--- /dev/null
+
+
+\section{The EMCal HLT online chain}
+
+The EMCal L0 or L1 hardware trigger decisions provide the input
+for a dedicated on-line event processing chain running on the HLT cluster,
+where further refinement based on criteria using the full event reconstruction
+information is performed.
+In fact, the detector optical link transports the raw data to the
+Read-Out Receiver Card (RORC) in the local data collector of the data acquisition system,
+which sends a complete copy of the readout
+to a set of specialized nodes in the HLT cluster (FEP or Front End Processors).
+Each FEP node is equipped with RORC cards in analogy to
+the collector nodes used by the data acquisition.
+The FEP nodes are physically linked to the detector hardware and reflect the geometrical
+partitioning of each ALICE sub-system.
+The 10 full-size super-modules are read out using 2 Read-Out Control Units (RCUs) for a total
+of 20 optical links running into the HLT FEPs.
+The reduced-size super-modules were installed prior to the 2012 LHC run and are not discussed in the present report.
+In addition to the 20 links from the super-module readout, the HLT receives also a copy of the L0/L1 trigger data stream
+via an additional optical link from the EMCal jet trigger unit (STU) data collector.
+The different stages of data processing are then performed by the
+software analysis chain executed on the HLT cluster:
+a set of general purpose nodes (Computing Nodes or CNs)
+perform the higher level operations on the data streams which have been
+already pre-processed on the FEPs at the lower level.
+The EMCal software components form a specialized sub-chain
+executed at run time together with all other ALICE sub-systems
+participating in the HLT event reconstruction.
+
+\begin{figure}[ht]
+\begin{center}
+\includegraphics[width=23pc]{figures/chain-new-eps-converted-to.pdf}
+\caption{\label{f1} Functional diagram of the EMCal online reconstruction components (signal processing, data structure makers, and clusterizers) shown in green.
+The EMCal chain is fed by the detector raw data. Trigger components are shown in red. EMCal-specific triggers operate on the calorimeter clusters
+and perform TPC track-matching when needed (electron and jet triggers). Monitoring components are shown in blue and live in a separate monitoring chain.
+The EMCal triggers are evaluated within the Global Trigger which is aware of the full HLT trigger logic of the other ALICE detectors.
+}
+\end{center}
+\end{figure}
+
+The functional units of the EMCal HLT online chain are presented in Figure \ref{f1} where
+the online reconstruction, monitoring, and trigger components
+%with their relevant classes
+are shown together with their relevant data paths.
+The lower-level EMCal online component ({\it RawAnalyzer}) is fed by the detector front end electronics
+and performs signal amplitude and timing information extraction.
+Intermediate components ({\it DigitMaker}) use this information to
+build the digitized data structures needed for the clusterizer components to operate on the cell signals.
+Alternatively, the digitized signals can be generated via monte carlo simulations ({\it DigitHandler}).
+
+At the top of the EMCal reconstruction chain, the digits are summed by the {\it Clusterizer} component to produce the cluster data structures.
+The calorimeter clusters are then used to generate the different kinds of EMCal HLT trigger information:
+a single shower trigger ($\gamma$) with no track matching, an electron trigger using the matching with a corresponding TPC track,
+and a jet trigger also using the TPC tracks information and the V0 multiplicity dependent threshold.
+
+The trigger logic generated by the EMCal chain is evaluated
+(together with the outputs of the HLT trigger components coming from other ALICE detectors)
+within the HLT Global Trigger which produces the final high level decision based on the reconstructed event.
+The ALICE data acquisition system will then discard, accept or tag the event according to the HLT decision.
+
+For performance and stability reasons, the full on-line HLT chain contains only analysis
+and trigger components. On the other hand, monitoring components typically make
+heavy use of histogramming packages and ESD objects, hence they are kept in a separate chain.
+The isolation of the monitoring from the reconstruction chain gives additional robustness since
+a crash in a monitoring component will not affect the reconstruction chain and the data taking.
