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.
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.
High energy photon induced interactions at the LHC
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, we performed using the CMS data first measurement of two-photon muon pair production as well as first ever observation of W boson pairs produced by photon fusion. Now our group is leading unique investigations of quartic couplings between photons and W and Z bosons.
External collaborators: CMS FSQ analysis group and the group of A. Szczurek (Krakow).
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.
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.
Search for exotic decays of the Higgs boson to a pair of new light bosons with two muons and two b jets in final states
We search for exotic decays of a Higgs boson to a pair of new light bosons, H->a1a1, where one of the light bosons decays to a pair of muons and the other one decays to a pair of b quarks. Such signatures are predicted in a number of well motivated extensions of the standard model, including the next-to-minimal supersymmetry and generic two Higgs doublet models with an additional scalar singlet.
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.
Search for Higgs bosons in the ll tau tau final state with the CMS experiment at the LHC
A resonance consistent with the stanadard model Higgs boson with mass of about 125 GeV was discovered in 2012 by the CMS and ATLAS experiments at the LHC. Using the available dataset (2011+2012 LHC runs) evidence was later found of the existence of the SM-predicted decay into a pair of tau leptons. The CP3 Louvain group has been involved in the channel where the Higgs boson is produced in association with the Z boson and decays into a pair of tau leptons.
A search for additional Higgs bosons in the general framework of models with two Higgs doublets (2HDM) was then performed by the same CP3 group using the same final state and the full Run-1 data. Models with two Higgs doublets feature a pseudoscalar boson, A, two charged scalars (H+-) and two neutral (h0 and H0) scalars, one of which is identified with the 125 GeV SM-like Higgs resonance. In some scenarios the most favored decay chain for the discovery of the additional neutral bosons is H0-->ZA-->llττ (or llbb). The search was carried out in collaboration with another group in CP3 who looks at the llbb final state.
An update of both the SM search and the exotic one is expected using the Run-2 dataset using more advanced techniques and by adding the llee and llmumu channels.
Search for long-lived heavy neutral leptons with CMS
Many well motivated new physics extensions of the SM include new particles whose decay width is very small and hence have a decay length which is macroscopic. One very attractive and minimal extension of the standard model is one with right-handed neutrinos with Majorana masses below the electroweak scale (low scale see-saw). This addition is able to generate both the light neutrino masses and the baryon asymmetry of the universe via low scale leptogenesis. In what is probably the most studied model that invokes the low scale seesaw, the Neutrino Minimal Standard Model , one of the three right-handed neutrinos is a dark matter candidate. A large allowed region of phase space for right handed neutrinos spans masses between 1 and 50 GeV with corresponding lifetimes (cτ) ranging from 10^3 to 10^-4 m. For higher masses the right handed neutrino basically decays promptly and for lower masses the probability that it decays within the detector volume is virtually zero thus giving rise to missing transverse momentum in the detector. These latter two extreme cases can be captured experimentally by standard searches at the general purpose LHC experiments, while the intermediate case is the natural target of the so-called “displaced” searches, which are highly peculiar and challenging analyses at the LHC in high demand for dedicated data reconstruction tools in order to extend their sensitivity. We intend to search for long-lived sterile neutrinos decaying at displaced vertices into a charged light lepton and hadrons. A fundamental ingredient of this search is the identification of charged tracks emerging from highly displaced vertices.
Search for massive long-lived charged particles with the CMS detector at the LHC
The CMS detector at the LHC is used to search for yet unobserved heavy (mass >100 GeV/c$^2$), long-lived (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. These features are a consequence of their high mass and the relatively limited LHC collision energy. Two experimental techniques are used to identify such hypothetical heavy and low-velocity particles: the measurement of the ionization energy loss rate using the all-silicon tracker detector and the time-of-flight measurement with the muon detectors.
