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Register a new userPrecise predictions for $tbar{t}gamma/tbar{t}$ cross section ratios at the LHC
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With the goal of increasing the precision of NLO QCD predictions for the $ppto tbar{t} gamma$ process in the di-lepton top quark decay channel we present theoretical predictions for the ${cal R}= sigma_{tbar{t}gamma}/sigma_{tbar{t}}$ cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of $sqrt{s} = 13$ TeV. Fully realistic NLO computations for $tbar{t}$ and $tbar{t}gamma$ production are employed. They are based on matrix elements for $e^+ u_e mu^- bar{ u}_mu bbar{b}$ and $e^+ u_e mu^- bar{ u}_mu bbar{b}gamma$ processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the $W$ gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain $1%-3%$ percent precision on ${cal R}$. Moreover, for differential cross section ratios theoretical uncertainties in the range of $1%-6%$ have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, ${cal R}$ at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the $t$-$bar{t}$-$gamma$ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.
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The total cross section for top quark pair production close to threshold in e+e- annihilation is investigated. Details are given about the calculation at next-to-next-to-leading logarithmic order. The summation of logarithms leads to a convergent expansion for the normalization of the cross section, and small residual dependence on the subtraction parameter nu. A detailed analysis of the residual nu dependence is carried out. A conservative estimate for the remaining uncertainty in the normalization of the total cross section from QCD effects is $lesssim pm 3%$. This makes precise extractions of the strong coupling and top width feasible, and further studies of electroweak effects mandatory.
The top-quark is the heaviest known particle of the Standard Model (SM); its heavy mass plays a crucial role in testing the electroweak symmetry breaking mechanism and for searching for new physics beyond the SM. In this paper, we determine the top-quark pole mass from recent measurements at the LHC at $sqrt{S}=13$ TeV center-of-mass energy to high precision by applying the Principle of Maximum Conformality (PMC) to the $tbar{t}$ pQCD production cross-section at NNLO. The PMC provides a systematic method which rigorously eliminates QCD renormalization scale ambiguities by summing the nonconformal $beta$ contributions into the QCD coupling constant. The PMC predictions satisfy the requirements of renormalization group invariance, including renormalization scheme independence, and the PMC scales accurately reflect the virtuality of the underlying production subprocesses. By using the PMC, an improved prediction for the $tbar{t}$ production cross-section is obtained without scale ambiguities, which in turn provides a precise value for the top-quark pole mass. The resulting determination of the top-quark pole mass $m_t^{rm pole}=172.5pm1.2$ GeV from the LHC measurement at $sqrt{S}=13$ TeV is in agreement with the current world average cited by the Particle Data Group (PDG). The PMC prediction provides an important high-precision test of the consistency of pQCD and the SM at $sqrt{S}=13$ TeV with previous LHC measurements at lower CM energies.
We have implemented a code for Z + n jets production in ALPGEN, with Z decays into several final states, including l+ l- and t tbar. The MLM prescription is used for matching the matrix element with the parton shower, including in this way the leading soft and collinear corrections. In order to demonstrate its capabilities, we perform a combined analysis of Z -> t tbar and Z -> t tbar j production for a heavy leptophobic gauge boson. It is found that the effect of the extra jet cannot only be accounted for by a K factor multiplying the leading-order cross section. In fact, the combined analysis for Z -> t tbar and Z -> t tbar j presented improves the statistical significance of the signal by 25% (8.55 sigma versus 6.77 sigma for a Z mass of 1 TeV), compared with the results of an inclusive analysis carried out on the same sample of t tbar + t tbar j events.
The top quark pair production $sigma_{tbar{t}}$ is measured in pp collisions at a center-of-mass energy of 5.02 TeV. The analyzed data have been collected by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 27.4 /pb. The measurement is performed by analyzing events with at least one charged lepton. The measured cross section is 69.5 $pm$ 8.4 pb. The result is in agreement with the expectation from the standard model. The impact of the presented measurement on the gluon distribution function is illustrated through a quantum chromodynamic analysis at next-to-next-to-leading order.
We consider the top quark charge asymmetry in the process $pp to tbar{t}+gamma$ at the 13 TeV LHC. The genuine tree level asymmetry in the $qbar{q}$ channel is large with about -12%. However, the symmetric $gg$ channel, photon radiation off top quark decay products, and higher order corrections wash out the asymmetry and obscure its observability. In this work, we investigate these effects at next-to-leading order QCD and check the robustness of theoretical predictions. We find a sizable perturbative correction and discuss its origins and implications. We also study dedicated cuts for enhancing the asymmetry and show that a measurement is possible with an integrated luminosity of 150 fb$^{-1}$.
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