The Tevatron collider — the world’s second most powerful particle accelerator behind CERN's Large Hadron Collider (LHC) — was shut down in 2011. Now, from beyond the grave, it’s revealing properties of the Higgs boson.
The American accelerator, situated at Fermilab in Illinois, ran from 1983 until 2011. During that time, it showed teasing glimpses of the Higgs, but ultimately it was the LHC that finally found the elusive particle. The discovery primarily identified the particle’s mass as 125 giga-electron volts—about 133 times the mass of the proton.
But like any particle, the Higgs boson has other properties, too, such as quantised amount of angular momentum (referred to by physicists as spin) and a kind of symmetry known as parity (which can be either even or odd), which dictate how it decays into different particles. The Standard Model predicts that the Higgs should have zero spin and positive parity but experimental results were required to confirm that’s the case.
Results from the LHC already suggest that to be the case, but analysis of data from Tevatron soon to be published in Physical Review Letters helps confirm the spin-parity of the Higgs. Science explains how researchers from Fermilab went about it:
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.... (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.
The new results from Tevatron confirm with more certainty than those from the LHC that the Higgs has zero spin and positive parity; though, arguably, the LHC did get there first. That’s understandable as many people working on Tevatron shifted to the LHC to perform new experimental work rather than analysing old data.
Regardless of firsts, it’s probably the last exciting glimpse of the Higgs that Tevatron will see. With the baton now firmly passed on to the LHC, our further understanding of the Higgs will come from Europe, not the US. [arXiv via Science]
Internal view of the Tevatron collider by Steve Krave under Creative Commons licence