Browsing by Subject "VIOLATION"

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Now showing items 1-10 of 10
  • The CMS collaboration; Tumasyan, A.; Adam, W.; Eerola, P.; Forthomme, Laurent; Kirschenmann, H.; Österberg, K.; Voutilainen, M.; Bharthuar, Shudhashil; Brücken, Erik; Garcia, F.; Havukainen, J.; Heikkilä, Jaana; Kim, Minsuk; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Lotti, Mikko; Luukka, P.; Martikainen, Laura; Myllymäki, Mikael Erkki Johannes; Ott, Jennifer; Pekkanen, Juska; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Viinikainen, Jussi; Petrow, H.; Tuuva, T. (2021)
    A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV.
  • Huitu, K.; Kärkkäinen, T. J.; Maalampi, J.; Vihonen, S. (2018)
    The triplet scalars (Delta = Delta(++), Delta(+), Delta(0)) utilized in the so-called type-II seesaw model to explain the lightness of neutrinos, would generate nonstandard interactions (NSI) for a neutrino propagating in matter. We investigate the prospects to probe these interactions in long baseline neutrino oscillation experiments. We analyze the upper bounds that the proposed DUNE experiment might set on the nonstandard parameters and numerically derive upper bounds, as a function of the lightest neutrino mass, on the ratio the mass M-Delta of the triplet scalars, and the strength vertical bar lambda(phi)vertical bar of the coupling phi phi Delta of the triplet Delta and conventional Higgs doublet phi. We also discuss the possible misinterpretation of these effects as effects arising from a nonunitarity of the neutrino mixing matrix and compare the results with the bounds that arise from the charged lepton flavor violating processes.
  • Huitu, Katri; Koivunen, Niko (2018)
    The models with the gauge group SU(3)(c) x SU(3)(L) x U(1)(x) (331 models) have been advocated to explain why there are three fermion generations in nature. As such, they can provide partial understanding of the flavor sector. The hierarchy of Yukawa couplings in the Standard Model is another puzzle that remains without a compelling explanation. We propose to use the Froggatt-Nielsen mechanism in a 331 model to explain both fundamental problems. It turns out that no additional representations in the scalar sector are needed to take care of this. The traditional 331 models predict unsuppressed scalar flavor-changing neutral currents at tree level. We show that they are strongly suppressed in our model.
  • Huitu, Katri; Keus, Venus; Koivunen, Niko; Lebedev, Oleg (2016)
    ATLAS and CMS have reported an excess in the flavor violating decay of the Higgs boson, h -> mu tau. We show that this result can be accommodated through a mixing of the Higgs with a flavon, the field responsible for generating the Yukawa matrices in the lepton sector. We employ a version of the Froggatt-Nielsen mechanism at the electroweak scale, with only the leptons and the flavon transforming non-trivially under the corresponding symmetry group. Non-observation of charged lepton flavor violation (LFV) in other processes imposes important constraints on the model, which we find to be satisfied in substantial regions of parameter space.
  • Fujikawa, Kazuo; Tureanu, Anca (2018)
    The idea that the Majorana neutrino should be identified as a Bogoliubov quasiparticle is applied to the seesaw mechanism for the three generations of neutrinos in the Standard Model. A relativistic analog of the Bogoliubov transformation in the present context is a CP-preserving canonical transformation but modifies charge conjugation properties in such a way that the C-noninvariant fermion number-violating term (condensate) is converted to a Dirac mass term. Puzzling aspects associated with the charge conjugation of chiral Weyl fermions are clarified.
  • Fujikawa, RIKEN; Tureanu, Anca (2021)
    In the analysis of neutron-antineutron oscillations, it has recently been argued in the literature that the use of the i gamma(0) parity n(p)(t, -(x) over right arrow) = i gamma(0)n(t, -(x) over right arrow), which is consistent with the Majorana condition, is mandatory and that the ordinary parity transformation of the neutron field n(p)(t, -(x) over right arrow) = i gamma(0)n(t, -(x) over right arrow) has difficulties. We show that a careful treatment of the ordinary parity transformation of the neutron works in the analysis of neutron-antineutron oscillations. Technically, CP symmetry in a mass diagonalization procedure is important and the two parity transformations i gamma(0)-parity and gamma(0)-parity are compensated for by the PauliGursey transformation. Our analysis shows that either choice of parity gives the correct results for neutronantineutron oscillations if carefully treated.
