How do you find a Higgs boson? A crash course.
Scientists are using the $10 billion Large Hadron Collider in their search for the Higgs boson. Understanding how it works explains why it has proven so hard to find the Higgs boson.
If that price tag sounds a little steep – even in the quest to find the elusive Higgs boson, which could answer the longstanding question of why the fundamental building blocks of matter have mass – consider what it is trying to do.
If theories are correct, the Higgs boson existed only during the first millionth of a millionth of a second after the Big Bang some 13.6 billion years ago. As the universe cooled, all the Higgs bosons decayed into other particles. That means to find it, scientists have to make it themselves, recreating the high energies that existed when the universe was only a millionth of a millionth of a second old.
To do this researchers use the LHC to accelerate protons to within a whisker of the speed of light, then steer them into head-on collision. In the usual shorthand, that makes the LHC a proton collider.
"Proton collider" is a misnomer, however, because the collisions actually occur among particles that make up protons, such as quarks and the gluons that bind the quarks together, says Lawrence Sulak, a physicist at Boston University who is working with one of the two Higgs-hunting teams. It's the particular combination of quarks that gives a proton its positive charge – the trait that allows scientists to use powerful magnets to guide and focus the LHC's proton beams.
As if to make matters more difficult, even though physicists speak of colliding beams of protons, in reality, the beams are mostly empty space. If an atom was the size of Earth, a proton in its nucleus would only measure about 2,000 feet across. The three quarks that typically make up the proton would be no more than four inches across, and probably a lot smaller, says Meenakshi Narain, a physicist at Brown University in Providence, R.I., and a member of the other team hunting for the Higgs boson.
Other processes go on inside the proton as well, but it's the collision among quarks and gluons, which binds the quarks -- that physicists are actually smashing.
At the LHC, the main show takes place in the collider's circular main ring, a racetrack with a circumference of nearly 17 miles and up to 300 feet below ground. But the facility also takes advantage of every type of particle accelerator developed in the past 115 years to first harvest the protons from hydrogen gas, then accelerate them incrementally through three increasingly powerful accelerators.
Only in the final stage do the protons reach energy levels high enough to allow the LHC's main ring to give them their final precollision energy.
Beams are made from bunches of protons – 100 billion protons to the bunch, and just over 2,800 bunches per beam. The large number of protons offsets the mostly-empty-space problem, significantly raising the probability of collisions.
Using powerful magnets to steer the protons around two lines running in opposite directions, the beams are at last brought into focus at each of two mammoth detectors the size of a cathedral's nave. This is where the collisions take place. By the time the beams are focused for collision, each is about half the width of a human hair. The detectors that track the collision debris must be able to locate the telltale debris trails to within half the width of a human hair.
The engineers who keep the machine going with such precision "are real magicians," says Dr. Sulak.
The collisions take place about once every 50-billionths of a second.
"That means you have to look at these collisions every 50 nanoseconds and decide whether you want one or not," says James Proudfoot, a senior physicist at the Argonne National Laboratory in Argonne, Ill. "That's a huge challenge, because there's a prodigious amount of background" – detector signals from collision debris that researchers must sift through to pick out the truly interesting signals.
[Editor's note: The original version of this story gave the incorrect title for James Proudfoot's position at the Argonne National Laboratory.]
The signals of interest get stored for detailed analysis. And how do you know you have spotted a Higgs boson? Among other things, theory predicts that a decaying Higgs boson should produce an array of secondary particles gathered into five distinct groups, or channels. It's not enough to spot one or two, although that will suggest the hunt may soon be over. All five channels must be present, and in the predicted proportions.
If the proportions are off, or the number of channels is different, researchers certainly can claim a discovery, but it will be up to the theorists to figure out what it is.
On Wednesday morning, the world will get the latest official word from scientists at the European Organization for Nuclear Research (CERN) in Geneva as to what the data tells them and how confident they are in their results.