Particle physicists create artificial atoms for research purposes

Laser pulses make pions dance

In the laboratories of modern physics the elementary components of matter are studied. To do this, scientists sometimes build artificial atoms to help them understand the laws of matter. A research team at the Paul Scherrer Institute (Villigen/AG) uses a specifically modified helium atom to determine the exact mass and other properties of pions. Pions could help to understand more precisely where atomic nuclei get their mass from.

This device was used at the Paul Scherrer Institute to produce pionic helium atoms for research purposes
Image: PSI

It was in 1947 when the British physicist Cecil Powell and colleagues discovered a new particle – the pion – in the upper earth's atmosphere. This particle is created when cosmic rays from the vastness of the universe hit the Earth's atmosphere. Three years after the discovery of the pion, Powell received the Nobel Prize. The detection of this particle is difficult because it decays quickly. Its lifetime is only a little more than a billionth of a second. Pions decay so quickly that most of the particles have transformed in other particles by the time they reach the surface of the Earth. Only pions with particularly high energy make it to the Earth.

Pions can also be created artificially. One of the world's leading pion sources is located in Switzerland at the Paul Scherrer Institute (PSI), one of the large research facilities of the Swiss Federal Institute of Technology (ETH). PSI in Villigen (AG) is a much sought-after place for scientists dedicated to researching the pion. Some of them joined together years ago in the PiHe collaborationwith the goal of determining the mass and other properties of the pion as accurately as possible. Beside the PSI the Max Planck Institute for Quantum Optics and CERN are involved in the PiHe collaboration. Recently the PiHe researchers published their latest findings in the journal 'Nature'.

Determine the mass of the pion 100 times more precisely

PSI has a long history of research into the pion. In 2016, physicists here determined the mass of the pion with the best accuracy to date, namely 139.57077 MeV/c2 (by an uncertainty of not more than ± 0.00018 MeV. The particle is thus a good 270 times heavier than the electron and about 7 times lighter than the proton. However, the scientists of the PiHe collaboration are not satisfied with this precision. "We want to determine the mass 100 times more precisely than is known today", says Prof. Dr. Anna Soter, former technical coordinator of the PiHe experiment and now a researcher at ETH Zurich.

Since the lifetime of the pion is very short, sophisticated experimental arrangements are required for a precision measurement of the mass of the pion.. Researchers use a trick: They use pions that stop in matter and are captured in atomic orbits hereby replacing an electron of the atom. The captured pion in these so-called exotic atoms undergoes a rapid cascade, while emitting X-rays characteristic to the mass of the particle. In this way the pion mass can be determined, but only up to a certain accuracy. The reason: The pions of the exotic atoms are however extremely short lived an then absorbed by the nuclei, usually within times much shorter than a billionth of a second. The fast destruction of the pions prevent usually all further precision experiments.

Enough time for the measurement of pions

Here the PiHe collaboration has now made great progress: The collaboration investigated helium atoms where the electron was replaced by a pion ('pionic helium'). For the first time, the scientists found an unlikely long lived state in this exotic atom for the first time, and probe it with laser spectroscopy. However, not all pionic helium atoms are long-lived, but those two percent in which the pion is in the so-called Rydberg state. This state can be imagined as a circular orbit far away from the atomic nucleus. Such atoms have a lifetime of 7 to 8 billionths of a second. This is a thousand times longer than the lifetime of atoms where the pion is not in the Rydberg state. This lifetime is still short, but long enough to measure certain properties of the pion with laser beams.

The researchers have succeeded in using a laser to excite the pions to a special energy jump. “We have performed for the first time ever the laser excitation of a meson (a state with a quark and an antiquark) - in our case a pion. This is an important first step towards being able to determine the mass of pions more precisely,” says Anna Soter. “The next step will be to measure the transition energy of the pions precisely with the resonance frequency with our laser. From this energy we can then determine the mass of the pion, ultimately with a 100 times higher precision than in present experiments.”

The results of the PiHe experiment so far are therefore an intermediate step on the way to an even more precise determination of the mass of the pion. This new experiment requires a lower density target to study the collision effects caused by other helium atoms, and other, more narrow atomic transitions will be also probed by the PiHe collaboration.

What is the mass of the atomic nucleus?

But why is it so important to know the mass of the pion so precisely? Pions are the lightest hadrons – that are bound states of quarks. While “everyday” hadronic matter, protons and neutrons, contain three quarks, pions contain only two, a quark and an antiquark. The mass of hadrons from its composite particles is difficult to calculate: the mass of the three quarks in protons and neutrons makes up only one percent of the nucleon’s mass, the rest comes from the elusive strong interaction. Besides being the simplest hadronic systems with only two quark constituents, pions also play an important role in mediating the strong interaction in nuclear matter.

If the role of the pions in the nucleus of the atoms could be clarified in the near future, a large circle would be closed: It would then be clear that the pions that were discovered in the upper Earth's atmosphere some 75 years ago are of central importance for the existence of the material world. The pions would then perhaps provide an answer to the 250-year-old question that Doctor Faust had asked but not answered in Johann Wolfgang von Goethe's drama of the same name, namely "what holds the world together at its core".

Author: Benedikt Vogel

To produce pionic helium, one of the two electrons of the helium atom is replaced by a pion. This artificially created atom can then be examined with a laser beam.
To produce pionic helium, one of the two electrons of the helium atom is replaced by a pion. This artificially created atom can then be examined with a laser beam.Image: PiHe Collaboration

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