---

---

Important!

The information on this page can be very helpful, but if you are running root (not necessarily root4star), please use the StEpdGeom class.  It has all the geometrical information you will ever need.
It is part of the StEpdUtil package (on RCAF, 'cvs co StRoot/StEpdUtil'), which also includes StBbcGeom (the geometry for the BBC, obviously), and StEpdEpFinder, which finds the Event Planes for you.

---

---

One often wants the phi and eta for a given tile. 

In the attached file TestGeometry.txt there are two Double_t arrays that you can cut and paste into your code.  It is a root macro. (drupal does not allow .C extension, so it is uploaded as .txt)

Double_t EpdTilePhi[2][12][31]

• first index is EW=0/1  - 0 means East wheel and 1 means West wheel
• second index is PP (position number)
• third index is TT (tile number)

Double_t EpdTileEta[2][31]

• first index is EW
• second index is PP

The macro also produces some pictures so you can check the validity of the geometry.  Pictures are attached below.  When you are looking to the west, be careful because -x points to the right!

---

The numbering of the supersector positions (PP) are such that it follows a clock face, when seen by particles flying into the detector.  I.e. when viewed from (0,0,0) in the TPC.  If you look at an EPD wheel from the outside, the numbering goes "counter-clockwise."  See description in the StEpd software documentation for more discussion of this.

Keep in mind the official STAR Coordinate system:

• +x points South
• +y points up (thank goodness)
• +z points West

Some very useful sketches of STAR detectors with coordinate system may be found here.  Probably the most useful image is reproduced below:

---

EpdTileCenterRadii_v4.ods is the spreadsheet which was used to calculate the geometric center of the various tiles. This center location is not trivial because the supersector design makes all but the first of the tiles asymmetric, where one side is higher than the other. An image is embedded in the spreadsheet where the basic trig calculations are done. The goal was to get these values for the psuedorapidity, so, for simplicity, the supersector was assumed to be centered on the y coordinate, which is true for no actual supersectors. Thus, if one wants the proper phi coordinate it is necessary to rotate the coordinates by (15 degrees)/2 for the first sector of the first supersector.

Additionally this spreadsheet was used for calculating the eta of the edges of each tile, as will be in the EPD NIM paper. A version of such a table was in the EPD construction proposal, but there appears to be some mistake in the calculation.

The scintillator of the EPD is assumed to start 4.6cm from the beamline, as designed. For reasons unclear to me the construction proposal has 4.5cm for this distance, but the difference is negligible. In reality there is a 1.65mm-thick epoxy gap between the tiles. This is not part of the calculation. One tile is assumed to end along the center of the gap and the next tile begins along the same line.