LHC对轻量CP奇数标量的双顶关联生产探测

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"这篇论文探讨了在大型强子对撞机(Large Hadron Collider, LHC)中探测轻量CP奇数标量的可能性，特别是在双顶夸克关联生产过程中的敏感性。研究者使用了基于喷射子结构的重建技术，并通过简化的模型方法来描述标量的相互作用。他们还关联了获得的限制与不同模型，如I型和II型两希格斯双胞模型(2HDM)以及非最小超对称标准模型(NMSSM)中的耦合。尽管对于解释银河中心伽马射线过量的CP奇数标量，LHC实验能够施加严格的限制，但对NMSSM中轻CP奇量标量的直接探测仍面临挑战。" 文章详细阐述了CP奇数标量在标准模型扩展中的重要角色，特别是近期它们在解释银河中心的弥漫性伽马射线过量方面的重要性。由于这类标量不直接耦合于规范玻色子，因此很难直接检测，目前的实验约束主要来自LEP的直接搜索和LHC的初步运行。研究者采用了双顶夸克关联生产的策略，这是一种在高能粒子碰撞中寻找新物理信号的有效方法。双顶生产提供了探测这些难以捉摸粒子的机会，因为它们可能在该过程中通过非标准相互作用参与。论文中，研究团队利用了喷射子结构分析技术，这是一种在高能粒子碰撞事件中重构基本粒子轨迹的方法，尤其适用于寻找与标准模型预测不同的信号。他们采用简化的模型框架来参数化CP奇数标量的性质，这允许他们将实验观测到的限制与理论模型中的耦合进行比较。通过这种方式，他们能够评估LHC在不同质量范围内的探测能力，并将这些结果与I型和II型2HDM以及NMSSM的理论预测相对比。结果表明，LHC在与双顶夸克相关的生产中对那些可能解释银河中心伽马射线过量的CP奇数标量具有高度敏感性，能够提供严格的限制条件。然而，尽管如此，直接探测NMSSM中的轻量CP奇数标量仍然是一个挑战，暗示着需要更高级别的分析或者更高能量的对撞机实验来实现这一目标。这项研究强调了在粒子物理学中寻找新物理现象的重要性，特别是那些可能解释宇宙中未解之谜，如暗物质和伽马射线过量的粒子。通过持续改进探测技术和理论模型，科学家们希望能够揭示出标准模型之外的更深层次的物理定律。

Eur. Phys. J. C (2015) 75 :498 Page 3 of 15 498

[GeV]

A/h

20 40 60 80 100 120

[pb]σ

−1

= 14 TeVs

htt

A, gtt

(a)

[GeV]

A/h

0 50 100 150 200 250 300 350 400 450 500

a.u.

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

= 14 TeVs

= 20 GeV

= 100 GeV

= 20 GeV

= 100 GeV

(b)

Fig. 1 a Comparison of the leading-order cross section for t

tA(solid

line)andt

th (dashed line)in pp collisions at

√

s = 14 TeV as a func-

tion of Higgs boson mass. In both cases a value of g

= 1 is assumed.

b Comparison of the Higgs boson p

between t

tA(black)andt

th (red)

for two different values of the Higgs boson mass, 20 GeV (solid)and

100 GeV (dashed)

section at a mass of 20 GeV, and only about a factor of 2 larger

at a mass of 120 GeV [44]. This difference results from the

presence of the extra γ

factor in the interaction between

a CP-odd Higgs boson and the top quark, compared to the

case of a CP-even Higgs boson. Another consequence of

the different interaction is that a CP-odd Higgs boson has a

substantially harder p

spectrum compared to the CP-even

case, particularly at low mass, as illustrated in Fig. 1b. This

is a key feature exploited in this analysis, as discussed in

Sect. 4.

A large sample of t

t+jets background events is gener-

ated including tree-level diagrams with up to two addi-

tional partons in the 5F scheme (i.e. including b- and c-

quarks). To avoid double-counting of partonic conﬁgura-

tions generated by both the matrix-element calculation and

the parton shower, a parton–jet matching scheme (“MLM

matching”) [45] is employed. The sample is normalised to

a cross section of 990 pb obtained using Top++ v2.0 [46]

at next-to-next-to-leading order (NNLO) in QCD, including

resummation of next-to-next-to-leading logarithmic (NNLL)

soft gluon terms [47–51], and using the MSTW 2008

NNLO [52,53]PDFset.Thet

t+jets sample is generated

inclusively, but events are categorised depending on the

ﬂavour content of additional particle jets in the event (i.e.

jets not originating from the decay of the t

t system). Par-

ticle jets are reconstructed with the anti-k

[54–56] algo-

rithm with a radius parameter R = 0.4 and are required

to have p

> 15 GeV and |η| < 2.5. Events where at least

one such particle jet is matched within R < 0.4toab-

hadron with p

> 5 GeV not originating from a top quark

decay are generically labelled as t

t +≥1b events. Similarly,

events where at least one such particle jet is matched within

R < 0.4toac-hadron with p

> 5 GeV not originating

from a W boson decay, and that are not labelled already as

t +≥1b, are labelled as t

t +≥1c events. Events labelled as

either t

t +≥1b or t

t+≥1c are generically referred to below

as t

t+HF events, where HF stands for “heavy ﬂavour”. We

do not apply dedicated corrections to the normalisation of

t+HF events, since Run 1 searches at the LHC [57]showed

that the LO prediction from Madgraph5 using the same set-

tings as us is consistent with data within ∼20 %, and a larger

systematic uncertainty will be assumed in this study. As in

Ref. [57], a ﬁner categorisation of t

t+HF events is consid-

ered for the purpose of assigning systematic uncertainties

associated with the modelling of heavy-ﬂavour production

in different topologies. In this way, a distinction is made

between events with only one extra heavy-ﬂavour jet satisfy-

ing the above cuts (referred to as t

t +b or t

t +c), events with

two extra heavy-ﬂavour jets (referred to as t

t +b

b or t

t +c ¯c)

and events with one extra heavy-ﬂavour jet containing two b-

or c

-hadrons (referred to as t

t + B or t

t +C). The remaining

events are labelled as t

t+ light-jet events, including those

with no additional jets.

Additional background samples corresponding to t

tW,

tZand t

production, where h

is the SM Higgs boson,

are also produced. The t

tW sample is generated requiring at

least one W boson in the event to decay leptonically, and is

normalised to the corresponding LO cross section, 0.404pb,

times a k-factor of 1.4 [58]. The t

tZ sample is generated

requiring Z → q ¯q decays and is normalised to the cor-

responding LO cross section, 0.353 pb, times a k-factor of

1.3 [58]. Finally, the t

sample is generated assuming

= 125 GeV and requiring h → b

b decays. It is nor-

malised to the NLO cross section [59–61], 0.611 pb, times

the h

→ b

b branching ratio of 57.7 % [62–65], collected

in Ref. [66]. In these samples Z → q ¯q and h

→ b

decays are performed by Madgraph5 and top quarks and

W bosons are decayed by Pythia.

123

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