LHC ATLAS实验：13 TeV pp碰撞中双光子和缺失横向动量的超对称性搜索

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"这篇实验物理学文章是ATLAS合作团队在大型强子对撞机(LHC)上使用ATLAS探测器进行的一项研究，旨在通过分析质子-质子碰撞产生的包含两个光子和缺失横向动量的最终状态来寻找超对称性。这项研究基于2015年s = 13 TeV的质心能量条件下收集的3.2fb-1数据。文章报告了标准模型背景估计为0.27-0.10 + 0.22个事件，并且在信号区域内没有发现任何事件。由此，他们得出了一个模型无关的95％置信水平(CL)上限，即可见横截面的上限为0.93 fb，暗示可能存在的超出标准模型的新物理现象。在一种特定的超对称模型——规范化介导的超对称性破裂的广义模型中，这个结果设定了类似bino的次轻质超对称粒子的质量下限为1650 GeV，同时排除了质量较轻的类似wino的中性动物的胶态简并八位位态。" 这篇开放获取的文章详细介绍了在欧洲物理杂志C(Eur.Phys.J.C)上发表的研究成果，其DOI为10.1140/epjc/s10052-016-4344-x。文章的核心是利用ATLAS探测器在LHC上的实验数据，对包含双光子和丢失的横向动量的事件进行超对称粒子的搜寻。在13 TeV能量的质子-质子碰撞数据中，研究者运用数据驱动和模拟方法评估了标准模型背景，并在没有检测到任何信号事件的情况下，对新物理现象的横截面给出了限制。这项研究对超对称性理论提出了严格的检验，特别是在一种特殊的超对称破缺机制下，它对超对称粒子的质量范围进行了限定，为理解宇宙中的暗物质和解决标准模型中的某些问题提供了重要的参考。搜索超对称性的方法涉及到对高能粒子碰撞产生的事件进行仔细分析，包括对光子的探测和对丢失动量的测量。光子是高能粒子相互作用的产物，而丢失的横向动量可能源于未被直接检测到的中性粒子，如可能存在的超对称粒子。通过对这些数据的深入分析，研究者可以推断出超对称粒子的存在或不存在的可能性，以及它们可能的性质和质量范围。此外，ATLAS合作团队的这项工作也展示了现代粒子物理学研究中的数据分析技术，包括背景估计的不确定性处理、统计方法的应用以及对新物理模型的理论框架。这些研究不仅有助于推进我们对基本粒子和宇宙学的理解，而且也为未来的粒子物理实验设定了新的标准和挑战。

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Table 1 Requirements deﬁning the signal region (SR) and the W γγ

CR referred to in Sect. 6

SR W γγ CR

2 Tight photons with

> 75 GeV

2 Tight photons with p

> 50 GeV

1 e or μ with p

> 25 GeV

φ

min

(jet, p

miss

)>0.5 φ

min

(jet, p

miss

)>0.5

miss

> 175 GeV 50 < E

miss

< 175 GeV

eff

> 1500 GeV N (jets)<3

eγ

/∈ 83–97 GeV

mass benchmark points, leading to the deﬁnition of a single

signal region (SR). The selection requirements for this SR

are shown in Table 1.

6 Background estimation

Processes that contribute to the Standard Model background

of diphoton ﬁnal states can be divided into three primary

components. The largest contribution to the inclusive dipho-

ton spectrum is the “QCD background”, which can be further

divided into a contribution from two real photons produced

in association with jets, and a “jet-faking-photon” contribu-

tion arising from γ +jet and multijet events for which one

or both reconstructed photons are faked by a jet, typically

by producing a π

→ γγ decay that is misidentiﬁed as

a prompt photon. An “electron-faking-photon background”

arises predominantly from W , Z, and t

t events, possibly

accompanied by additional jets and/or photons, for which

an electron is misidentiﬁed as a photon. Electron-to-photon

misidentiﬁcation is due primarily to instances for which an

electron radiates a high-momentum photon as it traverses the

material of the ATLAS inner detector. Last, an “irreducible

background” arises from W γγ and Z γγ events. These back-

grounds are estimated with a combination of data-driven and

simulation-based methods described as follows.

The component of the QCD background arising from real

diphoton events (γγ) is estimated directly from diphoton

MC events, rescaled as function of E

miss

and the number

of selected jets to match the respective distributions for the

inclusive diphoton sample in the range E

miss

< 100 GeV.

While this background dominates the inclusive diphoton

sample, it is very steeply falling in E

miss

, making it small rel-

ative to backgrounds with real E

miss

for E

miss

 100 GeV,

independent of the reweighting.

The component of the QCD background arising from jets

faking photons and the background arising from electrons

faking photons are both estimated with a data-driven “fake-

factor” method, for which events in data samples enriched in

the background of interest are weighted by factors parame-

terizing the misidentiﬁcation rate.

To estimate the jet-faking-photon fake-factor, the jet-

faking-photon background is enriched by using an inverted

isolation requirement, selecting events only if they contain

one or more non-isolated photons. The relative probability of

an energy cluster being reconstructed as an isolated, rather

than non-isolated, photon is known as the photon-isolation

fake factor, and is measured in an orthogonal “non-tight”

sample of photons. The selection of this sample requires that

all the tight photon identiﬁcation requirements be satisﬁed,

with the exception that at least one of the requirements on

the calorimeter variables deﬁned only with the ﬁrst (strip)

layer of the electromagnetic calorimeter fails. This leads to

a sample enriched in identiﬁed (non-tight) photons that are

actually π

s within jets. The correlation between the isola-

tion variable and the photon identiﬁcation requirements was

found tobesmall and to have no signiﬁcantimpact on the esti-

mation of the jet-faking-photon fake-factor. The fake factors

depend upon p

and η, and vary between 10 and 30 %. The

jet-faking photon background is then estimated by weighting

events with non-isolated photons by the applicable photon-

isolation fake factor.

The electron-faking-photon background is estimated with

a similar fake-factor method. For this case, the electron-

faking-photon background is enriched by selecting events

with a reconstructed electron instead of a second photon.

Fake factors for electrons being misidentiﬁed as photons are

then measured by comparing the ratio of reconstructed eγ

to ee events arising from Z bosons decaying to electron–

positron pairs, selected withinthemass range of 75–105 GeV.

The electron-faking-photon background is then estimated by

weighting selected eγ events by their corresponding fake

factors, which are typically a few percent.

The irreducible background from W γγ events is esti-

mated with MC simulation; however, because it is a poten-

tially dominant background contribution, the overall normal-

ization is derived in a γ γ control region (W γγ CR) as fol-

lows. Events in the W γγ CR are required to have two tight,

isolated photons with p

> 50 GeV, and exactly one selected

lepton (electron or muon) with p

> 25 GeV. As with the

SR, events are required to have φ

min

(jet, p

miss

)>0.5, so

that the direction of the missing transverse momentum vector

is not aligned with that of any high-p

jet. To ensure that the

control sample has no overlap with the signal region, events

are discarded if E

miss

> 175 GeV. While these requirements

target W γγ production, they also are expected to select

appreciable backgrounds from t

tγ , Z γ and Z γγ events, and

thus additional requirements are applied. To suppress t

tγ

contributions to the W γγ CR, events are discarded if they

contain more than two selected jets. To suppress Zγ contri-

butions, events are discarded if there is an e–γ pair in the

events with 83 < m

eγ

< 97 GeV. Finally, to suppress Z γγ

123

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LHC ATLAS实验：13 TeV pp碰撞中双光子和缺失横向动量的超对称性搜索

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