For 30 years, physicists around the world have been trying to reconstruct how life-giving particles formed in the very early universe. ALICE is their mightiest effort yet.
creating a universe isn’t the job of the Large Hadron Collider . Most of the back-breaking science—singling out and tracking Higgs bosons, for example—from the world’s largest particle accelerator happens when it launches humble protons at nearly the speed of light.
When physicists first split an atomic nucleus in the early 20th century, this was as far as they got. Protons, neutrons, and electrons formed the entire universe’s mass—well, those, plus dashes of short-lived electrically charged particles like muons. But calculations, primitive particle accelerators, and cosmic rays striking Earth’s atmosphere began to reveal an additional menagerie of esoteric particles: kaons, pions, hyperons, and others that sound as if they’d give aliens psychic powers.
This isn’t a recipe you can test in a home oven. In units of the everyday world, the temperature in a hadronic system is about 3 trillion degrees Fahrenheit—100 thousand times hotter than the center of the sun. The best appliance for the job is a particle accelerator. For the brief flickers for which the quantum matter exists in the world, physicists can watch the plasma materialize in what they call “little bangs.”
At ALICE’s heart lies a 39-foot-long solenoid maganet, coiled around a thermal shield and a number of fast-trigger detectors.The closer in time to the big bang that you travel, the less the universe resembles your familiar one. As of this writing, the James Webb Space Telescope has possibly observed galaxies from around 320 million years after the big bang.
CERN physicists Urs Wiedemann and Federico Antinori meet me in their office. Wiedemann is a theoretical physicist by background; Antinori is an experimentalist, presiding over heavy-ion collision runs. Studying QGP requires the talents of both. This transition happened in reverse in the very early universe as it cooled down past the Hagedorn temperature. The quarks and gluons clumped together, forming the protons and neutrons that, in turn, form the atoms we know and love today.
“You can say, ‘I understand how an electron interacts with a photon,’” says Wiedemann, “but that doesn’t mean that you understand how a laser functions. That doesn’t mean that you know why this table doesn’t break down.”With the likes of SPS, scientists could look at droplets of QGP and confirm they existed. But if they wanted to actually peer inside and see their properties at work—to examine them—they’d need something more powerful.
To date, more than 2,000 physicists from 40 different countries have been involved with the decades-long experiment.CERN physicist Nima Zardoshti greets me beneath that mural: ALICE’s detector, the QGP-watcher, depicted in a pastel-colored mural. Zardoshti leads me inside, past a control room that wouldn’t look out of place in a moon-landing documentary, around a corner covered in sheet metal, and out to a precipice. A concrete shield caps it, several stories below.
Lead ions make fine ingredients. A lead-208 ion has 82 protons and 126 neutrons, and both of those are “magic numbers” that help make the nuclei as spherical as nuclei can become. Spherical nuclei create better collisions. First those particles crash through silicon chips not unlike the pixels in your smartphone. Then the particles pass through a time projection chamber: a cylinder filled with gas. Still streaking at high energy, they shoot through the gas atoms like meteors through the upper atmosphere. They knock electrons free of their atoms, leaving brilliant trails that the chamber can pick up.
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