## Posts Tagged ‘detector’

### A second chance at sight

Monday, February 17th, 2014

This article appeared in symmetry on February 4, 2014.

Silicon microstrip detectors, a staple in particle physics experiments, provide information that may be critical to restoring vision to some who lost it.

In 1995, physicist Alan Litke co-wrote a particularly prescient article for Scientific American about potential uses for an emerging technology called the silicon microstrip detector. With its unprecedented precision, this technology was already helping scientists search for the top quark and, Litke wrote, it could help discover the elusive Higgs boson. He further speculated that it could perhaps also begin to uncover some of the many mysteries of the brain.

As the article went to press, physicists at Fermilab announced the discovery of the top quark, using those very same silicon detectors. In 2012, the world celebrated the discovery of the Higgs boson, aided by silicon microstrip detectors at CERN. Now Litke’s third premonition is also coming true: His work with silicon microstrip detectors and slices of retinal tissue is leading to developments in neurobiology that are starting to help people with certain kinds of damage to their vision to see.

“The starting point and the motivation was fundamental physics,” says Litke, who splits his time between University of California, Santa Cruz, and CERN. “But once you have this wonderful technology, you can think about applying it to many other fields.”

Silicon microstrip detectors use a thin slab of silicon, implanted with an array of diode strips, to detect charged particles. As a particle passes through the silicon, a localized current is generated. This current can be detected on the nearby strips and measured with high spatial resolution and accuracy.

Litke and collaborators with expertise in, and inspiration from, the development of silicon microstrip detectors, fabricated two-dimensional arrays of microscopic electrodes to study the complex circuitry of the retina. In the experiments, a slice of retinal tissue is placed on top of one of the arrays. Then a movie—a variety of visual stimuli including flashing checkerboards and moving bars—is focused on the input neurons of the retina, and the electrical signals generated by hundreds of the retina’s output neurons are simultaneously recorded. This electrical activity is what would normally be sent as signals to the brain and translated into visual perceptions.

This process allowed Litke and his collaborators to help decipher the retina’s coded messages to the brain and to create a functional connectivity map of the retina, showing the strengths of connections between the input and output neurons. That in itself was important to neurobiology, but Litke wanted to take this research further, to not just record neural activity but also to stimulate it. Litke and his team designed a system in which they stimulate retinal and brain tissue with precise electrical signals and study the kinds of signals the tissue produces in response.

Such observations have led to an outpouring of new neurobiology and biomedical applications, including studies for the design of a retinal prosthesis, a device that can restore sight. In a disease like retinitis pigmentosa or age-related macular degeneration, the eye’s output system to the brain is fine, but the input system has degraded.

In one version of a retinal prosthesis, a patient could wear a small video camera—something similar to Google Glass. A small computer would process the collected images and generate a pattern of electrical signals that would, in turn, stimulate the retina’s output neurons. In this way, the pattern of electrical signals that a naturally functioning eye would create could be replicated. The studies with the stimulation/recording system are being carried out in collaboration with neurobiologist E. J. Chichilnisky (Salk Institute and Stanford University) and physicist Pawel Hottowy (AGH University of Science and Technology, Krakow). The interdisciplinary and international character of the research highlights its origins in high energy physics.

In another approach, the degraded input neurons—the neurons that convert light into electrical signals—are functionally replaced by a two-dimensional array of silicon photodiodes. Daniel Palanker, an associate professor at Stanford University, has been using Litke’s arrays, in collaboration with Alexander Sher, an assistant professor at UCSC, who completed his postdoctoral work with Litke, to study how a prosthesis of this type would interact with a retina. Palanker and Sher are also researching retinal plasticity and have discovered that, in patients whose eyes have been treated with lasers, which can cause scar tissue, healthy cells sometimes migrate into an area where cells have died.

“I’m not sure we would be able to get this kind of information without these arrays,” Palanker says. “We use them all the time. It’s absolutely brilliant technology.”

