ATLAS探测器在pp碰撞中搜索希格斯玻色子对的最新结果

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"使用ATLAS探测器在pp碰撞下pp碰撞的bb¯τ+τ-衰减通道中搜索共振和非共振希格斯玻色子对产生" 在高能物理领域，希格斯玻色子的发现是粒子物理学标准模型中的一个里程碑。这篇研究论文详细介绍了ATLAS合作组在2015年和2016年利用欧洲核子研究中心的大型强子对撞机(LHC)记录的36.1 fb-1的pp碰撞数据，进行的一项针对希格斯玻色子对产生的实验性搜索。这次搜索主要关注的是bb¯τ+τ-衰变通道，其中τ轻子可以是通过强子或中微子的衰变路径。研究中，科学家们分析了两种不同类型的希格斯玻色子对产生：共振生产和非共振生产。非共振生产是指两个独立的希格斯玻色子无关联地产生，而共振生产则涉及某种新的粒子，如超对称粒子或额外维度理论中的粒子，它们可能以共振方式产生希格斯玻色子对。在非共振生产中，数据没有显示出显著超出预期背景的信号。因此，他们设定了一个上限，即非共振希格斯玻色子对产生的横截面乘以分支比率不超过30.9 fb，这是标准模型预测值的12.7倍，置信水平为95%。这意味着如果存在这样的过程，其强度必须在这个限制内。对于共振生产，研究者们特别关注了扩展的希格斯扇区模型，比如基于两个双峰的模型以及Randall-Sundrum大引力子模型。通过对数据的分析，他们能够限制这些模型的参数空间。例如，在简化hMSSM（最小超对称标准模型）中，当tanβ=2时，质量范围在305 GeV至402 GeV之间的新粒子X（可能是某种超对称粒子）的共振产生被排除。同时，对于Randall-Sundrum模型，当k/M′Pl=1时，质量范围在325 GeV至885 GeV的GKK引力子也被排除。这项工作不仅对理解希格斯玻色子的性质和可能的超出标准模型的新物理现象提供了有价值的限制，而且也展示了ATLAS探测器在高精度物理测量中的能力。通过这种复杂的数据分析，科学家们不断探索宇宙的基本粒子和力量，希望揭示隐藏在自然界的深层次结构。

lep

had

(τ

had

) SR is allowed to vary by 29% (35%)

relative to the normalization derived in the Z → μμ þ

heavy-flavor control region in order to account for accep-

tance differences between the two. An additional 20%

normalization uncertainty in the Z → ee þ light-flavor

background, related to the misidentification of electrons

as taus, is derived by comparing data and simulation in a

Z → ee control region with 0 or 1 b-tagged jets. The ZH

(ttH) background normalization is varied by 28% (30%)

based on ATLAS measurements [98,99]. The normaliza-

tions of the remaining minor backgrounds taken from

simulation are allowed to vary within their respective

cross-section uncertainties.

The uncertainty in the modeling of backgrounds due to

jets being misidentified as τ

had-vis

is estimated by varying

the fake factors and fake rates within their statistical

uncertainties and varying the amount of true-τ

had

back-

ground subtracted. Based on studies with simulated t

t and

W þ jets events, a systematic uncertainty is assigned to

cover the difference in the gluon and quark flavor compo-

sition of jets misidentified as a τ

had-vis

between the signal

region and the fake-τ

had

enriched sample, parametrized as a

function of the τ

had-vis

identification BDT score. The

uncertainty in the extrapolation of FFðQCDÞ to the signal

region is estimated from the difference between the

nominal FFs and alternative ones, calculated either in

the SS region for the τ

lep

had

channel or a multijet enriched

region, where Δϕðτ

had-vis

; τ

had-vis

Þ > 2.0, in the τ

had

case. Similarly, changes in the fake-τ

had

determination

when varying the t

t control region m

requirement in

simulation and data are used to estimate a systematic

uncertainty in both the fake factors and fake rates. The

overall effect of these uncertainties on the fake-τ

had

back-

ground estimate leads to an 8.4% variation of the NR signal

strength, predominantly due to the true-τ

had

subtraction in

the t

t control region and the composition of the fake τ

had

Theory uncertainties in the signal acceptance are calcu-

lated by independently varying the renormalization and

factorization scales, the choice of PDF and each PDF set by

its uncertainties. The uncertainty in the parton shower is

taken into account by comparing the default H

ERWIG

with P

YTHIA

8. Uncertainties in the underlying event,

initial-state radiation and final-state radiation are accounted

for by changing the P

YTHIA

tune, but are small. The effects

of various categories of uncertainty on the measured

nonresonant signal strength corresponding to the expected

upper limit at 95% C.L. are summarized in Table II. The

individual sources of uncertainty making up the categories

listed in the table are grouped together in the final fit to

determine their correlated combined effect on the signal

strength. For all signal hypotheses, the statistical uncer-

tainties dominate.

For each signal model considered, a profile-likelihood

fit [100] is applied to the BDT score distributions

simultaneously in the three SRs to extract the signal cross

section, along with the t

t and Z þ heavy-fla vor n ormal-

izations. The lattermost is constrained by including the

dedicated control region in the fit. All sources of system-

atic and s tatistical uncertainty in the signal and back-

ground models ar e implemented a s deviations from the

nominal model, scaled by nuisance parameters that are

profiled in the fit. None of th e d ominant nu isance

parameters are significantly constrained or pulled relative

to their input value by the fit. The BDT score distributions

for the nonresonant search and the G

signal are shown

in Fig. 1 after performing the fit and assu min g a back-

ground-only hypothe sis . The acceptance time s efficiency

for the NR HH signal is 4.2% (2.9%) in t he combined SLT

and LTT τ

lep

had

(τ

had

) channel over the full BDT

distribution, decreasing to 3.3% (2.4%) for the two most

sensitive BDT bins. As no sig nifi cant excess over the

expected background is obser ved, upper limits are set on

nonresonant and reso nant Higgs boson pair production at

95% C.L. using the CL

method [101].

Table III presents the upper limits on the cross section for

nonresonant HH production times the HH → bbττ branch-

ing ratio, and comparisons with the SM prediction. The

observed (expected) limit is 30.9 fb (36.0 fb), 12.7 (14.8)

times the SM prediction. In order to compare with previous

results, the BDTs are trained and applied to the signal

sample without reweighting the m

spectrum to

Refs. [9,31], giving an observed (expected) limit of

37.4 fb (33.5 fb), 15.4 (13.8) times the SM prediction.

The results of searches for resonant HH production are

presented as exclusion limits on the cross-section times the

HH → bbττ branching ratio as a function of the resonance

mass. The expected and observed limits for narrow-width

TABLE II. The percentage uncertainties on the simulated

nonresonant signal strength, i.e., the simulated NR HH yield

assuming a cross-section times branching fraction equal to the

95% C.L. expected limit of 14.8 times the SM expectation.

Source Uncertainty (%)

Total 54

Data statistics 44

Simulation statistics 16

Experimental uncertainties

Luminosity 2.4

Pileup reweighting 1.7

had

16

Fake-τ estimation 8.4

b tagging 8.3

Jets and E

miss

3.3

Electron and muon 0.5

Theoretical and modeling uncertainties

Top 17

Signal 9.3

Z → ττ 6.8

SM Higgs 2.9

Other backgrounds 0.3

PHYSICAL REVIEW LETTERS 121, 191801 (2018)

191801-5

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ATLAS探测器在pp碰撞中搜索希格斯玻色子对的最新结果

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2.Z_1=(¯C+¯(B+D+¯A))(A+B+¯(C¯D)的对偶式；反函数式为。

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