Researchers at the Brookhaven National Laboratory recently created the heaviest exotic antimatter hypernucleus ever observed.
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The antimatter hydrogen-4 hypernucleus consists of an
antiproton, two antineutrons, and an antilambda particle. |
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.