Atlantis: Visualization Tool in Particle Physics F.J.G.H. Crijns 2 , H. Drevermann 1 , J.G. Drohan 3 , E. Jansen 2 , P.F. Klok 2 , N. Konstantinidis 3 , Z. Maxa 3 , D. Petrusca 1 , G. Taylor 4 , C. Timmermans 2 1) CERN Laboratory, Geneva, Switzerland 2) Radboud University Nijmegen and NIKHEF, Nijmegen, The Netherlands 3) University College London, London, Great Britain 4) University of California, Santa Cruz (CA), USA E-mail: [email protected]Abstract Modern collision experiments in particle physics produce huge amounts of particles during a collision, which makes the analysis of such events extremely complex. A good visualization tool can be used to help the interpretation of events. Besides, the hu- man perception of displayed data must also be con- sidered. By using projections and transformations that are adapted to the specific properties of detector and particles and by using special techniques during dis- play, it is sometimes surprisingly easy to view spe- cific details and do a much better analysis. This article describes special features of a visualiza- tion tool, the event display Atlantis, which has been under development for about twenty years. Cur- rently the program is used for the analysis of sim- ulated data in the ATLAS detector and of experi- mental data from a test set-up, the Combined Test Beam of the ATLAS experiment at the CERN Lab- oratory. 1. Introduction 1.1. Particle Physics In short, particle physics explores what matter is made of and what forces hold it together. The necessary tools for this exploration are accelerators, which accelerate particles to almost the speed of light, and detectors to measure what happens when the particles interact. Currently a new accellerator, the Large Hadron Collider (LHC), which will be the world’s most powerful accellerator, is under construction at the CERN Laboratory in Geneva, Switzerland. It will be placed underground in a ringshaped tunnel with a length of 27 km. LHC has two particle beams that will collide at specific interaction points. Detectors will be placed around those points to take the data of collisions, one of those detectors is the ATLAS detector. The ATLAS collaboration, consisting of over 100 laboratories and over 1000 physicists, de- velops hardware and software for the experiment. Figure 1: The events to be produced by LHC will have thousands of trajectories, all originating from nearly the same point. Which hit belongs to which trajec- tory? Which hit is ”noise” and doesn’t belong to a trajectory? Which trajectories are fake, which trajec- tories are not found by the reconstruction programs? Simulation of collisions by Monte Carlo tech- niques is used to develop and optimize new experi- ments. Hardware and software for accellerator and detectors are developed according to the results. Fi- nally, data are taken and analysed. These steps to- gether may take up to 20 years! And for all those steps the visualization of detector and data is cru- cial. The data that become available via the detector are
Transcript
Atlantis: Visualization Tool in Particle Physics
F.J.G.H. Crijns2, H. Drevermann1, J.G. Drohan3, E. Jansen2, P.F. Klok2,N. Konstantinidis3, Z. Maxa3, D. Petrusca1, G. Taylor4, C. Timmermans2
1) CERN Laboratory, Geneva, Switzerland2) Radboud University Nijmegen and NIKHEF, Nijmegen, The Netherlands
3) University College London, London, Great Britain4) University of California, Santa Cruz (CA), USA
Modern collision experiments in particle physicsproduce huge amounts of particles during a collision,which makes the analysis of such events extremelycomplex. A good visualization tool can be used tohelp the interpretation of events. Besides, the hu-man perception of displayed data must also be con-sidered.By using projections and transformations that areadapted to the specific properties of detector andparticles and by using special techniques during dis-play, it is sometimes surprisingly easy to view spe-cific details and do a much better analysis.This article describes special features of a visualiza-tion tool, the event display Atlantis, which has beenunder development for about twenty years. Cur-rently the program is used for the analysis of sim-ulated data in the ATLAS detector and of experi-mental data from a test set-up, the Combined TestBeam of the ATLAS experiment at the CERN Lab-oratory.
1. Introduction
1.1. Particle Physics
In short, particle physics explores what matteris made of and what forces hold it together. Thenecessary tools for this exploration are accelerators,which accelerate particles to almost the speed oflight, and detectors to measure what happens whenthe particles interact.
