ATLAS探测器发现希格斯玻色子新衰变模式：H→aa→4b在13 TeV pp碰撞中的实验研究

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本研究论文详细探讨了在CERN大型强子对撞机（LHC）的ATLAS探测器上，对希格斯玻色子（Higgs boson）的一种独特衰变模式进行的搜索，即Higgs玻色子通过与W或Z玻色子关联产生，并进一步衰变为两个自旋为零的粒子a，最终形成H→aa→4b衰变通道。a玻色子的质量被设定在20-60 GeV范围内，它可以迅速或具有平均寿命cτa高达6毫米的平均寿命，通过b夸克衰变成四个b夸克。这项实验基于2015年和2016年LHC运行期间ATLAS探测器收集的质子-质子（pp）碰撞数据，碰撞能量为√s = 13 TeV，对应的数据集集成亮度为36.1 fb^-1。实验的目标是验证标准模型（Standard Model, SM）之外的异常Higgs玻色子衰变可能性，因为通常情况下，Higgs玻色子倾向于衰变为已知的基本粒子，而非这种新的自旋零粒子a。搜索过程中，研究者着重关注那些符合W或Z玻色子伴随产生的特征，即存在一到两个带电轻子（电子或μ子）以及多个由b夸克衰变产生的喷射。然而，分析结果显示没有发现超过标准模型背景预测的重大事件。在95%的置信水平下，对于迅速衰变的a玻色子，WH（W boson and Higgs boson）和ZH（Z boson and Higgs boson）组合产生的横截面乘以H→aa→4b的分支比上限分别为，当a玻色子质量ma在20 GeV时为3.0 pb，而在60 GeV时降低至1.3 pb。对于寿命较长的a玻色子，最严格的限制在cτa约为0.4毫米时分别为1.8 pb和0.68 pb。此研究不仅验证了当前粒子物理学理论，还对可能存在的新物理现象提供了有力的约束，特别是那些可能导致希格斯玻色子非标准衰变的潜在模型。通过这种方式，科学家们继续探索宇宙的基本结构和粒子相互作用的边界，深化我们对希格斯机制和粒子间作用的理解。

JHEP10(2018)031

The number of fake or non-prompt leptons satisfying the tight criteria can then be

calculated by inverting the matrix deﬁned by the two equations:

= N

+ N

, N

= ε

+ ε

where N

) is the number of events in data satisfying the loose (tight) lepton selection,

) is the number of events with a real prompt (fake or non-prompt) lepton in the

loose lepton sample, and ε

(ε

) is the eﬃciency for these events to fulﬁl the tight lepton

selection. By generalizing the resulting formula to extract ε

, a weight is assigned to

each event selected in the loose lepton data sample, providing a prediction for both the

yields and the kinematic distributions of the fake and non-prompt lepton background.

When applying the matrix method in the case of high jet and b-tagged jet multiplicities,

the number of events in data satisfying the loose and tight lepton selections is signiﬁcantly

reduced, leading to large ﬂuctuations in the background predictions. In order to mitigate

this problem, instead of tagging the jets by applying the b-tagging algorithm, their proba-

bilities to be b-tagged are parameterized as a function of the jet p

. This allows all events

in the sample before b-tagging is applied to be used in predicting the normalization and

shape of the background from fake or non-prompt leptons after b-tagging. The tagging

probabilities are derived using an inclusive sample of fake or non-prompt leptons and the

resulting predictions of this background estimate are in agreement with those obtained by

applying the b-tagging algorithm and have greatly reduced statistical uncertainties.

In the dilepton channel, the background contribution from fake or non-prompt leptons

is very small and is estimated from simulation and normalized to data in a control region

with two same-charge leptons.

5 Event categorization

Events satisfying the object selection are categorized into analysis regions according to the

number of leptons, jets and b-tagged jets. The regions enhanced in signal H → aa → 4b

events relative to the backgrounds are referred to as signal regions (SRs). The other regions,

referred to as control regions (CRs), are used to constrain the background predictions and

related systematic uncertainties (see section 7) through a proﬁle likelihood ﬁt to the data

(see section 8). The signal and backgrounds are derived consistently in the signal and

control regions in a combined ﬁt. The discrimination of signal from background is further

enhanced in the signal regions by using multivariate techniques, as described in section 6.

