Detector commissioning, operation and data processing

Modern experimental particle physics requires the use of extremely complex detectors, readout electronics and associated services (e.g. power supply, gas, cooling and safety systems). The behavior of built detectors must be deeply understood before their data can be used to extract physics measurements. This process, called "commissioning", is performed by means of functionality tests of increasing complexity aiming at delivering a device with an understood response in its final working environment. Researchers of CP3 have been involved in the commissioning of several large-scale detectors (e.g. the CMS silicon tracker, the NA62 Gigatracker) and prototypes (e.g. the Calice detector, test-beam devices, etc.).

After commissioning, operation of a complex particle detector is only possible if tools to configure, control and monitor the entire detecting system are developed and deployed. We have experience in the development of such monitoring tools (from Detector Control and Safety to Data Quality Monitoring) and are taking an active role in day-to-day operations of detectors in our facilities and at CERN (both for technical and coordination aspects).

The operation aspect goes beyond these purely "online" aspects.
Many stages of data processing are necessary to go from the fundamental data produced by particle detectors (and their associated auxiliary systems) to a physics measurement. These are aspects that have to be handled "offline" and in some case will have an impact on online activities later on. The quality and precision of physics measurements heavily depends on the following items:
  • Data reconstruction methods:
    They are necessary to transform the generally large amount of detector raw data into information about the identity and kinematic properties of particles.
  • Calibration and alignment:
    Detectors and higher level reconstructed data needs to be tuned in order to lead to accurate results.
  • Trigger:
    The statistics available for an offline analysis as well as the ability to estimate accurately detector acceptances, event selection inefficiencies and backgrounds depends on the quality of the experiment online event selection, called the trigger.

The large amount of data produced by modern high energy physics experiments as well as the complexity of the detectors require complex computing solutions (both hardware and software wise) to perform the data processing steps outlined above. For that purpose, we deployed and maintain a large-scale computing cluster.


CMS Tracker commissioning and performance assessment

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.
Physics performance of the detector are being constantly assessed and optimized as new data comes.
Members of UCL are playing a major role in the understanding of the silicon strip tracker and in the maintenance and development of the local reconstruction code.

External collaborators: CMS tracker collaboration.

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.


Gigatracker is in the core of one of the spectrometers used in NA62. It's composed of three planes of silicon pixels detectors assembled in a traditional way: readout electronics bump bonded on silicon sensors. Each plane is composed by 18000 pixels 300 um x 300 um arranged in 45 columns and readout by 10 chips. The particularity of this sensor is that its timing resolution should be better than 200 ps in order to cope with high expected rate (800 MHz). Another particularity is its operation in vacuum.

CP3 is involved in several aspects in the production and operation of this detector.

1) Production of 25 GTK stations that will be used during the NA62 Formula: 0 run

2) Operation of GTK during data taking: time and spatial calibration, efficiency studies, effects of radiation, ....

3) Track candidates reconstruction, simulation.

4) Signal development of the signal in the sensor. We use both commercial programs (i.e. TCAD by Synopsys) as well as software developed by us to study the expected signal in this sensor.

Imaging with cosmic-ray muons

The general goal of this project is to develop muon-based radiography or tomography (“muography”), an innovative multidisciplinary approach to study large-scale natural or man-made structures, establishing a strong synergy between particle physics and other disciplines, such as geology and archaeology.
Muography is an imaging technique that relies on the measurement of the absorption of muons produced by the interactions of cosmic rays with the atmosphere.
Applications span from geophysics (the study of the interior of mountains and the remote quasi-online monitoring of active volcanoes) to archaeology and mining.

We are part of the EU-funded H2020 network INTENSE where we coordinate the Muography work package, which brings together particle physicists, geophysicists, archaeologists, civil engineers and private companies for the development and exploitation of this imaging method.

We are using the local facilities at CP3 for the development of high-resolution portable detectors.
We also participate to the MURAVES collaboration, now merged into the MIVAS collaboration, through algorithmic and data-analysis aspects like the implementation of time-of-flight capabilities, the analysis of control data for the optimization of the reconstruction algorithms, and the understanding of physics and instrumental backgrounds by data-driven and simulation techniques.

