13 TeV pp碰撞中寻找新物理现象：同味反电荷轻子对+喷流+失稳对的研究

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在《JHEP03(2018)076》这篇论文中，研究团队来自CMS合作组织，他们在s = 13 TeV的质子-质子（pp）碰撞数据中进行了深入的探索，目标是寻找超越标准模型（SM）的新物理现象。这些现象表现在最终状态中，具体特征是包含两个具有相反电荷、相同风味的轻子（如电子或μ子）、喷射束以及显著的缺失横向动量（transverse momentum，记为MET，代表未观测到的粒子动量）。该分析利用了2016年在大型强子对撞机（LHC）上收集的35.9 femtobarn-1（fb-1）的积分亮度，这是 CMS 探测器在高能运行期间的数据。研究方法主要依赖于对轻子对的不变质量（invariant mass）的分析，目的是发现与 Z 玻色子质量相匹配的动态边缘（kinematic edge），或者可能的共振结构，这可能暗示着新粒子的存在。动态边缘搜索特别关注的是那些对强力反应敏感的粒子生成，而共振搜索则同时考虑了强力和弱力产生的潜在新物理效应。实验结果表明，观测到的事件数量与标准模型的预测吻合，这意味着在此次数据分析中没有观察到显著的新物理信号。进一步的，论文还对几种超对称（Supersymmetry, SUSY）模型进行了约束。例如，在胶态糖对生产的规范介导的超对称断裂（GMSB）模型中，基于最轻中性介子的质量，如果存在介导的 Z 玻色子衰变链，胶态的质量范围在1500-1770 GeV已被排除在95%置信水平之外。在电弱焦化中性中性粒生产模型中，若最轻中性粒的质量为零，焦化粒子的最大质量上限被设定在610 GeV。而在电弱中性-中性-中性产生的GMSB模型中，考虑到不同的衰变模式，中性物质的最高质量范围限制在500-650 GeV。最后，论文提到在一个特定模型中，由于底部对称性的成对产生和衰变链，双轻子不变质量（dilepton invariant mass）的分析结果也得到了讨论。然而，由于空间限制，详细的分析结果并未在此概述，但可以推测这同样是对超对称理论参数空间的一次细致探索。总结来说，这篇论文是CMS在13 TeV LHC运行期间对新物理现象进行的一项重要实验，尽管没有发现明显的突破性信号，但其对标准模型的验证和对超对称等潜在新理论的限制，为未来的粒子物理学研究提供了有价值的数据和限制条件。

JHEP03(2018)076

and > 250 GeV. Although the selections for the EW VZ and HZ regions are mutually

exclusive, they are not necessarily exclusive with respect to the strong-production SR se-

lections. For interpretations of the analysis results, either the strong-production or the EW

regions are considered depending on the signal model.

The baseline SR in the edge search requires m

> 20 GeV, at least two jets, p

miss

150 GeV, M

(``) > 80 GeV, and the two jets with the highest p

to have a separation in φ

from ~p

miss

of at least 0.4. A ﬁt is performed in this baseline region to search for a kinematic

edge in the m

spectrum. A counting experiment is also performed in seven bins of m

excluding the range used for the on-Z search. These are summarized in table 1.

A likelihood discriminant is used to distinguish between events originating from dilep-

tonically decaying top quark pairs and other sources. The observables used for the likeli-

hood discriminator are p

miss

, the p

of the dilepton system, |∆(φ)| between the leptons,

and an observable called Σm

. The latter is the sum of the invariant masses of the two

lepton and b-tagged jet systems, and should have an endpoint at 2

M(t)

− M(W)

for

events resulting from top quark pairs. To calculate Σm

, all pairings of a lepton with

a jet are considered, and the pairing with the minimum invariant mass is selected. This

process is repeated for the remaining lepton and jets, and the sum of the invariant masses

of the two lepton-jet pairs is deﬁned as Σm

. If b-tagged jets are present, they are given

priority in the calculation of both lepton-jet systems; i.e., if one or more b-tagged jets are

present, Σm

between the leptons and the b-tagged jet(s) is minimized ﬁrst, and then

the remaining (b-tagged) jets are considered for the minimization of the sum Σm

of the

second lepton. To calculate the likelihood discriminant, the probability density functions

of the four observables are determined by ﬁts in the diﬀerent-ﬂavor (DF) control sample

using the same kinematic requirements as the same-ﬂavor (SF) SR except removing the

(``) selection. The respective ﬁt functions are a sum of two exponential functions

for p

miss

, a second-order polynomial for |∆(φ)|, and a Crystal Ball (CB) function [59] for

both the dilepton p

and Σm

distributions. A likelihood function is constructed, and

its negative logarithm is taken as the discriminator value. Two categories of events are

deﬁned: “tt-like,” with a discriminator value less than 21 and an eﬃciency of 95% for tt

events, and “not-tt-like,” which is composed of the remainder of the events.

