Laser spectroscopy of long-lived pionic helium atoms

This page gives layperson explanations to the recent paper Laser spectroscopy of pionic helium.

The special significance of pions in the history of the development of particle physics and the story of the early pioneers in the French article in Futura Sciences by journalist Laurent Sacco.

Informative English descriptions in Physics World by journalist Edwin Cartlidge and Physics Today by journalist Johanna Miller and Sci-News.

Nice German articles in Spektrum by journalist Jan Dönges and in Frankfurter Allgemeine Zeitung by journalist Manfred Lindinger, concise article in Die Welt.

Dutch article in New Scientist by journalist Jean-Paul Keulen. Articles in India International Times, Spanish 20 minutos and Moncloa, Hungarian Origo and and BEOL, Czech Science mag, Russian nplus1

Theoretical descriptions and references are provided in a specialist 2014 paper.

What is a pion?

The protons and neutrons that make up the atomic nuclei in normal matter are each made of 3 quarks. Quarks have an interesting characteristic: they cannot exist alone in isolation. No one has ever observed a single bare quark. But 3 quarks together can coexist very stably in the form of a proton. The fact that protons are stable is of course vital to the existence of life!

Mesons are radically different from protons or neutrons as they are particles made of a quark and an antiquark. The antiquark is the antimatter counterpart of the quark. Among all the mesons, charged pions are the longest-lived and most stable variety. Still “stable” is a relative word, the charged pion’s lifetime is only 40 millionth of a second, or 26 nanoseconds. So every charged pion is destined to soon decay with this lifetime, usually into two particles called a muon and a neutrino.

Charged pions aren't unusual objects - they are constantly being produced 10 km or more up in the atmosphere of the earth when cosmic rays from outer space collide with the gas molecules that make up the atmosphere. The newly created pions then speed downwards, but most of them decay before they reach sea level. If you fly in an airplane or go skiing in a high mountain, after some time your body may be hit by a charged pion.

The story of its discovery

Pions are important for another reason. The neutron was discovered in 1932 in the University of Cambridge. This immediately triggered a minor crisis because, people couldn’t figure out how the protons and the newly discovered neutrons in the atomic nucleus are bound together. The known laws of physics at the time suggested that the positively charged protons in the nucleus would repel each other and fly apart. A far stronger force (today we call it strong interaction) had to exist to counteract this and keep the nucleus together.

So the meson was predicted in 1935 as the particle or “field” that acts as a kind of glue between protons and neutrons. The particle was predicted to have a mass 200 times heavier than the electron. After the Second World War, a group of scientists from Bristol went to the top of the Pyreness mountains and placed photographic emulsions. Among the photographs of particles traveling through the emulsion, they found a new particle. This was the charged pion. Many other mesons have been discovered over the last 70 years, the latest very exotic ones in facilities like CERN's Large Hadron Collider (LHC) or KEK's Super-B factory.

The "problem"

   Negatively charged pions, if you tried to study them by allowing them to come to rest in matter, normally immediately react with the atomic nuclei and disappear in a trillionth of a second, or 1 picosecond. Charged pions were the first particles to be artificially produced in accelerators in the late 1940’s so we know quite a lot about them. Many varieties of atoms containing pions were studied between the 1950's and early 90's to understand the strong interaction. All these pionic atoms formed in dense materials had lifetimes of a few picoseconds.

Because of this extremely short lifetime and fragility, we haven’t been able to study pions using the most precise and modern measurement methods that are available. We still know less about pions compared to the normal protons, neutrons, electrons, or even antiprotons and positrons (the antimatter counterparts of antiprotons and electrons) for which we can produce atoms with much longer lifetimes.

The idea of this work was to produce an atom containing a pion that has a lifetime >1000 times longer than any other pionic atom that could be experimentally synthesized in a practical way. This would allow measurement techniques involving lasers to be employed, so that the fundamental characteristics of the pion, such as its mass, could be studied with a far higher precision than before. This would then allow us to set upper limits on any new phenomenon involving pions that have not been previously predicted by the Standard Model.

The metastable (or long-lived) pionic helium atom

    A normal helium atom in a child’s balloon for instance, is composed of an atomic nucleus and two electrons. In metastable pionic helium (which is the atom we newly verified or identified), one of the electrons is replaced by a negatively charged pion. The pion orbits the nucleus at a distance of several tens picometers. This is far enough so the pion escapes being rapidly absorbed by the nucleus as is the case with practically every other pionic atom that physicists can produce. In addition to this, the remaining electron acts as a kind of shield, protecting the pion during collisions with other helium atoms. Because of these special features, the long-lived pionic helium atom can survive for many nanoseconds - this may seem short, but it is more than 1000 times longer than other pionic atoms that can be formed in dense materials (confusingly, there is a two-body object called "pionic helium" which has the same name but is an entirely different thing. The atom we studied and verified is properly called "long-lived", "three-body", "neutral" or "metastable" pionic helium to distinguish it from other things with similar names.)

