Dark matter: The mysterious middle child of the universe. As early as the 1880s, scientists have been theorizing about the likelihood of some other matter besides that which we see around us every day. This “dark matter” is thought to account for some 85% of the total matter in the universe and about a quarter of its total energy density. Despite its calculated prevalence, scientists have never been able to measure it.
Don’t Be Such A WIMP
Unlike the atoms that make up your phone, your shoes, and that pizza you had for lunch, dark matter is believed to be non-baryonic in nature, meaning it isn’t made of the same stuff, namely protons and neutrons. Instead, scientists theorize that dark matter is composed of subatomic particles that we have yet to directly observe or classify. One of the methods used to potentially explain dark matter involves weakly-interacting massive particles or WIMPs. These comparatively massive particles interact with baryonic matter only through the weak nuclear force and gravity. While they have never been directly observed in nature, scientists over at the Italian Gran Sasso National Laboratory believe that these particles might just be observable under the right conditions.
Not only are the interactions that could reveal dark matter to us incredibly rare, but they happen on the subatomic level and are difficult to record. External interference from radiation or electronics could skew the measurements and generate a false positive, so the interior of the testing chamber needs to be thoroughly isolated from the outside world. That’s what a team of scientists led by Italian physicist Elena Aprile has set out to do. The project, dubbed XENON1T, involves a massive two-metric-ton vat of liquid xenon.
Xenon belongs to a class of elements called noble gases. Under standard conditions, noble gases are all colorless, odorless, largely unreactive, and exist as single, unpaired atoms. Nuclear decay is often used in carbon-14 dating and similar measurements using uranium and lead. Radioactive decay happens through two primary methods: Alpha decay and beta decay. With beta decay, a decaying atom can release a positron or a neutron, or it may capture an electron. Any of these processes alter the identity of the atom, turning it into that of a derivative element.
When an atom undergoes electron capture, it strips an electron from one of its lower orbitals and fuses it with a proton to create a neutron. The resulting nucleus drops a rank on the periodic table, and to fill the gap left by the hijacked electron, another electron is plucked from a higher orbital to take its place. When the second electron jumps orbitals, a photon and a neutrino are released. The process of electron capture is a fairly common form of atomic decay. What is far less common is a dual electron capture, where two low-orbital electrons are absorbed into the nucleus, dropping the atom two slots on the periodic table.
This is where WIMPs come in. The team over at XENON1T speculate that interactions between baryonic particles (the everyday stuff) and non-baryonic particles (dark matter) could trigger these exceedingly rare double-captures. The only two recorded cases of a double capture involved xenon atoms. So, scientists filled a fancy bucket with two tons of liquid xenon and designed sensors to search for the two key processes that identify these reactions: Ionization and scintillation. Professor Aprile and her team believe that reasonable detection of these two indicators under the present conditions will confirm the existence of dark matter.
Currently, the closest result generated by XENON1T has been just shy of the acceptable error margin. Should the experiment yield successful results and transform a xenon atom into a tellurium atom, we will have our first confirmation of dark matter, all thanks to a decay process with a half-life about a trillion times the life of the universe itself.