# Exotic particle searches

“It seems as though it would be hard to top discovering a new particle, and contributing to a Nobel prize, but that’s exactly what scientists at CERN are hoping to do now that ‘Long Shutdown 1’ is coming to an end. Some of Europe’s finest minds, including many from the UK, are eagerly anticipating the chance to harness the improved facilities as they come online over the next few months. Two years of downtime for the accelerators have been used as an opportunity for consolidation and maintenance, but there have also been some major upgrades that will see the Large Hadron Collider (LHC) operating at even higher energies next year. Particle collisions at higher energies can create heavier particles, which will never have been seen before.” (quote from the UK Science & Technology Facilities Council)

This STFC article excerpt succinctly outlines the consensus among particle physicists that searches for New Physics at the LHC in Run-2 are arguably the #1 priority in HEP today. In the next several years, the main thrust of our research effort will be on this quest for exotic particles with the ATLAS experiment. In particular, we will be conducting searches for anomalous signals in final states with $WW$, $WZ$, $ZZ$ dibosons (thus exploring the consistency of the Electroweak Symmetry-breaking mechanism with the Higgs boson), right-handed charged bosons and heavy neutrinos (that offer a plausible solution to the Dark Matter puzzle), as well as Dark Matter particles decaying to boosted jets in association with an electroweak boson. The group’s expertise includes searches for heavy resonances, reconstruction of hadronic boosted jets and final states with dibosons, and statistical tools.

We are studying jet substructure using advanced multi-variate and machine-learning techniques, that can be used for the tagging of vector bosons or other exotic particles decaying hadronically. In parallel, we are exploring the deployment of some of these algorithms into the High-Level Trigger Farm in a so-called Trigger-Level Analysis in order to exploit the significantly larger Level-1 bandwidth and increase the search sensitivity into the lower mass range.

Recent public results include:

• "Search for heavy Majorana or Dirac neutrinos and right-handed W gauge bosons in final states with two charged leptons and two jets at $\sqrt{s}$= 13 TeV with the ATLAS detector”, submitted to JHEP, arXiv:1809.11105
• "Search for light resonances decaying to boosted quark pairs and produced in association with a photon or a jet in proton-proton collisions at $\sqrt{s}$= 13 TeV with the ATLAS detector”,  Phys. Lett. B, 2018.09.062, arXiv:1801.08769
• “Searches for heavy $ZZ$ and $ZW$ resonances in the $\ell \ell q q$ and $\nu \nu q q$ final states in $pp$ collisions at  $\sqrt{s}$= 13 TeV with the ATLAS detector”, JHEP 03(2018)009
• "Searches for diboson resonances with boson-tagged jets in $pp$ collisions at $\sqrt{s}$= 13 TeV with the ATLAS detector”, Phys. Lett. B 777(2017)91
• "Decorrelated Jet Substructure Tagging using Adversarial Neural Networks", Phys. Rev. D 96, 074034 (2017)

• “Searches for heavy diboson resonances in $pp$ collisions at  $\sqrt{s}$= 13 TeV with the ATLAS detector”, JHEP 09 (2016) 173
• "Combination of Run-1 exotic searches in diboson final states at the LHC", JHEP04 (2016) 155

The group consists of Xanthe Hoad, Dr Christos Leonidopoulos, Andreas Søgaard, Dr Antonia Strübig, Neofytos Themistokleous, Elena Villhauer, and Dr Ben Wynne.

The activities are led by Dr Christos Leonidopoulos.