8TeV质子-质子碰撞中中性色八元弱三重标量粒子搜索：首次实验限制

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在"JHEP09(2015)201"这篇论文中，研究团队利用了来自欧洲核子研究组织(CERN)的大型强子对撞机(LHC)上CMS实验收集的数据，进行了一项针对中性色八元组弱三重标量粒子(Theta零号，Θ0)的搜寻。这些粒子是理论中的假设，属于八重态三重态的框架。质子-质子碰撞的能量达到了8 TeV，对应的数据集拥有19.7 femtobarn-1的积分光度。研究关注的是Θ0的成对生产，其中每个粒子分别衰变为一对b夸克射流和一个Z玻色子加上一个射线。Z玻色子随后会进一步衰变为电子或μ子对。实验的目的是验证这种过程是否符合标准模型的预期，标准模型是物理学的基础理论，它在粒子物理学中扮演着核心角色。通过对实验数据的分析，研究人员发现观测到的事件数量与标准模型的预测相吻合，这意味着在当前实验条件下没有找到Θ0粒子的显著信号。然而，他们进一步计算了在95%置信水平下，对Θ0粒子质量的横截面与分支比的乘积的上限，这是一个定量评估新粒子可能性的重要指标。令人关注的是，研究者还设定了两种不同理论情形下的Θ0质量的95%置信下限，分别是623 GeV和426 GeV。这是首次在实验上对这类八重三重轴模型中的粒子质量设置明确的限制，对于这类理论模型的进一步发展具有重要意义。这项工作不仅检验了现有理论的预言，也为探索未知的粒子物理领域提供了关键的实验约束，同时也在间接支持或排除可能的新物理现象。未来的研究将继续在更高的能量和更精细的测量中寻找这样的新粒子，以扩展我们对基本粒子和宇宙的理解。

JHEP09(2015)201

4 Backgrounds

The dominant background processes are Z + jets and tt + jets. The contributions from

other processes, including diboson+jets and multijet, are small. The samples used for all

background processes are generated at tree level with the MadGraph program, inter-

faced with pythia v6.4 [33] for showering and hadronization. The Z + jets background is

normalized to the next-to-next-to-leading-order cross section using fewz v2.1 [34]. The

tt + jets and tW+jets (single top quark) backgrounds are normalized to the next-to-next-

to-leading-logarithm cross sections [35, 36]. The diboson+jets backgrounds are normalized

to the next-to-leading-order (NLO) cross section from the mcfm v5.8 [37] calculation. Full

simulation of the CMS detector is implemented using the Geant4 package [38].

4.1 Z + jets

More than half of the background events come from Drell-Yan (DY) production in asso-

ciation with jets. There are two points in estimating the background that require special

attention. First, care must be taken in estimating the yield of Z+ ≥3 jets events as the sim-

ulation may not correctly model the kinematic properties of multijets. Second, the event

yield and shape in the Z + b-jets process, where the kinematic properties of b jet pairs and

the fraction of heavy-ﬂavor jets in inclusive Z + jets might also suﬀer from mis-modeling

in the simulation. The following paragraphs describe how the data are used to improve on

the estimates from simulations.

To address the ﬁrst point, the event yield from the simulation is multiplied by a

correction factor to normalize it to data in a control region. This control region is deﬁned

such that the dilepton mass is between 80 and 100 GeV and there are at least three jets,

none of which is b tagged. The correction factor is found to be 0.98 ± 0.12 (0.91 ± 0.12)

for the electron (muon) channel. The correction is applied to all signal rectangle regions.

The 12% uncertainty comes from b tagging, JES, and pileup systematic uncertainties in

the control region, added in quadrature, while the statistical uncertainty is negligible. The

Z + jet mass in the Z + ≥3 jets (one b tag) control region is plotted in ﬁgure 4 after the

correction factor has been applied. Agreement between the data and simulated sample is

observed in the Z + ≥3 jets (no b tag) and Z + ≥3 jets (one b tag) control regions.

The second point requires a diﬀerent approach since control regions that include two

or more b jets may suﬀer from signal contamination. We take a two-step approach. First,

the simulated Z+ b-jets events are weighted by the ratio of the k-factor (NLO cross section

divided by LO cross section) for Z + 2 b-jets to that of Z+2 jets, using mcfm [39]. The

ratios vary from 1.09 to 2.56 in the b jet pair mass range of 20 GeV to 1.8 TeV. In the

simulation, about 20% of Z+jets events in the signal region have at least two jets originating

from b quarks. In the second step, the remaining diﬀerence between data and simulation is

evaluated as a function of b jet pair mass in the non-signal, oﬀ-diagonal regions (sidebands)

in the b jet pair mass and Z + jet mass plane. The uncertainty in the Z+heavy-ﬂavor jets

processes is taken from this diﬀerence or from the uncertainty in the CMS cross section

measurement [40], whichever is larger. The uncertainty varies from 20 to 50%.

– 6 –

JHEP09(2015)201

Figure 4. The Z + jet mass distribution in the Z + ≥3 jet (one b tag) control region after the

appropriate correction factor has been applied, for the electron channel (left) and muon channel

(right). The panels at the bottom show the ratio of data to background simulation, with the band

representing the systematic uncertainty including the normalization uncertainty obtained from the

Z + ≥3 jet (no b tag) control region.

The uncertainty in the shape of the other variable that is used to deﬁne the signal

search regions, the Z + jet mass distribution, is estimated by comparing the distribution in

the simulated Z + jets sample with those of several MadGraph samples that are produced

with factorization or renormalization scales and matrix element parton shower matching

thresholds varied up and down by a factor of two. The maximum diﬀerence between

the nominal distribution and the varied distributions is taken as an uncertainty. The

uncertainty varies from 2 to 55%.

4.2 tt + jets and tW + jets

After accounting for the Z + jets background, most of the remaining background events

come from t

t + jets, with a smaller contribution from tW+jets production.

Both tt + jets and tW+jets processes can yield ﬁnal states containing an opposite-sign

eµ pair. In this analysis, we are considering only opposite-sign, same-ﬂavor lepton pairs,

and thus the data containing an opposite-sign eµ pair can be used to calibrate the tt + jets

and tW+jets backgrounds. The overall event yield in the simulation is multiplied by a

factor of 1.07 ±0.13 to normalize it to data in the control region containing an eµ pair with

the mass between 60 and 120 GeV (Z

eµ

) and at least three jets, at least two of which are b

tagged. The uncertainty comes from b tagging, JES, and pileup systematic uncertainties

in the control region, added in quadrature, while the statistical uncertainty is negligible.

The b jet pair and Z

eµ

+jet mass distributions in this control region are plotted in ﬁgure 5

after the normalization factor is applied.

4.3 Diboson + jets

A few percent of the background events come from diboson+jets processes. The simulated

diboson cross sections are found to be in agreement with those measured by CMS [41, 42].

Therefore we make no correction to the event yields.

– 7 –

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