Project C - Search for new particles and symmetries

Inclusive searches for new particles in supersymmetric or other extensions of the standard model, exclusive searches and parameter determination in specific supersymmetric models, background estimation

G. Herten, H. Ita, K. Jakobs, H. Rzehak, J. J. van der Bij 


Subjects for PhD theses


 C.1: Inclusive study of final states with jets, leptons and missing transverse momentum (Herten)


  • Optimisation of inclusive SUSY searches at √s = 13-14 TeV (Herten)

    Inclusive searches for SUSY particles with jets , leptons and missing transverse momentum offer a powerful method  to detect strongly produced squarks and gluinos. The present analyses will be adapted to and optimized for  the higher center-of-mass energies of 13-14 TeV. The higher energy and luminosity require new background estimations with data-driven methods. In the first step we will investigate  specific models, like CMSSM or “simplified models”, followed by a systematic evaluation of pMSSM models as a second step.
  • Improving the sensitivity in searches with boosted jets (Herten)

    The topic for a thesis is to improve the sensitivity and reach in searches for new particles with so called “boosted jets”. Such jets emerge for example from decays of W- or Higgs-bosons at high transverse momentum, which might be produced in the decay of heavy SUSY particles with a subsequent decay into jets. These jets overlap due to the high momentum of the parent particle. By including boosted jets into the analyses one can therefore improve the reach in particle searches especially for high particle masses and  large mass differences, e.g. for gluinos and squark production at the highest LHC energies.
  • Study of final states with two same sign leptons (Herten)

    The study of final states with two leptons of the same charge provides a powerful method to detect new particles, like gluinos and squarks. The background from standard model processes is small for this event topology. A thesis offers the chance to detect gluinos or squarks with relatively low luminosity at 13-14 TeV. At the same time there are many synergies with studies of vector boson scattering, and production of WW, W+W+ or W-W- final states. 


C.2  Search for supersymmetric partners of third generation quarks


  • Search for pair production of sbottom and stop pairs at √s=13-14 TeV (Jakobs)

    This thesis is devoted to the searches for sbottom and stop pairs in the decay modes stop stop →tχ1010 or sbottom sbottom →bχ1010 at the new centre-of-mass energies to explore new mass ranges. Both the gluino induced as well as the direct production will be considered. The selection criteria have to be adapted and optimised for the new centre-of-mass energy and properties of signal and background processes.

  • Search for stop pair production in final states with tau leptons at a centre-of-mass energy of 13-14 TeV (Herten, Jakobs, Rzehak)

    A PhD thesis will focus on the search for stop pairs via decays in light staus or sneutrinos. Search strategies and selection criteria will be developed and optimized. Furthermore, the results will be interpreted with respect to different SUSY models.


C.3 Background estimation


  • Measurement of the ratio of production cross sections of Z(→l+l-)+jets /γ+jets (Herten)

    One of the main irreducible backgrounds to inclusive searches for supersymmetric particles, especially for final states with jets and missing transverse momentum without leptons, is the Z+jets production with a subsequent Z→νν decay. This background can be estimated from data using the Z→l+l--production, but this is statistically limited due to the small branching ratio. Using the ratio of the Z→l+l-+jets /γ+jets production and the assumption that QCD corrections are similar in both processes one can obtain a more precise estimation of the  Z→vv background. 


  • Background Processes of pp → W(Z)+(≥ n)-Jets at NLO in QCD (Ita)

    With the aim of providing predictions including QCD corrections for the data analysis of the LHC experiments, the available matrix elements for the pp → W(Z)+(≥ n)-jets signature will be integrated in to the Sherpa parton-shower. This will allow to obtain a full-event simulation. The key signature will be missing energy in association with multiple jets. First simulations are planned for transfer factors between signal and control regions. Initially these will be obtained by NLO-QCD computations at parton level for background studies for supersymmetry searches. The combination of the available matrix elements with the parton shower poses a significant challenge given the large number of subprocesses that have to be handled simultaneously. Theoretical structures, such as unitarity, on-shell methods and methods to deal with color degrees effectively will be exploited to deal with the computational and numerical challenges.


  • Vector-boson production in association with b-Quarks at NLO-QCD accuracy (Ita)

    It is planned to compute the production of vector bosons in association with light as well as heavy jets, that is W(Z)+bb ̄+(≥ 1)-Jets at next-to-leading order accuracy in perturbative QCD. Initially the available matrix elements for W(Z)+(4 ≥)-Jets will be used to approximate the behaviour of b-quarks. Later mass effects will be included, which requires the extension of the matrix elements currently available. The extensions are planned within the framework of the BlackHat library. The phenomenological aims include the study of backgrounds to super partners of the third generation quarks, which are an important topic of the experimental groups in Freiburg. Furthermore, the novel results will be used for the analysis of the irreducible Wbb backgrounds in the search for H → bb decays. 


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