8 TeV质子-质子碰撞中高能共振到Z和希格斯玻色子研究

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"这篇研究文章发表在Physics Letters B 748 (2015) 255–277，由CMS合作团队完成，报告了在质子-质子碰撞中寻找高能窄质量共振体的工作。该研究集中在8 TeV质心能量的碰撞数据上，这些数据是2012年CMS实验采集的，总集成光度为19.7 fb^-1。" 在高能物理领域，尤其是粒子物理学中，寻找新的物理现象和粒子一直是科学家们的重要任务。本文提出的实验方法是探索可能存在的高能窄质量共振体，这种共振体在衰变时会生成一个Z玻色子和一个希格斯玻色子。Z玻色子和希格斯玻色子都是标准模型中的基本粒子，Z玻色子负责传递弱相互作用，而希格斯玻色子则解释了其他基本粒子获得质量的机制。分析过程中，研究者关注的是衰变成合并的喷注对和τ轻子对的最终状态。喷注是高能碰撞后产生的强相互作用粒子簇，而τ轻子对则是Z和H玻色子衰变的产物。由于研究的共振体质量范围从0.8 TeV延伸到2.5 TeV，这意味着产生的Z和H玻色子具有相当大的动量，这种高能状态通常会导致喷注的合并以及τ轻子的高速运动，形成所谓的"提升"拓扑。 CMS实验，全称为Compact Muon Solenoid，是位于欧洲核子研究中心（CERN）大型强子对撞机（LHC）的一个国际合作实验项目。在这个实验中，研究人员利用2012年的数据，通过复杂的分析技术来检测和识别可能的共振信号，以期发现超出标准模型的新物理现象。关键词包括CMS、物理学、共振产生、喷注、τ轻子以及提升拓扑，这些都是实验研究的关键要素。CMS实验的目的是通过精确测量粒子碰撞的产物，以深入理解基本粒子的性质和宇宙的基本力，寻找可能揭示新物理的线索。这篇研究是高能物理领域的一次重要尝试，通过寻找高能质子-质子碰撞中Z和希格斯玻色子衰变产生的窄质量共振，期望能揭示未知的物理现象或粒子，进一步扩展我们对宇宙基本构成的理解。

CMS Collaboration / Physics Letters B 748 (2015) 255–277 259

Fig. 1. Observed distributions of m

for the all-leptonic channels along with the

corresponding MC expectations for signal and background, as well as background

estimation derived from data: (top) τ

category; (middle) τ

category; (bottom)

category. Ten equal-size histogram bins cover the region from 0 to 2.5 TeV,

while a single bin is used at higher m

because of the limited number of MC and

data events. The signal cross section is scaled by a factor of 5.

small contributions from potential signal resonances. In the case

of the semileptonic channels, we show in Fig. 3 the distribution of

jet

for data and MC at the preselection level. The black line, rep-

resenting

the ﬁt to data, is obtained by the sum of Eqs. (3) and

(4), with the tt shape as obtained from the control sample, the tt

normalization

is ﬁxed to MC scaled by the two SFs, and the other

components are free in the sideband ﬁt. An overall agreement be-

tween

data and prediction is observed. The background prediction

in the signal region is 156 ±26 events, with an observation of 151

events, for the τ

channel and 204 ± 31 events, with an obser-

vation

of 203 events, for the τ

channel.

In the all-hadronic channel, for events where the leading jet

satisﬁes the requirement τ

< 0.75, a plane is deﬁned using the

jet

and m

ττ

variables and four regions are considered, as shown

in Fig. 4. Most of the signal events are expected in region A, while

regions B, C, D are dominated by background events. Studies of the

correlation factors for simulated events and in regions orthogonal

Fig. 2. Observed distributions of m

for the semileptonic channels along with the

corresponding MC expectations for signal and background, as well as background

estimation derived from data: (top) τ

category; (bottom) τ

category. Ten

equal-size histogram bins cover the region from 0 to 2.5 TeV, while a single bin

is used at higher m

because of the limited number of MC and data events. The

signal cross section is scaled by a factor of 5.

to the signal region show that the variables m

jet

and m

ττ

are es-

sentially

uncorrelated. In this case, the total number of background

events in the region A can be estimated as:

bkg

=(N

)/N

. (5)

The method described by Eq. (5), called “ABCD method”, gives a

background prediction in the signal region that has been checked

to be insensitive to possible signal contamination in the regions B,

C, D.

Fig. 5 shows the observed distributions of m

in the τ

chan-

nel,

along with the corresponding MC expectations for signal and

background. The low number of events in regions B, C, D is not suf-

ﬁcient

to derive the shape of the distribution in the signal region

using the ABCD method. We use the results from this method to

compute the cross section upper limits, which are obtained with-

out

assumptions about the shape of the distributions. The ABCD

method is checked using an alternative background estimation

technique, where tt, W + jets and Z + jets background contri-

butions

are given by Eq. (2), while the QCD multijet background

is estimated from a control sample of events where at least one τ

candidate fails the isolation requirement. The same control sample

is used to obtain the shape of the QCD distribution in the signal

region presented in Fig. 5.

8. Systematic uncertainties

The sources of systematic uncertainty in this analysis, which

affect either the background estimation or the signal eﬃciencies,

are described below.

For

the signal eﬃciency, the main uncertainties come from the

limited number of signal MC events (3–10%), the integrated lumi-

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8 TeV质子-质子碰撞中高能共振到Z和希格斯玻色子研究

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