ATLAS探测器在13 TeV质子碰撞中寻找超重长寿命电R强子：最新实验结果

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在《物理学快报B》(Physics Letters B)的760卷第647-665页中，一篇由ATLAS合作团队发表的文章详细描述了他们在欧洲核子研究中心（CERN）的大型强子对撞机（LHC）上进行的一项研究。这项研究利用了在s = 13 TeV质子-质子碰撞数据集，总量达到了3.2 fb-1，目标是探索可能存在的重（massive）且寿命较长的带电R强子（R-hadrons）。R强子是一种理论预言中的新型强子，它们带有重质量但具有不同于标准模型粒子的特性，如可能的慢速移动和大电离损失。研究基于对这类特殊粒子的观测指标，即它们的离子化损失显著以及较光速显著减缓的传播速度。这些特征使得R强子的探测成为可能，因为它们在穿过物质时产生的效应会与常规带电粒子有所不同。然而，实验结果显示并未观察到与预期背景的显著偏离，这表明在当前数据中并未找到明显的R强子信号。文章提供了95%置信水平上的上限，限制了在600 GeV至2000 GeV质量范围内长寿命R强子的生产截面。此外，实验还排除了某些特定情况下可能的超对称粒子，如胶子ino（gluino）、bottom squark（底夸克喷射）和top squark（顶夸克喷射），其质量上限分别为1580 GeV、805 GeV和890 GeV，这些结果对于粒子物理的标准模型和超对称理论框架构成了进一步的约束。这篇研究是高能物理领域对未知粒子的探索尝试，它不仅验证了现有理论的精确性，也增加了我们对可能超出标准模型的新物理现象的理解。尽管没有发现R强子，但它为未来的实验设计和理论构建提供了有价值的数据基础。未来随着更多数据的积累和更先进的分析技术，这种类型的搜索将继续在寻找新物理的过程中扮演关键角色。

650 The ATLAS Collaboration / Physics Letters B 760 (2016) 647–665

Fig. 3. Data (black dots) and background estimates (red solid line) for m

(left) and m

βγ

(right) for the gluino R-hadron search (1000 GeV). The green shaded band illustrates

the statistical uncertainty of the background estimate. The blue dashed lines illustrate the expected signal (on top of background) for the given R-hadron mass hypothesis.

The black dashed vertical lines at 500 GeV show the mass selection and the last bin includes all entries/masses above. (For interpretation of the references to colour in this

ﬁgure legend, the reader is referred to the web version of this article.)

as well as βγ and β, respectively, via m = p/βγ . The minimum

mass requirements, m

min

βγ

and m

min

, are set to correspond to a

value about 2σ below the nominal R-hadron mass value, given

the mass resolution expected for the signal.

The

total selection eﬃciency depends on the sparticle mass and

varies between 9% and 15% for gluino and top-squark R-hadrons

and

6% to 8% for bottom-squark R-hadrons. The lower eﬃciency

for bottom squarks is expected, as R-hadrons are most likely pro-

duced

in mesonic states, where those with down-type squarks

tend to be neutral more often than those with up-type squarks,

due to light-quark production ratios of u : d : s ≈ 1 : 1 : 0.3 [12]

during

hadronisation. The expected signal yield and eﬃciency, es-

timated

background and observed number of events in data for

the full mass range after the ﬁnal selection are summarised in

Table 3.

5. Background estimation

The background is evaluated in a data-driven manner. First,

probability distribution functions (pdf) in the momentum, and also

in the β and βγ values, are determined from data. These pdfs

are produced from candidates in data, which have passed the ini-

tial

selection mentioned earlier, but fall in sidebands of the signal

region, as described below. Background distributions in m

and

βγ

are obtained by randomly sampling the pdfs derived above

and then using the equation m = p/βγ . These mass distributions,

which are normalised to the data events outside the signal region

(i.e. not passing both mass requirements of the hypothesis in ques-

tion),

are shown in Fig. 3 along with the data and expected signal

for the 1000 GeV gluino R-hadron mass hypothesis.

Each

R-hadron mass hypothesis has a different selection, and

therefore corresponding individual background estimates are pro-

duced

accordingly. The momentum pdf is produced from events

that pass the momentum cut, but fail the β and βγ requirements

in Table 1 for the chosen R-hadron mass hypothesis, but nonethe-

less

have β<1 and βγ < 2.5. The β and βγ pdfs are produced

by selecting events which pass the respective β and βγ selec-

tion

and have momentum in the range 50 GeV < p < 200 GeV.

Since momentum is correlated with |η|, any correlation between

|η| and β (βγ ) will lead to a correlation between momentum and

Table 2

Summary

of all studied systematic uncertainties. Ranges indicate a dependency on

the R-hadron mass hypothesis (from low to high masses).

Source Relative uncertainty

[±%]

Theoretical uncertainty on signal 14–57

Uncertainty on signal eﬃciency 20–16



Trigger eﬃciency 2



QCD uncertainty (ISR, FSR) 14



Pile-up 7–1



Pixel βγ measurement 1–3



Calorimeter β measurement 10–2

Luminosity 5

Uncertainties on background estimate 30–43

β (βγ ), invalidating the background estimate. The size and impact

of such correlations are reduced by determining the three pdfs in

ﬁve equal-width bins of |η|. This procedure also ensures that dif-

ferent

detector regions are treated separately.

6. Systematic uncertainties

The systematic uncertainties are obtained from data, whenever

possible. The two major uncertainties for which this is not the case

are cross sections and ISR, the latter being folded with the trigger

eﬃciency curve obtained from data to produce the overall E

miss

trigger eﬃciency. The individual contributions are outlined below

and summarised in Table 2.

6.1. Theoretical cross sections

Signal cross sections are calculated to next-to-leading order in

the strong coupling constant, including the resummation of soft

gluon emission at next-to-leading-logarithmic accuracy (NLO +

NLL) [38–40]. The nominal cross section and the uncertainty are

taken from an envelope of cross-section predictions using differ-

ent

PDF sets and factorisation and renormalisation scales, as de-

scribed

in Ref. [41]. This prescription results in an uncertainty of

14% (at 600 GeV) rising to 24% (at 1600 GeV) and to 32% (at

2000 GeV) for gluino R-hadrons and marginally larger values for

squark R-hadrons.

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