External collaborators: UGent; INTENSE Research & Innovation Staff Exchange network (Japan, Switzerland, Italy, France, Hungary); MIVAS Collaboration (France and Italy) including CNRS (France), INFN (Italy), INGV(Italy).

Luminosity calibration of the CMS detector

We contribute to the offline absolute calibration of the luminometry system of the CMS detector, by analysing the dedicated "Van der Meer scan" data at different center-of-mass energies and collision types (p-p, p-Pb, Pb-Pb).

As a related task, we also contribute to the data-driven inference of the true amount of "pile-up" collisions.

External collaborators: CMS Luminosity Physics Object Group.

NA62 computing

NA62 will look for rare kaon decays at SPS accelerator at CERN. A total of about $10^{12}$ kaon decays will be produced in two/three years of data taking. Even though the topology of the events is relatively simple, and the amount of information per event small, the volume of data to be stored per year will be of the order of ~1000 TB. Also, an amount of 500 TB/year is expected from simulation.

Profiting from the synergy inside CP3 in sharing computer resources our group is participating in the definition of the NA62 computing scheme. CP3 will be also one of the grid virtual organization of the experiment.

External collaborators: INFN (Rome I), University of Birmingham, University of Glasgow.

Particle Identification with ionization energy loss in the CMS experiment at the LHC

The CMS detector at the LHC can be used to identify particles via the measurement of their ionization energy loss. The sub-detectors that have provided so far useful information for this experimental technique are the silicon strip tracker and the pixel detectors. Identification of low momentum hadrons and detection of new exotic massive long-lived charged particles have all benefited from this experimental method. Members of UCL pioneered this technique in the early LHC times and have been developing the tools for its use and calibration. Since 2010 particle identification with ionization energy loss has been the basis of the CMS inclusive search for new massive long-lived charged particles, which has been providing the most stringent and model-independent limits existing to date on any model of new physics predicting such particles.

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.

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.

Past projects

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ASTERICS is a test platform designed for the radiation testing of digital circuits.
It has been developed by the TIMA lab (Grenoble, France). The aim is to acquire the competence and develop further this tester.

CMS Tracker commissioning at the Magnet Test Cosmic Challenge

The so called Magnet Test Cosmic Challenge (MTCC) was the first comprehensive operational and functional test of the CMS experiment. the MTCC took place in the first months of 2006 and was a slice test in which a small fraction of all the CMS detection equipment was operated in the 4 T solenoid of the experiment. Cosmic rays detected in the muon chambers were used to trigger the readout of all detectors in the global CMS data acquisition system. Prior to data taking, the detectors and their readout electronics were tuned and synchronized with dedicated software procedures. Local reconstruction was carried out online and offline in all sub-detectors for event selection and monitoring purposes. Global reconstruction, linking different sub-detectors, was performed mainly offline. A number of monitoring and visualization tools were also used for validation purposes and monitoring. One of the main goals of the MTCC was the validation of the hardware alignment system functionality.
At the MTCC, UCL had a leading role in the preparation, operation and offline data analysis related to the silicon strip tracker detector.

FROG: software for detector and event visualization

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

Hector, the simulator

Hector is a fast, multi-purpose simulator for the path of particles into beamlines.

It is build to be adaptative to any beamline and is already used by people from CMS (as a part of the official software) and ATLAS at the LHC and STAR at RHIC.

It is written in C++ and uses the ROOT framework to provide nice tools for analysis.

Results were cross-checked versus MAD-X, the official software used by the LHC machine group. It already allowed to obtain many estimations in terms of acceptance and resolutions of very forward detectors, as well as the effect of beamline misalignements and other related important topics related to forward detectors.

In collaboration with Xavier Rouby (Freiburg, D).

Measurement of detector material with particles and application to the Tracker of the CMS experiment at the LHC

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%.

