ATLAS探测器揭示13 TeV下矢量玻色子融合中不可见希格斯衰变的最新限界

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本文档探讨了使用ATLAS探测器在大型强子对撞机(LHC)的13 TeV质心能量下，通过矢量玻色子融合(VBF，Vector Boson Fusion)过程对不可见希格斯玻色子(Higgs boson)的衰变进行实验搜索。希格斯玻色子是粒子物理学中的关键粒子，特别是在标准模型(Standard Model)中，它与其它基本粒子如W和Z玻色子相互作用。研究中，实验特征表现为一对高能喷注，它们的不变质量在约1 TeV和100 GeV范围内，同时伴有缺失的横向动量，这是由于希格斯玻色子衰变为无法直接探测的粒子所导致的。数据分析基于ATLAS实验收集的36.1 fb$^{-1}$ pp(质子-质子)碰撞数据。在信号区，观察到了2252个事件，这一数值与背景估计相符合。对于一个假设的125 GeV标量粒子，标准模型预测的希格斯玻色子衰变成不可见粒子的分支比(BR，Branching Ratio)上限在95%置信水平下被限制为0.37，这个结果低于预期的0.28，这意味着这种衰变过程可能并未在实验中得到明显的观测。文章进一步将这一发现纳入了希格斯门户模型(Higgs Portal Model)，这是一个理论框架，其中暗物质粒子(WIMP，Weakly Interacting Massive Particles)可能通过与希格斯玻色子的耦合来解释。在该模型中，限制可转化为对WIMP-核子散射截面的边界。此外，研究还考虑了额外标量玻色子(Scalar Bosons)的无形衰变，这些玻色子的质量不超过3 TeV，其截面与分支比的上限范围在0.3至1.7 pb之间。这项工作不仅验证了标准模型的预测，也为暗物质粒子的研究提供了重要线索，尤其是当考虑到可能存在的新物理现象时。它强调了对希格斯玻色子衰变性质的深入探索在粒子物理学中的重要意义，并为未来实验设计和理论模型的优化提供了有价值的数据参考。

The ATLAS Collaboration / Physics Letters B 793 (2019) 499–519 503

Fig. 2. Distribution of event yields in the Z (top) and W (bottom) control regions.

The postﬁt normalizations for m

(left) and E

miss

(right) are summed over the sub-

samples.

The E

miss

distributions start at 180 GeV as indicated. The observed data N

(dots)

are superimposed on the sum of the backgrounds B (stacked histogram with

shaded systematic uncertainty bands). The breakdown of the B is given in the lower

left box in each panel. The bottom panels show the ratios of N to B with the sys-

tematic

uncertainty band shown on the line at 1. The “other,” as listed in Table 1,

contribute a few events at low values of m

and E

miss

, and are omitted. The last

bin in each plot contains the overﬂow.

The postﬁt value of ν

fake

(the product ρ

fake

·ν

fake

) is the abso-

lute

number of e fake events in the W

high

low

) subsamples.

Since there is a ν

fake

parameter for each bin i, the m

shape is

also predicted. Apart from determining the ρ

fake

value, which is

ﬁxed in the ﬁt, F

eν

is not part of the ﬁt model. We note that the

high

-W

low

samples are split by charge, because W

production

is not symmetric in pp collisions. However, the same ν

fake

param-

eter

is used for both charges because the e fakes are expected to

be symmetric in charge since they originate mostly from multijet

events.

The remaining processes—top quarks, dibosons, multijets—

contribute

negligibly to the SR (called “other” in Table 1). The

ﬁrst two are estimated with MC using nominal cross sections. The

multijet contribution is very small, but it is a diﬃcult process to

estimate. It is a potentially dangerous background because those

events that pass the E

miss

selection are mostly due to instrumen-

tal

effects.

The billionfold-or-more reduction of multijets after the event

selection makes it impractical to simulate, so a data-driven method

based on a rebalance-and-smear strategy [72]is used. The assump-

tion

is that the E

miss

is due to jet mismeasurement in the detector

response to jets and neutrinos from heavy-ﬂavor decays [73,74].

Using the jet-triggered sample, the jet momenta are rebalanced by

a kinematic ﬁt, within their experimental uncertainties, to obtain

the balanced value of the jets’ p

. The rebalanced jets are smeared

according to jet response templates, which are obtained from MC

and validated with dijet data. The rebalance-and-smear method

predicts both the shape of the E

miss

distribution and the absolute

normalization. The procedure is veriﬁed in a φ

-sideband vali-

dation

region (VR) with 95% purity of QCD multijet events. This

VR is deﬁned by 1.8 < | φ

|< 2.7 and the loosening of the other

requirements (| η

|> 3, m

> 0.6 TeV, and allow a third leading

jet with 25 < p

< 50 GeV, but no other jets with p

> 25 GeV). The

Fig. 3. Distribution of event yields in the multijet validation region for m

(left) and

miss

(right). The m

plot shows the 100< E

miss

< 120 GeV subset of the right plot

as indicated by the arrow. The N observed data (dots) are superimposed on the

sum of the B backgrounds (stacked histogram). The systematic uncertainty band

applies only to the multijet component. The statistical uncertainties are relatively

large because of the weighting of the trigger samples with large prescale values.

See the caption of Fig. 2 for other plotting details.

comparison of the predictions and the data in the VR shows good

agreement (Fig. 3). The multijet component is obtained using the

rebalance-and-smear method with the associated systematic un-

certainty

bands, while the non-multijet components are obtained

using MC.

6. Uncertainties

Experimental and theoretical sources of uncertainties as well as

the correlations between the various sources are described. The re-

sulting

impact of the uncertainties on the yields and on the signal

sensitivity is summarized later in Table 2.

Experimental sources of uncertainty are due mainly to the jet

energy scale and resolution [75], E

miss

soft term [76], and lep-

ton

measurements [42,43]. In order to reduce ﬂuctuations due to

limited MC sample size, the uncertainties in number of expected

events for the variations of jet energy scale and resolution for the

strong and electroweak background samples are averaged. This is

motivated by the similarities of the kinematics and the detector ef-

fects

for the two production processes for each m

bin. The uncer-

tainty

related to lepton identiﬁcation or veto has a non-negligible

(negligible) effect on α

(α

) because of the following scenarios.

The W

ν

background is signiﬁcant in the SR, which results in an

uncertainty for the cases related to the lepton veto. The Z



back-

ground

is negligible in the SR, because the selection requires there

to be no leptons.

The following experimental sources have small or negligible

impact in the ﬁnal result. The pileup distribution and luminosity

[77,78]have a relatively small impact. The trigger eﬃciency mod-

eling,

for both the lepton triggers for the CR and E

miss

triggers for

the SR, are not listed in Table 2. Their impact on the events yields

was at the 1% level and their impact on the signal sensitivity are

found to be negligible.

Theoretical sources of uncertainty are due mainly to scale

choices in ﬁxed-order matrix-element calculations. For the back-

ground

processes, QCD scales are varied for the resummation

scale (resum.), renormalization scale (renorm.), factorization scale

(fact.), and ckkw matching scale. The ﬁrst three scales in the list—

technically

called q

, μ

, respectively—are varied by a factor

of two [79,80]. For the ckkw matching scale between the matrix

element and the parton shower [60], the central value and the

considered variations are 20

+10

−5

GeV. The higher-order electroweak

corrections to the strongly produced W or Z are found to be neg-

ligible.

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