Litke’s physics-inspired technology is continuing to play a role in the development of neurobiology. In 2013, President Obama announced the BRAIN—Brain Research through Advancing Innovative Neurotechnologies—Initiative, with the aim of mapping the entire neural circuitry of the human brain. A Nature Methods paper laying out the initiative’s scientific priorities noted that “advances in the last decade have made it possible to measure neural activities in large ensembles of neurons,” citing Litke’s arrays.

“The technology has enabled initial studies that now have contributed to this BRAIN Initiative,” Litke says. “That comes from the Higgs boson. That’s an amazing chain.”

### Heat: Adventures in the World’s Fiery Places

Friday, February 8th, 2013

Heat: Adventures in the World's Fiery Places (Little Brown, 2013). If you haven't already fallen in love with the groundbreaking science that's taking place at RHIC, this book about all things hot is sure to ignite your passion.

Bill Streever, a biologist and best-selling author of Cold: Adventures in the World’s Frozen Places, has just published his second scientific survey, which takes place at the opposite end of the temperature spectrum. Heat: Adventures in the World’s Fiery Places features flames, firewalking, and notably, a journey into the heart of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.

I accompanied Streever for a full-day visit in July 2011 with physicist Barbara Jacak of Stony Brook University, then spokesperson of the PHENIX Collaboration at RHIC. The intrepid reporter (who’d already tagged along with woodland firefighters and walked across newly formed, still-hot volcanic lava—among other adventures described in the book) met with RHIC physicists at STAR and PHENIX, descended into the accelerator tunnel, and toured the refrigeration system that keeps RHIC’s magnets supercold. He also interviewed staff at the RHIC/ATLAS Computing Facility—who face the challenge of dissipating unwanted heat while accumulating and processing reams of RHIC data—as well as theorists and even climate scientists, all in a quest for understanding the ultrawarm.

The result is an enormously engaging, entertaining, and informative portrayal of heat in a wide range of settings, including the 7-trillion-degree “perfect” liquid quark-gluon plasma created at RHIC, and physicists’ pursuit of new knowledge about the fundamental forces and interactions of matter. But Streever’s book does more: It presents the compelling story of creating and measuring the world’s hottest temperature within the broader context of the Lab’s history, including its role as an induction center during both World Wars, and the breadth and depth of our current research—from atoms to energy and climate research, and even the Long Island Solar Farm.

“Brookhaven has become an IQ magnet, where smart people congregate to work on things that excite geniuses,” he writes.

Streever’s own passion for science comes across clearly throughout the book. But being at “the top of the thermometer” (the title of his final chapter, dedicated in part to describing RHIC) has its privileges. RHIC’s innermost beam pipes—at the hearts of its detectors, inside which head-on ion collisions create the highest temperature ever measured in a laboratory—have clearly left an impression:

“… I am forever enthralled by Brookhaven’s pipes. At the top of the thermometer, beyond any temperature that I could possibly imagine, those pipes explore conditions near the beginning of the universe … In my day-to-day life, bundled in a thick coat or standing before my woodstove or moving along a snow-covered trail, I find myself thinking of those pipes. And when I think of them, I remember that at the top of the thermometer lies matter with the audacity to behave as though it were absolutely cold, flowing like a perfect liquid…”

There’s more, a wonderful bit more that conveys the pure essence of science. But I don’t want to spoil it. Please read and share this book. The final word is awe.

The book is available for purchase through major online retailers and in stores.

-Karen McNulty Walsh, BNL Media & Communications Office

### A Change of Pace

Monday, February 4th, 2013

Some physicists and engineers from Purdue and DESY, and me, at the beamline we used to test new pixel designs

Every so often, a physicist needs a vacation from doing data analysis for the Higgs boson search. A working vacation, something that gets you a little closer to the actual detector you work on. So last week, I was at the DESY laboratory in Hamburg, Germany, helping a group of physicists and engineers study possible changes to the design of individual pixels in the CMS Pixel Detector. (I’ve written before about how a pixel detector works.) We were at DESY because they had an electron beam we could use, and we wanted to study how the new designs performed with actual particles passing through them. Of course, the new designs can’t be produced in large scale for a few years — but we do plan to run CMS for many, many years to come, and eventually we will need to upgrade and replace its pixel detector.

