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Changeset 113 in svn


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Timestamp:
Dec 28, 2008, 2:18:48 AM (16 years ago)
Author:
Xavier Rouby
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partial update

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trunk/paper
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2 edited

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  • trunk/paper/notes.tex

    r100 r113  
    11\documentclass[a4paper,11pt,oneside,onecolumn]{article}
    2 \usepackage[english]{babel}
     2%\usepackage[english]{babel}
    33\usepackage[ansinew]{inputenc}
    44\usepackage{abstract}
     
    5151a general purpose experiment. The simulation includes a tracking system, embedded into a magnetic field, calorimetry and a muon
    5252system, and possible very forward detectors arranged along the beamline.
    53 The framework is interfaced to standard file format (e.g. Les Houches Event File) and outputs observable analysis data objects, like missing transverse energy and collections of electrons or jets.
    54 The simulation of detector response takes into account the detector resolution, and usual reconstruction algorithms for complex objects, like FastJet. A simplified preselection can also be applied on processed data for trigger emulation. Detection of very forward scattered particles relies on the transport in beamlines with the Hector software. Finally, the FROG 2D/3D event display is used for visualisation of the collision final states.
     53The framework is interfaced to standard file formats (e.g. Les Houches Event File) and outputs observable analysis data objects, like missing transverse energy and collections of electrons or jets.
     54The simulation of detector response takes into account the detector resolution, and usual reconstruction algorithms for complex objects, like \textsc{FastJet}. A simplified preselection can also be applied on processed data for trigger emulation. Detection of very forward scattered particles relies on the transport in beamlines with the \textsc{Hector} software. Finally, the \textsc{Frog} 2D/3D event display is used for visualisation of the collision final states.
    5555An overview of \textsc{Delphes} is given as well as a few use-cases for illustration.
    5656\vspace{1cm}
    5757
    58 \saythanks
     58\noindent
     59\textit{Keywords:} \textsc{Delphes}, fast simulation, LHC, smearing, trigger, \textsc{FastJet}, \textsc{Hector}, \textsc{Frog}
     60\vspace{1cm}
     61
     62%\saythanks
    5963
    6064\section{Introduction}
     
