Giacomo Bruno, Jérôme de Favereau, Christophe Delaere, Pavel Demin, Andrea Giammanco, Vincent Lemaitre, Fabio Maltoni, Olivier Mattelaer, Sébastien Wertz

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.

Giacomo Bruno

A search for a yet-unobserved baryon number violating top quark decay has been performed using data collected in 2011 and 2012 by the CMS experiment at the LHC. This search was motivated by a theoretical work from the UCL-CP3 phenomenology group, who have noticed that the existence of physics beyond the standard model would imply, under certain conditions, baryon number violation both in the production of top quarks and in their decay process. In the latter case top quarks would decay with a certain branching fraction into a lepton and two jets. The CP3 Louvain experimental group has searched for such decays in a final state containing a pair of top quarks, where the second top quark experiences a SM hadronic decay. No evidence of such an exotic decay has been found and limits have been set at the level of per mille on the branching fraction of the top quark.
More recently, the CP3 Louvain group has been preparing a new search for boosted same sign top quark pairs, possibly accompanied by additional ligh-flavor jets. This is also a signature of baryon number violation. Notable models where such topologies can be realized are supersymmetric ones with R-parity violation.

Giacomo Bruno, Jérôme de Favereau, Andrea Giammanco, Vincent Lemaitre

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.