Abstract
The combined vertexing and tracking performance of the innermost part of the FCC-ee experiments must deliver outstanding precision for measurement of the track momentum together with an impact parameter resolution exceeding by at least a factor five that typically achieved at LHC experiments. Furthermore, precision measurements require stability and fiducial accuracy at a level which is unprecedented in collider experiments. For the innermost vertex layers these goals translate into a target hit resolution of approximately 3 mu hbox {m} together with a material budget of around 0.2% of a radiation length per layer. Typically this performance might be provided by silicon-based tracking, together with a careful choice of a low-mass cooling technology, and a stable, low-mass mechanical structure capable of providing measurements with a low enough systematic error to match the tremendous statistics expected, particularly for the run around the Z resonance. At FCC-ee, the magnetic field will be limited to approximately 2 T, in order to contain the vertical emittance at the Z pole, and a tracking volume up to relative large radius is needed. The technological solution could be silicon- or gaseous-based tracking, in both cases with the focus on optimising the material budget, and particle identification capability would be an advantage. Depending on the global design, an additional silicon tracking layer could be added at the outer radius of the tracker to provide a final precise point contributing to the momentum or possibly time-of-flight measurement. Current developments in monolithic and hybrid silicon technology, as well as advanced gaseous tracking developments, provide an encouraging road map towards the FCC-ee detector. The current state of the art and potential extensions will be discussed and a generic call for technology which could have a significant impact on the performance of an FCC-ee tracking and vertexing detector is outlined.
Highlights
The tracking volume which makes up the innermost part of any FCC-ee detector must be capable of delivering outstanding performance across the full acceptance, down to approximately 120 mrad, and full momentum range, typically with full efficiencies down to 300 MeV/c and 98% or better for muons down to 100 MeV/c transverse momentum
An overview of proposed detector layout and performance requirements can be found in the FCC-ee Conceptual Design Report (CDR) [1] and in this issue [2]
A driving factor in the design is that the magnetic field will be limited to approximately 2 T, in order to contain the vertical emittance at the Z pole, and a tracking volume up to relatively large radius will be needed
Summary
The tracking volume which makes up the innermost part of any FCC-ee detector must be capable of delivering outstanding performance across the full acceptance, down to approximately 120 mrad, and full momentum range, typically with full efficiencies down to 300 MeV/c and 98% or better for muons down to 100 MeV/c transverse momentum. The transverse momentum of, for instance, muons from Z decays relies on the asymptotic term, whereas tracks from, for instance, the K ∗ for the channel illustrated in Fig. 2 have typical transverse momenta of 3-4 GeV This resolution results in typical primary and secondary vertex resolutions (both transverse and longitudinal) of 3 and 7 μm [13]. The τ lifetime measurement, which will allow a precise test of lepton τ μ universality, will be based on 1012 τ pairs and will reach an expected statistical precision of 0.001 fs, corresponding to a few tens of nanometres on the flight distance This will set stringent requirements on the offline alignment and overall radial scale of the vertex detector
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