Late on the night of June 24, 2012, Sven Kreiss (GSAS ’14), a physics graduate student at NYU, pooled two sets of fresh evidence from the ongoing research. He and 3,000 other scientists were collaborating on the A Toroidal LHC Apparatus, or ATLAS, one of the main detectors at the Large Hadron Collider in Europe. On that night, he was the first to see the collective data cross the 5-sigma finish line. The next day, he emailed his adviser, NYU Associate Professor of Physics Kyle Cranmer, to share the good news. His reply, by email: “Holy shit.”
The existence of the Higgs boson was proposed in 1964, along with the Higgs field, which purportedly gives all fundamental particles their mass. (François Englert and Peter W. Higgs were jointly awarded the 2013 Nobel Prize in Physics for this “theoretical discovery.”) The boson fits into what’s called the Standard Model of particle physics, a theory laying out the fundamental forces and particles thought to exist. By 2000, everything on the chart had been discovered except the Higgs boson, the lynchpin of the whole structure. If it did not exist, physicists would have to find some new explanation for decades of experimental results.
The problem is, it takes a huge jolt to get the Higgs field to cough up a Higgs boson, a particle that hasn’t roamed freely since shortly after the Big Bang. So physicists had to build a machine capable of providing that jolt. The Tevatron particle accelerator at Fermilab in Illinois had enough power but could not produce enough particle collisions to make a reliable Higgs discovery before it shut down in 2011. It passed the baton to the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, in Switzerland.
The LHC works by sending two beams of protons in opposite directions around a 17-mile ring underground. The protons collide at nearly the speed of light inside giant detectors, including ATLAS and Compact Muon Solenoid, or CMS, where they create new particles. In a nuclear explosion, E=mc2 dictates that a small amount of mass is converted into a large amount of energy, because c, the speed of light, is a big number. “Here we do the reverse process,” Cranmer explains. “We take an enormous amount of energy, and with these collisions, we hope to produce a very small amount of mass, enough for something like a Higgs boson.”
While producing the Higgs is hard, detecting it is harder. Approximately one in a billion collisions will create the particle, and the detectors must be programmed to record only the collisions that look promising. ATLAS keeps about 300 of the 20 million that occur each second. Then the collisions must be analyzed. If a Higgs is produced, it decays almost immediately into smaller particles, such as two photons or four electrons, which are what the machine detects. Cranmer says that there are 10 to 20 decay possibilities, with teams of 50 to 100 people around the world focusing on each.
Once the teams have analyzed their data, they must combine their results into one analysis. “It’s a fairly intricate puzzle,” Cranmer says. This is where he made his most central contribution, by developing what he calls a collaborative statistical modeling framework. ATLAS and CMS each have 3,000 people, and everyone has a different piece of that data puzzle. Cranmer found ways to integrate their individual findings so that overall progress toward that 5-sigma goal could be clearly assessed. “It’s statistical analysis at a level that I don’t think has ever been done before,” he says.
The Higgs has been found, but there are more blanks to fill. Now physicists must nail down properties other than its mass and its spin, such as how often it decays or how strongly it interacts with other particles.
The character of this boson may signal how to extend the Standard Model, which cannot offer a complete picture of the world. Scientists hope the Higgs will offer clues to theories of supersymmetry or dark matter, answering big questions about the structure of the universe.
In college, Cranmer was torn between studying the physics of the very big or the very small. He went small, but he notes that in high-energy physics, “There’s this funny way in which the two extremes tie back into each other.”