+
+\subsection{Reconstruction components}
+
+As shown in Figure \ref{f1} the EMCal HLT analysis chain provides all the
+necessary components to allow the formation of a trigger
+decision based on full event reconstruction. The following
+subsections are devoted to a detailed discussion of each processing
+stage, starting from the most basic, i.e. signal extraction, to
+the highest stage: the HLT trigger decision.
+
+\subsubsection{RawAnalyzer}
+The {\it RawAnalyzer} component extracts energy and timing information for each calorimeter cell.
+Extraction methods implemented in the offline code (AliRoot) typically
+use least squares fitting algorithms, and cannot be used in online processing for
+performance reasons.
+Conversely, the HLT signal extraction is done without need of fitting
+using two possible extraction methods. The first method, referred to as {\it kCrude},
+simply produces an amplitude using the difference between the maximum and the minimum values
+of the digitized time samples and associates the time bin of the maximum as the signal arrival time.
+The {\it kCrude} method was used during the 2011 data taking: it has the advantage
+of being extremely fast and fully robust since no complex algorithms are used. On the other hand,
+it produces a less accurate result than the processing of the full signal shape.
+An alternative method ({\it kPeakFinder}) evaluates the amplitude
+and peak position as a weighted sum of the digitized samples. This approach is
+not as fast as {\it kCrude} but is a few hundred times faster than least squares fitting.
+
+\subsubsection{DigitMaker}
+The {\it DigitMaker} component essentially transforms the raw cell signal amplitudes
+produced by the {\it RawAnalyzer} into digit structures by processing the cell coordinates
+and by the application of dead channel maps and the appropriate gain factors (low and high-gain).
+
+\subsubsection{Clusterizer}
+The {\it Clusterizer} component merges individual signals (digits) of adjacent cells
+into structures called clusters.
+At transverse momenta $p_T>1$~GeV/c most of the clusters are associated to electromagnetic
+showers in EMCal from $\pi^0$ and $\eta$ mesons decays.
+Other sources of electromagnetic showers are direct photons
+and electrons from semi-leptonic decays of $c$ and $b$ hadrons.
+Since the typical cluster size in the EMCal can vary according to the detector occupancy due to shower
+overlap effects, which are much different for {\it pp} and heavy-ion collisions,
+clustering algorithms with and without a cutoff on the shower size are available (both in offline and in the HLT)
+to optimize the cluster reconstruction for the different cases.
+Events originating from {\it pp} collisions tends to generate
+smaller, spherical and well-separated clusters in the EMCal, at least up to 10 GeV/c.
+At higher transverse momenta, overlapping of the showers requires a shape analysis
+to extract the single shower energy.
+Above 30 GeV/c the reconstruction can be performed only with more sophisticated
+algorithms such as isolation cuts to identify direct photons.
+
+
+\begin{figure}[ht]
+\begin{minipage}{16pc}
+%\includegraphics[width=18pc]{clus-comp.eps}
+\includegraphics[width=16pc]{figures/emcal_cluster_match_nxn-eps-converted-to.pdf}
+\caption{\label{f5} Reconstruction efficiency for the $N\times N$ algorithm (cutoff) in offline and HLT. The notation $(A) \rightarrow (B)$ indicates
+the fraction of clusters found using method A that are also found using method B (data from run 154787, period LHC11c).}
+\end{minipage}\hspace{2pc}
+\begin{minipage}{16pc}
+\includegraphics[width=16pc]{figures/emcal_cluster_match_v1-eps-converted-to.pdf}
+\caption{\label{f6}Reconstruction efficiency for the $V1$ algorithms (no cutoff) in offline and HLT. The notation $(A) \rightarrow (B)$ indicates
+the fraction of clusters found using method A that are also found using method B (data from run 154787, period LHC11c).}
+\end{minipage}
+\end{figure}
+
+The identification of an isolated single electromagnetic cluster in the EMCal can be performed using different
+strategies: summing up all the neighboring cells around a seed-cell over threshold until no more cells are found
+or adding up cells around the seed until the number of clustered cells reaches the predefined cutoff value.
