In its day, the circular Tevatron produced about 10 million proton-antiproton collisions per second - about 200 collisions per second were recorded at each detector for further analysis (Image: Fermilab)
by Lisa Grossman
The Tevatron may now be defunct, but it is still detangling the nature of matter from beyond the grave. The late particle-smasher’s two main experiments, CDF and DZero, have released the most precise measurement yet of the mass of the W boson, one of the fundamental particles in the standard model of particle physics.
The new measurements, combined with earlier data from other detectors, places the W boson’s mass at 80.385 gigaelectronvolts (GeV), plus or minus 0.015 GeV. The measurement puts constraints on the mass of the Higgs boson – the long-sought missing piece that would complete the standard model and explain why all other particles have mass – placing it right where experimentalists want it.
The standard model is our current best picture of the menagerie of particles and forces that make up the universe and explains how they interact. But it cannot predict exactly how much each of those particles weighs; it can only describe how their masses are related to each other.
The Tevatron, which was housed at Fermilab in Batavia, Illinois, and turned off in September 2011, and the still-running Large Hadron Collider at CERN, near Geneva, Switzerland, have hunted the Higgs boson directly. Last December, physicists at the LHC announced tentative hopes that the Higgs had finally turned up, with a mass of about 125 gigaelectronvolts (GeV).
But thanks to the relationships laid out in the standard model, physicists can also predict at which mass the Higgs should show itself based on the masses of other particles, like the W boson and the top quark.
“The mass of the W boson is correlated with the mass of the top quark, which we here at Fermilab can measure very precisely,” says Dmitri Denisov, spokesman for the DZero experiment. And if the Higgs exists, the mass of the W boson is also correlated with the Higgs mass, he says.
Weight and see
Previous to the latest result, DZero and CDF – which use different and independent techniques to make similar particle measurements – had placed the W boson’s mass at 80.4 gigaelectronvolts, give or take about 0.045 GeV. That was the most precise measurement of the W boson’s mass at the time, but it was still not good enough, Denisov says. Taken together with the top quark, the earlier W boson measurement pointed to a Higgs that weighed about 90 GeV, a mass that had already been searched through and excluded by previous experiments.
That opened the door to possible physics beyond the standard model. But now, armed with about half the data the Tevatron produced over the course of its 28-year lifetime, the standard model is looking healthy again.
“Before this measurement, we thought maybe it’s not the standard model,” Denisov says. “But now it’s all fitting together nicely.” He recalls one comment when the team saw their latest result: “Let’s drink to the standard model, it wins!”
The result is consistent with a Higgs weighing between 115 and 127 GeV, which fits with the standard model’s predictions, and with CERN’s tentative findings. But it also allows the Higgs to be as massive as 152 GeV, which would require some extra particles or other deviations from the standard model.
Denisov says the Fermilab teams still have half of the Tevatron’s data to comb through, so their precision will probably improve. The LHC collaborations are also refining their searches, and expect to have enough data to rule the Higgs boson in or out by the end of this year.
“In the next six to 12 months at most, it will be clarified,” Denisov says. “But if [the Higgs] will be found here, then I will drink a second time. Because then everything will really fit.”………..
Read more: newscientist.com