LHC上的 Slepton 对生产：NLO + NLL 计算与恢复改进的 PDF

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"这篇论文‘LLO在NLO + NLL的LHC中生产瘦子对，并具有提高的部分密度’探讨了在大型强子对撞机（LHC）上，使用新奇的部分子密度（PDF）进行高精度的 Slepton 对（如Selectron/smuon和Stau）产生的计算。该研究考虑了恢复改进的矩阵元素，尽管这些改进只适用于简化数据集。通过应用分解方法，研究人员旨在减少数据集缩减带来的影响，避免在NNPDF方法中PDF不确定性的异常副本问题，同时保持规模不确定性的降低。这种方法对于LHC Run II期间的 Slepton 对生产截面的可靠性有着显著提升，尤其是对于左手的Selectron/smuon、右手的和最大混合的Stau产量。" 在高能物理领域，粒子对的产生是理解基本粒子性质和相互作用的关键。LHC作为世界上最强的粒子加速器，其碰撞事件提供了大量研究新物理现象的机会。在LHC中，Sleptons（超对称粒子的轻子对应物）的对生产和探测是检验超对称理论的一个重要途径。然而，由于Sleptons的预期寿命极短，它们几乎不会在实验中直接观察到，因此需要精确的理论预测来分析其产生的信号。 NLO（Next-to-Leading Order）和NLL（Next-to-Leading Logarithm）是量子色动力学（QCD）计算中的术语，分别表示比领先阶更高的阶别校正和包含更高阶的对数项。这些计算有助于更准确地预测粒子对产生的概率，从而帮助实验者在海量数据中识别出可能的Slepton信号。矩阵元素的恢复改进意味着在计算中包含了更高阶的QCD效应，而PDF的改进则意味着更好地考虑了初始状态夸克和胶子的能量分布。 NNPDF（Next-to-Next-to-Leading Order Parton Distribution Functions）是一种广泛使用的PDF参数化方案，但它可能会导致PDF不确定性估计中的异常副本。文中提到的分解方法解决了这个问题，确保了PDF不确定性的合理评估。同时，通过使用阈值改进的PDF，可以部分抵消矩阵元素中由恢复贡献引起的效应，这有助于更准确地预测Slepton对的生产率。这篇论文为在LHC上进行的Slepton对产生的理论计算提供了一个更为精确和可靠的方法，尤其是在处理NLO + NLL级别的计算时。通过这些改进，物理学家可以更有效地设计实验策略，提高对潜在超对称信号的敏感性，进一步推动对标准模型之外新物理的理解。

JHEP03(2018)094

as they were only determined by ﬁtting data from deep-inelastic scattering (DIS), the

Drell-Yan (DY) process, jet production etc. using NLO partonic matrix elements [38–40].

For consistency, hadronic cross sections should be computed with the same level of

accuracy in both the partonic cross section and the PDFs. With the publication of the

NNPDF30 nll disdytop PDFs [41], this has now become possible at the NLO+NLL level.

However, since only a reduced number of partonic cross sections that usually enter global

ﬁts are available with NLO+NLL precision, only a subset of the corresponding data sets,

i.e. on DIS, DY and top pair production, could be used in the determination of these

threshold-resummation improved PDFs. The consequence is that the PDF uncertainty

band, produced with the NNPDF replica method, is actually larger, not smaller than with

globally ﬁtted NLO PDFs. One of the examples used in ref. [41] is in fact slepton pair

production, calculated with RESUMMINO. However, only one invariant-mass distribution

for left-handed selectrons of mass 564 GeV is studied there.

The NNPDF30 nll disdytop PDFs have subsequently been employed to investigate the

eﬀect of threshold-resummation improved PDFs on squark and gluino production [42]. The

authors showed that the total central cross sections were modiﬁed both in a qualitative and

quantitative way, illustrating the relevance and impact of using threshold-resummation im-

proved PDFs. They introduced in particular a factorisation (K-factor) method, described

in detail below, that allows to combine the impact of NLO+NLL threshold resummation

on the reduced central PDF ﬁt with the smaller uncertainty of the global NLO PDF ﬁt,

leading to approximately consistent NLO+NLL hadronic cross sections. This method also

allows to avoid the problem of exceedingly large, small or even negative cross sections

induced by outlier replicas and the lack of positivity constraints on the NNPDF PDFs.

The replica method is known to be particularly problematic for resummation calculations,

which rely on a transformation of PDFs to Mellin space from all regions of x, including

those, where they are not well constrained [31].

The purpose of this paper is therefore threefold. First, our NLO+NLL predictions for

slepton pair production are updated to the current LHC collision energy of 13 TeV. Second,

we employ with NNPDF3.0 an up-to-date global set of PDFs, that is now also based on

ATLAS and CMS data from jet, vector-boson and top-quark production [43]. This allows

us to reach our third and central goal of studying the impact of threshold-resummation

improved PDFs not only on diﬀerential, but also on total slepton pair production cross

sections and for a variety of SUSY scenarios.

The remainder of this paper is organised as follows: in section 2 we describe our theo-

retical approach using the K-factor method and how we combine NLO+NLL resummation

eﬀects with global PDF and also scale uncertainties. In section 3, we present numerical

results for diﬀerential and total cross sections of left-handed ﬁrst and second generation

sleptons in graphical and tabular form. We do this for pp collisions of 13 TeV centre-of-

mass energy and various slepton masses relevant for Run II of the LHC. Similar results are

presented in section 4 for third-generation sleptons, i.e. right-handed or maximally mixed

staus. Our conclusions are given in section 5.

– 2 –

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