MUON GEOMETRY CONVENTIONS

Pushpa Bhat, 8-June-1998

(A) At the 4-June-1998 Muon Software Meeting, we made the following decisions:

o Use Mike Fortner's indexing scheme, shown below. Indexing is effected by
the use of MuoIndex objects. All offline software must use these objects for
indexing.

o MuoIndex objects will be used by MuoGeometer and MuoChannelGeometer as keys 
to gain access to the MuoBaseGeometry database. Access to the MuoBaseGeometry
directly is NOT allowed. 

o Each processed hit will contain a MuonIndex object.


An updated description of Mike Fortner's indexing scheme is reproduced below 
preceded with some notes of explanation.

(See Muon Index Utilities for link to Fortner's scheme and code.)


(B) Conventions/comments

The muon system is regarded as a series of "barrels" (of rectangular shape) 
and a set of "disks", in analogy to the description of the Silicon vertex 
detector. PDTs form the barrel, while MDTs and scintillators form the disks.

 o Positioning	
	o The position of a PDT is defined by the location of its center.

	o The position of an MDT is defined by the (right-angled) corner 
	  closest to the origin.  (There is proposal to change this to
	  the center of the MDT octant, which is what is in the TZ file.)

	o The position of a Pixel octant in BaseGeometry can defined as
	  for MDT, i.e., the same corners (at 3 O' clock, 6 O' clock etc.)

	o Rotations are specified in terms of Euler angles. Imagine a 
	  coordinate system, attached to an object, initially coincident with 
	  the global coordinate system. Euler angles are:
		1. alpha: Rotate about z-axis
		2. beta:  Rotate about new y-axis
		3. gamma: Rotate about new z-axis

		REMEMBER: Z-Y-Z!

 o Regions
	o PDTs are central. One PDT occupies one octant and one barrel.
	  (For Central Muon system sides, one PDT occupies two octants and
	  one barrel. 0+7, 3+4)
	o MDTs are north and south. One MDT occupies one octant.
	o Pixels are north and south and are divided into octants.

 o Octants
	o Central: look towards +z and begin counting, counterclockwise, from
	           global phi = 0; that is, octant 0 is along +x and +y.   

	o North, south: look from the origin towards increasing |z| and begin
		        counting, counterclockwise, from global phi = 0.

 o PDTs
	o Identified by  (_region, _type) = (0,0)

	o Wires are transverse to the beam (z-direction)

	o _eta indexes a wire, starting from the lowest z coordinate 
	  of the PDT and increasing in +z.

	o _phi is UNUSED (_phi = 0), since that information is provided by
	  the octant number.

	o _tube is UNUSED (_tube = 0), since that information is provided by
	  the _eta number.

 o MDTs
	o Identified by  (_region, _type) = (1,0), (2,0)

	o Wires are transverse to the beam

	o _eta indexes an 8-tube MDT unit. Each tube contains one wire.

	o _phi is UNUSED (_phi = 0), since that information is provided by
	  the octant number.

	o _tube indexes a tube within an 8-tube MDT unit, starting from the 
	  origin outwards.

 o Scintillators
	o Identified by  (_region, _type) = (1,1), (2,1)

	o _eta, _phi indexes a pixel in a given layer (which contains a single
	  plane). _eta is the pixel index in the radial direction.  
	  There are a maximum of 12 eta rings of pixels. Each eta ring
	  contains a pixel of a given size. Therefore, there are 12 different
	  scintillator sizes. Each size is identified, in the geometry 
	  database, by a scintillator type number, which is not to be confused
	  with _type number, which distinguishes PDTs and MDTs from
	  scintillators.  There are also hole where Scintillators are missing.
	  These locations have to be saved in the Geometry database.
	  _phi index runs from 0 to 9 in each octant, in the dame direction
	  as global phi increases.


(C) MuoIndex
	
MuoIndex : 

       MuoSectionIndex : 
              _region	// 0 = central, 1 = north, 2 = south

              _type	// 0 = wire chamber, 1 = scintillator

              _layer	// 0 = A-layer, 1 = B-layer, 2 = C-layer

              _octant	// 0-7 octant, 0 along +x axis toward +y axis
			// Central A-layer: vertical PDTs occupy 2
			// octants. Octant 0 spans octants 0 and 7; 
			// octant 3 spans octants 3 and 4. So octants
			// 4 and 7 are invalid indices for this layer

              _barrel	// Central A-layer: 1,2,3

			// Central B/C-layer: 0,1,2,3,4

			// Central B/C, octants 5,6: 0,1,3,4


       MuoCellIndex :
              _plane	// Scint: 0, Wires: increasing away from the origin
			// Scints have 1 plane only, hence _plane = 0

			// Wire A-Layer : 0,1,2,3

			// Central A (Octants 5,6) : 0,1,2

			// Wire B/C-Layer : 0,1,2

              _eta	// Central (PDTs): increases in +z, 
			// Forward (MDTs): increases in r

              _phi	// Wire: 0, Scint: 0-9, increases in global phi
			// Scint. has 10 phi divisions per octant.

              _tube	// Scint with 2 PMT: 0-1, MDT: 0-7