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| 1 : | xella | 1.1 | % |
| 2 : | %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% | ||
| 3 : | % SiTrack description | ||
| 4 : | %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% | ||
| 5 : | % | ||
| 6 : | \subsection{SiTrack} | ||
| 7 : | The SiTrack LVL2 algorithm adopts a combinatorial pattern recognition | ||
| 8 : | approach to reconstruct tracks starting from SPs formed in the ID | ||
| 9 : | silicon detectors. | ||
| 10 : | |||
| 11 : | \par In order to perform SP combinations, these are first of all grouped into sets from which | ||
| 12 : | the entries of each combination will be then extracted; the grouping is implemented in SiTrack, | ||
| 13 : | using the idea of ``logical layers''. These correspond to a list of physical detector layers, | ||
| 14 : | i.e. barrel layers and end-cap disks, and are labeled with increasing numbers moving away from | ||
| 15 : | the beam line. The same physical layer can be included in more logical layers, to increase the | ||
| 16 : | robustness of the track finding process. To provide a tangible example, the first logical layer | ||
| 17 : | adopted for the reconstruction of high $p_T$ isolated leptons includes the innermost two pixel | ||
| 18 : | barrel layers and the innermost pixel end-cap disk. | ||
| 19 : | |||
| 20 : | \par Once the SP have been associated to the logical layers they belong to, the track reconstruction | ||
| 21 : | algorithm proceeds through the following five steps: | ||
| 22 : | \begin{itemize} | ||
| 23 : | \item track seeds formation; | ||
| 24 : | \item optional primary vertex reconstruction along the beam line; | ||
| 25 : | \item track seeds extension; | ||
| 26 : | \item extended seeds merging; | ||
| 27 : | \item clone removal. | ||
| 28 : | \end{itemize} | ||
| 29 : | The formation of track seeds corresponds to a combinatorial pairing of SP coming from the | ||
| 30 : | innermost two logical layers. For each seed, the extrapolation to the beam line is evaluated, | ||
| 31 : | using a straight line approximation; this process is depicted in Fig. \ref{fig_sitrack_rphi}. | ||
| 32 : | \begin{figure}[htb] | ||
| 33 : | \begin{center} | ||
| 34 : | \ifpdf | ||
| 35 : | \includegraphics[width=0.4\textwidth]{figures/lvl2_sitrack_rphi12.pdf} | ||
| 36 : | \includegraphics[width=0.4\textwidth]{figures/lvl2_sitrack_rphi23.pdf} | ||
| 37 : | \else | ||
| 38 : | \includegraphics[width=0.4\textwidth]{figures/lvl2_sitrack_rphi12.eps} | ||
| 39 : | \includegraphics[width=0.4\textwidth]{figures/lvl2_sitrack_rphi23.eps} | ||
| 40 : | \fi | ||
| 41 : | \end{center} | ||
| 42 : | \caption{\label{fig_sitrack_rphi} Pictorial scheme of the SiTrack combinatorial strategy for | ||
| 43 : | track seeds formation (left) and track seeds extension (right).} | ||
| 44 : | \end{figure} | ||
| 45 : | At this point a cut on the transverse impact parameter is applied. This cut, meant to reduce | ||
| 46 : | the number of seeds to be further processed, is particularly important, as it fixes the lowest | ||
| 47 : | reconstructible track $p_T$ value. | ||
| 48 : | |||
| 49 : | \par The subsequent step is the reconstruction of the position of the primary interaction vertex | ||
| 50 : | along the beam line, used to reject tracks not coming from the primary interaction. The vertex | ||
| 51 : | reconstruction is performed filling a histogram with the longitudinal impact parameter of the | ||
| 52 : | seeds and searching for histogram maxima; more vertex candidates can be retained and seeds not | ||
| 53 : | pointing to any of the reconstructed vertexes are discarded. This step is optional and, | ||
| 54 : | as an example, is used for jet tracks reconstruction, while it is skipped in case of low | ||
| 55 : | multiplicity event topologies, e.g. for the reconstruction of single isolated leptons. | ||
| 56 : | Each retained seed is extended, as depicted in Fig. \ref{fig_sitrack_rphi}, extrapolating it to | ||
| 57 : | the outer logical layers and forming one or more SP triplets for each seed; extensions are | ||
| 58 : | selected applying a cut on the distance between the outer SP and the extrapolated seed. Each | ||
| 59 : | extended seed is then fitted by a straight line in the longitudinal plane and parametrized as a | ||
| 60 : | circle in the transverse plane. | ||
| 61 : | |||
| 62 : | \par At this point, all the extensions found for each seed must be merged into a single full track, | ||
| 63 : | grouping the triplets having similar track parameters after the fit. The full track is thus | ||
| 64 : | defined as formed by the union of the SP from all the merged extensions. All the triples not | ||
| 65 : | involved in the merging process are discarded, while track parameters are re-evaluated for the | ||
| 66 : | full track, fitting it with a straight line in the longitudinal plane and a circle in the | ||
| 67 : | transverse one. | ||
| 68 : | |||
| 69 : | \par Two full tracks obtained from different track seeds may still share most of their SP; these | ||
| 70 : | tracks are defined as clones. To eliminate these ambiguous cases, only the clone track containing | ||
| 71 : | the largest number of SP is retained; in case more clone tracks contain the same number of space | ||
| 72 : | points, the one with the lowest $\chi^2$ value prevails. The retained full tracks are finally | ||
| 73 : | refit using one of the available common fit tool. | ||
| 74 : |
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