Data analysis in HEP experiments

Through the analysis of data taken in high energy physics experiments we seek to determine the properties of elementary particles and the probabilities of occurrence of natural phenomena only visible at the scale of the attometer (or electroweak scale). We also search for new phenomena such as the production of new particles (predicted or not by a given theory) or anomalous cross sections or distributions. These measurements can also be used to extract or fit fundamental constants within the context of a given theoretical model, thereby providing the experimental inputs required to address phenomenology of elementary particles.

Most of the time, the processes under study are very rare and suffer from large background contaminations that need to be filtered out by sophisticated online trigger systems and through additional criteria applied on stored data. In order to achieve an optimal strategy for the signal selection, the use of specific observables from the basic reconstructed quantities from the detector is mandatory. The quality of these observables relies on the good calibration of the experimental set-up and on the quality of particle identification and reconstruction software. Furthermore this optimization must be realized while keeping a good understanding of the trigger and reconstruction efficiencies and while keeping the level of systematic uncertainties as low as possible.

Analyses strategies are therefore numerous and depend on the goal(s) to be achieved. They range from very simple and robust (e.g., cut-based analyses techniques) to more elaborated selection strategies which make use of kinematic fitting, multivariate analyses (such as artificial neural networks), or the so-called matrix element method.

Members of CP3 have been actively involved in the analysis of data collected by former CHARM II, CHORUS, ALEPH, DELPHI, H1 and ZEUS experiments. Since many years, members of CP3 participate to the analysis of the data collected by the CMS experiment at the LHC. They cover a wide range of physics topics, from top quark and Higgs boson studies to searches for SuperSymmetry and exotic particles. More recently, the analysis of the data collected by the fixed-target NA62 experiment at the SPS has also begun.

Advanced tools for simulation and data analysis of general interest for the HEP community, like DELPHES and MoMEMta, have been developed and are being maintained at CP3, although released as open-source code.


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)

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.

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.

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.

The final state containing two Z bosons decaying into a pair of leptons and a pair of neutrinos has been exploited by the CMS experiment at the LHC to produce a number of results related to the Higgs boson, including measurements of related standard model cross sections.

Constraints have been set on the total width of the 125 GeV Higgs boson, using its relative on-shell and off-shell production and decay rates to a pair of Z bosons, where one Z boson decays to an electron or muon pair, and the other to an electron, muon, or neutrino pair. The analysis is based on the data collected by the CMS experiment at the LHC in 2011 and 2012. A simultaneous maximum likelihood fit to the measured kinematic distributions near the resonance peak and above the Z-boson pair production threshold leads to an upper limit on the Higgs boson width of < 22 MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass of 125.6 GeV.

A search for heavy Higgs bosons in the H → ZZ → 2l2ν decay channel, where l = e or µ, has also been performed using data collected in 2015 at the center of mass energy of 13 TeV. No significant excess is observed above the background expectation. The results are interpreted to set exclusion limits on a number of extensions of the standard model scalar sectors: models with an additional electroweak singlet, as well as Type-I and Type-II two-Higgs doublets models.

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, 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).

The NA62 experiment in the North Area of the CERN SPS is now fully operational and taking data. The plan is to collect the highest statistics ever reached for Formula: 0 decays, of the order of Formula: 1 events in the fiducial decay region of the detector until the end of 2018. This high-intensity and high-precision setup makes it possible to probe a number of ultra-rare or forbidden decay channels. Of particular interest to the CP3 group are the LFV/LNV Formula: 2 and Formula: 3 modes.

Many BSM theories predict some degree of LFV, including Supersymmetry or the introduction of massive neutrinos. Furthermore, there are indirect hints for New Physics in the flavor sector, e.g. in the semileptonic decays of B-mesons. Explanations for the observed discrepancies predict effects of LFV in kaon decays. These particular LFV/LNV Formula: 4 processes which at present are not covered by another experiment provide an attractive opportunity to test the SM. Any observable rate for one of these modes would constitute unambiguous evidence for New Physics. Considering the statistics that will be available at NA62 the current limits on their branching-ratios could be improved by at least one order of magnitude.

External collaborators: University of Birmingham.

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) 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.

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 and an update is expected using the Run-2 dataset.

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.

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.

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.

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 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.

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 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 GeV, taking advantage of a "reference run" in Nov.2015 (CMS-TOP-16-015). This measurement, in addition to be useful as a reference for our future 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.

External collaborators: Pedro Silva and Marta Verweij (CERN).

Recent publications


Object definitions for top quark analyses at the particle level
CMS Collaboration
Public experimental note. 6th June.
Inclusive tt cross section at sqrt(s)=5.02 TeV using final states with at least one charged lepton
CMS collaboration
4th April.
Search for light bosons in decays of the 125 GeV Higgs boson in proton-proton collisions at sqrt(s) = 8 TeV
Khachatryan, Vardan and others
[Abstract] [PDF]
Refereed paper. 10th January.


Search for electroweak production of charginos in final states with two tau leptons in pp collisions at sqrt(s) = 8 TeV
CMS Collaboration
[Abstract] [PDF] [Journal]
Refereed paper. 1st December.
Constraints on the Higgs boson width from off-shell production and decay to Z-boson pairs
Khachatryan, Vardan and others
[Abstract] [PDF] [Journal]
Refereed paper. 6th October.
Search for long-lived charged particles in proton-proton collisions at sqrt(s) = 13 TeV
Khachatryan, Vardan and others
[Abstract] [PDF] [Journal]
6th October.
Cross section measurement of t-channel single top quark production in pp collisions at sqrt(s) = 13 TeV
CMS Collaboration
[Abstract] [PDF]
Refereed paper. Public experimental note. 4th October.
First measurement of the top quark pair production cross section in proton-proton collisions at sqrt(s)=5.02 TeV
CMS collaboration
Public experimental note. 16th June.
Combination of cross-section measurements of associated production of a single top quark and a W boson at sqrt(s)=8 TeV with the ATLAS and CMS experiments
ATLAS and CMS collaborations
Public experimental note. 24th May.
Search for H(WW)H(bb) decays using the 2015 data sample
CMS collaboration
Public experimental note. 28th April.
Search for A/H to Z(ll)+H/A(bb) with 2015 data
CMS collaboration
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]
Refereed paper. 28th April.
Measurement of the differential cross section for t-channel single-top-quark production at sqrt(s)=13 TeV
CMS Collaboration
Public experimental note. 27th April.
Summary results of high mass BSM Higgs searches using CMS run-I data
CMS Collaboration
Public experimental note. 22nd April.
Search for a heavy scalar boson decaying into a pair of Z bosons in the 2ℓ2ν final state
CMS Collaboration
Public experimental note. 22nd April.