What do you actually do at a testbeam? You sit there as close to 24 hours a day as you can — in shifts, of course. You take data. You change which new design is in the beam, or you change the angle, or you change the conditions under which it’s running. Then you take more data. And you repeat for the entire week.

So do any of the new designs work better? We don’t know yet. It’s my job to install the software to analyze the data we took, and to help study the results, and I haven’t finished yet. And yes, even “working on the detector” involves analyzing data — so maybe it wasn’t so much of a vacation after all!

### Gluon Walls: A New Form of Matter?

Tuesday, January 8th, 2013

Theoretical physicist Raju Venugopalan

We sat down with Brookhaven theoretical physicist Raju Venugopalan for a conversation about “color glass condensate” and the structure of visible matter in the universe.

Q. We’ve heard a lot recently about a “new form of matter” possibly seen at the Large Hadron Collider (LHC) in Europe — a state of saturated gluons called “color glass condensate.” Brookhaven Lab, and you in particular, have a long history with this idea. Can you tell me a bit about that history?

A. The idea for the color glass condensate arose to help us understand heavy ion collisions at our own collider here at Brookhaven, the Relativistic Heavy Ion Collider (RHIC)—even before RHIC turned on in 2000, and long before the LHC was built. These machines are designed to look at the most fundamental constituents of matter and the forces through which they interact—the same kinds of studies that a century ago led to huge advances in our understanding of electrons and magnetism. Only now instead of studying the behavior of the electrons that surround atomic nuclei, we are probing the subatomic particles that make up the nuclei themselves, and studying how they interact via nature’s strongest force to “give shape” to the universe today.

We do that by colliding nuclei at very high energies to recreate the conditions of the early universe so we can study these particles and their interactions under the most extreme conditions. But when you collide two nuclei and produce matter at RHIC, and also at the LHC, you have to think about the matter that makes up the nuclei you are colliding. What is the structure of nuclei before they collide?

We all know the nuclei are made of protons and neutrons, and those are each made of quarks and gluons. There were hints in data from the HERA collider in Germany and other experiments that the number of gluons increases dramatically as you accelerate particles to high energy. Nuclear physics theorists predicted that the ions accelerated to near the speed of light at RHIC (and later at LHC) would reach an upper limit of gluon concentration—a state of gluon saturation we call color glass condensate.* The collision of these super-dense gluon force fields is what produces the matter at RHIC, so learning more about this state would help us understand how the matter is created in the collisions. The theory we developed to describe the color glass condensate also allowed us to make calculations and predictions we could test with experiments. (more…)

### VERTEX 2012

Friday, October 5th, 2012

Never mind my complaints about travel, VERTEX 2012 was a very nice conference. There were a lot of interesting people there, mostly much more expert than me on the subject of vertex detectors. (I’ve written before about how tracking works and how a pixel detector works. In general, a vertex detector is a high-precision tracker designed to measure exactly where tracks come from; a pixel detector is one type of vertex detector.) My talk was about the current operations of the CMS pixel detector; you can see me giving the talk at right, and the (very technical) slides are here. Other talks were about future development in on-detector chip and sensor technology; this work is likely to affect the next detectors we build, and the upgrades of our current detectors as well.

The location of the conference — Jeju, Korea — was also very nice, and we got an afternoon off to see some of the island. The whole island is volcanic. The central mountain dominates the landscape, and there are lots of grass-covered craters. Sunrise peak, at left, erupted as recently as 5,000 years ago, but it seemed pretty quiet when we were there.

Overall, the conference was a great opportunity to meet people from all over the world and learn from them. And that’s really why we have to travel so far for these things, because good people work everywhere.

### Cleaning the world’s biggest machine

Tuesday, March 6th, 2012

Today I spent much of my time crawling around on hands and knees, picking pieces of rubbish from the innards of the ATLAS detector. It’s just one of those things that comes with the job and gives you a different view of the experiment (literally.) Before we start taking data we need to make sure that the ATLAS cavern is clean and safe. I call this process “Grooming the Beast”.

