Though you are allowed to go down and see CMS on a guided tour, you have to go through a long process to be down there for any length of time or go near to the detector. This is because it is an extremely dangerous machine in an extremely dangerous environment.
CMS has a number of very fragile pieces, including the beam line. You wouldn’t think that a steel pipe, even though it is narrow, would be in danger of being damaged, but here’s the problem: the beam of particles in the LHC is so precisely aligned, that if the beam pipe gets shifted by a millimeter or less, that can cause particle in the beam to hit the pipe, which creates a lot of stray, unwanted radiation.
That square tube in the middle is a casing for the beam pipe. They really don’t want anyone to bump it on accident! Original picture.
In addition to being careful of the mechanical pieces, one must look out for high voltages and magnetic fields. There are voltages of over 10,000V present when CMS is turned on, which won’t necessarily kill you (but it could), but it would certainly hurt, regardless.
Original picture.
The 4 tesla magnetic field, when it is turned on, is enough to destroy phones, credit cards, etc. if you get too close.
And that’s only the detector itself! The collisions create lots of radiation, which contaminates the cavern. While the beam is on, it is enough to make your death extremely probable. When the beam is off, there is still residual radiation that gets absorbed by the material in the experimental cavern.
The LHC is cooled with liquid helium and nitrogen, so if there were ever a leak, you would suffocate because the liquid will turn into gas, which displaces the oxygen in the air.
That cloud on top is helium gas.
All of these dangers means that you need to be sufficiently trained to avoid or deal with them before you are allowed to work in the experimental hall. You have to take a handful of online courses going over a number of safety measures, as well as attend training sessions in person. I had to take a class to lean how to use a self-rescue mask in the case of oxygen deficiency. To pass the class, I had to put on my mask in under 40 seconds in a realistic setting inside a mock-up of the LHC tunnel. I also have to attend a full-day course on radiation safety. In the near future, I will be going to a class to learn how to operate various different kinds of fire extinguishers. Valuable life skills, right?
Yes, I got to look that dorky too.
It is not until you have passed all relevant classes that you can then obtain the necessary equipment to enter the cavern. You need a hard hat, steel-toed shoes, and your CERN ID card.
Original picture.
You also need your personal dosimeter. This is a device that tracks how much radiation you have received, to make sure you haven’t gotten too high of a dose.
Original picture.
Finally, you get a biometric scan, so that you can get through the security doors.
Original picture.
But it is quite a reward when you get to see all of it!
Hello everyone! I am so happy that you are interested in particle detectors, and CMS in particular. If you are very new, I would recommend reading up on particle physics and the LHC first.
Otherwise, if you’re ready, we can continue!
What is a particle detector?
A particle detector, on its most basic level, is exactly what it sounds like: it is a machine designed to detect high-energy particles which pass through it. Particles are far too small to ever actually see them with our eyes, so we need to use machines to alert us to their presence. The particles pass through material in the detector and interact with it. We then can observe the changes of the detector material, which gives us information about the particle itself.
The materials used in these detectors varies widely, because different materials interact differently with different particles and so can tell us what type of particle is passing through. Some detectors use gas, while others use liquids. Crystals (below, bottom) and semi-conductors (below, top) are also common materials.
Due to all these differences, it is very important to select the proper type of particle detector in order to achieve the goals of your experiment. Many projects may use multiple types within one experiment.
CMS, which stands for Compact Muon Solenoid (I’ll explain the name in a bit), is one of the four main particle detectors which runs at the LHC. The other are ATLAS, LHCb, and ALICE.
What does it do?
I have often heard the purpose of CMS compared to smashing a watch. Imagine you have a watch, and you would like to know how it works. First, you would likely observe the watch an see how it behaves over time. Then you might make predictions about how the watch will continue to act in the future. This is what theoretical physicists do. But if you really want to find out about how the watch works at the deepest level, you can shatter the watch and then examine the pieces that come out. By examining the pieces and seeing how they might have fit together, you can deduce how it works.
CMS does much the same thing, except that instead of watches, it uses particles. It detects all of the particles that come out of collisions at the LHC, so that we can learn about the fundamental constituents of the universe.
Some scientists don’t like that analogy, because we are really more interested in what the pieces of the watch can (and do) combine to make. One physicist puts it, “It is more as though we smash watches together in hopes that a cellphone will appear out of the collision energy.”
What does CMS mean?
So that sounds and looks like a huge machine! Why is it called compact? That is easily answered when it is compared to its sister experiment, ATLAS.
Photo credit to Piotr Traczyk.
As I mentioned in the previous section, some detectors are better at finding specific particles. The ‘M’ in CMS stands for muon, which is a type of particle that CMS is particularly good at finding.
