T-channel Single Top Production Research Articles

Evaluating master integrals in non-factorizable corrections to t-channel single-top production at NNLO QCD

We studied the two-loop non-factorizable Feynman diagrams for the t-channel single-top production process in quantum chromodynamics. We present a systematic computation of master integrals of the two-loop Feynman diagrams with one internal massive propagator in which a complete uniform transcendental basis can be built. The master integrals are derived by means of canonical differential equations and uniform transcendental integrals. The results are expressed in the form of Goncharov polylogarithm functions, whose variables are the scalar products of external momenta, as well as the masses of the top quark and the W boson. We also gave a discussion on the diagrams with potential elliptic sectors.

Non-factorisable contribution to t-channel single-top production

We compute the non-factorisable mathcal{O} ( {alpha}_s^2 ) corrections to t-channel single-top quark production at the LHC. These peculiar corrections arise because of interactions between the heavy- and the light-quark lines and appear for the very first time at next-to-next-to-leading order in perturbative QCD. We find that the non-factorisable corrections change the single-top production cross section and the relevant kinematic distributions in this process by about half a percent.

On non-factorisable contributions to t-channel single-top production

We compute the non-factorisable contribution to the two-loop helicity amplitude for t-channel single-top production, the last missing piece of the two-loop virtual corrections to this process. Our calculation employs analytic reduction to master integrals and the auxiliary mass flow method for their fast numerical evaluation. We study the impact of these corrections on basic observables that are measured experimentally in the single-top production process.

Top-quark mass determination from t-channel single top production at the LHC

We study the determination of the top-quark mass using leptonic observables in t-channel single top-quark production at the LHC. We demonstrate sensitivity of transverse momentum of the charged lepton on the input top-quark mass. We present predictions at next-to-next-to-leading order (NNLO) in QCD with narrow width approximation and structure function approach. Further corrections due to parton shower and hadronization, non-resonant and non-factorized contributions are discussed. To reduce impact of SM backgrounds we propose to use the charge weighted distribution for the measurement, i.e., differences between distributions of charged lepton with positive and negative charges. By modeling both signal and background processes, we found the projections for (HL-)LHC to be promising, with a total theoretical uncertainty on the extracted top-quark mass of about 1 ∼ 2 GeV.

Precise predictions for single-top production: the impact of EW corrections and QCD shower on the t-channel signature

In this work we calculate and provide precise predictions for the signature that is typically exploited at the LHC for the measurement of t-channel single-top production: 1 lepton, 1 light jet, 1 b-jet, ɆT and no additional jets or leptons. We apply the cuts that define the fiducial region and we take into account all the contributions to this signature; not only those from resonant t-channel single-top production. On the one hand, we calculate the complete-NLO corrections, i.e., all NLO effects of QCD and EW origin. On the other hand, we study in detail the impact of a QCD parton shower for the fiducial region we consider. We provide predictions in different approximations at the inclusive level and for several differential distributions. Our study demonstrates the relevance of both EW corrections and shower effects for obtaining precise and reliable theoretical predictions for the single-top-production fiducial region.

Determination of the TMD gluon density in a proton using recent LHC data

Unintegrated (or transverse momentum dependent, TMD) parton distributions in a proton are important in high-energy physics. Using the latest LHC data on the hadron production in pp collisions, we determine the parameters of the initial TMD gluon density, derived in the framework of quark-gluon string model at the low scale μ0 ~ 1 − 2 GeV and refine its large-x behavior using data on the tt̅ production at $\sqrt s = 13\,{\rm{TeV}}$. Then, by using the Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation, we extend the obtained TMD gluon density to the whole kinematic region. We tested the proposed TMD gluon density to the inclusive Higgs production in different decay modes, t-channel single top production at the LHC and to the proton structure functions $F_2^c(x,\,{Q^2})$ and $F_2^b(x,\,{Q^2})$ in a wide region of x and Q2. Good agreement with the latest LHC and HERA data is achieved.

Effective operators in t-channel single top production and decay

The production of a single top quark in the t-channel and its subsequent decay is studied at NLO accuracy in QCD, augmented with the relevant dimension-6 effective operators from the Standard Model Effective Theory. We examine various kinematic and angular distributions for proton-proton collisions at the LHC at 13 TeV, in order to assess the sensitivity to these operators, both with and without the top quark narrow width approximation. Our results will be helpful when devising strategies to establish bounds on their coefficients, including the amount of CP violation of the weak dipole operator.

