Searches for new gauge bosons, Z' & W' and heavy neutrinos

Do new fundamental forces show up at the LHC the way the Z and Higgs bosons did? According to superstring theories, which propose to unify all fundamental forces, including gravity, there is room for new forces to be mediated by new gauge bosons, known as Z’ and W’. The W’ boson is also predicted by theories aiming at restoring parity (left-right) symmetry at high energies. This work consists of: (i) a detailed study and implementation into MC generators of various theories beyond the SM, (ii) an analysis of ATLAS data, taken at the highest available energies, and a comparison to simulation data. You will make use of one of the following processes:

  1. Search for Z’ in pp → Z’+X → l+l-+X. The so-called Drell-Yan process constitutes the main SM  background. The analysis consists in searching for a bump in the distribution of invariant mass of di-leptons Mll. At high masses top pair production is non-negligible. Two strategies are followed to determine the SM background: (i) using MC simulation and (ii) making use of data-driven methods. With the latter method you will investigate the best functions or make use of the so called “sliding-window” method to fit the data. In addition to searching for new particle  resonances you can look for other phenomena, one of which is the investigation of lepton substructure at some high energy scale, leading to a non-resonant excess at high masses. Eventually, you will interpret the data in terms of various Z’ or substructure models.  If you are more interested in theory/phenomenology, you can investigate new models, by studying their free parameters, implement the interesting models into MC generators,  and perform feasibility studies at the LHC. If you are fast and still eager you can test some models yourself with real data at hand.
  2. Search for W’ (and heavy Majorana neutrino) in pp → W’+X → l±+ν+X → l±+MET+X. You will follow a similar procedure as above. Due to the missing neutrino the signal is less prominent than in the resonance structure expected for a Z’. The distorted resonance shape is known as a Jacobian peak and, instead of the invariant mass, you will deal with the transverse mass built from the measured lepton and missing energy. The most important background stems from the real and virtual production of the W boson. You will interpret your results within W’ models, the Sequential SM one and the WR predicted by left-right symmetric models. What about the option to search for a heavy Majorana neutrino, N. For example in decays of a  WR where the N decays to a charged lepton and 2 jets through a virtual WR. You need to master both leptons and jets. As a bonus the SM background is expected to be very small as the 2 leptons in the final state have the same electric charge, in contrast to the cases above.

    Bildet kan inneholde: skråningen, rektangel, skrift, plott, parallell.


  3. Search for a DM-aware new Z’ gauge boson. You will search simultaneously for a new gauge boson Z’ and DM through the process pp→Z’+MET+X→ l+l-+MET+X, the missing transverse energy MET being due to DM particles. Non-SUSY simplified models assume, in addition to DM, some mediator which can be, among others, a scalar or a vector.  A new search mechanism, a generalisation of the mono-Z search to arbitrary vector boson mass, is proposed along 3 production mechanisms: (Left) Dark Higgs model, with a Z’ in association with a dark Higgs hD coupling to DM; (Middle) Light vector model with a Z’ coupling to dark states χ1 (DM) and χ2 (unstable); (Right) Light vector with inelastic Effective Field Theory model operating at an unknown new scale of new physics Λ.

    1. If you are interested in theory/phenomenology, you can investigate the DM models above by studying their free parameters: variation of the Z’ and DM masses within ranges compatible with experimental, astrophysical and cosmological constraints. You will work within the LHC DM group where experimental physicists from ATLAS, CMS and non-LHC experiments, together with theoreticians, collaborate to shed light on DM. You may implement new interesting models into MC generators and perform feasibility studies at the LHC by comparing to SM expectations. If you are fast and still eager you can test some models yourself with real data at hand.
    2. You can also start the other way around: analyse real data, search for Z’ and DM, and interpret your results in terms of the simplified models above. See for example the Z’ analysis above.
Emneord: CERN, LHC, ATLAS, Dark Matter, Higgs boson
Publisert 15. mars 2021 14:26 - Sist endret 12. aug. 2022 12:42


Omfang (studiepoeng)