寻找右旋规矩玻色子WR：ATLAS探测器13 TeV质子-质子碰撞分析

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"在TeV与ATLAS探测器发生碰撞时，寻找右手规矩玻色子衰减成高动量重中微子和带电轻子" 这篇论文详细探讨了在左对称模型（Left-Right Symmetric Models）的理论框架内，如何通过ATLAS探测器寻找右旋规矩玻色子WR（Right-handed Gauge Boson）。该探测器位于大型强子对撞机（Large Hadron Collider, LHC），在2015年至2017年间收集的质子-质子碰撞数据中，质心能量为13TeV，总集成光度达到了80 fb^-1。WR是一种假想的基本粒子，它是标准模型的扩展，与已知的左旋规矩玻色子W相对应。研究的主要目标是观测WR衰变成右旋重 neutrino (NR) 和一个带电轻子（电子或μ子）的过程。这一过程的独特之处在于它涉及大半径喷流（large-radius jets），特别是那些包含电子的喷流。利用这些特征可以有效地筛选信号，因为它们有助于降低背景噪声。论文中的搜索策略特别关注了信号事件的拓扑结构，通过对这些结构的选择，可以减少与预期信号不匹配的背景事件。分析结果表明，没有发现与标准模型预测的重大偏离，这意味着未观察到WR存在的明显证据。因此，研究人员根据实验数据在WR和NR的质量平面上设置了限制。对于质量范围在0.1-1.8 TeV的NR，WR的质量上限被限制在3.8-5 TeV之间。这项研究的结论是，虽然目前的分析并未发现WR粒子，但其设定的限制为进一步探索超出了标准模型的新物理现象提供了重要的参考。这不仅有助于验证左对称模型的预言，也为理解宇宙的基本力和粒子性质提供了新的视角。文章发表在《Physics Letters B》杂志的第798期，卷号为134942，是一篇开放获取的文章，可供公众免费查阅。编辑是M. Doser。该研究的提交日期为2019年4月30日，经过修订后于8月4日接受，最终在2019年9月18日在线发布。

The ATLAS Collaboration / Physics Letters B 798 (2019) 134942 5

Table 2

Object

selection criteria. The signiﬁcance of the transverse impact parameter is deﬁned as the transverse impact

parameter d

divided by its uncertainty, σ

, of tracks relative to the primary vertex with the highest sum of

track p

. The longitudinal impact parameter z

is multiplied by sinθ , where θ is the polar angle of the track.

Electron channel Muon channel

Lepton:

>26 GeV >28 GeV

|η||η| < 1.37 or 1.52 < |η| < 2.47 < 2.5

Leading lepton quality Medium [61], isolated [61]Medium[62], isolated [62]

Subleading lepton quality Medium, no isolation Medium, no isolation

Transverse impact parameter signiﬁcance

|/σ

< 5.0 |d

|/σ

< 3.0

Longitudinal impact parameter

|sin θ<0.5mm

Trimmed large-R jet:

>200 GeV

|η| <2.0

Mass

> 50 GeV None

• R

between the subleading lepton and the large-R jet is re-

quired

be less than 1, in order for the lepton to be inside the

jet.

• φ between the leading lepton and the large-R jet is required

to be greater than 2, in order to have a balanced topology be-

tween

the N

and the lepton from the W

decay.

• In order to reduce the Z +jets background, events with a dilep-

ton

invariant mass of less than 200 GeV are vetoed, and the

φ between the leading and subleading leptons is required to

be greater than 1.5.

After applying these requirements, simulation studies show that

the background consists mainly of tt and Z +jets processes (includ-

ing

off-shell Z /γ

∗

production), while contributions from W +jets,

single-top-quark and multijet processes are negligible. No require-

ments

on b-tagged jets are applied, as the W

in the signal can

decay to a top and bottom quark pair.

