ATLAS探测器揭示13TeV质子-质子碰撞中长寿命重电荷粒子搜寻

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本文探讨了在欧洲核子研究组织（CERN）的大型强子对撞机（LHC）上，利用ATLAS探测器进行的一项深入研究。这项研究的焦点是寻找在质子-质子碰撞中产生的带电的长寿命重粒子。ATLAS实验团队在2015年和2016年收集的数据样本，对应于一个综合发光度（即撞击能量下产生的总事件数量）为36.1 femtobarns^-1（fb^-1）。标准模型（SM）预测，这类粒子的速度远低于光速，因此它们的电离效应显著高于同等质量但电荷单位相同的粒子。研究者们利用ATLAS探测器的像素子系统来精确测量穿越像素探测器的所有重建带电粒子的离子化能量损失。通过这种技术，他们能够识别出那些在碰撞后具有异常离子化轨迹的信号，这可能是长寿命重粒子存在的迹象。由于这些粒子的特殊性质，它们可能会在探测器中留下独特的痕迹，即使经过较长的时间也能够在数据中显现出来。论文《Search for heavy charged long-lived particles in proton-proton collisions at √s = 13 TeV using an ionisation measurement with the ATLAS detector》发表在《物理快报B》杂志上，于2019年发布，展示了实验结果以及对可能的新物理现象的分析。文章接收日期为2018年8月14日，修订后的版本于同年10月3日收到，最终接受时间为10月29日，全文在线发布日期为11月1日。作者们通过对实验数据的细致分析，不仅验证了标准模型的预测，还可能揭示超出标准模型的新物理现象，如超新星、暗物质候选者或者额外维度等。这项工作对于理解宇宙的基本结构和粒子物理学的前沿理论有着重要意义。这项研究不仅测试了现有的理论框架，也为未来的粒子物理学研究开辟了新的探索路径。

100 The ATLAS Collaboration / Physics Letters B 788 (2019) 96–116

Table 1

Summary

of the different selection requirements applied to the signal region (SR), the valida-

tion

region (VR), and the control regions (CR).

p-CR dE/dx-CR

SR VR for SR for VR for SR for VR

Trackmomentum[GeV] > 150 50–150 > 150 50–150 > 150 50–150

miss

[GeV] > 170 > 170 < 170

Ionisation [MeV g

−1

] > 1.8 < 1.8–

the stable R-hadron region in which muons are not vetoed, the

isolation selection is tightened to 5GeV.

least two pixel clusters, after discarding the cluster with the

highest ionisation, must be included in the truncated mean calcu-

lation

of dE/dx to ensure it is robust. The relative uncertainty in

the momentum measurement must be less than 50%. The speciﬁc

ionisation of the candidate track measured by the pixel detector

must be larger than 1.8MeVg

−1

. Relative to inclusive gener-

ated

R-hadron events with a mass of 2000 GeV, the eﬃciency for

events to pass all selections, including the trigger, is 12% for stable

R-hadrons and 19% for those with a lifetime of 10 ns.

7. Background estimation

The expected background contains tracks from SM processes in-

cluding

vector boson, top-quark, and multi-jet production. Tracks

from any SM particle can be measured with high dE/dx due to

the unlikely sampling of multiple measurements from the long

tail of the Landau distribution, from overlapping particles deposit-

ing

charge in the same pixels, or from spurious pixel hits from

low-momentum particles being incorrectly assigned to the high-

momentum

track. To correctly estimate both the rate of high-

momentum

tracks in events with large E

miss

and the probability

of measuring a high ionisation energy for those tracks, the back-

ground

is fully estimated from data.

template for the momentum distribution of background

tracks in signal region (SR) events is obtained from a control

region (p-CR) in which the ionisation requirement is inverted,

dE/dx < 1.8MeVg

−1

, while all other track-level and event-

level

selections are applied.

