Vincent Lemaitre
Director of CP3
Centre for Cosmology, Particle Physics and Phenomenology - CP3
Université catholique de Louvain
2, Chemin du Cyclotron - Box L7.01.05
B-1348 Louvain-la-Neuve
+32 10 47 3241
UCL member card
People responsibilities
Research scientists
Jérôme de Favereau (FNRS), member since October 2002
CP3 website developement and maintenance, issue tracking system, general support, cluster maintenance, openstack deployement, CMS detector upgrade among other things...

Pavel Demin (UCL),
I'm working for the CMS experiment. My current responsibilities: developing physics analysis software for CMS; heavy flavor jet tagging based on electron identification; enabling computing GRID at UCLouvain/CP3.

Sébastien Brochet, member since March 2015
I'm a postdoctoral researcher in particle physics, member of the CMS collaboration. My main interest is in the search for physics beyond the Standard Model linked to the top quark. For this, I've directly contributed to the search for spin 1 and spin 0 resonances decaying to top-antitop during the Run 1 of the LHC. Currently, I'm working in a model-independent strategy to search for new physics, with concrete applications to the measurement of the ttH coupling, as well as the study of HH resonances.

Jesus Vizan Garcia, member since March 2012
Experimental particle physicist working in the CMS experiment. My main activity is the search for the Higgs boson produce in association with a Z boson in the llbb final state. I also work in the optimization of the techniques to identify b-jets.

PhD students
Brieuc François (IISN), member since September 2012
Search for physics beyond the standard model in the llbb+MET topology with CMS data. This study will be extended to the search for the rare process ttH.

Sébastien Wertz (FRIA), member since September 2013
Experimentalist with the CMS collaboration. Searches for non-resonant new physics in the llbb+MET topology (hh and ttbar processes). Also involved in the development of new tools easing the use of the Matrix Element Method.

Research statement
I am interested in looking for any phenomena Beyond the Standard Model. In particular, in the scalar sector related to the electroweak symmetry breaking.

The search for scalar particles predicted by model Beyond the Standard Model will require at least four conditions:

1) A deep and good understanding of the CMS detector, in particular, the jet or particle flow algorithms and b tagging techniques.

2) The developments of sophisticated analyses techniques, including state of the art event generators such as Madgraph/Madevent with embedded tools such as Madweight.

3) The understanding of top physics as a source of background but also, possibly, a source of signal when the higgs is produced in association with top(s) quarks or if it decays in top quark pairs.

4) A close collaboration with theorists for any possible interpretation of a given observed excess or deficit of events, in a given final state topology.

My research activity is therefore mainly focussed on these four points, with an emphasis on points 2, 3 and 4.

In particular, for point 2, I am taking part to the development of Madweight from the experimental viewpoint, namely, the study of the limitation of the method due to the limited understanding of the so called transfer functions. For point 3, I am particularly interested by the top quark pair production in PP interaction and by the photoproduction of asociated W boson and top quark. In both cases, the fully leptonic topology from the leptonic decays of the two w bosons offer interesting possibilities. Phenomenology of the scalar sector is particularly rich. Together with my theoretical colleagues, the associated possible phenomenology of a two Higgs Doublet Model is under study.

This research is made in very close collaboration with my other experimental colleagues, the CP3 postdocs and last but not least the PhD students which are under my supervision.

Another interest of mine is to maintain an activity in R&D related to silicon detectors. This activity will however decrease with the approaching data taking period of CMS. In this context, I am involved in two projects, the first related to the RD50 collaboration, which concerns the development of Cz silicon detectors which could be good candidates for the CMS tracker upgrade. Another project is related to the development of fundamental semiconductor structures based on the SOI technology, in collaboration with the engineering faculty of the university. My personnal involvement here is somewhat limited to a supervision work, in order to guaranty a good quality of the services related to the irradiation under different beams and doses. These irradiations are being guaranteed by the physicists, engineers and technicians of the Center and of Research of the Cyclotron (CRC) of the university.
I am involved in the following research directions:

a C++ software package to compute Matrix Element weights: MoMEMta

MoMEMta is a C++ software package to compute Matrix Element weights. Designed in a modular way, it covers the needs of experimental analysis workflows at the LHC. MoMEMta provides working examples for the most common final states (Formula: 0, WW, ...). If you are an expert user, be prepared to feel the freedom of configuring your MEM computation at all levels.
MoMEMta is based on:

- C++, ROOT, Lua scripting language
- Cuba (Monte-Carlo integration library)
- External PDFs (LHAPDF by default)
- External Matrix Elements (currently provided by our MadGraph C++ exporter plugin)

Advanced Multi-Variate Analysis for New Physics Searches at the LHC

With the 2012 discovery of the Higgs boson at the Large Hadron Collider, LHC, the Standard Model of particle physics has been completed, emerging as a most successful description of matter at the smallest distance scales. But as is always the case, the observation of this particle has also heralded the dawn of a new era in the field: particle physics is now turning to the mysteries posed by the presence of dark matter in the universe, as well as the very existence of the Higgs. The upcoming run of the LHC at 13 TeV will probe possible answers to both issues, providing detailed measurements of the properties of the Higgs and extending significantly the sensitivity to new phenomena.

Since the LHC is the only accelerator currently exploring the energy frontier, it is imperative that the analyses of the collected data use the most powerful possible techniques. In recent years several analyses have utilized multi-variate analysis techniques, obtaining higher sensitivity; yet there is ample room for further improvement. With our program we will import and specialize the most powerful advanced statistical learning techniques to data analyses at the LHC, with the objective of maximizing the chance of new physics discoveries.