Currently a new accellerator, the Large HadronCollider (LHC), which will be the world’s mostpowerful accellerator, is under construction at theCERN Laboratory in Geneva, Switzerland. It will
be placed underground in a ringshaped tunnel witha length of 27 km. LHC has two particle beams thatwill collide at specific interaction points. Detectorswill be placed around those points to take the dataof collisions, one of those detectors is the ATLASdetector. The ATLAS collaboration, consisting ofover 100 laboratories and over 1000 physicists, de-velops hardware and software for the experiment.
Figure 1: The events to be produced by LHC will havethousands of trajectories, all originating from nearlythe same point. Which hit belongs to which trajec-tory? Which hit is ”noise” and doesn’t belong to atrajectory? Which trajectories are fake, which trajec-tories are not found by the reconstruction programs?
Simulation of collisions by Monte Carlo tech-niques is used to develop and optimize new experi-ments. Hardware and software for accellerator anddetectors are developed according to the results. Fi-nally, data are taken and analysed. These steps to-gether may take up to 20 years! And for all thosesteps the visualization of detector and data is cru-cial.The data that become available via the detector are
indirectly measured points on particle trajectories.They are called hits and consist of points or linesin 3D space, depending on the type of subdetectorused. From those data the original trajectories arereconstructed. Reconstruction is a tough job, con-sidering the tens of thousands of hits from a typicalinteraction.
Figure 2: The ATLAS detector. The various detectorcomponents can be seen in this cut away view of thedetector. Only a few green chambers of the MuonSystem are shown on the greyish support structure.(picture by CERN/ATLAS)
Parts of the detector are placed within strongmagnetic fields, which cause the trajectories of elec-trically charged particles to be curved. This curva-ture helps in the computation of the energy andmass of the particle and the reconstruction of thetotal event.The different subdetector systems of the ATLAS de-tector, starting from the collision point, are the In-ner Tracker, the Calorimeter System and the MuonSystem. The different systems detect different typesof particles. The order in which the systems areplaced is guided by the penetration capability of theparticles: lesser penetrating particles are detectedin the Inner Tracker, fully penetrating particles aredetected during traversal of the Muon System.
See references [2], [3] and [4].
1.2. Human perception
Human perception plays an important role inthe visualization of the data. To be able to dis-
tinguish between small objects, only a very limitedset of colours can be used. Also the selection ofthe background is very important when displaying amultitude of graphics elements. E.g., thin white andyellow lines on a dark blue background can hardlybe distinguished.
See reference [5].
2. Atlantis
The development of a specific visualization toolfor the ALEPH experiment at CERN started aboutfifteen years ago by Hans Drevermann. He devel-oped a Fortran version, DALI, that has been usedintensively for ALEPH event data. For the newATLAS experiment, DALI, with the experience ofmany years of use, has been adapted and convertedby Gary Taylor to Atlantis, written in JAVA.
Atlantis is used to display both experiment data,i.e. the hits that are detected by the detector sub-systems, and reconstructed data, i.e. the trajecto-ries of the particles that have been computed fromthe hits.Since the LHC accellerator and the ATLAS detectorare still under construction, “realistic” experimentaldata are generated by using Monte Carlo techniquesand then “polluted” by noise hits. These simulatedexperiment data are used to test the reconstruction,which converts hits to trajectories to full event.Thus Atlantis can be used to check simulation andreconstruction programs and eventually to checkreal events for physics phenomena.
Apart from “standard” operations (zoom, move,rotate, rubberband, pick, select, scale, etc.), At-lantis has a choice of projections and transforma-tions that can be applied to the data. Those pro-jections and transformations make use of the spe-cific properties of detector subsystems and of thecurved trajectories of electrically charged particlesthat cross magnetic fields.
See reference [1].
2.1. Coordinates
Two coordinate systems are used in Atlantis.Cartesian coordinates (X,Y,Z) are used as follows:the Z axis coincides with the beam axis, i.e. thedirection of the incoming particles, the X axis runshorizontally and the Y axis runs vertically.Additionally a (φ, η, ρ) coordinate system
is used with ρ=√
X2 + Y 2, φ=arctan(Y/X),η=arctan(ρ/Z).
2.2. Projections
Some typical projections that are used in At-lantis are described.
