## [Research] Unknown forces could be hiding in the values of fundamental constants

The values of fundamental constants of nature, dubbed as physical constants, might be affected by new elementary particles and unknown forces of nature, according to a study by an international group of researchers that includes Designated Assistant Professor Teppei Kitahara at KMI and Institute for Advanced Research, Nagoya University and at IPNS, KEK (cross appointment). The findings were published in the journal Physical Review Letters.

Scientists continuously strive to develop and refine theories that explain the world and the universe around us. One extremely successful theory has been the Standard Model of particle physics, which predicts how elementary particles behave, interact, and eventually form larger structures such as atomic nuclei, atoms, and molecules.

In many cases, the predictions of the Standard Model are found to be in agreement with the results of very precise measurements on particles, atoms and molecules. However, the results of some recent experiments hint towards behavior that might be explained by some higher law of physics that has yet to be discovered. This has stimulated physicists to develop new theoretical models that could possibly describe the putative ‘new physics.’

Theoretical models – including the celebrated Standard Model – take the form of mathematical equations that reflect the laws of nature. The essential ingredients among these equations are the physical constants, such as the mass of the electron and the speed of light. For example, if the mass of the electron were chosen slightly too large, the theoretical predictions would no longer agree with experimental results.

In fact, the physical constants are determined – or rather adjusted – such that theoretical predictions match experimental results as closely as possible. This adjustment of the physical constants is repeated every few years by CODATA, the Committee on Data for Science and Technology. It involves about one hundred results from different types of precision measurements, which are compared with theoretical predictions by the Standard Model for these measurements – everything under the assumption that the Standard Model by itself is valid.

This research points out that this assumption is violated as soon as one contemplates hypothetical elementary particles or forces that might exist beyond the Standard Model. So, one might argue whether it is correct to use these physical constants in combination with theoretical models that try to describe such new phenomena.

At the same time, recent CODATA adjustments revealed that – for unknown reasons – some experimental data are not fully consistent with the found physical constants, for example, the proton charge radius data and the hydrogen 2S–8D transition. These problems were mitigated by increasing the uncertainty of these experimental data so that the inconsistency became insignificant.

But what if the inconsistency were due to new physical phenomena? This research provides an answer to this question: the result points out that instead of increasing uncertainties, one can also modify the theoretical predictions by adding the hypothetical effect of a new elementary particle. The theory that describes this new particle is then characterized by its own fundamental constants, for example the mass of the new particle. It is found that these new constants can be determined along with the known physical constants using the same adjustment procedure.

This research has examined six possible new-physics models that postulate, for example, the existence of particles akin to the famed Higgs boson. Remarkably, in one of these models the introduction of a new particle made the inconsistencies disappear. The new particle also causes the values of some of the existing physical constants to change substantially from their present CODATA values.

Designated Assistant Professor Teppei Kitahara commented:

To put it bluntly, we have been looking for new physics based on the physical constants determined by assuming the Standard Model. This research shows that if there is a certain new physics, this usual framework is not reliable. The value of this research lies in the realization that the precision measurements, now used to determine physical constants, can also be used to search for new physics. However, our results should not be interpreted as the discovery of a new particle. There are measurement data from particle accelerator experiments that rule out some of the hypothetical models we have proposed.