“Although important nuclear physics work was to go on in laboratories such as ours had become – and we had to cut down to a lower energy group – it was not fundamentally opening up new insights on the structure of matter. That required you to be in a higher league.” -John Henry Carver
Okay, before we get into speculating, the short answer is maybe. And that’s amazing! Let’s recap the basics, and bring you up to speed of what could be the Tevatron’s last — and possibly greatest — hurrah.
This is the Tevatron. For decades, it was the most powerful particle accelerator in the world, creating proton-antiproton collision at speeds exceeding 99.99% the speed of light in vacuum! Recently, however, it’s been relegated to #2 in energy behind the new Large Hadron Collider (LHC), whose protons can travel that tiny bit extra 0.007% faster than Fermilab’s can. The Tevatron soon will also fall to #2 in terms of the total number of collisions it has collected data on, and its run is imminently coming to an end.
The accelerator at Fermilab has been absolutely amazing, however, make no mistake.
This is the standard model of elementary particles. Three of them: the bottom quark, top quark, and the tau neutrino, were first discovered at Fermilab!
Additionally, Fermilab has discovered a myriad of composite particles — mostly combinations of quarks and gluons — and is famous these days for competing with the LHC to find the Higgs.
And if you’ve ever worked in particle physics, today’s xkcd comic will be all too familiar to you.
After all, we have two things working against us:
- We are looking for a huge number of possibly interesting effects, and
- we get a whole bunch of “junk” data, that we have to make responsible cuts to in order to not have it swamp our signal.
There are dangers to both of these. First off, these particles and the laws governing their interactions tell us what they ought to do, on average. In other words, let’s say I make a Z0-particle. I know that 3.33% of the time, it should decay to an electron-antielectron, 3.33% it should decay to a muon-antimuon, and 3.33% to a tau-antitau (and 90% to other stuff, which isn’t important for now). Now, if I make 100 Z0, and I see that 3 decayed to electron-positrons, 2 decayed to muon-antimuons, and 6 decayed to tau-antitaus, what does that tell me?
Well, according to statistics, this particular combination is pretty unlikely, and so if this were my only data, I would conclude that this could be a sign of new physics! Of course, this could also be a sign that I need more data, and that if I did 1000 trials, I might get numbers where this effect disappears.
There’s also a problem with cuts: if I make irresponsible cuts, I could, for instance, be throwing away some real events that made this effect disappear, also. This problem gets worse as you get closer to the energy limit of your accelerator. Those of you who’ve followed high-energy physics for some time may remember this…
LEP was the Large-Electron-Positron collider (which was the predecessor to the LHC, the Large Hadron Collider). And LEP’s energy topped out at 114 GeV. And wouldn’t you know, they saw some interesting effects right up at that high-energy limit! Some people still claim that the Higgs is probably right there, at about 114 GeV, and if we had only spent a few more months running LEP at full speed, we would’ve discovered it long ago.
But it could also be a problem of either low statistics or of irresponsible cuts. We normally demand a certain level of significance in particle physics far beyond what other disciplines require: 5 standard deviations before a result is considered “robust”. A result three standard deviations away from normal is interesting, but requires a lot more verification, and announcing a two-sigma (or two standard deviation) result should get you laughed out of the room.
And so today, there was an announcement and a talk given by Viviana Caviliere, and I went to download the slides…
Fermilab has released an interesting result! It’s being reported in the New York Times, there’s a paper on arxiv.org, Sean Carroll has a take (and more selected images than I’ve pulled), and you yourself will be able to watch the talk here when it’s posted. What’s the deal?
Well, based on the standard model, when we smash protons and antiprotons together at various energies, we should produce jets of particles, W’s, and Z’s in certain proportions, and carrying certain proportions of the energies. They are shown in the graph below in the variously-colored sections.
But that isn’t what Fermilab saw! Up in between the energies of 120 GeV and 160 GeV, it saw an excess of events, as fitted to that slightly raised blue line!
First off, it’s important to state right now what it isn’t: this is not the Higgs.
But it could be indicative of a different particle, one we didn’t anticipate!
But you’ve been paying attention, and so I know you’re dying to know the answer to this: how significant is this result?
Well, it’s a three-sigma result. Which means, if this is a real effect, we’re seeing the first signs of it. But it could also mean that when we collect more data, or when we go back and redo our cuts, this effect will disappear. And this “raised blue bump”…
…will fall back down to the red line.
Of course, if it is real, the big question is, what kind of new physics or new particle is this showing us? I am sure there will be new papers with new theories out any day… but this is honestly what particle physics sorely needs right now, data pointing us in new, interesting directions! Time will tell whether this result holds up or not, but if it does, it could be Fermilabs last, and greatest, most unexpected discovery!