almost complete version
This page gives an overview of the Run II muon detector, including a brief description of the layout, coverage, and expected resolutions. Also, some comparisons to the Run I detector will be included, as will some discussions of backgrounds.
Muon reconstruction utilizes the inner tracking, the calorimeter, and the muon detector elements themselves to identify muons and determine their energy. The muon detector consist of scintillator and drift tubes, with effectively complete coverage out to |eta| of 2. As seen in the layout, the detector is split at |eta| of 1 into a central and forward system. Each has 3 layers (usually called A,B,C with A between the calorimeter and iron and the other two outside the iron) of drift tubes (called PDTs in the central and MDTs in the forward; they differ in size with the PDTs being 4 x 2.5 inch rectangles while the MDTs are 1 cm squares with the PDTs also have time division measurement). There is also 2 or 3 layers of scintillator coverage with the forward scintillators sometimes called pixels, the central A-layer counters called a-phi, and the BC counters called the cosmic cap. Scintillator time is read out with both a 15-20 ns "trigger" gate and a 80-100 ns "readout" gate.
Run I top event 79 and top event 417 show the typical response of the calorimeter and the central muon PDTs; this will be similar for Run II muons. In particular, while the Run II PDT efficiencies should be better, there will still be cases where the PDT hits are lost (or wrong) due to the passage of other particles (our top event 417 is an example of this as its a-layer hits were due to a delta ray). We will also need to pattern recognize two close muons, such as in event 79.
Detector coverage will be almost complete for high pt muons, with more than 85% hitting at least 2 layers and more than 95% hitting at least 1 layer (|eta|<2). For a-layer muons, their is no coverage for phi from 225 to 315 degrees for |eta|<1 giving a geometric acceptance of about 85%. Need figures for coverage.
The detector thickness varies from 5-9 interaction lengths in the calorimeter, and 7-9 in the iron. The thickness is shown in: Thickness versus Theta . The plot indicates the thin spots at the CC-EC and CF-EF gaps. It does not show the thin spot at phi=110 degrees in the central (due to the main ring pipe) or the small loss of material on the bottom for cables. Energy loss follows thickness, with 1 interaction length in the calorimeter or iron equivalent to 0.25 or 0.23 GeV/c energy loss respectively. This gives a minimum energy of about 1.6 GeV for a muon to exit the calorimeter, and about 3.3 GeV to exit the iron.
Muon momentum will be measured using both the inner tracking system and the muon toroids. The momentum resolution (at eta=0) will be about .02+.002pt for the inner tracking and .18+.003p for the muon toriods (where the terms are added in quadrature). The first term is due to multiple scattering and increases with |eta|.
Muon backgrounds in Run I were from cosmic ray muons and combinatorics. For Run II, we have the additional ability to trigger and reconstruct lower pt muons which hit only the a-layer. As the detector is thinner, a significant number of muons will be produced by punchthroughs which will be in time, but with energy and direction exiting the calorimeter which are not in agreement with the inner tracking's values. A Preliminary Punchthrough Study has been done use paramterizations but GEANT-based studies are needed to fully understand this background and how to minimize it.
Cosmic ray muon backgrounds will be reduced from their Run I values of a few percent for isolated muons by improved scintillator timing and better central tracking information. In particular, most muon will strike at least two scintillation counters with time resolutions of about 1 ns for the smaller a-phi and pixel counters, and 2-3 ns for the large outer counters. This will easily discriminate between entering and exiting muons.
Combinatorics were a significant background for Run I muons in the forward directions. The rate in muon detector elements will be reduced by the new shielding, with the overall combinatoric background also being reduced to a (hopefully) insignificant level by having a more capable central tracker. Combinatorics will remain a significant source of muon triggers. Many of the detector hits will be due to interactions at low angles which produce particles (mainly neutrons and gammas) which exit ("sneakthrough") into the muon system. As they have longer path lengths, and often slower velocities, such hits will be out of time. This is seen in a A-phi Run I timing study , and is also observed in Run II Monte Carlo events.
Last modified: May 3, 2000
