A laser as a weighing scale for elementary particles
In collaboration with French colleagues of Laboratoire Kastler Brossel in Paris, researchers of VU University Amsterdam and the FOM Foundation have succeeded in very accurately comparing the masses of the electron and the proton. What made this mass comparison so special was that it was realised with the help of lasers. This method had already been postulated by scientists forty years ago, but it could only be realised now thanks to a combination of advanced experimental techniques – besides lasers also hydrogen ions - and theory. The result of the mass comparison by Jurriaan Biesheuvel and his colleagues, which was realised with an accuracy of three billionths, was published on 27 January 2016 in the authoritative journal Nature Communications.
Precision measurements with lasers on supercold molecules
The researchers 'weighed' the elementary particles by using a small number of molecular hydrogen ions. These molecules consist of two protons and one electron with the electron functioning as a 'spring' that connects the two protons with each other (figure 1). The mass of the particles determines the frequency of vibration.
The molecules can be set into vibration with a laser of the correct colour. From the colour of the laser the researchers could very precisely derive the vibrational frequency. To ensure that the temperature of the molecules would not disrupt this measurement, the researchers held the molecules in place with electric fields while the molecules were cooled using a second laser to -272.14 °C, just one degree above absolute zero.
For the last step in the weighing process the advanced theory of quantum electrodynamics was applied. Researchers could use this to precisely 'translate' the vibrational frequencies measured into the masses of the electron and proton.
Hidden natural forces and higher dimensions
The high accuracy of the 'optical weighing scale' used also makes the instrument sensitive for as yet undiscovered, mysterious phenomena, such as a possible hidden fifth force of nature and 'rolled up' higher dimensions. The effects of these would only be manifest at the very small scale of atoms and molecules and would, if they exist, result in a deviation of the measured masses of the proton and the electron.
The results from Amsterdam are, however, in exact agreement with mass determinations previously carried out using other techniques and for which the accuracy is still two to thirty times higher. An important conclusion of the researchers is therefore that the effects of a fifth force of nature or higher dimensions are at least one billion times smaller than the normal electromagnetic forces between electrons and protons.
The Dutch and French groups are currently preparing a follow-up experiment that must be fifty times more accurate than the current one. According to Dr. Jeroen Koelemeij, project leader at VU University Amsterdam, the results of that experiment are anticipated with excitement: "The improved measurements will not only be far more sensitive for possible, as yet undiscovered, physical phenomena, but they will also serve as an important second opinion for the completely different techniques currently used for the most accurate mass determinations of protons and electrons.”
The research in Amsterdam is being funded by the FOM Foundation, Technology Foundation STW, the European COST Action Ion traps for tomorrow’s applications, and the Dutch-French Van Gogh Programme.
Reference
Probing QED and fundamental constants through laser spectroscopy of vibrational transitions in HD+. J. Biesheuvel, J.-Ph. Karr, L. Hilico, K.S.E. Eikema, W. Ubachs, and J.C.J. Koelemeij, Nature Communications 7, 10385 (2016). DOI: 10.1038/ncomms10385
Information
Jeroen Koelemeij, +31 20 598 78 92.