[atlas] / groups / Trigger / TriggerCSC / Tracking / SiTrack.tex Repository: Repository Listing atlas

# Annotation of /groups/Trigger/TriggerCSC/Tracking/SiTrack.tex

 1 : xella 1.1 % 2 : %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 3 : % SiTrack description 4 : %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 5 : % 6 : \subsection{SiTrack} 7 : The SiTrack LVL2 algorithm adopts a combinatorial pattern recognition 8 : fparodi 1.2 approach to reconstruct tracks starting from space points formed in the ID 9 : xella 1.1 silicon detectors. 10 : 11 : fparodi 1.2 \par In order to perform space point combinations, these are first of all grouped into sets from which 12 : xella 1.1 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 : fparodi 1.2 \par Once the space point have been associated to the logical layers they belong to, the track reconstruction 21 : xella 1.1 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 : fparodi 1.2 The formation of track seeds corresponds to a combinatorial pairing of space point coming from the 30 : xella 1.1 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 : fparodi 1.2 the outer logical layers and forming one or more space point triplets for each seed; extensions are 58 : selected applying a cut on the distance between the outer space point and the extrapolated seed. Each 59 : xella 1.1 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 : fparodi 1.2 defined as formed by the union of the space point from all the merged extensions. All the triples not 65 : xella 1.1 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 : fparodi 1.2 \par Two full tracks obtained from different track seeds may still share most of their space point; these 70 : xella 1.1 tracks are defined as clones. To eliminate these ambiguous cases, only the clone track containing 71 : fparodi 1.2 the largest number of space point is retained; in case more clone tracks contain the same number of space 72 : xella 1.1 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 :