Piecing together muons

To piece together many of the particles produced by Tevatron collisions, physicists at DZero must first piece together the muons they decayed into.

To understand the subatomic world, physicists must piece together a picture of the particles made in a high-energy collision using information from detectors as large as apartment buildings. The particles recorded by a detector may themselves be the decay products of unstable particles, like a Higgs boson or a top quark, that don’t reach the detector directly. A key particle in this multi-layered puzzle is the muon, a heavy cousin of the electron, that would travel a few kilometers before decaying when it was produced in a Tevatron collision. As a charged particle that packs penetrating power, it leaves a clean signature in the detector that can help physicists piece together the bigger picture. The outermost layers of modern collider detectors are designed just to find muons, and a recent effort at DZero describes, in detail, the process of identifying muons from the detector hardware readout to benefit other experiments with muon detectors.

The DZero muon system includes three layers, each a combination of tracking chambers that accurately determines the muon’s location and large scintillation counters with fast readout that measure the muon’s arrival time to within a few nanoseconds. Large magnetized blocks of iron fill the space between the first two layers, allowing for a momentum measurement within the muon system. A notable feature of the DZero muon system is that the magnetic field was reversible and systematically flipped, allowing DZero analyses to be particularly sensitive to differences between muons and antimuons.

A new document describes the algorithms used to piece together information from all the individual layers of the muon system into the three-dimensional path a muon might have taken through the detector. Since a muon’s signal could be mimicked by other particles, additional criteria are defined that help select real muons while rejecting backgrounds. The paper details these methods and provides the selection efficiency and background rejection rates of the standard sets of DZero muon identification criteria. The wealth of information provided about muon reconstruction and identification is not just crucial historical documentation for DZero analyses, but provides information that will help improve future particle detection systems.

Mike Cooke

These physicists made major contributions to this analysis.
This array of scintillating panels, part of one layer of the DZero forward muon system, provides the muon’s arrival time to within a few nanoseconds.