Finally, the ‘S’ in CMS stands for solenoid, which is a particular type of magnet. Now this magnet is truly ridiculous. It is the largest super conducting magnet ever built, and weighs over 12,000 tonnes. It operates at a temperature that is one degree warmer than outer space, and produces a magnetic field of 4 Tesla, which is 100,000 times stronger than the earth’s magnetic field. It has to be built strong enough so that its own magnetic field does not rip it apart, so it contains almost twice as much iron as the Eiffel Tower!
The magnetic field curves the path of the particles that pass through it, and has a stronger effect on particles with more momentum. So based on the curvature of the particle’s path, we can deduce the particle’s momentum.
How was it built?
Construction of CMS actually began back in 1998, and it was ‘finished’ in 2008. Of course upgrades of the detector will never really be finished, so it is still under construction now, in a sense. The whole thing is 70 feet long and 52 feet tall. It weighs almost 14,000 tons! Over 3,000 scientists from about 200 institutions in 39 different countries collaborated to design and construct it.
Note the person in the bottom center, for scale.
One of the most difficult and expensive parts of the project was digging a cavern in the mountainside, so that CMS could be placed underground. Once the cavern had been built, CMS was lowered 300ft in to the ground, piece by piece.
The project cost was around $1 billion dollars. Now that sounds like a lot, but let’s put it in perspective: you could build two of these detectors for the same amount that the U.S. spends on one B-2 stealth bomber.
The layers of CMS
CMS is kind of like an onion – there are lots of layers! The goal is for no particle to pass through this series of layers without being detected. Since different detectors are good at seeing different things, many different layers are needed. They are, from the inside out: the tracker, electromagnetic calorimeter, hadronic calorimeter, and muon system. A schematic can be seen below. There are a large number of other, smaller subdetectors, but these are the main ones.
So here’s the tricky part: getting all of it hooked up! I have found that it is difficult to determine just how many cables went in to CMS. The best estimate that I can come up with comes from a technician who worked on the cabling of the muon system, which I am going to say is about one fifth of the whole detector. He calculated that there are approximately 1400 kilometers of cables in that section. That means that there are about 7500 kilometers, or approximately 4500 miles, of cables in the detector! When you see things like the picture below, you start to believe it…
You may notice that all these cables are held together with zip ties. Some quick math will tell us that a zip tie is placed about once every 30 centimeters (1 foot), but that it usually holds more than one cable. We can say this averages to one zip tie for every meter of cable. This leads to the realization that there must be around 7.5 million zip ties in the experiment! I once heard it said that the person who had made the single largest contribution to physics was the person who invented zip ties. Now I am starting to believe it!
Particles collide in the center of CMS 40 million times per second. This means that the electronics on the detectors have to be able to read out data once every 25 nanosecond (or 25 billionths of a second). That’s fast! Getting everything to work so quickly and so precisely is quite the challenge.
Usually, there is actually more than one collision at a time since it is actually bunches of protons being accelerated towards one another. So there are typically around 600 million ‘events’ happening per second. The raw data for one event is about one megabyte, so that comes out to be around 600 terabytes of data that the electronics have to read out every single second.
However much we would like to be able to save all of that data, it just isn’t possible. To cut down on the volume we use something called a trigger, which is basically a process which tells us which events are interesting enough to keep and analyze and which are not.
This process actually happens twice. The first time, the data is cut down from the 600 million events per second that the detectors pick up, to around 100,000 events per second that are actually reconstructed. Then, more advanced algorithms are used to pick out 100-200 interesting events per second. These events are then written to servers at the CERN Data Centre at a rate of 1 gigabyte per second.
All of this amounts to about 5 petabytes of data a year. To analyze all this data, a worldwide computing network is used, and then the data can be accessed by scientists all over the world.
So that’s about it! If you have any further questions, please let me know!
Hello everyone! Prepare yourselves for some knowledge.
This post, while originally intended to be the first part of ‘All About CERN‘, sort of took on a life of its own and became worthy of an entire post to itself!
Physics is a difficult subject, but I tried to make it approachable. So if you are new to particle physics, then this is a good place to start! I highly encourage you to follow some links, watch the videos, or maybe even do some research on your own!
I’ll start from the very beginning:
What is physics?
Physics is the science of matter and energy. It is also the science of forces (interactions between energy and matter) and motion (the result of a force). Four simple concepts.
To learn about these four concepts, we ask questions. We develop theories to answer these questions, and then test the theories through observation and experimentation. Through this process, our knowledge about the world around us grows, allowing us to ask deeper, more specific questions.
Most, if not all, people know that everything around us is made up of molecules. Further, molecules are made up of atoms, and atoms are made up of protons, neutrons, and electrons. Maybe less commonly known is that protons and neutrons are made up of things called quarks.
So far, we don’t think that quarks and electrons are made up of anything else – meaning that they are fundamental constituents of the universe.
