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 \section{Spin Sorting\label{sec:spindb}} Each of the bunches in the two RHIC beams is polarized independently, and the polarization pattern can change from fill to fill. An accurate record of the spin state of each bunch crossing in a fill is essential for any spin analysis. The polarization pattern for the rings is set by the RHIC Collider-Accelerator Department (C-AD) and broadcast through the CDEV \cite{Barton:2003sh} control and monitoring system. The pattern is formatted as a list of 360 8 bit numbers, one for each of the time buckets in RHIC, and is defined in terms of the bunch crossings at the 12 o'clock position in the RHIC ring. The beam experiences an odd number of spin flips in between 12 o'clock and the STAR IP at six o'clock, so the beam polarizations at STAR are flipped relative to the broadcast record. The interpretation of each bit is given in Table \ref{tbl:spin8}. \begin{table} \begin{center} % \begin{ruledtabular} \begin{tabular}{c|l} \hline bit & meaning \\ \hline \hline 0 & yellow beam filled\\ \hline 1 & yellow beam polarized up\\ \hline 2 & yellow beam polarized down\\ \hline 3 & yellow beam unpolarized\\ \hline 4 & blue beam filled\\ \hline 5 & blue beam polarized up\\ \hline 6 & blue beam polarized down\\ \hline 7 & blue beam unpolarized\\ \hline \end{tabular} % \end{ruledtabular} \end{center} \caption{Significance of each of the bits in an eight bit spin record.} \label{tbl:spin8} \end{table} A given bunch from the Blue ring always collides with the same bunch from the Yellow ring at a specific interaction point in the RHIC ring over the course of a fill. C-AD has historically configured the beams so that bunch zero from the Blue beam collides with bunch zero from the Yellow beam at the PHENIX IP, and as a result the PHENIX experiment sees only one abort gap in its fill patterns. The situation is different at STAR, where the RHIC toggle mode'' defines the pairs of bunches from the Blue and Yellow beams that collide. The toggle mode is typically set once at the beginning of the RHIC data-taking period, and is encoded implicitly in the spin patterns broadcast by CDEV. After accounting for the spin flip and the toggle mode, an analysis must map the event IDs recorded at the experiment to the 7 bit bunch crossing IDs defined by CDEV in order to determine the spin state of a given event. The STAR Trigger receives this information from RHIC for every event, but analyses have observed occasional inaccuracies in the feed that render it unreliable. Instead, STAR uses a more robust 48 bit counter synchronized to the 9.4 MHz RHIC clock to uniquely identify every event. The 7 bit bunch crossing IDs can be expressed in terms of this 48 bit counter as % \begin{equation} \mathrm{7~bit~ID} = \left(\mathrm{48~bit~counter} + \mathrm{offset}\right)~mod~120 \end{equation} % The offset is calculated for each STAR run by generating 120 histograms of triggers versus the 7 bit bunch IDs for each possible value of the offset, comparing these histograms to a histogram of the intended spin pattern at STAR, and searching for a minimum in the $\chi^2$ distribution. An example of the overlap between the trigger rate versus bunch crossing ID and the spin pattern after the correct offset has been applied is shown in Figure \ref{fig:bxing-offset}. The offsets for each STAR run are calculated once and uploaded to the STAR Calibrations DB which allows them to be applied by all STAR spin analyses. Further details of the spin state determination can be found in Reference \cite{spin-db-website}. \begin{figure} \includegraphics[width=1.0\textwidth]{figures/bxing-offset-determined} \caption{Distribution of events (blue) versus the corrected 7 bit bunch crossing IDs at the STAR interaction point. The yellow bars indicate the bunch crossings where both beams have filled bunches according to the intended spin patterns broadcast by CDEV.} \label{fig:bxing-offset} \end{figure}