Narrow Results

The forward–backward asymmetry observed in the top quark pair production at the Fermilab Tevatron points toward the existence of beyond the standard model physics. We have studied the top quark pair production in the TeV energy electron–positron linear collider to the leading order of the noncommutative parameter Θ_μν in the noncommutative standard model. We have made a detailed laboratory frame analysis of the time-averaged cross-section, polar, azimuthal angular distributions, transverse momentum and rapidity distributions, polar (forward–backward) and azimuthal asymmetries of the top-quark pair production in the presence of earth's rotation. We investigated their dependence on the orientation angle of the noncommutative vector η and the noncommutative scale Λ and found that those deviates from the standard model distributions significantly. The azimuthal distribution which is flat in the standard model deviates largely for η = π/2 and Λ = 700 GeV at the fixed machine energy E_com = 1000 GeV. We found that the polar distribution deviates largely from the standard model distribution for η = π/2 and Λ = 500 GeV. The azimuthal asymmetry A_ϕ which is zero in the standard model can be as large as 4% for Λ = 500 GeV and η = π/2 at the fixed machine energy E_com = 1000 GeV. Assuming that the future TeV linear collider will observe A_ϕ = ±0.01 we find Λ≤750(860) GeV corresponding to η = π/2. Similarly, corresponding to polar asymmetry A_FBz = 0.5078 (which deviates from the standard model prediction by 1%), we find Λ≤760 GeV at the fixed machine energy E_com = 1000 GeV for η = π/2.

We review the present situation in top quark physics, in these early days of Run II of the LHC. We take mostly a Standard Model perspective, showing recent results, and review the key concepts and results of the associated theoretical predictions. The issues we discuss are the top quark mass, top quark pair and single top production, production in association with other particles, charge asymmetry and top quark decay.

We present results on top quark physics from the CDF and D0 collaborations at the Fermilab Tevatron collider. These include legacy results from Run II that were published or submitted for publication before mid-2014, as well as a summary of Run I results. The historical perspective of the discovery of the top quark in Run I is also described.

In recent papers, a microscopic model for the SM Higgs mechanism has been proposed, and an idea how to determine the 24 quark and lepton masses of all three generations has emerged in that framework. This idea is worked out in detail here by accommodating the fermion masses and mixings to microscopic parameters. The top quark mass can be given in terms of the Fermi scale and of certain exchange couplings of isospin vectors obeying a tetrahedral symmetry. The observed hierarchy in the family spectrum is attributed to a natural hierarchy in the microscopic couplings. The neutrinos will be shown to vibrate within the potential valleys of the system, thus retaining very tiny masses. This is related to a Goldstone effect inside the internal dynamics. A discussion of the quark and lepton mixing matrices is also included. The mixing angles of the PMNS matrix are calculated for an example set of parameters, and a value for the CP-violating phase is given.

A model is presented where the Higgs mechanism of the Standard Model is deduced from the alignment of a strongly correlated fermion system in an internal space with A₄ symmetry. The ground state is constructed and its energy calculated. Finally, it is claimed that the model may be derived from a field theory in 6 + 1 dimensions.

The Higgs boson $H$ has the largest coupling to the top quark $t$ among the standard model (SM) fermions. This is one of the ideal places to investigate new physics beyond SM. In this work, we study the potential of determining Higgs boson $CP$ properties at the LHC and future 33 TeV and 100 TeV $pp$ colliders by analyzing various operators formed from final states variables in $t\bar{t}H$ production. The discrimination power from SM coupling is obtained with Higgs boson reconstructed from $H\to \gamma \gamma$ and $H\to b\bar{b}$. We find that $t\bar{t}b\bar{b}$ process can provide more than $3\sigma$ discrimination power with $300{\mathrm{\text{fb}}}^{-1}$ integrated luminosity in a wide range of allowed Higgs to top couplings for the LHC, the 33 TeV and 100 TeV colliders. For $t\bar{t}\gamma \gamma$ the discrimination power will be below $3\sigma$ at the LHC, while for 33 TeV and 100 TeV colliders, more than $3\sigma$ sensitivity can be reached.