Precise luminosity measurement in CMS

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.

Reconstruction and identification of hadronic tau decays with CMS at the LHC

The current experimental program of the CMS experiment contains many analyses which look for a Formula: 0 lepton in the final state. The decay of Higgs boson into Formula: 1 leptons is one of the few decay channels which can be used to observe or exclude a low mass neutral scalar boson that is predicted by the Standard Model as well as by many Beyond SM scenarios. Additionally, the observation of a charged Higgs boson, which for masses below 200 GeV preferably decays into a Formula: 2 lepton and a neutrino, would represent a unique clue to both the origin of mass and the deeper symmetries in Nature.
The Formula: 3 lepton can be useful also in many other analyses beyond the Higgs sector, e.g. to test the lepton/flavour universality.

Being the heaviest lepton, Formula: 4 can decay either to Formula: 5 or electron (``leptonic Formula: 6'') or to lighter hadrons (``hadronic Formula: 7''). Most of the Formula: 8 leptons decay hadronically (65 %). In hadronic decays, there is an odd number of charged hadrons possibly accompanied by neutral hadrons (due to charge conservation), forming together so-called Formula: 9 jet. Finally, there is always at least one neutrino (two for leptonic modes) among the Formula: 10 decay products.

Given that bulk of Formula: 11 decays are non-leptonic, the efficient reconstruction and identification of Formula: 12 jets is of crucial importance for the CMS physics program.

At CMS, the Formula: 13 decay products are reconstructed from Particle Flow (PF) objects. In the PF approach, the information from all sub-detectors is combined to identify and reconstruct all particles from collision, namely charged and neutral hadrons, photons, muons and electrons. The Formula: 14 reconstruction starts from jets.

The main reconstruction algorithm at CMS is "Hadron plus strips" (HPS). It combines PF electromagnetic particles into strips (due to broadening of calorimeter depositions from photon conversions) in order to reconstruct Formula: 15 candidates. Those are combined with charged hadrons to reconstruct visible Formula: 16 decay products.

Several identification criteria are applied to the Formula: 17 candidates: isolation (how much momentum is carried by jet constituents that cannot be associated with Formula: 18 decay products) and rejection against electrons and muons. All discriminators exist in cut- or MVA-based form and have several working points with different values of Formula: 19 reconstruction efficiency and rejection against fake Formula: 20 candidates.

The aim of this project is to maintain and improve the performance of the CMS tau reconstruction and identification algorithms.

Reconstruction of high energy muons in the CMS experiment at the LHC

The detection of TeV muons is a fundamental ingredient of a number of key analyses performed by the CMS experiment at the LHC collider, like the search for new high-mass resonances decaying into di-muons or one muon and one neutrino. Muons with an energy of a few hundred GeV or more experience catastrophic energy losses in the material they traverse. These energy losses have a very significant negative imact on the most important parameters of the muon energy measurement distribution: central value, resolution, and tails.

In order to mitigate these effects, a new muon reconstruction algorithm, called DYnamic Truncation (DYT), has been developed. The DYT identifies the muon position measurements that are produced after a catastrophic energy loss. The inclusion of these measurements in the muon track fit is responsible for the degradation of the muon energy measurement. The identification of such measuremnts is based on the level of incompatibility between the position measurement itself and the expected position obtained using the previous measurements.

Simulation of the CMS silicon tracker

The Tracker Simulation group is responsible for the Geant-based simulation of the Pixel and Strip Tracker response, material budget and geometry description.
Members from CP3 are concentrating on various aspects of the validation with data. We also share the convenership of the group.

External collaborators: CMS tracker collaboration.

Triggers for top quark physics

Our group is involved in the trigger design and maintenance for the LHC Run II in order to select the most interesting collision events for topologies with a lepton and a b quark with the CMS detector. Given the final state, this trigger is of particular interest for top quark physics, but it is also relevant to many other analyses (including the single top plus Higgs search ongoing in our group). Studies are carried out to implement the latest CMS developments in physics object definitions into the trigger path as well as to optimize the selections for the best efficiency and the rate possible. Our group is responsible for the b-tagging part together with organizing the whole activities within the CMS top quark group. Andrey Popov is official contact person between the Top Quark group and the Trigger group.

