Exotic atoms are those in which one or more of the constituents of normal atoms have been replaced by an exotic particle, such as an antimatter particle. These atoms can then be probed to search for any tiny discrepancies in their properties from those predicted by models using techniques that underpin the world’s most accurate timekeepers, atomic clocks — and thereby opening a window on the foundations of physics. Writing in Nature, Hori et al.1 are the first to report laser excitation of helium atoms in which one electron has been replaced by a subatomic particle called a pion.
The interest in exotic atoms arises from the fact that they often facilitate the most basic experimental strategy used in physics: changing a single parameter or component in an otherwise complex system, to observe the effect. In practice, this is not as simple as it might seem. Different particles can have different masses or charges, and might interact with their surroundings differently in other subtle ways. However, such subtleties often add to the value of exotic atoms.
As the techniques needed to study exotic atoms improve, increasing numbers of scientists are working with these atoms to investigate the fundamental properties of nature. A good example of this is the ‘proton radius puzzle’, which arose from a study of muonic hydrogen2 — a hydrogen atom in which the electron has been replaced by a subatomic muon particle (muons have similar properties to electrons, but have about 200 times greater mass).
Muonic hydrogen was used to determine a key property of the proton known as the charge radius, but the value obtained was about seven standard deviations away from the expected value at the time. The value obtained using muonic hydrogen has since been independently confirmed in a study of ordinary hydrogen3, and also in experiments in which electrons are scattered from protons4, potentially clarifying the true value of the proton radius and thus solving the puzzle. Nevertheless, muonic hydrogen aptly illustrates how exotic, sometimes short-lived, atomic systems can be used to poke holes in seemingly well-established results.
An important feature of exotic atoms that adds to their utility as probes for fundamental physics is that they are bound systems (energy is needed to pull their components apart), with multiple internal energy states. Transitions between these states are therefore amenable to study by laser spectroscopy, the most precise measurement tool in the physics toolkit. The study of transitions in atoms — and particularly in the hydrogen atom — is an ongoing effort that has spanned more than two centuries. It inspired Niels Bohr’s groundbreaking model of the atom in the early twentieth century, for example, and has driven much of the development of quantum mechanics.
Today, atomic transitions are the foundation on which all time measurements are built: a transition in the caesium-133 atom provides a reference value that underpins the International System of Units definition of the second. Therefore, spectroscopic techniques for measuring transitions are constantly being refined, and the best measurements can now reach staggering precisions — to almost 18 significant figures5. The most precise measurements of exotic atoms still lag some way behind, but a measurement on antihydrogen (the bound state of an antiproton and an antielectron) achieved a milestone precision of almost 12 significant figures6, thus paving the way for extremely sensitive tests of the fundamental properties of antimatter.
In their seminal work, Hori et al. record the first observation of a transition in a ‘pionic’ helium atom. In such an atom, one of the two electrons of a helium atom has been replaced by a subatomic particle called a pion. Pions were discovered7 by Cecil Powell and co-workers in 1947, but their existence was first predicted8 in 1935 by Hideki Yukawa. They belong to the family of subatomic particles known as mesons, which are made up of a quark and an antiquark; quarks are the particles that make up protons and neutrons.
Pions are short-lived particles that come in positively charged, negatively charged and neutral varieties. The negatively charged pions used by Hori et al. have a lifetime of only 26 nanoseconds when isolated. It is thus no small feat that the experimenters not only succeeded in replacing an electron in helium atoms with a pion, but also observed the resulting exotic atom undergo a quantum transition. A further difficulty is that the lifetime of the pion in the exotic atom can be reduced to picoseconds because of its vicinity to the atom’s nucleus.
The authors prepared pionic helium atoms by firing a beam of pions at a liquid-helium target. In Hori and colleagues’ experiment, the helium target was cooled to a cryogenic temperature of about 2 kelvin. This allowed some pions to be captured in a weakly bound state of pionic helium, in which the pion was sufficiently far from the nucleus to be shielded from it by the remaining electron (Fig. 1). The resulting exotic atom therefore retained a lifetime of nanoseconds, which is long enough for a laser pulse to excite the nascent exotic atom.
Using a short pulse (0.8 nanoseconds) of infrared laser light, Hori and colleagues provoked a transition of the pion. This resulted in the ejection of the remaining electron, leaving a short-lived system consisting of just a pion bound to a helium nucleus. The pion was then absorbed by the nucleus, leading to the latter’s break-up (fission).
The pion transition was detected by carefully removing large ‘background’ signals from the experimental data; this background was associated with fission products from short-lived states of pionic helium, or was generated by the pion beam itself. That left a signal from just three transitioning pionic helium atoms per hour, or an estimated three per billion exotic atoms produced. Despite the challengingly low numbers, the laser-induced signal for the transition was clearly detected, and the laser frequency at which it occurred (and which corresponds to the energy change of the transition) could be determined with an absolute precision of about five significant figures.
This result builds on Hori and colleagues’ extensive experience of studying another exotic helium-type atom, in which one electron was replaced by an antiproton9. That work resulted in, among other things, the most precise determination so far of the ratio of the mass of the antiproton to that of the electron. However, the authors had to overcome further challenges to study pionic helium.
For example, the lifetimes of pionic helium atoms are shorter than those of antiprotonic helium and the widths of lines in its spectrum are broadened, because the excited exotic atoms undergo many collisions in the relatively dense liquid-helium target. It might be possible to reduce the density of the liquid helium to resolve these somewhat incalculable effects, although this would also lower the signal rate. It remains to be seen whether extrapolation to zero density, where collisional effects are minimized, is practicable.
Nevertheless, Hori and colleagues’ work opens up a whole new experimental system for further exploration. If the above challenges can be overcome, such exploration might enable the accuracy of the mass of the negative pion to be improved by a factor of 10–100, for instance; currently, this mass is known to only six decimal places. The experiment thus paves the way to fresh insights into the fundamental constituents of nature.