arXiv subjects
Publications and source records attributed to Tomasz Stebel.
We present our results on soft gluon resummation in the invariant mass threshold applied to the associated production of a top quark pair with a W boson at the LHC in the Mellin space formalism.
In the following we present our recent results on the resummation of soft gluon corrections to the $pp\rightarrow t\bar{t}H$ 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 $pp\rightarrow t\bar{t}H$ cross section.
We perform threshold resummation of soft gluon corrections to the total cross section and the invariant mass distribution for the process $pp \to t\bar{t}H$. 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.
The Lam-Tung relation breaking coefficient $A_{\mathrm{LT}} = A_0 - A_2$ in the Drell-Yan dilepton angular distributions in the $Z^0$ boson mass region at the LHC is analyzed in the $k_T$-factorization approach. This observable was recently measured with high precision by ATLAS collaboration. Within the $k_T$-factorization approach we perform an approximate ${\cal O}(α_{\mathrm{em}}α_s^2)$ calculation of the off-shell parton hard matrix elements in which we include the leading tree level contributions of valence quarks and off-shell gluons: the $q_{\mathrm{val}} g^* \to qZ^0$ channel and the $g^*g^* \to q\bar q Z^0$ channel. The resulting $A_{\mathrm{LT}}$ exhibits high sensitivity to the gluon transverse momentum distribution (TMD). Several gluon TMDs are probed derived from the CCFM and BFKL evolution equations, and given by QCD-inspired phenomenological parameterizations. The ATLAS data favor a simple "Weizsäcker-Williams" (WW) hard gluon TMD with the asymptotic behavior of one-gluon exchange at large gluon transverse momenta and moderate $x$. It is verified that the proposed approach with the WW gluon TMD describes well also the $A_0$ and $A_2$ angular coefficients at the $Z^0$ peak, as well as the Drell-Yan dilepton mass distribution at lower masses. We conclude that inclusion of gluon transverse momentum effects improves description of the angular distributions of Drell-Yan dileptons and that the Drell-Yan scattering provides an excellent probe of the parton TMDs.
The forward Drell--Yan process in $pp$ scattering at the LHC at $\sqrt{S}=14$ TeV is considered. We analyze the Drell--Yan structure functions assuming the dominance of a Compton-like emission of a virtual photon from a fast quark scattering off the small $x$ gluons. The color dipole framework is applied to perform quantitatively the twist decomposition of all the Drell--Yan structure functions. Two models of the color dipole scattering are applied: the Golec-Biernat--Wüsthoff model and the dipole cross section obtained from the Balitsky--Fadin--Kuraev--Lipatov evolution equation. The two models have essentially different higher twist content and the gluon transverse momentum distribution and lead to different significant effects beyond the collinear leading twist description. It is found that the gluon transverse momentum effects are significant in the Drell--Yan structure functions for all Drell--Yan pair masses $M$, and the higher twist effects become important for $M \lesssim 10$ GeV. It is found that the structure function $W_{TT}$ related to the $A_2$ angular coefficient and the Lam--Tung observable $A_0 -A_2$ are particularly sensitive to the gluon $k_T$ effects and to the higher twist effects. A procedure is suggested how to disentangle the higher twist effects from the gluon transverse momentum effects.
In the following we present our results on resummation of invariant mass threshold corrections for the $2 \to 3$ type hadronic production processes in the Mellin moment space formalism. This method is applied to the associated Higgs boson production process $pp \to t \bar{t} H$ at the LHC. The results for the total cross section, the differential distribution with respect to the invariant mass and their uncertainties are presented.
Quantum Telescope is a recent idea aimed at beating the diffraction limit of spaceborne telescopes and possibly also other distant target imaging systems. There is no agreement yet on the best setup of such devices, but some configurations have been already proposed. In this Letter we characterize the predicted performance of Quantum Telescopes and their possible limitations. Our extensive simulations confirm that the presented model of such instruments is feasible and the device can provide considerable gains in the angular resolution of imaging in the UV, optical and infrared bands. We argue that it is generally possible to construct and manufacture such instruments using the latest or soon to be available technology. We refer to the latest literature to discuss the feasibility of the proposed QT system design.
