Mother Nature's Bumps

Last week, Dr Sam Harper visited us to talk about his PhD work at the CDF experiment at Fermilab.

Sam was an undergraduate student at Imperial HEP before he moved to Oxford to do his PhD (or whatever it is they call them), so it was a bit of a homecoming for him.

The talk was about a study of events at the Tevatron collider with two prominent electrons/positrons emerging from the collision of protons and anti-protons. This is a channel with good sensitivity to New Physics, because the events themselves are clean and well-defined, the expectation from the Standard Model is well understood, and because if there is something New that is contributing to the events, you can use the electrons to figure out its mass, although that does depend on the way the New thing turns into electrons.

Sam went through the different aspects of the work, from the motivation and analysis procedure through to the final "mass spectrum" that he found from the electrons. It is the mass spectrum which encodes any signs of New Physics, and in particular, any unexpected "bumps" in the spectrum (there is only one you expect to see, a huge bump at 90 GeV due to the "Z boson" discovered a couple of decades ago at CERN) would be a smoking-gun signal, if significant.

For an example of an undisputed, definitely significant, unexpected bump in the equivalent type of event, at the high energy frontier of more than three decades ago, check out the J particle discovery mass spectrum!

Getting to the mass spectrum was just two-thirds of the talk though. Because events occurring in the detector gradually build the mass spectrum up over time, there is always going to a be an inherent bumpiness in the spectrum at any one moment. So the rest of his talk dealt with a full statistical analysis, which demonstrated how likely it was to see such "accidental bumps" in the spectrum and of what size. The human eye has a natural tendency to see patterns even when there is nothing there, so you need an objective analysis like this to tell you if there might be something New there or not.

Whether there are any significant bumps, and other details of the study can be found at Sam's home page for the analysis.


All of this did remind us of other "bumps" in Tevatron data which have been the recent focus of discussion in physicists' blogs and the popular press.

The moral of this story is I think, that when Mother Nature responds to the questions you have asked her, listen carefully, and don't pick and choose from her answers....