M2: Search for new phenomena through the study of vector-bosons polarization at LHC with the ATLAS experiment
Context:
The proposed internship and PhD subject uses the polarization states of gauge bosons pairs (W and Z) produced by scattering in proton–proton interactions at the LHC at energies of 13 to 13.6 TeV as probes ito look for signs of new physics. This search is one of the major goals of the LHC, following the observation of the Higgs boson in 2012.
The Higgs mechanism is an essential ingredient of the Standard Model that ensures the calculated crosssection for gauge-boson scattering does not diverge at high energies, i.e. in the regime where new particles or interactions are expected. Validating these calculations through the experimental study of polarized vector-boson scattering (VBS) is therefore complementary to measurements based on the direct detection of the Higgs boson. In particular, the scattering of longitudinally polarized gauge bosons is closely linked to the Higgs mechanism. These processes are, however, among the rarest ever measured at the LHC, and their study is still in its infancy. It could reveal signs of new physics. Thus, very recently, thanks to the work of the LAPP team, a major milestone was reached with a first measurement of the longitudinal polarization states of scattered W and Z bosons [1], based on a partial data sample.
Summary of the internship work:
The proposed M2 internship follows directly from this result and will make use of all the LHC Run 3 data available at the time: for the first time, the candidate will have access to the complete dataset collected by the ATLAS experiment from 2015 to 2026, i.e. an unprecedented integrated luminosity of 450 fb⁻¹.
The planned work will focus on improving the techniques used to separate the longitudinal and
transverse polarization states of the bosons, by exploiting novel artificial intelligence (AI) methods. The primary goal will be to check, with more data, whether the small yield of longitudinally polarized W bosons measured in the previous result corresponds to a statistical fluctuation or is confirmed. Its confirmation could be a sign of new physics.
The candidate’s work will be integrated into the analysis group of the international ATLAS collaboration working on this WZ final state, a group coordinated by the LAPP team. This will ease the student’s onboarding into the analysis and enhance the visibility of his work. The student may also interact with theorists.
Possible opening towards a PhD subject:
The internship may be extended into a PhD thesis whose main goal will be to carry out the measurement of the polarization states of the vector bosons produced in WZ scattering, exploiting the full ATLAS dataset at 13 and 13.6 TeV. This will require the development and use of advanced AI algorithms, aimed at increasing the sensitivity of the analysis to the extremely rare signal events being searched for. The measurements will then be compared with Standard Model predictions to search for possible signs of new physics. The results will be published in a peer-reviewed international journal.
These developments will also build on phenomenological work carried out in collaboration with theorists at the international level; the LAPP group already has close connections with the theoretical experts in this field.
Furthermore, the proposed study will constitute an excellent preparation for the analyses of the High-Luminosity LHC (HL-LHC) phase, planned after the ongoing long shutdown. Indeed, the measurement of polarized vector-boson scattering is one of the two major goals of the HL-LHC phase, alongside the study of the Higgs boson self-coupling. The subject is therefore highly promising.
Within the ATLAS collaboration, the student will also be involved in more technical developments. At the candidate’s choice, this may take the form of contributions to the group’s hardware projects, which will then be in the deployment phase at CERN for the start of the HL-LHC. Work related to the understanding of detector responses and performance, such as lepton detection, or jet reconstruction and identification of their substructure, as part of the improvement of algorithms for the HL-LHC, may also be considered.
The geographical proximity of LAPP to CERN will facilitate the student’s direct involvement in the life of the experiment, with participation in collaboration meetings as well as in the preparations for the HL-LHC start-up in 2030.
Finally, the candidate’s work will be an integral part of the European network COMETA, which brings together members of the ATLAS experiment as well as members of the CMS experiment and theorists, thus offering the candidate even greater potential visibility for their work.
Name of supervisor: Emmanuel SAUVAN
Email: sauvan@lapp.in2p3.fr
Experiment: ATLAS