In this thesis we focus on the theoretical subtleties of the top-quark mass ($m_t$) determination, issue which persists in being highly controversial. In the first part of the thesis, we investigate the presence of linear renormalons in observables that can be employed to determine the top mass. We considered a simplified toy model to describe $W^{*} \rightarrow t \bar{b} \to Wb \bar{b}$. The computation is carried out in the limit of a large number of flavours ($n_f$), using a new method that allows to easily evaluate any infrared safe observable at order $α_s(α_s n_f)^n$ for any $n$. The second part of the thesis is devoted to the comparison of several NLO generators for top-pair production, implemented in the POWHEG BOX framework, that differ by the level of accuracy employed to describe the top decay. The impact of the shower Monte Carlo programs, used to complete the NLO events generated by POWHEG BOX, is also studied. In particular, we discuss the two most widely used shower Monte Carlo programs, i.e. Pythia 8.2 and Herwig 7.1, and we present a method to interface them with processes that contain decayed emitting resonances.
Inspecting the power of parton showering the generated events by comparing the events generated in NLO+PS to NLO and NNLO without parton shower in $e^+e^-$ at 91.2 GEV
We perform a detailed study of NLO parton shower matching uncertainties in Higgs boson pair production through gluon fusion at the LHC based on a generic and process independent implementation of NLO subtraction and parton shower matching schemes for loop-induced processes in the Sherpa event generator. We take into account the full top-quark mass dependence in the two-loop virtual corrections and compare the results to an effective theory approximation. In the full calculation, our findings suggest large parton shower matching uncertainties that are absent in the effective theory approximatio…
We propose a new approach for combining next-to-leading order (NLO) and parton shower (PS) calculations so as to obtain three core features: (a) applicability to general showers, as with the MCatNLO and POWHEG methods; (b) positive-weight events, as with the KrkNLO and POWHEG methods; and (c) all showering attributed to the parton shower code, as with the MCatNLO and KrkNLO methods. This is achieved by using multiplicative matching in phase space regions where the shower overestimates the matrix element and accumulative (additive) matching in regions where the shower underestimates the matrix …
本文介绍了在克拉科夫IFJ PAN建立一个完整的NLO级部分子簇射蒙特卡罗程序的计划。讨论了四个问题：（1）将标准的包含共线分解扩展为一个新的完全排他的方案;（2）在新方案中重建LO Parton Shower;（3）将排他的NLO修正包含到硬过程中;（4）将排他的NLO修正包含到演化（阶梯）部分中。
We present the first combination of NLO QCD matrix elements for di-Higgs production, retaining the full top quark mass dependence, with a parton shower. Results are provided within both the POWHEG-BOX and MadGraph5_aMC@NLO Monte Carlo frameworks. We assess in detail the theoretical uncertainties and provide differential results. We find that, as expected, the shower effects are relatively large for observables like the transverse momentum of the Higgs boson pair, which are sensitive to extra radiation. However, these shower effects are still much smaller than the differences between the Born-i…
Perturbative calculations of the static QCD potential have the $u=3/2$ renormalon uncertainty. In the multipole expansion performed within pNRQCD, this uncertainty at LO is known to get canceled against the ultrasoft correction at NLO. To investigate the net contribution remaining after this renormalon cancellation, we propose a formulation to separate the ultrasoft correction into renormalon uncertainties and a renormalon independent part. We focus on very short distances $Λ_{\rm QCD} r \lesssim 0.1$ and investigate the ultrasoft correction based on its perturbative evaluation in the large-$β…
We present a selection of results from our recent study of $pp\to t\bar{t}W^\pm$ production matched to partons showers at NLO QCD at the LHC. Theoretical predictions are obtained at perturbative orders $\mathcal{O}(α_s^3α)$ and $\mathcal{O}(α_sα^3)$, where the different contributions are studied first separately at the inclusive level before being combined within a realistic two same-sign lepton signature. We investigate in detail uncertainties originating from missing higher-order corrections and from the parton-shower matching scheme employed.
Matching NLO QCD computations and parton shower simulations
提出了一种将QCD过程的次领先级（NLO）计算与部分子簇射蒙特卡罗（MC）模拟相匹配的方法。该方法具有以下特点：完全互斥事件的产生，根据MC模型进行强子化;总互斥率精确到NLO;分布的NLO结果在$α_S$中展开后恢复;硬发射处理为NLO计算，而软/共线发射由MC模拟处理，具有与MC相同的对数精度;并且硬和软/共线发射区域之间的匹配是平滑的。生成具有负权重的事件的一小部分，但是以合理的效率取消权重仍然是可能的。该方法是澄清使用一个简单的玩具模型，并说明了应用程序的hadroproduction的W $^+$ W $^-$对。
We present results for Higgs boson pair production with variations of the trilinear Higgs boson self-coupling at next-to-leading order (NLO) in QCD including the full top quark mass dependence. Differential results at 14 TeV are presented, and we discuss the implications of anomalous trilinear couplings as well as differences between the PYTHIA 8.2 and HERWIG 7.1 parton showers in combination with POWHEG. The implementation of the NLO QCD calculation with variable Higgs boson self-coupling is made publicly available in the POWHEG-BOX-V2 Monte Carlo framework. A simple method for using the new …
Power corrections and renormalon resummation for the average thrust
红外功率修正的平均推力< T >在e+e-湮灭的框架下分析的重整化，激发类似的骨架膨胀QED和BLM方法。通过对“重胶子”的重整化积分，得到了一个重整化方案不变的结果。我们发现，一个已知的次领先阶（NLO）的计算和实验之间的差异的主要部分可以解释由高阶微扰项的再确认。这一事实并不排除红外有限耦合解释与一个实质性的1/Q功率项。将正则化微扰和加上1/Q项与实验数据拟合得到alpha_s^{MSbar}（M_Z）=0.110 \pm 0.006。
