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.
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.
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.
The CMS experiment is used to study the di-muon invariant mass spectrum. These spectra allow searches for high-mass unstable particles (resonances) to be performed in a yet unexplored high mass range.
High-mass resonances decaying into muon pairs are predicted in a number of models beyond the Standard Model of the fundamental interactions. Notable examples are heavy neutral gauge bosons predicted by grand unification theories, as well as gravitons arising in the Randall-Sundrum model of extra dimensions.
The first search for high mass resonances was published in JHEP by CMS using the data acquired in 2010. Updated results were produced using part of the 2011 dataset in Summer 2011. By combining di-electron and di-muon data, CMS excluded the existence of resonances predicted by a number of theoretical models with masses below about 2 TeV. These limits are the most stringent to date.
The UCL CP3 group contributed to these two early CMS publications by being one of the three teams of the CMS Collaboration that regularly analyzed new data, optimising the muon isolation criteria and conducting a full study of a mild excess observed in the low-mass region (at ~120 GeV) in both the di-electron and the di-muon channels.
Since 2012 the activity of the UCL-CP3 group is limited to the exploration of a matrix-element approach to this search. Preliminary results show that the exploitation of the full kinematical information of the di-muon events can give some sizable improvements over the classical one that uses just the di-muon invariant mass. In addition, the group develops a new algorithm for measuring the energy of TeV-muons (for details, please read the dedicated project). This algorithm is expected to bring improvements in both the di-muon and single muon+missing energy searches starting from 2014.
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.