November 6: Beneath our feet on this warm November night, we have realized the ancient dream of turning lead into gold. Well, metaphoric gold. This week, the LHC replaced the protons in the accelerator with lead ions. Tonight, for the first time, the beams of ions collided. The lead collisions produce something called a quark-gluon plasma*, a rare scientific thing that actually is as complicated and exotic as it sounds.
Don't let the people who study it fool you; they'll try to convince you in afternoon tea and cookie colloquiums that it's easy to understand. But even among the particle physicists on ATLAS, those who work on lead-ion physics are a small subset, and among the rest of us, it is not unusual to hear people talk about quark-gluon plasma as though it were an ancient, mysterious substance more easily admired from afar than talked about in detail.
Quark-gluon plasma, in fact, is an ancient substance -- older than alchemists, older than the Earth, older than gold. In the beginning of the universe, before atoms existed, before protons and neutrons had established their separate identities, the quarks and gluons that would eventually become the protons, neutrons, and chocolate bars of our lives zipped freely along in a plasma-like substance. Or they flowed like a liquid. Or -- well, that's why we're reproducing this substance in our laboratory -- we want to know how it behaves.
We're not the first to hunt for quark-gluon plasma. The RHIC accelerator at Brookhaven National Lab turned real gold ions into metaphoric gold and opened a world of exploration that the LHC hopes to further by colliding ions with more than 10 times more energy. While the ALICE detector is particularly designed to study these collisions, we'll look at them on ATLAS, too. It won't be simple to look at them; because each lead ion has 82 protons and 126 neutrons in it, the collisions will be dramatically more complicated than collisions from two single protons.
On this quiet night, however, the data isn't processed yet, so we can't start analyzing the events. We have a few moments to sit back and appreciate the collisions from a more philosophical standpoint. What would the ancient alchemists think if we could tell them that the secret to turning lead into something else was to push it at nearly the speed of light? And how would they feel if they knew that our goal was not to turn the metal into a shiner, yellower one -- but to understand the original, primordial, shimmering colored soup from which our universe was born?
* We expect and hope! Still to be verified, of course.