On Wednesday, July 4, physicists at CERN had big news to announce: the discovery of a new subatomic particle. Its properties analyzed so far bear a strong resemblance to the highly sought after Higgs boson, predicted by the Standard Model of physics. While numerous media outlets eagerly declared it official, researchers at the Large Hadron Collider insisted on waiting for further analysis before calling the new particle the Higgs. Regardless of the outcome of these next steps, the boson has already caught the attention of the public far beyond the particle physics world.
When seven out of ten trending subjects on Twitter relate to particle physics, even while America is in the throes of its Independence Day celebrations, you know something special has happened. Wednesday morning, July 4, two teams working at CERN revealed the highly anticipated results of their respective experiments. Both the CMS and ATLAS groups arrived at the same conclusion, adding credence to their announcement: the discovery of a totally new subatomic particle, one that might preserve the extremely effective, but incomplete, Standard Model of physics.
The Standard Model describes matter in terms of fermions, the particles of ordinary matter that have already been identified: electrons, quarks, and neutrinos, among others. These are thought to interact by exchanging “messenger particles”, called bosons. The Standard Model describes matter very precisely, when it works. But there are some questions it is unable to answer. Why, for example, do these subatomic particles have mass? And why should this mass differ among particles? To resolve these questions, scientists in the 1960s proposed a messenger particle, the Higgs boson, that would be responsible for giving particles the property of mass.
Detecting a Boson
For decades, physicists have had an idea of what they needed to find, to either confirm the Standard Model, or reveal the cracks in its finish. But how do you hunt a Higgs? By detecting its decay. A Higgs boson won’t stick around long enough to be detected directly, but physicists can look for the particles formed when it disintegrates. In CERN’s particle accelerator, the Large Hadron Collider (LHC), two beams of protons are sent flying around the 17-mile-long ring of the underground facility. When protons smash into each other, new particles are produced by the energy of the collision. One is predicted to be the boson proposed by Scottish physicist Peter Higgs in 1964.
When Higgs’ hypothesized particle decays, it is expected to break down into other elementary particles. These could be two gamma ray photons, or four leptons (electrons or muons), or an array of other particle decay products. The ATLAS and CMS detectors were optimized to read the signal of the first two scenarios, explains Nikola Makovec, a researcher on the ATLAS project. All ten channels—or modes of particle disintegration—will be used to characterize the new particle by the end of 2012, he says, but detection of the photons and leptons are “more than sufficient to claim the discovery of a new particle today.”
What the Data Showed
What the CERN physicists saw in the data for these two readouts was an excess of particles, compared to background levels, in the mass range that had been indicated for the predicted Higgs boson. Previous experiments had allowed scientists to narrow down the possible window to somewhere around 125 gigaelectron volts (GeV), the unit used to measure mass/energy (one and the same, according to Einstein) of subatomic particles. CMS, using a calorimeter with more than 75,000 crystals of lead tungstate, detected a new particle with a mass of 125.3 +/- 0.6 GeV. Halfway around the particle accelerator, on the opposite side of the LHC, the ATLAS experiment relied on a calorimeter made with lead and liquid argon to pick up the trace of the Higgs. Their analysis yielded a mass for the newfound particle of about 126.5 GeV. A very exciting correspondence of results for the competing, yet mutually confirming, teams.
It is this confirmation—both between the groups (who knew nothing of each other’s results during the analysis) and between the two channels examined for the Higgs signal—that allowed leaders at CERN to justify the announcement of a major discovery today. The convention in particle physics is to require a very low probability that the findings could be due only to chance; specifically, a chance of one in three million, expressed as a value of “5 sigmas”. ATLAS achieved this value, but the CMS results, at 4.9 sigmas, might have been just shy of the necessary level of certainty. The extremely close correspondence between the experiments, though, gave CERN sufficient confidence in the data to celebrate the discovery of a new particle.
A New Particle Discovered
But that is all that CERN said. Much of the media has been quick to pin down the newcomer. “We got it,” said the scientific publisher Nature, via Twitter. “The Higgs Boson.” Well, yes, maybe. Probably, even. At the announcement event held Wednesday in Paris, at the headquarters of France’s National Center for Scientific Research (CNRS), French members of the CERN teams did not restrain their enthusiasm, but did refrain from calling the new particle the Higgs boson. Philippe Chomaz, director of the Institute for Research on the Fundamental Laws of the Universe, part of the French Alternative Energies and Atomic Energy Commission (CEA), preferred to say that they had discovered a particle that “strongly resembles the Higgs.”
Expectations are very high that the evidence will continue to point in this direction. Still, scientists searching for the elusive subatomic particle insist that it will take much more data and analysis to know its exact nature. Sandrine Laplace, a researcher on the ATLAS experiment, underlined the fact that, in the scientific process, when something is “proved”, it usually happens by disproving something else, and progressively eliminating the alternatives. The new particle will need to be analyzed further regarding its mass, size, spin, and the way it interacts with other particles, explained Laplace – enough work to keep the LHC’s Higgs researchers busy for a number of years. Only with this information, which will be made possible by the massive amounts of data coming out of the LHC, thanks to a ramping up of the accelerator’s energy this year, will physicists be able to say whether this new boson is the very one predicted by Higgs.
Higgs or Not, a Boson that Fired the Imagination
When asked why the announcement was being made now, even though scientists are not yet sure if the particle is the Higgs boson itself, Sandrine Laplace seemed mildly exasperated: “Because we’re sure we’ve discovered a new particle!” To the scientific community, this is no less exciting. If it is the Higgs, it will resolve a major question left unanswered by the Standard Model; if it isn’t, a whole new physics could be on the verge of opening up. “Whatever this boson is,” declared Philippe Chomaz, “this is a new area in the subatomic world. And that’s absolutely fantastic.”
The excitement surrounding this announcement – in the papers, on social networks, on the TV news – suggests that both the media and the general public agree with Chomaz. The vast majority, though, cannot be assumed to understand the complex physics underyling the importance of this discovery. Why, then, has the Higgs boson so gripped imaginations around the world? Perhaps it’s the novelty of something new, something genuinely, fundamentally new in our understanding of the nature of the universe. Discoveries like this, Philippe Chomaz reminded the press, only come around a few times per century.
It also likely has to do with the possibility of witnessing a scientific revolution firsthand, something made possible only recently by the participatory nature of the Web 2.0. Never before could so many follow so closely the extremely specialized work of a group of particle physicists. Websites devoted to the experiments, blogs, Facebook and Twitter accounts, both personal and institutional, even comics have all been used to bring this science up from the underground lair of the LHC and out into the open. By emerging into the light of day, this research not only contributes to greater understanding of science, but allows the world to share in the great sense of wonder that comes with discovery.
Find out more:"The Last Boson?" In this video, physics Nobel laureates give Nature their view on the discovery at CERN. http://www.youtube.com/watch?v=iWPZYIp5KiIPhysicists declare victory in Higgs hunt http://www.nature.com/news/physicists-declare-victory-in-higgs-hunt-1.10940 “What in the World is a Higgs Boson?”, New York Times http://thelede.blogs.nytimes.com/2012/07/04/what-in-the-world-is-a-higgs-boson/?smid=tw-shareHiggs Boson explained using sugar and ping-pong balls http://www.bbc.co.uk/news/science-environment-18712914