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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2010)

Many of the proposed solutions to the hierarchy and naturalness problems postulate new ``partner'' fields to the Standard Model (SM) particles. Determining the spins of these new particles will be critical in distinguishing among the various possible SM extensions, yet proposed methods rely on the underlying models. We propose a new model-independent method for spin measurements which takes advantage of quantum interference among helicity states. By looking at the azimuthal angular dependence of the differential cross section in the production followed by decay of a new particle species one can determine its spin by looking at the various cosine modes. We demonstrate that this method will be able to discriminate scalar particles from higher spin states at the ILC, and discuss application to higher spins and possible uses at the LHC. Supersymmetry and Universal Extra Dimensions models prove problematic at the LHC because missing energy signatures result in too many unknowns while reconstructing events. However, warped extra dimension models in certain setups allow for events whose kinematics can be fully reconstructed. In such scenarios, the heavy spin-2 Kaluza-Klein (KK) graviton provides a unique signature with a $cos{(4 phi)}$ mode.We study the feasibility of this approach to measuring the spin of the KK graviton in the warped Randall-Sundrum Model at the LHC.

In chapter 5 of this thesis, taking a phenomenological approach, we study a color sextet scalar at the LHC. We focus on the QCD production of a color sextet pair $Phi_6bar{Phi}_{6}$ through $gg$ fusion and $qbar{q}$ annihilation. Its unique coupling to $bar{psi^c}psi$ allows the color sextet scalar to decay into same-sign diquark states, such as $Phi_6to tt/tt^*$. We propose a new reconstruction in the multijet plus same sign dilepton with missing transverse energy samples ($bb+ell^pmell^pm+cancel{E}_T+Nj$, $Ngeq 6$) to search for on-shell $ttbar{t}bar{t}$ final states from sextet scalar pair production. Thanks to the large QCD production, the search covers the sextet mass range up to 1 TeV for 100 fb$^{-1}$ integrated luminosity.

    Cover page: Spin Determination and Physics Beyond the Standard Model at the LHC and ILC
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2011)
    In anomaly-mediated supersymmetry breaking, superpartners in a hidden sector have masses that are proportional to couplings squared and so naturally freeze out with the desired dark matter relic density for a large range of masses. We present an extremely simple realization of this possibility, with WIMPless dark matter arising from a hidden sector that is supersymmetric QED with NF flavors. Dark matter is multicomponent, composed of hidden leptons and sleptons with masses anywhere from 10 GeV to 10 TeV, and hidden photons provide the thermal bath. The dark matter self-interacts through hidden sector Coulomb scatterings that are potentially observable. In addition, the hidden photon contribution to the number of relativistic degrees of freedom is in the range ΔNeff∼0-2, and, if the hidden and visible sectors were initially in thermal contact, the model predicts ΔN eff∼0.2-0.4. Data already taken by Planck may provide evidence of such deviations. © 2011 American Physical Society.
      Cover page: WIMPless dark matter in anomaly-mediated supersymmetry breaking with hidden QEDCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2014)
      We present a new technique for distinguishing the hadronic decays of boosted heavy particles from QCD backgrounds based on wavelet transforms. As an initial exploration, we illustrate the technique in the particular case of hadronic W boson decays, comparing it to the "mass drop" cut currently used by the LHC experiments. We apply wavelet cuts, which make use of complementary information, and in combination with the mass drop cut result in an improvement of ∼7% in discovery reach of hadronic W boson final states over a wide range of transverse momenta. © 2014 The Author(s).
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2015)
        We present a new probe of strongly coupled electroweak symmetry breaking at the 14 TeV LHC by measuring a phase shift in the event distribution of the decay azimuthal angles in massive gauge boson scattering. One generically expects a large phase shift in the longitudinal gauge boson scattering amplitude due to the presence of broad resonances. This phase shift is observable as an interference effect between the strongly interacting longitudinal modes and the transverse modes of the gauge bosons. We find that even very broad resonances of masses up to 900 GeV can be probed at 3σ significance with a 3000 fb-1 run of the LHC by using this technique. We also present the estimated reach for a future 50 TeV proton-proton collider.
          Cover page: Probing strong electroweak symmetry breaking dynamics through quantum interferometry at the LHC
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2012)
          We present a model with dark matter in an anomaly-mediated supersymmetry breaking hidden sector with a U(1)×U(1) gauge symmetry. The symmetries of the model stabilize the dark matter and forbid the introduction of new mass parameters. As a result, the thermal relic density is completely determined by the gravitino mass and dimensionless couplings. Assuming nonhierarchical couplings, the thermal relic density is Ω X∼0.1, independent of the dark matter's mass and interaction strength, realizing the WIMPless miracle. The model has several striking features. For particle physics, stability of the dark matter is completely consistent with R-parity violation in the visible sector, with implications for superpartner collider signatures; also the thermal relic's mass may be ∼10GeV or lighter, which is of interest given recent direct detection results. Interesting astrophysical signatures are dark matter self-interactions through a long-range force, and massless hidden photons and fermions that contribute to the number of relativistic degrees of freedom at big bang nucleosynthesis and cosmic microwave background. The latter are particularly interesting, given current indications for extra degrees of freedom and near future results from the Planck observatory. © 2012 American Physical Society.
            Cover page: WIMPless dark matter from an anomaly-mediated supersymmetry breaking hidden sector with no new mass parametersCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2012)
            This document proposes a collection of simplified models relevant to the design of new-physics searches at the Large Hadron Collider (LHC) and the characterization of their results. Both ATLAS and CMS have already presented some results in terms of simplified models, and we encourage them to continue and expand this effort, which supplements both signature-based results and benchmark model interpretations. A simplified model is defined by an effective Lagrangian describing the interactions of a small number of new particles. Simplified models can equally well be described by a small number of masses and cross-sections. These parameters are directly related to collider physics observables, making simplified models a particularly effective framework for evaluating searches and a useful starting point for characterizing positive signals of new physics. This document serves as an official summary of the results from the Topologies for Early LHC Searches workshop, held at SLAC in September of 2010, the purpose of which was to develop a set of representative models that can be used to cover all relevant phase space in experimental searches. Particular emphasis is placed on searches relevant for the first 50-500 pb 1 of data and those motivated by supersymmetric models. This note largely summarizes material posted at http://lhcnewphysics.org/, which includes simplified model definitions, Monte Carlo material, and supporting contacts within the theory community. We also comment on future developments that may be useful as more data is gathered and analyzed by the experiments. © 2012 IOP Publishing Ltd.
              Cover page: Simplified models for LHC new physics searches
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