搜索高能质子-质子碰撞中tt共振的新方法

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"在高能物理领域，CMS合作组发表了一篇关于在质子-质子碰撞中寻找重共振衰变为顶夸克-反顶夸克对的研究文章。这项研究利用了LHC上的CMS探测器在质心能量为13TeV的质子-质子碰撞数据，数据集对应于2.6fb^{-1}的积分数。分析集中在高度增益的轻子+喷气和全强子最终状态，这两种通道都是为了捕捉大型共振体中顶夸克的高洛伦兹增强效应。经过详尽的分析，未发现任何显著的共振信号，因此对重共振的生产截面设定了上限，并且相对于之前在8TeV能量下进行的分析，对于质量超过2TeV的共振体的排除限得到了显著提升。这篇文章的关键词涵盖了标准模型之外的新物理现象和强子对撞实验。" 本文详细探讨了在大型强子对撞机(LHC)上进行的质子-质子碰撞实验中，如何通过CMS探测器寻找超出标准模型的新粒子，即重共振体。标准模型是粒子物理学的基石，但无法解释所有观测到的现象，如暗物质和宇宙中的物质-反物质不对称性，因此科学家们一直在寻找可能存在的新物理现象。在本研究中，特别关注那些可能衰变成顶夸克对的重粒子，因为顶夸克作为最重的基本粒子，其相互作用可能揭示标准模型以外的物理过程。实验方法主要包括在两种不同最终状态下的数据分析：轻子+喷气和全强子状态。轻子+喷气通道涉及到一个带电轻子（电子或μ子）和至少四个喷流，而全强子通道则包含多个强子喷流，不包含明显的电磁簇射。这两种通道的选择是为了覆盖不同的共振体可能衰变的模式，并充分利用CMS探测器的探测能力，尤其是对于高洛伦兹因子的顶夸克的探测。通过对2.6fb^{-1}的碰撞数据进行分析，没有观察到任何明显的超出预期的顶夸克对生产，这意味着没有发现新的强相互作用粒子的直接证据。然而，通过设置生产截面的上限，研究人员能够排除某些质量范围内的共振体存在。这些限制对于质量大于2TeV的粒子尤其重要，因为它们比之前的8TeV分析提供了更强的排除力。这种改进归因于更高的质心能量和更大的数据样本。总结来说，这篇开放获取的文章展示了在LHC上进行的前沿粒子物理研究，其目的是探索标准模型之外的未知领域。通过在13TeV能量下对质子-质子碰撞的详尽分析，对新物理现象的搜索进一步推进了我们对基本粒子和宇宙基本力量的理解。

JHEP07(2017)001

and M

had

are the means of the corresponding mass distributions. The values of M

lep

, M

had

, and σ

had

are derived using a sample of simulated events in which all four

partons of the ﬁnal state top quark decay products are matched to a reconstructed jet

used in the hypothesis. In each event, the tt pair reconstructed with the smallest value of

(labeled χ

min

) is chosen. In events with a t-tagged jet, M

had

is given by the mass of

the large-radius jet calculated using the soft drop algorithm. This choice is made because,

compared to the conventional jet mass, the soft dropped mass is much less dependent on

the jet p

, and therefore on the resonance mass in a given signal hypothesis. Moreover,

this provides greater discrimination between background and signal.

Events in the signal region are required to have χ

min

< 30 for all lepton+jets categories.

This upper threshold on χ

min

reduces the contribution of events from non-tt background

processes and maximizes the expected sensitivity of the analysis to new resonances.

Finally, to further enhance sensitivity, events are categorized according to the number

of t-tagged and b-tagged jets as follows: events with one t-tagged jet (1 t tag); events

with zero t-tagged jets and at least one b-tagged jet (0 t tag, 1 b tag); and events with zero

t-tagged and b-tagged jets (0 t tag, 0 b tag).

5.2 Fully hadronic channel

The fully hadronic channel requires that at least two jets satisfy kinematic and t tagging

selection criteria. The data were collected online with a trigger requiring the scalar sum of

the AK4 jet energies (H

) to be larger than 800 GeV. The trigger selection has an eﬃciency

of above 95%, as measured in simulation, for events that satisfy the oﬄine requirement

> 1000 GeV. The event reconstruction is performed using only AK8 jets. The two

leading jets are required to have p

> 500 GeV, rapidity |y| < 2.4, and both are required

to be t tagged. A back-to-back topology is selected by requiring the azimuthal separation

of the two leading jets to satisfy |∆φ| > 2.1.

