Monday, 28 June 2010

Science: the other bit

I've been working in Japan for 2 weeks now and I've spent most of that time working on an experiment. Of that time about 1 day was spent taking data, the rest was spent working on analysis. Analysis is the bit of science that most people forget. People look at the LHC and expect it to run for a while and then spit out an answer. Does the Higgs exist? yes or no. Are there extra-spatial dimensions, how many? Can lepton flavour be violated? maximally or only in rare cases?

These are all questions that the LHC may answer, but not immediately, not even quickly in anything other than a scientific time frame and the reason for this is analysis. Analysis probably accounts for most of the time spent on an experiment. Before the LHC was even finished being built people had been running analysis for years and will run it for many more years once it shuts down. The reason for this is that every last bit of data has to be accounted for.

The experiment I have been working on for the last 2 weeks has been the calibration of a scintillation detector, to do this a radioactive source is placed near the detector and the detectors response is measured. This measurement then produces a plot that looks a little like this:
The big peak on the right is caused by Cesium 137. This peak lies at 662keV,  and is used to calibrate devices because the position of this spectral line can be calculated from what we know about the nuclear make up of cesium.

In this case it would seem to be a fairly simple analysis but it isn't a complete analysis, what about the two smaller peaks to the left of the big 662KeV peak, what is the huge peak near zero? The smaller two are called the backscatter peak and the Compton edge respectively, both are caused by scattering processes that the gamma ray (the source of the big peak) can undergo and ultimately mean not all the energy is accounted for hence the lower energy peaks. This leaves only the big left hand peak, caused by hard x-rays emitted when electrons move in their orbits (de-excite), gamma rays are caused by entire nuclei de-exciting hence their higher energy.

The above paragraph is a reasonable, qualitative analysis of the above graph but by no means is a full, rigorous analysis. For that the position of each peak would have to be calculated (for several other sources as well as cesium), carefully modelled using the expected curve which is then fitted to the data, the parameters produced can then be inspected and checked against the initial assumptions to see that there were no obvious differences from the expected.

The process above is what I did over the last two weeks. Two weeks of analysis for less than 2 days of data taking.

The LHC has orders of magnitude more complexity and a nearly unimaginably large amount of data taken every second (at the raw data rate it produces several thousand times more data than can be written to hard drive using current technology). The analysis of its data has been planned meticulously for the last decade, in fact the entire LHC has been modelled and simulated many times for various different possible physics scenarios (even black holes) over the last decade just so we'll have an idea of how it would look. These simulations have produced analysis procedures that can be applied to the data as soon as it exists. 

Once the initial data analysis is done it may be revisited several times: there are people in my lab working on data taken at the Zeus experiment (an smaller version of the LHC based in Hamburg, Germany). Zeus stopped taking data in 2007, 3 years later it is still being looked at and depending on what gets discovered at the LHC another round of analysis may begin: signals that were too weak to be noticed may be looked for with different techniques or better initial parameters.

Analysis is where the real science happens: experiments just make something to analyse. Already people are analysing the LHC: checking its calibration, looking for early signs of the Higgs. But the data taken now may be revisited a hundred times before it's fully understood, the meaning of ever last wrinkle, dimple and bump fully understood by which time the next set of experiments will be firing, fusing and flickering towards the next data set.

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