Top decays with flavor changing neutral Higgs interactions at the LHC

The top quark, discovered at the FERMILAB TEVATRON collider in 1995, is the heaviest known elementary particle. Today, ten years later, still relatively little is known about its properties. The strong and weak interactions of the top quark are not nearly as well studied as those of the other quarks and leptons. The strong interaction is most directly measured in top quark pair production. The weak interaction is measured in top quark decay and single top quark production, which remains thus far unobserved. The large top-quark mass of about 175 GeV/c2 suggests that it may play a special role in nature. It behaves differently from all other quarks due to its large mass and its correspondingly short lifetime. The top quark decays before it hadronises, passing its spin information on to its decay products. Therefore, it is possible to measure observables that depend on the top quark spin, providing a unique environment for tests of the Standard Model and for searches for physics beyond the Standard Model. This report summarises the latest measurements and studies of top quark properties and rare decays from the TEVATRON in Run II.With more than 1fb−1 of luminosity delivered to each experiment, CDF and DØ, top quark physics at the TEVATRON is at a turning point from first studies to precision measurements with sensitivity to new physics. An outlook onto top quark physics at the Large Hadron Collider (LHC) at CERN, planned to begin operation in the year 2007, is also given.KeywordsHiggs BosonLarge Hadron ColliderSystematic UncertaintyHadron ColliderIntersect Storage RingThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We consider the possibility that the heavier CP-even Higgs boson~($H^0$) in the minimal supersymmetric standard model (MSSM) decays invisibly into neutralinos in the light of the recent discovery of the 126 GeV resonance at the CERN Large Hadron Collider (LHC). For this purpose we consider the minimal supersymmetric standard model with universal, non-universal and arbitrary boundary conditions on the supersymmetry breaking gaugino mass parameters at the grand unified scale. Typically, scenarios with universal and nonuniversal gaugino masses do not allow invisible decays of the lightest Higgs boson~($h^0$), which is identified with the $126$ GeV resonance, into the lightest neutralinos in the MSSM. With arbitrary gaugino masses at the grand unified scale such an invisible decay is possible. The second lightest Higgs boson can decay into various invisible final states for a considerable region of the MSSM parameter space with arbitrary gaugino masses as well as with the gaugino masses restricted by universal and nonuniversal boundary conditions at the grand unified scale.The possibility of the second lightest Higgs boson of the MSSM decaying into invisible channels is more likely for arbitrary gaugino masses at the grand unified scale. The heavier Higgs boson decay into lighter particles leads to the intriguing possibility that the entire Higgs boson spectrum of the MSSM may be visible at the LHC even if it decays invisibly, during the searches for an extended Higgs boson sector at the LHC. In such a scenario the nonobservation of the extended Higgs sector of the MSSM may carefully be used to rule out regions of the MSSM parameter space at the LHC.

We investigate the observability of the lightest Higgs boson in the gluon-fusion channel at the CERN Large Hadron Collider (LHC) in the minimal supersymmetric Standard Model with explicit CP-violating mixing among three neutral Higgs bosons. The lightest Higgs boson with its mass less than 130 GeV can be detected at the LHC via its gluon-fusion production followed by the decay into two photons. The explicit CP violation can suppress both the production cross section and the two-photon decay branching fraction so significantly that the signal cross section may be more than ten times smaller than the SM signal. This reduction factor can be as small as 1/40 if the lightest Higgs boson mass is 115 GeV and its production cross section at LEP2 is more than 90 % that of the SM case.

We review the prospects for studies of the top quark at the LHC. Members of the working group who have contributed to this document are: A.Ahmadov, G.Azuelos, U.Baur, A.Belyaev, E.L.Berger, W.Bernreuther, E.E.Boos, M.Bosman, A.Brandenburg, R.Brock, M.Buice, N.Cartiglia, F.Cerutti, A.Cheplakov, L.Chikovani, M.Cobal-Grassmann, G.Corcella, F.del Aguila, T.Djobava, J.Dodd, V.Drollinger, A.Dubak, S.Frixione, D.Froidevaux, B.Gonzalez Pineiro, Y.P.Gouz, D.Green, P.Grenier, S.Heinemeyer, W.Hollik, V.Ilyin, C.Kao, A.Kharchilava, R. Kinnunen, V.V.Kukhtin, S.Kunori, L.La Rotonda, A.Lagatta, M.Lefebvre, K.Maeshima, G.Mahlon, S.Mc Grath, G.Medin, R.Mehdiyev, B.Mele, Z.Metreveli, D.O'Neil, L.H.Orr, D.Pallin, S.Parke, J.Parsons, D.Popovic, L.Reina, E.Richter-Was, T.G.Rizzo, D.Salihagic, M.Sapinski, M.H.Seymour, V.Simak, L.Simic, G.Skoro, S.R.Slabospitsky, J.Smolik, L.Sonnenschein, T.Stelzer, N.Stepanov, Z.Sullivan, T.Tait, I.Vichou, R.Vidal, D.Wackeroth, G.Weiglein, S.Willenbrock, W.Wu

