13 TeV质子-质子碰撞中重共振到双希格斯或希格斯-W/Z的搜索

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"在13 TeV的质子-质子碰撞中寻找重共振衰变成两个希格斯玻色子或希格斯玻色子和W或Z玻色子" 这篇研究论文详细介绍了CMS（Compact Muon Solenoid）合作组进行的一项实验，目的是在13 TeV质心能量的质子-质子碰撞中探测可能存在的高能重共振现象。这些共振可能衰变成两个希格斯玻色子（HH）或者一个希格斯玻色子（H）和一个W或Z玻色子（V）。这种寻找重共振的实验对于理解基本粒子物理的潜在新现象至关重要，因为它可能揭示超出标准模型的新物理理论。论文考虑的衰变通道包括HH→bbτ+τ− 和 VH→qqτ+τ−，其中H代表希格斯玻色子，V代表W或Z玻色子，τ是τ轻子，b是底夸克，q是轻夸克。这些衰变过程涉及到复杂的粒子相互作用，特别是希格斯玻色子的产生和衰变，以及W和Z玻色子的产生。通过对碰撞数据的分析，研究团队可以探索这些粒子间的关联，从而可能发现新的粒子或物理现象。数据分析基于CMS合作组在13 TeV质心能量下收集的大量质子-质子碰撞数据，总集成光度为35.9 fb-1。为了从背景噪声中提取信号，研究者采用了先进的子结构技术，这种方法能够区分由矢量玻色子衰变到夸克、希格斯玻色子衰变到底部夸克以及由夸克或胶子诱导的喷注所产生的不同类型的粒子簇。此外，还应用了针对τ轻子对高度增强的事件的专门优化重建技术，以提高信号识别的敏感性。尽管进行了详尽的分析，但最终观察到的数据与标准模型预测的结果相符，没有发现显著的超出预期的信号。这意味着在0.9到4 TeV的质量范围内，未观测到任何明显的新共振。这一结果虽然没有直接发现新的粒子，但对排除某些理论模型的假设起到了重要作用，同时为未来的高能物理实验提供了更明确的指导。这项工作是在JHEP（Journal of High Energy Physics）上发表的开放获取文章，其发表流程包括2018年8月提交，2018年12月接受，最后于2019年1月发布。这项研究的严谨性和深度反映了高能物理领域的研究标准，对于理解宇宙的基本组成部分和力以及可能存在的新物理现象具有深远意义。

JHEP01(2019)051

son. To discriminate against backgrounds, the jets are required to have small values of τ

and events are divided into categories of high-purity (HP) if τ

< 0.4, and low-purity (LP)

if 0.4 < τ

< 0.75. A jet is V tagged if it fulﬁlls the soft-drop jet mass and τ

requirements.

The normalization scale factors 0.99 ±0.06 for the HP category and 0.96 ±0.11 for the LP

category [56] are applied to simulated events with genuine hadronic boson decays. Higgs

boson jet candidates are classiﬁed according to the number of subjets (1 or 2) that pass the

b tagging selection. Subjet b tagging is not used for jets compatible with W or Z candidates

and no N-subjettiness requirement is applied to the Higgs boson candidate jet. If neither

the N-subjettiness nor the b tagging requirements are satisﬁed, the event is discarded.

Events are divided into categories depending on the number of identiﬁed τ

candidates

(1 or 2), and on the classiﬁcation of the large jet cone, i.e. either HP or LP in τ

, or either

1 or 2 b-tagged subjets.

Since the undetected neutrinos carry a signiﬁcant fraction of the momentum in the

ττ system, signal events are expected to have a large ~p

miss

, thereby justifying the use of

triggers that require large p

miss

or H

miss

. A stringent oﬄine requirement of greater than

200 GeV is applied to the reconstructed p

miss

, to ensure a stable trigger eﬃciency and to

suppress the background contribution from multijet events. Events with top quark pairs or

single top quarks are suppressed by removing events in which any AK4 jet not overlapping

with the AK8 jet is b-tagged.

