20 July 2009

Signal to Noise ratio

This post is from my Summer Research summary which can be found here. Its long and probably a bit boring to many of you, but if you are interested in how I've been spending my time, check it out. I am only publishing this because I am proud of the analogy about "Where's Waldo" and wanted to share that with a broader audience. I had to post all of it, because the analogy alone wouldn't make any sense without some background.

Any time I hear this phrase (Signal to Noise Ratio), I always think of the guy who once came to my house to work on my cable service. He told me they can sit in their truck and measure the amount of background noise that is leaking from bad connections or improperly insulated wires. They even once told me the signal was turned up/amplified too much which was causing my On Demand problems. (it generally wouldn't work and kept giving some sort of error code)

In particle physics, when you talk about signal to noise, you are not too far away from this same idea. During particle collisions, there are processes that occur in which we are not really interested. This is called background. That doesn't mean its not important, it simply means for the particular process at which you are looking, its just not something you want to observe. You would like for your detector to measure this background so that you can then calculate a value for it and subtract it from the actual signal. If you tune out all of the background, you won't get an accurate depiction of the event in which you are interested. The signal is a value which is predicted (theoretically) by the Standard Model and can be verified (with a particle accelerator) experimentally. If you know the value of your signal, and by know I mean verify a theoretical prediction experimentally, you can then go on to look for new physics above and beyond the energy level at which you are working.

While driving from Oklahoma City to Stillwater, I had an epiphany on how to explain the concept of "signal to noise ratio". Think about the popular children's books and games called "Where's Waldo?" Remember those? (try it by double clicking the picture to find Waldo) You stare at a picture looking for a goofy-faced kid who is wearing a red and white striped sweater with a similarly-colored knit cap. You look and look and look until finally he pops out of the background, plainly obvious and you wonder "why didn't I seem him sooner?" The key is the red and white striped sweater. If not for that, it would be nearly impossible for you to see Waldo. He would blend into the background.This is especially true as you advance to harder and harder levels of the game. There are more and more people in the picture, therefore Waldo is harder and harder to spot.

Studying the Z boson, as we are, is the "putting on of the sweater". We are painting a better picture of what the signal, the actual Z boson looks like. When we advance to the next level of the game, i.e. searching for the Higgs, we will have a better understanding of what the background looks like so scientists may then look at whats left and determine whether there is evidence for the Higgs or not. If not, the Standard Model will have to be revised.

Let me know what you think and as usual, thanks for reading.


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