ATLAS Collaboration / CERN
This diagram shows the results of a proton-on-proton collision in the Large Hadron Collider's ATLAS detector last September, with four muons indicated by red tracks. Such a result could be consistent with the Standard Model with or without the Higgs boson, depending on the analysis of multiple events.
By Alan Boyle
Has the Higgs boson finally been detected? It's almost gotten to the point that if a discovery of some sort doesn't come out of next week's update on the multibillion-dollar subatomic search, it'll be a big surprise. But how far will the announcement go, and what will it mean for the future of physics?
To refresh your memory, the Higgs boson is the only fundamental subatomic particle predicted by theory but not yet detected. It's thought to play a role in endowing some particles, such as the W and Z boson, with mass ... while leaving other particles, such as the photon, massless. The Higgs mechanism, proposed by British physicist Peter Higgs and others in the 1960s, could have played a role in electroweak symmetry breaking, which was a key event in the rise of the universe as we know it.
The Higgs boson is so key to the current understanding of fundamental physics that Nobel-winning scientist Leon Lederman nicknamed it the "God Particle"Â â" a term that has been making other physicists wince ever since. Another religion-tinged cliche would be to call it the "holy grail of particle physics," as CERN physicist John Ellis has. He says finding the Higgs is a key goal for the $10 billion Large Hadron Collider.
"That's one thing that we're really looking forward to with the LHC," Ellis told me five years ago. "In fact, back when we persuaded the politicians to stump up the money to build the thing, that's probably what we told them."
Last December, the teams reported that they saw "tantalizing hints" of the Higgs' existence at a mass of around 125 billion electron volts, or 125 GeV. But the confidence in those results was not yet high enough to claim a discovery.Â Now the teams behind the collider's CMS and ATLAS experiments have collected higher piles of data, at higher energy levels, sparking higher expectations.
An hour-long BBC Horizon documentary focuses on the hunt for the Higgs boson.
The 5-sigma fetish
When physicists talk about their confidence, they talk in terms of statistical "sigma" levels. The higher the sigma, the less likely that the results are just a fluke. In particle physics, 3 sigma constitutes strong evidence, but it takes 5 sigma to accept the results as a discovery. At the 5-sigma level, statisticians say there's roughly one chance out of 3 million that you're leaping to the wrong conclusion, as opposed to a 1-in-1,000 chance at the 3-sigma level.Â That distinction makes a big difference when you're sifting through billions upon billions of proton-on-proton collision reports.
Last year, the best that the LHC teams could do was 3.6 sigma for ATLAS, and 2.6 for CMS. Now physicists are looking for a 5.
For three weeks, the teams have been running the numbers on their experimental results in secret, so as to avoid any chance that one analysis will influence the other. Their results are to be announced during a presentation at the CERN nuclear research center in Geneva, which will be webcast starting at 9 a.m. CEST (3 a.m. ET) on July 4. Although no official word has leaked out, the unofficial word is that someone looking for a discovery could get to the magic number.
"Reports from the experiments indicate that at least one of them, if not both, will reach the 5 sigma level of significance for the Higgs signal, when they combine 2011 and 2012 data and the most sensitive channel. So, this will definitely be the long-awaited Higgs discovery announcement, and party time for HEP [high-energy physics] physicists," Columbia mathematician Peter Woit wrote on his Not Even Wrong blog a week ago.
Since then, other physicist-bloggers have been fine-tuning the expectations. Here's a selection:
- On the Resonaances blog, physicist Adam Falkowski (a.k.a. Jester) has a countdown clock ticking toward the Higgs discovery. "It is not clear, at least to me, if either of the two experiments will pass the 5-sigma fetish. But it does not really matter. ... What's going to change next Wednesday is that the status of the Higgs will be upgraded from 'almost certain' to 'beyond reasonable doubt.'"
- On Quantum Diaries, Southern Methodist University physicist Aidan Randle-Conde advises against trying to combine the data from the two teams to get to 5 sigma. "With all this pressure to get as much out of the data as possible, it's tempting to move too quickly and do what we can to get a discovery, but now is not the time to rush things," he writes.
- On theÂ ViXra Log, Philip Gibbs says that when CERN's researchers report their progress, "it is likely that the main question they are investigating will switch from 'Is there a Higgs Boson?' to 'Is it the Standard Model Higgs boson?'"
- On a blog titled "Of Particular Significance,"Â Rutgers physicist Matt Strassler advises caution,Â but also suggests getting "the cases of champagne ready, in case the time has finally come to pop the corks." He points out that a discovery announcement would by no means be the end of the story. "Even if we see strong evidence of a Higgs-like particle ... the correct understanding of that particleÂ â" in particular, determining whether it is or isn't a 'simplest Higgs'Â â" may take years."
- As we approach H-Hour, you can expect to hear more via all these outlets as well as other blogs such as Cosmic VarianceÂ and "A Quantum Diaries Survivor."
Physicist Gigi Rolandi discusses the Higgs search in a CERN video.
Hedging on the Higgs
What Strassler and Gibbs are saying is important: Technically speaking, CERN is unlikely to announce that the Higgs boson has been definitively discovered. It's more likely that physicists will talk about a new particle that has a signature consistent with the Higgs but has to be investigated further.
CERN hinted at that approach last week in the news release announcing Wednesday's webcast. "It's a bit like spotting a familiar face from afar," said the center's director general, Rolf Heuer. "Sometimes you need closer inspection to find out whether it's really your best friend, or actually your best friend's twin."
Gigi Rolandi, a senior research physicist at CERN, used a similar analogy in a video released this week, referring to crops of corn (which he calls maize, as most Europeans do), wheat (which he calls corn) and poppy flowers. Some particles are as easy to spot as a red poppy in a wheat field, he said. But not the Higgs.Â "The search for the Higgs is more similar to looking for a single plant of maize among many, many corn plants, than looking for a poppy among the corn," he said.
We'll get a foretaste of Wednesday's proceedings on Monday, when Fermilab is due to provide its final update on the Higgs boson search, based on the full set of data from the now-closed Tevatron. Will Fermilab try to steal some of CERN's thunder, at least for a couple of days? Stay tuned....
Previous episodes in the Higgs hunt:
- Ups and downs for Higgs boson buzz
- Cartoons visualize the Higgs boson
- Can physicists crack the big puzzle?
- Flash graphic: Inside the Big Bang Machine
- Flash graphic: Michio Kaku on LHC nightmares and dreams
- Msnbc.com's special report on the Large Hadron Collider
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding theÂ Cosmic Log pageÂ to your Google+ presence. You can also check outÂ "The Case for Pluto,"Â my book about the controversial dwarf planet and the search for new worlds.