+
+The first approach is more suitable for an accurate reconstruction. A further improvement to this
+clustering algorithm would be the ability to unfold overlapping clusters as generated from the
+photonic decay of high-energy neutral mesons, however this procedure usually requires computing intensive fitting algorithms.
+
+Such performance penalty must be avoided in the online reconstruction so the cutoff
+technique is preferred. In the EMCal HLT reconstruction a cutoff of 9 cells is used (according to the
+geometrical granularity of the single cell size), so the clusterization is performed into a
+square of $3\times3$ cells. The cutoff and non-cutoff algorithms are referred to as $N\times N$ and $V1$, respectively.
+
+In {\it pp} collisions the response of the two methods is very similar since the majority of clusters are well separated, while
+in {\it PbPb} collisions, especially in central events, the high particle multiplicity requires the use of the cutoff (or unfolding in offline)
+to disentangle the cluster signals from the the underlying event to avoid the generation of artificially large clusters.
+
+The quality of the EMCal online clusterizer algorithms implemented in the HLT chain were checked against offline,
+as shown in Figures \ref{f5} and \ref{f6} where it can be seen that the performance is in a reasonable agreement in all cases.
+The low point at 1.25 GeV is due to bad towers, which are assigned an energy of 1 GeV.
+%The HLT clusterizer does not have the capability to remove bad clusters.
+Bad clusters are removed in later stages of the analysis, but that is not yet reflected in Figures \ref{f5} and \ref{f6}.
+This effect leads to an excess of clusters that are found by the HLT clusterizer, but not by the offline clusterizer.\\
+
+Since the EMCal HLT reconstruction is mainly targeted for triggering, a small penalty in the accuracy of the energy reconstruction of the clusters is accepted
+as a trade off in favor of faster performance, and for this reason the cutoff clustering method was used, especially in {\it PbPb} collisions.
+
+\subsection{Trigger components}
+
+The online HLT chain is capable of producing trigger decisions based on full
+event reconstruction. In terms of EMCal event rejection the following relevant trigger observables
+have been implemented:
+
+\begin{itemize}
+\item neutral cluster trigger
+\item electron and jet trigger
+\end{itemize}
+
+
+\subsubsection{Cluster trigger}
+The single shower triggering mode is primarily targeted to trigger on photons and neutral mesons.
+In all collision systems, the high level trigger post-filtering can improve
+the hardware L0 and L1 trigger response by using the current bad channels map information
+and calibration factors (which could be recomputed directly in the HLT).
+
+\subsubsection{Electron trigger}
+For this trigger the cluster information reconstructed online by the EMCal HLT analysis
+chain is combined with the central barrel tracking information to produce complex event selection
+as a single electron trigger (matching of one extrapolated track with an EMCal cluster.
+% TCA - this shouldn't be in this section, if it is included...
+%, and requiring a match between the cluster energy and the track momentum) and a full jet trigger
+%(matching of multiple tracks with a jet patch in the EMCal).
+Performance and accuracy studies of the track matching component developed for this purpose
+have been done using simulated and real data taken during the 2011 LHC running period.
+Results are shown in Figures \ref{f7} and \ref{f8} where the cluster - track residuals
+in azimuth and pseudo-rapidity units are to be compared with a calorimeter cell size of
+$0.014\times 0.014$.
+
+\begin{figure}[hb]
+\begin{minipage}{16pc}
+\includegraphics[width=15.5pc]{figures/Fig4dPhi_performance-eps-converted-to.pdf}
+\caption{\label{f7}
+Distribution of the residuals in azimuth ($\Delta\phi$) for the EMCal cluster and central barrel tracks
+obtained using the HLT online chain for run 154787 (LHC11c), ~ 70 k events reconstructed.
