ATLAS探测器揭示13 TeV下希格斯玻色子与顶夸克对的关联观察

97 浏览量更新于2024-07-16 收藏 1.39MB PDF 举报

在2018年的《物理学报B》第784期中，ATLAS合作团队发表了一篇关于在大型强子对撞机（LHC）的13 TeV质心能量下，观察希格斯玻色子（Higgs boson）与顶夸克对（ttH）关联产生的研究文章。这项研究利用了ATLAS探测器记录的大规模质子-质子碰撞数据，其综合光度达到了79.8 fb$^{-1}$。实验关注了希格斯玻色子的多种衰变模式，包括bb（底夸克对）、WW⁎、τ+τ-、γγ（光子对）和ZZ⁎（Z玻色子对）。研究人员观察到的显著性为5.8个标准偏差，这超过了预期的4.9个标准偏差，显示出ttH过程的信号强度高于背景噪声。这一结果与标准模型（SM）的预测相一致。通过将13 TeV的数据集与7 TeV（4.5 fb$^{-1}$）和8 TeV（20.3 fb$^{-1}$）的数据集相结合进行分析，观察到的总显著性提升到了6.3个标准偏差，而预期值为5.1个标准偏差，进一步证实了ttH事件的观察。测量的总ttH生成截面，假设标准模型的分支比例，得出的结果是670±90（统计）-100 + 110（系统）fb。这个数值提供了对希格斯机制和顶夸克相互作用的重要定量验证，对于理解粒子物理学的标准模型以及可能的新物理现象具有重要意义。这篇论文不仅展示了高能物理实验中的精确测量技术，还反映了当前粒子物理研究中对希格斯粒子性质和基本力之间关系的深入探索。它代表了大型对撞机实验在发现新粒子和验证理论预言方面所取得的里程碑式成果，为未来的粒子物理实验计划提供了宝贵的数据基础和理论支持。

176 The ATLAS Collaboration / Physics Letters B 784 (2018) 173–191

Table 1

Observed

number of events in the different bins of the H → γγ and H → ZZ

∗

→ 4 searches, using 13 TeV data corresponding to an integrated luminosity of 79.8 fb

−1

The observed yields are compared with the sum of expected t

tH signal, normalised to the SM prediction, background from non-t

tH Higgs boson production and other

background sources, with the systematic uncertainties assigned to the observed result in the H → γγ analysis, and expected systematic uncertainties in the H → ZZ

∗

→ 4

analysis. The numbers for H → γγ are counted in the smallest m

γγ

window containing 90% of the expected signal. The numbers for H → ZZ

∗

→ 4 are derived in a

four-lepton mass window of 115 GeV < m

4

< 130 GeV. In the H → γγ analysis, the background yield is extracted from the ﬁt with freely ﬂoating signal. The BDT bins are

in descending order of signal purity.

Bin Expected Observed

tH (signal) Non-t

tH Higgs Non-Higgs Total Total

H → γγ

Had 1 4.2 ±1.10.49 ±0.33 1.8 ±0.56.4 ±1.310

Had 2 3

.4 ±0.70.7 ±0.67.5 ±1.111.6 ±1.514

Had 3 4

.7 ±0.92.0 ±1.732.9 ± 2.239.6 ± 3.247

Had 4 3

.0 ±0.53.2 ±3.155.0 ±2.861±567

Lep 1 4

.5 ±1.00.24 ±0.09 2.2 ± 0.66.9 ±1.27

Lep 2 2

.2 ±0.40.27 ±0.10 4.6 ±0.97.1 ±1.07

Lep 3 0

.82 ±0.18 0.30 ±0.13 4.6 ±0.95.7 ±0.95

→ ZZ

∗

→ 4

Had 1 0.169 ±0.031 0.021 ±0.007 0.008 ± 0.008 0.198 ±0.033 0

Had 2 0

.216 ±0.032 0.20 ±0.09 0.22 ±0.12 0.63 ±0.16 0

Lep 0

.212 ±0.031 0.0256 ±0.0023 0.015 ±0.013 0.253 ±0.034 0

put to a likelihood ﬁt that extracts the t

tH yield. The expected

dominant uncertainties in the cross section are due to the parton-

shower

modelling affecting the acceptance of the selection, and

to the cross-section uncertainty in the Higgs boson plus heavy-

ﬂavour

background (about 10% each). The leading experimental

uncertainty arises from the calibration of the jet energy scale (6%).

The expected and observed numbers of events are presented in

Table 1. No event is observed. The expected signiﬁcance is 1.2 stan-

dard

deviations.

4. Combination

The t

tH searches in the H → γγ and H → ZZ

∗

→ 4 decay

channels are combined with the H →b

b and multilepton searches

from Refs. [10,15]. These analyses use a dataset corresponding to

an integrated luminosity of 36.1 fb

−1

√

s = 13 TeV, and ﬁnd

observed (expected) signiﬁcances of 1.4 (1.6) standard deviations

for H → b

b and 4.1 (2.8) for the multilepton search. The combina-

tion

is performed using the proﬁle likelihood method described in

Ref. [54], based on simultaneous ﬁts to the signal regions and con-

trol

regions of the individual analyses. The overlap between the

selected events in the different analyses is found to be negligi-

ble.