The

dE/dx distribution, in a few bins of momentum,

is ob-

tained

for the expected background from a low-E

miss

data sample

in which E

miss

< 170 GeV. Inverting the E

miss

requirement relative

to the high-E

miss

SR minimises signal contamination in this con-

trol

region (dE/dx-CR), and the lack of correlation between E

miss

and dE/dx for high-momentum SM tracks allows the dE/dx distri-

bution

of the expected background to be derived from low-E

miss

events which pass all other selections. Since the E

miss

trigger

thresholds varied as a function of time for the collected data, the

events in this control region are reweighted so that the ratio of

low-to-high E

miss

events is constant versus time.

The

momentum and dE/dx distributions obtained in the con-

trol

regions (CR)s are used as templates to calculate the shape

of the expected mass distribution of candidate tracks from back-

ground

events. Apair of p and dE/dx values is obtained by ran-

domly

sampling from the p-CR distribution, and then randomly

sampling from the dE/dx-CR distribution in the appropriate p-bin.

The mass for each pair of p and dE/dx values is calculated as de-

scribed

in Section 5. The resulting background mass distribution

is normalised to data in the region where m < 160 GeV, in which

To account for the dependence of dE/dx on momentum up to the Fermi plateau.

The most probable energy loss reaches a constant value, the Fermi plateau, at

large βγ .

signal was previously excluded [48,56], before the high ionisation

requirement is imposed.

The

procedure for estimating both the normalisation and shape

of the expected background is validated in a low-momentum val-

idation

region (VR) in which the momentum of tracks is required

to be between 50 GeV and 150 GeV. The differences between the

selections applied to the SR, CR, and VR are shown in Table 1. The

control and validation regions are independently produced for both

the metastable and stable R-hadron SRs. The expected mass dis-

tributions

in the two validation regions, along with the observed

data, are shown in Fig. 3. Good agreement between the data and

the prediction in the VR validates the background estimation pro-

cedure.

8. Systematic uncertainties

The background estimation technique described in the previ-

ous

section relies on the lack of correlation between several key

kinematic variables in background events. The largest uncertain-

ties

in the central value of the background estimate come from

possible residual correlations. In particular, the residual correlation

between η and dE/dx results in an uncertainty in the size of the

background estimate ranging from 15% at the lowest mass values

to 30% at the highest mass values. This uncertainty is assessed by

comparing the nominal background estimate with an estimate per-

formed

in η bins. Additionally, an uncertainty of 1%–25% in the

background yield arises from residual correlations between p and

dE/dx for tracks entering the background calculation. This is esti-

mated

by reweighting the p template from the p-CR by the differ-

ence

in the p distribution between tracks with high and low dE/dx

the low-E

miss

region. Similarly, the residual correlation between

miss

and dE/dx is probed by rescaling the template dE/dx dis-

tribution

with a scale factor obtained from the difference between

the dE/dx distributions in the VR for tracks in events with high

miss

and low E

miss

. This uncertainty ranges from 3% to 12% on

the background expectation in different mass windows.

the background is fully estimated from data, detector or

data-taking conditions which affect the measurement of dE/dx

are

accounted for, as long as the luminosity proﬁle of the con-

trol

regions matches that of the signal region. The reweighting

of the dE/dx-CR control region achieves this. A conservative un-

certainty

in the time-dependence of the dE/dx measurement is

assessed by comparing the background estimate with and with-

out

the reweighting, which results in an additional uncertainty of

3%–18% on the background yields. The limited numbers of events

in the control regions contribute 6% uncertainty. Other uncertain-

ties

in the background estimate are below 5%, including an un-

certainty

in the shape of the dE/dx tail from the CR and in the

different fractions of muons between the CR and SR.

The

uncertainty in the expected number of signal events is

dominated by the estimation of the production cross-section of

gluino–gluino pairs; the calculation of the cross-section and its un-

certainty

is described in Section 4. The uncertainty ranges from

14% for gluino masses of 600 GeV to 36% for masses of 2200 GeV.

An additional uncertainty in the number of produced signal events

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