Tevatron: “My money is on the false alarm at the moment”

A somewhat excited BBC report of potentially ground-breaking interim results from Cern LHC rival accelerator, Tevatron, is undermined by the statistical possibility of a false alarm.

The Guardian’s tame particle physicist, at Cern, Jon Butterworth explains.

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.

I’d tend to agree with his assessment

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…

And, once again, here’s a good introduction to the Standard Model of Particle Physics from Cern News – it’s the first in a series of videos.

As I’ve said, of course there may still be further wrinkles ahead…


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