+}
+\end{minipage}\hspace{2pc}
+\begin{minipage}{16pc}
+\includegraphics[width=16pc]{figures/Fig5dEta_performance-eps-converted-to.pdf}
+\caption{\label{f8}
+Distribution of the residuals in pseudo-rapidity ($\Delta\eta$) for the EMCal cluster and central barrel tracks
+obtained using the HLT online chain for run 154787 (LHC11c), ~ 70 k events reconstructed.
+}
+\end{minipage}
+\end{figure}
+
+
+In addition to the extrapolation of the track from the central barrel
+to the EMCal interaction plane and the matching with a compatible nearby cluster,
+the electron trigger component must finally perform particle identification
+to issue a trigger decision. The selection of electron candidates is done
+using the $E/pc$ information where the energy is measured from the
+EMCal cluster and the momentum from the central barrel track.
+The trigger component is initialized with default values
+for the cut of $0.8< E/pc <1.3$. The default cuts are stored in the HLT
+conditions database and can be overridden via command line arguments
+at configuration time (usually at start of run).
+
+The performance of the electron trigger was studied using {\it pp} minimum
+bias data at 7 TeV with embedded $J/\Psi$ events.
+Figure \ref{f9} shows the good agreement of the $E/pc$ distributions
+obtained with the track extrapolation - cluster matching
+performed using the online algorithms compared to the ESD-based tracking (red).
+
+\begin{figure}[ht]
+\includegraphics[width=24pc]{figures/Fig6HLTEoverP_performance-eps-converted-to.pdf}
+\begin{center}
+\caption{\label{f9}
+$E/pc$ distributions obtained with the track extrapolation - cluster matching
+via the online algorithms compared to the ESD-based tracking (red).}
+\end{center}
+\end{figure}
+
+To determine the possible improvement of the event selection
+for electrons with energies above 1~GeV, AliRoot simulations of the HLT chain using LHC11b10a {\it pp} minimum bias data
+at 2.76 GeV and the EMCal full geometry (10 super-modules) have been used. These studies
+have shown that at least a factor 5 to 10 in event selection can be gained compared to the single shower trigger, as shown in Figure \ref{f10}.
+
+\begin{figure}[ht]
+\includegraphics[width=24pc]{figures/Fig7Events_performance-eps-converted-to.pdf}
+\begin{center}
+\caption{\label{f10}
+Improvement in the event selection for $E_{e^-}>$~1~GeV from AliRoot simulation (anchor to LHC11b10a) with minimum bias {\it pp} at $\sqrt{s}=2.76$~TeV (EMCal full geometry).
+The red points are obtained with the requirement of one hit in one of the silicon pixel (SPD) layers to reject a higher fraction of photon conversions.
+}
+\end{center}
+\end{figure}
+
+
+\subsubsection{Jet trigger}
+
+The EMCal online jet trigger component was developed to provide
+an unbiased jet sample by refining the hardware L1 trigger decisions.
+In fact, the HLT post-processing can produce a sharper turn on curve
+using the track matching capabilities of the online reconstruction chain.
+In addition, a more accurate definition of the jet area than the one provided by the hardware L1 jet patch,
+can be obtained choosing a jet cone based on the jet direction calculated online.
+The combination of the hadronic and electromagnetic energy provides a measurement of
+the total energy of the jet by matching the tracks identified as part of the jet with
+the corresponding EMCal neutral energy.
+
+The use of the HLT jet trigger also allows a better characterization
+of the trigger response as a function of the centrality dependent threshold
+by re-processing the information from the V0 detector directly in HLT.
+
+Performance considerations, due to the high particle multiplicity
+in {\it PbPb} collisions, impose that the track extrapolation is done only geometrically
+without taking into account multiple scattering effects
+introduced by the material budget in front of the EMCal.
+The pure geometrical extrapolation accounts for a speedup factor of 20 in the
+execution of the track matcher component with respect to the
+full-fledged track extrapolation used in {\it pp} collisions.
+
+The identification of the jet tracks is performed using the anti-$k_T$
+jet finder provided by the FastJet package.
+
+The EMCal jet trigger was only partially tested during the 2011 data taking period
+and will be fully commissioned for the LHC {\it pPb} run period in 2012.
