Electromagnetic deflection effects in the integrated luminosity measurement at the CEPC

TL;DR

The paper assesses electromagnetic deflection effects on the integrated luminosity measurement at the CEPC Z0 pole, focusing on EM deflection of initial (EMD1) and final (EMD2) state particles. Using GuineaPig simulations with post-CDR beam parameters, it quantifies how these effects bias Bhabha counts by altering four-momenta and the crossing angle. It shows a total uncorrected luminosity bias of about $6\times10^{-3}$, with EMD1 driving a crossing-angle reduction near $140\mu$rad and EMD2 contributing ~ $1.4\times10^{-3}$; corrections rely on precise crossing-angle measurements from central processes (e.g., di-muon events) and potential LABS-acolinearity diagnostics. The study outlines experimental strategies to determine corrections within the target $10^{-4}$ precision and discusses residual uncertainties and future calibration approaches.

Abstract

In order to ensure measurement of the integrated luminosity with a relative precision of $\mathrm{10^{-4}}$ at the $\mathrm{Z^{0}}$ pole at CEPC, numerous systematic effects have to be quantified and, if possible, corrected for. Here we discuss the impact of electromagnetic fields of incoming bunches on the initial state electrons and positrons (EMD1) as well as on the Bhabha scattering final states (EMD2). Both effects change four-momenta of the final state particles, leading to a modification of the Bhabha count in the luminometer. These effects are quantified in simulation, together with their stability with respect to the beam parameters variations. Possible correction methods based on experimental measurements with the CEPC detector are discussed on a conceptual level.

Figures (12)

• Figure 1: Illustration of the EMD1 effect on initial state in the laboratory frame. Colliding bunches of electrons (positrons) are indicated in blue (red).
• Figure 2: Illustration of the change of momentum of colliding electron-positron system along the x-axis in the laboratory frame.
• Figure 3: Illustration of the effective reduction of the crossing angle per beam due to EMD1.
• Figure 4: Illustration of the smearing in polar angles ($\mathrm{\Delta \theta_{BH}}$) of the Bhabha final states due to EMD1, without biasing the mean.
• Figure 5: Illustration of the EMD1 dependance on the azimuthal angle $\phi_0$ of Bhabha final states.