Just this month, the CDF experiment at Fermilab saw a bump in their data at an energy of 140-150 GeV suggesting that they had seen a new type of particle. But does it really exist and, if so, what is it?
The result was at the 3.2 sigma level, which in statistics means that it is about three standard deviations away from the null hypothesis--or about a 6 in 10,000 chance that the signal is just a statistical fluctuation. That's a small chance but particle physicists have high standards when it comes to this sort of thing.
In particle physics, a 3 sigma result is often described as "evidence" for something but it takes a 5 sigma result to claim "discovery." The trouble is, there are lots of 3 sigma results in particle physics that go away with more data, usually because of some unsuspected systematic error. That doesn't change the likelihood that this result is correct but says that 3 sigma is not actually a discovery, just a promising hint.
So now we've dealt with the idea that this might not be anything, let's play with a fun speculation advanced by Fermilab's Dan Hooper at today's plenary session at the American Physical Society April meeting.
One thing that physicists know is that it isn't the Higgs particle. But Hooper suggested that the bump could be a new particle representing a new type of force. The particle would generally be described as a Z' (pronounced Z prime) or, equivalently, the force could be called a fifth force, different from electromagnetism, gravity, the weak force, or the strong force.
To make sense, the particle must interact modestly with quarks and extremely weakly with leptons--a so-called leptophobic Z'. But if that is the case, it needs a bunch of other exotic particles to exist or else the Standard Model of particle physics goes haywire. One of those particles would have the properties of a dark matter particle. Furthermore, it would have a mass and cross section just where a bunch of other experiments hint (see our previous story about this).
One role for this Z' could be to mediate between dark matter and the matter we can see--between the dark and the light, so to speak. And that opens up a whole new world of exploration.
Remember that this is just speculation about what the particle could be if it exists but that isn't guaranteed. Expected at the end of the summer is a new analysis that will include about four times the amount of data and that should make the situation much clearer. But wouldn't it be cool if we found a gateway to dark matter in the final year of running of the Tevatron?