Scientists create bizarre form of nuclear antimatter

Give a scientist a world-class particle accelerator and he will study many counterintuitive phenomena. In the collisions they produce, classical physics falls by the wayside and quantum mechanics reigns supreme. Matter and antimatter appear and disappear with unbridled abandon. Particles and temperatures not seen in the universe since the Big Bang appear. And complicated atomic nuclei form and decay.

None of these phenomena are visible to the human eye, but they are all part of a recent achievement by a team of researchers at Brookhaven National Laboratory. They have created the heaviest exotic antimatter hypernucleus ever seen.

It wasn’t easy and required a multitude of steps to achieve. Just understanding their achievement requires a long list of steps – each one difficult and simply overwhelming when combined.

Exotic hydrogen

Nuclei are located 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. In fact, the defining property of hydrogen is a nucleus with only one proton. Nuclei with two protons are helium.

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

Even rarer and not found in nature is hydrogen-4, which consists of one proton and three neutrons. Hydrogen-4 was created in the laboratory and has a lifetime of 1.4 x 10^-22 seconds. It decays by releasing a neutron and becomes 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 with strange names: up, down, charm, strange, top, and bottom. In protons (two up quarks and one down quark) and neutrons (one up quark and two down quarks), only up and down quarks are found. The other quarks live for only a fraction of a second and do not occur in nature. They were last found in abundance in the universe less than a second after it was formed.

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

Quarks can group together in groups of three to form what are known as “baryons.” The most common baryons are protons and neutrons. Quarks can also group together in quark/antimatter quark pairs. These particles are called “mesons.” Other quark combinations have been observed in recent years, but these combinations are unusual and do not occur in nature.

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

Antimatter

The kind of matter you and I are made of is just one form of material that can exist. Another is a material called “antimatter.” Antimatter is a substance that is the opposite of ordinary matter. When matter and antimatter meet, they annihilate each other, releasing a tremendous amount of energy. When one gram of matter and antimatter are combined, the energy released is comparable to the atomic explosion in Hiroshima.

Antimatter was proposed in 1928 and first observed in 1932. The first form of antimatter observed was the positron, the antimatter equivalent of the electron. The positron has many of the same properties as the electron, but with the opposite electric charge. Since then, antimatter versions of the proton (1955) and the neutron (1956) have been observed. Antimatter equivalents of most 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. This means that as long as antimatter is created in complete isolation from matter, it is possible to form antimatter atoms. In complete isolation, it would be possible to form antimatter planets, galaxies, people, and antimatter versions of every matter object 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 then collide them. The range of accelerated nuclei spans the periodic table, but the collisions between nucleons in these collisions are about 200 GeV (or an energy equivalent to the mass energy of about 213 protons). All kinds of extreme processes occur in these high-energy collisions, including the creation of lambda particles as well as many antimatter baryons.

In 2010, STAR researchers created what they call a tritium hypernucleus, consisting of a proton, a neutron, and a lambda particle. They also created the antimatter version (antiproton, antineutron, and antilambda). In this latest 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, the scientists examined over six billion collisions and obtained 24 matter-hydrogen-4 matter hypernuclei and 16 antimatter hypernuclei.

The data collected was robust enough to examine them and look for differences between matter and antimatter in the hydrogen-4 hypernuclei. This study was motivated by the fact that, although our theories suggest that matter and antimatter should be present in equal amounts, our universe is made entirely of matter. Researchers suspect that in the early universe, matter was preferred over antimatter by a tiny amount. (The hypothetical excess was one part in a billion.) So researchers often study matter and antimatter, looking for clues as to what caused that original imbalance.

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