CMS实验在13 TeV质子-质子碰撞中搜索四个顶级夸克

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"在CMS实验中，通过分析2.6 fb^-1的13 TeV质子-质子碰撞数据，对包含四个顶级夸克（tt'tt）的事件进行了搜索。研究涉及单轻子（电子或μ子）+喷流和反对称双轻子（μ+μ-, μ±e∓, 或e+e-）+喷流通道。利用增强的决策树技术，综合全球事件和喷流特性信息，区分tt和tt'的产生。观测到的事件数量与背景和标准模型预测相符，从而在95%置信水平下对标准模型中tt'tt的生成截面设定了上限，上限值为94 fb，预期限制为118 fb。这比CMS之前发布的在相同符号双轻子通道中的搜索结果有所改进，其限制为69 fb（95%置信度，预期71 fb）。这些是对tt产量的最严格的限制至今。" 这篇物理论文发表在《Physics Letters B》上，探讨了在质子-质子碰撞中寻找四个顶级夸克的标准模型产生的过程。研究使用了CMS（紧凑型缪子螺线管）探测器在13 TeV能量下的数据，该数据集对应于2.6 fb^-1的综合光度。在搜索过程中，主要关注了两种事件类型：单轻子事件和反对称双轻子事件，这两种类型能提供关于顶级夸克生成的重要线索。分析采用了先进的机器学习方法——增强的决策树，这种技术可以有效地结合事件的整体特征和喷流性质，以区分可能的tt'tt生成与其他背景事件。经过一系列的选择标准后，观测到的事件数量与理论预测和背景噪声相吻合，没有发现超出预期的信号。通过对这些数据的分析，研究者在95%的置信水平下，对标准模型下tt'tt的生成截面设定了上限，上限值为94 femtobarns（fb），而预期的限制是118 fb。这个结果是对之前CMS实验在相同符号双轻子通道中搜索结果的改进，之前限制为69 fb（预期71 fb）。这表明对于tt'tt的生成，目前的实验限制已经显著收紧，为理解顶级夸克的物理特性及其相互作用提供了更精确的限制。这项研究加深了我们对高能粒子碰撞中顶级夸克生成的理解，特别是在探索超出标准模型的新物理现象方面。通过更严格的限制，科学家们可以更加精确地测试现有的物理理论，并可能揭示潜在的新物理现象。

340 The CMS Collaboration / Physics Letters B 772 (2017) 336–358

Fig. 3. Distribution of the event-level BDT discriminants D

dil

for the combined

dilepton (μ

−

+μ

∓

−

) event sample for 4–5 jets (top), 6–7 jets (middle),

and ≥8 jets (bottom). The vertical bars show the statistical uncertainty in the data.

The predicted background distributions from simulation are shown by the shaded

histograms. The hatched area shows the size of the dominant systematic uncer-

tainty

in the simulation, which comes from the choice of the matrix-element (ME)

factorization and renormalization scales used in the simulation. The electroweak

(EW) histogram is the sum of the Drell–Yan and W boson+jets backgrounds. The

expected SM t

tsignal contribution is shown by the open histogram, multiplied by

a factor of 20.

(renormalization and factorization scale) and

+6.2%

−6.4%

(PDF). The

effect of uncertainties in the cross sections for the other back-

grounds

were checked and found to be negligible.

3. The

uncertainties from trigger, lepton identiﬁcation, and lepton

isolation corrections, which are included as nuisance parame-

ters

in determining the upper limit. Combined, these give an

uncertainty of 1.2% in the single-muon channel, 3.7% in the

single-electron channel, 4.3% in the μμ channel, 4.6% in the

μe channel, and 4.8% in the ee channel.

The

shape uncertainties are:

1. The

uncertainty from the choice of the factorization and renor-

malization

scales in the calculation of the matrix element of

the hard-scattering process, which is estimated by the maxi-

mum

variation in the D

dil

or D

distribution obtained when

each scale is changed separately by a factor of 1/2 and 2,

excluding unphysical anticorrelated combinations. This proce-

dure

is performed separately for the t

tsignal and the t

tback-

ground.

In addition, alternative t

tsamples are used to estimate

the impact of a change in the scale at the parton-shower level,

taking into account the uncertainty in α

for the hadroniza-

tion [25].

The differences in the distributions with respect to

the nominal ones are taken as the uncertainty. The uncertainty

in the matrix-element scale is the dominant systematic uncer-

tainty

in the analysis.

2. Differences

in the simulation of t

tfrom the choice of the

matrix-element generator, which is estimated by comparing

the nominal t

t simulation using powheg +pythia 8to samples

generated using MadGraph + pythia 8 with MLM matching

[16]. The difference relative to the nominal simulation is used

to estimate the uncertainty from this source.

3. The uncertainty in the fraction of ttbbevents in the t

tback-

ground,

which is estimated using the uncertainty in the mea-

sured

cross section ratio σ

ttbb

: σ

tjj

[42] that was used to

correct the ttbb content of the t

t simulation. An anticor-

uncertainty in the measured cross section ratio of

(σ

tjj

−σ

ttbb

): σ

tjj

is applied simultaneously to the light-quark

fraction to maintain the total t

tcross section.

4. The uncertainties in the jet energy scale and the jet energy

resolution [49], which are estimated by varying these within

their uncertainties by ±1 standard deviation. A similar method

is used to estimate the uncertainty from the inelastic proton–

proton

cross section and the procedure used in the pileup

reweighting. These uncertainties have very little inﬂuence on

the ﬁnal limit.

5. The

uncertainty in the corrections to the values of the b tag-

ging

CSV discriminator, where three categories of systematic

uncertainty are applied for each jet ﬂavor: the jet energy scale,

purity of the data sample used to derive the corrections, and

the statistical uncertainties derived from the ﬁts used in the

method. The uncertainty in the b tagging correction caused by

the jet energy scale is treated as fully correlated with the jet

energy scale uncertainty described above. Typical magnitudes

of each of these individual uncertainties on the corrections to

the b tagging CSV discriminator are 10–50% before the ﬁt, de-

pending

on the number of jets and b jets in the event. A full

description of these corrections can be found in Ref. [41].

Each

systematic source was attributed a nuisance parameter in the

limit determination.

7. Results

No deviation from the background-only simulation, which in-

cludes

tproduction and negligible single top, t

t+H/Z/Wbo-

son,

Drell–Yan+jets, and W+jets backgrounds, is observed in the

dil

or D

distributions. An upper limit is derived for the t

production

cross section using the asymptotic approximation of

the CL

method provided in Refs. [50–54]. The signal and back-

ground

distributions are ﬁtted using a simultaneous maximum-

likelihood

method. The normalization uncertainties are included

using log-normal functions and the shape uncertainties are in-

cluded

as Gaussian-distributed nuisance parameters. The expected

and observed 95% CL upper limits from the two analyses and

their combination are listed in Table 1. For the combination of

the single-lepton and opposite-sign dilepton results, the system-

atic

uncertainties attributed to the integrated luminosity, jet en-

ergy

scale, and modeling of the pileup contribution are assumed

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