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\subsection{Hits and clusters}
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A track passing through a particular doublet layer produces
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scintillation light in one or at most two fibre channels.
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For each channel ``hit'', the tracker data aquisition system records
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the channel number, $n$, and the pulse height.
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After calibration, the pulse height is recorded in terms of the number
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of photo-electrons ($n_{\rm pe}$) generated in the Visible Light
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Photon Counter (VLPC) illuminated by the hit channel.
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Occassionally, showers of particles or noise can cause three or more
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neighbouring channels to be hit.
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The term ``clusters'' is used to refer to an isolated hit, a doublet
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cluster and a multi-hit cluster.
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The position of a hit in the doublet-layer coordinate system may be
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determined from the channel number.
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For isolated hits, the measured coordinate $\alpha \in {u, v, w}$ is
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\alpha = c_p (n - n_0)\,;
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where $n_0$ is the channel number of the central fibre and $c_p$ is
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the channel pitch given by:
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where $f_d$ is the fibre diameter ($f_d = 350\,\mu{\rm m}$) and $f_p =
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$ is the fibre pitch ($f_p = 427\,\mu{\rm m}$ see figure
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For clusters in which two channels are hit (``doublet clusters'', see
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figure \ref{Fig:Clust}), the measured coordinate is given by:
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\alpha = c_p \left[ \frac{( n_1 + n_2)}{2} - n_0 \right]\,;
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where $n_1$ and $n_2$ are the channel numbers of the two hit fibres.
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For a multi-hit cluster (clusters with more than two neighbouring
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channels), the measured position is determined from the pulse-height
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weighted mean of the fibre positions:
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\frac{\sum_i n_{{\rm pe}i}n_i}{\sum_i n_{{\rm pe}i}}
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where the subscript $i$ indicates the $i^{\rm th}$ channel.
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The pulse-height for doublet and multi-channel clusters is determined
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by summing the pulse height of all the hits that make up the cluster.
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\includegraphics[width=0.9\linewidth]%
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{04-Reconstruction/04-01-Hits-and-clusters/Figures/clusterRES.eps}
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Channel overlap as simulated in G4MICE; fine-tuning reduces the
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error associated to doublet clusters.
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The ``measurement vector'', ${\bf m}$ is defined as:
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where $\alpha$ is given above and, in the absense of additional
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information, $\beta = 0$.
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The corresponding covariance matrix is given by:
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\underline{\underline{V_m}} =
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\sigma^2_\alpha & 0 \\
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where $\sigma^2_\alpha$ and $\sigma^2_\beta$ are the variance of
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$\alpha$ and $\beta$ respectively.
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The variance on $\alpha$ for a single-hit cluster is given by:
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\sigma^2_m = \frac{c^2_p}{12} \, .
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For a doublet-cluster, the variance is given by:
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\sigma^2_m = \frac{\Delta^2_\alpha}{12} \, ;
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where $\Delta_\alpha = ?$ is the length of the overlap region between
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neighbouring fibre channels (see figure \ref{Fig:Clust}).
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For multihit clusters, the variance is given by
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\sigma^2_m = \frac{??}{??} \, .
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The variance of the perpendicular coordinate, $\beta$, depends on the
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effective length, $l_{\rm eff}$ of the fibre (see figure ?? and
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Appendix ??) and is given by:
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\sigma^2_\beta = \frac{l^2_{\rm eff}}{12} \, ;
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l_{\rm eff} = ?? \; .