Slipperiest particle gets away again

June 26, 2003

The most elusive particle in physics has skipped even further out of reach. Pinning down the Higgs boson, or proving that it does not exist, would be a huge step towards understanding why our universe has mass. But fresh predictions from Fermilab, home to the world's most powerful particle accelerator, have dashed hopes of achieving that goal for at least the next 6 years.

Current understanding of the universe is summarised in physicists' standard model, but this lacks any explanation for why things have mass. The popular Higgs theory says that a gooey "Higgs field" pervades the universe and endows matter with mass through the effect of Higgs particles. So finding the Higgs has become a matter of urgency: its discovery would confirm the theory, while disproving its existence would pave the way for a new theory, such as a slew of higher dimensions. "It is one of the most important discoveries in science," says Al Goshaw, former spokesman for one of the collaborations working at Fermilab's Tevatron accelerator in Batavia, Illinois.

Previous experiments have shown that if the Higgs exists, it must have a mass of between 114 and 211 gigaelectronvolts (GeV). The challenge is to pinpoint the mass within that window, or scan the whole window and show that the Higgs is not there. Physicists do this by smashing particles together at different energies and scrutinising the showers of particles that result.

In 2000, researchers at the Large Electron Positron collider (LEP) at CERN in Switzerland caught a whiff of the Higgs at a mass of 115 GeV, just before they switched LEP off for good. But their statistics were not good enough to draw any firm conclusions (New Scientist, 9 September 2000, p 4). With CERN's new accelerator, the Large Hadron Collider (LHC), not due to start running until 2007, scientists there were forced to pass the baton to Fermilab's Tevatron.

At the Tevatron, physicists smash together beams of protons and antiprotons and analyse the products for traces of the Higgs. Higgs particles are difficult to distinguish from background noise, so a high rate of collisions is crucial for a clear signal.

Until last week Tevatron researchers believed they would be able to generate enough collisions at high enough energies to confirm or reject LEP's signal within a few years. But estimates of the Tevatron's likely future activity that have now been passed to the US Department of Energy, which provides its funding, reveal that this is highly unlikely.

If the estimates are correct, it will be virtually impossible to squeeze enough collisions out of the Tevatron to settle whether 115 GeV is the mass of the Higgs. One problem is that the antiproton beams the accelerator uses are turning out to be tricky to produce, after previous plans to recycle antiprotons from old experiments did not work. The 20-year-old Tevatron is also feeling its age, which is leading to long periods of down time spent waiting for repairs. "Everyone is disappointed and frustrated," says Judy Jackson of Fermilab.

The admission pushes the earliest date for concrete proof to 2009, when the LHC should have collected enough data to nail or reject the Higgs at 115 GeV. If the Higgs is not found at that mass, the LHC should also be powerful enough to scan the rest of the possible masses up to 211 GeV.
Author: Celeste Biever

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New Scientist issue: 28th June 2003

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