CMS实验：13 TeV质子-质子碰撞中Zγ共振的搜寻

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"在13 $$ \sqrt {s} = 13 $$ TeV的质子-质子碰撞中，CMS合作组利用轻子和强子最终态搜索Zγ共振的实验方法。这一分析利用了2016年在大型强子对撞机(LHC)上收集的质心能量为13 TeV的碰撞数据，数据集对应于35.9 fb-1的综合光度。实验旨在探测可能衰变为Z玻色子和光子的共振体。在轻子通道中，通过电子或介子对来重建Z玻色子候选事件。而在强子通道中，采用了大半径射流的子结构分析和先进的b夸克标记技术来识别包含Z玻色子轻夸克或b夸克衰变产物的射流。结合这两个通道的结果，对0.35至4.0 TeV质量范围内的窄和宽自旋-0共振体进行了解释，给出了关于这种共振体生产截面乘以其至Zγ分支分数的最严格的限制。这项研究发表在JHEP09(2018)148期刊上，是开放访问的。" 这篇研究论文详细阐述了一种新的方法，用于在高能粒子碰撞中寻找Zγ共振。共振体的探测对于理解超出标准模型的新物理现象至关重要，因为它们可能预示着未知粒子的存在。在13 TeV的能量下进行质子-质子碰撞，可以提供足够的能量来创建可能的新型强相互作用粒子，这些粒子在衰变时可能会产生Zγ对。论文中，研究人员首先在轻子通道中分析数据，利用电子或μ子对来识别Z玻色子。这个方法依赖于精确测量这些轻子的动量和能量，以重构Z玻色子的质量。然后，在强子通道中，研究人员利用射流子结构技术，特别是对大半径射流的分析，来识别可能包含Z玻色子衰变产物的喷注。这里，b夸克标记技术被用来进一步确认这些射流是否源于Z玻色子。结合两个通道的数据，研究团队能够对不同质量范围内的自旋-0共振体设定上限，这包括窄共振（可能对应于单个新粒子的产生）和宽共振（可能表示更复杂的物理过程）。这些限制是对现有粒子物理理论的有力测试，有助于排除或确认超出标准模型的假设。通过对35.9 fb-1的碰撞数据的深入分析，该研究提供了迄今为止对Zγ共振体最严格的限制。这样的限制对未来的粒子物理实验具有指导意义，可以指导实验设计和理论预测，以更好地探索可能存在的新物理现象。由于该研究是开放获取的，全球的研究人员都可以访问并利用这些结果进行进一步的理论和实验工作。

JHEP09(2018)148

the same pseudorapidity range as the leading lepton. The photon in the eeγ (µµγ) channel

is required to satisfy p

> 65 (40) GeV and |η| < 2.5. Electrons and photons in the ECAL

barrel-endcap transition region (1.44 < |η| < 1.57) are excluded from the analysis. In the

eeγ channel, the p

thresholds on the electrons and photons in the ECAL endcap region are

increased to 70 GeV, in order to ensure a fully eﬃcient trigger. Photons are required to be

separated from lepton candidates by ∆R > 0.4, to reduce the background from ﬁnal-state

radiation in Z → `` events. The invariant mass of the dilepton system is required to be

50 < m

< 130 GeV. The minimum dilepton mass requirement suppresses contributions

from pp → γγ

∗

events, where an internal conversion of a photon produces a lepton pair.

Finally, we require the ratio of the photon p

to m

Zγ

to be greater than 0.27. This require-

ment suppresses backgrounds due to jets misidentiﬁed as photons, without signiﬁcant loss

in the signal eﬃciency and without introducing a bias in the m

Zγ

spectrum. We search for

resonances in the m

Zγ

spectrum above 300 (250) GeV in the electron (muon) channel.

For the Jγ channel, the photon candidates are required to have p

> 200 GeV and

to fall within the barrel ﬁducial region of the ECAL (|η| < 1.44). Events with a photon

reconstructed in the endcap region suﬀer from high γ+jets background and do not add to

the sensitivity of the analysis; therefore they are not considered. Photon candidates in the

event are required to be separated from large-radius jets by a distance of ∆R > 1.1, which

guarantees that the photon isolation cone is not contaminated with the jet constituents.

To identify Z boson candidates in the Jγ channel, the reconstructed large-radius jet

mass, evaluated after applying a jet pruning algorithm [63, 64], is used. The jet pruning

reclusters the jet constituents and eliminates soft, large-angle QCD radiation, which oth-

erwise contributes signiﬁcantly to the jet mass. The pruning algorithm reclusters each jet

starting from its original constituents with the Cambridge-Aachen (CA) algorithm [65] and

discards soft and wide-angle recombinations in each step of the iterative CA procedure.

