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Register a new userAssociated $t bar{t} H$ production at the LHC: theoretical predictions at NLO+NNLL accuracy
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We perform threshold resummation of soft gluon corrections to the total cross section and the invariant mass distribution for the process $pp to tbar{t}H$. The resummation is carried out at next-to-next-to-leading-logarithmic (NNLL) accuracy using the direct QCD Mellin space technique in the three-particle invariant mass kinematics. After presenting analytical expressions we discuss the impact of resummation on the numerical predictions for the associated Higgs boson production with top quarks at the LHC. We find that NLO+NNLL resummation leads to predictions for which the central values are remarkably stable with respect to scale variation and for which theoretical uncertainties are reduced in comparison to NLO predictions.
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In the following we present our recent results on the resummation of soft gluon corrections to the $pprightarrow tbar{t}H$ cross section at the LHC. The resummation was carried out at next-to-next-to-leading-logarithmic (NNLL) accuracy using the Mellin space technique. Obtained results were matched to the NLO cross section. We show that the resummation leads to reduction of scale-variation uncertainty of the total $pprightarrow tbar{t}H$ cross section.
We present results of a computation of NLO QCD corrections to the production of an off-shell top--antitop pair in association with an off-shell $text{W}^+$ boson in proton--proton collisions. As the calculation is based on the full matrix elements for the process $text{p}text{p}to {text{e}}^+ u_{text{e}},mu^-bar{ u}_mu,tau^+ u_tau,{text{b}},bar{text{b}}$, all off-shell, spin-correlation, and interference effects are included. The NLO QCD corrections are about $20%$ for the integrated cross-section. Using a dynamical scale, the corrections to most distributions are at the same level, while some distributions show much larger $K$-factors in suppressed regions of phase space. We have performed a second calculation based on a double-pole approximation. While the corresponding results agree with the full calculation within few per cent for integrated cross-sections, the discrepancy can reach $10%$ and more in regions of phase space that are not dominated by top--antitop production. As a consequence, on-shell calculations should only be trusted to this level of accuracy.
We discuss lepton charge asymmetries in $t bar t$ and $t bar t gamma$ production at the LHC, which can be measured in the semileptonic decay channel $t bar t to W^+ b , W^- bar b to ell^+ u b , q bar q bar b$ (or the charge conjugate). Considering several variants of a new physics scenario with a light colour octet, it is seen that for $t bar t$ these asymmetries may have a sensitivity competitive with the dilepton asymmetry already measured. For $t bar t gamma$ the new leptonic asymmetries, as well as the $t bar t$ charge asymmetry, will reach their full potential with the high luminosity LHC upgrade. These asymmetries can pinpoint deviations at the $3sigma-4sigma$ level for new physics scenarios where the charge asymmetries already measured in $t bar t$ production agree within $1sigma$.
We study effects of CP violation in the associated production of a charged Higgs boson and a top quark at the LHC, $pp to tH^pm + X$. We calculate the CP violating asymmetry between the total cross section for $H^+$ and $H^-$ production at next-to-leading order in the MSSM, and perform a detailed numerical analysis. In the production the asymmetry is of the order of 20%. The asymmetry in the production and any subsequent decay of an on-shell charged Higgs boson is to a good approximation the sum of the asymmetry in the production and the asymmetry in the decay. We consider subsequent decays of $H^pm $ to $t b$, $ u_tau tau^pm$ and $W h^0$. In the case with $H^pm to t b$ decay, mainly due to CP violating box graphs with gluino, the asymmetry can go up to $sim$ 12%.
We study the production of $(t+bar{t}) tilde{g}$ at the hadron colliders in an R-parity ($R_{p}$) violating supersymmetric model. This process provides us with information not only about $R_{p}$ violation, but may also help us in detecting the supersymmetry itself. It is possible to detect an $R_{p}$ violating signal (with single gluino production) at the future hadron colliders, such as Fermilab Tevatron Run II or CERN Large Hadron Collider (LHC), if the parameters in the supersymmetric $rlap/ R_{p}$ interactions are not too small, e.g. for $m_{tilde{g}}=1$ TeV, $lambda^{}=0.1$, still hundreds of events are produced at LHC with luminosity $30 fb^{-1}$. Even if we could not detect a signal of $ rlap/R_{p}$ in the experiment, we get stringent constraints on the heavy flavour $rlap/R_{p}$ couplings. In addition to the minimal supersymmetric standard model we have also considered some models with a heavy gluino as the lightest supersymmetric particle.
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