We are part of a network of European institutions whose goal is to foster the development and exploitation of Advanced Multi-Variate Analysis (AMVA) for New Physics searches. The network offers extensive training in both physics and advanced analysis techniques to graduate students, focusing on providing them with the know-how and the experience to boost their career prospects in and outside academia. The network develops ties with non-academic partners for the creation of interdisciplinary software tools, allowing a successful knowledge transfer in both directions. The network studies innovative techniques and identifies their suitability to problems encountered in searches for new physics at the LHC and detailed studies of the Higgs boson sector.

External collaborators: University of Oxford, INFN, University of Padova, Université Blaise Pascal, LIP, IASA, CERN, UCI, EPFL, B12 Consulting, SDG Consulting, Yandex, MathWorks.

Angular asymmetries in ttW production

We take advantage of the large statistics being recorded by the CMS experiment in Run 2 to launch a systematic study of angular asymmetries in the ttW process, which have a potentially large sensitivity to non-SM effects.
In synergy with the CP3 phenomenology group, we aim at reporting our results in a form that can be easily translated in EFT constraints.

Development of a framework for fast simulation of a generic collider experiment: Delphes

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.

Search for non-resonant new physics in ttbar production

The lack of observed resonances produced at the LHC motivates finding new ways of searching for BSM phenomena. This project aims at discovering possible non-resonant New Physics affecting the production of Top quark pairs, by means of a dedicated analysis of data recorded by the CMS experiment. The New Physics effects are modeled using an effective field theory (EFT), whose parameters are to be measured or constrained in a global fit.

The analysis is conducted in close collaboration with phenomenologists to ensure the approach is theoretically sound and future-proof.

Search for nonresonant Higgs boson pair production in the llbb+MET final state

The discovery of a Higgs boson (H) by the ATLAS and CMS experiments fixes the value of the self-coupling λ in the scalar potential whose form is determined by the symmetries of the Standard Model and the requirement of renormalisability. Higgs boson pair production is sensitive to the self-coupling and will play a major role in investigating the scalar potential structure.

This project consists in a search for nonresonant Higgs boson pair production via gluon fusion in the final state with two leptons, two b jets and missing transvere energy – gg → H(bb) H(WW) asking for the leptonic decay of the W's. The analysis is conducted in close collaboration with phenomenologists to ensure the approach is theoretically sound and future-proof.

Search for resonant Higgs pair production in the llbb+MET final state

The recent discovery of a scalar boson compatible with the Standard Model (SM) Higgs boson opened new windows to look for physics beyond the SM (BSM). An example of newly accessible phenomenology is the production of resonances decaying into two SM Higgs bosons (h) predicted by several theory families such as additional Higgs singlet/doublet or warped extra dimension.

This project consists in a search for spin-0 or spin-2 resonances produced via gluon fusion in the final state with two leptons, two b-jets and missing transverse energy – gg → X → h(bb) h(WW) asking for the leptonic decay of the W's. In particular, we are probing a mass range between 260 GeV and 900 GeV.

The CMS silicon strip tracker upgrade

Development of the "phase II" upgrade for the CMS silicon strip stracker.

More precisely, we are involved in the development of the uTCA-based DAQ system and in the test/validation of the first prototype modules. We take active part to the various test-beam campaigns (CERN, DESY, ...)

This activity will potentially make use of the cyclotron of UCL, the probe stations and the SYCOC setup (SYstem de mesure de COllection de Charge) to test the response to laser light, radioactive sources and beams.

The final goal is to take a leading role in the construction of part of the CMS Phase-II tracker.

External collaborators: CRC and CMS collaboration.

World LHC Computing Grid: the Belgian Tier2 project

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.

About 10 PBytes of data are produced every year by the experiments running 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 the CMS experiment alone in 2016 amount to about 1500 kHepSpec06 of computing power, 90 PB of disk storage and 150 PB 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 would involve the 6 Belgian Universities involved in CMS. The Tier2 project consists of contributing to 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 to integrate, bring on the GRID, and share resources with other scientific computing projects. The projects currently integrated in 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.

Show past projects.
Publications in CP3
Showing 5 publications over 54. Show all publications.
All my publications on Inspire


Search for H(WW)H(bb) decays using the 2015 data sample
CMS collaboration
[Full text] prepared for Moriond 2016
Public experimental note. 28th April.


Higgs pair production at the High Luminosity LHC
CMS collaboration
[Full text] CMS-PAS-FTR-15-002
Public experimental note. 29th October.


Study of HH production with H → bb, H → WW → lν lν for an upgraded CMS detector at the HL-LHC
C. Delaere, B. Francois, V. Lemaitre, A. Mertens, M. Vidal Marono
[Full text] CMS AN-2014/141
Private experimental note. 1st December.
Evidence for the direct decay of the 125 GeV Higgs boson to fermions
Chatrchyan, Serguei and others
[Abstract] [PDF] [Journal] Subjects: High Energy Physics - Experiment (hep-ex)
Journal reference: Nature Physics 10 (2014) 557
DOI: 10.1038/nphys3005
Report number: CMS-HIG-13-033, CERN-PH-EP-2014-004

Refereed paper. 1st July.
Measurement of the production cross sections for a Z boson and one or more b jets in pp collisions at $sqrt{s}$ = 7 TeV
Chatrchyan, Serguei and others
[Abstract] [PDF] [Journal] [Full text] Published in JHEP 1406 (2014) 120
Refereed paper. 10th February.

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