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| Pionic helium: In this experiment, a pion – shown here with one orange and one blue particle representing its quark and anti-quark – replaces one of the two electrons in the helium atom. This new metastable atom is then excited with laser light (shown here in red) to probe its properties. (Courtesy: Max Planck Institute of Quantum Optics/Thorsten Naeser) |
After eight years of ongoing research, a group led by Masaki Hori, senior physicist at the Max Planck Institute of Quantum Optics in Garching, Germany, has now succeeded in a challenging experiment: In a helium atom, they replaced an electron with a pion in a specific quantum state and verified the existence of this long-lived "pionic helium" for the very first time. The usually short-lived pion could thereby exist 1000 times longer than it normally would in other varieties of matter. Pions belong to an important family of particles that determine the stability and decay of atomic nuclei. The pionic helium atom would enable scientists to study pions in an extremely precise manner using laser spectroscopy.
How was the pion-Helium achieved?
To achieve this, the Helium atom was cooled to almost absolutely Zero Temperature. At this temperature, Helium becomes a superfluid and is able to climb the walls of the container. Then, to create the pionic Helium, researchers shot the Helium atom with pions.
Pions are similar to protons and neutrons but are composed of only one quark and antiquark. They can be either positive, negative or neutral depending on the different compositions of the quarks and antiquarks. While it is not possible to swap a negative quark for an electron and get a stable atom, researchers have now shown that it is possible dor a very short amount of time.
The challenge that the team faced :
The challenge the team struggled with for eight years was proving that such a pionic helium atom exists in a tank filled with extremely cold, superfluid helium. In the helium atom, the pion behaves like a very heavy electron. It can only jump between discrete quantum states, like climbing steps on a ladder. The team had to find a long-lived state and a very special quantum leap which they could excite with a laser and which would kick the pion into the helium nucleus and destroy the atom. Then the team could detect the debris from the breakup of the nucleus as a "smoking gun" (see figure). However, the theoreticians couldn't exactly predict at which light wavelength the quantum leap would occur.
So the team had to install three complex laser systems, one after the other, until they were successful.
Research advantages it could provide:
The ability to make artificial atoms containing exotic particles in place of electrons is giving physicists a new way of probing fundamental interactions. This recent work could shed light on the nature of both pions and neutrinos – tiny, neutral particles for which certain attributes, including mass, remain relatively poorly understood.
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