Browsing by Subject "Standard model"

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  • The CMS collaboration; Sirunyan, A. M.; Eerola, P.; Kirschenmann, H.; Pekkanen, J.; Voutilainen, M.; Havukainen, J.; Heikkilä, J. K.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Luukka, P.; Mäenpää, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2018)
    The production of a Z boson, decaying to two charged leptons, in association with jets in proton- proton collisions at a centre- of- mass energy of 13 TeV is measured. Data recorded with the CMS detector at the LHC are used that correspond to an integrated luminosity of 2.19 fb - 1. The cross section is measured as a function of the jet multiplicity and its dependence on the transverse momentum of the Z boson, the jet kinematic variables ( transverse momentum and rapidity), the scalar sum of the jet momenta, which quantifies the hadronic activity, and the balance in transverse momentum between the reconstructed jet recoil and the Z boson. The measurements are compared with predictions from four different calculations. The first two merge matrix elements with different parton multiplicities in the final state and parton showering, one of which includes oneloop corrections. The third is a fixed- order calculation with next- to- next- to- leading order accuracy for the process with a Z boson and one parton in the final state. The fourth combines the fully differential next- to- next- to- leading order calculation of the process with no parton in the final state with next- to- next- to- leading logarithm resummation and parton showering.
  • Stendahl, Alex (Helsingin yliopisto, 2020)
    The Standard model of particle physics has been very successful in describing particles and their interactions. In 2012 the last missing piece, the Higgs boson, was discovered at the Large Hadron Collider. However even for all its success the Standard model fails to explain some phenomena of nature. Two of these unexplained phenomena are dark matter and the metastability of the electroweak vacuum. In this thesis we study one of the simplest extensions of the Standard model; the complex singlet scalar extension. In this framework the CP-even component of the singlet mixes with the Standard model like Higgs boson through the portal operator to form new mass eigenstates. The CP-odd component is a pseudo-Goldstone boson which could be a viable dark matter candidate. We analyse parameter space of the model with respect to constraints from particle physics experiments and cosmological observations. The time evolution of dark matter number density is derived to study the process of dark matter freeze-out. The relic density of the Dark Matter candidate is then calculated with the micrOmegas tool. These calculations are then compared to the measured values of dark matter relic density. Moreover, the electroweak vacuum can be stabilised due the contribution of the singlet scalar to the Standard Model Higgs potential. We derive the β-functions of the couplings in order to study the renormalisation group evolution of the parameters of the model. With the contribution of the portal coupling to the β-function of the Higgs coupling we are able to stabilise the electroweak vacuum up to the Planck scale. The two-loop β-functions are calculated using the SARAH tool.
  • The CMS collaboration; Sirunyan, A. M.; Tumasyan, A.; Eerola, P.; Forthomme, Laurent; Kirschenmann, H.; Österberg, K.; Voutilainen, M.; Brücken, Erik; Garcia, F.; Havukainen, J.; Karimäki, V.; Kim, Minsuk; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Lehti, S.; Lindén, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Luukka, P.; Tuuva, T. (2021)
    A search is performed for the rare decay W-+/--> pi(+/-)gamma in proton-proton collisions at root s = 13 TeV. Data corresponding to an integrated luminosity of 137 fb(-1) were collected during 2016 to 2018 with the CMS detector. This analysis exploits a novel search strategy based on W boson production in top quark pair events. An inclusive search for the W-+/--> pi(+/-)gamma decay is not optimal at the LHC because of the high trigger thresholds. Instead, a trigger selection is exploited in which the W boson originating from one of the top quarks is used to tag the event in a leptonic decay. The W boson emerging from the other top quark is used to search for the W-+/--> pi(+/-)gamma signature. Such decays are characterized by an isolated track pointing to a large energy deposit, and by an isolated photon of large transverse momentum. The presence of b quark jets reduces the background from the hadronization of light-flavor quarks and gluons. The W-+/- -> pi(+/-)gamma decay is not observed. An upper exclusion limit is set to this branching fraction, corresponding to 1.50 x 10(-5) at 95% confidence level, whereas the expected upper exclusion limit is 0.85(-0.29)(+0.52) x 10(-5). (C) 2021 The Author(s). Published by Elsevier B.V.