Measuring the α-particle charge radius with muonic helium-4 ions.

The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S-2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering1, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle2-5, in line with recent determinations of the proton charge radius6-9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.

High-resolution dual comb spectroscopy using a free-running, bidirectional ring titanium sapphire laser.

We report the first measurement of resolved molecular absorption lines with dual-comb spectroscopy using a Kerr-lens mode-locked bidirectional Ti:sapphire ring laser cavity. A 3 nm broad spectrum has been recorded in 5.3 ms with a spectral resolution of ≈ 1 GHz (0.05 cm-1) corresponding to a relative spectral resolution of 2.5 × 10-6. The measurement of spectrally resolved molecular absorption lines have been demonstrated on the oxygen A-band at 394 THz (760 nm, 13 000 cm-1) and was obtained with two free-running 100 fs Ti:sapphire trains of pulses without the need for active phase stabilization protocol nor real-time or post-processing correction. This work demonstrates that the bidirectional laser configuration enables a sufficient level of absolute and mutual coherence for dual-comb spectroscopy of resolved molecular absorption lines. Considering the high versatility of Ti:sapphire emission spectral range (from 600 to 1100 nm) with high-peak powers, the here reported results pave the way for Dual-Comb spectroscopy in the UV range at mW average output power using a standalone set-up, in the aim to extend its applicability for atmospheric remote-sensing.

New Measurement of the 1S-3S Transition Frequency of Hydrogen: Contribution to the Proton Charge Radius Puzzle.

We present a new measurement of the 1S-3S two-photon transition frequency of hydrogen, realized with a continuous-wave excitation laser at 205 nm on a room-temperature atomic beam, with a relative uncertainty of 9×10^{-13}. The proton charge radius deduced from this measurement, r_{p}=0.877(13) fm, is in very good agreement with the current CODATA-recommended value. This result contributes to the ongoing search to solve the proton charge radius puzzle, which arose from a discrepancy between the CODATA value and a more precise determination of r_{p} from muonic hydrogen spectroscopy.

Improved x-ray detection and particle identification with avalanche photodiodes.

Avalanche photodiodes are commonly used as detectors for low energy x-rays. In this work, we report on a fitting technique used to account for different detector responses resulting from photoabsorption in the various avalanche photodiode layers. The use of this technique results in an improvement of the energy resolution at 8.2 keV by up to a factor of 2 and corrects the timing information by up to 25 ns to account for space dependent electron drift time. In addition, this waveform analysis is used for particle identification, e.g., to distinguish between x-rays and MeV electrons in our experiment.