• DOI:10.1140/epjp/s13360-021-02265-3
• Corpus ID: 236447803

Challenges for FCC-ee luminosity monitor design

@article{Dam2021ChallengesFF,
  title={Challenges for FCC-ee luminosity monitor design},
  author={Mogens Dam},
  journal={The European Physical Journal Plus},
  year={2021},
  volume={137},
  pages={1-10}
}

• M. Dam
• Published 27 July 2021
• Physics
• The European Physical Journal Plus

For cross section measurements, an accurate knowledge of the integrated luminosity is required. The FCC-ee physics programme at and around the Z pole sets the ambitious precision goal of $$10^{-4}$$ 10 - 4 on the absolute luminosity measurement and one order of magnitude better on the relative measurement between energy scan points. The luminosity is determined from the rate of Bhabha scattering, $$\mathrm {e^+e^- \rightarrow e^+e^-}$$ e + e - → e + e - , where the final state electrons and positrons are detected in dedicated monitors covering small angles from the outgoing beam directions. The constraints on the luminosity monitors are multiple: ( i ) they are placed inside the main detector volume only about 1 m from the interaction point; ( ii ) they are centred around the outgoing beam directions and do not satisfy the normal axial detector symmetry; ( iii ) their coverage is limited by the beam pipe, on the one hand, and by the requirement to stay clear of the main detector acceptance, on the other; ( iv ) the steep angular dependence of the Bhabha scattering process imposes a precision on the acceptance limits at about 1  $$\upmu $$ μ rad, corresponding to an absolute geometrical precision of $${\mathcal {O}}(1\,\upmu \text {m})$$ O ( 1 μ m ) on the monitor radial dimensions; and v ) the very high bunch-crossing rate of 50 MHz during the Z-pole operation calls for fast readout electronics. Inspired by second-generation LEP luminosity monitors, which achieved an experimental precision of $$3.4 \times 10^{-4}$$ 3.4 × 10 - 4 on the absolute luminosity measurement (Abbiendi et al. in Eur Phys J C 14:373–425, 2000), a proposed ultra-compact solution is based on a sandwich of tungsten-silicon layers. A vigorous R&D programme is needed in order to ensure that such a solution satisfies the more challenging FCC-ee requirements.