    6468% - 3) permet de comparer
    6569
    66 A fast simulation of a typical \textsc{lhc} multipurpose detector response can be used to obtain more realistic observables and fast approximate estimates of signal and background rates for specific channels. \textsc{Delphes} includes the most crucial detector apects as jet reconstruction, momentum/energy smearing for leptons, photons and hadrons and missing transverse energy. Starting from ``particle-level" information, the package provides reconstructed jets, isolated leptons, photons, reconstructed charged tracks, calorimeter towers and the expected transverse missing energy. Although this kind of approach yields much realistic results than a simple ``parton-level" analysis, a quick simulation comes at the expense of detector details. Therefore, the interactions not simulated in \textsc{Delphes} are: secondary interactions, multiple interactions, photon conversion, electron Bremsstrahlung, magnetic field effects, detector dead materials.\\
    67 
    68 The simulation package proceeds in two stages. The first part is executed on the generated events. ``Particle-level" informations are read from input files and stored in a {\it \textsc{gen}} \textsc{root} tree. Three varieties of input files can currently be used as input in \textsc{Delphes}. In order to process events from many different generators, the standard Monte Carlo event structure StdHep can be used as an input. Besides, \textsc{Delphes} can also provide detector response for events read in "Les Houches Event Format" (\textsc{lhef}) and \textsc{root} files obtained using the {\bf h2root} converter program. This first stage is performed using three C++ classes: {\verb HEPEVTConverter }, {\verb LHEFConverter } and {\verb STDHEPConverter }. Afterwards, \textsc{Delphes} performs a simple trigger simulation and reconstruct "high-level objects". These informations are organised in classes and each objects are ordered with respect to the transverse momentum. The output of the various C++ classes is stored in the {\it Analysis} tree. The program is driven by a datacard (data/DataCardDet.dat) which allow a large spectrum of running conditions by modifying basic detector parameters, including calorimeter and tracking coverage and resolution, thresholds or jet algorithm parameters.\\
    69 
     70Experiments at high energy colliders are very complex systems, in several ways. First, in terms of the various detector subsystems, including tracking, central calorimetry, forward calorimetry, and muon chambers, with different principles, technologies, geometries and sensitivities. Then, due to the requirement of a highly effective online selection (i.e. a \textit{trigger}) which is subdivided into several levels for an optimal reduction factor, but based only on partial data. Finally, in terms of the experiment software with different data format (like \textit{raw} or \textit{reconstructed} data), many reconstruction algorithms and particle identification schemes.
     71
     72This complexity is handled by large collaborations of thousands of people, which restrict the availability of the data, software and documentation to their members. Real data analyses require a full detector simulation, including the various detector inefficiencies, the dead material, the imperfections and the geometrical details. Moreover, detector calibration and alignment are crucial. Such simulation is very complicated, technical and slow. On the other hand, phenomenological studies, looking for the observability of given signals, may require only fast but realistic estimates of the observables.
     73
     74A new framework, called \textsc{Delphes}~\cite{bib:Delphes}, is introduced here, for the fast simulation of a general purpose collider experiment.
     75Using the framework, observables can be estimated for specific signal and background channels, as well as their production and measurement rates, under a set of assumptions.
     76Starting from the output of event generators, the simulation of the detector response takes into account the subdetector resolutions, by smearing the kinematical properties of the visible final particles. Tracks of charged particles and calorimetric towers are then created.
     77
     78\textsc{Delphes} includes the most crucial experimental features, like (1) the geometry of both central or forward detectors; (2) lepton isolation; (3) reconstruction of photons, leptons, jets, $b$-jets, $\tau$-jets and missing transverse energy; (4) trigger emulation and (5) an event display (Fig.~\ref{fig:FlowChart}).
     79
     80\begin{figure}[!h]
     81\begin{center}
     82\includegraphics[width=\columnwidth]{FlowDelphes}
     83\caption{Flow chart describing the principles behind \textsc{Delphes}.}
     84\label{fig:FlowChart}
     85\end{center}
     86\end{figure}
     87
     88Although this kind of approach yields much realistic results than a simple ``parton-level" analysis, a fast simulation comes with some limitations. Detector geometry is idealised, being uniform, symmetric around the beam axis, and having no craks nor dead material. Secondary interactions, multiple scatterings, photon conversion and bremsstrahlung are also neglected.
     89%
     90
     91
     92The simulation package proceeds in two stages. The first part is executed on the generated events. ``Particle-level" informations are read from input files and stored in a {\it \textsc{gen}} \textsc{root} tree. Three varieties of input files can currently be used as input in \textsc{Delphes}. In order to process events from many different generators, the standard Monte Carlo event structure StdHep can be used as an input. Besides, \textsc{Delphes} can also provide detector response for events read in "Les Houches Event Format" (\textsc{lhef}) and \textsc{root} files obtained using the {\bf h2root} converter program. This first stage is performed using three C++ classes: {\verb HEPEVTConverter }, {\verb LHEFConverter } and {\verb STDHEPConverter }.
     93Afterwards, \textsc{Delphes} performs a simple trigger simulation and reconstruct "high-level objects". These informations are organised in classes and each objects are ordered with respect to the transverse momentum. The output of the various C++ classes is stored in the {\it Analysis} tree. The program is driven by a datacard (data/DataCardDet.dat) which allow a large spectrum of running conditions by modifying basic detector parameters, including calorimeter and tracking coverage and resolution, thresholds or jet algorithm parameters.\\
     94
     95Usual algorithms are applied for the reconstruction of complex objects, like the missing transverse energy or the jets originating from $b$ quarks or $\tau$ leptons.
     96
     97A simplified preselection can also be applied on processed data for trigger emulation.
     98All detectors are assumed to be symmetric with respect to the beam axis
    7099\section{Central detector simulation}
    71100
    72101\begin{figure}[!h]
    73102\begin{center}
    74 \includegraphics[width=\columnwidth]{detectorAng.eps}
    75 \caption{\small{detectorAng.eps}}
     103\includegraphics[width=\columnwidth]{Detector_Delphes_1}
     104\caption{Layout of the generic detector geometry assumed in \textsc{Delphes}. The innermost layer, close to the interaction point, is a ce
     105ntral tracking system (pink). It is surrounded by a central calorimeter volume (green) with both electromagnetic and hadronic sections. Th
     106e outer layer of the central system (red) consist of a muon system. In addition, two end-cap calorimeters (blue) extend the pseudorapidity
     107 coverage of the central detector. The actual detector granularity and extension is defined in the user-configuration card. The detector i
     108s assumed to be strictly symmetric around the beam axis (black line). Additional forward detectors are not depicted.}
    76109\label{fig:GenDet}
    77110\end{center}
    78111\end{figure}
    79112
    80 The overall layout of the general purpose detector simulated by \textsc{Delphes} is shown in figure \ref{fig:GenDet}. A central tracking system surrounded by an electromagnetic (\textsc{ecal}) and a hadron calorimeter (\textsc{hcal}). A forward calorimeter ensure a larger geometric coverage for the measurement of the missing transverse energy. The fast simulation of the detector response takes into account geometrical acceptance of sub-detectors and their finite energy resolution. No smearing is applied on particle direction.\\
     113\begin{figure}[!h]
     114\begin{center}
     115\includegraphics[width=0.5\columnwidth]{Detector_Delphes_3}
     116\caption{Profile of the layout assumed in \textsc{Delphes}. The extension of the various subdetectors, as defined in Tab.~\ref{tab:defEta}
     117, are clearly visible. Same colour codes as for Fig.~\ref{fig:GenDet} are applied. Additional forward detectors are not depicted.}
     118\label{fig:GenDet3}
     119\end{center}
     120\end{figure}
     121
     122\begin{figure}[!h]
     123\begin{center}
     124\includegraphics[width=0.6\columnwidth]{Detector_Delphes_2b}
     125\caption{Layout of the generic detector geometry assumed in \textsc{Delphes}. Open 3D-view of the detector with solid volumes. Same colour codes as for Fig.~\ref{fig:GenDet} are applied. Additional forward detectors are not depicted.}
     126\label{fig:GenDet2}
     127\end{center}
     128\end{figure}
     129
     130
     131The overall layout of the general purpose detector simulated by \textsc{Delphes} is shown in figure \ref{fig:GenDet}. A central tracking system surrounded by an electromagnetic and a hadron calorimeters. A forward calorimeter ensures a larger geometric coverage for the measurement of the missing transverse energy. The fast simulation of the detector response takes into account geometrical acceptance of sub-detectors and their finite resolution.
     132
     133\textcolor{red}{No smearing is applied on particle direction. (???)}\\
    81134
    82135Before starting to loop over events, the {\verb RESOLution } class loads all sub-detector resolutions and coverage from the detector parameter file. If no such file is provided, predifined values are used. The coverage of the various sub-systems used in the default configuration are summarized in table \ref{tab:defEta}.
     