We consider photon radiation in tbar-t events at the upgraded Fermilab Tevatron and the CERN Large Hadron Collider (LHC) as a tool to measure the electric charge of the top quark. We analyze the contributions of tbar-t-gamma production and radiative top quark decays to p-p, pbar-p -> gamma l^+/- nu bbar-b jj, assuming that both b-quarks are tagged. With 20~fb^{-1} at the Tevatron, the possibility that the ``top quark'' discovered in Run I is actually an exotic charge -4/3 quark can be ruled out at the 95% confidence level. At the LHC, it will be possible to determine the charge of the top quark with an accuracy of about 10%.

This thesis presents two research works carried out within the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC): the system validation of the scintillator section of the High Granularity Calorimeter (HGCAL), and a search for a heavy resonance coupling to top quarks and produced in association with top quark pairs in proton–proton collisions at center-of-mass energies of 13 and 13.6 TeV. For the High-Luminosity LHC (HL-LHC) phase, the endcap calorimeters of CMS will be replaced by the HGCAL, a high-precision sampling calorimeter designed to cope with the extreme radiation and pileup conditions expected at the HL-LHC. In regions where radiation levels remain moderate, the hadronic section employs scintillator tiles coupled to silicon photomultipliers (SiPM-on-tile) as active materials. Each HGCAL tilemodule, the basic detector unit of this region, accommodates up to 144 SiPM-on-tile channels read out by dedicated front-end electronics. The performance of several tilemodules was evaluated during test-beam campaigns at the DESY-II facility using 3 GeV electron beams. The measurements serve to validate the response and uniformity of the detector components and to confirm that they meet the design specifications for HL-LHC operation. A search for a new heavy resonance that couples exclusively to top quarks and is produced in association with a top quark pair is presented. The analysis uses proton–proton collision data collected with the CMS detector at center-of-mass energies of 13 TeV (2016–2018) and 13.6 TeV (2022). The search targets the final state with two oppositely charged leptons from the W boson decays of the nonresonant top quarks, and two high-energy jets reconstructing the hadronically decaying resonant top quarks. In this scenario, the two top quarks from the resonance decay are expected to be highly Lorentz-boosted. Their hadronic decays are reconstructed using jets clustered with a variable-radius algorithm and identified with a dedicated top quark tagger based on a boosted decision tree (BDT). Events are selected in a final state containing opposite-sign leptons and b-tagged jets, and the resonance mass is reconstructed from pairs of tagged top jets. The analysis probes resonance masses between 500 GeV and 4 TeV and searches for local excesses in the reconstructed mass spectrum. The results are interpreted in the context of models with vector-like, scalar and pseudoscalar resonances produced in association with top quarks, representing the first such interpretation performed with CMS data.

We consider the measurement of the trilinear couplings of the neutral Higgs bosons in the minimal supersymmetric standard model (MSSM) at a high energy [Formula: see text] linear collider in the light of the discovery of a Higgs boson at the CERN Large Hadron Collider (LHC). We identify the state observed at the LHC with the lightest Higgs boson [Formula: see text] of the MSSM, and impose the constraints following from this identification, as well as other experimental constraints on the MSSM parameter space. In order to measure trilinear neutral Higgs couplings, we consider different processes where the heavier Higgs boson [Formula: see text] of the MSSM is produced in electron–positron collisions, which subsequently decays into a pair of lighter Higgs boson. We identify the regions of the MSSM parameter space where it may be possible to measure the trilinear couplings of the Higgs boson at a future electron–positron collider. A measurement of the trilinear Higgs couplings is a crucial step in the construction of the Higgs potential, and hence in establishing the phenomena of spontaneous symmetry breaking in gauge theories.

