探索TeV尺度下的夸克合作伙伴异国情调

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"顶级合作伙伴的常见异国情调" 本文讨论了顶级夸克的Dirac-fermion合作伙伴在TeV范围内的搜索问题。这些夸克的搜索主要集中在将其衰减为电弱规格玻色子和希格斯以及标准模型夸克。通过分析几类基础模型，我们表明异乎寻常的衰减是常态，并且通常以显着的速率出现。知识点1：Dirac-fermion合作伙伴 Dirac-fermion合作伙伴是指夸克的合作伙伴粒子，它们是根据标准模型的扩展理论而提出的。这些夸克的存在可以解决层次结构问题，并且它们的质量通常在TeV范围内。知识点2：夸克的衰减模式夸克的衰减模式是指夸克衰减为其他粒子的过程。常见的衰减模式有两种，一种是衰减为电弱规格玻色子和希格斯，另一种是衰减为标准模型夸克。知识点3：复合希格斯模型复合希格斯模型是一种基于标准模型的扩展理论，它可以解决层次结构问题。该模型将希格斯粒子看作是一种复合粒子，而不是基本粒子。知识点4：奇异衰减通道奇异衰减通道是指夸克衰减为其他粒子的过程中，除了常见的衰减模式外的其他衰减模式。这些奇异衰减通道可以修改搜索策略，并影响搜索范围。知识点5：简化的方案简化的方案是指基于复合希格斯模型的简化模型，该模型可以解释夸克的衰减模式。该方案可以简化搜索策略，并且可以更好地理解夸克的衰减过程。知识点6：有效的拉格朗日值和基准有效的拉格朗日值和基准是指基于复合希格斯模型的数学公式，该公式可以描述夸克的衰减过程。这些公式可以帮助我们更好地理解夸克的衰减模式，并且可以指导搜索策略。知识点7：搜索策略搜索策略是指寻找夸克的方法。常见的搜索策略有两种，一种是寻找夸克衰减为电弱规格玻色子和希格斯，另一种是寻找夸克衰减为标准模型夸克。知识点8：基础模型基础模型是指标准模型的扩展理论，它可以解决层次结构问题。这些模型可以描述夸克的衰减模式，并且可以指导搜索策略。知识点9：衰减率衰减率是指夸克衰减的速率。常见的衰减率有两种，一种是夸克衰减为电弱规格玻色子和希格斯，另一种是夸克衰减为标准模型夸克。知识点10：轻量级新标量轻量级新标 scalar是指夸克衰减过程中出现的新的标量粒子。这些粒子可以影响搜索策略，并且可以指导夸克的衰减模式。本文讨论了顶级夸克的Dirac-fermion合作伙伴的搜索问题，并且提出了基于复合希格斯模型的简化方案。该方案可以指导搜索策略，并且可以更好地理解夸克的衰减模式。

JHEP06(2018)065

interactions of the pseudo-scalar a with SM particles can be parameterised as

(∂

a)(∂

a) −

−

fγ

f +

16π

µν

aµν

8π

µν

−,µν

16π

µν

16π

µν

egc

Zγ

8π

µν

. (2.4)

Note that we have written the couplings to the gauge bosons in the mass eigenstate basis

because the mass of a can well be around or below the electroweak (EW) scale, and we

deﬁne V

µν

= ∂

−∂

and

µν

= 

µνρσ

ρσ

. The couplings are, in general, independent,

but relations among them may exist depending on the origin of the pseudo-scalar a. In

underlying models where a is associated to a U(1) symmetry [30, 58], the couplings to

gauge bosons can be determined in terms of two parameters: one coupling, K

, to the

SU(2) bosons and one, K

, to hypercharge. Thus, we have the relations

= K

+ K

, K

= K

+ K

, K

Zγ

= K

− K

, (2.5)

where t

= tan(θ

) is the tangent of the Weinberg angle. The parameters K

W,B

are fully

determined in terms of the underlying theory. On the other hand, the couplings to the

fermions depend on the origin of the mass terms, i.e. the choice of top partners and the

values of the mixing couplings (more details will be provided in section 3). For a given

model and choice of the fermion couplings, the only other remaining parameters are the

decay constant f

, which controls the overall width and coupling strength, and the mass

, on which the BRs depend. The latter can be potentially very small, of the order of few

