Abstract
The measurement of the muon reconstruction effi ciency in the ATLAS experiment based on the and method using decay muons from J/ψ and Z resonances is presented. The effi ciency measurement is compared to its value as estimated from simulations in order to provide scaling factors to correct the residual mis-modeling of the ATLAS muon identification performance. This paper describes the measurement of the muon iden- tification e ffi ciency as determined by using the so-called and method at the J/ψ and Z resonances; a detailed description of these studies can be found in (1) and (2). The method allows to select an unbiased sample of muons by searching for an Inner Detector (ID) track (the probe) that, along with a well reconstructed muon (the tag) forms a system with invariant mass consistent with a di-muon resonance. By this procedure a sample of low pT probes (from the J/ψ !� + � decay) and high pT probes (coming from Z ! � + � decay) are selected in- dependently of the ATLAS Muon Spectrometer (MS) and can be used to measure the effi ciency for reconstructing a muon with MS measurement. Two categories of muons are reconstructed in ATLAS using the Muon Spectrometer data: Combined (CB) muons, that require the reconstruction of consistent tracks in the MS and in the ID, and Segment tagged (ST) muons, that give additional effi ciency as they can recover muons, typ- ically of low pT , which did not cross enough precision chambers to allow an independent momentum measure- ment in the MS. The two classes of muons are implemented in two different reconstruction chains, hereafter referred as chain 1 (Staco) and chain 2 (MuId). At the J/ψ resonance the tag and probe method suf- fers from a large background from muons originating from light meson decays in flight or from b and c semi-leptonic decays identified as tags that, associated to ID tracks, form a system with invariant mass close to the J/ψ mass. By re- quiring the probe to be associated with an energy deposit in the calorimeter consistent with a minimum ionizing par- ticle, a sample with low background contamination can be selected. These Calorimeter-tagged probes (CT probes) allow a measurement of the identification e ffi ciency in the MS al- most free from the biases induced by the background sub- traction procedure. The background contamination in the tag and probe selection at the Z resonance is very low and well understood theoretically, thus allowing for a simple signal yield extraction.
Highlights
This paper describes the measurement of the muon identification efficiency as determined by using the so-called “tag and probe” method at the J/ψ and Z resonances; a detailed description of these studies can be found in [1] and [2]
Two categories of muons are reconstructed in ATLAS using the Muon Spectrometer data: Combined (CB) muons, that require the reconstruction of consistent tracks in the MS and in the Inner Detector (ID), and Segment tagged (ST) muons, that give additional efficiency as they can recover muons, typically of low pT, which did not cross enough precision chambers to allow an independent momentum measurement in the MS
The selected tag and probe pairs are required to consist of opposite charge particles, well displaced in azimuth (∆φ(tag, probe) > 2) and associated to a common interaction vertex; the pairs which fall in an invariant mass window of 10 GeV around the best world average Z mass value [4] are used for the muon identification efficiency measurement
Summary
This paper describes the measurement of the muon identification efficiency as determined by using the so-called “tag and probe” method at the J/ψ and Z resonances; a detailed description of these studies can be found in [1] and [2]. The method allows to select an unbiased sample of muons by searching for an Inner Detector (ID) track (”the probe”) that, along with a well reconstructed muon (”the tag”) forms a system with invariant mass consistent with a di-muon resonance By this procedure a sample of low pT probes (from the J/ψ → μ+μ− decay) and high pT probes (coming from Z → μ+μ− decay) are selected independently of the ATLAS Muon Spectrometer (MS) and can be used to measure the efficiency for reconstructing a muon with MS measurement. In the high multiplicity of LHC events, an ID track can pass all the selection criteria in association with more than one tag in the event In such cases, in order to avoid testing the muon reconstruction result more than once for the same probe, a unique probe-to-tag association is chosen based on the minimum χ2 of common vertex fit.
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