The International Linear Collider is the future international particle accelerator. It will create high–energy particle collisions between electrons and positrons, their antimatter counterparts. It will be a great instrument for a deeper understanding of the fundamental interactions in the nature.
In 1999, a group of physicists from LPC joined the Linear Collider effort. They participated to the evaluation of the huge physics potential of the machine as well as to the preliminary design of an adequate detector (Tesla TDR).
Currently, the team is part of the CALICE and EUDET collaborations in charge of the various detector R & D.
We are involved in the Very Front End (VFE) electronics of the SiW calorimeter. We participate equally to the developpement of the software and reconstruction algorithms, especially in the LDC detector concept framework. Our favourite benchmark, both for full reconstruction and detector optimisation is the challenging measurement of the trilinear Higgs self–coupling.
CALICE, CAlorimeter for the LInear Collider Experiment, is a collaboration devoted to a high granularity calorimeter for the International Linear Collider. It consists of 200 physicists/engineers from 36 institutes and 10 countries, distributed over three regions (America, Asia and Europe).
The calorimeter is optimised for the Particle Flow measurement of the multi–jet final states for a center–of–mass energy of the machine ranging from 90 GeV to 1 TeV.
The CALICE Collaboration is actively developing the PFA calorimetry for ILC through simulation and algorithms studies, electronics R & D, detector prototype building and testing in test beam campaigns.
EUDET, Detector R & D towards the International Linear Collider, is a project supported by the European Union in the framework of the sixth Programme structuring the European Research Area. The project comprises 31 European partner institutes from 12 different countries working in the field of High Energy Physics.
THe EUDET programme offers an infrastructure supportting the european detector R & D effort for the International Linear Collider: test beam infrastructure, with access to the DESY and CERN testbeam facilities, infrastructure for the Tracking Detectors, calorimeter infrastructure, detector R & D network.
The group is involved in both collaborations (CALICE and EUDET) as far as the electromagnetic calorimetry aspects are concerned:
This work is also done in synergy with the software development for the detector optimization.
The group activities are the following
The International Linear Collider will address a wide scope of particle physics topics. As a short summary, the following ones may be quoted
The LPC physicists are part of the "Higgs boson physics" working group and concentrate mainly on the measurement of the Higgs self–coupling.
In the framework of the standard model, the generation of mass occurs through the Higgs mechanism.
The measurement of the trilinear self–coupling leads to am independent determination of the Higgs potential shape and offers a direct experimental test of the Higgs mechanism hypothesis.
The signal consists of a double Higgs–strahlung and
WW fusion as follows.
Less than 100 signal events are expected in one year of data taking, while background processes are producing millions of events.
The feasibility study for the measurement of the Higgs self–coupling has already been performed for a centre–of–mass energy of 500 GeV. The study pointed out the importance of both calorimeter and vertex detectors.
A new analysis has been performed at higher centre–of–mass energy of 800 GeV, where the WW–fusion process is more important.
The Research and Development in electronics is devoted to the treatment of analogous signals delivered by the sensors and to their digital shaping.
In the current design, a single, mixed analog / digital chip addresses both tasks.
The particle reconstruction and identification imposes a high granularity calorimeter. Consequently, the proposed SiW calorimeter has 50 million sensors.
The requirements for the first stage of the electronics (Very Front End) are ambitious: large dynamics range, low consumption, high integration to fit in a volume as small as possible and no cooling.
To solve these challenges, a chip is jointly designed by the LPC and LAL (Orsay) electronics engineers specialized in analog systems. The LPC effort is concentrated on the shaper and on the ADC. The LPC electronics engineers specialised in digital electronics are in charge of a testbench conception. This is a first step towards the elaboration of the complete design of the link between VFE and the data acquisition system.
The simplified scheme of a chip for a Very Front End dedicated to the SiW calorimeter is displayed below. It indicates the rough architecture and the basic blocks needed.
Synopsis of the VFE chip.
