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Accueil du site > Formation > Thèses - Stages de M2

Search for new phenomena with the ATLAS detector using final states with W and Z pairs

The M2 internship and thesis work focus on the study of final states with pairs of gauge bosons (a W and a Z) produced in proton-proton collisions at the LHC, as a probe to look for new physics phenomena. This is one of the main topics of research at the LHC, closely related to the Higgs mechanism.

The angular analysis of final states with a W and a Z boson associated with two jets gives access to the scattering of longitudinally polarised vector bosons, a process in which the Higgs mechanism plays an essential role because it ensures the good behaviour of the cross section with the energy. The proposed study probes in this way the Higgs mechanism in an energy regime, which is higher with respect to the one probed by the direct Higgs detection and where new phenomena are expected to occur. Furthermore measurements of the production of gauge boson pairs performed in an inclusive way or associated with two jets give access to potential anomalous triple and quartic couplings among gauge bosons and represent a promising avenue to discover new physics phenomena, even if the energy of the accelerator is not high enough to produce directly new particles. This is an interesting time to start a thesis work, because, after a shutdown of about three years for the upgrade of the LHC and the ATLAS detector, in May 2022, the data taking (Run 3) will resume at the centre-of-mass energy of 13.6 TeV, and is expected to last about there years. At the end of Run 3 the available data sample will increase by a factor three.

Summary of the internship work :

The internship work focuses on the inclusive production of a W and a Z boson, which decay in leptonic modes. It is expected that potential anomalous coupling between gauge bosons modify the polarisation state of the bosons. Angular observables allow the access to the polarisation state of the W and Z bosons. The aim of the internship is to study if the use of variables based on the production and decay angles of the gauge bosons improve the sensitivity to new physics phenomena. So far similar studies were performed only partially because are limited by the relative small size of the data sample The tool will be simulation of the signal and background processes that may be compared with data. Members of the LAPP group have already a long experience on this topic having led the effort in ATLAS WZ analyses with data samples collected at Run1 and 2 of the LHC. This expertise will facilitate the integration of the selected candidate in the analysis team.

Information about a possible PhD subject :

The internship may be continued into the preparation of a PhD thesis, normally from October 2022 to September 2024. The thesis subject will deal with the measurement of WZ final states accompanied by two jets with Run 3 data. The increased luminosity opens new possibilities to measure for the first time the polarization of the single boson and the spin correlation between the two bosons in vector bosons scattering. These measurements are of great importance and are not possible with present data sample for lack of statistics. New observables and analyses techniques based on the Machine Learning must be developed. The selected candidate will profit of the group expertise on this technique. The project proposed here is an excellent preparation to the analyses of the subsequent phases of the LHC program (High Luminosity LHC, HL-LHC) ; therefore this is a future-oriented subject, which is beneficial for a student who wishes to continue to work in the field. The geographical proximity of LAPP with CERN (about 50 km) 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. The thesis work requires also the implication of the student in a more technical task : contribution to the hardware projects of the group for HL-LHC (related to the construction of the new ATLAS inner tracking or to the upgrade of the liquid argon calorimeter) or to the understanding of the detector response and performance (lepton, γ detection, missing energy or jet reconstruction). An extended stay at CERN during the PhD thesis may be considered.

o Level : Master 2

o Supervisor : Lucia Di Ciaccio

o Phone : 04 50 09 16 24

o e-mail : Lucia.di-ciaccio@lapp.in2p3.fr

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