Back in April, Fermilab physicists detected a strange bump in their data that couldn't be explained by any known particles. The mystery remains unsolved, and it's looking more and more likely that we're on the verge of a major discovery.
We covered the initial announcements here and here, which offer more extensive background on the initial discovery. Very basically, the initial discovery was made during background analysis of the production of gauge boson pairs, which are elementary force-carrying particles. (If you need a refresher on the elementary particles, check out our field guide.)
This was all fairly routine stuff, which made the discovery of an unexpected energy bump in the data even more surprising. This bump could have been one of two things: some sort of mistake, or the signature of a previously undiscovered particle. The possibility that this bump was all just an error can't be discounted lightly - after all, it doesn't require us to invoke any new particles, which is always preferable in the absence of compelling evidence to the contrary.
Just how compelling does the evidence need to be? Physicists generally set the lower limit for a new discovery at what's known as a 5-sigma event - something that has a 99.99994% chance of being real. The initial energy bump was little more than a 3-sigma event, which carries with it a 99.73002% chance of being correct. To be sure, that only leaves about a .27% chance that the result was an error, but if you give physicists a choice between a .27% chance and rewriting physics...well, they're going to take the .27% chance every time.
But now the Fermilab physicists have had a chance to add even more data, and the bump hasn't gone away. In fact, the evidence for it has crept up to as much as 4.8-sigma, just short of the gold standard to be considered a discovery. And because Fermilab's CDF collaboration has been working for so long, it's unlikely that any obvious errors have been made. A very subtle systematic fault is still a possibility, but overall it's looking good that this really is a signal.
So just what is this new particle, assuming that's actually what it is? We know it's not the elusive Higgs boson, because its signature doesn't even remotely match what the Higgs would do at that energy level. The current best guess is that it's some sort of new gauge boson, or perhaps a pair made up of the well-understood W bosons and some other mysterious partner.
Still, the CDF team won't be able to confirm this particle's existence on their own - they'll need supporting evidence from Fermilab's other collaboration, DZero. However, the word is that DZero hasn't found the signature yet, which would tip things back towards this bump looking like some sort of artifact or ghost in the data. Either way, the Large Hadron Collider will also need to look for the particle, and it might prove the deciding vote if CDF and DZero's data can't agree.
Still, physicist Joanne Hewett of Stanford's SLAC National Accelerator Laboratory remains optimistic:
"It's very, very unlikely that this is just an artifact. We expect new discoveries. This is going to be the most exciting summer particle physics has had since 1974."
1974, for those who haven't memorized the dates of every particles' discovery, was the year physicists demonstrated the existence of the charm quark, which helped jump start a series of crucial breakthroughs in our understanding of particle physics.