ATLAS探测器发现125.5 GeV希格斯玻色子衰变为光子和Z玻色子的最新研究

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本研究论文标题为"在 sqrt(s)=7 TeV 和 8 TeV pp碰撞中搜寻Higgs玻色子衰变为光子和Z玻色子的实验结果与ATLAS探测器"，发表于《物理快报B》第732期，是ATLAS合作团队的一项关键工作。这项研究专注于对中性希格斯玻色子（Higgs boson）的衰变模式进行探索，特别是关注其通过H→Zγ (Z 衰变为两个轻子，如电子或μ子) 的途径。作者们利用了4.5 fb^-1的7 TeV质心能量下的pp碰撞数据和20.3 fb^-1的8 TeV质心能量下的pp碰撞数据，这些数据是在欧洲核子研究中心的大型强子对撞机(LHC)上由ATLAS探测器收集的。他们的目标是对希格斯玻色子的质量范围120-150 GeV内的这一特定衰变模式进行统计分析，以验证标准模型(SM)的预测。实验结果显示，对于质量约为125.5 GeV的希格斯玻色子，观察到的信号与标准模型的预期在三个最终状态粒子（光子和两个轻子）的不变质量(m_ℓℓγ)分布上是一致的。然而，95%置信水平下的上限结果表明，实际观测到的Higgs boson衰变为Zγ的信号强度是标准模型预测的11倍。这为理论和实验物理学提供了重要的约束，特别是对于Higgs boson的性质及其在高能物理中的角色。此外，论文还给出了8 TeV质心能量下，对于质量在120-150 GeV范围内的中性Higgs boson，衰变为Zγ过程的截面乘以分支比(BR)的95%置信水平上限，范围在0.13 pb到0.5 pb之间。这些限制进一步验证了希格斯机制的预测，并可能揭示超出标准模型的新物理效应。这项研究不仅提升了我们对希格斯玻色子行为的理解，也对粒子物理学的标准模型进行了严格的检验，为未来的高能物理实验提供了有价值的数据和参考。通过这样的精确测量，科学家们可以继续探索粒子宇宙的未知领域，寻找新的物理现象和理论突破。

ATLAS Collaboration / Physics Letters B 732 (2014) 8–27 11

Fig. 1. Three-body invariant-mass distribution for H → Zγ , Z → μμ (top) or

→ ee (bottom) selected events in the 8 TeV, m

= 125 GeV gluon-fusion sig-

nal simulation, after applying all analysis cuts, before (ﬁlled circles) and after (open

diamonds) the corrections described in Section 4.2. The solid and dashed lines rep-

resent the ﬁts of the points to the sum of a Crystal Ball and a Gaussian function.

photon direction and FSR corrections improve the invariant-mass

resolution by about 1% each, while the Z -mass constraint brings

an improvement of about 15–20%.

Fig. 1 illus

trates the distributions of m

μμγ

and m

eeγ

for

simulated signal events from gg

→ H at m

= 125 GeV after

all corrections. The m

eeγ

resolution is about 8% worse due to

bremsstrahlung. The m

γ

distribution is modelled with the sum

of a Crystal Ball function (a Gaussian with a power-law tail), rep-

resenting the core of well-reconstructed events, and a small, wider

Gaussian component describing the tails of the distribution. For

=125.5 GeV the typical mass resolution σ

of the core com-

ponent of the m

μμγ

distribution is 1.6 GeV.

4.3. Event classiﬁcation

The selected events are classiﬁed into four categories, based

on the pp centre-of-mass energy and the lepton ﬂavour. To en-

hance the sensitivity of the analysis, each event class is further

divided into categories with different signal-to-background ratios

and invariant-mass resolutions, based on (i) the pseudorapidity dif-

ference

η

Zγ

between the photon and the Z boson and (ii) p

the component of the Higgs boson candidate p

that is orthogo-

nal to the Z

γ thrust axis in the transverse plane [58]. Higgs boson

candidates are classiﬁed as high- (low-)p

candidates if their p

=|(



) ×

t| where

t = (



−



)/|



−



| denotes the thrust axis

in the transverse plane, and



are the transverse momenta of the photon and

the Z boson.