    173226\subsection{Jet reconstruction}
    174227
    175 Jets are reconstructed using a cone algorithm with $R=0.7$ and make only use of the smeared particle momenta. The reconstructed jets are required to have a transverse momentum above 20~GeV and $|\eta|<3.0$. A jet is tagged as $b$-jets if its direction lies in the acceptance of the tracker, $|\eta|<0.5$, and if it is associated to a parent $b$-quark. A $b$-tagging efficiency of $40\%$ is assumed if the jet has a parent $b$ quark. For $c$-jets and light/gluon jets, a fake b-tagging efficiency of $10 \%$ and $1 \%$ respectively is assumed.\\
    176 
    177 \subsection{{\it b}tagging}
    178 
    179 The simulation of the b-tagging is based on the detector efficiencies assumed (1) for the tagging of a b-jet and (2) for the mis-identification of other jets as b-jets. This relies on the TAGGING\_B, MISTAGGING\_C and MISTAGGING\_L constants, for (respectively) the efficiency of tagging of a b-jet, the efficiency of mistagging a c-jet as a b-jet, and the efficiency of mistatting a light jet (u,d,s,g) as a b-jet. The (mis)tagging relies on the particle ID of the most energetic particle within a cone around the observed (eta,phi) region, with a radius CONERADIUS.
     228Jets are reconstructed using a cone algorithm with $R=0.7$ and make only use of the smeared particle momenta. The reconstructed jets are required to have a transverse momentum above 20~GeV and $|\eta|<3.0$. A jet is tagged as $b$-jets if its direction lies in the acceptance of the tracker, $|\eta|<0.5$, and if it is associated to a parent $b$-quark. A $b$-tagging efficiency of $40\%$ is assumed if the jet has a parent $b$ quark. For $c$-jets and light/gluon jets, a fake $b$-tagging efficiency of $10 \%$ and $1 \%$ respectively is assumed.\\
     229
     230\subsection{$b$-tagging}
     231
     232The simulation of the $b$-tagging is based on the detector efficiencies assumed (1) for the tagging of a $b$-jet and (2) for the mis-identification of other jets as $b$-jets. This relies on the TAGGING\_B, MISTAGGING\_C and MISTAGGING\_L constants, for (respectively) the efficiency of tagging of a $b$-jet, the efficiency of mistagging a c-jet as a $b$-jet, and the efficiency of mistatting a light jet (u,d,s,g) as a $b$-jet. The (mis)tagging relies on the particle ID of the most energetic particle within a cone around the observed (eta,phi) region, with a radius CONERADIUS.
    180233
    181234\subsection{Tau identification}
     
