Times are changing at CERN. With the advent of the LHC the focus of the work has shifted towards the discoveries that will shape the coming decades of particle physics, including the search for the Higgs boson, understanding dark matter, and supersymmetry. To keep up with the changes CERN’s outreach program has evolved to include a new “Universe of Particles” exhibit, in the famous CERN globe. This will complement the role of CERN’s main visitor attraction, the Microcosm, which is to be transformed into a teaching aid. Shortly before this big announcement, Tingting, Steve and I headed to the Microcosm to see what we would find.
The Microcosm itself is huge, and its topics are diverse. At the entrance the visitors are given a short history of the site, and how CERN came to be the way it is today. For me, this is the most interesting and informative part of the experience. Between abstract lectures on quantum field theory, development of CPU intensive simulation, and hands-on hardware experience, we can gain most of the skills we need to be skilled physicists, but this is no substitute for experience. When we look to the future at what we can discover, and what we can expect to achieve (given that our funding must come from somewhere) we must look to the past to see how things were done then. It seems that CERN’s greatest hits came in the early 1980s when the massive gauge bosons were discovered. This confirmed one of the greatest predictions of the Standard Model and went a long way to completing our current view of the universe on the smallest scales. It’s easy in hindsight to see when and how that kind of discovery would take place, safe in the knowledge that we have a fairly good understanding of massive gauge bosons, but we must not become complacent. At the same time there was a long and diverse list of other models which didn’t fit the data, from technicolor to supersymmetry. In this respect, things haven’t changed a great deal in 25 years. The Standard Model predicts the Higgs boson, but there are at least a dozen other models of new physics, and nobody knows what the LHC will see. It’s an exciting time to be a physicist, but also time to plan for the future. When we do see new physics, what happens then? How are we going to repeat the success stories of the past and get even more impressive machines?
The next displays showcase the fundamental aspects of particle physics. Visitors are taken step by step from the scale of the visible universe, right down to the smallest scales we can probe. Then they are introduced to the four forces which apparently govern everything around us. None of this is particularly exciting to a particle physicist- it’s old news to us and it’s just the tip of the iceberg, a hint of something much more fascinating and rich. The next part of the Microcosm is perhaps the most exciting, as visitors get a chance to reproduce the experiments of Thomson, who discovered the electron, and Rutherford who discovered the nucleus. These experiments are remarkably simple. Take a beam of electron and pass them through electric and magnetic fields. Adjust the fields carefully so that the forces cancel out exactly and you find you have a straight beam where the charges and masses of the constituents are uniform. In other words, you have just “discovered” the lightest fundamental charged particle, the electron! In a similar way you can “discover” the nucleus in a scattering experiment. There is a radiation source which bombards a target, and by moving the target around at different angles you can see how the interaction rates change. Sure enough, there are a small but significant number of interactions at large angles, which is typical of a very small, very dense target, leading to the “discovery” of the nucleus!
However, most people’s favorite part of the Microcosm is the muon spark chamber exhibit. As high energy protons from outer space hit the upper atmosphere they generate huge numbers of particle, known as cosmic rays. Most of these don’t get very far. They either decay very quickly, or get slowed down or absorbed by the particles in the atmosphere. However muons do not tend to lose much energy. Left to their own devices they survive for around 2 millionths of a second, which is an impressively long time for a particle physicist. The spark chambers interact with these incoming muons, leaving tracks of lightning where the muon passes through. It’s a dramatic display of a phenomenon that has been known and understood for over half a century. However, when cosmic rays were first seen they were rather puzzling. First of all the particles that were observed were not quite the same as anything seen before. They looked just like heavy electrons (which is all a muon is.) Secondly, these muons seems to last a longer in flight than they did in the lab. This was strong evidence in favor of Einstein’s theory of special relativity. Steve and I decided this was an excellent opportunity for a discussion with Tingting and we spent 10 minutes discussing the muons and their path to Earth. Being able to use knowledge obtained in the classroom and the lab and applying it to real life situations helps clarify our thoughts, and makes it easier to appreciate how powerful our ideas can be.
The Microcosm continues to ask some of the most fundamental questions about the universe, leaning on the knowledge of world experts in the field. Questions include “Why is there more matter than antimatter?”, “What are neutrinos and why do they mix?” and “Where does mass come from? Why are there massive things, rather than just energy?”. These are questions that drive us as physicists, and we hope that our discoveries at the LHC will help to answer these questions, or at the very least, they will tell us where we need to look to get a definitive answer. These are fascinating questions, but they are a topic for another blog post.
The rest of the Microcosm is dedicated to the hardware that helps us to gather and analyze the data. There are plans and models of the LHC and the detectors, as well real parts of old experiments (including the apparatus that discovered the massive gauge bosons.) It’s a chance for visitors to get to grips with how the experiments are performed. These experiments require a huge amount of expertise to operate properly, from the second the beams are generated from hydrogen gas, through the acceleration and cooling processes, to the collisions, triggering, data acquisition and analysis. One of the highlights includes a life-size model of the LHC tunnel, where the magnets are shown in all their glory. The scale is impressive, and visitors are invited to imagine how much apparatus would fit in a tunnel 27km long.
The data acquisition and analysis demands of CERN have lead to huge breakthroughs in computing over the past decades. It is no secret that the world wide web was created at CERN. Even now we are making new ground with the GRID system. The GRID is a globally distributed computing network, giving analysts access to vast banks of computing power and disk space. In an experiment with a few hundred tracks per event, and thousands of events per minute there is an urgent need for large farms of CPUs to filter out the junk, keep the interesting data, and to process the data. This is why Steve and I dedicate so much of our working time to the trigger system on ATLAS.
Towards the end of the Microcosm the displays focus mainly on the hardware. There are excellent examples of bubble chamber photos, which show paths of particles as they pass through fluid, multiwire proportional chambers, which are used to track the paths of particles, quadrupole magnets, which are used to steer the beams, and electromagnetic calorimeters, which are used to measure energy depositions of ionizing particles. However, as new technologies become available, the hardware on show becomes more dated and less relevant. ATLAS uses state of the art pixel detector technology, but even now some of the other hardware is starting to look outdated. Already there are plans to upgrade large parts of the detector to improve performance. These upgrades are just around the corner and it leads me to consider what we might see in the Microcosm a few years from now. Will CERN continue to put fossils from our most impressive experiments in the Microcosm? If I visit in 20 years time could I expect to see some ATLAS old hardware on display? It’s both sad and reassuring to think that eventually this experiment will be an old (and perhaps fond) memory for thousands of physicists, as they embark on the next period of discovery.
It will be interesting to see how to the Microcosm changes over the next few years. At the moment it seems to cover so many aspects of the work at CERN that it can be a little overwhelming. The Microcosm looks at so many areas of research that none of them gets the full attention it deserves. To remove just one aspect would be to tell an incomplete story, but to add nothing would mean to leave the whole picture incomplete. Hopefully, the new exhibit in the globe will take some of the ideas presented in the Microcosm and explain them in a better, more up to date manner, allowing what is left of the Microcosm to blossom into something more substantial. The displays are high quality, and provide a lot of information, but there is certainly scope for improvement, and we still have a great deal to learn from past experiences.
The Microcosm website has more information about what is available and opening times. The Universe of Particles press release describes the new exhibition in the Globe, and gives tentative hints about the future of the Microcosm.