Table 2

Expect

ed signal (N

) and background (N

)yieldsina±5 GeV mass window around

= 125 GeV for each of the event categories under study. In addition, the ob-

served number of events in data (N

) and the FWHM of the signal invariant-mass

distribution, modelled as described in Section 4.2, are given. The signal is assumed

to have SM-like properties, including the production cross section times branching

ratio. The background yield is extrapolated from the selected data event yield in the

invariant-mass region outside the

±5 GeV window around m

=125 GeV, using an

analytic background model described in Section 6. The uncertainty on the FWHM

from the limited size of the simulated signal samples is negligible in comparison to

the systematic uncertainties described in Section 5.

√

[TeV]

 Category N

√

FWHM

[GeV]

8 μ high p

2.3 310 324 0.13 3.8

μ low p

,low|η| 3.7 1600 1587 0.09 3.8

μ low p

,high|η| 0.8600 6020.03 4.1

8 e high p

1.9 260 270 0.12 3.9

8 e low p

,low|η| 2.9 1300 1304 0.08 4.2

8 e low p

,high|η| 0.6 430 421 0.03 4.5

μ high p

0.440 400.06 3.9

μ low p

0.6 340 335 0.03 3.9

7 e high p

0.325 210.06 3.9

7 e low p

0.5 240 234 0.03 4.0

is greater (smaller) than 30 GeV. In the analysis of

√

s = 8TeV

data, low-p

candidates are further split into two classes, high-

and low-

|η

Zγ

|, depending on whether |η

Zγ

| is greater or less

than 2.0, yielding a total of ten event categories. Signal events are

typically characterised by a larger p

and a smaller |η

Zγ

| than

background events, which are mostly due to q

q → Z + γ events

in which the Z boson and the photon are back-to-back in the

transverse plane. Signal events with high p

or low |η| are en-

riched in VBF, VH and ttH events, in which the Higgs boson is more

boosted, and in gluon fusion events in which the leptons and the

photon are harder or more central in the detector than in signal

events with low p

and high |η|.Thisresultsinabetterγ in-

variant mass resolution for the high p

and low |η| categories,

which are also characterised by a better signal-to-background ra-

tio.

As an example, the expected number of signal and background

events in each category with invariant mass within a

±5 GeV win-

dow around m

=125 GeV, the observed number of events in data

in the same region, and the full-width at half-maximum (FWHM)

of the signal invariant-mass distribution, are summarised in Ta-

ble 2. Using this classiﬁcation improves the signal sensitivity of

this analysis by 33% for a Higgs boson mass of 125.5 GeV com-

pared to a classiﬁcation based only on the centre-of-mass energy

and lepton ﬂavour categories.

4.4. Sample composition

The main backgrounds originate from continuum Z +γ , Z → 

production, from radiative Z → γ decays, and from Z + jet,

→  events in which a jet is misidentiﬁed as a photon. Small

contributions arise from t

t and WZ events. Continuum Z + γ

events are either produced by qq in the t-oru-channels, or from

parton-to-photon fragmentation. The requirements m



> m

−

10 GeV, m

γ

> 115 GeV and R

γ

> 0.3 suppress the contribu-

tion from Z

→ γ , while the photon isolation requirement re-

duces the importance of the Z

+γ fragmentation component. The

latter, together with the photon identiﬁcation requirements, is also

effective in reducing Z

+ jets events.

In this analysis, the estimated background composition is not

used to determine the amount of expected background, which is

directly ﬁtted to the data mass spectrum, but is used to normalise

the background Monte Carlo samples used for the optimisation of

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ATLAS探测器发现125.5 GeV希格斯玻色子衰变为光子和Z玻色子的最新研究

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