One of the world's most elusive particles will stay hidden a while longer, it seems.
Scientists at the gigantic Large Hadron Collider (LHC) particle accelerator at the CERN physics lab in Switzerland have wrapped up — at least for 2011 — the kind of experiments that might have shown a glimpse of the long-sought Higgs boson particle.
The Higgs boson, which has been theorized but never observed, is thought to give all other particles their mass. Physicists have been hoping to see signs of it ever since they began colliding particles at the LHC in 2008. Yet there is still no sign of the Higgs.
"LHC is running fantastically, it's marvelous," said CERN particle physicist Christoph Rembser, who works on LHC's ATLAS experiment. "What is not that fantastic is that we've not yet seen anything new, and no new discoveries have been made."
Yet Rembser and others urged caution, saying that they knew in advance it would take time for enough data to accumulate to reveal new particles. [Wacky Physics: The Coolest Little Particles in Nature]
Not too much time, though: Another CERN scientist suggests that if the particle remains elusive next year, chances are it doesn't exist.
For 180 days this year, LHC was colliding protons together inside its 17-mile (27 kilometer) underground loop. The huge energies created when two of these particles smacked into each other head on at high speed were thought to be about right to give rise to exotic particles like the Higgs.
However, this week physicists called off LHC's proton-proton run for this year, intending to use the remaining months of 2011 to collide heavier lead ions (made of 82 protons and even more neutrons).
These crashes are so powerful they can essentially melt matter down into a primordial soup of its building blocks — tiny particles called quarks and gluons. Studying this quark-gluon soup could reveal more about how atoms formed at the beginning of the universe almost 14 billion years ago.
The "God particle"
When LHC starts up again next year, physicists intend to resume the search for the Higgs boson. This particle, sometimes called the "God particle" because of its importance, is thought to be associated with a partner Higgs field, which pervades the universe.
When other particles travel though this field, they acquire mass, just as an object traveling through a lake gets wet. This is the mechanism scientists think could explain why particles have mass.
The Higgs model is so successful it has been integrated into the Standard Model of particle physics, scientists' best working theory to describe the fundamental constituents of the universe. [Infographic: Nature's Tiniest Particles Dissected]
"Of all the new physics we're looking for [at the LHC], the Higgs boson is special in the sense that we've already included it in our calculations," CERN physicist Jonas Strandberg said. "For our theory to be right, we need the Higgs to exist. If it doesn't exist, we need something to replace it."
Cornering the Higgs
Just because LHC hasn't yet found the Higgs doesn't mean that it hasn't revealed anything about the particle. By searching for so long, the atom smasher has already eliminated some possible places the particle could be hiding.
"We know everything about the Higgs boson from our theory except one thing, which is which mass it has," Strandberg told LiveScience. "Depending on this mass, it has certain properties. We have excluded a lot of the possibilities for the Higgs. But what's left are the most likely possibilities. In that sense, we still have the most interesting window left. That will take a little bit longer to close."
Scientists can now say, with 99 percent certainty, that the Higgs mass isn't between 160 and 220 giga-electron volts, or GeV (for comparison, a proton has a mass of about 0.938 GeV). But other ranges, such as between 114 and 135 GeV, and above 500 GeV, are still in the running for the Higgs mass.
"By next year we'll have excluded all possible masses," Strandberg said. "I think if we don’t find it next year, the Higgs boson as we know it doesn't exist."
Although some physicists had hoped to find the Higgs boson sooner, most say they're not disappointed.
"What I would have hoped for, of course, is it would have been easier, but I am very satisfied because the results are very solid and the experiments are running fine," Rembser said. "There are no crying physicists at CERN. It's so much fun to investigate and look into the data, that the atmosphere currently at CERN is just fantastic."
And most researchers also professed a lack of surprise that the big prize hasn't come already.
"I thought it would take five years," said Harvard physicist Joao Guimaraes da Costa, part of the LHC's ATLAS team. "I think it actually is going very fast."
In fact, many physicists think the most probable mass of the Higgs lies within the lighter mass range that has not yet been probed to enough depth by LHC. For some, it would have been unexpected if the particle had already been found.
However, if the same null result still stands at this time next year, many more experts are likely to be surprised.
"If we don’t find it at all, this would be rather unexpected," Strandberg said. "This would really mean you have to rethink everything we know about it that we've been taught and that we think is true."