A general analysis of Wtb anomalous couplings**Supported by National Science Foundation of China (11275009, 11675002, 11635001), National Science Foundation (PHY-1315983, PHY-1316033) and DOE (DE- SC0011095)

We investigate new physics effects on the Wtb effective couplings in a model-independent framework. The new physics effects can be parametrized by four independent couplings, , , and . We further introduce a set of parameters x0, xm, xp and x5 which exhibit a linear relation to the single top production cross sections. Using recent data for the t-channel single top production cross section σt, tW associated production cross section σtW, s-channel single top production cross section σs, and W-helicity fractions F0, FL and FR collected at the 8 TeV LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our global fitting results show that the top quark effective couplings are strongly correlated. We show that (i) improving the measurements of σt and σtW is important in constraining the correlation of (,) and (,); (ii) and are anti-correlated, and are sensitive to all the four experiments; (iii) and are also anti-correlated, and are sensitive to the F0 and FL measurements; (iv) the correlation between and is sensitive to the precision of the σt, σtW and F0 measurements. The effective Wtb couplings are studied in three kinds of new physics models: the G(221) = SU(2)1 ⊗ SU(2)2 ⊗ U(1)X models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson’s mass (MW′) is less than several hundred GeV, but the constraint is loose if MW′ > 1 TeV. Furthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.

Measurements of single top quark cross sections in pp collisions at 8 TeV with the CMS detector

Measurements are presented of the single top quark production in proton-proton collisions at the LHC at a centre-of-mass energy of 8 TeV, using data collected with the CMS experiment during 2012. The first analysis, in the t-channel single top production mode, considers decay channels where the W comes from the top decays into electron-neutrino or muon-neutrino. It makes use of a template fit exploiting the pseudorapidity distribution of the recoil jet and the reconstructed top quark mass, using background estimates determined from control samples in data. The measurement of top/antitop cross section ratio is also presented.The second analysis, measuring the associated production cross section of single top quark and W boson, considers final states in which the associated W boson, as well as the one originating from the top quark, decay leptonically. Multivariate methods are used to separate the signal from the topologically similar top pair production background. Multivariate techniques are also adopted in the search for s-channel single top quark production finally reported, in order to discriminate the very small signal from the huge background processes.

Associated production of single top and Higgs at the LHC in the littlest Higgs model with T-parity

In the littlest Higgs model with T-parity (LHT), we study the t-channel single top production in association with a Higgs boson at 8 and 14 TeV LHC. We find that the cross section can be enhanced obviously in this model compared to the Standard Model. By performing a simple parton-level simulation through $$ pp\to t\left(\to {\ell}^{+}\nu b\right)h\left(\to b\overline{b}\right)j $$ at 14 TeV LHC, we find that the observability of the signal is promising in the favorable parameter space.

Top-quark physics at colliders

Results are reviewed which were obtained within the project Top-quark physics at colliders. These include QCD and weak-interaction corrections to hadronic tt¯ production, determinations of the the top-quark mass, predictions of the tt¯ charge asymmetry and tt¯ spin correlations, NLO QCD corrections to hadronic tt¯+1-jet production, and partial results for hadronic t-channel single-top production at NNLO QCD. In addition, some methodical developments made in this project are outlined, which are relevant for NLO and NNLO QCD calculations.

Probing anomalous t t ¯ Z $$ t\overline{t}Z $$ interactions with rare meson decays

Anomalous couplings of the $Z$ boson to top quarks are only marginally constrained by direct searches and are still sensitive to new particle dynamics at the TeV scale. Employing an effective field theory approach we consider the dimension-six operators which generate deviations from the standard-model vector and axial-vector interactions. We show that rare $B$ and $K$ meson decays together with electroweak precision observables provide strong constraints on these couplings. We also consider constraints from t-channel single-top production.