In this paper, we calculate rare top quark 2-body and 3-body decays in the littlest Higgs Model with T-parity (LHT). We find that the branching ratios of $t\to cX$$(X=g,\gamma ,Z,H)$ and $t\to c{X}_1{X}_2$$(X=g,\gamma ,Z,H,f)$ can be greatly enhanced in the LHT model in the allowed parameter space. The recent measurements of the decay $t\to cg$ can give a strong constraint on the LHT parameters and the decays of $t\to cgg$ and $t\to cb\bar{b}$ can be accessible at the LHC.

We have long speculated,${}^{2,12,50-62}$ that 6 top + 6 antitop quarks due to the relatively large size of the top-Yukawa coupling would bind exceptionally strongly by mainly Higgs exchange. Here we present a surprisingly simple “calculation” of the mass of this speculated bound state. Even a possible resonance in scattering of two such bound states is speculated. For the “calculation” of the masses it is crucial to assume, that our since long speculated principle “multiple point principle,”${}^{5-18}$ is true. This principle says: there are several vacua all having almost zero energy density. Further, we make an approximation of the Higgs Yukawa potential essentially replacing the exponential in it by a step-function. The new result means that there are now two independent calls for our bound state having the mass around 750 GeV required by our “new law of nature” the Multiple Point Principle. It should be remarked that in our picture there is no new physics in the sense of new fundamental particles, but the “multiple point principle” is new in the sense of being not yet accepted. Further, we get the same mass within uncertainties as earlier² but now from a completely different assumption, except for being from our “multiple point principle.” But the two masses are gotten from using different (speculative) vacua occurring in the pure Standard Model.

We collect and estimate support for our long speculated “multiple point principle”${}^{11,12,14-16,18-30,36-39}$ saying that there should be several vacua all having (compared to the scales of high energy physics) very low energy densities. In pure Standard Model, we suggest there being three “multiple point principle” low energy density vacua, “present,” “condensate” and “high field” vacuum. We fit the mass in our picture of a long speculated bound state${}^{8,67-80}$ of 6 top and 6 antitop quarks in three quite independent ways and get remarkably within our crude accuracy the same mass in all three fits! The new point of the present paper is to estimate the bound state mass in what we could call a bag-model estimation. The two other fits, which we review, obtain the mass of the bound state by fitting to the multiple point principle prediction of degenerate vacua. Our remarkable agreement of our three mass-fits can be interpreted to mean that we have calculated at the end the energy densities of the two extra speculated vacua and found that they are indeed very small. Unfortunately, the recently much discussed statistical fluctuation peak F(750)${}^{2-5}$ that has now been revealed to be just a fluctuation, very accidentally matches our fitted mass of the bound state remarkably well with the mass of this fluctuation 750 GeV.

The constraints from the measurements of the $B\to X_s\gamma$ decay rate on the parameter space of 3-Higgs Doublet Models (3HDMs), where all the doublets have nonzero vacuum expectation values, are studied at the next-to-leading order in QCD. In order to naturally avoid the presence of flavour changing neutral currents at the tree level, we impose two softly-broken discrete $Z_2$ symmetries. This gives rise to five independent types of 3HDMs that differ in their Yukawa couplings. We show that in all these 3HDMs (including the case of type-II-like Yukawa interactions) both masses of the two charged Higgs bosons $m_{H_1^{\pm }}$ and $m_{H_2^{\pm }}$ can be smaller than the top mass $m_t$ while complying with the constraints from $B\to X_s\gamma$. As an interesting phenomenological consequence, the branching ratios of the charged Higgs bosons decay into the $cb$ final states can be as large as $80\%$ when their masses are taken to be below $m_t$ in two of the five 3HDMs (named as Type-Y and Type-Z). This light charged Higgs boson scenario provides a hallmark 3HDM signature that cannot be realised in $Z_2$ symmetric 2-Higgs doublet models. We find that in the Type-Y and Type-Z 3HDMs the scenario with $90\mathrm{\text{GeV}}<m_{H_1^{\pm }}$, $m_{H_2^{\pm }}<m_t$ is ruled out by the direct searches at the LHC, but in the Type-Y 3HDM $80\text{GeV}<m_{H_1^{\pm }}<90\text{GeV}$ and $90\text{GeV}<m_{H_2^{\pm }}<m_t$ is allowed by $B\to X_s\gamma$ and direct searches at LEP2, Tevatron and LHC due to the reduced sensitivity of these searches to the degenerate case $m_{H_1^{\pm }}\approx m_{W^{\pm }}$. The cases where only one or both charged Higgs bosons are above the top quark mass are also naturally allowed in the both Type-Y and Type-Z 3HDMs.

