In a scientific ghost story, a U.S. atom smasher has made an important scientific contribution 3.5 years after it shut down. Scientists are reporting that the Tevatron collider in Batavia, Illinois, has provided new details about the nature of the famed Higgs boson—the particle that’s key to physicists’ explanation of how other fundamental particles get their mass and the piece in a theory called the standard model. The new result bolsters the case that the Higgs, which was discovered at a different atom smasher, exactly fits the standard model predictions.
“This is a very interesting and important paper, because it’s a different mechanism” for probing the Higgs’s properties, says John Ellis, a theorist at King’s College London and CERN who was not involved in the work. “This is the swan song” for the Tevatron, he says.
The Tevatron, a 7-kilometer-long ring-shaped collider at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, ran from 1983 until September 2011. It saw hints of the Higgs boson but never actually discovered the particle. That honor went to physicists working at the Large Hadron Collider (LHC), a 27-kilometer-long atom smasher at CERN, the European particle physics laboratory near Geneva, Switzerland. They announced their discovery in July 2012.
As soon as physicists at the LHC discovered the Higgs, they nailed down its mass: 125 giga-electron volts, or roughly 133 times the mass of the proton. But the particle has other characteristic properties, too. Like all fundamental particles, the Higgs has a fixed and quantized amount of angular momentum or spin. It also has a property of symmetry called parity, which can be either even or odd and which affects, for example, the way the Higgs can decay into other particles. According to the standard model, the Higgs should have zero spin and positive parity. However, it’s conceivable that the observed particle could have zero spin and negative parity or two units of spin and positive parity. Many physicists would be thrilled if the Higgs had such exotic “spin-parity,” as it would point to new phenomena not predicted by the standard model.
In fact, experimenters working with the two biggest particle detectors fed by the LHC—massive devices called ATLAS and CMS—have already shown with high certainty that the Higgs boson has zero spin and even parity. To do that, they studied the decay of the Higgs into familiar particles, such as a pair of photons or a pair of massive particles called Z bosons. From the angular distributions of those emerging daughter particles, physicists were able to determine the spin and parity of the parent Higgs.
Researchers working with the Tevatron data took a different tack. Instead of studying the decays of the Higgses, they looked for signs of a Higgs produced in tandem with a Z boson or a W boson, particles that convey the weak nuclear force, as they explain in a paper in press at Physical Review Letters. (The Higgs was assumed to decay into a pair of particles known as a bottom quark and an antibottom quark.) From the energies and momenta of the Higgs and its partner, researchers then calculated a quantity called the invariant mass for the pair. Were the Higgs and the partner born from the decay of a single parent particle, this quantity would be the mass of that parent. In actuality, the Higgs and its partner would emerge directly from the chaos of the particle collision, so the parent particle is purely hypothetical.
Nevertheless, by calculating the mass of that hypothetical parent particle, researchers were able to test for different combinations of spin and parity by proxy. If the Higgs had “exotic” spin-parity rather than the standard model characteristics, the observed invariant mass would be higher. So researchers working with the two particle detectors fed by the Tevatron—CDF and D0—searched for such high-invariant mass pairs. Finding none, they ruled out even more stringently exotic versions of the Higgs. So even though Tevatron physicists never conclusively observed the Higgs boson, they were able to put limits on its properties.
Technically, the new Tevatron limits are slightly stronger than the limits set by the LHC experiments, says Dmitri Denisov, a physicist at Fermilab who works on D0. But CERN’s Ellis says that ATLAS and CMS had already essentially settled the matter.
In fact, Tevatron researchers missed an opportunity to scoop their LHC counterparts on the spin and parity of the Higgs, Ellis says. Just weeks after researchers at the LHC had discovered the Higgs, Ellis and colleagues explained in a paper how the Tevatron teams might apply the invariant-mass technique to their archived data to take the “fast track” to testing the Higgs’s spin and parity. For technical reasons, the technique would be more sensitive on Tevatron data than on LHC data, they explained, because the Tevatron collided protons and antiprotons, whereas the LHC collided protons and protons. But in the end, the Tevatron analysis proceeded slowly, as CDF and D0 team members left to work on the LHC. “This result has somewhat of an ‘us too’ character rather than being first as we’d hoped,” Ellis says.
Denisov agrees that lack of people impeded progress. He notes that the whole idea could have been tried even before the Higgs was found: “If [Ellis] had come to us a year before we might have been able to determine the spin and parity of the Higgs even before it was discovered.”
For Higgs studies at the Tevatron, “this is basically it,” Denisov says. In the meantime, physicists working at the LHC are aiming to probe other properties of the Higgs with higher precision. In particular, they hope to measure to within a few percentage points how quickly the Higgs decays into different combinations of more-familiar particles and compare that with standard model predictions. Researchers say that work should take about 15 years.