The existence of this atom was however hypothetical until recently. In the 1960’s, pions were studied using devices called bubble chambers (the invention won the Nobel Prize in Physics in 1960). Big tanks were filled with liquid helium and kept near boiling conditions. Every few seconds, a piston expanded the volume of the tank. When pions traveled through the helium, small tracks of bubbles would appear along the particle trajectories, similar to airplane contrails. The tracks were photographed with film cameras. And looking at the developed photographs, research groups using facilities located in Chicago, Pittsburgh, and the USSR saw an anomaly which seemed to suggest that a small fraction of the pions were surviving longer than the normal trillionth of a second. The measurement was repeated in the beginning of the 1990's using newer methods in Vancouver. George Condo of the University of Tennessee in 1964 proposed the long-lived pionic helium atom as a way to explain this anomaly. But it wasn’t possible to verify this theory. After all, what is experimentally seen is that some pions are long-lived, that doesn’t necessarily mean the atoms are produced. There were at least 2 other plausible theories being discussed for decades that did not involve the existence of such an atom.

How to verify the atom's existence

The smoking-gun proof would be to carry out optical spectroscopy. Each atom that exists in nature has several characteristic atomic wavelengths at which the atom can absorb or emit light. These wavelengths are the atom’s “fingerprints” which are unique to that particular atom. For example, the element helium was first discovered, not on the earth but in one of the layers of the sun's atmosphere, called the chromosphere. A characteristic yellow component of light with a wavelength of 587.5 nm was observed in 1868 during a total solar eclipse in India. It was decided by scientists at the time that this special wavelength indicated the existence of an unknown element that was previously never seen on the earth. More than a decade later, this new element (called helium - related to the Greek word for sun) was discovered on the earth, within the material that erupted from the Mount Vesuvius volcano in Italy.

The difficulty is that this three-body pionic helium atom doesn’t live long enough to emit light and provide this fingerprint. If the atom doesn’t emit light, we can’t see it or detect that it exists. But there is another modern way which is heavily studied in laboratires like the Max Planck Institute for Quantum Optics; if we place the atoms within a very powerful beam of laser light with an intensity of megawatts of the correct wavelength, then we can possibly make the atom resonate within its short lifetime. No such laser resonance had ever been achieved for an atom containing a meson for many reasons, but it seemed that it might be possible for this special case. The laser was invented in 1960 (almost exactly 60 years ago), but we don't think people at the time would have imagined such an application.

Details of the experiment

Here the pion is orbiting the nucleus. The laser light forces the pion to undergo a quantum jump from an outer atomic orbit to an inner one. This causes the pion to eventually fall into the nucleus. The nucleus not being able to accomodate the addition of the pion breaks up into its constituent protons, neutrons, and deuterons. To repeat, when the laser wavelength is carefully tuned to the theoretically expected wavelength of pionic helium, and if the atom actually exists, we cause the orbiting pion to undergo a quantum jump, which leads to nuclear breakup and the emission of particles. As the probability of all this happening is low, we need to produce as large a sample of these atoms as possible, and a powerful and highly reliable laser to excite them. A sensitive detector is needed to measure the protons, neutrons, and deuterons.

We went to the world’s most powerful pion source located in the Paul Scherrer institute near Zurich in Switzerland. This machine called the 590 MeV ring cyclotron produces a beam of protons of 1.2 megawatt power which is allowed to strike a plate of graphite. This produces typically 20 million pions per second of a certain velocity at the location of our experimental target. But the accelerator was so powerful that the pion beam was overwhelmed by contamination of a few billion electrons per second travelling nearly at the speed of light. This completely blinded our sensitive detectors. So PSI staff installed a device that deflected away the relativistic electrons using electrodes that were biased at more than two hundred thousand volts. Numerous other adjustments reduced the contamination in the beam to levels where we could expect the experiment to work. The purified beam of pions was allowed to come to rest in a tank filled with superfluid helium where some of the pions formed long-lived pionic helium atoms.

Even with the powerful accelerator and laser beam, and a sensitive detector, our experiment could only expect to see 2 or 3 atoms per hour resonating with the laser and producing a signal. There were a thousand times larger backgrounds coming from pions that didn’t produce the atoms. Months of data accumulation were needed on a 24-h basis to search for and resolve the signal. This atom has several characteristic wavelengths from ultraviolet to infrared wavelengths. But not all the wavelengths would produce a signal. If a pion didn’t populate the state related to that particular wavelength, we would see nothing. Three transitions at ultraviolet and infrared wavelengths were attempted one by one. Each wavelength needed the development of its own specialized laser or at least a modified one, and months of measurements. To our surprise, pionic orbits that the theory predicted should be long-lived were instead found to produce no signal (the theory concerning how these atoms are formed are far from perfect because we don't know enough about the complex processes). Finally the wavelength at 1631 nm produced the signal. This happens to be a wavelength used for optical fiber telecommunication, and so many optical devices with superior characteristics were available.

The project took 8 years including the design and construction of the experiment. Many people were very patient with us. We thank all the staff of Paul Scherrer Institute, CERN, and Max Planck Institute for Quantum Optics.

The PiHe experiment installed in the PiE5 beamline of PSI
PiHe is an international collaboration of researchers from:
  • CERN, Geneva, Switzerland
  • Max Planck Institute of Quantum Optics, Garching, Germany
  • Paul Scherrer Institute, Villigen, Switzerland
  • Wigner Research Centre for Physics, Budapest, Hungary
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