Events are further categorized into six regions based on two criteria: the rapidity

diﬀerence (∆y) between the two AK8 jets and the number of jets with at least one b-

tagged subjet for the two highest p

jets. Events can contain 0, 1, or 2 jets with a b-tagged

subjet, and they are separated into bins of |∆y| < 1.0 and |∆y| > 1.0.

5.3 Tagging variables in lepton+jets and fully hadronic channels

The distributions of the two variables used in the t tagging algorithm, τ

and M

, are

shown in ﬁgure 2 for the lepton+jets channel (upper row) and the fully hadronic channel

(lower row). Each of the ﬁgures is obtained after removing the selection on the quan-

tity being plotted, while maintaining all other analysis-level selections. We observe good

agreement between data and simulation in the lepton+jets decay channel, where simulated

events are divided into contributions from generator-level top quarks and other jets from

t events and subdominant background processes. The fully hadronic channel also shows

good agreement between the simulated distribution and data. The small discrepancies do

not aﬀect the analysis, as it relies on data to estimate the NTMJ contribution to the back-

ground. Some discrepancy is visible at high values of τ

, however this region is excluded

by the selection used for t tagging.

– 8 –

JHEP07(2017)001

Events/bin

100

150

Data

Matched to top quark

Unmatched to top quark

=1 pb)σZ' 3 TeV (

CMS

(13 TeV)

-1

2.6 fb

< 210 GeV

| < 2.4, 110 GeV < Mη > 500 GeV, |

AK8 jets with p

lepton+jets

τJet

0 0.2 0.4 0.6 0.8 1

Data/bkg

0.5

1.5

Events/10 GeV

100

150

Data

Matched to top quark

Unmatched to top quark

=1 pb)σZ' 3 TeV (

CMS

(13 TeV)

-1

2.6 fb

< 0.69

τ| < 2.4, η > 500 GeV, |

AK8 jets with p

lepton+jets

Jet soft-drop mass [GeV]

0 50 100 150 200 250 300

Data/bkg

0.5

1.5

Events/bin

100

150

200

CMS

(13 TeV)

-1

2.6 fb

< 210 GeV

subjet b-tag, 110 < M

| < 2.4,η > 400 GeV, |

AK8 jets with p

fully hadronic

Data

QCD

=1 pb)σZ' 3 TeV (

τJet

0 0.2 0.4 0.6 0.8 1

Data/bkg

0.5

1.5

Events/10 GeV

100

150

CMS

(13 TeV)

-1

2.6 fb

< 0.69

τ| < 2.4, subjet b-tag, η > 400 GeV, |

AK8 jets with p

fully hadronic

Data

QCD

=1 pb)σZ' 3 TeV (

Jet soft-drop mass [GeV]

0 50 100 150 200 250 300

Data/bkg

0.5

1.5

Figure 2. Distributions of the N-subjettiness ratio, τ

, and the soft dropped mass, M

, for AK8

jets in data and simulation, after the signal selection. For lepton+jets, with p

> 500 GeV (upper

row). For the fully hadronic ﬁnal state, with p

> 400 GeV and subjet b tag (lower row). The

distribution of τ

(left) is shown after the selection 110 < M

< 210 GeV, and the distribution

of M

(right) is shown after the selection τ

< 0.69. The lepton+jets channel plots compare

data to background simulation, where the latter is divided into contributions from jets matched at

the generator level to top quarks and other jets in top pair or W+jets events. The fully hadronic

channel plots compare data to tt and QCD multijet simulation. Contributions from a benchmark

narrow Z

signal model are shown with the black dashed lines. In obtaining the ﬁnal results, NTMJ

production is estimated from data, and simulated QCD multijet events are not used. In all plots,

the error bars include only statistical contributions.

6 Background model and normalization

In this section, we describe the sources of the SM background and methods of background

estimation for both the lepton+jets and fully hadronic channels. We then introduce the

sources of systematic uncertainty considered in this analysis. Finally, we describe the

treatment of the backgrounds and uncertainties in the maximum likelihood ﬁt that is used

to determine the total yield of SM processes and in the statistical analysis of data.

– 9 –

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