UPDATE (July 4): They really did discover something. During a live CERN announcement on Wednesday morning, representatives from the CMS and ATLAS detectors at the LHC detailed the discovery (to a 4.9 and 5 sigma significance in the CMS and ATLAS detectors respectively) of a previously unknown massive boson that resembles the Higgs. History has just been made.
ORIGINAL: After four years of high-energy particle demolition inside the detectors of the Large Hadron Collider (LHC), are physicists on the verge of announcing one of the most significant discoveries of our time? If you’ve seen this morning’s headlines, then you’d think the answer is a huge yes. But in typical quantum physics style, we’ll have to wait a little longer for definitive proof for the elusive Higgs boson.
So why all the excitement?
On Wednesday (July 4), scientists heading two major experiments at the LHC plan to announce their most recent findings at a physics conference in Australia with accompanying meetings in Geneva, Switzerland. What’s more, senior scientists at European Organization for Nuclear Research (CERN) are hinting that there is strong evidence in their data that suggests the Higgs boson exists.
For the last year or so there have been “hints” of a Higgs detection, then those hints turned into “potential evidence.” Now, will we finally get word of a bona fide discovery?
“I agree that any reasonable outside observer would say, ‘It looks like a discovery,’” CERN physicist John Ellis told The Associated Press. “We’ve discovered something which is consistent with being a Higgs.”
The Higgs boson is the last piece of the physics Standard Model, a collection of theories that underpin all modern physics. The Higgs particle is theorized to mediate mass — like a photon (also a boson) mediates the electromagnetic force, i.e., light — and creates the “Higgs field” that must pervade the entire Universe, endowing matter with mass.
If the LHC didn’t detect signs of the Higgs particle, its non-discovery would turn modern physics on its head. But physicists are an inquisitive bunch, so a non-discovery would be just as exciting (if not more so) than a discovery. But for all the Higgs doubters out there, it’s looking more and more likely the Higgs does exist and the Standard Model is as robust as physicists always thought.
So when the announcement comes from ATLAS and CMS physicists on Wednesday, will we get the definitive proof we’ve been (not-so-)patiently waiting for?
In the world of high-energy physics, it’s not a question of slamming particles together and then photographing a Higgs boson screaming away from the carnage. Countless billions of collisions need to be recorded and the resulting spray of particles tracked. Like a photograph, more photons are needed to make the image appear defined and bright. If just a few photons hit the photographic paper, a very vague and fuzzy image is the result. The longer you leave the photograph under the light, more photons are collected and the better the image becomes.
This is basically what the LHC scientists are doing. They repeat the same experiment again and again and collect the huge quantities of data to gradually build an “image” of the kinds of particles produced inside the LHC as it smashes protons together at near the speed of light. Over time, statistical spikes start to appear in the data, suggesting particles of a certain energy (or mass) are being detected.
One statistical spike, at around the energy of one predicted variety of Higgs boson, has been growing stronger and more defined over the months, but at what point does that “spike” become a discovery and not just background noise? As this is a lesson in statistics of huge numbers, physicists have a way of categorizing how strong the signal is.
So far, the strength of this particular Higgs signal hasn’t exceeded 4.3-sigma — which relates to a 99.996 percent chance of the signal being real (and a 0.0004 percent chance that it’s just noise). A 5-sigma signal, on the other hand, is regarded as the “Gold Standard” in particle physics, relating to a 99.99994 percent chance that the signal is real (and only a 0.00006 percent chance of it being noise). Only when the signal hits that magical 5-sigma standard can a discovery be announced.
This is why Ellis says that to any “reasonable outside observer” Wednesday’s announcement will appear to confirm a Higgs boson discovery, but to particle physicists, the signal may be just shy of the 5-sigma mark.
There is another possibility. By combining the results of both the CMS and ATLAS detectors, CERN can check the results of one against the other. In the pursuit of the Higgs, they also combine the data from both (which is how the previous 4.3-sigma signal was derived). On Wednesday, however, we’re not going to see a combined signal from both detectors.
“Combining the data from two experiments is a complex task, which is why it takes time, and why no combination will be presented on Wednesday,” said CERN spokesman James Gillies.
So this opens up another possibility: perhaps one of the detectors has a 5-sigma signal and the other does not. This could be the source of the ambiguity. Regardless, we may be getting close and few people are doubting that these results suggest a Higgs discovery will soon be confirmed.
“Only the most curmudgeonly will not believe that they have found it,” said cosmologist Sean Carroll.
But keep the Champagne on ice, that “discovery” announcement may still be some time off.
Image: The massive CMS detector in the LHC (CERN/LHC/CMS)