Top quark pair production and decay at hadron colliders: Predictions at NLO QCD including spin correlations

The CERN Large Hadron Collider (LHC) is designed to collide proton beams of unprecedented energy, in order to extend the frontiers of high-energy particle physics. During the first very successful running period in 2010--2013, the LHC was routinely storing protons at 3.5--4 TeV with a total beam energy of up to 146 MJ, and even higher stored energies are foreseen in the future. This puts extraordinary demands on the control of beam losses. An un-controlled loss of even a tiny fraction of the beam could cause a superconducting magnet to undergo a transition into a normal-conducting state, or in the worst case cause material damage. Hence a multi-stage collimation system has been installed in order to safely intercept high-amplitude beam protons before they are lost elsewhere. To guarantee adequate protection from the collimators, a detailed theoretical understanding is needed. This article presents results of numerical simulations of the distribution of beam losses around the LHC that have leaked out of the collimation system. The studies include tracking of protons through the fields of more than 5000 magnets in the 27 km LHC ring over hundreds of revolutions, and Monte-Carlo simulations of particle-matter interactions both in collimators and machine elements being hit by escaping particles. The simulation results agree typically within a factor 2 with measurements of beam loss distributions from the previous LHC run. Considering the complex simulation, which must account for a very large number of unknown imperfections, and in view of the total losses around the ring spanning over 7 orders of magnitude, we consider this an excellent agreement. Our results give confidence in the simulation tools, which are used also for the design of future accelerators.

Associated production of the Higgs boson and a single top quark in the littlest Higgs model at Large Hadron Collider

We investigate the potential of the compact muon solenoid detector at the CERN Large Hadron Collider (LHC) to discriminate between two theoretical models predicting anomalous events with jets and large missing transverse energy, minimal supersymmetry, and little Higgs with T parity. We focus on a simple test-case scenario, in which the only exotic particles produced at the LHC are heavy color-triplet states (squarks or T quarks), and the only open decay channel for these particles is into the stable missing-energy particle (neutralino or heavy photon) plus a quark. We find that in this scenario, the angular and momentum distributions of the observed jets are sufficient to discriminate between the two models with a few inverse femtobarns of the LHC data, provided that these distributions for both models and the dominant standard model backgrounds can be reliably predicted by Monte Carlo simulations.

The top quark is the heaviest known fundamental particle. As it is the only quark that decays before it hadronizes, it allows us to probe the properties of bare quarks at the Large Hadron Collider. Highlights of a few recent precision measurements by the ATLAS Collaboration of the top quark using 13 TeV and 8 TeV collision data will be presented: top-quark pair and single top production cross sections including differential distributions will be presented alongside measurements of top-quark properties, including results using boosted top quarks, probe our understanding of top-quark production in the TeV regime. Measurements of the top-quark mass and searches for rare top quark decays are also presented.

The Compact Linear Collider (CLIC) is a mature option for a future electron-positron collider operating at centre-of-mass energies of up to 3 TeV. CLIC will be built and operated in a staged approach with three centre-of-mass energy stages currently assumed to be 380 GeV, 1.5 TeV and 3 TeV. This contribution discusses the prospects for precision measurements of top-quark production and properties at CLIC. Based on a dedicated centre-of-mass energy scan around the top-quark pair production threshold, the top-quark mass can be determined with a precision of about 50 MeV in a theoretically well-defined manner. Other approaches to extract the top-quark mass at the first stage of CLIC make use of ISR photons or the direct reconstruction of the top quarks. The large number of top-quark pairs produced also allows competitive searches for Flavour Changing Neutral Current (FCNC) top-quark decays to charm quarks. At the high-energy stages of CLIC the top Yukawa coupling and CP properties in the ttH coupling can be studied, as well as the top-quark pair production through the vector-boson fusion. Expected limits on new physics contributions described in terms of Effective Field Theory (EFT) operator coefficients are presented, showing sensitivity of CLIC to mass scales up to 100 TeV.

Top quark pair production and decay at a polarized photon collider

In this paper, we consider the processes of single top-quark production in association with dark matter particles through the t-channel in simplified models with scalar and pseudoscalar mediators. Within the framework of these models, we analyze the angular correlations that arise in the top quark production process in association with dark matter, comparing them with the production of a top quark in the Standard Model. We consider angular correlations of kinematic variables, which define admissible regions in momentum space and can be used experimentally to distinguish between the fraction of the total momentum carried away by the scalar or pseudoscalar mediator. We demonstrate that scalar mediator significantly changes the rest frame of the well-known angular correlation observable and changes the distributions. In this case, this variable provides clear separation between the SM and DM contribution and can be used to increase the sensitivity of future DM analyses.