Towards Lamb shift spectroscopy of antihydrogen atoms at the GBAR $\mathrm{\bar{H}}$ beam line
T.A. Tanaka*, P.P. Blumer, G. Janka, B. Ohayon, C. Regenfus, P. Crivelli, N. Kuroda  on behalf of the GBAR collaboration, R. Tsukida, T. Higuchi and K.S. Tanaka
*: corresponding author
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Pre-published on: March 17, 2025
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Abstract
A microwave (MW) spectroscopy experiment has been proposed to directly measure the resonant frequency of $2S_{1/2}-2P_{1/2}$ Lamb shift transition in antihydrogen ($\mathrm{\bar{H}}$) atoms. The spectroscopy opens up the possibility of deriving the charge radius of antiproton ($\mathrm{\bar{p}}$) using a beam of $\mathrm{\bar{H}}$ atoms with a kinetic energy of a few keV travelling under a magnetic field-free environment.
The requisite spectroscopy apparatuses have been developed and installed in the $\mathrm{\bar{H}}$ beam line at the GBAR experiment, where the production of $\mathrm{\bar{H}}$ beam at 6.1 keV was demonstrated through a charge exchange reaction of a $\mathrm{\bar{p}}$ beam passing through a positronium (Ps) cloud.
The spectroscopy setup is composed of a MW spectrometer and a Lyman-$\alpha$ photon detector.
The MW spectrometer consists of two consecutive MW apparatuses which have a relatively large borehole of 30 mm diameter, and each MW apparatus comprises a pair of parallel plate electrodes as its inner conductor and a rectangular box as its outer conductor.
Downstream to the MW spectrometer, the Lyman-$\alpha$ detector has been installed to count the $\mathrm{\bar{H}}$ atoms remaining in the $2S$ state after interacting with the MW E-field.
Towards the $\mathrm{\bar{H}}$ Lamb shift spectroscopy, we present here a characterization of the MW spectrometer, an evaluation of the detection efficiency of the Lyman-$\alpha$ detector, and a precision expected in the first line shape measurement of the $\mathrm{\bar{H}}$ Lamb shift.
DOI: https://doi.org/10.22323/1.480.0044
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