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In the analysis, kaons, pions and protons are identified through their
specific energy loss in the main TPC. The yields are extracted
by fitting distributions of quality-selected tracks, with a
multiple-gaussian fit. Because energy loss is a function of total momentum
only, the choice was made to do the fits in bins of total momentum
and transverse momentum . The yields are then corrected for
acceptance and efficiency (including kaon decay) and a coordinate
transformation is made to a rectangular grid in rapidity and
. To obtain rapidity spectra, the measured spectra are
integrated by summing the measured yields in the interval where a
reliable measurement is available and extrapolating to high (and in
some cases low) .
Technically, the process is divided into the following steps:

- Fill histograms from ROOT micro-DST
- Fit histograms to extract raw yield
- Calculate correction for acceptance and decay
- Calculate reconstruction efficiency correction
- Apply corrections and do transformation
- Make final plots

The most intricate step is the actual fitting and I will spend some
time explaining the considerations that went into this. In terms of
processing power, probably each of the four first steps are equal. Note
also that the correction for acceptance, decay and efficiency is split
into two parts for technical reasons. The reconstruction efficiency is
calculated from embedding, which is inefficient when efficiencies
are low, so the acceptance and decay correction is calculated
separately from a larger statistics sample of pure simulation.
Not mentioned explicitly is the calculation of systematic
uncertainties. These are calculated by varying some parameters of the
fit that are not fitted to the data in every bin separately (see
Section 3.3.4). In practice, this means that the fitting
step is carried out multiple times and that the final steps include
systematic error propagation.

** Next:** Analysis steps
** Up:** A practical guide to
** Previous:** Introduction/scope
Marco van Leeuwen
2009-01-14