带你了解CERN 之 粒子狂欢 CERN · Party in ATLAS

上次我们跟随粒子们穿过了不同大小、不同作用的各种管道。现在，随着大家的速度都达到了光速，是时候和其他粒子一起狂欢了!

Last time we followed the particles and travelled through the tubes, now reaching light speed, it’s time to party with other ones!

在此之前，如果你有兴趣先了解一下CERN、LHC 和ATLAS 的话，欢迎查看附属文章，也许对接下来的神奇旅程有帮助哦！

Before we start, if you are interested in knowing more about CERN, LHC and ATLAS, please feel free to check out the attached articles. They might help you enjoy the next magical journey more!

图为ATLAS探测仪的内部场景，对比图中人像可以看出这个机器的巨大

The inside of ATLAS detector, the size of person compared to the surrounding shows the humongous size of the machines

千里来相会

A thousand miles to meet

在LHC的真空管中，每个质子束由2808个群组成。每个群开始时都有12亿个质子，大约几厘米长，几毫米宽*。但是质子束的大小也会在整个环中变化，因为会在不同位置受到挤压和膨胀。

Inside the LHC’s vacuum tubes, each beam of protons has 2808 bunches. Each bunch begins with 1.2 hundred billion protons at the start and is a few cm in length and a mm wide*. However, the bunch size is not always constant throughout the ring, they get squeezed and expanded at different parts of ring.

当这些质子束相遇时，在 2000亿个质子之间约有40次的碰撞发生在。你看，即使是质子，遇到命运中的那个也是相当的难得啊。这些质子束平均每秒交叉3000万次，因此每秒大约有10亿次碰撞，每次质心能量为13TeV。

When the bunches meet each other, up to 40 collisions happen between 200 billion particles. See, it hard to meet the one in fate, even for protons. On average the bunches cross about 30 million times per second, so collisions happen 1 billion times per second with a center-of-mass energy of 13 TeV.

图为ATLAS在2015年7月3日第一次j记录到的13TeV质子-质子碰撞

The first 13TeV proton-proton event recorded by ATLAS in 3 June 2015

ATLAS“触发”系统会作为重要数据的过滤器，将LHC 40 MHz的交叉事件率降低了4万倍，只选择需要的数据进行下一步分析*。

The ATLAS ‘trigger’ system, acting as a filter for the important data, reduces LHC bunch-crossing event rate of 40 MHz by 40 thousand so only the required data is selected for further analysis*.

天选之子 – 质子

The Chosen One – Protons

LHC只对同类的粒子束进行加速，这里选了是质子或者铅离子。正好它们都是强子，所以对撞机就叫了LHC。选质子的原因是粒子必须带电才能被电磁系统控制，并且处于稳定状态，而这就把选择范围缩小到了电子、质子、离子和它们的反粒子。
The LHC only accelerate beams of the particles of the same kind, either protons or lead ions are chosen here, which are hadrons, hence calling the collider the LHC. Reason being the particles have to be charged in order to be controlled by the electromagnetic system and in stable states, which limits the particle to electrons, protons, ions and their anti-particles.

在基本所有情况下，使用质子是因为它们的重量。由于它们的重量是电子的2000倍，因此在每次同步辐射时，它们能比更轻的粒子保持更多的能量。因此，要产生最高能量的碰撞，较重的粒子将是更好的选择。

For the majority of the times, protons are used because of their heavy mass. Being 2000 times more massive than electrons, they are able to maintain much more of their energy per turns of synchrotron radiation than lighter particles. Therefore, to have the highest energy collisions, heavier particles would be the better choice.

产生这些超高能量粒子，LHC每年的总功耗为600GWh，在2012年以4TeV运行的LHC最大功耗为650GWh。经过刚结束的这次大型维修保养后，耗能可能会提升到每年750 GWh *。

Producing these ultrahigh energy particles, the LHC has a total power consumption of 600GWh per year, with a maximum of 650GWh in 2012 for the LHC running at 4TeV. After the recent Long Shutdown 2 for renewal and maintenance, it likely to increase to 750 GWh per year*.

CERN的总能耗几乎是20万人口的日内瓦州能源产量的一半。

The total energy consumption of CERN is almost half of the energy production in Geneva canton, which has a population of 200 thousand.

图为CERN在2010-2017年的能耗，其中2013-2014因为大型维护降低了部分耗能，但再重新升级后，能耗增加了原本的20%。因此，在CERN不断的升级下，耗能会越来越高。

The energy consumption of CERN between 2010 and 2017, which inclues the Long Shutdown in 2013 to 2014. Though less used during it, energy consumption increased by 20% after. So as CERN upgrades more and more of its technology, its going to keep on inceasing the bill as well. 

