ATLAS探测器在质子-质子碰撞中寻找b喷射双喷射共振的新研究

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"这篇论文是ATLAS合作组在《Physics Letters B》期刊上发表的研究成果，标题为“Search for resonances in the mass distribution of jet pairs with one or two jets identified as b-jets in proton–proton collisions at √s=13TeV with the ATLAS detector”。研究基于大型强子对撞机(Large Hadron Collider, LHC)的ATLAS探测器在质子-质子碰撞数据，数据集的积分光度为3.2fb⁻¹，质心能量为13TeV。文章主要目的是在双喷射不变质量谱中寻找一或两个被识别为b喷射的喷射对的共振现象。文章内容概述了对高能共振体的搜寻，这些共振体可能出现在具有b喷射标识的双喷射系统的质量分布中。在分析的3.2fb⁻¹的数据样本中，没有发现异常现象，即没有观测到与理论预测不符的信号。根据这些结果，研究者排除了95%置信水平下质量在1.1至2.1TeV的兴奋b夸克（b∗ quarks）以及1.1至1.5TeV的无轻子耦合Z'玻色子（leptophobic Z' bosons）。同时，他们还排除了有效截面从大约0.4pb到0.001pb的高斯信号形状的贡献。通过这项研究，ATLAS探测器进一步限制了超出标准模型的新物理粒子的存在范围，这有助于我们理解基本粒子间的相互作用和宇宙的基本结构。该研究是高能物理学领域的重要进展，为探索未知的物理现象提供了重要的数据支持。" 这篇论文详细分析了在高能量质子-质子碰撞中，尤其是在b喷射产生的双喷射系统中的质量分布，目的是寻找可能存在的新粒子或者共振态。虽然没有发现显著的异常信号，但这种否定结果同样意义重大，因为它对已知和假设的粒子模型进行了严格的检验，并且给出了新的物理过程的限制条件。这有助于科学家们在标准模型框架外继续探索宇宙的基本构建块。

232 The ATLAS Collaboration / Physics Letters B 759 (2016) 229–246

Fig. 4. Dijet mass spectra overlaid with the ﬁts to the background function together with the results from BumpHunter and benchmark signals scaled by a factor of 50. The

most discrepant region is indicated by the two blue lines. The lower panels show the signiﬁcances per bin of the data with respect to the background ﬁt, in terms of the

number of standard deviations, considering only the statistical ﬂuctuations. The distributions are shown for the (a) “1b” and (b) “2b” categories. (For interpretation of the

references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

outcomes is evaluated using the ensemble of Poisson outcomes

across all intervals scanned, by applying the algorithm to many

pseudo-data samples drawn randomly from the background ﬁt.

Without including systematic uncertainties, the probability that

ﬂuctuations of the background model would produce excesses at

least as signiﬁcant as those observed in the data, anywhere in the

distribution, is greater than 60% in the “1b” and “2b” categories.

Thus, there is no evidence of localized contributions to the mass

distribution from BSM phenomena.

6. Systematic uncertainties

Uncertainties in the parameters of the ﬁtted background func-

tion

Eq. (1) are evaluated by ﬁtting the ansatz to pseudo-data

drawn via Poisson ﬂuctuations around the ﬁtted background

model. The uncertainty in the prediction in each m

bin is taken

to be the root mean square of the function value for 10000 gen-

erated

pseudo-experiments. To estimate an uncertainty due to the

choice of background parameterization, one additional degree of

freedom, z

log(z)

, is appended as a multiplicative factor to the

nominal ansatz (Eq. (1)), and the difference between the estimated

parameters from the two ﬁts is taken as an uncertainty.

The

uncertainty in the jet energy scale is estimated using vari-

ous

methods in 8TeVdata, corrected to the new centre-of-mass

energy by taking the difference between the 8 TeV and 13 TeV

runs

into account using MC simulation [28]. The jet energy scale

uncertainty used in this analysis relies on a set of three nuisance

parameters [34]. For untagged jets it is within the range 1–5% for

jet transverse momenta greater than 200 GeV.

The

relative additional uncertainty in the energy scale of

b-tagged jets is estimated using the MC samples and veriﬁed with

data following the method described in Ref. [35]. The ratio r

trk

the sum of track transverse momenta inside the jet to the total

jet transverse momentum measured in the calorimeter is used for

this estimate. The double ratio of r

trk

from data and simulation is

formed and compared for inclusive jets and b-jets. The estimated

relative additional uncertainty for jets with 200 < p

< 800 GeV is

found to be less than 2.6%, and this value is subsequently used in

the higher p

regions. This relative uncertainty is applied in ad-

dition

to the nominal jet energy scale uncertainty. The maximum

uncertainty for b-tagged jets is estimated to be 6% and is conser-

vatively

applied to all p

regions.

The

uncertainty in the jet energy resolution is estimated using

the same method as the untagged jet energy scale uncertainty and

relies on an additional Gaussian smearing of the reconstructed jet

energies in MC simulation. For jets with p

> 50 GeV, the uncer-

tainty

is less than 2%.

The

uncertainty introduced by the application of the b-tagging

algorithm is the largest systematic uncertainty in the analysis. The

uncertainty in the measured tagging eﬃciency of b-jets is esti-

mated

by studying t

t events in 13 TeV data for jet p

up to

200 GeV [31]. The uncertainties in the measured rate of mistag-

ging

c-jets and light-ﬂavour jets are estimated in 8TeVdata. The

uncertainties are extrapolated to 13 TeV, taking into account the

addition of the new IBL system as well as reconstruction and tag-

ging

improvements. An additional term is included to extrapolate

the measured uncertainties to the high-p

region of interest. This

term is calculated from simulated events by considering varia-

tions

on the quantities affecting the b-tagging performance such

as the impact parameter resolution, percentage of poorly measured

tracks, description of the detector material, and track multiplicity

per jet. The dominant effect on the uncertainty when extrapolat-

ing

at high-p

is related to the different tagging eﬃciency when

smearing the tracks impact parameters based on the resolution

measured in data and simulation. The difference in the impact pa-

rameter

resolution is due to effects from alignment, dead modules

and additional material not properly modelled in the simulation.

The impact of the b-tagging eﬃciency uncertainty increases with

jet p

and reaches 50% above 2TeV.

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