未来线性对撞机中I型2HDM中性希格斯玻色子搜寻研究

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"这篇研究论文探讨了在未来的线性对撞机中，特别是在I型两-Higgs双态模型（Type-I 2HDM）框架下，寻找中性希格斯玻色子（包括标量H和伪标量A）的可能性。通过对撞机在500和1000 GeV能量运行时，主要关注的信号过程是电子正电子对撞产生AH对，随后AH衰变为ZHH，最终Z衰变为双喷流jj，两个Higgs玻色子衰变为b夸克对bbar。文章中，作者分析了几种不同的质量假设场景，并强调了信号过程主要受A→ZH（在高$\tan\beta$和足够的Higgs玻色子质量分裂时）以及H→bbarb在Type-I模型中大衰变率的驱动。事件生成后，使用ILC上的SiD探测器模拟来研究探测器响应。此外，考虑了初始态辐射（ISR）和束辐射（Beamstrahlung）对结果的影响。" 这篇研究详细分析了在标准模型类似情景下I型2HDM的特性，特别是在高能量线性对撞机环境下的表现。2HDM是一种扩展标准模型的理论，它包括了两个Higgs双态，允许存在多个希格斯玻色子，增加了粒子物理研究的复杂性和可能性。在I型2HDM中，只有上色粒子与额外的希格斯场耦合，这一特性影响了希格斯玻色子的生产和衰变模式。研究中，通过模拟对撞产生的希格斯粒子，作者评估了在不同质量和能量条件下的可观察性。对于信号过程$e^-e^+\rightarrow AH\rightarrow ZHH\rightarrow jjb\bar{b}b\bar{b}$，其中jj表示由Z玻色子衰变产生的双喷流，而$b\bar{b}$对则是Higgs玻色子的b夸克衰变产物。由于在高$\tan\beta$值下，A至ZH的衰变更为显著，同时H至b夸克对的衰变也占有重要地位，这使得该过程成为研究的关键。通过ILC的SiD探测器进行的模拟，可以更准确地预测实验中可能观测到的信号，同时也考虑了实际实验中可能出现的ISR和Beamstrahlung效应，这些因素会对探测结果产生影响。这项工作为未来线性对撞机实验提供了重要的理论基础，有助于更好地理解希格斯粒子的性质和2HDM模型的潜在验证。

Eur. Phys. J. C (2019) 79 :419 Page 3 of 14 419

Table 1 Higgs coupling to up-type quarks, down-type quarks and lep-

tons in types allowed by the Z

symmetry. The superscript i is a gener-

ation index



Type I 



Type II 



Type X 



Type Y 



Table 2 ρ

factors corresponding to different ﬂavor-conserving

2HDM types

III X Y

cot β −tan β cot β −tan β

cot β cot β cot β cot β



cot β −tan β −tan β cot β

+iv

−1



− ρ

dγ

d + ρ

¯uγ

−ρ



γ





A, (2)

where ρ

factors are provided in Table 2.

As seen in Table 2, coupling factors of different types dif-

fer dramatically leading to considerable differences in phe-

nomenological features of different types [18]. According to

Table 2, factors of the type-I acquire the same values (cot β)

resulting in an interesting environment in both low and high

tan β regions. Searching for the Higgs bosons in the type-I

in this study is highly motivated by the large enhancement

the H → b

b channel receives at low tan β values.

2 Signal process

The process chain e

−

→ AH → ZHH → jjb

b is

assumed as the signal process in this study and the Type- I

2HDM is assumed as the theoretical framework to beneﬁt

from possible enhancements in low tan β regime. jj is a

pair of jets resulting from the Z boson hadronic decay Z →

jj. After the pseudoscalar Higgs boson undergoes the decay

channel A → ZH, both of the scalar CP-even Higgs bosons

experience the decay mode H → b

b which receives a large

enhancement due to the cot β factor in the Higgs–fermion

coupling factors as shown in Table 2 and thus, is dominant

in low tan β regime as long as the scalar Higgs mass m

below the threshold of the on-shell top quark pair production.

The initial e

−

collision is assumed to occur at the center-

of-mass energies of 500 and 1000 GeV at a linear collider.

2.1 Benchmark points

Several benchmark points with different mass hypotheses are

assumed in the parameter space of the 2HDM as shown in

Table 3. Benchmark points corresponding to the two assumed

center-of-mass energies are simulated and analyzed sepa-

rately. The scalar Higgs boson mass m

is assumed to vary

in range 150–250 GeV and the mass splitting between the

H and A Higgs bosons is assumed to range from 50 to 100

GeV. tan β is set to 10 in all scenarios resulting in a consider-

able enhancement in the assumed scalar Higgs boson decay

channel.

2.2 Theoretical constraints

To ensure that the considered scenarios are consistent with

theoretical constraints, potential stability [25], perturbativity

and unitarity [26–29] of each scenario is checked with the

use of 2HDMC 1.7.0 [30,31] and the allowed range for

parameter is provided in Table 3.

2.3 Experimental constraints

The assumed masses of the Higgs bosons are consistent

with results of 86 analyses as checked by HiggsBounds

4.3.1 [32] and HiggsSignals 1.3.0 [33].

The experimental constraint [34,35], based on the mea-

surement performed at LEP [36], limits the deviation of

the parameter ρ = m

cos θ

)

−2

from its SM value.

To respect this constraint, the Higgs bosons A and H

are

assumed to have the same masses in all of the benchmark

points. This is because of the demonstration provided in

[37,38] that concludes the deviation of the ρ parameter from

its SM value is negligible if any of the conditions

= m

, m

= m

, (3)

is met.

As reported in [39], the ATLAS collaboration has excluded

mass regions m

= 310−410, 335−400, 350–400 GeV for

= 150, 200, 250 GeV respectively at tan β = 10 in the

type-I. Both CMS and ATLAS also put the limit m

> 350

on the pseudoscalar Higgs mass for tan β<5 in the type-

I[40,41]. Another study by ATLAS collaboration excludes

the mass range m

= 170 − 360 GeV for tan β<1.5in

this type [42]. It is obvious from Table 3 that the assumed

scenarios are consistent with the current constraints resulting

from direct LHC observations. In Fig. 1 the selected bench-

mark points are shown in the (m

) plane together with

the current LHC exclusion contour at tan β = 10 [39]. What

is important here is that the selected points are chosen in a

way to cover regions where the mass difference between the

scalar and pseudo-scalar Higgs bosons is below the Z boson

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