We present a twist expansion of differential cross-sections of the forward Drell-Yan process at the high energies. The expansion of all invariant form-factors is performed assuming GBW saturation model and the saturation scale plays the role of the hadronic scale of OPE. Some expilicit predictions for LHC experiments are given. It is shown also how the Lam-Tung relation is broken at twist 4 what provides a sensitive probe for searching of higher twists.
We perform resummation of soft gluon corrections to the total cross section for the process $pp \to t\bar{t}H$. The resummation is carried out at next-to-leading-logarithmic (NLL) accuracy using the Mellin space technique, extending its application to the class of $2 \to 3$ processes. We present an analytical result for the soft anomalous dimension for a hadronic production of two coloured massive particles in association with a colour singlet. We discuss the impact of resummation on the numerical prediction for the associated Higgs boson production with top quarks at the LHC.
Forward Drell-Yan process at the LHC probes the proton structure at very small Bjorken-$x$ and moderate hard scales. In this kinematical domain higher twist effects may be significant and introduce sizeable corrections to the standard leading twist description. We study the forward Drell-Yan process beyond the leading twist approximation within the color dipole model framework that incorporates multiple scattering effects. We derive the Mellin representation of the forward Drell-Yan impact factors for fully differential cross-sections. These results combined with the color dipole cross-section of the saturation model are used to perform twist expansion of the Drell-Yan structure functions at arbitrary transverse momentum $q_T$ of the Drell-Yan pair and also of the structure functions integrated over $q_T$. We also investigate the Lam-Tung relation, find that it is broken at twist 4 and provide explicit estimates for the breaking term.
Forward Drell-Yan process at the LHC is a sensitive tool for investigating higher twist effects in QCD. The expansion of all Drell-Yan structure functions is performed assuming GBW saturation model and the saturation scale plays the role of the hadronic scale of OPE. We show that the Lam-Tung relation is broken at twist 4. The results open the way for a forthcoming analysis of multiple scattering and higher twist effects.
We apply the method of ratios to search for geometrical scaling in the charm production in deep inelastic scattering. To this end we use recent combined data from H1 and ZEUS experiments. Two forms of geometrical scaling are tested: originally proposed scaling which results from Golec-Biernat-Wusthoff model and scaling motivated by a dipole representation which takes into account charm mass. It turns out that in both cases some residual scaling is present and charm mass inclusion improves scaling quality.
We use recently proposed method of ratios to assess the quality of geometrical scaling in deep inelastic scattering for different forms of the saturation scale. We consider original form of geometrical scaling (motivated by the Balitski-Kovchegov (BK) equation with fixed coupling) studied in more detail in our previous paper, and four new hypotheses: phenomenologically motivated case with $Q^2$ dependent exponent $λ$ that governs small $x$ dependence of the saturation scale, two versions of scaling (running coupling 1 and 2) that follow from the BK equation with running coupling, and diffusive scaling suggested by the QCD evolution equation beyond mean field approximation. It turns out that more sophisticated scenarios: running coupling scaling and diffusive scaling are disfavored by the combined HERA data on $e^+p$ deep inelastic structure function $F_2$.
We propose a method to assess the quality of geometrical scaling in Deep Inelastic Scattering and apply it to the combined HERA data on $γ^{\ast}p$ cross-section. Using two different approaches based on Bjorken $x$ binning and binning in $γ^{\ast}p$ scattering energy $W$, we show that geometrical scaling in variable $τ\sim Q^{2} x^λ$ works well up to Bjorken $x$'s 0.1. The corresponding value of exponent $λ$ is 0.32 -- 0.34.
We analyze geometrical scaling in deep inelastic scattering using experimental data from HERA ep collider. Parallel analyses are performed for energy and Bjorken-x data binnings. In particular, value of parameter λwhich governs x-dependence of saturation scale is found for both binnings: λ_En = 0.352 \pm 0.008 and λ_Bj = 0.302 \pm 0.004. We use the following method: λ_En is found as a value for which ratios σ^{W1}_{γ*p}(τ)/σ^{W2}_{γ*p}(τ) are closest to 1 (σ^{Wi}_{γ*p} is a photon-proton cross section with definite energy W_i and τ= Q^2 x^λ is a scaling variable); λ_Bj is found analogously using ratios of cross sections with definite Bjorken-x variable. We show also that GS is present for x<0.2.