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The Proton Size Puzzle: Testing QED at Extreme Wavelengths

Total Cost €


EC-Contrib. €






 QED-PROTONSIZE project word cloud

Explore the words cloud of the QED-PROTONSIZE project. It provides you a very rough idea of what is the project "QED-PROTONSIZE" about.

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Project "QED-PROTONSIZE" data sheet

The following table provides information about the project.


Organization address
address: DE BOELELAAN 1105
postcode: 1081 HV

contact info
title: n.a.
name: n.a.
surname: n.a.
function: n.a.
email: n.a.
telephone: n.a.
fax: n.a.

 Coordinator Country Netherlands [NL]
 Project website
 Total cost 2˙497˙664 €
 EC max contribution 2˙497˙664 € (100%)
 Programme 1. H2020-EU.1.1. (EXCELLENT SCIENCE - European Research Council (ERC))
 Code Call ERC-2015-AdG
 Funding Scheme ERC-ADG
 Starting year 2016
 Duration (year-month-day) from 2016-09-01   to  2021-08-31


Take a look of project's partnership.

# participants  country  role  EC contrib. [€] 
1    STICHTING VU NL (AMSTERDAM) coordinator 2˙497˙664.00


 Project objective

A key component of the Standard Model is Quantum Electrodynamics (QED). QED explains e.g. the anomalous magnetic moment of the electron and small energy shifts in the energy structure of atoms and molecules due to vacuum fluctuations. After decades of precision measurements, especially laser spectroscopy in atomic hydrogen, QED is considered the most successful and best-tested theory in physics. However, in 2010 precision spectroscopy in muonic-hydrogen (where the electron is replaced with a muon) has lead to discrepancies in energy level structure that cannot be accounted for. If QED is considered correct, then one way of interpreting the results is that the size of the proton is different in normal (electronic) hydrogen by as much as 4% (a 7 sigma effect) compared to muonic hydrogen. Despite great theoretical and experimental efforts, this 'proton size puzzle' is still unsolved.

I propose to perform precision spectroscopy in the extreme ultraviolet near 30 nm in the helium ion, to establish an exciting new platform for QED tests and thereby shed light on the proton-size puzzle. The advantages of helium ions over hydrogen atoms are that they can be trapped (observed longer), QED effects are more than an order of magnitude larger, and the nuclear size of the alpha particle is better known than the proton. Moreover, the CREMA collaboration has recently measured the 2S-2P transition in muonic He (both 3He and 4He isotopes) at the Paul Scherrer Institute. Evaluation of the measurements is ongoing, but could lead to an 8 fold (or more) improved alpha-particle radius, so that it is no longer limiting QED theory in normal He. I will use several ground-breaking methods such as Ramsey-comb spectroscopy in the extreme ultraviolet to measure the 1S-2S transition in trapped normal electronic He, with (sub) kHz spectroscopic accuracy. This will provide a unique and timely opportunity for a direct comparison of QED in electronic and muonic systems at an unprecedented level.


year authors and title journal last update
List of publications.
2019 Nicolas Hölsch, Maximilian Beyer, Edcel J. Salumbides, Kjeld S. E. Eikema, Wim Ubachs, Christian Jungen, Frédéric Merkt
Benchmarking Theory with an Improved Measurement of the Ionization and Dissociation Energies of H 2
published pages: 103002, ISSN: 0031-9007, DOI: 10.1103/physrevlett.122.103002
Physical Review Letters 122/10 2019-09-18

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