The CMS silicon strip tracker is the largest device of its type ever built. There are 24244 single-sided micro-strip sensors covering an active area of 198m2. While first data from collisions are coming in, the physics performances of the detector are being assessed and optimized. Members of UCL are playing a major role in the understanding of the silicon strip tracker and in the finalization of all tools needed for its configuration, control, monitoring and calibration. We are sharing the convener-ship of the tracker detector performance group (DPG).
External collaborators: CMS tracker collaboration.
The amount and distribution of the material composing a particle detector that measures the trajectories of charged particles must be known with high accuracy for two main reasons: 1) avoid any bias in the measurements of the momentum of charged particles and 2) provide an accurate Monte Carlo simulation of the detector. A novel method for measuring the material of a generic tracking apparatus has been developed. The method exploits the multiple scattering experienced by charged particles while they sail through the detector. The method relies on the precise position measurement of the crossing points provided by the tracking detectors. The method is completely general and can be applied to any experiment equipped with detectors with good enough space resolution. The material of the CMS Silicon Strip Tracker has been measured with this technique to a precision at the level of 10%.
The detection of TeV muons is a fundamental ingredient of a number of key analyses (e.g. search for new high-mass di-muon resonances) to be performed by the CMS experiment at the LHC collider. In the CMS experiment, the resolution on the measurement of the energy and direction of O(TeV) muons is dominated by the precision of the crossing point measurement performed by the muon chambers (including the alignment accuracy) and by the catastrophic energy losses in the material traversed by the muon. A new algorithm for reconstructing high energy muons has been developed. The algorithm aims at improving both the purity of the measurements associated to the reconstructed muon track and at rejecting the measurements produced following a catastrophic energy loss, which would bias the muon measurement. The algorithm has been proved to reduce significantly the non-Gaussian tails in the muon energy resolution, while leaving the width of the core distribution unchanged.
Muons are particles that can be identified and measured with high precision by the CMS detector at the LHC. CMS can therefore be used to study the invariant mass spectrum of di-muon pairs and search for high-mass unstable particles (resonances) in a yet unexplored high mass range. High-mass resonances decaying into muon pairs are predicted in a number of models beyond the Standard Model of the fundamental interactions. Notable examples are heavy neutral gauge bosons predicted by grand unification theories, as well as gravitons arising in the Randall-Sundrum model of extra dimensions. The first search for high mass resonances was published in JHEP by CMS using the data acquired in 2010. Updated results were produced using part of the 2011 dataset in Summer 2011. By combining di-electron and di-muon data, CMS has excluded the existence of resonances predicted by a number of theoretical models with masses below about 2 TeV. These limits are the most stringent to date. S. Basegmez, G. Bruno and D. Pagano of the UCL CP3 group have contributed to this analysis by being one of the three teams of the CMS Collaboration that has regularly analyzed new data to produce the updated invariant mass spectrum, by setting up the technique for identifying isolated muons and by computing the significance, including the look-elsewhere effect, of an excess observed at 120 GeV in both the di-electron and the di-muon channels. In addition, a new technique for measuring high energy muons, which is the fundamental ingredient of the entire analysis, has been developed in the past year by the team. This technique, which has been proved to be the most robust against the catastrophic energy losses that can be experienced by muons, is expected to be adopted in the search starting from the 2012 run.
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.