The asymptotic approximation used in the ﬁt is veriﬁed with

pseudo-experiments, and the results are corrected if necessary. The

effect of systematic uncertainties in the predicted yields and distri-

butions

is incorporated into the statistical model through nuisance

parameters. The correlation scheme of all systematic uncertainties

between the H →b

b and multilepton analyses, as well as the cor-

relation

scheme of the theory uncertainties between all channels

are the same as in Ref. [10]. Since the H → γγ and H → ZZ

∗

→

4 analyses employ improved reconstruction software compared

with the H → b

b and multilepton analyses, the correlations be-

tween

the experimental systematic uncertainties are evaluated for

each source individually. Some components of the systematic un-

certainties

in the luminosity, the jet energy scale, the electron/pho-

ton

resolution and energy scale, and in the electron reconstruction

and identiﬁcation eﬃciencies are correlated between the channels.

All Higgs boson production processes other than t

tH, including

Higgs boson production in association with a single top quark, are

considered as background and their cross sections are ﬁxed to the

SM predictions [37]. The respective cross-section uncertainties are

considered as systematic uncertainties. The total t

tH cross section

is extracted assuming SM branching fractions and using the detec-

tor

acceptance and eﬃciencies predicted from the t

tH simulation

Table 2

Summary

of the systematic uncertainties affecting the combined t

tH cross-section

measurement at 13 TeV. Only systematic uncertainty sources with at least 1% im-

pact

are listed. The fake-lepton uncertainty is due to the estimate of leptons from

heavy-ﬂavour decay, conversions or misidentiﬁed hadronic jets. The jet, electron,

and photon uncertainties, as well as the uncertainties associated with hadronically

decaying τ -leptons, include those in reconstruction and identiﬁcation eﬃciencies,

as well as in the energy scale and resolution. The Monte Carlo (MC) statistical un-

certainty

is due to limited numbers of simulated events. More detailed descriptions

of the sources of the systematic uncertainties are given in Refs. [10,15].

Uncertainty source σ

/σ

[%]

Theory uncertainties (modelling) 11.9

t + heavy ﬂavour 9.9

tH 6.0

Non-t

tH Higgs boson production 1.5

Other background processes 2

Experimental uncertainties 9

Fake leptons 5

Jets, E

miss

4.9

Electrons, photons 3

Luminosity 3

τ -leptons 2.5

Flavour tagging 1

MC statistical uncertainties 4

discussed above. The respective uncertainties are included in the

ﬁt.

A combination is also performed with the t

tH searches based

on datasets corresponding to integrated luminosities of 4.5 fb

−1

√

s = 7 TeV and 20.3 fb

−1

√

s = 8TeV[16]. The com-

bined

observable is the signal strength μ = σ /σ

. The SM cross-

section

expectations σ

and branching ratios used in the 7 and

8 TeV analyses are updated with the values in Ref. [37], while

their uncertainties are not changed. Theoretical uncertainties in

the SM cross-section prediction for t

tH are included in the signal-

strength

extraction. The branching-fraction uncertainties and the

uncertainties due to missing higher-order corrections in the t

cross-section

prediction are correlated between the 7 and 8TeV

and

13 TeV analyses. Furthermore, the relevant uncertainties in the

electron/photon energy scale and resolution are correlated.

5. Results

Table 2 shows a summary of the systematic uncertainties in the

13 TeV t

tH production cross-section measurement. The dominant

uncertainties arise from the modelling of the t

t+ heavy-ﬂavour

processes in the H → b

b analysis [15] and the modelling of the

tH process, which affects the acceptance of the selection in all

剩余18页未读，继续阅读

weixin_38705640

粉丝: 8
资源: 953

ATLAS探测器揭示13 TeV下希格斯玻色子与顶夸克对的关联观察

与ATLAS探测器关联产生希格斯玻色子和顶夸克对的证据

使用ATLAS探测器搜索在多轻子最终状态下具有顶级夸克对的希格斯玻色子的相关产生

ATLAS探测器在多轻子最终状态中寻找与顶夸克对关联产生的希格斯玻色子

使用ATLAS探测器搜索在XH→qqbb最终状态下衰减为希格斯玻色子和新粒子X的共振

使用ATLAS探测器在s = 13 $$ \ sqrt {s} = 13 $$ TeV下搜索希格斯玻色子的Zγ衰减模式和pp碰撞中的新的高质量共振

用ATLAS探测器搜索希格斯和Z玻色子衰变分别为ϕγ和ργ

用光阑和改良的希格斯联轴器校正di-希格斯玻色子的产生

在具有希格斯或Z玻色子的事件中搜索直接的顶级副产生，并在与ATLAS探测器发生s = 13 $$ \ sqrt {s} = 13 $$ TeV pp碰撞的情况下丢失横向动量

搜索与向量玻色子相关联产生的希格斯玻色子，并在s = 13 $$ \ sqrt {s} = 13 $$ TeV的pp碰撞中，在H→aa→4b通道中的H→aa→4b通道中衰减成两个自旋零粒子 探测器

ATLAS探测器在13 TeV pp碰撞中搜索希格斯玻色子与暗物质关联

最新资源

搜索与向量玻色子相关联产生的希格斯玻色子，并在s = 13 $$ \ sqrt {s} = 13 $$ TeV的pp碰撞中，在H→aa→4b通道中的H→aa→4b通道中衰减成两个自旋零粒子探测器