Madweight is a algorithm to automatically reweight experimental events with the squared matrix element, and therefore provides the required computation techniques for a practical application of the matrix element method. We also study the usefullness of MadWeight to estimate differential cross-section via the marginal distributions of the the experimental weights .
External collaborators: Pierre Artoisenet (Ohio state university).
Observability of new phenomenological models in High Energy experiments is delicate to evaluate, due to the complexity of the related detectors, DAQ chain and software. Delphes is a new framework for fast simulation of a general purpose experiment. The simulation includes a tracking system, a magnetic field, calorimetry and a muon system, and possible very forward detectors arranged along the beamline. The framework is interfaced to standard file format from event generators and outputs observable analysis data objects. The simulation takes into account the detector resolutions, usual reconstruction algorithms for complex objects (FastJet) and a simplified 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.
External collaborators: Severine Ovyn, Xavier Rouby from pfease company.
The top quark is the heaviest elementary particle discovered so far, and many properties related to this quark are still to be understood. Its observation and mass measurement at Tevatron highlighted the uncommon nature of this quark. The fact that its electroweak decay is faster than the hadronization time scale implies that the top quark exists only as a free quark, so that the effects from new physics should show up very clearly by comparing measurements with the precise Standard Model preditions. Its "re-discovery" at LHC will be a major milestone for the experiments, since the complexity of the final state demands a fairly good knowledge of the experimental apparatus and a certain degree of control of the backgrounds. Its expected large coupling to Higgs bosons will also be relevaant for the searches for higgs sectors beyond the Standard Model.
External collaborators: CMS collaboration.
The Fast Simulation of CMS is an object-oriented subsystem of the general CMS C++ based software. Event production rates are of the order of 100 times faster than the corresponding Full Simulation ones, with nonetheless comparable accuracy for most of the physics objects typically considered in the analyses. It produces data samples in the same format as the one used by the Geant4-based (henceforth Full) Simulation and Reconstruction chain; the output of the Fast Simulation of CMS can therefore be used in the analysis in the same way as other ones. The Fast Simulation is intended to be used for most of the physics analyses, in particular, for those requiring a generation of many samples to scan an extended parameter space of the physics model (e.g. SUSY), those involving a consideration of large cross section backgrounds and samples of manageable size can only be produced by events skimming based on the final reconstructed objects, or those for which in general a large computation time is foreseen. We share the convenership of the group.
At LHC, the Z boson can be produced in association with one or two b-quarks, which is here refereed as b(b)Z production. This process has been seen for the first time at LHC, and measurement of it is an important test of QCD calculations. For the first time, we observed the Z+b final state and measured of the Z+b/Z+j cross-section ratio in 35.9/pb of pp collisions at 7 TeV, using particle flow jets and simple secondary vertex b-tagging algorithm in the definition of the signal.Emphasis is put on kinematic properties of the jets. With more luminosity, we are working on the measurement of the cross-section for the b(b)Z process, with the identification of one or two b-jets.
External collaborators: Anne-Marie Magnan (IC London), Alexandre Nikitenko (IC London), Natalie Heracleous (Aachen-I), Adrian Perieanu (Aachen-I).
The goal of this work is to explore the phenomenology of a two Higgs doublet model with a custodial symmetry. It provides topologies that are unusual in the models usually considered in the literature. Notably, it opens the possibility of having an inverted mass spectrum with respect to MSSM with the lightest Higges being a pseudo-scalar and an heavier triplet of charged and neutral higgses.
External collaborators: Simon de Visscher (Zurich university).
High-energy photon-photon and photon-proton interactions at the LHC offer interesting possibilities for the study of the electroweak sector up to TeV scale and the search for processes beyond the Standard Model. After in-depth exploratory studies, first investigations of anomalous photon interactions in CMS are being performed.
External collaborators: CMS forward physics analysis group, R. Schicker (Heidelberg) and A. Szczurek (Krakow).
