A monitor showing the first ultra high-energy collisions.
Credit: AFP/File, Fabrice Coffrini
LONDON: The world's most powerful particle smasher has finally weighed in on the likely mass of the most wanted particle in physics - the Higgs boson.
And signals in its particle detectors suggest that they may already be in the process of spotting the elusive particle.
The Higgs boson is said to be the last missing piece of the standard model of particle physics which, since the early 1970s, has effectively explained particles and forces. But at the beginning, it had a major flaw: none of the particles possessed mass. As all real-world objects have some heft, researchers came up with a theoretical particle to solve the mass problem. Of the boson's several inventors, only the name of Peter Higgs stuck.
Particle physicists have been looking in vain for it ever since. One problem is that because the standard model doesn't predict particle masses, a key identifier in detectors.
Where the Higgs could be hiding
An earlier particle smasher at CERN near Geneva, Switzerland, known as the Large Electron Positron collider (LEP) ruled out potential Higgs masses up to 114 giga-electronvolts (GeV), or about 114 times the mass of a proton.
For the past 25 years, the Tevatron at Fermi National Accelerator Laboratory in Batavia, Illinois, which collides particles at an energy of 2 tera-electronvolts (TeV), or 2000 GeV, has been narrowing the likely range from the top. Recent experiments suggested the likely range to be between 114 and 158 GeV. The Large Hadron Collider, which replaced LEP, started its first real run last year, colliding particles at 7 TeV.
Last week at the International Europhysics Conference on High-energy Physics, the LHC's ATLAS and CMS experiments placed new, stronger limits on where the Higgs could be hiding. According to the latest CMS results, the expected Higgs is no heavier than 149 GeV.
Too early to be excited
But a potential sighting of the particle itself has raised the volume of the buzz surrounding the findings. Both CMS and ATLAS report "an excess" of particles that could stem from Higgs decays, right around a mass of 140 GeV. It could be the first glimpse of the Higgs, or it could be nothing - more data will tell.
The search for the Higgs depends on separating the particles produced by a Higgs boson from a background of particles produced through other processes in the standard model. Right now, ATLAS gives the potential signal a significance of 2.8 sigma - or put another way, there's a 1 in 400 chance of getting this kind of excess of potential Higgs bosons just by a statistical accident.
In the statistics-heavy world of particles, researchers need a 5 sigma significance (0.00006% chance of being a fluke) to claim discovery. "Two sigma effects come and go all the time," said Robert Roser, co-spokesperson of the CDF experiment on the Tevatron. "I think it's too early to be excited."
Need twice as much data
Even so, the hints were enough to energise ATLAS physicists, and Bill Murray, who heads Higgs searches for ATLAS, said it was a "huge relief" to find that CMS was seeing a similar signal. Still, he warned: "It's quite a tricky measurement. You need to think hard about whether you have understood the background."
Because CMS and ATLAS make the same assumptions about the background, there is still a chance they are making the same mistake. "You can very easily be fooled," CERN director Rolf-Dieter Heuer cautioned in a press conference this week, speaking of the vagaries of statistical analysis.
Both collaborations also reported a few cleaner potential Higgs events, but they haven't seen enough of them to say anything definite. "Clearly what we need is more data," said Murray, adding that twice as much could firm up the signal. Already, the CMS and ATLAS collaborations have collected 20% more data than they analysed for this conference, and the hoard should double by the end of August.
Heuer, meanwhile, is looking ahead to when the LHC can give the final word on the particle. "We can settle the Shakespeare question of the Higgs boson - to be or not to be - by the end of next year," he said.
It is a huge relief that
It is a huge relief that bill murray heads the Higgs searches for ATLAS.
Gravity already deciphered
No collision 'on earth' can bring out Higgs. I literally mean 'on earth', because these are not the condition which can take us back to the so-called big bang. Further, Higgs is theory. In practice gravity is due to particles which have been overlooked by the SM, because it does not acknowledge the STR much. My site gives a chronology of the developments in my research efforts; don't be too much disappointed by the absence of details or by an absence of neatly painted instruments (and then, the site really is quite like a blog), the details shall be published independently with the publication of my US patent application by the US Patent Office. Although, I feel, even then there would be so much more to explore after our world view change.