I review calculations of soft-gluon corrections for top-quark production in hadron collisions. I describe theoretical formalisms for their resummation and for finite-order expansions. I show that soft-gluon corrections are dominant for a large number of top-quark processes. I discuss top–antitop pair production as well as single-top production, including total cross sections and differential distributions, and compare with data from the LHC and the Tevatron. I also discuss top-quark production in association with charged Higgs bosons, Z bosons and other particles in models of new physics.

Electron–positron annihilation into a pair of top quarks is considered at the energy of the future collider CLIC. Polarization components of the top quark are calculated with the $\gamma t\bar{t}$ and $Zt\bar{t}$ interactions which follow from the Lagrangian of the effective field theory of the Standard Model. The polarization vector as a function of the scattering angle is calculated and averaged components of polarization are analyzed as functions of anomalous coupling constants. Extrema of these observables (maxima, minima and saddle points) are studied as functions of the $e^{+}{e}^{-}$ energy.

The distributions of the bottom quark in the process $e^{+}{e}^{-}\to t\bar{t}\to b{W}^{+}\bar{t}$ are considered at the $e^{+}{e}^{-}$ energy corresponding to the first construction stage of the Compact Linear Collider. The cross-sections of $e^{+}{e}^{-}\to t\bar{t}\to b{W}^{+}\bar{t}$, as functions of the $b$-quark energy and angle with respect to the direction of the electron beam, are derived and calculated. The effects of physics beyond the Standard Model are included via the modified $\gamma t\bar{t}$ and $Zt\bar{t}$ couplings which naturally appear in effective field theories. In addition to the cross-sections, the energy and angular asymmetries are calculated. The dependence of these observables on the $e^{+}{e}^{-}$ energy is calculated, and features of this dependence are investigated.

In this paper, the triple top quark production processes have been investigated and calculated in the scope of the Standard Model. The cross-sections for different channels are provided for the proton–proton collision energies of $\sqrt{s}=14$ and 100 TeV. The importance of the electroweak contribution has been demonstrated. For the main channels, the interference between the gluon and the weak bosons mediated contributions is negative and significant, therefore the complete set of diagrams has to be taken into account. Estimated total rates are about 1.9 fb for $\sqrt{s}=14$ TeV and 530 fb for $\sqrt{s}=100$ TeV. A simple estimation of the uncertainty of the calculated cross-sections gives about 20%. The integrated luminosity of 3 ab${}^{-1}$ at HL-LHC allows expecting about 5700 events, giving a chance to detect this rare process.

In this paper, we study the top quark pair events production in pp collisions in the $\ell +\mathrm{\text{jets}}$ channel at the energy of $\sqrt{s}=14$ TeV for Standard Model as well as new physics processes. We explore the usage of semi-boosted topologies where the top quark decays into a high-transverse momentum (boosted) hadronic W-jet and an isolated b-jet and study their performance in the $t\bar{t}$ events kinematic reconstruction. An important event fraction is recovered and the correlation of selected kinematic variables between the detector and particle level is studied. Quality of the reconstructed mass line shape of a hypothetical scalar resonance decaying into $t\bar{t}$ is evaluated and compared for regimes of a different degree of the transverse boost. Unfolding performance is checked in terms of comparing the excess of events in spectra before and after the unfolding, concluding with the proof of a signal significance loss after the unfolding procedure for both energy and angle related observables, with possible applications for current LHC experiments.

