PROCADIS

The aim of this project is to obtain a new determination of the proton charge distribution radius Rp. Today Rp is determined from the elastic electron-proton scattering, from the spectroscopy of atomic hydrogen, and, very recently, from the one of muonic hydrogen. This last measurement gives a very precise Rp value, which is in disagreement (larger than 4 sigma) with the precedent ones. The goal of the PROCADIS project is to measure new transition frequencies in atomic hydrogen to clarify the puzzle between the QED calculations in hydrogenlike systems and the experimental results. The first measurement of the proton radius Rp has been obtained from the electron-proton scattering with a relative accuracy of 2%, which is limited by the difficulty of the data analysis. The determination of the proton radius from the hydrogen frequency measurements uses the energy shift due to the finite proton size. This finite size modifies the coulomb potential and the level energies, mainly for S states (for S states the electron has a non-zero probability to be inside the proton). This effect contributes to the Lamb shifts of the S levels. Several optical frequencies of hydrogen are now known with a fractional accuracy better than 10-11. On the theoretical side, major progress has been performed in the last 15 years for the QED calculations of the Lamb shift in hydrogen. Then it is possible to extract from the comparison between the experiment and the theory a value of the proton radius Rp with an accuracy of about 1% which is currently in agreement with the one obtained from electron-proton collisions. The third method to determine the proton charge radius Rp uses the muonic hydrogen atom (an atom formed with a proton and a muon, µ-p). The muon being 207 times heavier than the electron, the size of the muon orbits is reduced by the same factor. Then the energy shift due to the finite proton size is much larger than in electronic hydrogen. This experiment has been performed at the Paul Scherrer Institute (PSI), in the framework of an international collaboration including Hänsch team in Munich and our group in Paris. In July 2009, this experiment has given a very precise value (accuracy of about 0.2%), but in disagreement by about 4.4 sigma with the hydrogen result. This discrepancy may result from the QED calculations in normal and muonic hydrogen, or from the measurements of the optical frequencies in hydrogen. The aim of the PROCADIS project is to measure the 1S-3S and 1S-4S transitions in hydrogen. Presently these two transitions are not used to determine Rp. These new data will give the possibility to clarify the hydrogen results and to improve the Rp accuracy. Presently the Rp value derived from the hydrogen measurements is mainly deduced from the comparison of the 1S-2S frequency with the 2S-nS/D frequencies (n=8, 12), which have been measured in our group. As these last measurements are limited by an important and non-linear light shift, we have begun the study of the 1S-3S transition, which can be observed with a negligible light shift. Unfortunately the experiment is limited by the low intensity of the UV source at 205 nm used for the 1S-3S excitation. In the PROCADIS project, we plan to build a new UV source at 205 nm, replacing two frequency doubling stages of a titanium sapphire laser at 820 nm with a frequency sum of a commercial UV source at 266 nm and a titanium sapphire laser at 896 nm. With this scheme, it is possible to increase the UV power at 205 nm by a factor ten, and, consequently, significantly improve the signal to noise ratio of the 1S-3S spectrum. The objective of the PROCADIS project is to measure the 1S-3S frequency with an uncertainty of 1 kHz, which corresponds to a relative uncertainty of 0.6% for the determination of Rp. Moreover, with this new laser scheme, it is possible to excite the 1S-4S transition at 194 nm and obtain new data to determine the proton radius Rp.