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Home page > PhD fellowships > Proposed PhD subjects 2016-2019 > ATLAS : Search for New Physics with the ATLAS detector using W and Z final states

ATLAS : Search for New Physics with the ATLAS detector using W and Z final states

by Luc Frappat - 17 December 2015

Topics:  LHC Physics, ATLAS collaboration

Proponents:  Lucia Di Ciaccio

Address:  LAPP - 9, chemin de Bellevue - BP 110 - 74941 ANNECY-LE-VIEUX CEDEX

Phone:  + 33 (0)450 09 16 24

Contact Email:  lucia.di-ciaccio@lapp.in2p3.fr


The proposed thesis subject will use final states with a pair of gauge bosons (W and Z) produced in proton-proton collisions at the LHC, at an energy in the centre of mass of 13 TeV, as a probe to look for New Physics phenomena. The project is based, ultimately, on the analysis of the full data sample (≈ 100 fb-1 that will be collected in the next three years by the ATLAS detector (Run 2).

After the Higgs discovery in 2012, the Run 2 of the LHC opens the exploration of a new land of discovery and even though no signal of New Physics may show up, the results that will be obtained in Run 2 will pave the road of the discipline at medium and long term.

The PhD student will participate to this challenging and important program with her/his thesis work.

The first step of the proposed program is to clarify the small excess of events that was seen in LHC-Run 1 data, in the invariant mass distribution at about 2 TeV, in the analysis of final states with a pair of gauge bosons (W or Z) decaying to a pair of jets. The originality of the work proposed here is to look at events where both gauge bosons decay leptonically. These events have the advantage to be affected by a much reduced background, even though they suffer from a much smaller branching ratio. In particular, the use of fully leptonic final states will be mandatory, if the Run 1 excess will be confirmed by the early Run 2 analyses.

The discovery of a new resonance at the LHC would obviously be a major event, but even if no signal of New Physics in the form of a resonant state will be confirmed, final states with a W and Z remain among the most important and reliable probes of New Physics via the study of the anomalous gauge boson self-interactions.

If the new resonance is not found, the thesis work will focus on the cross section measurement of final states with a W and a Z plus an additional gauge boson (γ or Z). These events allow to access quartic gauge boson couplings. This measurement has not yet been done at the LHC and a good sensitivity is expected for the proposed final state with 100 fb-1 of data at an energy of 13-14 TeV. At the same time the search proposed here is an excellent preparation to the analyses for the subsequent phases of the LHC program, which is actively being pursued already now; therefore this is a future-oriented subject, which is beneficial for a student who wishes to continue to work in the field.

The measurement proposed here requires the implication of the student in the understanding of the detector response and performance (lepton, γ detection and missing energy reconstruction) as well as frequent interactions with theorists.

Members of the LAPP group have already a long experience in the study of final states with weak gauge bosons having led the effort in ATLAS in the 8 TeV and now 13 TeV WZ diboson analyses. Moreover, the geographical proximity of LAPP with CERN will facilitate the direct implication of the student in the life of the experiment including shifts and collaboration meetings. The contacts with theoreticians are also facilitated by the presence of the theoretical physics laboratory LAPTh in the same building.