The ﬁnal discriminating observable used in the analysis is the

reconstructed mass of the W

candidate, m

reco

. In the electron

channel, the selected large-R jet corresponds to the N

candidate,

and therefore the W

candidate four-momentum is obtained by

adding the large-R jet and the leading electron four-momenta. In

the muon channel, the N

candidate four-momentum is obtained

by adding the four-momentum of the selected large-R jet to that of

the muon contained in the jet. The W

candidate four-momentum

is obtained by adding the N

candidate four-momentum to that of

the leading muon. In both cases, if there is more than one large-R

jet

in the event, the large-R jet with the largest mass is used.

Based

on the range of m

reco

, control and signal regions (CR,

SR) are deﬁned as speciﬁed in Table 3. The CR is deﬁned in a

region of low reconstructed m

reco

corresponding to a parameter

space excluded by previous searches [14]. The background in the

SR is evaluated from a combined ﬁt of MC and data events in the

CR (described in Section 6). To test the performance of large-R

jets

containing electrons, a validation region (VR) is deﬁned with

a selection orthogonal to the CR and the SR. This requires a muon

balanced in p

by a large-R jet with an electron inside. By con-

struction,

the VR is dominated by t

t events decaying dileptonically

to eμ ﬁnal states.

Fig. 3, good agreement is observed between data and sim-

ulation

in the m

reco

distributions in the control regions of the

electron and muon channels, as well as in the validation region.

In the bottom-right plot, the selection eﬃciency times acceptance

is shown for different signal samples. The eﬃciency decreases for

lower m

and higher m

values. The largest ineﬃciency arises

from the diﬃculty of electron reconstruction close to hadronic ac-

tivity,

which is discussed in the next section. The probability of

producing an off-shell W

increases with the mass. This can result

Table 3

Deﬁnition

of signal, control and validation regions.

Region Range of m

reco

Lepton ﬂavour

Signal region (SR) > 2TeV Same ﬂavour

Control region (CR) < 2TeV Same ﬂavour

Validation region (VR) All Mixed ﬂavour (leading:

muon; subleading: electron)

in a less boosted N

, explaining the drop in signal eﬃciency for

higher m

values.

5. Performance of large-R jets containing electrons

A distinguishing feature of this search is the use of large-R

jets

containing electrons as a proxy for N

in the electron chan-

nel.

Since the large-R jet construction procedure is based on en-

ergy

clusters calibrated at the hadronic scale, the effect of a non-

negligible

fraction of EM clusters in the large-R jet needs to be

investigated. The analysis does not use the kinematic properties of

the identiﬁed electron inside the large-R jets to reconstruct the

or W

invariant masses, but uses the mass of the large-R jet,

which includes the associated electron clusters. The presence of

real hadronic activity close to an electron may affect the recon-

struction

of the electron.

The

jet mass and energy scales, JMS and JES, deﬁned as the

average of the ratio of the mass or energy of the reconstructed

and corresponding generator-level large-R jets, are used to study

the effect of including the large EM-cluster of the electron in

the jet reconstruction. The matching between detector-level and

generator-level large-R jets is performed with R

< 0.75. The

generator-level jet is obtained by clustering stable ﬁnal-state parti-

cles

(with lifetime greater than 30 ps) except muons and neutrinos

using the same jet algorithm, radius parameter and trimming used

at the detector-level. The JMS and JES of the selected large-R jets

for a few representative signal samples are shown in Fig. 4 as a

function of the ratio of the energy of the electron to the energy of

the large-R jet. This ratio can be considered a proxy for the elec-

tromagnetic

energy fraction in the large-R jet. Constant values of

JES and JMS within a few percent of unity indicate that the large-R

jet

has only a weak dependence on the fraction of electromagnetic

energy inside the jet, and thus no particular additional corrections

are required for the signal large-R jets. Typical numbers for the

large-R jet mass resolution (JMR) in signal events are about 4-6%

in the electron channel and about 7-14% in the muon channel,

while the large-R jet energy resolution (JER) is about 3-5% GeV

in both channels. As opposed to the muon channel, in the electron

channel the large-R jet does contain the electron as a compact and

high energy deposit. This implies a more precise angular distribu-

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