On March 30th, humanity is scheduled to start running the biggest, baddest particle collider ever constructed, the one that makes its closest competitor, Fermi's Tevatron, so out-of-date that the current plan is simply to shut the Tevatron down. But Brookhaven's RHIC, which can't even reach the Tevatron's energies, will be kept running indefinitely.
If that doesn't make sense, it's just one of a large collection of things associated with particle physics that may not be very intuitive. Like the fact that we've built a series of detectors in the Large Hadron Collider that are designed to pick up signs of particles that we've never seen before, and tiny bits of dark matter that don't actually interact with anything. Or the fact that we need a bunch of detectors for this in the first place. Fortunately, for the last month or so, I've had a variety of physicists explaining matters to me in language that even a biologist could understand. In a series of articles, I'll try to explain the process of physics so that, as the LHC gears up, you can appreciate why scientists think it's a pretty big deal.
The basics of smashing atoms
|All about particle smashers|
|Part I: Everything you ever wanted to know about particle smashers (but were afraid to ask)|
|Part II: How huge particle detectors actually detect tiny particles|
|Part III: The software brains behind the particle colliders|
The recent generation of particle accelerators rely on essentially identical principles: take an atom, strip off all its electrons, and use the charge that remains as a lever to boost it to speeds that are a rounding error away from the speed of light. Two beams of identical particles are sent around a ring-shaped accelerator in opposite directions and, at specific points, the beams are made to cross, creating the possibility of collisions. For the LHC, this primarily involves the simplest atomic nucleus, the single proton of hydrogen. RHIC uses gold, with 79 protons and 90 neutrons, although the LHC is capable of using lead, which is similar in mass to gold. (We'll get into the whys of the two weights shortly.)
The atoms end up going so close to the speed of light that distance contracts along the path of their travel—if you could watch the gold atom shoot past from the side, it would look like a disk. (In fact, Brookhaven scientists had to change their images from spheres to disks after complaints from anal-retentive audience members [my term, not theirs].) It's moving so fast that the huge repulsion that might otherwise result from the large collections of positive charges doesn't even have a chance to slow things down. "By the time these guys realize they should experience charge repulsion," said Brookhaven's Peter Steinberg, "it's all over."
Two protons bring a total of six quarks into the collision, yet a large collection of particles results from their collision. Two gold ions bring in over 150 protons and even more neutrons, and over 10,000 particles come streaming out—physicist Todd Satogata said that, to scale, it looks "like a Kush ball the size of a three-story house." Where's all this stuff coming from?
When particles collide, they bring a tremendous amount of energy to a screaming halt. It all has to go somewhere, and some of it ends up radiating off as massless photons. But, because mass and energy are equivalent, a certain amount of the energy gets converted directly to mass and kinetic energy, creating the particles that spray from the collisions.
Why the LHC is a big deal
Here's where we get into the difference between RHIC's use of gold and the LHC's use of protons. Since energy and mass are proportional, the heavier the particle you hope to create, the more energy you have to pump into the collisions. So, for example, if a particle's mass is the equivalent of a trillion electron volts, you have to have collisions with at least that much energy to have any chance of producing it. As the energy involved in the collisions increases beyond this point, you have an increasing chance of producing the particle in question.
For well over a decade, we've been stuck with collisions that are just under 2TeV, provided by Fermi Lab's Tevatron. That was easily enough to produce the top quark, and may be within the range of a major hypothetical particle, the Higgs boson. But the probabilities of spotting the Higgs in the Tevatron are pretty low, which means that long runs and lots of data would be needed to know for sure.
At its maximum energy, the LHC will produce collisions that are nearly an order of magnitude more powerful than the Tevatron's, and it will produce a lot more of them—it has a higher luminosity, as physicists put it. The Higgs will be easily within reach, and any number of more exotic hypothetical particles should definitely be possible.
What's the point of a low-powered collider like RHIC?
At its highest energies, Brookhaven's Relativistic Heavy Ion Collider couldn't even produce a top quark. Why's it considered so valuable that it'll keep going after the Tevatron's scheduled to shut down? As Brookhaven's Steinberg put it, the LHC is a particle machine, but RHIC is a quantum chromodynamics machine—it studies the interactions among fundamental particles when they're present in bulk. The LHC can do that as well, with its ability to accelerate lead and an entire detector, ALICE, devoted to studying these collisions. But that wasn't what it was built for, so new physics like the Higgs and dark matter particles will always be a higher priority.
RHIC's top priority has always been the nuclear program. Although the collisions of heavy ions also produce a fair share of unusual particles like strange quarks, their key feature is the fact that they take lots regular quarks, which are normally bundled in the protons and neutrons of the nucleus, and pack them into an incredibly dense space at extremely high energy. Under these conditions, the forces that keep quarks and gluons neatly bundled into these particles break down, and they diffuse into a dense particle soup. That soup lasts a total of 30 yoctoseconds—long enough for light to cross the width of about 10 protons—before escaping in a miniature fireball.
This quark-gluon plasma hasn't been seen with any regularity since shortly after the big bang. In fact, Brookhaven scientists were fairly cautious about claiming that they had even created it. Even though physicists had been talking as if it had existed for years, the paper that provided the best evidence to date was only published this year.
There are two interesting aspects to these collisions. The first is the behavior of the quark-gluon plasma, which appears to act as a perfect quantum liquid, with properties that Brookhaven staff suggested made the odd behavior of Bose-Einstein condensates look positively tame. The other is that, as the quarks rocket out of the collision site, they will re-condense into familiar (and strange) nucleons. If enough of these end up travelling in close proximity, they can also condense into a regular nucleus (or antinucleus). These processes can help us understand how the world of regular matter gets put together.
RHIC also does some collisions using protons to study phenomena that may not require the huge energies available at the LHC. One of these involves a search for the proton's spin, which is the sum of the spins of its component quarks. That may sound nice and intuitive, but there's apparently nothing intuitive about the physical process that performs that addition. Intriguingly, the search for source of the spin has come up short so far. "We've figured out a lot of places it isn't," said Steinberg, who called it "the most fascinating null result I'm aware of."
We're arranging to talk to someone about the proton experiments. In the meantime, check in tomorrow as we discuss how the huge detectors at the LHC and RHIC manage to identify the tiniest of particles, and use that information to reconstruct what happens in the instants after collisions take place.