Abstract
The capability to determine the FCC-ee centre-of-mass energies (ECM) at the ppm level using resonant depolarization of the beams is essential for the Z line shape measurements, the W mass and the possible observation of the Higgs boson s-channel production. A first analysis (Blondel A et al Polarization and centre-of-mass energy calibration at FCC-ee. arXiv:1909.12245) demonstrated the feasibility of this programme, conditional to careful preparation and a number of further developments. The existing simulation codes must be unified; the analysis and design of the instrumentation must be developed; and a detailed planning must be developed for the simultaneous and coordinated operation of the accelerator, of the continuous polarization and depolarization measurements, and of the beam monitoring devices, ensuring a precise extrapolation from beam energies to centre-of-mass energy and energy spread.
Highlights
A unique feature of circular lepton colliders is the precision with which the beam energies can be calibrated by means of resonant depolarization (RD)
A cornerstone of the FCC-ee physics program lays in the precise measurements of the W and Z masses and widths, as well as of lepton forward–backward asymmetries around the Z pole
The RD provides an instantaneous precision on beam energy that can be as good as ±1 ppm at the Z energies, and serves as basis for a running mode where such measurements are made about 5 times an hour on pilot bunches for both electrons and positrons
Summary
A unique feature of circular lepton colliders is the precision with which the beam energies can be calibrated by means of resonant depolarization (RD). For the standard monochromatization scheme using opposite sign horizontal dispersion (A in Fig. 1) the average centre-of-mass boost varies across the bunch while the ECM spread is reduced; theses variations and reduction must be measured by dividing the sample of muons along the horizontal axis—the vertex resolution of ±3μm is much smaller than the horizontal beam size. The measurements of the variation of the centre-of-mass boost, ECM and ECM spread across the bunch will verify the proper realization of monochromatization; this is essential for the interpretation of the e+e− → H physics result This experiment should be scheduled after the Higgs run, since the Higgs mass must be known, and preferably be developed with the Z machine at the Z pole where statistics of muon pairs is 300 times higher. The main challenge remains to obtain high luminosity and monochromatization level simultaneously
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