Eight months ago, physicists working with the world's biggest atom smasher—Europe's Large Hadron Collider (LHC)—created a sensation when they reported that they had discovered a particle that appeared to be the long-sought Higgs boson, the last missing piece in their standard model of particles and forces. Today, those researchers reported that the particle does indeed have the basic predicted properties of the standard model Higgs boson, clinching the identification.
"It sure does look like the standard model Higgs boson, you bet," says Sally Dawson, a theorist at Brookhaven National Laboratory in Upton, New York, who was not involved with the measurements.
It's a big step, at least semantically. Ever since the new particle was reported last July, officials at the home of the LHC—the European particle physics laboratory, CERN, near Geneva, Switzerland—have taken great care to describe the new thing as a "Higgs-like particle." Now, a CERN press release calls the particle "a Higgs boson." "That's a big deal for the community," Dawson says.
To make the positive identification, researchers relied not on dental records, but on observations of how the Higgs boson decays into combinations of other, more familiar particles. Key characteristics of the Higgs include its spin and its parity, a symmetry property. They can be determined by looking at correlations in the particle directions when, for example, a Higgs boson decays into two particles called Z bosons, each of which then decays into two particles called muons.
Although not yet entirely conclusive, current measurements show that the new particle has no spin (as opposed to 1 or 2 quantum units of it) and positive parity, researchers reported today at a meeting in La Thuile, Italy. That's exactly what the standard model predicts for the Higgs boson. The measurements were made by teams working independently with two massive particle detectors fed by the LHC, which are known as ATLAS and CMS. The teams simultaneously discovered the Higgs last summer. They have analyzed roughly twice as much data now as they had analyzed then.
Ironically, most physicists had been hoping for more than the standard model's plain vanilla Higgs. "That was certainly my feelings," says Daniel Green, a member of the CMS collaboration from Fermi National Accelerator Laboratory in Batavia, Illinois. "Of course, we want to discover something new."
In fact, the new results raise the prospect that the only new thing that the $5.5 billion LHC will produce will be the standard model Higgs boson, an outcome some physicists have described as their nightmare scenario. Physicists working with the LHC say that it's too early to rule out further discoveries. The LHC has been taking data since only 2010 and has collected less than 1% of the data researchers hope to obtain by 2030. Moreover, the LHC has been running at half-energy because of faulty connections between its massive superconducting magnets. It has just shut down for 2 years of repairs that will enable it to run at full energy starting in 2015. "This is a voyage of discovery and we're still in the shallows," Green says.
Once the LHC comes back on, one of the first things that researchers will look for is other Higgs bosons. The standard model includes only one of them. But more-elaborate theories—such as one known as supersymmetry, which posits a more massive partner for every known particle—suggest there could be several. "Why would there be only one Higgs?" Dawson says. "Why wouldn't there be two, three, whatever? There's no reason to preclude it." Dawson says that she won't be willing to give up on the hope for something new until the LHC has collected about a tenth of the aimed-for data set, sometime around 2020.
Until then, CERN officials will surely continue to issue press releases about "a Higgs boson"—and hope that something else comes up before they have to start talking about "the Higgs boson."