# A Liquid Hydrogen Target for the MUSE Experiment at PSI

## TL;DR

This paper reports the design, construction, and successful testing of a liquid hydrogen target for the MUSE experiment at PSI, achieving high stability and low systematic uncertainties for precise proton charge radius measurements.

## Contribution

It introduces a specialized liquid hydrogen target with enhanced temperature and density stability for scattering experiments, improving systematic control.

## Key findings

- Target operated successfully during commissioning at PSI.
- Temperature stability achieved at 0.01 K, density stability at 0.02%.
- Material constraints effectively minimized background and systematic uncertainties.

## Abstract

A 280 ml liquid hydrogen target has been constructed and tested for the MUSE experiment at PSI to investigate the proton charge radius via simultaneous measurement of elastic muon-proton and elastic electron-proton scattering. To control systematic uncertainties at a sub-percent level, strong constraints were put on the amount of material surrounding the target and on its temperature stability. The target cell wall is made of $120\,\mu$m-thick Kapton, while the beam entrance and exit windows are made of $125\,\mu$m-thick aluminized Kapton. The side exit windows are made of Mylar laminated on aramid fabric with an areal density of $368\,$g/m$^2$. The target system was successfully operated during a commissioning run at PSI at the end of 2018. The target temperature was stable at the 0.01 K level. This suggests a density stability at the $0.02\,$% level, which is about a factor of ten better than required.