+
+\subsection{Monitoring components}
+
+The role of the EMCal HLT reconstruction in {\it pp} collisions is targeted mainly on
+the monitoring functions since the expected event sizes are small enough for
+the complete collision event to be fully transferred to permanent storage.
+\begin{figure}[h]
+\includegraphics[width=26pc]{figures/all-monitor-eps-converted-to.pdf}
+\begin{center}
+\caption{\label{f2} Output from the EMCal HLT monitoring component. Top left: cluster energy spectra as a function of the reconstructed cluster energy;
+bottom left: cluster position in $\eta$ and $\phi$ coordinates; bottom right: cluster time distribution; top right: number of cells per cluster vs cluster
+energy. LHC11b period, $\sqrt{s}=7$ TeV {\it pp} data, 10~kEvent analyzed.}
+\end{center}
+\end{figure}
+
+In this respect, two monitoring components have been developed and deployed in the online chain.
+The first component currently monitors reconstructed quantities, such as the cluster energy spectra and timing,
+the cluster position in the $\eta$ and $\phi$ coordinates, and the number of cells per cluster
+as a function of the cluster reconstructed energy as shown in Figure \ref{f2}.
+
+\begin{figure}[h]
+\includegraphics[width=24pc]{figures/l0clusters-eps-converted-to.pdf}
+\begin{center}
+\caption{\label{f3} Energy spectrum for all clusters reconstructed by the EMCal (black points) superposed with the triggered cluster spectrum
+(i.e. clusters reconstructed which also carry the L0 hardware trigger bit set, red points). }
+\end{center}
+\end{figure}
+
+The second component re-evaluates the EMCal hardware trigger decisions by recalculating
+the cluster energy spectrum for all the clusters with the L0 trigger bit set as shown
+in Figure \ref{f3}. The L0 turn on curve can then be calculated online as the ratio between
+the triggered and the reconstructed cluster spectra and monitored for the specific run.
+
+
+No recalculation of hardware L1 trigger primitives was possible
+during the 2011 data taking since the optical link from the EMCal L1 trigger unit
+could only installed during the 2011-2012 winter shutdown of the LHC hence
+the software development for the L1 trigger monitoring is still underway.
+
%through three steps: \textbf{simulation}, \textbf{reconstruction}
%and \textbf{analysis}. This steps are explained in the next sections.
-This document is addressed to those who want to work with
-EMCal software and the different tasks needed to have the data taken ready to be analyzed. It is divided in 2 blocks: a first one with the description of the procedures needed cook the data and a second one with the reconstruction and simulation offline code.
+This document is addressed to those who want to work with the
+EMCal software. It explains the different steps to have the data taken ready to be analyzed. It is divided in 2 blocks: a first one with the description of the procedures needed to cook the data and a second one with the reconstruction and simulation offline code.
-For a fast introduction on the code and how it works you can have a look to the EMCal for beginners guide \cite{EMCAL:beginners}. Some other interesting references are the AliRoot primer\cite{ALIROOT:doc} , the offline AliRoot page \cite{ALIROOT}, and the installation page from Dario Berzano \cite{ALIROOT:berzano}
+For a fast introduction on the code and how it works you can have a look to the EMCal for beginners guide \cite{EMCAL:beginners}. Some other interesting references are the AliRoot primer~\cite{ALIROOT:doc}, the offline AliRoot page \cite{ALIROOT}, and the installation page from Dario Berzano \cite{ALIROOT:berzano}.
-\subsection{Mechanical description - Federico}
+\subsection{Mechanical description of the EMCAL - Federico}
+The chosen technology is a layered Pb-scintillator sampling calorimeter with a longitudinal pitch of 1.44 mm Pb and 1.76 mm scintillator with longitudinal
+Wavelength Shifting Fiber (WLS) light collection. The full detector spans $\eta$ = -0.7 to $\eta$= 0.7 with an azimuthal acceptance of $\Delta\phi~107^\circ$
+and is segmented into 12,288 towers, each of which is approximately projective in $\eta$ and $\phi$ to the interaction vertex.