  • Das, Debottam; Ghosh, Kirtiman; Mitra, Manimala; Mondal, Subhadeep (2018)
    We consider an extension of the standard model (SM) augmented by two neutral singlet fermions per generation and a leptoquark. In order to generate the light neutrino masses and mixing, we incorporate inverse seesaw mechanism. The right-handed neutrino production in this model is significantly larger than the conventional inverse seesaw scenario. We analyze the different collider signatures of this model and find that the final states associated with three or more leptons, multijet and at least one b-tagged and ( or) t-tagged jet can probe larger RH neutrino mass scale. We have also proposed a same-sign dilepton signal region associated with multiple jets and missing energy that can be used to distinguish the present scenario from the usual inverse seesaw extended SM.
  • Tureanu, Anca (2018)
    We formulate the quantum field theory description of neutron-antineutron oscillations in the framework of canonical quantization, in analogy with the Bardeen-Cooper-Schrieffer theory and the Nambu-Jona-Lasinio model. The physical vacuum of the theory is a condensate of pairs of would-be neutrons and antineutrons in the absence of the baryon-number violating interaction. The quantization procedure defines uniquely the mixing of massive Bogoliubov quasiparticle states that represent the neutron. In spite of not being mass eigenstates, neutron and antineutron states are defined on the physical vacuum and the oscillation formulated in asymptotic states. The exchange of the baryonic number with the vacuum condensate engenders what may be observed as neutron-antineutron oscillation. The convergence between the present canonical approach and the Lagrangian/path integral approach to neutron oscillations is shown by the calculation of the anomalous (baryon-number violating) propagators. The quantization procedure proposed here can be extended to neutrino oscillations and, in general, to any particle oscillations.
  • 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. (2019)
    A search is presented for resonant production of second-generation sleptons ((mu) over tilde (L),(nu) over tilde (mu)) via the R-parityviolating coupling. gimel'(211) to quarks, in events with two samesign muons and at least two jets in the final state. The smuon ( muon sneutrino) is expected to decay into amuon and a neutralino (chargino), which will then decay into a second muon and at least two jets. The analysis is based on the 2016 data set of proton- proton collisions at root s = 13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 35.9 fb(-1). No significant deviation is observed with respect to standard model expectations. Upper limits on cross sections, ranging from 0.24 to 730 fb, are derived in the context of two simplified models representing the dominant signal contributions leading to a same- sign muon pair. The cross section limits are translated into coupling limits for a modified constrained minimal supersymmetric model with. gimel'(211) as the only nonzero R-parity violating coupling. The results significantly extend restrictions of the parameter space compared with previous searches for similar models.
  • 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.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2018)
    A search for new physics is presented in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from b quarks, and missing transverse momentum (p(T)(miss)). The search uses proton-proton collision data at root s = 13 TeV amounting to 35.9 fb(-1) of integrated luminosity collected using the CMS detector in 2016. Hypothetical signal events are efficiently separated from the dominant t (t) over bar background with requirements on p(T)(miss) and transverse-mass variables. No significant deviation is observed from the expected background. Exclusion limits are set in the context of simplified supersymmetric models with pair-produced top squarks. For top squarks, decaying exclusively to a top quark and a neutralino, exclusion limits are placed at 95% confidence level on the mass of the lightest top squark up to 800 GeVand on the lightest neutralino up to 360 GeV. These results, combined with searches in the single-lepton and all-jet final states, raise the exclusion limits up to 1050 GeV for the lightest top squark and up to 500 GeV for the lightest neutralino. For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1300 GeV for top squarks and up to 800 GeV for the lightest neutralino. The results are also interpreted in a simplified model with a dark matter (DM) particle coupled to the top quark through a scalar or pseudoscalar mediator. For light DM, mediator masses up to 100 (50) GeV are excluded for scalar (pseudoscalar) mediators. The result for the scalar mediator achieves some of the most stringent limits to date in this model.