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.
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.
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.
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.
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.
The general goal of this project is to develop muon-based tomography (“muography”), an innovative multidisciplinary approach to study geological structures, establishing a strong synergy between geophysics and particle physics.
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 international networks (G-ENDEAVOR, European Muography Network) that bring together particle physicists and geophysicists for the development and exploitation of high-resolution portable detectors.
We are using the local facilities at CP3 (e.g., the gRPC cosmic test bench) for further hardware developments.
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: G-ENDEAVOR and European Muography Network (Japan, Italy, France, UK, Hungary); MIVAS Collaboration (France and Italy) including CNRS (France), INFN (Italy), INGV(Italy).
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 (CMS-TOP-16-015 and CMS-TOP-16-023). This measurement, in addition to be 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 (arXiv:1709.07411 [nucl-ex]), 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.
External collaborators: David D'Enterria, Pedro Silva and Marta Verweij (CERN).
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.
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.
The top quark is the heaviest elementary particle discovered so far, and many properties related to this quark are still to be understood.
Its observation and mass measurement at Tevatron highlighted the uncommon nature of this quark. The fact that its electroweak decay is faster than the hadronization time scale implies that the top quark exists only as a free quark, so that the effects from new physics should show up very clearly by comparing measurements with the precise Standard Model preditions.
Its "re-discovery" at LHC will be a major milestone for the experiments, since the complexity of the final state demands a fairly good knowledge of the experimental apparatus and a certain degree of control of the backgrounds. Its expected large coupling to Higgs bosons will also be relevaant for the searches for higgs sectors beyond the Standard Model.
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.
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.
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.
The matrix element reweighting method attempts to compute the full likelihood of an observed event given a theoretical model. The method therefore measures the degree of compatibility of the event with the given model using as much information as available. MadWeight is a tool that fully automatize the computation of the event likelihood for any model implemented in MadGraph, by performing phase-space integration and providing a framework for taking into account the experimental resolution on the observed final state objects.
This project aims at validating the matrix element reweighting technique implemented in MadWeight on a number of benchmark searches. In some cases, the final goal is the efficient identification of background events. The final states that are being considered are: Zbb, single top, ttbar resonances and dimuon resonances.