大型强子对撞机双光子过量实验探索与新粒子研究

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"这篇论文是关于大型强子对撞机(LHC)实验中双光子过量的现象引发的理论探讨，作者们分析了这一异常现象可能的解释，并提出了实验上探测新粒子和新物理效应的可能性。" 正文: 大型强子对撞机(LHC)在运行期间观测到的双光子过量引起了科学家们的关注，因为这可能是新物理学的迹象。论文中提到，如果这种过量可以归因于一个质量约为750 GeV的新粒子X s，那么为了保持物理的自洽性，需要存在更多的新粒子来解释这个信号。这些新粒子可能与自然性（Naturalness）理论有关，意味着它们可能参与顶级伙伴（top partner）的衰变过程。顶级伙伴是一种假设中的粒子，它在超对称理论（SUSY）中扮演着重要角色，能帮助解决标准模型中的自然性问题。在这种情况下，X s 可能在顶级伙伴衰变过程中产生，如't′粒子衰变为t夸克和X s，而X s则主要衰变为两个 gluons (gg)。这种衰变模式可能导致在13 TeV的SUSY搜索数据中，观察到低缺失能量（Missing Transverse Energy, MET）和/或多轻子的多喷事件，这些事件可以通过重新解释现有数据来探测。另外，如果X s 是一种奇特（strange）约束状态，其衰变可能会伴随异常数量的软质子（soft tracks）或软射流（soft jets）。这种特性可以为实验提供识别新粒子的独特标志。论文还讨论了其他可能的共振，如双核子、光子+射流以及双射流等，这些可能在750 GeV附近或之上出现，甚至可能观察到隐藏胶球（hidden glueball）的信号。同时，论文提出了一种假设，即如果过量的双光子实际上是长寿命粒子衰变成光子对或电子对的结果，那么可能会在X s 或125 GeV希格斯粒子的衰变中看到独特的光子转换模式。此外，可能存在这样的情况：硬的“光子”与狭窄的、仅由前向介子探测器（HCAL）隔离的“喷流”相互作用，经过校正喷射流的电磁起源能量后，会在750 GeV处形成峰值。这篇论文深入探讨了LHC实验中双光子过量的可能成因，提出了一系列实验策略和方法，以期在现有的或即将进行的实验中寻找这些新粒子的证据。这些理论研究对于理解基本粒子物理学的前沿问题，尤其是超对称性和自然性问题，具有重要的意义。

JHEP06(2016)082

where the summation runs over all types of fermions, y

is the Yukawa coupling to η,

is the number of ﬂavors, t

is the Dynkin index of the colored fermions under SU(3)

(normalized to 1/2 for fundamental representation) and Q

is the electric charge of the

fermion. The branching ratio of η to two photons is given by

Br(η → γγ) ≈









. (2.4)

For N

copies of an SU(2) singlet quark t

of charge 2/3, plus its vector-like partner

Br(η → γγ) ≈ 0.4%. For N

copies of SU(2) doublets Q

= (t

, b

) of charge (2/3, −1/3)

and their vector-like partners, Br(η → γγ) ≈ 0.15%. These two choices will be our bench-

mark models in the following discussions. We will also assume that heavy quarks are all

degenerate in mass for simplicity.

The production cross section of η, when we add the vector-like color triplets with a

common y

and m

, is approximately

pp→η

≈ 400 fb

eﬀ

, (2.5)

where v = 246 GeV is the Higgs vev.

In models with singlet quarks t

, N

eﬀ

= N

, while

in models with doublets Q

, N

eﬀ

= 2N

In the case of t

or Q

, this can be translated into a bound on the combination of the

parameters,

(

eﬀ

(245 GeV)/(1.4 → 1.9) t

eﬀ

(245 GeV)/(2.2 → 3.2) (t

, b

)

, (2.6)

where the lower (higher) number in the denominator corresponds to 3 (6) fb for σ(pp → X

In cases with t

decaying to t + η or to t(b)+ SM gauge bosons, the new quarks have to

be heavier than ∼ 1 TeV, as we will discuss below, and so y

eﬀ

& 6. To keep the theory

perturbative, (i.e. the one-loop correction to y

smaller than y

we need y

. 16π

In general one can obtain any values of κ

, κ

desired, by including multiple particles

with diﬀerent masses, hypercharges, etc., and including colorless particles. Therefore one

must view the κ

, κ

as free parameters, only requiring that they not be so large as to

violate perturbativity in the eﬀective theory.

Since κ

is induced at one loop, this type of model tends to give η a width below 1 GeV,

and does not immediately explain how a resonance could have a width of tens of GeV, as

ATLAS’s excess seems tentatively to suggest. However, the model is easily modiﬁed. For

instance, a renormalizable interaction of η with invisible particles could greatly increase

The prefactor of 400 fb is the cross-section for a 750 GeV Standard Model Higgs in the limit that

→ ∞. We computed this in Madgraph with the Higgs eﬀective theory (heft) model ﬁle [10] and

assigned a K factor ∼ 2 (for 14 TeV, the K factor for ggh of a 750 GeV Higgs is around 1.8 [11]).

Note added. In a previous version we used a slightly stronger criterion; ﬁgure 1 now has a slightly

weaker perturbativity constraint, but of course this is not a sharp boundary in any case.

– 4 –

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