Laser focus on mesons

The first demonstration of laser spectroscopy of a meson, achieved at PSI's πE5 beamline, opens up new avenues for precision studies of ‘exotic atoms’.

Almost 60 years ago to the day, on 16 May 1960, Theodore Maiman made a breakthrough that would change the world: he successfully operated the first functional laser. It was not long until the realisation that these novel sources of light enable not only novel technologies, but also new levels of high-precision spectroscopy. And lasers did deliver on that promise, most famously in the spectroscopy of hydrogen and a long string of other atomic systems. Reporting this week in Nature [1], a collaboration led by Masaki Hori from the Max Planck Institute of Quantum Optics in Garching, Germany, describes unique experiments performed at PSI, in which for the first time atomic lines of not a lepton — such as electrons or muons — have been resolved, but those of a meson. This achievement points the way to harnessing laser spectroscopy for exploring the properties and behaviour of mesons with unprecedented precision.

Mesons are subatomic particles composed of a valence quark and an antiquark. The lightest and longest-lived among them are pions, consisting of up–down quark−antiquark pairs, which mediate nucleon–nucleon interactions. Shortly after their discovery in 1947, it was found that negatively charged pions can be part of ‘exotic’ atoms or ions, in which one electron is replaced with a π⁻ meson. The creation of such pionic atoms opens up the possibility, in principle, to study pions with the toolset of atomic spectroscopy. In practice, however, such studies are notoriously difficult, for the short lifetime of these atoms, on the order of a picosecond or less, and the challenges associated with producing them in sufficiently high numbers. These limitations have prevented so far in particular detailed studies of the pion mass beyond 1-ppm precision.

Long live the pions

The collaboration, which included Anna Sótér, who now works at PSI and at ETH Zurich, and PSI laser engineer Andreas Dax, has found ways to addressing both issues, the low density and the short lifetime of the pionic atoms. They decided to work with pionic helium (π⁴He⁺), which consists of an α particle, an electron, and a π⁻ meson. For this effective three-body system, a metastable state with nanosecond lifetime has been predicted, which they had identified in earlier theoretical and simulation work as the most promising candidate for laser spectroscopy. Just that metastable π⁴He⁺ has never been unambiguously observed — until now.

To observe direct spectroscopic signatures of π⁴He⁺, Sótér and colleagues relied on the pion source with the worldwide highest average intensity, the πE5 beamline of the 590-MeV ring cyclotron at PSI, which has been used before for standard-setting experiments with pionic atoms. In the present experiments, the intense pion beam was directed into a superfluid-helium target, where a few percent of the pions are captured in an atomic collision, such that π⁴He⁺ is formed, with the pion orbiting the helium ion in a highly excited Rydberg state.

After their creation, there was a narrow window to probe the π⁴He⁺ atoms with sub-nanosecond laser pulses. The limited time was dictated not only by the lifetime of the atoms, but also by the repetition rate at with π⁻ mesons arrive from the cyclotron. Its accelerating radiofrequency field oscillates at 50.63 MHz, such that there is less than ~20 ns between pulses. All factors considered, there was a 1:1000 chance that a laser pulse would coincide with a π⁴He⁺ atom. Once such an event happens, the laser initiates an electromagnetic cascade process that ends with the nucleus absorbing the π⁻ meson and undergoing fission. The emerging neutron, proton and deuteron fragments can then be measured using a scintillator array.

Third time's a charm

Before these tell-tale signatures of a laser-induced resonance in the atom could be detected, one further complication had to be tackled: if was not clear precisely which transitions could be excited with the laser pulse. The team expected that the principal quantum number of the state would be around 16, and the orbital angular momentum one unit less. Which still left them with a handful of options, each of which to test would require the installation of a complex laser system and typical measurement times on the order of hundreds of hours. After failing to record signals for the (16, 15) → (17, 14) and (16, 15) → (16, 14) transitions, the persistence would pay off. For the (17, 16) → (17, 15) transition they recorded an unmistakable signal of more than seven standard deviations (see the figure).

This first demonstration of laser spectroscopy of pionic helium atoms, together with the verification of the existence of metastable π⁴He⁺, opens up new possibilities in meson research. In particular, a new high-precision determination of the charged-pion mass should become possible. That mass was traditionally measured by looking at the deflection of trajectories in a magnetic field for pions produced in accelerators, or by detecting the x-ray emission during the de-excitation cascade from excited pionic atoms. Laser spectroscopy might offer an improvement in precision by one to two orders of magnitude. Such experiments will require measurements are different helium pressures though, such that the ‘vacuum value’ can be deduced. This sort of experiment with gaseous rather than the denser superfluid helium should be possible, now that the relevant transitions frequencies are known. Furthermore, pion precision measurements could also set new bounds on any beyond-standard-model forces involving mesons.

On to another exotic flavour

For Sótér, however, the next focus is on the laser spectroscopy of another kind of exotic atoms. In January 2020, she has started an SNSF Ambizione fellowship, with the goal of using muonium, which consist of a positive muon and an electron, for novel investigations of antimatter gravity and in high-precision spectroscopic studies. Hosted by the group of Prof. Klaus Kirch, she will develop technology in his labs at ETH Zurich and use the unique capabilities at PSI for performing muon experiments.