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As part of their research on antimatter, researchers at CERN have demonstrated the atypical and unexpected behavior of a hybrid atom, consisting of both matter and antimatter, when immersed in superfluid helium (a temperature close to absolute zero). The results of this experiment pave the way for a new way of measuring the mass of antiparticles, with unprecedented precision.
According to the standard model of particle physics, matter and antimatter differ only by the electric charge of their particles. For example, proton and antiproton carry an opposite charge, but remain identical in all other respects. Past experiences confirm this theory a priori. That said, the experimental methods used so far might not be sensitive enough to detect the minute differences that might exist between particles and their antiparticles. Scientists are thus trying to develop more precise measurement techniques.
with the surrounding matter, under penalty of immediate annihilation. Antimatter atoms were magnetically levitated in vacuum chambers for spectroscopic measurements. Other experiments have confined antiprotons in ion traps made up of electric and magnetic fields, explains Masaki Hori, a physicist at the Max Planck Institute for Quantum Optics, who uses spectroscopy to study antimatter. He and his team succeeded in combining matter and antimatter in the same helium atom, in order to increase the precision of the measurements. But their recent discovery could offer even greater possibilities.
A more stable antiproton in helium superfluid
The creation of hybrid atoms is at the heart of the ASACUSA experiment at CERN, which aims to study the fundamental differences between matter and antimatter that . The antiprotonic helium in question here was created by replacing one of the electrons of the helium atom with an antiproton (which is also negatively charged); This was achieved using CERN’s Antimatter Decelerator, which shoots slowed down antiproton beams into cold helium gas which. Most antiprotons annihilate rapidly on contact with surrounding matter, but a small number of them combine with helium to form hybrid atoms. The mass of the antiproton is then measured using laser spectroscopy, a level of precision never before achieved.
When atoms are placed in a liquid, their spectral lines optics which correspond to the electronic transitions are strongly enlarged compared to those of isolated atoms, because of the intense interactions existing between the molecules of the liquid which. This increase can reach a factor of more than a million, which is detrimental to high resolution spectroscopy analyses. This happens even in superfluid helium, which is yet the thinnest, coldest and most chemically inert transparent liquid. However, the exact position of the resonance line on the frequency scale, as well as its shape, make it possible to deduce the properties of the atom studied.
The team of the ASACUSA experiment, led by Masaki Hori, set out to observe how a hybrid atom behaved when it too was immersed in superfluid helium. To do this, they mixed the antiprotons from the decelerator with liquid helium cooled to a temperature close to absolute zero (-95 C ). They found that its spectral line retained a sub-gigahertz width. Against all expectations, the structure remained stable long enough to be studied by spectroscopy: protected by the electronic layer of the helium atom, the antiproton was not immediately destroyed.
An approach applicable to other exotic particles
To achieve this result , the team examined the hybridized helium atoms at different temperatures by spectroscopy. As soon as the temperature fell below the critical temperature of 2.2 Kelvin (i.e. -95,95 C) At which helium enters a superfluid state, the narrow antiproton spectral lines become more narrow. Let us recall in passing that superfluidity describes a state of matter in which it behaves like a fluid devoid of any viscosity. We do not yet know how the striking change in the spectral lines of the antiproton occurs in such an environment and what physically happens in the process. We were surprised by it ourselves, recognizes Hori.
If the phenomenon remains clarified, it opens the way to new possibilities of measurement. The narrowing of the spectral lines is such that the hyperfine structure defined by the minute changes and divisions of the energy levels of the atom can be resolved. This made it possible to solve the hyperfine structure resulting from the spin-spin interaction between the electron and the antiproton with a relative spectral resolution of two parts on 106, even if the antiprotonic helium resided in a dense matrix of atoms of normal matter , underline researchers in Nature.
Concretely, this implies that other hybrid helium atoms, composed of various other antimatter particles, could be created in the same way in superfluid helium, to be studied by laser spectroscopy with high spectral resolution. Scientists could thus accurately determine the masses of these particles.
The sharp spectral lines could also be useful for detecting antiprotons and antideutrons in the radiation cosmic. Superfluid helium detectors could serve as a support for future experiments and be adapted to the capture and analysis of antiparticles from space, explains Hori. However, many technical challenges must be overcome before these methods complement existing methods, notes the physicist.