The ADC is requested to have a very low consumption (25mW/36 channels). AMS Cmos 0.35 µm technology is very commonly employed by the car industry and will be supplied in the future. The characteristics of this technology (thicker and exclusively with CMOS transistors) should improve the chip integration and consumption.
Various development of the Very Front End for the electromagnetic calorimeter have been batched in Austriamicrosystems fondery (0,35µm CMOS). Some layout are displayed below.
|Common layout with LAL-Orsay|
An alternate version based on a "ADC with slope" has been developed. Although this version does not fulfill all the requirements, it lead to an operational chip used for the testing of the EUDET prototype with beam.
In the meanwhile several ADC pipeline designs have been developped and 10-bit pipeline ADC prototypes have been produced in a 0.35micro CMOS technology. This ADC is a building block of the very-frontend electronics dedicated to the electromagnetic calorimeter. Based on a 1.5-bit per stage resolution architecture, it reaches the 10-bit accuracy at a sampling rate of 4MSamples/s with a consumption of 35mW. The Integral and Differential Non-Linearity obtained are respectively within pm 1LSB and pm 0.6 LSB, and the measured noise is 0.47 LSB r.m.s. The performance obtained confirms that the pipeline architecture ADC is suitable to the requirements of the readout electronics of the electromagnetic calorimeter.
Views of the Active Sensor Unit (ASU). One side there are the wafer glued on, a wafer consists in 6 X 6 silicon diodes of 1 cm2 each. The dedicated aera to welcome the embeded very-front-end chips is visible on the other side.
A test bench has been conceived and implemented in order to evaluate the performance of the chips. This is a mandatory feature for both the tracking and understanding the sources of problems and also for editing the performance sheets. This work, at the frontier of the analogical and numerical electronics, was realised by the service of electronics from LPC. It uses the know-how acquired through the developments for the LHC experiments.
A highly granular electromagnetic calorimeter prototype based on tungsten absorber and sampling units equipped with silicon pads as sensitive devices for signal collection has been constructed. The full prototype will have in total 30 layers and be read out by about 10000 Si cells of 1 × 1 cm2. A module consisting of 14 layers and depth of 7.2X0 at normal incidence, with in total 3024 channels of 1 cm2, was tested with electron beam at CERN in 2007 and 2008, after DESY and CERN during 2006.
Analysis performed on the data colected during the test-beam periods has been done in order to extract its performance with respect to position resolution, response inhomogeneity and transverse containment. The next data taking will be done at FNAL in 2009.
Radiography of the electromagnetic calorimeter with most active regions in red and the inactive area in yellow.
In some events, a large quantity of energy may be deposited in the proximity of the guard-rings and a signal of high amplitude appears on all peripheral pads. This phenomena may be observed on the event display of the energy deposited in the different lauers of the electromagnetic calorimeter. The yellow square is cleary visible.
These so-called "square events" are due to capacitive coupling which exists between the guard rings and the peripheral pads. Sophisticated Simulation of the guard-rings has been performed with dedicated package (SILVACO) which to take into account the material structure (celling, doping, etc.). This is the first step to design new guard-rings geometries in collaboration with the fondery partners. In parallel, a test-bench including charge injector and micro-manipulators has been developped at LPC to characterize produced wafer in collaboration with the LLR-école polytechnique.
In many aspects, event visualisation is an useful tool. The reconstruction in a high granularity calorimeter requires new algorithms as well as track–cluster association. The particle flow algorithms have to be checked and tuned in a very detailed way, especially in the region where the geometry description is complex .
In this context, 3D visualisation tools are necessary and very powerful. Such a tool is CALIMERO ( CALorimeter IMAgE for RecO ), under development at LPC.
The graphics is handled by OpenGL and OpenGL Utility Toolkit (GLUT). The graphics interfaces are created with Fast Light ToolKit (FLTK), while the overall framework is written in C++.
CALIMERO is based on OpenGL and KLTK libraries. The goal is to provide a tool to debug and improve the tuning/understanding of the reconstruction algorithms. It propose a 3D event navigator with a simple grammar (MSQL–like), which allows the selection of every possible combination of parts of the detector and reconstructed objects. The parameters characterizing a particular visualisation can be to downloaded for a later re–use in a future session.
