A hybrid matter—an antimatter helium atom containing an antiproton, the
proton's antimatter equivalent in place of an electron, has an unexpected
response to laser light when immersed in superfluid helium, reports the
ASACUSA collaboration at CERN. The result, described in a paper published
today in the journal Nature, may open doors to several lines of research.
"Our study suggests that hybrid matter–antimatter helium atoms could be used
beyond particle physics, in particular in condensed matter physics and
perhaps even in astrophysics experiments," says ASACUSA co-spokesperson
Masaki Hori. "We have arguably made the first step in using antiprotons to
study condensed matter."
The ASACUSA collaboration is well used to making hybrid matter–antimatter
helium atoms to determine the antiproton's mass and compare it with that of
the proton. These hybrid atoms contain an antiproton and an electron around
the helium nucleus (instead of two electrons around a helium nucleus) and
are made by mixing antiprotons produced at CERN's antimatter factory with a
helium gas that has a low atomic density and is kept at low temperature.
Low gas densities and temperatures have played a key role in these
antimatter studies, which involve measuring the response of the hybrid atoms
to laser light in order to determine their light spectrum. High gas
densities and temperatures result in spectral lines, caused by transitions
of the antiproton or electron between energy levels, that are too broad, or
even obscured, to allow the mass of the antiproton relative to that of the
electron to be determined.
This is why it came as surprise to the ASACUSA researchers that, when they
used liquid helium, which has a much higher density than gaseous helium, in
their new study, they saw a decrease in the width of the antiproton spectral
lines.
Moreover, when they decreased the temperature of the liquid helium to values
below the temperature at which the liquid becomes a superfluid, i.e. flows
without any resistance, they found an abrupt further narrowing of the
spectral lines.
"This behavior was unexpected," says Anna Sótér, who was the principal Ph.D.
student working on the experiment and is now an assistant professor at ETHZ.
"The optical response of the hybrid helium atom in superfluid helium is
starkly different to that of the same hybrid atom in high-density gaseous
helium, as well as that of many normal atoms in liquids or superfluids."
The researchers think that the surprising behavior observed is linked to the
radius of the electronic orbital, i.e. the distance at which the hybrid
helium atom's electron is located. In contrast to that of many normal atoms,
the radius of the hybrid atom's electronic orbital changes very little when
laser light is shone on the atom and thus does not affect the spectral lines
even when the atom is immersed in superfluid helium. However, further
studies are needed to confirm this hypothesis.
The result has several ramifications. Firstly, researchers may create other
hybrid helium atoms, such as pionic helium atoms, in superfluid helium using
different antimatter and exotic particles, to study their response to laser
light in detail and measure the particle masses. Secondly, the substantial
narrowing of the lines in superfluid helium suggests that hybrid helium
atoms could be used to study this form of matter and potentially other
condensed-matter phases. Finally, the narrow spectral lines could in
principle be used to search for cosmic antiprotons or antideuterons (a
nucleus made of an antiproton and an antineutron) of particularly low
velocity that hit the liquid or superfluid helium that is used to cool
experiments in space or in high-altitude balloons. However, numerous
technical challenges must be overcome before the method becomes
complementary to existing techniques for searching for these forms of
antimatter.
Reference:
Anna Sótér et al, High-resolution laser resonances of antiprotonic helium in
superfluid 4He, Nature (2022).
DOI: 10.1038/s41586-022-04440-7