Monochromatization interaction region optics design for direct s-channel Higgs production at FCC-ee

TL;DR

This paper presents a detailed design study of the interaction region optics for monochromatization at FCC-ee, aiming to reduce energy spread for precise Higgs measurements without significant luminosity loss.

Contribution

It introduces a new interaction region optics configuration for monochromatization at FCC-ee, improving energy spread reduction techniques for Higgs physics.

Findings

Optimized optics configurations for monochromatization at FCC-ee.

Simulation results show effective energy spread reduction.

Assessment of beam dynamics and beamstrahlung effects.

Abstract

The FCC-ee offers the potential to measure the electron Yukawa coupling via direct s-channel Higgs production, $e^+ e^- \rightarrow \text{H}$, at a centre-of-mass (CM) energy of ~125 GeV. This measurement is significantly facilitated if the CM energy spread of $e^+ e^-$ collisions can be reduced to a level comparable to the natural width of the Higgs boson, $\Gamma_{\text{H}}$ = 4.1 MeV, without substantial loss in luminosity. Achieving this reduction in collision-energy spread is possible through the "monochromatization" concept. The basic idea is to create opposite correlations between spatial position and energy deviation within the colliding beams, which can be accomplished in beam optics by introducing a nonzero dispersion function with opposite signs for the two beams at the interaction point. Since the first proposal in 2016, the implementation of monochromatization at the FCC-ee has been continuously improved, starting from preliminary parametric studies. In this paper, we present a detailed study of the interaction region optics design for this newly proposed collision mode, exploring different potential configurations and their implementation in the FCC-ee global lattice, along with beam dynamics simulations and performance evaluations including the impact of "beamstrahlung."