Protons collide at CERN’s CMS experiment Image by Thomas McCauley, Lucas Taylor/CERN.
What does the Higgs boson mean to physics, and how will particle physicists know when they’ve found it?
Evidence of the elusive Higgs Boson may be peeking out from some very dense data, but scientists are not ready to conclude that they’ve found it, according to an announcement in a crowded auditorium Tuesday morning at CERN, the European Organization for Nuclear Research.
Scientists from two separate experiments there — C.M.S. and Atlas — said they have found promising hints of Higgs, and evidence that the particle weighs about 125 billion electron volts. But, they say, they need more data to prove that it’s not a statistical fluke.
But let’s back up. The Higgs boson is an invisible particle believed to be responsible for endowing other particles — and thus by extension everything else in the universe — with mass. Finding Higgs would complete a vital piece of the puzzle of the standard model of particle physics by proving that there is an invisible “Higgs field,” which permeates all of space. As particles swim through the Higgs field, they take on mass, some more than others.
“It is responsible for the mass of fundamental particles,” said Michael Barnett, senior physicist at the Lawrence Berkeley National Laboratory and coordinator of education and outreach at CERN. “Without that, you don’t get the stars and the planets and the universe that we see today. In that sense, it’s responsible for our existence … Without it, you’d have this cold, dark universe.”
But Higgs is a shy, squirrely, fantastically difficult particle to find.
Here’s why: The Higgs boson decays in a fraction of a second, so you can never directly see it. To humans, it’s essentially invisible.
When scientists say that they’re looking for Higgs, what they’re really looking for are the products, or “modes” of this decay, the debris particles that indicate that Higgs was once there. It is believed that Higgs decays into a few different products or combinations of products. One of those combinations is two photons or two gammas. (That’s what physicists are referring to when they say “gamma gamma.”) It can also decay into two Z bosons, which in turn can decay into two muons and two electrons or a combination of those particles. So scientists aren’t looking for Higgs — they’re looking for the children and grandchildren of Higgs.
But here’s where it gets more complicated. The experiments are performed at CERN’s mammoth Large Hadron Collider, where protons charge toward one another at near light speeds and then smash together in carefully orchestrated, extraordinarily powerful collisions. The purpose is to mimic conditions from a trillionth of a second after the Big Bang. The hope: that these experiments will produce never-before-seen particles, like the Higgs boson.
In this particle collider, decay is occurring and photons are getting produced, Higgs or no Higgs. “You always have events that have nothing to do with the signal you’re looking for,” Barnett said. “What you’re looking for is more events from Higgs decaying into something.”
Barnett explains it this way: Imagine you’re talking to someone across the table in a noisy cafeteria. You try to hear what your lunch mate is saying, but it’s a struggle to filter his voice out from the 200 other background voices in the room. “We’re trying desperately to separate the signal from the noise,” Barnett says. In other words, the products of Higgs from other background products.
The mass of Higgs itself is unknown, but scientists announced today that they think they’ve found evidence that they’re seeing a signal for Higgs at 125 gigaelectronvolts (GeV.)
Using detectors at opposite sides of the collider — Atlas and CMS — scientists are starting to home in on a “mass region,” said Fabiola Gianotti, the spokeswoman and leader of the Atlas experiment. “We have restricted the most likely mass region for the Higgs boson to 115 to 130 GeV, and over the last few weeks, we have started to see an intriguing excess of events in the mass range around 125 GeV.”
This excess may be due to a fluctuation, but it could also be something more interesting, Gianotti said.
The “excess of events,” they hope, is Higgs signaling to them above the background of other particles decaying.
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