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\subsection{Space-point reconstruction}
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This section describes the space-point reconstruction, the algebra by
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which the cluster positions are translated in to tracker coordinates
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and, to some extent, the algorithm.
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\subsubsection{Selection of clusters that form the space-point}
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For each particle event, the clusters found within each doublet layer
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are ordered by fibre-channel number.
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Taking each station in turn, an attempt is made to generate a space
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point using all possible combinations of clusters.
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ThetThree clusters, one each from views $u$, $v$ and $w$, that make up
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a space point satisfy:
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n^u + n^v + n^w = n^u_0 + n^v_0 + n^w_0 \, ;
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where $n^u$, $n^v$ and $n^w$ are the fibre numbers of the clusters in
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the $u$, $v$ and $w$ views respectively and $n^u_0$, $n^v_0$ and
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$n^w_0$ are the respective central-fibre numbers (see Appendix
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A triplet space point is selected if:
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| (n^u + n^v + n^w) - (n^u_0 + n^v_0 + n^w_0) | < K \, .
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Once all triplet space-points have been found, doublet space-points
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are created from pairs of clusters from different views.
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\subsubsection{Crossing-position calculation}
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\paragraph{Doublet space-points}
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The position of the doublet space-point in station coordinates,
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${\bf r}_s$, is given by:
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{\bf r}_s & = & \left(
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& = & \underline{\underline{R}}_{SD1} {\bf m}_1
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& = & \underline{\underline{R}}_{SD2} {\bf m}_2
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where the measurement vector corresponding to the $i^{\rm th}$
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and the rotation matrix $\underline{\underline{R}}_{SDi}$ are defined
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in section \ref{HtsClstrs}.
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The simultaneous equations \ref{Eq:DSP1} and \ref{Eq:DSP2} contain two
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unkowns, $\beta_1$ and $\beta_2$.
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Equations \ref{Eq:DSP1} and \ref{Eq:DSP2} may be rewritten:
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{\bf m}_1 = \underline{\underline{R}}_{SD1}^{-1}
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\underline{\underline{R}}_{SD2} {\bf m}_2 \, .
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\underline{\underline{S}} & = & \underline{\underline{R}}_{SD1}^{-1}
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\underline{\underline{R}}_{SD2} \\
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equations \ref{Eq:DSP1} and \ref{Eq:DSP2} may be solved to yield:
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\beta_2 & = & \frac{\alpha_1 - s_{11} \alpha_2}{s_{12}} \\
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\beta_1 & = & s_{21} \alpha_2 + s_{22} \beta_2 \, .
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The position of the space-point may now be obtained from equation
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\ref{Eq:DSP1} or \ref{Eq:DSP2}.
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\paragraph{Triplet space-points}
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As shown in figure \ref{Fig:SenseArea}, the fibres layout is of one of
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In one case (right panel of figure \ref{Fig:SenseArea}), the centre of
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the channels, one in each of the three views, cross intersect at a
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In this case, the position of the crossing can be calculated as
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described in seection \ref{Para:DblSpPnt}.
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When the area of overlap of the three channels forms a triangle
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(figure \ref{Fig:SenseArea} left panel), the centre of area of overlap
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\bar{x} & = & \frac{2}{3}c_p \, {\rm ; and} \\