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First comments on a descoped/staged FCC-ee

Alain Blondel, Christophe Grojean, Patrick Janot, Guy Wilkinson

TL;DR

This note assesses a descoped FCC-ee as an alternative to the ESG's baseline FCC-ee, focusing on how reduced SR power and fewer interaction regions affect physics reach and precision. It shows that, with a longer operational program, the descoped option can match the baseline performance for most measurements, except the top-quark program which benefits from a staged top run. A staged ttbar run, potentially after decades, preserves key EWPO sensitivity and enhances the Higgs self-coupling reach when combined with FCC-hh data. The results inform strategic planning by highlighting upgrade pathways, the value of a four-IP option, and the role of funding dynamics in enabling the top-run upgrade.

Abstract

In response to its remit, the European Strategy Group (ESG) recommended the electron-positron Future Circular Collider (FCC-ee) as the preferred option for the next flagship collider at CERN; and a descoped FCC-ee as the preferred alternative option (with reduced synchrotron radiation (SR) power, without a run at the $\rm t \bar t$ threshold, and with only two interaction regions) in the event that the preferred option turns out not to be feasible. Upon request of the ESG, a basic comparison of the physics potential of the descoped option with that of the baseline version is presented in this short note. Our first observations about the alternative proposal of a descoped FCC-ee are that (i) the same performance as the baseline, apart from $\rm t \bar t$-run related, is achieved with the longer proposed period of operation; and (ii) the top run remains essential, but can be staged at a later date. The full appraisal of the consequences of descoping and a proposal for the best way to integrate possible staging wherever feasible, and possible improvements, will need further joint studies by the physics and accelerator groups.

First comments on a descoped/staged FCC-ee

TL;DR

This note assesses a descoped FCC-ee as an alternative to the ESG's baseline FCC-ee, focusing on how reduced SR power and fewer interaction regions affect physics reach and precision. It shows that, with a longer operational program, the descoped option can match the baseline performance for most measurements, except the top-quark program which benefits from a staged top run. A staged ttbar run, potentially after decades, preserves key EWPO sensitivity and enhances the Higgs self-coupling reach when combined with FCC-hh data. The results inform strategic planning by highlighting upgrade pathways, the value of a four-IP option, and the role of funding dynamics in enabling the top-run upgrade.

Abstract

In response to its remit, the European Strategy Group (ESG) recommended the electron-positron Future Circular Collider (FCC-ee) as the preferred option for the next flagship collider at CERN; and a descoped FCC-ee as the preferred alternative option (with reduced synchrotron radiation (SR) power, without a run at the threshold, and with only two interaction regions) in the event that the preferred option turns out not to be feasible. Upon request of the ESG, a basic comparison of the physics potential of the descoped option with that of the baseline version is presented in this short note. Our first observations about the alternative proposal of a descoped FCC-ee are that (i) the same performance as the baseline, apart from -run related, is achieved with the longer proposed period of operation; and (ii) the top run remains essential, but can be staged at a later date. The full appraisal of the consequences of descoping and a proposal for the best way to integrate possible staging wherever feasible, and possible improvements, will need further joint studies by the physics and accelerator groups.
Paper Structure (4 sections, 1 figure, 1 table)

This paper contains 4 sections, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Scaling measurement uncertainties at a descoped FCC-ee
  3. A staged top run
  4. Summary and outloook

Figures (1)

  • Figure 1: From slide 100 of Ref. Fabiola2025: details about the descoped FCC-ee (dubbed "staged FCC-ee" in the table) and the other colliders discussed in the ESG drafting session, in particular: instantaneous luminosity per IP for ZH productions, duration of operation, capital cost, operation cost, path towards higher energies.