or display the mathematical parameters and the other one the physical parameters. At startup no helix is defined and all you see is the wire frame of the TPC. To define a helix fill out the respective fields and press the Apply button. If you give the mathematical parameters the physical parameters are displayed and vice versa. The physical parameters also require you to define the strength of the magnetic field. The transformation from mathematical to physical parameters and vice versa depend on it. There are many ways to manipulate the rendered helix, e.g. rotating, shifting and zooming. See the graphics section for more.