This page gives an overview of Run II muon identification, including a brief description of the detector, coverage, trigger, and expected resolutions. Also, some comparisons to the Run I muon ID will be included, as will some discussions of backgrounds. Preliminary descriptions of offline algorithms and analysis tools are also given along with a pointer to software documentation.
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. A Calibration overview describes expected detector resolutions.
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 75% hitting at least 2 layers and more than 90% 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%.
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.
The magnetic field plus the geometry of the muon toriods and shields is desrcibed in D0 Note 3874 (V. Koreshev, June 2001), and includes plots of the:
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|.
The muon L1 trigger will primarily use the central fiber tracker trigger matched to one or more elements in the muon system. The scintillator counter will provide a timing window of about 15-20 ns, while adding requirements of the wire chambers will reduce combinatorics. Additional requirements that some elements are outside the iron are included makes an effective pt cut at about 4 GeV/c in the central region. Four CFT thesholda can be used:
The muon L2 trigger has available the complete time information, including calibration constants and PDT deltaT, for the wire and scintillator elements. It also has scintillator hits which have been read out using the wide gate in addition to those channels which passed the narrow L1 gate. Among other things it can do, it will improve on the L1 correlation between the muon scintillator and wire hits, between the A-layer and BC-layer muon segments, and between the muon segments and the central tracking. The scintillator time resolution will be better due to using calibration and time-of-flight corrections allowing better discrimination between fast, slow, and out-of-time sources. L2 should also allow better discrimination between 1 and 2 muon events, some punchthrough rejection, and an invariant mass determination (though with modest resolution).
The muon L3 trigger will utilize aspects of the offline muon reconstruction. The muon L2 will act as a L3 "pre-processor" as it will define geographic regions where L3 should unpack and track. L3 muons will have more complete information on the vertex and inner tracking components which will yield an improved momentum resolution, and the ability to require that multiple muons came from the same vertex. Fits done to the muon detector elements will be essentially the same as in the final offline reconstruction, and requirments on matching the muon track to the inner tracking can reduce any remnant combinatorics plus punchthroughs. L3 will also use the calorimeter energy to reduce combinatorics, plus also separate muons into isolated and non-isolated. L3 will improve on L2's ability to seprate muon sources into prompt, slow, or out-of-time by fitting the available scintillator hits along a track to the particle's velocity. L3 can remove remnant cosmic ray muons both by their being out-of-time and by looking for evidence of a penetrating track on the opposite side of the detector. L3 can also clean up single muon events which L1 and L2 identified as dimuons, such as those which pass through the FAMUS-WAMUS overlap region.
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.
Muon identification uses both the muon detector, the inner tracking, and the calorimeter. The goal is to pattern recognize all muons, even those without any hits in the muon detector, and assign them quality flags.
Muon software is described either in the offline muon ID pages just given, or in Muon Online page. Muon Software Documentation gives a description of the muon raw data format, and of the classes and libraries used in muon online and offline applications. (The documentation is in a very preliminary stage).
Last modified: July 15, 2000