    236289\section{Visualisation}
    237290
     291
     292As an illustration, an associated photoproduction of a $W$ boson and a $t$ quark is shown in Fig.~\ref{fig:wt}. This corresponds to a $pp \rightarrow Wt \ +  \ p  \ + \ X$ process, where the $Wt$ couple is induced by an incoming photon emitted by one interacting proton. This leading proton survives from the photon emission and subsequently from the $pp$ interaction, and is present in the final state. The experimental signature is a lack of hadronic activity in one forward hemisphere, where the surviving proton escapes. The $t$ quark decays into a $W$ and a $b$. Both $W$ bosons decay into leptons ($W \rightarrow \mu \nu_\mu$ and $W \rightarrow \tau \nu_\tau$).
     293
     294\begin{figure}[!h]
     295\begin{center}
     296\includegraphics[width=\columnwidth]{Events_Delphes_1}
     297\caption{Example of $pp(\gamma p \rightarrow Wt)pY$ event. One $W$ boson decays into a $\mu \ \nu_\mu$ pair and the second one into a $\tau \ \nu_\tau$ pair. The surviving proton leaves a forward hemisphere with no hadronic activity. The isolated muon is shown as the blue vector. The $\tau$-jet is the cone around the green vector, while the reconstructed missing energy is shown in gray. One jet is visible in one forward region, along the beamline axis, opposite to the direction of the escaping proton.}
     298\label{fig:wt}
     299\end{center}
     300\end{figure}
     301
     302
    238303\section{Conclusion and perspectives}
    239304
     
    245310\section{User manual}
    246311
    247 The available code is a tar file which comes with everything you need to run the DELPHES package. Nevertheless in order to visualise the events with the FROG program, you need to install libraries as explained in {\it href="http://projects.hepforge.org/frog/}
     312The available code is a tar file which comes with everything you need to run the \textsc{Delphes} package. Nevertheless in order to visualise the events with the \textsc{Frog} program, you need to install libraries as explained in {\it href="http://projects.hepforge.org/frog/}
    248313
    249314\subsection{Getting started}
    250315
    251 In order to run DELPHES on your system, first download is sources and compile it:\\
     316In order to run \textsc{Delphes} on your system, first download is sources and compile it:\\
    252317\begin{quote}
    253318\begin{verbatim}
     
    261326
    262327
    263 \subsection{Running Delphes on your events}
     328\subsection{Running \textsc{Delphes} on your events}
    264329
    265330\subsubsection{Setting the run configuration}
     
    268333{\b The run card }\\
    269334
    270 Contains all needed information to run DELPHES
     335Contains all needed information to run \textsc{Delphes}
    271336\begin{itemize}
    272337 
    273338\item The following parameters are available: detector parameters, including calorimeter and tracking coverage and resolution, transverse energy thresholds allowed for reconstructed objects, jet algorithm to use as well as jet parameters.
    274339 
    275 \item Four flags, {\verb FLAG_bfield }, {\verb FLAG_vfd }, {\verb FLAG_trigger } and {\verb FLAG_frog } should be assigned to decide if the magnetic field propagation, the very forward detectors acceptance, the trigger selection and the preparation for FROG display respectively are running by DELPHES.
     340\item Four flags, {\verb FLAG_bfield }, {\verb FLAG_vfd }, {\verb FLAG_trigger } and {\verb FLAG_frog } should be assigned to decide if the magnetic field propagation, the very forward detectors acceptance, the trigger selection and the preparation for \textsc{Frog} display respectively are running by \textsc{Delphes}.
    276341 
    277342\item An example (the default detector card) can be found in {\verb files/DataCardDet.dat }
     