The H → aa → 4b decay chain is expected to have multiple b-tagged jets, often three

or four, satisfying the object selection. The dominant background arises from t

t events in

the single-lepton channel and Z + jets events in the dilepton channel, which can also have

diﬀerent jet and b-tagged jet multiplicities or leptons of diﬀerent ﬂavour in the case of the

dilepton channel. The regions are referred to as (n

`, n

j, n

b) indicating n

leptons, n

selected jets and n

b-tagged jets. The SRs contain at least three b-tagged jets and are (1`,

3j, 3b), (1`,4j, 3b) and (1`, 4j, 4b) for single-lepton events, and (2`, 3j, 3b), (2`, ≥4j, 3b)

and (2`, ≥4j, ≥4b) for same-ﬂavour dilepton events. The CRs are (1`, 3j, 2b), (1`, 4j, 2b),

(1`, ≥5j, 3b) and (1`, ≥5j, ≥4b) for single-lepton events, (2`, ≥3j, 2b) for same-ﬂavour

– 8 –

JHEP10(2018)031

SR SR SR

W + jets

t + light

t + b

Z + jets

t + c ¯c/b

t + b

(≥5j, ≥4b)(4j, 4b)(≥5j, 3b)(4j, 3b)(3j, 3b)(≥5j, 2b)(4j, 2b)(3j, 2b)

Single-lepton

Dilepton

same-ﬂavor

Dilepton

diﬀerent-ﬂavor

Figure 2. Deﬁnition of the signal and control regions (SR and CR, respectively) in the single-

lepton and dilepton channels. The main background component probed with the CR is indicated.

The vertical axis shows the lepton selection, while the horizontal axis shows the jet and b-tagged

jet multiplicities.

dilepton events, and (2`, ≥3j, 3b) and (2`, ≥4j, ≥4b) for diﬀerent-ﬂavour dilepton events.

The signal and control regions are summarized in ﬁgure 2, indicating the main background

sources probed in the CRs. Figure 3 summarizes the background composition in the signal

and control regions.

In the single-lepton signal regions, background t

t events can only satisfy the selection

criteria if accompanied by additional b-tagged jets. The t

t + light background is dominant

in the sample of events with exactly two or three b-tagged jets. The background processes

t+c¯c and t

t+b

b become more important as the jet and b-tagged-jet multiplicities increase.

In particular, the t

t + b

b background dominates for events with ≥5 jets and ≥4 b-tagged

jets. In the case of (1`, 3j, 3b) or (1`, 4j, 3b), the main sources of t

t background are events

with jets mistagged as b-tagged jets, particularly from W → cs decays, where the c-jet is

misidentiﬁed, and from W → τν, where the τ-lepton decays hadronically and is likewise

mistagged. In the case of (1`, 4j, 4b), the t

t background includes more events with genuine

b-quarks from gluon splitting to b

b pairs.

In the dilepton channel, the background is mainly composed of t

t and Z + jets events.

In the case of the t

t background, most events contain two prompt leptons from the leptonic

decays of the two W bosons, and two b-jets from the top-quark decays. Additional jets arise

from gluon splitting into b

b and c¯c and from jets from initial-state radiation and pile-up.

In each of these cases, the third and fourth b-tags in the event are from additional b-tagged

jets, or from the mistag of additional c- or light-jets. In the samples with exactly three

or four jets and exactly three b-tagged jets, the contributions of each of these sources is

similar. In the case of the sample with exactly four jets and exactly four b-tagged jets, the

contribution from events with real additional b-tagged jets, such as from gluon splitting

into b

b, dominates.

In the case of the Z + jets background, the dominant contribution is from Z bosons

produced in association with multiple heavy-ﬂavour jets, for example from gluon splitting

– 9 –

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