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Context

The first LHC run allowed to test the Standard Model of particle physics at an unprecedented high energy level, never reached in a particle collider. Many results were published using these LHC data among which the discovery of a Higgs boson that completes the particle content of the Standard Model. However, theoretical and experimental arguments show that this model still has limitations and needs to be extended, modified or replaced in order to give answers to the hierarchy problem, to describe the dark matter or to include the gravity in a coherent frame with the other interactions...

Theoreticians proposed several ideas to build theories beyond the standard model in order to solve some of these problems. Many of them like supersymetry, technicolor, or models with extradimensions lead to the prediction of massive particles that can be searched for in LHC data.

 

Signature

These new massive particles could give different signatures in the ATLAS detector. We choose to look for massive particle decaying into a top quark pair (semi-leptonic channel). Indeed the top quark, because of its properties like its high mass close to the electroweak breaking scale, plays an important role in numerous new physics models from Technicolor to extra-dimension theories. Moreover its signature at the LHC is sufficiently clear to maintain Standard Model background at a reasonable level.

 

Analysis

The originality of the analysis lies in the identification of the top pair final state and the reconstruction of the top pair mass. Indeed LHC collisions produce highly boosted top pairs from Standard Model processes or from new high mass particles decays. This leads to a new topology where top decay products (leptons and quarks) are quite collimated and reconstructed in one single object. New tools have thus to be developed to disentangle the different decay products and allow the identification, reconstruction and calibration of such boosted particles.

Once boosted top quark pairs are identified and their invariant mass reconstructed, the mass spectra from LHC data is compared to the one predicted by the standard model. If data and simulation are in agreement, limits can be set on the mass and the production cross-section of massive particles predicted by new models. An excess of top quark pairs with respect to the standard model would inversely sign the presence of new physics.

 

Results

Analysis results at 13 TeV with 36 fb-1 - run2 : 2015-2016 data - Eur. Phys. J. C 78 (2018) 565

mtt resolved e bcat3 fig 13e mtt resolved mu bcat3 fig 13f
mtt resolved e bcat3 fig 13e mtt resolved mu bcat3 fig 13f
limZpTC2 fig 14 limKKg fig 11b lim GKK fig 16

 

 

Dark Matter model interpretation 2018 results - ATLAS-CONF-2018-051 -

DMcouplinglimit fig 08
DMmassmedlimit axial fig 10a DMmassmedlimit vector fig 09a

 

Publications

 

Members

Members

  • Sabine Crépé-Renaudin (in charge of LPSC analysis)
  • Pierre-Antoine Delsart (jet substructure expert)

Previous analysis group members

  • Clément Camincher (2014-2017) : PHD
  • Benjamin Decheneaux (2010-2013) : PHD