There are some particles (specific types of quarks and heavier versions of the electron) that exist, but do not make up the matter we see around us because they decay quickly into lighter particles. There are still others that don’t make up matter because they do not interact with other particles very much (neutrinos). And finally, there are types of particles called gauge bosons, which also don’t make up matter, because they are responsible for carrying the forces through which particles interact. And remember, all these things are unimaginably small.
It occurs to me that many (if not most) of you are unfamiliar with the lab I am working at and what they are working on. I’m going to try to change that! If you aren’t usually one for a big dose of science, I ask that you stick with me. I am going to do my best to explain things in a way that everyone can understand. And it’ll be worth it, because scientists are doing completely mind-blowing things here.
If you are more curious and would like to learn more, I encourage you to follow some of the links I include! They will provide more detailed information. There is also a list of interesting links at the end, so you have that to look forward to.
CERN, or the European Organization for Nuclear Research, was founded in 1954. This multi-national endeavor is the world’s largest particle physics lab. It sits on the border between France and Switzerland, just outside of Geneva. It has about 2,500 permanent staff members, and annually hosts well over an additional 10,000 other visiting scientists and students.
CERN is in no way connected to military research of any kind. Any experiments that take place there pursue answers to questions in fundamental physics, and all records are 100% public.
Many of these experiments require high energy subatomic particles to study, so CERN maintains a large accelerator complex which accelerates different types of particles to supply to the various experiments, including protons, antiprotons, and lead ions. If you have heard the name CERN in the news over the past years, it is most likely due to the Large Hadron Collider (LHC) – the largest and most powerful particle accelerator and collider in the world.
What is the LHC?
Before going in to specifics on the LHC, I’ll address what exactly a particle accelerator is and why we need them.
You may not be aware, but only about 1% of particle accelerators world-wide are used for research. The other 99% are used for things like radiotherapy and industrial processing. Accelerators are used to scan cargo containers coming into the US for nuclear weapons. They are used to harden steel, toughen asphalt, sterilize food, kill cancer, seal chip bags, create computer chips… the list goes on. This amazing technology also allows us to delve down to the very deepest level of matter in order to study the universe around us.
Now particle colliders do exactly what you think they would – they collide particles. Very high energy particles. When these high energy particles smash in to each other, they actually come apart. The pieces then either decay or combine in to other particles. We then detect and study what comes out of these collisions in order to learn about the fundamental structure of the universe.
So why is the LHC such a big deal? After being switched on in 2008, it is the largest particle accelerator in the world and truly an amazing feat of physics, engineering, and technology.
This is what the beamline looks like, except underground.
It takes protons from regular old hydrogen gas, speeds them up to 99.9999991% the speed of light, and smashes them together. Here are some facts that I hope will blow your mind:
To put this in perspective, it takes about 20 minutes for me to drive from one side of the ring to the other, while it takes the protons .09 milliseconds. That’s crazy!
Here’s a great little video series put out by Fermilab, explaining things quite nicely:
The LHC is currently switched off for upgrades, and will be turned back on in late March. Last time, we discovered the Higgs boson lurking in the data we took. Who knows what will show up this time!
Why is any of this important?
I often get questioned about the usefulness of fundamental physics research. Many see it as so far removed from their everyday life – so far that they are unsure why they should care or why we should be funding these ventures. To those people, I usually have one of two responses.
First is the answer for the practical folks: that most of the technology developed in pursuit of particle physics has been seamlessly integrated in to their everyday lives, but they don’t even realize it. In order to do all of this big science, physicists have pushed the boundaries on modern technology and taken it further than ever before. Examples include superconducting wires to carry more electricity to homes, better ways to transmit and store data, more advanced computer and electrical chips, computing grids, advances in parallel processing and machine learning, the internet… Wait, what? That’s right! CERN invented the internet. So you have particle physics to thank for that.
The first ever proposal for the internet.
Second is the answer that I like better, and the reason that I am in particle physics. It is pure curiosity. We live in this amazing, mysterious, magnificent universe which holds so many secrets just waiting to be discovered. How can you not feel the allure of that knowledge just beyond your grasp? It is too much for me to stand idly by and let the universe run its course. I want to know how the world around us works, down to every last detail. I want to see more.
If I may make an analogy: It’s always nice to see a beautiful mountain off in the distance, but it’s even better to go and climb it.
So I hope I have at least piqued your interest in what is going on in the world of physics! If you have ANY questions or feel I have left something out, please contact me! But I have said enough for one post. Next time, I will talk about the CMS experiment: one of the large detectors at the LHC.
And that’s only the detector itself! The collisions create lots of radiation, which contaminates the cavern. While the beam is on, it is enough to make your death extremely probable. When the beam is off, there is still residual radiation that gets absorbed by the material in the experimental cavern.
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