[blog post] A sigma here, a sigma there…

11 05 2012

Whenever we come across a new result one of the first things we ask is “How many sigma is it?!” It’s a strange question, and one that deserves a good answer. What is a sigma? How do sigmas get (mis)used? How many sigmas is enough?

The name “sigma” refers to the symbol for the standard deviation, σ. When someone says “It’s a one sigma result!” what they really mean is “If you drew a graph and measured a curve that was one standard deviation away from the underling model then this result would sit on that curve.” Or to use a simple analogy, the height distribution for male adults in the USA is 178cm with a standard deviation of 8cm. If a man measured 170cm tall he would be a one sigma deviation from the norm and we could say that he’s a one sigma effect. As you can probably guess, saying something is a one sigma effect is not very impressive. We need to know a bit more about sigmas before we can say anything meaningful.

A typical distribution of simulated data with a one sigma band drawn.

A typical distribution of simulated data with a one sigma band drawn.

Keep reading…


6 05 2012

Mark took over for me on shift. Here’s his livebog of the experience!

Occasional Musings of a Particle Physicist

10:30pm: Quiet at CERN. Lots of rain outside. Soon I will go out in the rain as the ATLAS control room is on the other side of the road. Hope my swipe card access still works from last year….
Red Bull Count: 1
Espresso Count:…

11pm: Take over from previous shifter, LHC has cryo problems. Wonder if I’ll even see beam in the next 8 hours. Get in touch with trigger on call expert to confirm she does want to be called when the beams are back even if it’s 4am.

11:15pm: Crisis averted. Discovered US->Swiss power adapter left in office with minimal laptop battery remaining, quick dash across the road and all is well again.
Red Bull Count: 1
Espresso Count: 1

11:30pm: When I arrived LHC expected cryo conditions at midnight, this has now changed to 3am 😦

12:10am: ATLAS shift leader thinks there’ll be no collisions during…

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