Search for Higgs boson(s) within the Standard Model and beyond and also withing a minimal extension of the scalar sector (2HDM). The final state under study is a Z decaying into a lepton pair associated with two b-jets. This topology is sensitive to a light SM Higgs via the associate ZH production, as well as a middle mass range SM Higgs boson via the inclusive Higgs production followed by its decay into ZZ with one Z decaying into a lepton pair and the other into bbar. It is also very sensitive to the production of a non standard heavy Higgs boson decaying into Z plus A (pseudo scalar Higgs boson).
External collaborators: Nick van Remortel (Belgium, UA), Barbara Clerbaux (Belgium, ULB), and CMS collaboration.
The electroweak production cross section of single top quarks is an important measurement for LHC, being a potential window on "new physics" effects. After having produced the very first measurement at 7 TeV (in t channel) with 2010 data, now we aim at: 1) competing with Tevatron on |Vtb| extraction; 2) observe the tW process for the first time; 3) study several differential distributions in order to test the existing models. We currently have the convenership of the group.
Development of silicon sensors (strixels) for CMS tracker upgrade for very high luminosity at LHC. This activity is making usr of the cyclotron of UCL, the probe stations and the SYCOC set-up: SYCOC stands for "SYstem de mesure de COllection de Charge". This system is intended to measure charge collection efficiency of semiconductor detectors with both a laser and radiactive sources. This installation is used in the characterization of semiconductor detectors in order to study its radiation hardness. This project is done in collaboration with RD50 team at CERN.
External collaborators: CRC, Frank Hartman (Karlsruhe) and RD50 and CMS collaboration.
The matrix element reweighting method attempts to compute the full likelihood of an observed event given a theoretical model. The method therefore measures the degree of compatibility of the event with the given model using as much information as available. MadWeight is a tool that fully automatize the computation of the event likelihood for any model implemented in MadGraph, by performing phase-space integration and providing a framework for taking into account the experimental resolution on the observed final state objects. This project aims at validating the matrix element reweighting technique implemented in MadWeight on a number of benchmark searches. In some cases, the final goal is the efficient identification of background events. The final states that are being considered are: Zbb, single top, ttbar resonances and dimuon resonances.
The World LHC Computing GRID (WLCG) is the worldwide distributed computing infrastructure controlled by software middleware that allows a seamless usage of shared storage and computing resources. One PByte of data is expected to be produced every year by the CMS detector at the LHC collider. This data must be processed (iterative and refined calibration and analysis) by a large scientific community that is widely distributed geographically. Instead of concentrating all necessary computing resources in a single location, the LHC experiments have decided to set-up a network of computing centers distributed all over the world. The overall WLCG computing resources needed by CMS alone in 2010 amount to about 25,000 CPUs, 25,000 TB of disk storage and 35,000 TB of tape storage. Working in the context of the WLCG translates into seamless access to shared computing and storage resources. End users do not need to know where their applications run. The choice is made by the underlying WLCG software on the basis of availability of resources, demands of the user application (CPU, input and output data,..) and privileges owned by the user. Back in 2005 UCL proposed the WLCG Belgian Tier2 project that was endorsed by the 6 Belgian Universities involved in CMS. The Tier2 project consists of contributing to the set-up of the WLCG by building two computing centers, one at UCL and one at the IIHE (ULB/VUB). The UCL site of the WLCG Belgian Tier2 is deployed in a dedicated room close to the cyclotron control room of the IRMP Institute and is currently a fully functional component of the WLCG. The UCL Belgian Tier2 project also aims at integrating, bringing on the GRID and sharing resources with other scientific projects. The scientific projects related to or directly integrated on the UCL computing cluster are the following: MadGraph/MadEvent, NA62 and Cosmology.
External collaborators: CISM (UCL), Pascal Vanlaer (Belgium, ULB), Lyon computing centre, CERN computing centre.