+The towers are grouped into super modules of two types: full size which span $\Delta\phi=20^\circ$ and 1/3 size which span $\Delta\phi$ = 6.67$^\circ$.
+There are 10 full size and 2, 1/3-size super modules in the full detector acceptance (Fig. \ref{fig:emcal-full}).
-\subsection{How EMCal works - Terry}
+\begin{figure}[ht]
+\begin{center}
+\includegraphics[width=1.0\textwidth]{figures/emcal-full.jpg}
+\end{center}
+\caption{\label{fig:emcal-full} Azimutha view from the A-side (opposite to the di-muon arm) of the full EMCal as installed into the ALICE detector.
+The two 1/3-size super-modules are visible at ~9 o'clock position.}
+\end{figure}
-EMCAL basic units are cells/towers (Pb-scintillator sandwich of about 70 layers). We have 12 SuperModules (4 in 2010, 10 in 2011-2012) composed of 24 (phi direction) x 48 (eta direction) cells (except last 2 SuperModules made of 8 cells in phi direction). Particles traversing the calorimeter, in particular photons and electrons, will deposit energy in different towers. The EMCAL reconstruction measures such energy per tower, forms clusters of cells produced by a given particle, and if possible matches them with particles detected by the tracking detectors in front of EMCAL (charged particles).
+
+The super module is the basic structural units of the calorimeter.
+These are the units handled as the detector is moved below ground and rigged during installation.
+
+
+Fig. \ref{fig:emc-sm}
+%shows a super module with its external mechanical structure stripped away to illustrate the stacking of modules within the super module.
+shows a full size super module with $12\times24$ modules configured as 24 strip modules of 12 modules each.
+The supporting mechanical structure of the super module hides the stacking into a nearly projective geometry which can be inferres by the differnt tilt
+of the strip modules going from the left to the right part of the picure. The electronics integration pathways are also visible.
+Each full size super module is assembled from $12 \times 24$ = 288 modules arranged in 24 strip modules of 12 modules each.
+
+\begin{figure}[ht]
+\begin{center}
+\includegraphics[width=1.0\textwidth]{figures/emc-sm.png}
+\end{center}
+\caption{\label{fig:emc-sm} View of one EMCal super-module during the installation into the ALICE detector. The cradle holds the 24 strip modules into a mechanically rigid unit.
+Each strip module holds 12 unit modules. On the right side the two electronics crates are visible. }
+\end{figure}
+
+Each module has a rectangular cross section in the $\phi$ direction and a trapezoidal cross section in the $\eta$ direction with a full taper of 1.5$^\circ$.
+The resultant assembly of stacked strip modules is approximately projective with an average angle of incidence of less than 2$^\circ$ in $\eta$ and
+less than 5$^\circ$ in $\phi$. An assembled strip module is shown in Fig. \ref{fig:strip}.
+
+\begin{figure}[ht]
+\begin{center}
+\includegraphics[width=0.5\textwidth]{figures/strip-module.jpg}
+\end{center}
+\caption{\label{fig:strip} View of a fully assembled strip module. The photo shows the APD+CSP package and copper shielding monunted light guide fixture. On the right part
+of the photo the LED UV optical fiber distribution system is visible. Each strip module, is cabled via 3 T-Cards visible in the center of the assembly.}
+\end{figure}
+
+
+
+The smallest building block of the calorimeter is the individual module illustrated in Fig. \ref{fig:module}
+Each individual module contains $2\times 2$ = 4 towers built up from 77 alternating layers of 1.44 mm Pb and 1.76 mm polystyrene,
+injection molded scintillator. White, acid free, bond paper serves as a diffuse reflector on the scintillator
+surfaces while the scintillator edges are treated with TiO2 loaded reflector to provide tower to tower optical
+isolation and improve the transverse optical uniformity within a single tower.
