ATLAS实验中带电希格斯玻色子与顶夸克关联搜索及H±→τν衰变研究

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"这篇研究论文发表在Physics Letters B 759 (2016) 555–574上，由ATLAS合作团队完成，报告了在大型强子对撞机(LHC)的ATLAS探测器中对带电希格斯玻色子的搜索结果。该搜索是针对质子-质子碰撞数据进行的，这些数据是在能量为13 TeV的碰撞中收集的，总和起来相当于3.2 fb-1的综合光度。" 在标准模型之外，带电希格斯玻色子是许多超出标准模型理论，如超对称理论中的预测粒子。这项研究关注的是与单个顶夸克关联并通过H±→τν衰变的带电希格斯玻色子的产生。在这种衰变过程中，带电希格斯玻色子会衰变成一个τ轻子和一个中微子，而顶夸克则会经历强子衰变。这种特定的最终状态特点是有τ轻子的强子衰变、缺失的横向动量（由于中微子未被探测到）以及没有高横向动量的电子和介子。数据分析显示，观测到的数据与标准模型预测的背景流程一致，没有发现明显的带电希格斯玻色子信号。因此，研究人员进行了统计分析，以设置在95%置信水平下带电希格斯玻色子的产生截面乘以其分支分数σ（pp→[b] tH±）×BR（H±→τν）的上限。这个上限在玻色子质量范围200至2000 GeV之间变化，从1.9 pb到15 fb不等。此次搜索的结果优于之前基于8 TeV质子-质子碰撞数据得到的限制，进一步限制了带电希格斯玻色子的存在可能性。这样的研究对于理解基本粒子物理学中的未知领域，特别是可能存在的新物理现象，具有重要意义。通过对带电希格斯玻色子的探索，科学家们可以更深入地了解希格斯机制，以及可能存在的超对称性等新的物理理论。

558 The ATLAS Collaboration / Physics Letters B 759 (2016) 555–574

Fig. 2. Fake factors parameterised as a function of p

and the reconstructed τ

had

decay mode, for a b-tagging score of the τ

had-vis

candidate below (left, light-ﬂavour region)

or above (right, heavy-ﬂavour region) the chosen threshold. The τ

had

decay mode is referred to as 1p0n, 1p1n, 1pXn, 3p0n or 3pXn, for a p-prong decay with n neutral

particles (π

), where X ≥ 2(1) for 1pXn (3pXn) decays. The threshold value for the b-tagging score is set to keep enough entries in each of the two bins, separately for each

type of τ

had

decay. The errors shown arise from the statistical uncertainty for a given p

bin.

Fig. 3. Distribution of m

in the control region enriched with W → τν events, which

differs from the nominal event selection by the requirements that m

< 100 GeV

and

that the number of b-tagged jets be zero. The W → τν background is nor-

malised

to the data through an overall scale factor. The total (statistical and system-

atic)

uncertainties in the SM prediction are shown in the lower plot.

The backgrounds with a true τ

had

are estimated using simula-

tion.

The two dominant processes, t

t and W → τν, are validated

in two dedicated control regions, which differ from the nominal

event selection by the requirements that m

< 100 GeV (instead of

> 50 GeV) and that the number of b-tagged jets be either at

least two (for the control region enriched with t

t events) or zero

(for the control region enriched with W → τν events). The latter

is also used to correct the overall normalisation of the simulated

W → τν background. The m

distributions that are predicted and

measured in these two background-enriched control regions are

displayed in Figs. 3 and 4. The relative signal contamination in the

control region enriched in W → τν events is about two orders of

magnitude smaller than the expected fraction of H

→ τν events

in the signal region. The control region enriched in t

t events has a

small overlap with the signal region, however the expected signal

contamination is negligible, about one order of magnitude smaller

than the expected fraction of H

→ τν events in the signal re-

gion.

The

expected number of background events in the signal region

is shown in Table 1, together with the hypothetical contribution

from charged Higgs bosons with a mass of 200 or 1000 GeV, and

with σ (pp →[b]tH

) × BR(H

→ τν) set to the prediction from

the hMSSM scenario for tan β = 60 (for a given mass, the expected

signal yield increases quadratically with tan β). The calculation of

the production cross section is based on Refs. [22,75–78], while

HDECAY [79] is

used for computing the branching fraction. The sig-

Fig. 4. Distribution of m

in the control region enriched with t

t events, which differs

from the nominal event selection by the requirements that m

< 100 GeV and that

the number of b-tagged jets be at least two. The total (statistical and systematic)

uncertainties in the SM prediction are shown in the lower plot.

Table 1

Expected

event yields for the backgrounds and a hypothet-

ical

signal after all selection criteria, and comparison

with 3.2fb

−1

of data. The values shown for the signal as-

sume

a charged Higgs boson mass of 200 or 1000 GeV, with

a cross section times branching fraction σ (pp →[b]tH

) ×

BR(H

→ τν) corresponding to tanβ = 60 in the hMSSM

benchmark scenario. The uncertainties include statistical

and systematic components.

Sample Event yield

True τ

had

t & single-top-quark 590 ±170

→ τν 58 ±14

→ ττ 6.4 ±2.0

diboson (WW

, WZ, ZZ)4.3 ±1.3

Misidentiﬁed e

, μ → τ

had-vis

40 ±6

Misidentiﬁed jet

→ τ

had-vis

196 ±24

All backgrounds 900

±170

(200 GeV), hMSSM tanβ =60 175 ±28

(1000 GeV), hMSSM tanβ =60 2.0 ±0.2

Data 890

nal acceptance at 200 (1000) GeV is 1.5% (12%), as evaluated with

respect to simulated samples where both the τ -lepton and the as-

sociated

top quark decay inclusively. The event yield observed in

3.2fb

−1

of data is also shown in Table 1 and found to be consis-

tent

with the expectation for the background-only hypothesis.

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