CSC Strip, Wire, and Chamber Orientation

18-Feb-98
Authors:
D. Acosta, R. Breedon, Y. Bonushkin, T. Ferguson, J. Hauser, J. Hoftiezer, T.Y. Ling, P. Padley, O. Prokofiev, B. Tannenbaum


Introduction Single Chamber Layout Overall CSC Layout  Unique Strip & Wire IDs 
Muon Bending Direction Strip Stagger Trigger Eta/Phi Decoding CMSIM Variables

Introduction

It actually took a couple of years to come up with a consistent labeling scheme for the CSC muon stations (ME1/1, ME1/2, etc.). However, there has been no generally accepted finer-grain labeling scheme for strips, wires, and chambers positions in phi. The reason for this is that for most purposes, e.g. the DAQ readout of the CSC system, the relative orientation and labeling of CSC strips, wires, and chambers does not much matter - they can always be re-oriented or re-labeled in software. For the trigger, on the other hand, we must be able to form stubs from multi-layer coincidences, and derive space coordinates and vector directions easily for track finding and for correlation with the calorimetry. These matters lead us to propose a scheme by which any strip or wire in the system can be uniquely identified (sort of a URL for the CSC system) and its position in various coordinate systems understood. There also seems to be a need to explain the various coordinates in one self-consistent place.

Caveats:

Single-Chamber, Standalone Layout

The CSC chambers are constructed as trapezoidal objects, and during assembly they lay on tables so that electronics can be mounted on top. In this configuration, the top plane of wires and strips (i.e. those closest to the electronics) is layer 1, and the bottom plane is layer 6. As per Hoftiezer agreement, wire number increases from inner radius to outer radius (1:n, or 0:n-1 when convenient). Strip number increases from left to right (1:n, or 0:n-1 when convenient) when one looks from the small "inner" end toward the large "outer" end of the chamber, as shown in the picture below:

In this picture, anode electronics is mounted on the right side, while HV signals are fed in on the left side. Note Bene: For P0, plane 1 is resting on the table (argh!)

Layout of CSC Chambers in CMS

The CMS detector lies on the North side of the LHC ring. The x-axis (phi=0) points toward the center of the ring (South), while the y-axis is vertical and to complete the right-handed coordinate system, the z-axis points West. The magnetic field points along the z-axis:

Stations 1 and 2 chambers are mounted on iron disks on the sides closest to the interaction point. Conversely, in stations 3 and 4 chambers are mounted on iron disks on the sides away from the interaction point. This is shown in the figure below, which is also figure 4.6.3 in the CMS Muon TDR. Therefore, in the West endcap, strip numbers in stations 1 and 2 are in increasing phi order, and strip numbers in stations 3 and 4 are in decreasing phi order. In the East endcap, strip numbers in stations 1 and 2 are in decreasing phi order, while strip numbers in stations 3 and 4 are in increasing phi order.

Unique Channel IDs for Chambers, Strips, and Wires

In addition to the usual notation for endcap muon station and ring (e.g. ME1/3), a unique channel ID for a strip or wire requires the index of a chamber within a phi ring of chambers from 1 to 18 or 36, an index for the layer number from 1 to 6, an index or letter to distinguish wires from strips, and a strip or wire number.

The index of a chamber within a phi ring of chambers is defined to be in increasing phi order from 1 to n (18 or 36). Because the muon track finder needs to match CSC with Barrel chambers as well as possible in the overlap region, CSC chamber 1 starts at 15 degrees in phi in the trigger.

The layer number index starts at 1 for the layer closest to the electronics, as explained above.

To distinguish wires from strips, one can either precede the wire number with a W and strip number with an S, or if numbering is required, strips are defined as type T=1, and wires are defined as type T=2.

Strip and wire numbers are defined as explained above, i.e. wire number increases from inner radius to outer radius (1:n, and trip number increases from left to right (1:n when one looks from the small "inner" end toward the large "outer" end of the chamber.

Therefore, we define the format for identifying a particular strip in the CSC system: MEs/r/C/L/{S,W}n, where

For example: is denoted by any of the following: ME2/1/3/4/2/37, or ME2/1/C3/L4/W37, or ME2/1-C3-L4-W37. Any of these forms are acceptable - the first is most convenient for a computer program do-loop, while the last form is most readable.

The Direction of Muon Bending

It is well known that the bending direction of endcap muons reverses along the muon trajectory - initially, the muon crosses the +z solenoidal field lines, but around station 1 the magnetic field lines diverge in the +r direction and the muon crosses the field lines in the opposite direction. From looking at the picture above, one can tell that the positive muons travelling in either direction (East or West) will first bend in the -phi direction and then reverse toward the +phi direction in travelling through the muon system.

The Orientation of Strip Staggering

The CSC chambers contain strips milled on every anode and cathode panel. Except for ME1/1 (which is not staggered), strip 1 is indented by 1/2-strip in layers 1 (top), 3, and 5; with respect to strip 1 in layers 2, 4, and 6 (bottom), as shown:

(The way that this is accomplished in chamber manufacturing is that the bare cathode panels are staggered 1/4-strip toward the left when viewed with strips up and looking from small end toward the large end (as in the picture above), while the anode panels are staggered 1/4-strip toward the right. In the assembled chamber, however, the strips appear on the bottom of the panels, and the order of the panels from top to bottom is CACACA(blank), where C=cathode and A=anode.)

Decoding Channel IDs into Eta and Phi (Trigger)

The Sector Receiver card in the Muon Track Finder electronics must decode a precise phi and eta position from the hit wire and strip numbers. In the eta coordinate there is a straightforward lookup table, but in the West endcap (+eta) the eta value decreases as wire number increases from inner to outer radii. In the East endcap, eta increases with increasing wire number, since it is a negative number. The lookup table is also somewhat non-linear.

In the phi position coordinate, one takes the 15 degree offset (2pi/24) in any case. Then there are two cases, depending on whether the strips are in increasing or decreasing phi order within a chamber (as discussed above):

  1. Increasing phi order: Phi = 2pi/24 + [k+(m/Nstrip)]*dPhi
  2. Decreasing phi order: Phi = 2pi/24 + [k+1-(m/Nstrip)]*dPhi

Correspondence with CMSIM Variables

The 4-letter CMSIM label and "CMS detector identifying numbers" for CSC muon stations are (see cmthit.for or mcd2r.for): The numbering scheme for strips is based upon a view of the chamber just as in the figures above, i.e. from the small (inner) radius toward the outer, except that instead of a physical point of view where electronics is on "top", we are looking at the chamber from the interaction region, i.e. the "top" is the layer closest to the interaction region. The strip numbering scheme is then where n=40 for a chamber with 80 strips, etc. Obviously there is no strip 0. (However, some hits currently show strip=0, ahem...)

Wire, plane, and chamber (in phi) numbers are deduced from the global coordinates. More details later...

Return to UCLA-CMS trigger web page here.