+The Pb-scintillator stack in a module is secured in place by the static friction between individual
+layers under the overall load of ~350 kg. The module is closed by a skin of 150 $\mu$m thick stainless
+steel screwed by flanges on all four transverse surfaces to corresponding front and rear aluminum plates.
+This thin stainless skin is the only inert material between the active tower volumes.
+The internal pressure in the module is stabilized against thermal effects,
+mechanical relaxation and long term flow of the Pb and/or polystyrene by a customized
+array of 5 non-linear spring sets (Bellville washers) per module.
+In this way, each module is a self supporting unit with a stable mechanical lifetime of
+more than 20 years when held from its back surface in any orientation as when mounted in a strip module.
+
+\begin{figure}[ht]
+\begin{center}
+\includegraphics[width=0.5\textwidth]{figures/module1.png}
+\end{center}
+\caption{\label{fig:module} The first 1.5$^\circ$ tapered module of the EMCal generation II prototype produced in EU shown.
+The module's internal compression is mantained by a set of 5 Bellville washers (non linear springs) acting between the top and bottom containment
+Al plates to prevent the delamination of the internal Pb-scintillator sandwich.
+}
+\end{figure}
+
+All modules in the calorimeter are mechanically and dimensionally identical. The front face dimensions of the
+towers are $6\times6$ $cm^2$ resulting in individual tower acceptance of $\Delta\eta\times\Delta\phi=0.014\times0.014$ at $\eta$=0.
+The EMCal design incorporates a moderate detector average active
+volume density of ~5.68 g/cm$^3$ which results from a 1:1.22 Pb to scintillator ratio by volume.
+This results in a compact detector consistent with the EMCal integration volume at the chosen detector thickness of 20.1 radiation lengths.
+%Practical considerations, including the total assembly labor cost,
+%suggest reducing the total number of Pb/scintillator layers thus decreasing the sampling frequency.
+
+As described above, the super module is the basic building block of the calorimeter.
+Starting with 288 individual modules which are rather compact and heavy, the main engineering task is to create a super module structure which is rigid,
+with small deflections in any orientation yet does not require extensive, heavy external stiffening components that would reduce the volume available for the active detector.
+The solution adopted for the ALICE EMCal is to develop a super module crate which functions not as a box for the individual modules
+but rather an integrated structure in which the individual elements contribute to the overall stiffness.
+The super module crate is effectively a large I-beam in which the flanges are the long sides of the crate and the 24 rows of strip modules together.
+This configuration gives to the super module good stiffness for both the 9 o'clock and 10 o'clock locations. For the 12 o'clock location,
+the I-beam structure of the super module is augmented by a 1 mm thick stainless steel forward sheet (traction loaded),
+which controls the bending moment tending to open the crate main sides, and helps to limit deflection of strip modules.
+Ridges are provided on the interior surfaces of the crate to allow precision alignment of the strip modules at the correct angle.
+The stiffness given by this I-beam concept allows the use of non-magnetic light alloys for main parts of the super module crate.
+Parts of the super module crate will be made mainly from laminated 2024 aluminum alloy plates.
+The two main sides (flanges of the I-beam) of the crate will be assembled from 2 plates, 25 mm and 25 mm thick,
+bolted together and arranged so as to approximately follow the taper of the 20 degree sector boundary. Each of the 24 rows of a super module contain 12 modules as described above.
+Each of the modules is attached to a transverse beam by 3.4 mm diameter stainless steel screws.
+The 12 modules and the transverse beam form a strip module. The strip module is 1440 mm long, 120 mm wide, 410 mm thick.
+The total weight of the strip module is approximately 300 kg and like module, it is a self supporting unit.
+The transverse beam, which is the structural part of the strip module, is made from cast aluminum alloy
+with individual cavities along its length where the fibers emerging from towers are allowed to converge.
+The casting process is well suited to forming these cavities and the overall structure, saving considerable raw material and machining time.