所以，有一个问题 : 这么耗能耗钱地进行这样的实验真的值得吗? 新的科研发现基本都需要很长的时间和精力才有可能诞生，而且会有失败的几率并且无用武之地。那首先关注全球变暖这样关于生存的问题是否会更有价值?

So, here’s a question: is it really worth it to spend so much energy and money to run experiments like this? When new discovery takes so long to happens and may not be very useful. Would it more worthy to focus on problems like global warming first?

ATLAS 舞厅

The ATLAS Dance Floor

ATLAS探测器便是粒子相遇舞蹈的地方，主要由四个部分组成:内部探测器、量热计、介子谱仪和磁体系统，组建成一个直径25米、长度44米的圆柱体。当高能质子束穿过LHC到达时，它们会在ATLAS的中心碰撞，产生新的粒子，随后被不同的组件跟踪、引导和测量。
The ATLAS detector where the particles meet and dance, there are four main components to it: The Inner Detector, the Calorimeters, the Muon Spectrometer and the Magnet System. These components surround the beam forming a cylinder 25m in diameter and 44m in length. As high energy proton beams arrive from the LHC, they collide at the center of ATLAS producing new particles which are tracked, guided and measured by the various components.

图为ATLAS探测仪的内部结构，由整整7层互相包裹组成

The inner structure of the ATLAS detector, made of 7 layers wrapped around each other

这里将了解一下探测粒子最主要的部分：内部探测器和介子谱仪。

Here we are going focus on the most important parts for particle detection: the internal detector and the muon spectrometer.

内部探测器 Inner Detector

内部探测器(ID)是这些高能碰撞中产生的粒子所接触到的第一部件。它的主要功能是追踪从碰撞中产生的带电粒子。

The Inner Detector (ID) is the first part of ATLAS encountered by the particles produced in these high energy collisions. Its primary function is to track electrically charged particles as they emerge from the collision.

ID包含三个探测器子系统：像素探测器、半导体跟踪器和跃迁辐射跟踪器。虽然每个子系统都有专门的设计，但它们都通过诱导入射粒子的能量损失来跟踪粒子——主要是通过电离。

The ID contains three detector sub-systems: the Pixel Detector, the Semiconductor Tracker and the Transition Radiation Tracker. Although each of these sub-systems have a specialized design, they all track particles by inducing energy loss from the incident particle – primarily through ionization.

这三个系统结合在一起可以连续跟踪粒子的轨迹。整个ID被封装在一个与束轴平行的2T磁场中，由一个超薄的超导螺线管磁铁控制。它们对粒子的跟踪能找到每个粒子独一无二的动量，由此可以得到粒子们的身份。

The three systems combine for the continuous tracking of particle trajectories. The entire ID is housed within a 2T magnetic field parallel to the beam axis by a thin super-conducting solenoidal magnet. The system combination is able to find the momentum of the particles, which is unique to each type, and therefore their identities.

图为ATLAS探测仪的横切面示意图，其中最下端绿色和黑色部分就是最重要的监测区了。虽然体积相比之下较小，但至关重要

The cross section of the ATLAS detector, the green and black parts at the lower end of the picture are the detection region. Although smaller in volume compared to other parts, but crucial in experiments

像素探测器(PD)和半导体跟踪器(SCT)都是硅基固态电离探测器。

The Pixel Detector (PD) and Semiconductor Tracker (SCT) are both silicon-based, solid-state ionization detectors.

    作为半导体的硅有两种载流子：电子和“空穴”(正电荷载流子)。经过一系列的化学处理就可制成两种类型的半导体材料。n型材料有电子在原子晶格周围自由移动，而p型材料则有“洞”在晶格结构周围通过。

    As a semiconductor, silicon contains two types of charge carriers: electrons and ‘holes’ (positive charge carriers). After a number of chemical processes, two types of semiconducting materials are created. N-type has electrons move freely around the atomic lattice, while P-type material has the ‘holes’ passing around the lattice structure.

PD和SCT的它们的传感器就采用了这两种材料，创建了一个三层系统，两种材料各一层，然后在它们之间有一个耗竭区，用于两种材料中自由粒子形成中性原子。

The PD and SCT utilize both these materials in their sensors, creating a tripled-layered system, with a layer of each material and a depletion region between them for the formation neutral atoms from free particles in the materials.

当带电粒子穿过耗竭区时，它将使中性原子电离，产生电子或空穴对，然后分别向正极和负极移动。随后会有电流脉冲出现，表示带电粒子的产生。通过电子芯片读出后，这些信息就会被存为二进制的字符串了。

As a charged particle moves through the depletion region it will ionize the neutral atoms creating electron or hole pairs, which will move toward the positive and negative electrodes respectively. A current pulse will be measured indicating the presence of a charged particle, read out through electronic chips then stored as a binary string.

