Top-quark Transverse-momentum Distributions Research Articles

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at [Formula: see text] using the ATLAS detector.

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tbar{t} system and event-level kinematic observables in proton–proton collisions at a centre-of-mass energy of sqrt{s}=8,mathrm{TeV}. The observables have been chosen to emphasize the tbar{t} production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb^{-1}, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

High-Precision Differential Predictions for Top-Quark Pairs at the LHC.

We present the first complete next-to-next-to-leading order (NNLO) QCD predictions for differential distributions in the top-quark pair production process at the LHC. Our results are derived from a fully differential partonic MonteCarlo calculation with stable top quarks which involves no approximations beyond the fixed-order truncation of the perturbation series. The NNLO corrections improve the agreement between existing LHC measurements [V. Khachatryan etal. (CMS Collaboration), Eur. Phys. J. C 75, 542 (2015)] and standard model predictions for the top-quark transverse momentum distribution, thus helping alleviate one long-standing discrepancy. The shape of the top-quark pair invariant mass distribution turns out to be stable with respect to radiative corrections beyond NLO which increases the value of this observable as a place to search for physics beyond the standard model. The results presented here provide essential input for parton distribution function fits, implementation of higher-order effects in MonteCarlo generators, as well as top-quark mass and strong coupling determination.

NNLL threshold resummation for top-pair and single-top production

I discuss threshold resummation at NNLL accuracy in the standard moment-space approach in perturbative QCD for top-pair and single-top production. For top quark pair production I present new approximate NNLO results for the total cross section and for the top quark transverse momentum and rapidity distributions at 8 TeV LHC energy. I discuss the accuracy of the soft-gluon approximation and show that the NLO and NNLO approximate results from resummation are practically indistinguishable from exact NLO and partial NNLO results. For single top production I present new approximate NNLO results for the total cross sections in all three channels at the LHC and also for the top quark transverse momentum distributions in t-channel production and in top-quark associated production with a W boson. For both $$t\bar t$$ and single-top production the agreement of theoretical results with LHC and Tevatron data is excellent.

Resummation prediction on top quark transverse momentum distribution at largepT

We study the factorization and resummation of $t$-channel top quark transverse momentum distribution at large ${p}_{T}$ in the standard model at both the Tevatron and the LHC with soft-collinear effective theory. The cross section in the threshold region can be factorized into a convolution of hard, jet and soft functions. In particular, we first calculate the next-to-leading order soft functions for this process, and give a renormalization group improved cross section by evolving the different functions to a common scale. Our results show that the resummation effects increase the next-to-leading order results by about $9%\ensuremath{\sim}13%$ and $4%\ensuremath{\sim}9%$ when the top quark ${p}_{T}$ is larger than 50 and 70 GeV at the Tevatron and the 8 TeV LHC, respectively. Also, we discuss the scale independence of the cross section analytically, and show how to choose the proper scales at which the perturbative expansion can converge fast.