In this talk, we report on a recent next-to-leading order QCD calculation of the production of a W boson in association with three jets at hadron colliders. The computation is performed by combining two programs, BlackHat for the computation of the virtual one-loop matrix elements and Sherpa for the real emission part.
Automated computations of electroweak corrections using Sherpa+Recola
考虑到实验精度和LHC可以测量的众多过程，电弱（EW）校正变得越来越重要。这就要求EW校正的自动化，特别是在多用途蒙特卡罗发生器中实现，以便对这些贡献进行系统研究。在这些程序中，我介绍了Sherpa Monte Carlo中矩阵元素生成器Recola的实现。Sherpa+Recola的组合允许在原则上以NLO QCD和EW精度计算标准模型中的任何过程。除了固定阶NLO计算，Sherpa的所有功能，如部分子簇射或强子化仍然可用。
Precise theoretical predictions are vital for the interpretation of Standard Model measurements and facilitate conclusive searches for New Physics phenomena at the LHC. In this contribution I highlight some of the ongoing efforts in the framework of the Sherpa event generator to provide accurate and realistic simulations of Standard Model production processes. This includes the automated evaluation of NLO QCD and NLO EW corrections and their consistent consideration in the parton-shower evolution of scattering events. Particular emphasis is given to examples and applications involving the prod…
In this paper a complete generalisation of the Catani-Seymour dipole subtraction method to next-to-leading order electroweak calculations is presented. All singularities due to photon and gluon radiation off both massless and massive partons in the presence of both massless and massive spectators are accounted for. Particular attention is paid to the simultaneous subtraction of singularities of both QCD and electroweak origin which are present in the next-to-leading order corrections to processes with more than one perturbative order contributing at Born level. Similarly, embedding non-dipole-…
Measurements of $t\bar{t} H$ production in the $H\to b\bar{b}$ channel depend in a critical way on the theoretical uncertainty associated with the irreducible QCD $t\bar{t}+b$-jet background. We introduce a new $pp\to t\bar{t}b\bar{b}$ POWHEG generator in the 4F scheme based on POWHEG-BOX-RES and on OpenLoops for fast evaluation of the scattering amplitudes. We present predictions and uncertainties for $t\bar{t}+b$-jet observables at the 13 TeV LHC. We also consider theoretical uncertainties related to the POWHEG matching method and to the parton shower (PS) modelling, with emphasis on $g\to b…
We consider the production of a pair of $Z$ bosons at the LHC and study the inclusion of EW corrections in theoretical predictions at fixed order and based on multijet-merged parton-shower simulations. To this end we present exact NLO EW results for $pp\to e^+e^-μ^+μ^-$, and, for the first time, for $pp\to e^+e^-μ^+μ^-j$, and compare them to the EW virtual and NLL Sudakov approximation. We then match the exact NLO EW result to the resummed Sudakov logarithms to achieve an improved NLO EW + NLL $\text{EW}_\text{sud}^\text{exp}$ result. Further, we discuss the inclusion of the above EW correctio…
In this article we present the next-to-leading order QCD predictions for $Wb\bar{b}+n$-jet ($n=0,1,2,3$) production at the Large Hadron Collider with $\sqrt{s}=13$ TeV. We work in the four-flavor number scheme with a non-vanishing bottom-quark mass and include all subprocesses at leading electroweak order as well as all heavy-fermion-loop effects. We show the impact of QCD corrections for total as well as differential cross sections and make an assessment of theoretical uncertainties of $Wb\bar{b}$ production viewed as an irreducible background to $H(\rightarrow b{\bar b})W$ studies. For the c…
Four-Jet Production at the Large Hadron Collider at Next-to-Leading Order in QCD
本文给出了在大型强子对撞机上产生四个喷流的截面，在QCD耦合中为次领先级。我们使用的BlackHat库与SHERPA和最近开发的算法组装原始振幅成彩色装饰的振幅。我们采用ATLAS在研究\sqrt{s} = 7 TeV pp碰撞中多喷流事件时所使用的切割。我们包括非微扰校正的估计并与ATLAS数据进行比较。我们存储在一个框架，允许廉价的计算额外的结果为不同的选择规模或部分子分布的中间结果。
We present for the first time resummed predictions at NLO + NLL' accuracy for the Durham jet-resolution scales $y_{n,n+1}$ in multijet production in $e^+e^-$ collisions. Results are obtained using an implementation of the well known CAESAR formalism within the SHERPA framework. For the 4-, 5- and 6-jet resolutions we discuss in particular the impact of subleading colour contributions and compare to matrix-element plus parton-shower predictions from SHERPA and VINCIA.
We use the SHERPA Monte Carlo generator to simulate the process $e^+e^-\rightarrow\mbox{hadrons}$ using matrix elements with up to six partons in the final state. Two samples of SHERPA events are generated. In the "LO" sample, all final states are generated with leading order matrix elements; in the "NLO" sample, matrix elements for final states with up to four partons are generated at next-to-leading order, while matrix elements for final states with five or six partons are generated at leading order. The resulting samples are then passed through the ALEPH detector simulation. We compare the …
We present next-to-leading order (NLO) calculations including QCD and electroweak (EW) corrections for $2\ell2ν$ diboson signatures with two opposite-charge leptons and two neutrinos. Specifically, we study the processes $pp\to e^+ μ^- ν_{e}\barν_μ$ and $pp\to e^+ e^-ν\barν$, including all relevant off-shell diboson channels, $W^+W^-$, $ZZ$, $γZ$, as well as non-resonant contributions. Photon-induced processes are computed at NLO EW, and we discuss subtle differences related to the definition and the renormalisation of the coupling $α$ for processes with initial- and final-state photons. All c…
Theoretical predictions for ttbb production are of crucial importance for ttH measurements in the H->bb channel at the LHC. To address the large uncertainties associated with the modelling of extra QCD radiation in ttbb events, in this paper we present a calculation of pp->ttbbj at NLO QCD. The behaviour of NLO corrections is analysed in a variety of observables, and to assess theoretical uncertainties we use factor-two rescalings as well as different dynamic scales. In this context, we propose a systematic alignment of dynamic scales that makes it possible to disentangle normalisation and sha…
We present theoretical predictions for the production of top-quark pairs in association with jets at the LHC including electroweak (EW) corrections. First, we present and compare differential predictions at the fixed-order level for $t \bar t$ and $t \bar t+$jet production at the LHC considering the dominant NLO EW corrections of order $\mathcal{O}(α_S^2 α)$ and $\mathcal{O}(α_S^3 α)$ respectively together with all additional subleading Born and one-loop contributions. The NLO EW corrections are enhanced at large energies and in particular alter the shape of the top transverse momentum distrib…