+
+In addition to functioning as a convenient structural unit which offers no interference with the active volume of the detector
+and forming the web of the I-beam structure of the super module, the transverse beam of the strip module provides protection for the fibers,
+a structural mount for the light guide, APD and charge sensitive preamplifier and a light tight enclosure for these elements.
+
+
+\subsection{Functional description of the EMCAL - Terry}
+
+**** need some additional info on PE APDs *********
+%EMCAL basic units are cells/towers (Pb-scintillator sandwich of about 70 layers). We have 12 SuperModules (4 in 2010, 10 in 2011-2012) composed of 24 (phi direction) x 48 (eta direction) cells (except last 2 SuperModules made of 8 cells in phi direction).
+
+Particles traversing the calorimeter, in particular photons and electrons, will deposit energy in different towers.
+The EMCAL reconstruction measures such energy per tower, forms clusters of cells produced by a given particle,
+and if possible matches them with particles detected by the tracking detectors in front of EMCAL (charged particles).
+
+Scintillation photons produced in each tower are captured by an array of 36 Kuraray,
+Y-11, double clad, WLS fibers that run longitudinally through the Pb/scintillator stack.
+Each fiber terminates in an aluminized mirror at the front face end of the module and is integrated into a polished,
+circular group of 36 at the photo sensor end at the back of the module.
+The fiber bundles are pre-fabricated and inserted into the towers after the module mechanical assembly is completed.
+The 36 individual fibers are packed into a circular array 6.8 mm in diameter and held in place inside a custom injection
+molded grommet by Bicron BC-600 optical cement. An optical quality finish is applied to the assembled bundle using a diamond polishing machine.
+At the other end of the bundle, individual fibers are similarly polished and mirrored with a sputtered coat
+of aluminum thick enough to ensure the protection of the inner mirror.
+The response of the Al-coated fiber is considerably flatter with an overall increase in efficiency in the range of about 25\% in the vicinity of shower maximum
+(i.e. the location of the highest energy deposition for an electromagnetic shower).
+This number accounts for material immediately in front of the detector; which ranges between 0.4 and 0.8 radiation lengths,
+and assumes 5.5 - 6.0 radiation lengths for shower maximum for 10 GeV photons.
+At this depth in the detector, the mirrored fiber response is very uniform does not contribute to the non-linearity of the detector as a whole.
+
+Other factors which can significantly impact the electromagnetic performance of the calorimeter,
+include scintillator edge treatment and the density of the wavelength shifting fiber readout pattern and the material
+chosen for the interlayer diffuse reflector. For scintillator edge treatment and fiber density, advantage was taken from the
+extensive studies made by the LHCb collaboration for their ECAL. In particular, a diffuse reflector edge treatment was adopted,
+such as that obtained with Bicron Titanium Dioxide loaded white paint (BC622A) with a total fiber density of about one fiber per $cm^2$.
+In the case of the interlayer diffuse reflector, a white, acid free, bond paper was used in place of the Teflon based commercial TYVEK.
+While TYVEK produces slightly better surface reflectivity, its coefficient of friction is too low to permit its use in this
+design where the module's mechanical stability depends somewhat on the interlayer friction.
+
+The 6.8 mm diameter fiber bundle from a given tower connects to the APD through a short light guide/diffuser
+with a square cross section of 7 mm $\times$ 7 mm that tapers slowly down to 4.5 mm $\times$ 4.5 mm as it mates (glued)
+to the 5 mm $\times$ 5 mm active area of the photo sensor.
+The 4 pre-fabricated fiber bundles are inserted into the
+towers of a single module.
+
+The selected photo sensor is the Hamamatsu S8664-55 Avalanche Photo Diode ************
+
+This photodiode has a peak spectral response at a wavelength of 585 nm compared to an emission peak of 476 nm for the Y-11 fibers.
+However, both the spectral response and the quantum efficiency of the APD are quite broad with the latter dropping from the maximum by only ~5\% at the WLS fiber emission peak.
+At this wavelength, the manufacturer's specification gives a quantum efficiency of 80\%.
git://git.uio.no / u / mrichter / AliRoot.git / commitdiff