External collaborators: CMS TOP PAG, CMS TSG.



CMS Luminosity Measurement for the 2017 Data Taking Period
CMS Collaboration
[Full text] Physics Analysis Note CMS-LUM-17-004, Presented at the 6th Annual Conference on Large Hadron Collider Physics, LHCP18, Bologna, Italy, 4-9 June, 2018
Public experimental note. 6th June.


Particle-flow reconstruction and global event description with the CMS detector
CMS collaboration
[Abstract] [PDF] [Journal] [Full text] Submitted to JINST.
Refereed paper. 21st August.
CMS Luminosity Measurement for the 2016 Data Taking Period
CMS Collaboration
[Full text] Physics Analysis Note CMS-LUM-17-001
Public experimental note. 26th March.


CMS Luminosity Calibration for the pp Reference Run at sqrt(s) = 5.02 TeV
CMS Collaboration
[Full text] Physics Analysis Note CMS-LUM-16-001
Public experimental note. 1st December.
Reconstruction and identification of τ lepton decays to hadrons and ν$_τ$ at CMS
Khachatryan, Vardan and others
[Abstract] [PDF] [Journal]
Refereed paper. 6th October.


Tau reconstruction and identification in CMS during LHC run 1
CMS Collaboration
[Full text]
Public experimental note. 1st December.


Data preparation for the Compact Muon Solenoid experiment
Roberto Castello on behalf of CMS collaboration
[Full text]
Contribution to proceedings. 4th July.
Alignment procedures for the CMS Silicon Tracker detector during pp collisions
Roberto Castello on behalf of CMS collaboration
[Full text]
Contribution to proceedings. 4th July.
Alignment of the CMS tracker with LHC and cosmic ray data
The CMS collaboration
[Full text] Published on Journal of Instrumentation
Refereed paper. 19th June.
The Fast Simulation of the CMS Experiment
Andrea Giammanco
[Full text] Proceedings of the 20th International Conference on Computing in High Energy and Nuclear Physics (CHEP2013)

Journal of Physics: Conference Series 513 (2014) 02201

Contribution to proceedings. 9th June.


Studies of Tracker Material
The CMS Collaboration
[Full text]
Public experimental note. 8th February.


CMS Tracking Performance Results from early LHC Operation
CMS collaboration
[Abstract] [PDF] [Journal] [Full text] Published in Eur.Phys.J.C70:1165-1192,2010.
Refereed paper. 21st December.
Precise Mapping of the Magnetic Field in the CMS Barrel Yoke using Cosmic Rays
Chatrchyan, Serguei and others
[Abstract] [PDF] [Journal]
12th February.


Alignment of the CMS Silicon Tracker during Commissioning with Cosmic Rays
CMS Collaboration
[Abstract] [PDF] [Journal] [Full text] CMS PAPER CFT-09-003
Published in JINST

Refereed paper. 26th December.
Commissioning and Performance of the CMS Pixel Tracker with Cosmic Ray Muons
CMS Collaboration
[Abstract] [PDF] [Journal] [Full text] CMS-CFT-09-001.
Published in JINST

