Hundreds of physicists crowded into a seminar room at CERN, the European particle physics laboratory near Geneva on Tuesday, breaking into applause as Fabiola Gianotti and Guido Tonelli, who lead separate teams at CERN’s Large Hadron Collider (LHC), revealed evidence for the particle amid the debris of hundreds of trillions of proton collisions at the machine.
Thousands more tried to watch online although the live feed crashed shortly after the meeting started.
First postulated in the mid-1960s, the Higgs boson has become the most coveted prize in particle physics. Its discovery would rank among the most important scientific advances of the past 100 years and confirm how elementary particles acquire mass.
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While the results are not conclusive -- the hints of the particle could fade when the LHC collects more data next year -- they are the strongest evidence so far that the Higgs particle is there to be found.
“We have narrowed down the region where the Higgs particle is most likely to be, and we see some interesting signals, but we need more data before we can reach any firm conclusions,” said Gianotti, who heads the team that works on the collider’s enormous Atlas detector. “It’s been a busy time, but a very exciting time.” Finding the Higgs boson has been a major goal for the GBP10bn LHC after a less powerful machine at CERN called LEP failed to find the missing particle before it closed for business in 2000. The hunt was joined by scientists at the Tevatron collider near Chicago, who will present their own results early next year.
The Higgs boson is the signature particle of a theory published by six physicists within a few months of each other in 1964. Peter Higgs at Edinburgh University was the first to point out that the theory called for the existence of the missing particle.
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According to the theory, an invisible energy field fills the vacuum of space throughout the universe. When some particles move through the field they feel drag and gain weight as a result. Others, such as particles of light, or photons, feel no drag at all and remain mass-less.
Without the field -- or something to do its job -- all fundamental particles would weigh nothing and hurtle around at the speed of light. That would spell disaster for the formation of familiar atoms in the early universe and rule out life as we know it.
While the field is thought to give mass to fundamental particles, including quarks and electrons (the two kinds of particles that make up atoms), it accounts for only one or two percent of the weight of an atom itself, or any everyday object. That is because most mass comes from the energy that glues quarks together inside atoms.
To hunt for the Higgs boson, physicists at the LHC sift through showers of subatomic debris that spew out when protons collide in the machine at close to the speed of light. Most of the energy released in these microscopic fireballs is converted into well known particles that are identified by the collider’s giant detectors.
Occasionally, the collisions might create a Higgs boson, but it is expected disintegrate immediately into more familiar particles. To find it, scientists must look for telltale “excesses” of particles that signify Higgs boson decays. They appear as bumps, or peaks, in the data.
Speaking at the seminar, the scientists said they had narrowed down the range of masses the Higgs boson could have -- particle masses are measured in gigaelectronvolts (GeV), where one GeV is roughly equivalent to the mass of a proton, a subatomic particle found in atomic nuclei.
The CMS has excluded all Higgs masses except 115-128Gev, so between them the two experiments leave only 115-127GeV as the most probable Higgs mass, with both teams seeing signals between 124 and 126GeV.
Particle physicists use a “sigma” scale to grade the significance of results, from one to five. One and two sigma results are unreliable because they come and go with statistical fluctuations in the data. A three sigma result counts as an “observation”, while a five sigma result is enough to claim an official discovery. There is less than a one in a million chance of a five sigma result being a statistical fluke.
At Tuesday’s seminar, the Atlas team reported a 2.3 sigma bump in their data that could be a Higgs boson weighing 126GeV, while the CMS team reported a 1.9 sigma Higgs signal at a mass of around 124GeV. There is a 1% chance that the Atlas result could be due to a random fluctuation in the data.
Oliver Buchmueller, a physicist on the CMS experiment, said: “We see a small bump around the same mass as the Atlas team and that is intriguing. It means we have two experiments seeing the same thing and that is exactly how we would expect a Higgs signal to build up.” Early next year, the Atlas and CMS teams will pool their results, a move that should see the signals strengthen. Both teams are expected to need around four times as much data before they can finally confirm whether or not the Higgs boson exists. That might be difficult to collect before the end of next year, when the machine is due to close for at least a year for an upgrade before it can run at its full design power.
“There is definitely a hint of something around 125GeV but it’s not a discovery yet. We need more data! I’m keeping my champagne on ice,” said Jeff Forshaw, a physicist at Manchester University, England. “It should be said this is a fantastic achievement by all concerned. The machine has been working wonderfully and it is great to be closing in on the Higgs so soon.” The director general of CERN, Rolf-Dieter Heuer, said: “I find it fantastic that we have the first results in the search for the Higgs, but keep in mind these are preliminary results. The window for the Higgs mass gets smaller and smaller, however it is still alive. It’s intriguing hints in several channels, in two experiments, but we have not found it yet, we have not excluded it yet.” If the glimpse of the Higgs boson turns into a formal sighting next year, it may be one of several Higgs particles outlined in a radical theory of nature called super symmetry. The theory, which says that every known type of particle has an undiscovered twin, is popular among many physicists because it explains how some of the forces of nature might have behaved as one in the early universe. Unifying these fundamental forces was a feat that eluded Einstein to the grave.
Dick Hagen, a physicist at Rochester University who helped to develop the Higgs theory in 1964, said: “Einstein once said that God may be subtle but he is not perverse. Today’s results seem to favour the simplest manifestation of [the Higgs mechanism], and that is very gratifying as it coincides with the choice we made in 1964 -- not to mention the more personal issue that more complicated versions could easily fail to appear in the lifetimes of its principal authors.” © Guardian News & Media 2011