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.
Madweight is a algorithm to automatically reweight experimental events with the squared matrix element, and therefore provides the required computation techniques for a practical application of the matrix element method.
We also study the usefullness of MadWeight to estimate differential cross-section via the marginal distributions of the the experimental weights .
External collaborators: Pierre Artoisenet (Ohio state university).
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.
At LHC, the Z boson can be produced in association with one or two b-quarks, which is here refereed as b(b)Z production. This process has been seen for the first time at LHC, and measurement of it is an important test of QCD calculations.
For the first time, we observed the Z+b final state and measured of the Z+b/Z+j cross-section ratio in 35.9/pb of pp collisions at 7 TeV, using particle flow jets and simple secondary vertex b-tagging algorithm in the definition of the signal.Emphasis is put on kinematic properties of the jets. With more luminosity, we are working on the measurement of the cross-section for the b(b)Z process, with the identification of one or two b-jets.
External collaborators: A.Gilbert, A.-M.Magnan, A.Nikitenko (Imperial College London); N.Heracleous, A.Perieanu (RWTH Aachen); M.Musich, E.Migliore (University of Torino), and CMS Collaboration.
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.