CMS实验在13TeV质子-质子碰撞中搜索重共振至τ轻子对
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"在s = 13 $$ \sqrt {s} = 13 $$ TeV的质子-质子碰撞中搜索衰落到tau轻子对的重共振"
这篇研究论文详细探讨了在质子-质子碰撞实验中，对高能重共振粒子衰变为τ轻子对的搜寻。实验在欧洲核子研究中心（CERN）的大型强子对撞机（LHC）上使用CMS（紧凑μ子线圈）探测器进行，数据集对应于2.2 fb -1的综合光度，即质子-质子碰撞的能量为13TeV。研究的目标是寻找可能超出标准模型预期的新型强相互作用粒子，这些粒子可能在高能量下产生并衰变成τ轻子对。
论文中提到的"Open Access"标签意味着这篇研究是公开可获取的，研究人员和公众无需付费即可阅读和使用其研究成果。文章发表在JHEP（Journal of High Energy Physics）的2017年2月刊，编号048，并由Springer出版。论文的接收、接受和发表时间分别为2016年11月20日、2017年1月25日和2017年2月9日。
在分析过程中，科研团队没有发现与标准模型预测显著不符的现象，这意味着观测到的数据与理论预测一致。不过，他们根据实验结果设置了95%置信水平的限制，这限制了可能存在的新型重共振粒子的性质。例如，对于顺序标准模型（Sequential Standard Model, SMS），研究排除了质量低于2.1 TeV的Z'玻色子存在，因为这样的Z'玻色子如果衰变为τ轻子对，其存在应该在实验中有所体现，但并未观察到。这个结果扩展了之前在这个最终态上的限制。
此外，对于一个特殊的理论框架——顶色辅助的彩色染料模型（Top Color-Assisted Technicolor Model, TC2），该模型预言Z'玻色子更倾向于与第三代费米子耦合。基于实验数据，质量低于1.7 TeV的Z'玻色子被排除，这是至今为止对这种模型给出的最严格的限制。
这项工作不仅对理解基本粒子物理提供了重要的限制，还为探索超出标准模型的新物理现象提供了宝贵的参考。通过不断的实验和数据分析，科学家们持续地测试着现有的理论模型，并寻找可能揭示宇宙新奥秘的线索。
JHEP02(2017)048
We do not apply a selection requirement on the mass variable of eq. (5.1), but instead
utilize it to search for a broad enhancement in the m(τ
1
, τ
2
, ~p
miss
T
) distribution consistent
with new physics.
The product of signal acceptance and efficiency for Z
0
→ ττ events varies with the Z
0
boson mass. The τ
µ
τ
h
and τ
h
τ
h
channels provide the largest product of acceptance and
efficiency, while the τ
e
τ
µ
channel has the lowest. For the combination of all four channels
the product of acceptance and efficiency amounts to 6.3, 13, and 14%, respectively, for
m (Z
0
SSM
) = 0.5, 1.5, and 3.0 TeV.
6 Background estimation
To estimate the background contributions in the signal region, techniques based on data
control regions are employed wherever possible. The strategy when using such a technique
is to modify the standard event selection requirements in order to define samples enriched
with events from specific sources of background. These control regions are used to model
the m(τ
1
, τ
2
, ~p
miss
T
) shape of the backgrounds and to measure the probability for an event
to satisfy the selection requirements. Contributions from additional background sources
in a given control region are subtracted using their predictions from simulation. The
background estimation methods based on data control regions are validated by determining
the ability of the method, applied to simulated samples, to predict correctly the true
number of background events and the shape of the m(τ
1
, τ
2
, ~p
miss
T
) distribution. In some
cases, for backgrounds with misidentified leptons, the background estimation methods are
also validated with the data. In such tests, the agreement between the relevant quantities
is always within the statistical uncertainties, and is at the level of 20% or better, depending
on the channel. In cases where an approach based on a data control region is not possible,
or for backgrounds with small expected contributions, we rely on simulation. For the most
relevant SM contributions evaluated from simulation, we verify that the MC prediction is
in agreement with the data in background-enhanced regions.
The QCD multijet background is relevant for both the τ
h
τ
h
and τ
`
τ
h
channels, where
it represents more than 80% or 20% of the total background, respectively. The contri-
bution from QCD multijet events in the τ
e
τ
µ
channel is approximately 5% (< 1%) for
m(τ
1
, τ
2
, ~p
miss
T
) > 85 (300) GeV. In the τ
h
τ
h
final state, this background is evaluated
from the like-sign ττ mass distribution (> 98% purity of QCD multijet events), which
is scaled using the opposite-sign-to-like-sign ratio measured in a control region where the
E
miss
T
requirement is inverted (E
miss
T
< 30 GeV). In the τ
e
τ
h
final state, the QCD multijet
background is evaluated using the mass shape reconstructed from a data sample with a
nonisolated τ
h
. This mass shape is weighted by the probability for a jet to satisfy the
τ
h
isolation criterion, which is measured from a sample of like-sign ττ candidates. The
systematic uncertainties in these estimates are discussed in section 7.
Background from W+jets events is important in the τ
`
τ
h
channels, representing about
40 (45)% of the total background when ` is an electron (muon). The contribution from
W+jets events in the τ
e
τ
µ
and τ
h
τ
h
channels is <1% for m(τ
1
, τ
2
, ~p
miss
T
) > 300 GeV. To
estimate this background, we take the mass distribution of events selected in data with
– 6 –
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