This week the CDF experiment at the Tevatron proton-antiproton collider caused a stir with this paper and a special seminar. This is a common situation in scientific research. The result is (a) really important if confirmed and (b) in need of confirmation before everyone is certain of it.

If you want more than that standard summary, I'm going to try to justify parts (a) and (b). But first the data. The main plot in the paper is this:

On the left you see crosses and some solid, coloured-in histograms.

The horizontal axis is the mass you get from adding together two jets of particles produced, along with a W boson, in a proton-antiproton collision. The vertical axis is the number of times this configuration (dijet mass+W boson) occurred while CDF was recording data. The coloured histograms represent the expectation and the crosses are what actually happened. You can see that at masses of about 150 GeV/c2 this configuration happened more often than expected.

If you subtract all the histograms (except the red bit) from the data you get the crosses in the plot on the right. The first bump is made by jets from a W or Z boson (the red bit on the left). So those are expected, they are collisions when two bosons are made, and one of them decays to give two jets.

The second bump on the right is what all the fuss is about.

A bump like this could be a sign that a new particle (a bit like the W boson but nearly twice as massive) was made, and decayed to two jets. There is no such particle known to science. It would be wonderful. It doesn't fit very neatly into any expected theory (though there are already several new theoretical papers eager to explain it). It would represent a massive breakthrough in our understanding of fundamental physics (though it would take quite some time to actually understand it!).

That's the reason for interest and excitement.

There are a few reasons for the caution. Firstly there is a chance that this is just a statistical fluke.

If the histograms and data are exactly right, the paper quotes a one-in-ten-thousand (0.0001) chance that this bump is a fluke. That's pretty small; although bear in mind that lots of distributions like this get plotted. If you plot 100 different distributions, the chances become about one in a hundred (0.01) that you'll see something odd in one of them. See xkcd's jelly beans for an illustration of the effect.

Still, that's pretty small. And it's not clear to me that there really are 100 different distributions (or flavours of jelly bean) as interesting as this one.

Another worry is that neither the data nor the histograms are exact. The energy of the jets is only known to within 3% on average for example*. Also you need to understand the scatter (resolution) of the dijet mass, as well as how solid the theory expectation is. When CDF considered a lot of factors like this (known as systematic uncertainties), they say probability of the bump being a false alarm is raised by a factor of eight.

Changing these things really can create or destroy bumps, so CDF will have spent a long time studying effects like this very carefully. Still, it is very hard, sometimes impossible, to reliably assign probabilities to systematic uncertainties.

CDF have more data to look at, their rivals on the Tevatron, D0, have plenty of data too, and pretty soon the experiments (like my own) at the Large Hadron Collider will be able to say something about this as well. This is not a mystery which will last long. Either it will be the last and greatest breakthrough from a fantastic machine, or it will have been another false alarm on the frontiers of physics.

My money is on the false alarm at the moment, but I would be very happy to lose it. And I reserve the right to change my mind rapidly as more data come in! That's all part of the fun...

*I wrote a bit about jet energy scale studies and how they affect searches like this here.

And someone (Tommaso Tabarelli de Fatis, a CMS collaborator) even made a little animation to show (approximately) how an energy scale shift can destroy the bump. Although the shifts here go up to 7%, much bigger than the 3% CDF quote, it is still a good illustration of the trickiness of systematic uncertainties.