Contact
Name
Alexandre Mertens
Position
Former member
PhD student in CP3 from September 2011 to March 2017
Projects
I am involved in the following research directions:

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.

Fast Simulation of the CMS experiment

A framework for Fast Simulation of particle interactions in the CMS detector (FastSim) has been developed and implemented in the overall simulation, reconstruction and analysis framework of CMS. 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 data and Full Simulation samples. FastSim is used in several physics analyses in CMS, in particular those requiring a generation of many samples to scan an extended parameter space of the physics model (e.g. SUSY) or for the purpose of estimating systematic uncertainties. It is also used by several groups to design future sub-detectors for the Phase-II CMS upgrades.
Related activities at UCL include the integration with the Full Simulation in the simulation of the electronic read-out ("digitization") and of the pileup of events from other proton-proton collisions, both in-time and out-of-time; the performance monitoring; and the overall maintenance and upgrade of the tracking-related code. Matthias Komm is current L3 convener of Tracking in FastSim, and Andrea Giammanco has been main responsible of the FastSim project from 2011 to 2013.

Search for Higgs boson(s) in CMS at the LHC in the llbb topology

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).

Similar selection (but outside of the Z window) is also sensitive to H->aa->llbb, with "a" a generic light scalar.

External collaborators: CMS collaboration.

Study and optimization of b-tagging performances in CMS

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. Furthermore, we are involved in the re-optimization and improvement of the Combined Secondary Vertex (CSV) tagger for the 2012 analyses.

External collaborators: Strasbourg CMS group, CMS collaboration.

Past projects
Hide past projects.

I am involved in the following research directions:

Advanced Techniques for Fast Simulation and Data Analysis in High Energy Physics (ASAP)

The discovery of the 125GeV Higgs boson by the LHC experiments has finally opened a new era in the exploration of the TeV scale. The physics programs of CMS and ATLAS aim far beyond the simple discovery, and vigorously pursue the full characterization of the newly discovered state and the full exploration of the TeV scale in search of new phenomena. A key lesson drawn from first two years of LHC running is that most probably first discoveries and then identification of new states/interactions will not be easy. On the one hand, model-independent searches in simple topologies such as single/multi lepton at high transverse momenta have not shown any hint of new physics so far. On the other, topologies with jets and/or missing transverse energies, much more challenging experimentally, do strongly depend on the underlying theoretical models so that efficiently identifying signal enhanced regions of the phase space is quite involved. In this context, multi-variate techniques have become more and more central in the analysis of data from hadron collider experiments, to maximally exploit the information available on the signal and on the backgrounds. Amongst the most advanced techniques and certainly the most powerful one from the theoretical point of view, the so called matrix element method stands out. The main goal of this proposal is to advance the use and the scope of the matrix-element method so to significantly extend the range of physics applications at the LHC to the search of new physics. First we aim at providing the experimental HEP community with complete and automatic simulation tools, such as MadWeight/MoMEMta and Delphes, that overcome the technical limitations of the method. Second we propose to test and apply the new tools to current analyses in signatures that involve final state leptons and b-jets. Finally, we explore new and original applications of the method to both model-dependent or model-independent searches of new physics at the LHC.

External collaborators: CMS collaboration.

Search for Higgs boson(s) in CMS at the LHC in the llbb+MET topology

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 lepton pair associated with two b-jets and missing transverse energy. This topology is sensitive to a light SM Higgs via the H(bb)H(WW) production (via loop or Higgs self-coupling diagrams) and via the associated production of ttbar pairs and invisible particles. Requiring additional b jets (thus defining the llbbbb+MET final state) allows also the very interesting studies of a standard model Higgs produced in association with a top quark pair and It is also sensitive to the production of non standard heavy Higgs bosons decaying into ZA or WA, in the WAWA and ZAZA channels.

In particular, we are studying both resonant and non-resonant Higgs-pair production.

External collaborators: CMS collaboration.

Publications in CP3
All my publications on Inspire

2016

Search for A/H to Z(ll)+H/A(bb) with 2015 data
CMS collaboration
[Full text] Prepared for Moriond 2016
Public experimental note. 28th April.
Search for Neutral Resonances Decaying into a Z Boson and a Pair of b Jets or Tau Leptons
CMS collaboration
[Abstract] [PDF] [Journal] [Full text] Published in Phys. Lett. B 759 (2016) 369
Refereed paper. 28th April.
Summary results of high mass BSM Higgs searches using CMS run-I data
CMS Collaboration
[Full text] CMS Physics Analysis Summary CMS-PAS-HIG-16-007,
Public experimental note. 22nd April.

2015

Search for H/A decaying into Z and A/H, with Z-->ll and A/H-->bb or A/H-->tau tau
CMS Collaboration
[Full text] CMS-PAS-HIG-15-001
Public experimental note. 1st December.
Higgs pair production at the High Luminosity LHC
CMS collaboration
[Full text] CMS-PAS-FTR-15-002
Public experimental note. 29th October.

2014

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.
Generic search for a Two Higgs Doublet Model in the Z(ll)+bb final state
R. Castello, A. Caudron, C. Delaere, T.A. du Pree, A. Mertens
[Full text] CMS AN-2014/029
Restricted access (CMS internal analysis note)

Private experimental note. 24th November.

2013

DELPHES 3, A modular framework for fast simulation of a generic collider experiment
de Favereau, J. and others
[Abstract] [PDF] [Journal] Published in JHEP
Refereed paper. 25th July.


[UCLouvain] - [SST] [IRMP] - [SC]
Contact : Jérôme de Favereau
Research
Job opportunities Postdoctoral Position on the CMS tracker upgrade for HL-LHC
EOS be.h : 10 PhD positions