The pruned jet mass (m

pruned

) is computed from the sum of the four-momenta of the re-

maining constituents, which are corrected with the same factor as has already been used

in the generic jet reconstruction described above. A detailed description of the pruning

algorithm can be found in ref. [66]. For the signal selection, we require a Z candidate to

have 75 < m

pruned

< 105 GeV.

Finally, in the Jγ channel a requirement is imposed on the ratio of photon p

the reconstructed Zγ mass of p

Zγ

> 0.34, with the cutoﬀ chosen, based on a study

of simulated signals and backgrounds, to maximize the discovery potential for a narrow

resonance. We search for resonances with masses m

Zγ

> 650 GeV in this channel.

To further discriminate against the QCD multijet and γ+jets backgrounds in the Jγ

channel, we categorize the events according to the likelihood of a large-radius jet to contain

subjets originating from b quark fragmentation and to contain exactly two subjets. In

order to do so, we employ subjet b tagging and N-subjettiness [67] variables (τ

). The

N-subjettiness observable measures the spatial distribution of jet constituents relative to

candidate subjet axes in order to quantify how well the jet can be divided into N subjets.

Subjet axes are determined by a one-pass optimization procedure, which minimizes N -

subjettiness [68]. In particular, the ratio of 2- to 1-subjettiness, τ

= τ

/τ

, oﬀers an

excellent separation between the QCD jets and jets from vector boson decays [69], which

tend to have lower τ

values than the former.

– 7 –

JHEP09(2018)148

To infer the presence of b quark subjets within a large-radius jet, pruned jets are split

into two subjets by reversing the ﬁnal iteration of the jet clustering algorithm. These

subjets are classiﬁed according to the probability of their originating from b quarks, based

on results from the combined secondary vertex (CSVv2) b tagging algorithm [70, 71]. A

jet is identiﬁed as being consistent with the Z → bb decay when at least one of its subjets

satisﬁes the medium operating point of the CSVv2 algorithm, and the other subjet satisﬁes

the loose operating point. The medium and loose operating points correspond to 70 and

85% (20 and 50%) in the b jet tagging eﬃciency for p

< 300 GeV (p

= 1 TeV), and 1–

2% and 10–15% misidentiﬁcation probability of a light-ﬂavor jet, respectively. If an event

contains a Z → bb candidate, it is classiﬁed as “b tagged”. For the rest of the events, if the

large-radius jet has τ

< 0.45, we classify the event as “τ

tagged”. Otherwise, the event

is assigned to the “untagged” category. These three categories are mutually exclusive and

are combined for the ﬁnal result. The additional classiﬁcation according to the τ

value

enhances signal sensitivity by 10–15% at low to intermediate signal masses (up to ∼2 TeV),

relative to a previous analysis in the hadronic channels [32].

4 Background and signal modeling

4.1 Background modeling

Simulations in the ``γ channels indicate that 80–90% of the background remaining after

the full event selection is from SM Z boson production accompanied by initial-state photon

radiation, with the remainder mostly from Z+jets events, with a jet misreconstructed as a

photon. The background m

Zγ

distributions fall steeply and smoothly with increasing mass,

in these channels. Likewise, studies for the Jγ channel based on simulated background

samples and on the lower sideband of the jet mass distribution (50 < m

pruned

< 70 GeV)

in data show that the invariant mass distribution m

Zγ

of the SM background also falls

smoothly in this channel, and that the distributions of kinematic observables derived from

the lower jet mass sideband match those for the signal selection.

The background is measured directly in data, through unbinned maximum-likelihood

ﬁts to the observed m

Zγ

distributions, performed separately in each channel. The back-

ground in each channel is parametrized with an empirical function. Diﬀerent families of

functions inspired by the ones used in searches for beyond-the-SM phenomena in the di-

jet, multijet, diphoton, and VV channels at hadron colliders are evaluated in the signal

region using simulation (``γ channels) or the lower jet mass sideband (Jγ channel). Ex-

amples of these functions are: f (x) = P

(1 − x

1/3

)

(ln x)

[72], g(x) = P

(1 −

(ln x)

[73], h(x) = P

exp(P

x + P

) [74], where x = m

Zγ

√

s is the

center-of-mass energy (13 TeV), and the number of the ﬁt parameters P

shown is the

maximum order considered. The choice of the order within a family of ﬁtting functions

for the background distribution is made independently in each channel using the Fisher

F -test [75], which balances the quality of the ﬁt against the number of parameters. The

choice among the families of functions is optimized based on the results of the bias test

– 8 –

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