Several selection requirements are applied to remove SM backgrounds, such as meson

and baryon resonances, Z+jets, W+jets and tt and single top quark production. The

angular distance ∆R

τ τ

should be smaller than 1.5, in order to reject W+jets events in

which a jet misidentiﬁed as a τ lepton is typically spatially well-separated from the genuine

lepton. To further increase the signal purity, the di-tau mass, as estimated from the SVfit

procedure [64–66], should be between 50 and 150 GeV. The SVfit algorithm, based on a

likelihood approach, estimates the di-τ system mass using the measured momenta of the

visible decay products of both τ leptons, the reconstructed ~p

miss

, and the ~p

miss

resolution.

Finally, the resonance candidate mass m

, deﬁned as the invariant mass of the H → ττ

candidate and the hadronically decaying boson jet, is required to be larger than 750 GeV

in order to ensure full trigger and reconstruction eﬃciencies.

After these selection requirements, the selection eﬃciency of a radion signal in the 1

and 2 b tag categories is 1–6% in the τ

channel, and 3–10% in the `τ

channel, for

low and high resonance masses, respectively. The eﬃciencies for a W

signal passing the

V-tagging selection are 2–9% and 8–19% in the τ

and `τ

channels, respectively. Events

in which H → VV are found contribute up to an additional 20–30% of the total signal yield

in the `τ

channels, and less than 10% in the τ

channels.

6 Background estimation

The main sources of background originate from top quark pair production and from the

production of a vector boson in association with jets (Z+jets and W+jets), while minor con-

tributions arise from single top quark, diboson, and multijet production. These background

– 7 –

JHEP01(2019)051

Channel τ

LP τ

HP 1 b-tagged subjet 2 b-tagged subjets

`τ

0.96 ± 0.04 1.06 ± 0.06 1.00 ± 0.06 1.11 ± 0.15

Table 1. Normalization scale factors for top quark production for diﬀerent event categories, de-

pending on the V tagging and H tagging requirement applied. Uncertainties are due to the limited

number of events in the control regions and the uncertainty in the b tagging eﬃciency.

contributions are split into either tt and single top quark production (tt, t), or into V+jets

production. The latter includes Z+jets and W+jets, multijet, and SM diboson production.

The shape of the distribution of the top quark pair and single top quark background

is determined from simulation, while the normalization is determined from data through

dedicated control regions that are enriched in top quark events. Control regions having

a purity larger than 80% for top quarks are selected by inverting the b tag veto on the

AK4 jets and tightening the b tagging criteria. Events are separated according to the

requirements of large-cone jet identiﬁcation. Data are found to be well-described by sim-

ulations in terms of the jet and dijet resonance-mass distributions. Multiplicative scale

factors are used to correct for the diﬀerence in normalization of data and simulation in

the control regions, after subtracting the other background contributions. Scale factors

obtained in control regions of `τ

events are applied also to the τ

channels, where there

are fewer events. The normalization of top quark production processes in each region is

also corrected using the scale factors reported in table 1.

The estimated contribution from V+jets backgrounds is based on data, in regions

deﬁned by applying the complete signal selection apart from the jet-mass requirements.

Data are divided into the τ

and the `τ

channels. Two jet-mass sidebands (SB) are

deﬁned with jet masses in the range of 30–65 GeV for the low sideband (LSB), or above

135 GeV for the high sideband (HSB), and used to predict the background contribution in

the signal region (SR). Analytic functions are ﬁtted to the simulated distributions of the jet

mass, separately for V+jets and for top quark processes. The former is modeled by a third-

order polynomial or an exponential convoluted with an error function, and the latter with

a sum of one or two Gaussian functions to model the W and top quark distributions. The

background model, composed of the sum of the functions modeling the two backgrounds,

is ﬁtted to the data in the jet mass sidebands. In this unbinned likelihood ﬁt, only the

V+jets distribution shape and normalization parameters are left free to ﬂoat. The number

of expected events in the SR is then obtained by integrating the background components

in the jet mass window compatible with the signal hypothesis. An indicative ﬁt is shown

in ﬁgure 2 (left) for the HP τ

category of `τ

events. The procedure is repeated with an

alternative function used for modeling the V+jets jet mass, with the observed diﬀerence in

the normalization taken to be the associated systematic uncertainty. The expected number

of background events in each signal region is reported in table 2.

The distribution of the V+jets background resonance mass (m

) in the SR is estimated

from the SBs through a transfer function α(m

), which accounts for the small kinematical

diﬀerences and the correlations involved in the interpolation from the SBs to the SR, and

does not depend on the systematic uncertainties that aﬀect the simulated V+jets spectra,

– 8 –

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