    300365\subsubsection{Running the code}
    301366Create the above cards (data/mydetector.dat and data/mytrigger.dat)
    302 Create a text file containing the list of input files that will be used by DELPHES (with extension *.lhe, *.root or *.hep)
     367Create a text file containing the list of input files that will be used by \textsc{Delphes} (with extension *.lhe, *.root or *.hep)
    303368To run the code, type the following
    304369\begin{quote}
     
    309374
    310375
    311 \subsection{Running an analysis on your Delphes events}
    312 
    313 Two examples of codes running on the output root file of DELPHES are coming with the package
     376\subsection{Running an analysis on your \textsc{Delphes} events}
     377
     378Two examples of codes running on the output root file of \textsc{Delphes} are coming with the package
    314379\begin{enumerate}
    315 \item The {\verb Examples/Analysis_Ex.cpp } code shows how to access the available reconstructed objects and the trigger information The two following arguments are required: a text file containing the input DELPHES root files to run, and the name of the output root file. To run the code:
     380\item The {\verb Examples/Analysis_Ex.cpp } code shows how to access the available reconstructed objects and the trigger information The two following arguments are required: a text file containing the input \textsc{Delphes} root files to run, and the name of the output root file. To run the code:
    316381  \begin{quote}
    317382\begin{verbatim}
     
    320385  \end{quote}
    321386 
    322 \item The {\verb Examples/Trigger_Only.cpp } code permits to run the trigger selection separately from the general detector simulation on output DELPHES root files. An input DELPHES root file is mandatory as argument. The new tree containing the trigger information will be added in these file. The trigger datacard is also necessary. To run the code:
     387\item The {\verb Examples/Trigger_Only.cpp } code permits to run the trigger selection separately from the general detector simulation on output \textsc{Delphes} root files. An input \textsc{Delphes} root file is mandatory as argument. The new tree containing the trigger information will be added in these file. The trigger datacard is also necessary. To run the code:
    323388  \begin{quote}
    324389\begin{verbatim}
     
    329394\end{enumerate}
    330395
    331 \subsection{Running the FROG event display}
     396\subsection{Running the \textsc{Frog} event display}
    332397
    333398\begin{itemize}
    334 \item If the { \verb FLAG_frog } was switched on, two files were created during the run of DELPHES: {\verb DelphesToFrog.vis } and {\verb DelphesToFrog.geom }. They contain all the needed information to run frog.
    335 \item To display the events and the geometry, you first need to compile FROG. Go to the {\verb Utilities/FROG } and type {\verb make }.
     399\item If the { \verb FLAG_frog } was switched on, two files were created during the run of \textsc{Delphes}: {\verb DelphesToFrog.vis } and {\verb DelphesToFrog.geom }. They contain all the needed information to run frog.
     400\item To display the events and the geometry, you first need to compile \textsc{Frog}. Go to the {\verb Utilities/FROG } and type {\verb make }.
    336401\item Go back into the main directory and type {\verb ./Utilities/FROG/frog }.
    337402\end{itemize}
     
    339404\begin{thebibliography}{99}
    340405 
    341 \bibitem{Delphes} \textsc{Delphes}, hepforge:
     406\bibitem{bib:Delphes} \textsc{Delphes}, hepforge:
     407\bibitem{bib:FastJet} \textsc{Fast-Jet},
     408\bibitem{bib:Frog} \textsc{Frog},
    342409\end{thebibliography}
    343410
     
    352419CONERADIUS !
    353420
     421 in other words, the effect related to the particle showers that would happen in the calorimeters are not taken into account. We took the hypothesis that stable particles interacting electromagneticaly deposit their energies in the ECAL calorimeter and that the hadrons just interact with the HCAL
     422
    354423\end{document}
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