Back in April, rumors were flying around the particle physics community about a tiny "bump" spotted in the data from Fermilab's CDF collaboration that seemed to indicate the presence of new particle, possibly even the elusive Higgs boson -- that thing that gives mass to all the other particles, and the last remaining piece to be found to complete the Standard Model.
The rumored signal was sufficiently weak that most expected it wouldn't hold up under subsequent analysis. But last weekend, at a physics conference in France, CDF member Giovanni Punzi (PDF) showed the results after more data had been added to the mix, and the signal had strengthened instead of disappearing.
So it appears the rumors may be true: Fermilab does have compelling evidence for a possible new particle. And that is exciting news indeed.
SLAC National Accelerator Laboratory's Joanne Hewett wants to be clear about one thing, however: "This isdefinitely not the Higgs, in capital flashing neon letters," she told Discovery News. There's been a bit of confusion on that score, stemming from the fact that the data in question come from routine background analysis of the production of gauge boson pairs (force-carrying particles), part of the ongoing search for the Higgs.
A Signal from the Background
It's usually pretty mundane stuff in the background, but the studies need to be done, because the better physicists understand those backgrounds, the more precisely they can pick out potential Higgs signatures from the sea of data. And sometimes those background analyses can yield surprising results -- in this case, the CDF collaboration found an unexpected signal that got stronger with subsequent analysis.
That's how discoveries in particle physics are made. First you get a hint of a signal. That usually disappears when you add more data into the mix, but if it doesn’t -- as in this case -- you get a slightly stronger result, and so on, adding more data with each iteration and hoping to inch up gradually to the gold standard of "5 sigma" -- enough to announce a bona fide discovery.
This suspected new particle doesn't quite reach the golden threshold but it's getting awfully close.
Back when it was just a pesky 3.2 sigma, the naysayers expected it to go away, as most such signals do. It didn’t. "The signal persisted when they doubled the data," says Hewett. It’s now up to 4.8 sigma -- possibly dropping to 4.2 sigma once all the systematic uncertainties are figured in -- and that is much more difficult to dismiss.
It also helps the case that CDF is a mature collaboration: the machine and all its quirks are very well understood by now, significantly reducing the likelihood of the more obvious analytical errors. "It would be very hard at this point to simply attribute it to an energy miscalibration or something like that; if it is a systematic error, it's a subtle one," Caltech's Sean Carroll writes over at Cosmic Variance. "But it doesn’t look like an error; it looks like a signal."
Not the Higgs
So, if they're not sure what this possible new particle might be, how can Hewett and her cohorts be so sure that it's not the Higgs? In short, it's not leaving the right kind of signature. Physicists are only able to recognize the particles produced in these collisions by the particle decay patterns ("signatures") they leave behind.
Quarks only exist for fractions of a second before they decay into other secondary particles. This is partly determined by mass: a particle can't decay into things that are heavier than its own mass, since that would violate conservation of energy -- you can't make something out of nothing. So it will usually decay into the heaviest particle that is still less than its own mass. The same basic principle holds true for the Higgs boson.
Each quark has many different ways of decaying, so there are several possible signatures, and each must be carefully examined to determine which particles were present at the time of the collision. Decay patterns are like branching generations in the family tree, and every bit as complicated.
In this case, the final state, or "signature," we're dealing with is a lepton (for example, a muon or electron), a neutrino, and two hadron jets.
"Jets" appear because quarks can't exist in isolation; they must be bound inside hadrons. So whenever a quark is produced in a collision, it goes flying out of its host hadron. But before it can escape completely, it is surrounded by a spray of hadrons all traveling in more or less the same direction. By studying the jet spray, scientists can tell what kind of quark produced it.
According to the Standard Model, this is one of the signatures that can arise from the production of gauge boson pairs: either two W bosons, or a W and a Z boson pair. In both instances, one W in the pair can decay into a lepton and neutrino, and the other particle (W or Z) in the pair will decay into hadron jets.
Got that? I know, it's like trying to keep track of the ever-expanding Kardashian Klan, isn't it?
The point is, physicists understand particle decay signatures very well, including what a Higgs boson signature would look like at a mass of 150 GeV. Whatever this new thing might be, it just doesn't act like a Higgs, says Hewett: "It has the wrong behavior we'd expect to see at this mass."
A Higgs at that mass would decay to W bosons, but the "bump," or peak in the data that's causing all the fuss, isn't in the Ws -- it's in the hadron jets. And those hadron jets should have mostly bottom quarks if they came from a Higgs with 150 GeV mass, because the bottom quark is the heaviest particle lighter than 150 GeV. (The next heaviest is the top quark with a mass of 174 GeV.) Instead, the jets are a mix of all the different kinds of quarks.
Who Ordered That?
So, okay, it's not the Higgs. But it's pretty interesting and exciting in its own right, because the Standard Model doesn't predict it. It predicts a bunch of W+W and W+Z gauge boson pairs that then decay into leptons, neutrinos, and hadron jets. And that’s pretty much what scientists saw in the first peak of the graph (shown in red below). "The first peak is right where it should be," says Hewett.
The surprise is an unexpected second peak in the data (shown in blue on the right). That's just plain weird, and isn't predicted by any of the prevailing theoretical models. You want to be careful tossing around phrases like "new physics," but there's a good chance that's what we’re looking at here: the possibility of a new kind of gauge boson, most likely, or what Hewett describes as a "W + X" pair, with "X" being the unknown partner.
"It falls into the ‘Who ordered that?’ category," says Hewett, referencing the famous observation by I.I. Rabi back in the 1930s when the muon was first discovered. That, too, was completely unexpected, and forced physicists to rework the Standard Model.
This mysterious new signal could still turn out to be an artifact, mind you, particularly since "word on the street" (as Hewett puts it) is that CDF's sister collaboration at Fermilab, D-Zero, isn't seeing the same signal. D-Zero scientists haven't officially made an announcement but they are expected to weigh in later this week with their own results.
Most Exciting Summer Since 1974?
That's the next step: hearing from D-Zero and then waiting on results from the latest Large Hadron Collider data that will be announced over the summer. "If neither sees the bump, then I guess the CDF people just screwed up their backgrounds," Marc Sher, a physicist at the College of William and Mary, told Discovery News via email. "I'm not optimistic, since it is so ugly for theorists, but I said the same thing about the cosmological constant, so what do I know?"
Hewett is less pessimistic: "It’s very, very unlikely that this is just an artifact," she says. But Hewett and Sher agree that the upcoming particle physics conferences this summer will showcase results that put many such questions to rest.
"I'm not sure what to work on now, since anything I do might be wiped out next week," Sher jokes. "We've waited two decades for this moment, and lots and lots of questions will be answered within weeks."
"We expect new discoveries," says Hewett. "This is going to be the most exciting summer particle physics has had since 1974."
Image credits: Fermilab, CERN
Provided by Discovery Space
