The top quark (what’s that?) turns 25

Fermilab’s CDF and DZero experiments, which studied collisions produced by the Tevatron collider, announced their discovery of the top quark on March 2, 1995.

The top quark is the heaviest known elementary particle, as small as an electron but 340,000 times more massive! We don’t know how small those particles are, only that they are smaller than 1/1000th the size of a proton which itself is 1/100,000th the size of the smallest atom, hydrogen. Millionths, billionths … soon we’re talking small numbers!

The top quark discovery was announced at Fermilab on March 2, 1995, by two international teams each of about 600 physicists. It had been anticipated since its partner, the bottom quark, was discovered 18 years earlier, also at Fermilab, but its mass was not known. Experiments in Japan and Europe (CERN) sought it without success. It completes a table of six different types of quark particles. The lightest two are called up and down and make up protons and neutrons in all atoms.

While March saw the 25th anniversary of their discovery, not one top quark lives to celebrate that birthday. They die as soon as they are born, typically within a trillionth of a trillionth (10-24) of a second, decaying into a bottom quark and a couple of other elementary particles. Then the bottom quark decays in its turn into lighter particles in about a picosecond. It’s complicated. So, we have never really “seen” top quarks, but we can reconstruct them from the tracks their eventual decay products leave in our particle detectors. It’s like seeing the vapor trail of an airplane across the sky; you know a plane was there even if you didn’t see it.

Since Fermilab’s Tevatron was shut down in 2011, CERN’s Large Hadron Collider is the only place in the world making top quarks, thousands every second. Precisely measuring their mass and how they interact with other particles should help us understand matter and fundamental forces. They are naturally created in violent cosmic events, from the Big Bang to today in cosmic ray showers.

Top quarks decay before even emerging from the proton collisions in which they were created, so could not have practical applications. On the other hand, we probably owe our very existence to the top and bottom quarks being part of nature. Before those were found, some theorists said they should exist to allow matter and antimatter to behave a little differently. And in the latest theories, if the top quark mass were different by only 4%, the vacuum filling the universe would be unstable. Oops!

This is a version of an article that originally appeared in Positively Naperville.