In addition, two aggregate SRs are deﬁned for the edge search, integrating the mass

bins below and above the Z boson mass for the not-tt-like category.

6 Standard model background predictions

The backgrounds from SM processes are divided into three categories. Those that produce

DF pairs (e

∓

) as often as SF pairs (µ

∓

, e

∓

) are referred to as ﬂavor-symmetric (FS)

backgrounds. Among them, the dominant contribution arises from top quark pair produc-

tion; subleading contributions are also present from W

−

, Z/γ

∗

(→ ττ), tW single-top

quark production, and leptons from hadron decays. Data samples of DF events are used

to predict the SF background.

The remaining background categories contain ﬂavor-correlated sources of lepton pro-

duction that only contribute events with OCSF leptons. The dominant contributions at

– 9 –

JHEP03(2018)076

Strong-production on-Z (86 < m

< 96 GeV) signal regions

Region N

jets

b-jets

[GeV] M

(``) [GeV] p

miss

binning [GeV]

SRA b veto 2–3 =0 >500 >80 100–150, 150–250, >250

SRB b veto 4–5 =0 >500 >80 100–150, 150–250, >250

SRC b veto ≥6 =0 — >80 100–150, >150

SRA b tag 2–3 ≥1 >200 >100 100–150, 150–250, >250

SRB b tag 4–5 ≥1 >200 >100 100–150, 150–250, >250

SRC b tag ≥6 ≥1 — >100 100–150, >150

Electroweak-production on-Z (86 < m

< 96 GeV) signal regions

Region N

jets

b-jets

Dijet mass [GeV] M

[GeV] p

miss

binning [GeV]

VZ ≥2 =0 m

< 110 M

(``) > 80 100–150, 150–250, 250–350, >350

HZ ≥2 =2 m

< 150 M

(`b`b) > 200 100–150, 150–250, >250

Edge signal regions

Region N

jets

miss

[GeV] M

(``) [GeV] tt likelihood m

binning [GeV]

Edge ﬁt ≥2 >150 >80 — >20

tt-like ≥2 >150 >80 <21

20–60, 60–86, 96–150, 150–200,

200–300, 300–400, >400

not-tt-like ≥2 >150 >80 >21 same as tt-like

aggregate ≥2 >150 >80 >21 20–86, >96

Table 1. Summary of all SR selections.

lower p

miss

are from DY production in association with jets, where p

miss

arises from mis-

measurement of the jet energies. Data samples of photon events are used to predict this

DY+jets background.

The ﬁnal category comes from events with prompt neutrinos in addition to an OCSF

pair from a Z/γ

∗

boson. This includes WZ and ZZ production and processes with lower

cross section such as ttZ among others. These backgrounds are referred to as “Z+ν” and

can be important in the high-p

miss

signal bins.

6.1 Flavor-symmetric backgrounds

The method of estimating the FS backgrounds relies on the fact that, for such processes, SF

and DF events are produced at the same rate. This allows for prediction of the background

yields in the SF sample from those in the DF sample by application of an appropriate

correction factor, which is estimated from CRs in data. This factor corrects for diﬀer-

ent ﬂavor-dependent reconstruction and identiﬁcation eﬃciencies and for ﬂavor-dependent

trigger eﬃciencies, which can be diﬀerent for electrons and muons.

For cases where the DF contribution is of suﬃcient statistical power to make an accu-

rate prediction in the SF channel, a background estimate in the SF channel can therefore

be obtained by applying a multiplicative correction factor, R

SF/DF

, to the DF channel

yield. The correction is determined in two independent ways, both based purely on con-

trol samples in the data. The two results are then combined using the weighted average

according to their uncertainties to obtain the ﬁnal factor. The ﬁrst approach uses a direct

– 10 –

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13 TeV pp碰撞中寻找新物理现象：同味反电荷轻子对+喷流+失稳对的研究

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