The physics perspectives of the production and decay of single top quarks and top-quark pairs at the CERN Large Hadron Collider (LHC) are reviewed from a phenomenological point of view.

https://iopscience.iop.org/article/10.1088/0954-3899/35/8/083001/pdf

Top-quark physics at the LHC

The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem - is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D-brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades.

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The Hunt for New Physics at the Large Hadron Collider

The physics accessible at the high-luminosity phase of the LHC extends well beyond that of the earlier LHC program. This white paper, submitted as input to the Snowmass Community Planning Study 2013, contains preliminary studies of selected topics, spanning from Higgs boson studies to new particle searches and rare top quark decays. They illustrate the substantially enhanced physics reach with an increased integrated luminosity of 3000 fb-1, and motivate the planned upgrades of the LHC machine and ATLAS detector.

Physics at a High-Luminosity LHC with ATLAS

We systematically calculate various flavor-changing neutral-current top-quark processes induced by supersymmetry at the Large Hadron Collider, which include five decay modes and six production channels. To reveal the characteristics of these processes, we first compare the dependence of the rates for these channels on the relevant supersymmetric parameters, then we scan the whole parameter space to find their maximal rates, including all the direct and indirect current experimental constraints on the scharm-stop flavor mixings. We find that, under all these constraints, only a few channels, through cg -> t at parton level and t -> ch, may be observable at the Large Hadron Collider.

Supersymmetry-induced flavor-changing neutral-current top-quark processes at the CERN Large Hadron Collider

A search for flavor-changing neutral currents in top-quark decays t to Zq is performed in events produced from the decay chain t t-bar to Zq+Wb, where both vector bosons decay leptonically, producing a final state with three leptons (electrons or muons). A dataset collected with the CMS detector at the LHC is used, corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV. No excess is seen in the observed number of events relative to the standard model prediction; thus no evidence for flavor-changing neutral currents in top-quark decays is found. A combination with a previous search at 7 TeV excludes a t to Zq branching fraction greater than 0.05% at the 95% confidence level.

/pdf/search-for-flavor-changing-neutral-currents-in-top-quark-19nqiqjct3.pdf

Search for Flavor-Changing Neutral Currents in Top-Quark Decays $t \to Zq$ in $pp$ Collisions at $\sqrt{s}=8$ TeV

We consider the rare top quark decays in the framework of the top-color-assisted technicolor (TC2) model. We find that the contributions of top-pions and top-Higgs-bosons predicted by the TC2 model can enhance the SM branching ratios by as much as 6\char21{}9 orders of magnitude; i.e., in the extreme case, the orders of magnitude of branching ratios are $Br(\stackrel{\ensuremath{\rightarrow}}{t}cg)\ensuremath{\sim}{10}^{\ensuremath{-}5},$ $Br(\stackrel{\ensuremath{\rightarrow}}{t}cZ)\ensuremath{\sim}{10}^{\ensuremath{-}5},$ $Br(\stackrel{\ensuremath{\rightarrow}}{t}c\ensuremath{\gamma})\ensuremath{\sim}{10}^{\ensuremath{-}7}.$ With reasonable values of the parameters in the TC2 model, such rare top quark decays may be testable in future experiments. Therefore, rare top quark decays provide a unique way to test the TC2 model.

Rare top quark decays t→cV in the top-color-assisted technicolor model

In the framework of topcolor-assisted technicolor(TC2) model, we study a neutral toppion production process $e^{-}\gamma\to e^{-}\Pi^{0}_{t}$ in this paper. Our results show that the production cross section of $e^{-}\gamma\to e^{-}\Pi^{0}_{t}$ can reach the level of several tens fb, and over $10^{3}$ neutral toppion events can be produced in the planned $e^+e^-$ linear colliders each year. Therefore, such a toppion production process provides us a unique chance to detect toppion events and test the TC2 model. On the other hand, the cross section of $e^{-}\gamma\to e^{-}\Pi^{0}_{t}$ is about one order of magnitude larger than those of some similar processes in SM and MSSM(i.e., $e^{-}\gamma\to e^{-}H$ in SM and $e^{-}\gamma\to e^{-}H^{0}(A^0,h^0)$ in MSSM). So, we can easily distinguish the neutral toppion from other neutral Higgs bosons in SM and MSSM.

Production of the neutral top pion at the e gamma colliders

In technicolor models with a scalar doublet, we discuss the effect of the custodial isospin violation on the electroweak parameters S and T in one-loop perturbation theory. We find that the “high-energy” contributions to the parameter S could be reduced by the mass splitting of technifermions p and m. The leading contributions to the parameter T come from the technifermions p−m mass splitting and may exceed the experimental bounds. However, the value of parameter T can be reduced by adjusting the value of δ.

The discussion of electroweak parameters in technicolor models with a scalar doublet

At the energy of LEPII, we carry out the calculation in the process in the framework of the one-generation technicolor model. The results show that the process is immeasurably small in the one-generation TC model.

THE PROCESS $e^+ e^- \to \bar{t}c$ IN THE ONE-GENERATION TECHNICOLOR MODEL

By means of the effective Lagrangian approach we calculate the vertex corrections to the {ital Zb{bar b}} and {ital Z}{tau}{bar {tau}} vertices from sideways extended technicolor (ETC) boson exchange and diagonal ETC boson exchange in a realistic one-family technicolor (TC) model without exact custodial symmetry. We find that the {ital Z} partial width {Gamma}{sub {ital b}}={Gamma}({ital Z}{r_arrow}{ital b{bar b}}), branching ratio {ital R}{sub {ital b}}, and {tau} asymmetry parameter {ital A}{sub {tau}} increase, all in agreement with present experimental data.

Ling-De Wan

Papers

Rare top quark decays t→cV in the top-color-assisted technicolor model

Production of the neutral top pion at the e gamma colliders

The discussion of electroweak parameters in technicolor models with a scalar doublet

THE PROCESS $e^+ e^- \to \bar{t}c$ IN THE ONE-GENERATION TECHNICOLOR MODEL

Corrections to the Zbb̄ and Zττ̄ vertices in a realistic one-family technicolor model