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Typical working day..

Roman Pasechnik

Senior lecturer

Typical working day..

Searches for new physics phenomena with jet final states in the ATLAS detector


  • Eva Brottmann Hansen

Summary, in English

Collimated sprays of paticle, known as jets,are abundantly produced in high-energy proton-proton (pp) collisions, and are predicted by many theories of physics beyond the Standard Model. This thesis reports first on the calibration of jets with focus on the data-driven in situ calibration. Two searches for new physics phenomena in events with jets in the final state are then presented. The analyses use √s = 13 TeV pp collision data, collected by the ATLAS experiment at the LHC.
The response to small- and large-radius jets is measured in situ using data collected between 2015 and 2017. The jet energy scales are measured in events where a jet recoild against a calibrated reference object. The mass scale of large-radius jets is measured with two methods: one uses track jets to provide an independent measurement of the jet mass, the other uses events with a pair of top quarks, that are boosted and decay hadronically.

The first analysis is a search for massive coloured objects which are pair-produced and decay to two jets. The analysed data was recorded between 2015 and 2016, corresponding to an integrated luminosity of 36.7 fb-1. Two selections of four-jet events are defined; one with two b-tagged jets and one inclusive. No significant deviation from the expected background of Standard Model processes is observed in either event selection. The results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is a stop squark, which decays hadronically to two quarks through R-parity violating couplings. Limits are derived on the stop squarks mass and on the mass of pair-produced colour-octet resonances. The scenario where the two squarks decay to a b-quark and a lighter quarks is considered separately using the b-tagged event selection.
The second analysis is still ongoing and uses data collected between 2015 and 2018, which corresponds to an integrated luminosity of 139 fb-1. The search looks for a resonance, which could be a mediator between a confined hidden sector and the Standard Model sector. The mediator decays to two dark partons, which shower and hadronise according to the dynamics of the hidden sector. The search targets a selection of model parameters where all dark hadrons decay promptly to visible particles. The signature is a pair of "dark jets" with different characteristics than Standard Model QCD-jets. A mass-decorrelated tagger is defined based on the number of tracks associated with the jet. The tagger is designed to distinguish dark jets from the QCD background without distorting the smoothly falling background distribution.


  • Particle Physics

Publishing year




Document type



Lund University


  • Subatomic Physics


  • Particle physics
  • jets
  • dark matter
  • Supersymmetry
  • Hidden Sectors




  • Caterina Doglioni
  • Roman Pasechnik
  • Torsten Åkesson


  • ISBN: 9789178956852
  • ISBN: 9789178956845

Defence date

3 December 2020

Defence time


Defence place

Rydberg auditorium, Fysikum, Lund University. Join via zoom: Webinar ID: 654 2052 7407 Passcode: 643608


  • Niki Saoulidou (Associate Professor)