Scientists create bizarre form of nuclear antimatter

Give a scientist a world-class particle accelerator and they’re going to study lots of nonintuitive phenomena. In the collisions they generate, classical physics drops by the wayside and quantum mechanics rules. Matter and antimatter appear and disappear with wanton abandon. Particles and temperatures not commonly seen in the Universe since the Big Bang come out to play. And complicated nuclei form and fall apart. 

None of these phenomena can be seen by the human eye, yet all were components of a recent accomplishment by a group of researchers working at Brookhaven National Laboratory. They created the heaviest exotic antimatter hypernucleus ever seen.

It wasn’t easy, requiring a large number of steps to bring it about. Even simply understanding their achievement requires a laundry list of steps — each difficult and simply mind-boggling when performed together.

Exotic hydrogen

Atomic nuclei exist at the center of atoms. They generally contain a mixture of protons and neutrons. The exception is the nucleus of a hydrogen atom, which usually consists of a single proton. Indeed, the defining property of hydrogen is a nucleus with just one proton. Nuclei with two protons are helium.

However, while most hydrogen consists of a single proton, a few hydrogen nuclei contain a proton and a neutron. The name for this is hydrogen-2 (also called deuterium). On Earth, there is about one deuterium atom for every 6,200 regular hydrogen atoms (0.02%). Similarly, there is even a rarer form of hydrogen with one proton and two neutrons (called hydrogen-3, or tritium). Tritium nuclei make up about a billionth of a billionth of the hydrogen on Earth (10^-16 %). Like hydrogen, deuterium is stable, while tritium decays with a half-life of 12 years.

Even rarer, and not found in nature, is hydrogen-4, consisting of one proton and three neutrons. Hydrogen-4 has been created in the laboratory and has a lifetime of 1.4 x 10^-22 seconds. It decays by emitting a neutron and becoming tritium.

Quarks

Protons and neutrons are not the smallest objects known to science — that title belongs to smaller objects called “quarks.” There are six known types of quarks, all boasting odd names: up, down, charm, strange, top, and bottom. Only up and down quarks are found inside protons (two up quarks and one down quark) and neutrons (one up quark and two down quarks). The other quarks live for only a fraction of a second and are not found in nature. They were last common in the Universe less than a second after it began.  

Quarks were proposed in 1964 and data validating their existence was found in the 1970s. The initial theory proposed only three quarks (up, down, and strange). The others were discovered later (charm in 1974, bottom in 1977, and top in 1995).  

Quarks can bind together in groups of three to make particles called “baryons.” The most familiar baryons are the proton and neutron. Quarks can also group in quark/antimatter quark pairs. These particles are called “mesons.” Other combinations of quarks have been observed in recent years, but those combinations are uncommon and not seen in nature.

While protons and neutrons are the familiar baryons, scientists have made other baryons by combining three quarks together. One combination is called the Λ, or lambda, particle, which consists of the quark combination: up, down, and strange. The lambda baryon is unstable, decaying in only 3 x 10^-10 seconds.  Many other forms of baryons have been created and studied in particle accelerators.

Antimatter

The kind of matter that makes up you and me is only one form of material that can exist. Another is a material called “antimatter.” Antimatter is an antagonistic substance to ordinary matter. When matter and antimatter meet, they annihilate each other, releasing a tremendous amount of energy. If a gram of matter and antimatter are combined, the energy release is comparable to the atomic explosion at Hiroshima.  

Antimatter was proposed in 1928 and first observed in 1932. The first observed form of antimatter was the positron, which is the antimatter equivalent of the electron. The positron has many of the same properties as the electron but with the opposite electrical charge. Since then, antimatter versions of the proton (1955) and neutron (1956) have been observed. Antimatter equivalents of most of the known baryons have been observed, including the antimatter version of the lambda particle.

The laws governing antimatter are thought to be identical to those governing matter — meaning that as long as antimatter is made in complete isolation from matter, it is possible to make antimatter atoms. In full isolation, it would be possible to make antimatter planets, galaxies, people, and antimatter versions of any matter thing we have ever observed. Antihydrogen (an antiproton combined with an antimatter electron) was first observed in 1995, and antihelium nuclei were observed in 2011.

The STAR collaboration

The STAR collaboration at Brookhaven National Laboratory uses the Relativistic Heavy Ion Collider (RHIC) to accelerate atomic nuclei to very high energies and collide them together. The range of nuclei accelerated spans the periodic table, however, the collisions between nucleons in these collisions is about 200 GeV (or an energy equivalent to the mass-energy of about 213 protons.) In these highly energetic collisions, all manners of extreme processes occur, including the creation of lambda particles, as well as many antimatter baryons.

In 2010, STAR researchers created what is called a tritium hypernucleus, which means a proton, a neutron, and a lambda particle. They also created the antimatter version (antiproton, antineutron, and antilambda). In this most recent result, scientists created the heavier hydrogen-4 hypernucleus (proton, two neutrons, and a lambda particle), and its antimatter equivalent.  This is the highest-mass antimatter hypernucleus ever created, with a mass of about 3.92 GeV.

To extract the signal, scientists studied over six billion collisions, obtaining 24 matter hydrogen-4 matter hypernuclei and 16 antimatter ones.

The data collected was sufficiently robust to study, looking for matter/antimatter differences in the hydrogen-4 hypernuclei. This study was motivated by the fact that, despite our theories suggesting that matter and antimatter should exist in equal quantities, our Universe is composed entirely of matter.  Researchers hypothesize that somehow in the early Universe, matter was favored over antimatter by a very small amount. (The hypothesized excess was one part in a billion.) So, researchers often study matter and antimatter counterparts, looking for clues as to what created this primordial imbalance.

The STAR researchers report no difference in the properties of matter and antimatter hydrogen-4 hypernuclei. This is consistent with expectations. Researchers continue to study their data, looking for more subtle differences between matter and antimatter hypernuclei.