Illinois lab issues its last word on the long-sought particle

By Alexandra Witze

Web edition: July 2, 2012

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The now-shuttered Tevatron atom-smasher, in Illinois, smashed together protons and antiprotons to search, in part, for the long-sought Higgs particle.

Credit: Fermilab

As physicists in Europe prep for a major announcement about the long-sought Higgs particle, U.S. scientists are not going gentle into that good night. Researchers at the Fermi National Accelerator Laboratory in Illinois have squeezed out the last drop of analysis on their own data — and concluded that the Higgs, if it exists, should have a mass between 115 and 135 billion electron volts.

That range, reported July 2, is not substantially different from earlier reports. It fits with hints reported by the European laboratory CERN in December, which suggested the Higgs might have a mass around 125 billion electron volts (SN: 12/31/11, p. 10). A proton has a mass of around 1 billion electron volts.

The Fermilab report “is the strongest evidence of Higgs boson production at any collider,” says Dmitri Denisov, a physicist at the lab.

Still, the data aren’t strong enough to claim that the Higgs particle has been spotted. “I would be willing to bet your house, but not mine,” says Rob Roser, another Fermilab physicist.

Many physicists expect to hear convincing, or near-convincing, evidence of the Higgs’ existence on July 4, when CERN will report its latest data.

The Higgs is the last undiscovered fundamental particle in physics’ standard model, which describes the particles that make up the universe and the forces that govern their interactions. Several theorists, including the University of Edinburgh’s Peter Higgs, proposed the particle in 1964 as a signal of a field that would have slow down the motion of some elementary particles, giving them mass.

Fermilab’s data come from its now-shuttered Tevatron collider, which slammed beams of protons into their antimatter counterpart, antiprotons. Between 2001 and 2011(SN: 9/24/11, p. 22), it gathered data on 500 trillion such collisions. Each created a shower of particle debris that physicists can scour for traces of the Higgs, which if it exists would have emerged in a small fraction of collisions and then immediately decayed away into other particles.

Tevatron scientists have about the same amount of data as CERN has so far, but at much lower energies. That means far fewer Higgs particles should have been produced in Illinois than in Europe. Still, data from the two main Tevatron detectors suggest that there are more particle “events” in the 115 billion to 135 billion electron volt range than one would expect if the Higgs did not exist, Fermilab scientists reported at an in-house seminar.

Further analysis from Europe should pin down whether this signal is real or not. CERN has the mother of all atom smashers: The 27-kilometer-circumference Large Hadron Collider collides protons with protons at energies never before achieved. Project scientists will present their latest findings at a CERN seminar on the morning of July 4, Geneva time, and also later in the week at a high-energy physics conference in Melbourne, Australia. Rumors are flying that the 125 GeV signal may have gotten stronger with data from CERN’s 2012 data run.

If the Higgs is found, the question turns to whether its properties match predictions made by the standard model, or whether they are slightly different. “If we do find the Higgs, we can start to play these measurements off each other and decide if we have a standard model Higgs or if there’s new physics involved,” said Fermilab physicist Eric James.

Key to the July 4 announcement will be the statistical strength of any reported Higgs signal. Separately, each of the LHC’s two main Higgs experiments may not be able to achieve the desired “five-sigma” signal, meaning the probability is less than 1 in 3.5 million that a statistical fluke could create the same signal or a more extreme one. Combined, the experiments may be able to reach that threshold. (The Tevatron analysis from its two detectors is just under three sigma, meaning the chance is 1 in 550 that a statistical fluke could create something at least as unusual.)

How long it will take, though, remains a mystery. “I can tell you 2012 is the year” to learn if the Higgs exists or not, says Tom LeCompte of Argonne National Laboratory. “I can’t tell you July is the month.”