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.
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 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.
External collaborators: CMS collaboration.
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.
We are involved in the activities of the btag POG (performance object group) of CMS, in release and data validation and purity measurement. We are also interested in btagging in special cases like for colinear b-jets.
External collaborators: Strasbourg CMS group, CMS collaboration.
