Cosmology, Particle Physics and Phenomenology - CP3

We study the observation of single top quarks in photoproduction at the LHC as a way of detecting the presence of flavour changing neutral currents, predicted by some theories beyond the standard model.

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 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.

FROG is a generic framework dedicated to visualize events produced in particle collisions and detected by particle detectors.
It has been written in C++ and use OpenGL cross-platform libraries. It can be used to any particular physics experiment or detector design. The code is very light and very fast and can run on various Operating System. Moreover, FROG is self consistent and does not require installation of ROOT or Experiment software (e.g. CMSSW) libraries on user's computer.
It includes a lot of features based on an unique and powerful principle. Some of the functionalities are listed below :
3D and 2D visualization, graphical user interface, mouse interface, configuration files, production of pictures in various format, integration of personal objects.
One of the FROG application is to display events for one of the most complex physics experiment : the CMS experiment. But it works as well and even faster with smaller experiment like the Gastof detector.

Frog WebSite
CMS TWiki Page

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%.

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.

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).

Precise determination of the absolute luminosity is crucial for many measurements in CMS. The measurement of the exclusive two-photon production of muons pairs by CMS provides a powerful method to calibrate the integrated luminosity.

External collaborators: CMS forward physics analysis group, CMS luminosity group.

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.

A search for a yet-unobserved baryon number violation is performed using CMS data, following what suggested by the UCL-CP3 phenomenology group, who first proposed such a possibility in the top-quark system. Baryon number violation can manifest itself both in the production and in the decay process. In the latter case a top quark, produced in association with an anti-top, would decay with a certain branching ratio into a lepton and a W-boson. The analysis searches for such decays in a final state where both the other top quark and the W-boson decay hadronically, which is the most probable decay scenario. A first version of the analysis is close to being finalized and its results have been scheduled by CMS to be made public for the Summer 2012 conference.

The CMS detector at the LHC is used to search for yet unobserved heavy (mass >100 GeV/c$^2$), quasi-stable (lifetime > 1 ns), electrically charged particles, called generically HSCPs.
HSCPs can be distinguished from Standard Model particles by exploiting their unique signature: very high momentum and low velocity, due to their mass and the available LHC collision energy. Two experimental techniques are used to measure the velocity of such particles. They make use of the Silicon Tracker and of the Barrel Muon Drift Tube detectors.

UCL members lead the analysis since 2010, when the first HSCP paper was one of the first published LHC search papers. Updated results, using the 2011 dataset, were produced and submitted for publication.
The analysis, which is extremely model-independent and inclusive, doesn't find evidence of HSCP. It currently excludes the existence of stable gluinos, predicted by split supersymmetry, with a mass lower than about 1.1 TeV. This limit is the most stringent to date.

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.

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 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.

External collaborators: 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.

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 centres 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 centres, 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.