8TeV质子-质子碰撞中WZ衰变新共振的实验搜寻与上限设定

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在《物理快报B》(PhysicsLettersB, Vol. 740, Issue 1-2, pages 83-104, 2015)的文章中，CMS合作组织报告了一项对在8 TeV质子-质子（pp）碰撞中寻找新共振的研究。研究的焦点是这些新粒子可能通过WZ途径衰变为包含电子、缪子和中微子的全轻子终态。实验数据集基于19.5 femtobarns (fb-1) 的总光子亮度，这在大型强子对撞机(LHC)的运行中积累。标准模型(SM)的预期背景在实验结果中并未观察到显著的超额。研究人员针对一个扩展规范模型(Extended Gauge Model, EGM)中的W'玻色子进行了搜索，这是一种假设的额外弱规范玻色子，其生产截面和W'WZ耦合强度受到了严格的检验。根据模型预测，W'玻色子的生产横截面上限被设置在95%置信水平下，而理论上的预期质量上限为1.55 TeV，然而实际观测到的上限为1.47 TeV，这意味着尽管存在理论预测的可能性，但实验并未发现超出标准模型范围的新粒子信号。此外，研究还考虑了低能量尺度的 technicolor 模型，这是理论物理学中的一种设想，它试图解释强相互作用的自发对称性破缺现象。在这个背景下，作者设定了更严格的限制，对模型参数空间内的新粒子行为进行了约束。这些限制进一步验证了现有理论的稳定性，并排除了某些特定的 Technicolor 实例。这项工作对于粒子物理学界来说具有重要意义，因为它不仅测试了现有的理论预测，也为未来的实验设计和理论发展提供了有价值的输入。任何超越标准模型的新发现都将极大地推动我们对基本粒子和宇宙的理解。然而，当前的实验结果支持了SM的延续，但同时也为新物理的探索留下了开放的窗口。

CMS Collaboration / Physics Letters B 740 (2015) 83–104 87

Table 1

Minimum

requirements and search windows for each EGM W



mass point along with the number of expected background events (N

bkg

), observed events (N

obs

), expected



signal events (N

sig

), and the product of the signal eﬃciency and acceptance (ε

sig

×Acc.). The indicated uncertainties are statistical only.



mass (GeV) L

(GeV) M

window (GeV) N

bkg

obs

sig

×Acc. (%)

170 110 163–177 9.0 ±0.3818±11.33 ±0.09

180 115 172–188 38

±2 49 140 ±71.97 ±0.09

190 120 181–199 62

±1 76 371 ±14 2.6 ±0.1

200 125 190–210 81

±4 86 610 ±20 3.2 ±0.1

210 130 199–221 86

±3 101 786 ±23 3.9 ±0.1

220 135 208–232 91

±3 84 896 ±24 4.5 ±0.1

230 140 217–243 92

±4 80 977 ±25 5.2 ±0.1

240 145 226–254 91

±4841011±24 5.8 ±0.1

250 150 235–265 82

±1851021±23 6.4 ±0.1

275 162 258–292 73

±3 85 970 ±20 8.0 ±0.2

300 175 280–320 60

±1 74 858 ±16 9.6 ±0.2

325 188 302–348 56

±3 53 792 ±13 11.8 ±0.2

350 200 325–375 48

±3 37 699 ±10 13.9 ±0.2

400 225 370–430 32

±1 40 542 ±718.1 ±0.2

450 250 415–485 23

.1 ±0.8 26 399 ±521.5 ±0.2

500 275 460–540 16

.6 ±0.5 13 297 ±324.8 ±0.3

550 300 505–595 13

.2 ±0.6 14 220 ±227.6 ±0.3

600 325 550–650 10

.0 ±0.5 10 167 ±230.4 ±0.3

700 375 640–760 4

.7 ±0.2496.9 ±0.834.3±0.3

800 425 730–870 2

.8 ±0.2556.5 ±0.536.5±0.3

900 475 820–980 2

.1 ±0.2435.0 ±0.338.6±0.3

1000 500 910–1090 1

.4 ±0.1023.7 ±0.243.3±0.3

1100 500 1000–1200 0

.8 ±0.1015.9 ±0.146.8 ±0.3

1200 500 1080–1320 0

.58 ±0.08 1 10.77 ±0.07 49.1 ±0.3

1300 500 1108–1492 0

.56 ±0.08 1 8.20 ±0.04 56.1 ±0.3

1400 500 1135–1665 0

.60 ±0.08 1 5.64 ±0.03 57.3 ±0.3

1500 500 1162–1838 0

.57 ±0.08 1 3.76 ±0.02 57.5 ±0.3

1600 500 1190–2010 0

.56 ±0.08 1 2.56 ±0.01 57.7 ±0.3

1700 500 1218–2182 0

.50 ±0.08 1 1.782 ±0.009 57.6 ±0.3

1800 500 1245–2355 0

.44 ±0.07 1 1.255 ±0.007 58.0 ±0.3

1900 500 1272–2528 0

.39 ±0.07 0 0.844 ±0.005 55.0 ±0.3

2000 500 1300–2700 0

.36 ±0.07 0 0.595 ±0.003 54.7 ±0.3

Fig. 2. Limits at 95% CL on σ × B(W



→ 3ν) as a function of the mass of the

EGM W



(blue) and ρ

(red), along with the 1σ and 2σ combined statisti-

cal

and systematic uncertainties indicated by the green (dark) and yellow (light)

bands, respectively. The theoretical cross sections include a mass-dependent NNLO

K-factor. The thickness of the theory lines represents the PDF uncertainty associ-

ated

with the signal cross sections. The predicted cross sections for ρ

assume

that M

− 25 GeV and that the LSTC parameter sin χ = 1/3. (For inter-

pretation

of the references to color in this ﬁgure legend, the reader is referred to

the web version of this article.)

Finally, an additional uncertainty of 2.6% due to the measure-

ment

of the integrated luminosity is included [59]. Table 2 presents

a summary of the above systematic uncertainties.

Table 2

Summary

of systematic uncertainties. Values are given for the impact on signal and

background event yields. When the value of the uncertainty differs between the

different decay modes of the Wand Z bosons and/or between different W



masses

considered, a range is quoted in order to provide an idea of the magnitude of the

uncertainty, i.e. its impact.

Systematic uncertainty Signal impact Background impact

miss

resolution & scale 1–3% 1–23%

Muon p

resolution 1–3% 0.5–5%

Muon p

scale 1–2% 1–22%

Electron energy scale & resolution 0.5–2% 1.5–12%

Pileup 0.1–0.8% 0.5–5%

Electron trigger eﬃciency 2% 2%

Electron reconstruction eﬃciency 2% 2%

Electron ID & isolation eﬃciencies 1% 1%

Muon trigger eﬃciency 5% 5%

Muon reconstruction eﬃciency 2% 2%

Muon ID & isolation eﬃciencies 3% 3%

+jets – 30%

t – 15%

γ –50%

ZZ – 30%

WZ PDF – 5–10%

WZ scale – 5–15%

WZ MadGraph modeling – 5–30%

Luminosity 2.6% 2.6%

6. Results

As shown in Fig. 1, the data are compatible with the ex-

pected

SM background and no signiﬁcant excess is observed. Ex-

clusion

limits on the production cross section σ (pp → W



/ρ

→

WZ) × B(WZ → 3ν) are determined using a counting experiment

and comparing the number of observed events to the number of

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