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.
The CMS detector at the LHC can be used to identify particles via the measurement of their ionization energy loss. The sub-detectors that are expected to provide useful information for this experimental technique are the silicon strip tracker, the pixel detectors and the electromagnetic calorimeter. Identification of low momentum hadrons, improvement of electron identification and detection of new exotic heavy stable charged particles can all benefit from this experimental method. Members of UCL have explored for the first time this technique and have developed the tools for calibrating and measuring the ionization energy loss with the silicon strip tracker. Particle identification with ionization energy loss was commissioned on cosmic rays and on first LHC collisions: it has proved to perform extremely well allowing protons, kaons, as well as light resonances decaying into kaons and protons to be cleanly identified. This technique has also allowed the first search for new heavy stable charged particles. The pixel and electromagnetic calorimeter detectors are planned to be also used in order to further improve the current performance.
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.
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.
