New data suggest a lighter Higgs

The God particle has fewer places to hide.

New data offer evidence that the heft of the Higgs particle lies somewhere in the low end of the range being probed by particle colliders on two continents. The results also hint that the particle’s mass may be consistent with supersymmetry, a theory that gives every particle in the standard model of physics a much heavier partner.

The latest results come from two ongoing experiments at the Fermi National Accelerator Laboratory's Tevatron particle accelerator in Batavia, Ill., that suggest the elusive Higgs cannot have a mass between 158 billion and 175 billion electron volts. (1 billion electron volts, or 1 GeV, is just slightly heavier than the mass of a proton.) Ben Kilminster of Fermilab reported the finding July 26 at the International Conference on High Energy Physics in Paris.

Studies from the Large Electron-Positron Collider, which shut down in 2000 at the European research organization CERN, along with indirect constraints from both theory and experiments, had indicated that the Higgs could have a mass anywhere between 114 and 185 GeV. In late 2009, the two Tevatron experiments, known as CDF and DZero, excluded the range between 162 GeV and 166 GeV. With the new constraints, CDF and DZero have now ruled out nearly 25 percent of the mass range for the Higgs allowed prior to 2009, before the two experiments began weighing in on the proposed particle.

Dubbed the God particle because its existence would explain why some subatomic particles are weightless but others are not, the existence of the Higgs particle was proposed in 1964 by physicist Peter Higgs, who posited a quantum field pervading the vacuum of space. The field would slow down some particles traveling through it, causing them to acquire mass. Other particles, like photons, would be impervious to the field and continue to travel at the speed of light. Although the field couldn’t be detected directly, it could be excited at high energies to produce the elusive Higgs particle.

The newest limit on the particle’s mass “is important, as it demonstrates the power of the Tevatron experiments to search for the Higgs,” says theorist JoAnne Hewett of the SLAC National Accelerator Laboratory in Menlo Park, Calif. “They just keep getting better and better.”

However, Hewett notes, excluding the region between 158 to 175 GeV for the mass of the Higgs won’t have much effect on theory, because most physicists expect the Higgs to be lighter than 135 GeV.

“The most tantalizing region of 114 to140 GeV has yet to be explored,” Hewett notes.

That’s why intriguing, but not statistically significant, hints of a 140-GeV Higgs recorded by Tevatron’s CDF experiment “has the theory community abuzz,” Hewett says. Abid Patwa of the Brookhaven National Laboratory in Upton, N.Y., presented that data July 23 at the ICHEP meeting.

CDF cospokesman Rob Roser of Fermilab says, “We see something. It could be consistent with many things,” including a low-mass Higgs. “I am not sure I would agree that ‘the community is all abuzz,’ but perhaps the theorists are excited — they are an excitable lot.”

Physicists are intrigued by a low-mass Higgs because it would most easily fit into one of the simplest extensions of the standard model of particle physics, known as minimal supersymmetry. Supersymmetry supposes that every known particle has a heavier partner. The minimal version of that theory has the smallest possible number of such partners.

But even if the CDF signal at 140 GeV turns out to be a fluke, the new restrictions at higher masses will guide theorists in their work on the Higgs, asserts Roser. Physicists working on the CDF and DZero experiments discovered the new limits by independently sifting through more than 500 trillion collisions between protons and antiprotons generated since 2001 at the Tevatron. After searching for years, "We have finally crossed the threshold where every new piece of data allows us to explore new regions and brings us new knowledge" about the Higgs, says CDS cospokesman Giovanni Punzi of the University of Pisa and the National Institute of Nuclear Physics in Italy. "We are really at the point of starting to find the answer."

Physicists should be able to explore the full range of masses that the Higgs could still have using data expected to be acquired by CDF and DZero through the end of 2011, when the Tevatron is currently scheduled to shut down, Roser says. If the entire mass range is eventually ruled out, physicists will have to look for another explanation — a new type of force and particle — to explain how some subatomic particles got mass, Punzi notes.

The findings also heat up a David versus Goliath race to find the Higgs between the relatively small Tevatron facility and CERN’s powerful Large Hadron Collider near Geneva. The LHC now operates at 3.5 times the energy of the Tevatron and will ultimately double that amount. However, electrical problems have delayed operations at the LHC, and it hasn't acquired nearly as much data as initially hoped. Moreover, the LHC will shut down for upgrades in 2011 after running at only half its maximum energy. The LHC is not expected to operate at full capacity until 2013, which has prompted Tevatron scientists to request that the Illinois accelerator continue to run for three additional years, until 2014.