A transversal view of a WW production event is displayed below togethjer with some typical views obtained with CALIMERO.v.3.0 :
In the LCD concept detector framework, full simulation of the detector is performed in order to develop and to test new reconstruction algorithms for simple objects (photon, electron, and muon) as well as simpler (!) objects (hadron, tau, and jets).
The LPC effort concentrates on:
LDC is one of the concept detectors considered for the linear collider. It is a hermetic detector with a large volume devoted to the tracking (TPC) and compact calorimeters (ECAL and HCAL) placed inside the yoke of the magnet.
Full simulation based both on GEANT.3 and GEANT.4 (MOKKA) are available. A framework called MARLIN (Modular Analysis and Reconstruction for the LINear collider) is devoted to LCD. The informations are managed in a dedicated data model, LCIO, endorsed by the Linear Collider community. Many packages already exit and it is possible, even in this very preliminary stage, to perform the reconstruction of the data delivered by the full simulation of the detector.
A first attempt has been done with the measurement of the Higgs self–coupling which includes Higgs–strahlung and WW–fusion. The group is also developing a new approach with respect to the standard clusterisation for the photon identification (EMILE).
EMILE is an alternative to the standard clusteration method. EMILE identifies the pads coming from an electromagnetic shower. The software is based on a stochastic approach which does not pass by a formal clusterisation. This approach can be interesting as soon as two objectsare close one to the other.
The illustration displays, on the left–side, a charged pion near with a small energy photon. After the treatment, EMILE isolates the pads attached to the photon, presented within right–hand side. The colors indicate the true origin, green for the photon and red for the charged pion.
Alongside the fast simulation, analysis in the context of a full simulation is now on the way. Trilinear Higgs coupling measurement is a highly challenging measurement and a perfect benchmark.
With MARLIN and the attached collection of processors (MARLINRECO) we are now able to move towards physics analysis based on the full simulation and reconstruction of the detector. Many aspects are far from being solved, but the actual framework allows some preliminary studies.
Events corresponding to the double Higgs–strahlung and to the W fusion, which are governed by the trilinear Higgs coupling, were produced and completely simulated in the LDC detector with MOKKA. The reconstructed objects are clustered in six jets in order to form variables needed to discriminate to background. One of these variables is the angle between the directions of the two Higgs bosons in the rest frame of the virtual Higgs boson. The angular distribution is expected to be flat. The distribution obtained at the end of the full simulation chain reflects this characteristic as illustrated on the histograms.
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|International Linear Collider||www.linearcollider.org/cms/|
|ILC at DESY||ilc.desy.de|
|ILC at Fermilab||ilc.fnal.gov|
Le collisionneur linéaire à électrons est le prochain instrument de Physique des particules. Il permettra de sonder la structure de la matière à l'échelle du Tera-electron-Volt (TeV) et d'ouvrir une fenêtre sur l'évolution cosmologique de l'Univers. Le champ d'investigation est l'étude des processus créés lors des collisions à haute énergie entre les électrons et les positons, l'anti-matière partenaire de l'électron. Cette machine (International Linear Collider) est le fruit d'une concertation et d'un effort international; elle sera en mesure d'apporter des réponses claires à propos d'un très large évantail de question de la Physique des particules. A titre illustratif, nous pouvons citer
En 1999, un groupe de physiciens initia la participation du LPC à l'effort du Collisionneur Linéaire, en prenant part à l'évaluation du potentiel de Physique que peut offrir un tel instrument de Physique. Les premières investigations furent reportées dans le TDR Tesla.
Désormais, le groupe est, en autres, membre de la collaboration internationale CALICE, dédiée à la calorimétrie pour le Collisionneur Linéaire. Il prend part aussi au programme Européen nommé EUDET qui recouvre l'ensemble des sous-détecteurs (détecteur de vertex, trajectrographe, calorimètres, etc.)
L'activité du groupe se répartit entre
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