An intuitive, orthogonal projection along thebeam axis is given by the Y/X projection, whichprojects data on the plane through X and Y axes.See figure 3.
Figure 3: ATLAS detector and simple event with Y/Xprojection applied. The detector is represented by thecentral Inner Tracker (black), the Calorimeter System(green, red) and the outer Muon System (black andblue). Both hits (white, red) and reconstructed tra-jectories (green, yellow) are shown.
A similar orthogonal projection on the planethrough Y and Z axes, is not very useful. Moreuseful is a special projection, the ρ/Z projection,which projects the planes through the Z axis withvariable angle ρ onto a plane through the Z axiswith fixed angle ρ′.
Calorimeter data can be shown in the φ/η pro-jection as energy deposits of the particles in thecalorimeter layers. See figure 4.
Figure 4: The φ/η projection for calorimeter data: topleft shows the V’s of the V-plot, others show the en-ergy deposits in the successive calorimeter layers withV’s superimposed.
Data from 3D tracking chambers may be shownin a very special projection in φ and η, called theV-plot. For one point in space (with coordinates φ,η, ρ) a pair of points is displayed. In the case of par-ticles moving in a solenoidal magnetic field the twodisplayed points get the same φ as vertical positionand get two different horizontal positions namelyη ± k × (ρmax − ρ). The value of the gradient kis set by default but may be changed interactively.The parameter ρmax is set automatically dependingon the selected view. As k and ρmax are known, φ,η and ρ may be recalculated from the coordinatesof a pair of displayed points, which means that theV-plot is a true 3D image.
Figure 5: Creation of a V-plot: top left shows eventwith Y/X projection applied; top center shows eventwith zooming applied; top right has φ/η projection ap-plied for both hits and reconstructed trajectories andshows rubberband selection; bottom left has φ/η pro-
jection applied for reconstructed trajectories; bottomcenter shows area selected by rubberband with bothhits and reconstructed trajectories; bottom right showsselected area with reconstructed trajectories only.
See reference [5].
2.3. Transformations
Two useful transformations are described, thefisheye and the clock transformation.
Since so many trajectories originate from onesmall center point, it is hard to see the trajectoriesnear the center point. A circular fisheye transforma-tion can be applied, which means that the area nearthe center point is enlarged and the faraway areaof the muon system is shrunken. Circles aroundthe center point are transformed into circles witha different radius. Close to the center the radiusincreases, far from the center the radius decreases.Straight lines through the center remain straight.Note that this transformation, with blown-up InnerTracker and shrunken Muon System, allows to scru-tinize the area around the center point without los-ing sight of how trajectories continue into the MuonSystem. See figure 6.
Figure 6: As figure 3 but with additional fisheye trans-formation applied.
Another transformation that can be used e.g. to“untie” close trajectories, is the clock transforma-tion, an angular fisheye transformation which allows
a selected azimuthal region to be shown in detailwhile still displaying the full 3600.
Figure 7: As figure 3 but with both fisheye and clocktransformation applied.
2.4. Interactions
The canvas that shows the pictures generatedby the visualization tool can be divided in one ormore subwindows. Thus it is possible to have dif-ferent projections visible simultaneously in the sub-windows. Synchro cursors allow the same cursor po-sition to be shown in all projections. By defining apoint in one of the subwindows it can be displayed inthe other subwindows. This concept makes it pos-sible to find the hit or trajectory you located in onesubwindow in the multitude of hits and trajectoriesin another subwindow with a different projection,transformation and zooming applied.
3. Combined Test Beam
Since the construction of a detector in the LHCisn’t a simple job, a lot of testing is required bothin hardware and in software. This is done for detec-tor components and computer programs seperately,but in 2004 a Combined Test Beam of the ATLASsubdetector systems was constructed to be able totest the working of all types of detector componentstogether. Using finished and prototype hardware,a full section of the new detector was built aboveground and tested with various particle beams.
Atlantis was used as the primary visualizationtool for this setup and has already booked some re-markable successes, e.g. detector components thatwere positioned wrongly in the software, connec-tions that were exchanged in the hardware.
Figure 8: Checking the calibration of TRT hits inthe Combined Test Beam: left without calibration,right with calibration. (pictures by Thijs Cornelissen,NIKHEF)
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