UCL members have developed the ionization energy loss identification technique and have lead the CMS HSCP search since 2010, when the first HSCP paper became one of the first published LHC search papers. Updated results, using the 2011 dataset, were then published followed by a comprehensive paper including also searches for fractional and multiply-charged particles published using the full CMS Run-1 dataset. The results obtained by analysing the 2015 Run 2 data at 13 TeV have also been published.
The analysis, which is very inclusive, doesn't find evidence of HSCP. It currently excludes, among various models, the existence of quasi-stable gluinos, predicted by certain realizations of supersymmetry, and Drell-Yan-produced staus with masses lower than about 1.3 TeV and 350 GeV, respectively. These and the other limits set by the analysis are the most stringent to date. The CMS HSCP papers total to date more than 300 citations.
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.
Single top studies at LHC
The electroweak production cross section of single top quarks is an important measurement for LHC, being a potential window on "new physics" effects.
Past achievements of this group include the very first measurement at 7 TeV (in t channel) with 2010 data, followed by the most precise inclusive cross section measurements of t-channel cross section at 7, 8 and 13 TeV, and the first differential measurements at 13 TeV; the most precise |Vtb| extraction from single top in the world; the first measurement of W-helicity fractions in a single-top topology; the first observation of the tW production mode; the first measurement of single-top polarization in t channel; stringent limits on anomalous tWb, tgu, tgc couplings.
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.
Testing the sign of the coupling of the new H(125) boson to top quarks with the CMS data
During 2012, the CMS and ATLAS collaborations independently reported unambiguous evidence of the existence of a new particle of mass around 125 GeV.
Several analyses are ongoing to challenge the hypothesis that the new particle is the SM Higgs. We focus on the search for its production in association with a single top quark. Due to an effect of quantum interference, this process is strongly suppressed in the Standard Model while it gets enhanced if its couplings to the top quark and to the W boson have opposite sign. An observation of this production mode would therefore be a convincing proof that this new particle does not belong to the Standard Model.
We published the first search for this process using 8 TeV data, and we expect to achieve sensitivity to anomalous values of the top Yukawa phase during the LHC Run-II.
External collaborators: CMS collaboration.
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.
Top quarks in Heavy Ion collisions and other non-standard LHC datasets
The top quark, being the heaviest known elementary particle, is a powerful tool to test QCD.
The study of top quark pair production in Heavy Ion collisions at the LHC, making use of the dedicated Pb-Pb and p-Pb runs, will open a new road in the investigation of the Quark-Gluon Plasma.
This research project started with the first measurement of top-pair cross section in pp collisions at 5.02 TeV, taking advantage of a "reference run" in Nov.2015. This measurement, in addition to being useful as a reference for measurements in Pb-Pb and p-Pb collisions at the same center-of-mass energy per nucleon, also provides a significant broadening of the lever arm for global PDF fits making use of top-quark data.
More recently we reported the first observation of top quark production in p-Pb collisions, using the data at 8.16 TeV taken in Nov.2016, testing the models of nuclear modification of the gluon PDF at high Bjorken-x.
We are now preparing future analyses in PbPb collisions. We are also interested in differential analyses in pPb collisions (to constrain nuclear PDFs) and in searches in central exclusive production pp events.
External collaborators: David D'Enterria, Pedro Silva and Marta Verweij (CERN).
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.
Sirunyan, Albert M and others
[Abstract] [PDF] Submitted to Phys. Rev. D
Refereed paper. 27th November.
[Full text] Physics Analysis Summary HIG-18-009, Presented at the XXXIX International Conference on High Energy Physics, ICHEP 2018, Seoul, South Korea, 4-11 July, 2018
Public experimental note. 6th July.
[Full text] Physics Analysis Note CMS-LUM-17-002, Presented at the XXXIX International Conference on High Energy Physics, ICHEP 2018, Seoul, South Korea, 4-11 July, 2018
Public experimental note. 6th July.
[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.
Public experimental note. 5th March.