Searching for missing particles

The Standard Model of particle physics has been confirmed many times, but it is far from complete. Alan Duffy reports.

The proposed Future Circular Collider will dwarf the existing, already huge, Large Hadron Collider (LHC).

The proposed Future Circular Collider will dwarf the existing, already huge, Large Hadron Collider (LHC).

Charitos, Panagiotis

We have long sought to understand our world. That quest now involves projects larger than any one country. It may soon be at an end. But it has been far from elementary.

The Ancient Greeks believed the world comprised just five elements: earth, air, water, fire and aether.

In 1869, Russian chemist Dimtri Mendeleev added a deeper level of understanding – and complexity – when he proposed his Periodic Table of Elements. There are now 118 elements in all, and together they explain the chemical reactions and properties the Greeks noted of the world around us.

As the twentieth century began, we learned that all of these elements are just a combination of the three subatomic particles: electrons, protons and neutrons. Unlike before, this deeper but more fundamental revelation reduced the complexity of nature, while providing greater insights into its properties.

As that century – and its technology – advanced, so too did the number of “fundamental” subatomic particles. From pions to muons and more, a zoo of hundreds of strange short-lived particles (as well as their antiparticles) were uncovered.

In 1964, Nobel Prize-winning physicist Murray Gell-Mann set out to simplify things again by proposing that these particles were in fact all formed from quarks held together by gluons. Take two “up quarks” and one “down quark” and you get a proton; two down and one up forms a neutron. Order was restored.

Over the past decades the remaining fundamental building blocks of our world were identified, like the final pieces of a jigsaw puzzle, to form the Standard Model of Particle Physics. Combinations of quarks (held together by gluons) form hadrons such as protons and neutrons, orbited by a family of electron-like leptons each with a corresponding neutrino of almost imperceptible mass.

Interactions between these particles in the Standard Model are mediated by four “force-carrying” particles, most famously the photon of electromagnetism, as well as the W and Z bosons and the aforementioned gluon. In 2012 the long-predicted final particle, the Higgs Boson (aka “the god particle”) was uncovered at the Large Hadron Collider (LHC). The Standard Model was finished. It was a triumph. It was also incomplete.

There was no mention of gravity, or solid prediction for the dark matter that holds the galaxies together; and all efforts to explain the dark energy that appears to drive the accelerating expansion of the universe was off by 120 orders of magnitude. As before, it is likely the answer will come only after things get worse, with a more fundamental description of nature that clarifies an ever-messier situation.

The twenty-first century will see mammoth projects in all directions to find this more elementary description of our Universe. The LHC continues in an upgraded, higher intensity mode, while particle physicists have set their sights on the successor – a proposed $40 billion Future Circular Collider, 100 kilometres in circumference, that could access higher energies and hence probe smaller scales of nature to find hints of a more fundamental theory.

There are efforts to find the elusive particle that makes up dark matter by isolating sensitive detectors deep underground, waiting for an occasional collision to reveal this otherwise invisible side of our universe.

The size of the targets in these detectors vary depending on the mass of dark matter searched for, but experiments like SABRE in Australia have 50 kilograms of sodium iodide crystals, while Xenon1T has an entire tonne of the noble gas xenon as its target.

Expect the experiments to continue to grow and increase in sensitivity if dark matter continues to elude us. At some point the detectors may become large enough to detect the original ghost-like particle, the neutrino.

Because neutrinos can pass through a light year of lead before half would be stopped, you need a huge detector to have a chance of finding these rarest and most interesting high energy particles. They don’t come bigger than IceCube with an entire cubic kilometre of Antarctic ice laced with cameras that can detect the Cherenkov light from relativistic particles in the ice.

There are exciting prospects for a fourth neutrino, a so-called sterile neutrino, that may alleviate certain tensions between the expansion rate of the Universe today compared with just 380,000 years after the Big Bang, as revealed by the Cosmic Microwave Background.

There is a zoo of predicted particles that the following decades will find, or rule out, without even mentioning the prospect for one associated with gravity – the graviton.

Regardless of what we uncover, all of these efforts to detect new particles follow a successful scientific track record of greater complexity inferring a simpler, underlying truth. It’s elementary.

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
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