In this paper, we explore the usage of boosted as well as semi-boosted topologies in all-hadronic $t\bar{t}$ final states in simulated $pp$ collisions at $\sqrt{s}=14$TeV, with top quarks decaying into a boosted hadronic top-jet or a W-jet and an isolated b-jet. Correlations between selected kinematic variables and their shapes are studied for scalar and vector resonances decaying to a pair of top quarks, and also for a models of $t\bar{t}$-associated production with an invisible dark matter particle pair. Stacked signal$+$background samples have been investigated in terms of the ability to resolve an excess of events over the Standard Model background in terms of the $t\bar{t}$ invariant mass, top quark transverse momentum and other 1D and 2D spectra using a parameterized detector simulation. A 2D extension of the BumpHunter algorithm is proposed, resulting in an improved signal sensitivity in specific 2D areas. We identify the most promising variables with the largest signal significance and smaller sensitivity to experimental uncertainties related to the jet energy calibration. We compare to statistical tests computing the background-only hypothesis compatibility and a likelihood fit of the signal strength.

Numerical simulations of three and four top quark hadroproduction processes are carried out in the SMEFT model framework. The simulated data are used to derive expected theoretical constraints on Wilson coefficients of relevant SMEFT operators of dimension six. Obtained limits for both cases are discussed and compared in terms of processes’ sensitivity to possible BSM contribution. Results show that operator $O_{tt}^1$ is better constrained by the process of four top quark production, whereas other four operators $O_{QQ}^1$, $O_{Qt}^1$, $O_{Qt}^8$ and $O_{QQ}^8$, are similarly constrained in three and four top quark production processes. In all cases, the expected limits taken from the simultaneous analysis of the production of three and four top quarks are strengthened. Analytical expressions for the partial amplitudes of the processes $tt\to tt$ and $t\bar{t}\to t\bar{t}$ caused by the operators $O_{tt}^1$, $O_{QQ}^1$, $O_{Qt}^1$, $O_{Qt}^8$, $O_{QQ}^8$ were obtained for the first time. Based on the expressions of the obtained partial amplitudes, graphs of the perturbative unitarity boundary for the listed operators were drawn. The question of how kinematic cuts motivated by partial unitarity affect the resulting constraints on the Willson coefficients is addressed. It is shown that in all cases the limits are getting somewhat worse if such cuts are applied.

We evaluate the FCNC decays t → H⁰ + c at tree-level and t → γ + c at one-loop level in the context of Alternative Left-Right symmetric Models (ALRM) with extra isosinglet heavy fermions; in thr first case, FCNC decays occurs at tree-level and they are only suppressed by the mixing between ordinary top and charm quarks.

This paper summarizes top quark cross-section measurements at the Tevatron and the LHC. Top quark pair production cross-sections have been measured in all decay modes by the ATLAS and CMS collaborations at the LHC and by the CDF and D0 collaborations at the Tevatron. Single top quark production has been observed at both the Tevatron and the LHC. The t-channel and associated Wt production modes have been observed at the LHC and evidence for s-channel production has been reported by the Tevatron collaborations.

Almost two decades after its discovery at Fermilab's Tevatron collider experiments, the top quark is still under the spotlight due to its connections to some of the most interesting puzzles in the Standard Model. The Tevatron has been shut down two years ago, yet some interesting results are coming out of the CDF and D0 collaborations. The LHC collider at CERN produced two orders of magnitude more top quarks than Tevatron's, thus giving birth to a new era for top quark physics. While the LHC is also down at the time of this writing, many top quark physics results are being extracted out of the 7 TeV and 8 TeV proton proton collisions by the ATLAS and CMS collaborations, and many more are expected to appear before the LHC will be turned on again sometime in 2015. These proceedings cover a selection of recent results produced by the Tevatron and LHC experiments.