Refereed paper. 26th December.
Commissioning of the CMS Experiment and the Cosmic Run at Four Tesla
CMS Collaboration
[Abstract] [PDF] [Journal] [Full text] Published in JINST
Refereed paper. 21st December.
CMS Data Processing Workflows during an Extended Cosmic Ray Run
CMS Collaboration
[Abstract] [PDF] [Journal] [Full text] Published in JINST
Refereed paper. 21st December.
Commissioning and Performance of the CMS Silicon Strip Tracker with Cosmic Ray Muons
CMS Collaboration
[Abstract] [PDF] [Journal] [Full text] Published in JINST
Refereed paper. 21st December.
Stand-alone Cosmic Muon Reconstruction Before Installation of the CMS Silicon Strip Tracker
CMS Tracker Collaboration (W. Adam et al.).
[Abstract] [PDF] [Journal] [Full text] Published in JINST 4:P05004,2009
Refereed paper. 21st December.
Alignment of the CMS Silicon Strip Tracker during stand-alone Commissioning.
W. Adam et al.
[Abstract] [PDF] [Journal] Published in JINST 4:T07001,2009
Refereed paper. 21st December.
Delphes, a framework for fast simulation of a generic collider experiment
S. Ovyn and X.Rouby
[Abstract] [PDF] [Full text] Full description of the Delphes framework. Software manual in appendix
7th January.


The CMS Tracker Control System
A.Dierlamm, G.Dirkes, M.Fahrer, M.Frey, F.Hartmann, L.Masetti, O.Militaru, S.Youssaf Shah, R.Stringer, A.Tsirou
[Full text] Journal of Physics 119 (2008) 022019
Contribution to proceedings. 31st December.
The 2008 CMS Computing, Software and Analysis Challenge
The CMS Collaboration
[Full text] CMS IN-2008/044. 90 pp G. Bruno and L. Quertenmont among the "actual" authors (~40)
Private experimental note. 11th December.
The CMS experiment at the CERN LHC
The CMS Collaboration
[Journal] [Full text] The CMS Collaboration, "The CMS experiment at the CERN LHC", 2008 JINST 3 S08004, 361pp doi: 10.1088/1748-0221/3/08/S08004.
Refereed paper. 10th December.
Silicon Strip Tracker Detector Performance with Cosmic Ray Data at the Tracker Integration Facility
J.-L. Bonnet, G. Bruno, B. De Callatay, B. Florins, A. Giammanco, G. Gregoire, Th. Keutgen, D. Kcira, V. Lemaitre, D. Michotte, O. Militaru, K. Piotrzkowski, L. Quertermont, V. Roberfroid, X. Rouby, D. Teyssier et al. (>100 authors)
[Full text] W. Adam et al., "Silicon Strip Tracker Detector Performance with Cosmic Ray Data at the Tracker Integration Facility", CMS NOTE-2008/032.
Public experimental note. 10th December.
The CMS tracker operation and performance at the Magnet Test and Cosmic Challenge
S. Assouak, J.-L. Bonnet, G. Bruno, B. de Callatay, S. de Visscher, D. Favart, B. Florins, E. Forton5, A. Giammanco, G. Gregoire, S. Kalinin, D. Kcira, Th. Keutgen, V. Lemaitre, D. Michotte, O. Militaru, S. Ovyn, K. Piotrzkowski, X. Rouby, D. Teyssier, O. Van der Aa et al. (> 100 authors)
[Journal] [Full text] W. Adam et al., “The CMS tracker operation and performance at the Magnet Test and Cosmic Challenge”, 2008 JINST 3 P07006. doi: 10.1088/1748-0221/3/07/P07006.
G. Bruno co-editor

Refereed paper. 10th December.
Particle Identification with Energy Loss in the CMS Silicon Strip Tracker
A. Giammanco
[Full text] CMS-NOTE-2008-005
Public experimental note. 18th November.


Results from the Commissioning Run of the CMS Silicon Strip Tracker
Dorian Kcira
[Abstract] [PDF]
Contribution to proceedings. 3rd December.


Tagging b jets with electrons and muons at CMS
P. Demin, S. de Visscher, A. Bocci, R. Ranieri
[Full text] CERN-CMS-NOTE-2006-043
Public experimental note. 31st December.

[UCLouvain] - [SST] [IRMP] - [SC]
Contact : Jérôme de Favereau
Job opportunities Post-doctoral positions in BSM phenomenology at colliders and in astrophysics
Postdoctoral Position on advanced analysis techniques applied to the CMS di-Higgs study
Post-doctoral positions in collider phenomenology and particle astrophysics
EOS be.h : 10 PhD positions