Top-quark processes at NLO in production and decay

We describe the implementation of top production and decay processes in the parton-level Monte Carlo program MCFM. By treating the top quark as being on-shell, we can factorize the amplitudes for top-pair production, s-channel single-top production, and t-channel single-top production into the product of an amplitude for production and an amplitude for decay. In this way we can retain all spin correlations. Both the production and the decay amplitudes are calculated consistently at next-to-leading order in . The full dependence on the b-quark mass is also kept. Phenomenological results are presented for various kinematic distributions at the LHC and for the top quark forward–backward asymmetry at the Tevatron.

Four-dimensional formulation of the sector-improved residue subtraction scheme

Four years ago, one of us introduced a novel subtraction scheme [1] for the evaluation of double-real radiation contributions to cross sections at next-to-next-to-leading order (NNLO) in QCD. This approach, named SecToR Improved Phase sPacE for Real radiation (STRIPPER), has already found several non-trivial applications. In particular, it has allowed for the determination of NNLO corrections to hadronic top-quark pair production, fully differential top-quark decays, inclusive semileptonic charmless b-quark decays, associated Higgs boson and jet production in gluon fusion, muon decay spin asymmetry, and t-channel single-top production. Common to these calculations was the use of conventional dimensional regularization (CDR). In this publication, we present a complete formulation of the subtraction scheme for arbitrary processes with any number of colored partons in the final state, and up to two partons in the initial state. Furthermore, we modify the integrated subtraction terms of the double-real radiation to enable the introduction of the 't Hooft–Veltman version of dimensional regularization (HV), in which resolved states are four-dimensional. We demonstrate the correctness of our approach on the example of top-quark pair production in the gluon fusion channel.

Top-quark transverse-momentum distributions int-channel single-top production

I present approximate next-to-next-to-leading-order (NNLO) top-quark transverse momentum, pT, distributions in t-channel single-top production. These distributions are derived from next-to-next-to-leading-logarithm (NNLL) soft-gluon resummation. Theoretical results for the single top as well as the single antitop pT distributions are shown for LHC and Tevatron energies.

Lifting degeneracies in Higgs couplings using single top production in association with a Higgs boson

Abstract Current Higgs data show an ambiguity in the value of the Yukawa couplings to quarks and leptons. Not so much because of still large uncertainties in the measurements but as the result of several almost degenerate minima in the coupling profile likelihood function. To break these degeneracies, it is important to identify and measure processes where the Higgs coupling to fermions interferes with other coupling(s). The most prominent example, the decay of h → γγ, is not sufficient to give a definitive answer. In this paper, we argue that t-channel single top production in association with a Higgs boson, with h → bb, can provide the necessary information to lift the remaining degeneracy in the top Yukawa. Within the Standard Model, the total rate is highly reduced due to an almost perfect destructive interference in the hard process, W b → th. We first show that for non-standard couplings the cross section can be reliably computed without worrying about corrections from physics beyond the cutoff scale Λ ≳ 10 TeV, and that it can be enhanced by more than one order of magnitude compared to the SM. We then study the signal pp → thj(b) with 3 and 4 b’s in the final state, and its main backgrounds at the LHC. We find the 8 TeV run dataset to be sensitive to the sign of the anomalous top Yukawa coupling, while already a moderate integrated luminosity at 14 TeV should lift the degeneracy completely.

Production-decay interferences at next-to-leading order in QCD fort-channel single-top-quark production

We present a calculation of O(\alpha_s) contributions to the process of t-channel single-top production and decay, which include virtual and real corrections arising from interference of the production and decay subprocesses. The calculation is organized as a simultaneous expansion of the matrix elements in the couplings \alpha_{ew},\alpha_s and the virtuality of the intermediate top quark, (p_t^2-m_t^2)/m_t^2 ~ \Gamma_t/m_t, and extends earlier results beyond the narrow-width approximation.

Little Higgs models and single top production at the LHC

We investigate the corrections of the littlest Higgs(LH) model and the SU(3) simple group model to single top production at the CERN Large Hardon Collider(LHC). We find that the new gauge bosons $W_{H}^{\pm}$ predicted by the LH model can generate significant contributions to single top production via the s-channel process. The correction terms for the tree-level $Wqq'$ couplings coming from the SU(3) simple group model can give large contributions to the cross sections of the t-channel single top production process. We expect that the effects of the LH model and the SU(3) simple group model on single top production can be detected at the LHC experiments.