PD是ATLAS中最内层的粒子探测器，除了跟踪带电粒子，它还有提供重建次级顶点的关键功能。比如B-喷流，一种包含B强子衰变的喷流，的识别能帮助识别粒子。

The PD is the innermost particle detector on the ATLAS Experiment. Besides tracking charged particles it provides crucial functionality in the reconstruction of secondary vertices. A key example of this is in the identification of B-jets, which are jets containing the decay of a B hadron and used to help identify particles.

图为ATLAS探测仪内部探测器的结构图，是粒子碰撞后最初接触到的部分

The Inner Detector section of the ATLAS detector, which are first parts the particles encounter after their collisions

跃迁辐射跟踪器(TRT)是内部探测器三个子系统中最外层的一个。是一个由30万个漂移管组成的气体电离检测器。这些漂移管的直径为4毫米，每个中心都有一个31微米直径的镀金钨丝。

The Transition Radiation Tracker (TRT) is the outermost of the three sub-systems within the Inner Detector. The TRT is a gaseous ionization detector comprised of 300,000 drift tubes. These tubes are 4mm in diameter and at the center of each tube exists a gold-plated Tungsten wire of 31 μm diameter.

每个管壁会保持在一个负电压，而钨丝则保持在地电位，从而 产生一个穿过管道内部电场。每个管子都里有70%的氙气、27%的二氧化碳和3%的双原子氧的混合气体。*

The wall of each tube is kept at a negative voltage whilst the tungsten wire is kept at ground potential, inducing an electric field across the inside of the tube. Each tube is filled with a gas mixture of 70% Xenon, 27% Carbon Dioxide and 3% Diatomic Oxygen. *

当带电粒子通过漂移管时，它将通过与气体混合物中的原子电子的电磁作用来放弃能量。这一过程中会产生自由电子和离子对，然后漂移到阳极和阴极，在那产生可以被测量记录的电流。

As a charged particle moves through a drift tube it will deposit energy by interacting electromagnetically with atomic electrons in the gas mixture. This process produces free electron and ion pairs which will drift towards the anode and cathode, which will register a measurable current.

电流的大小将与入射粒子的能量和它产生的离子的数量成比例。由于电子移动所花费的时间，在发生电离的那一瞬间和测量到电流脉冲之间会有一个时间差。

The magnitude of the current will be proportional to the energy of the incident particle and the number of ions it produced. There will be a time lag between the moment of ionization and the measurement of the current pulse due to the time taken for the electrons to travel.

所以，知道了电子的漂移时间和漂移速度，就可以计算出粒子入射到管子上的角度和位置。通过一个粒子在多个漂移管的数据组合便可以追踪带电粒子的轨迹。

So, knowing this drift time and the drift velocity of the electrons allows for the calculation of the angle and position of the particle incident on the tube. Gathering all this data across each drift tube a particle passes through will track the trajectory of the charged particle.

介子光谱仪 Muon Spectrometer

ATLAS的最后一层是介子光谱仪(MS)。MS有4000个独立的腔室，总共5500 平方米，占据了ATLAS的大部分体积。它的主要部件是监测漂移管腔，专注于精确跟踪介子，介子是唯一能够通过整个探测器的带电粒子。
The final layer of ATLAS is the Muon Spectrometer (MS). With 4,000 individual chambers covering an area of 5,500 square meters, the MS occupies the majority of the volume of ATLAS. Its primary components are Monitored Drift Tube Chambers which are focused on the precision tracking of Muons, the only charged particles capable of passing through the entire detectors.

MS有超过35.4万根铝漂移管，每根直径为30毫米，中间有一根直径为50微米的阳极线。管内有93%的氩气和7%的二氧化碳的气体混合物。*

In total there are over 354,000 aluminum drift tubes, each 30mm in diameter with an anode wire 50μm in diameter down its centre. The tubes are filled with a gas mixture of 93% Argon and 7% Carbon Dioxide. *

MS和TRT的跟踪功能都基于同一原理。

The tracking functionalities of both the MS and TRT operate on the same principal.

图为ATLAS正面模型图，外部蓝色的管道为介子光谱仪，占据了整个监测仪器的大部分空间

The model of the front of ATLAS, the blue chambers on the outer layers are the muon spectrometer, which takes up the majority of ATLAS' total size

好啦，到这里

CERN、LHC 和ATLAS

应该都了解的差不多了吧！

感觉是不是很厉害很前卫呢？

那我们就一起期待它的下一个

振奋人心的大发现吧！

Now you know all this about 

CERN, LHC and ATLAS, 

isn’t it so magnificent  and cool? 

Let's look forward to its next exciting discovery!

图片和视频来自 CERN 官网

文中除*部分英文信息应用于CERN官网，

其余中英文内容为原创

Pictures and video from official website of CERN.

The * parts of English information was cited from official websites of CERN,

the rest of the Chinese and English content is original