GeV [30]. As an example, the couplings resulting from the SU(4)/Sp(4) model discussed

in section 3.2 are

= −1.6 , K

= 1.9 , K

= −2.3 , C

f6=t

= 1.9 , C

(

1.46 for Bm1

2.33 for Bm2

(2.6)

where from the UV model, we also ﬁx f

= 2.8 TeV, and only leave the mass m

as a free

parameter. They correspond to the underlying model M8, discussed in ref. [30]. The BRs

of a for this parameter choice (for Bm1) are shown in ﬁgure 2 (left) as a function of m

The BRs do not depend on the decay constant f

The pseudo-scalar a can be directly produced at the LHC in gluon fusion.

As the BRs

of a are determined as given in ﬁgure 2, bounds from ATLAS and CMS resonance searches

in the channels jj [61–63], t

t [64–66], b

b [67], τ

−

[68–71], µ

−

[72], W

−

[73–79],

ZZ [73, 74, 80–85], Zγ [86–88] and γγ [89–92] can be translated into bounds on the decay

constant f

, which controls the direct production cross section [30, 58]. The right panel in

ﬁgure 2 shows the resulting bounds on f

for the benchmarks Bm1 and Bm2 characterised

We give the eﬀective Lagrangian up to dimension 5 operators. Additional interactions can be generated

at higher order. See ref. [58] for couplings haa and hZa.

Top-associated production (t

ta) is also possible — in particular for light a [59]. Furthermore, light a

can result from h → aa or h → Za decays [60]. For the benchmark models considered in section 3, these

processes only yield weak bounds, however, as shown in ref. [58].

– 5 –

JHEP06(2018)065

Figure 2. Left: branching ratios of a as a function of the mass m

in the benchmark model Bm1

characterised by the parameters in eq. (2.6). Right: experimental lower bounds on f

from ATLAS

and CMS resonance searches on the benchmark models Bm1 (blue) and Bm2 (black) as compared

to the underlying theory value (red).

by the couplings given in eq. (2.6).

The red line corresponds to the estimated value

of f

from the underlying model (see section 3.2). As it can be seen, this value is not

excluded by current searches for almost all masses of m

(see refs.[30, 58] for bounds on f

in other models).

Indirect production of a, via the decay T → t a or B → b a, is therefore an independent

test of the models. A dedicated collider study of the signatures is beyond the scope of this

article, so we brieﬂy comment on the new allowed ﬁnal states in a way to relate them to the

standard top partner searches. While the BRs shown in ﬁgure 2 apply to a speciﬁc model,

the plot shows some typical behaviours which are generic for most models. We can thus

infer a general trend for the dominant ﬁnal states, depending on the mass range of the a:

• For m

> 2m

, the channel T → t a → tt

t has the dominant BR. This is a generic

expectation as long as a sizeable coupling to the tops is present. The resulting “tri-

top” and “esa-top” ﬁnal states are not searched for, but they will be eﬃciently covered

by existing 4-top searches, as shown in ref. [93].

• Below the t

t threshold, the dominant a decays are into hadrons (gg) or b

b. Thus,

T → t a decays provide similar ﬁnal states as standard channels like T → t V

had

and

T → t h → tb

b. However the mass reconstructions applied in current fully hadronic

decay searches typically focus on invariant mass reconstructions in the W /Z and

Higgs mass ranges. Thus the signal generated by T → ta is potentially being rejected

unless its mass is close to the one of the standard bosons. Note that a simple recast

was possible for some Run-I searches where the Higgs mass reconstruction was not

imposed [40].

• The ﬁnal state T → t a → tτ

−

could also arise from T → t h/t Z but is to our

knowledge currently not covered by any top partner searches. For VLQ masses much

The minor diﬀerence in bounds between Bm1 and Bm2 arises mainly from C

-dependent one-loop

contributions to a production in gluon fusion, and to a lesser extent from C

-dependent one-loop corrections

to the decay partial width of a into gg, Zγ, and γγ. For expressions of the BRs at one-loop, see ref. [30].

– 6 –

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