We started our shift with a briefing in the ATLAS Control Room.

The ATLAS detector is housed in the ALTAS cavern, just behind the Globe at CERN. The journey down is long (more than 100 meters) and convoluted, with all kinds of doorways, locks, passages and elevators. Work has been taking place in the cavern during the winter shutdown to make improvements and sort out minor problems with the detector. Is a piece of the hardware getting damaged by interactions with matter? This is an excellent time to replace it!

Some of the team survey the work ahead of them.

Cleaning the cavern just as people start to leave it may seem like an unusual thing to do, but it serves a very important purpose. There has been a lot of work to improve the detector during the shutdown, and this leaves some debris. The engineers clear up as much as they can as they go along, but the odd screw or piece of wire goes missing, and over the months this builds up. The real danger to the machine is metal debris. The detector contains large magnets and these can interact with metallic objects lying around. They need to be removed before we turn on and take data!

The problem with photographing ATLAS is that it's just too big and the cavern is too small.

The cleaning also serves a milestone in the life of the experiment. It serves as a reminder that the shutdown is over, the repairs are complete and that we need to look forward to the new data that’s going to arrive. It’s no coincidence that at the same time as we clean the cavern, we present our work at the Moriond conferences. (These two weeks are going to be my favorite of this year! So many interesting results, and getting the chance to poke around inside a large detector.)

We got treated to a coffee break halfway through! (Note the security guard preventing use of the "Goods In" entrance, and the retinal scanners in the green "Pedestrians" entrance.)

As you would expect, health and safety are very important in this process. To get access to the cavern I had to pass 4 levels of safety training, get a dosimeter to monitor radioactive dose, a hardhat with a light, and hard boots. In addition we had to register our names and phone numbers in case anything happened while we were down there. There is an elaborate key system in place as well, which is mainly for safety. We each take a key as we enter, and the beams cannot pass through the cavern until every key is returned. We each get a pass (either on our CERN ID cards or a magnetic key fob) that is linked to our names and dosimeters, so that if a key is not returned we know who is still in the cavern and when they entered. Safety isn’t the only concern though, these systems have the added advantage of protecting the machinery. Everyone who goes down to the cavern has to have safety training and the correct permission, which significantly improves the quality of all the work down there. If we knew we could just pop along at any time to fix a minor problem there would be people down in the cavern all the time!

Armed with a proton pack (I mean a vacuum cleaner) I take on the dirt and debris of the ATLAS cavern!

Most of the actual work involved picking pieces up off the floor and cleaning the areas that others can’t reach. It seems simple, but the shape and size of the detector make it very difficult. Balancing on one foot on a low friction floor as you lean into a crevice to see if that wire is actually attached to anything, while wearing a hard hat that makes your head bigger than usual is exactly as difficult as it sounds! For bonus points you can do this in a dark space with a special tool for grabbing objects in places too small for your hands to reach. Some of the “treasure” I found included a drill bit, a box of screws and tubes (“It’s good, but it’s not the Higgs”), a guide to some important looking apparatus, and some rusted metal in a box of rusty water. When faced with those objects it’s not always obvious what to do. Is it trash? Is it safe to move? Is somebody missing it?

"How can we make the ATLAS cavern even cooler than it already is?" "Put a scorpion like crane in it, of course!"

This is the first time I’ve seen the ATLAS detector in person and it’s impressive. But on the other hand, I couldn’t relate to it very easily. I saw some piece of the toroid and some piece of the muon system, but from the outside it looked like an amorphous chunk of wires and pipes. My officemate, Julia, pointed out the muon systems, and showed me the sensors they used to calibrate their position. In addition to this, they also have geodetic instrumentation in the cavern so that they can work out the position of any part of the detector. That’s some neat hardware to have 100 meters underground! I’d hoped to be able to recognize a lot more of the detector, but it’s just not that kind of experiment. It’s far too huge to appreciate in a single day.

Is this ATLAS? Or a space age submarine? Or the best clubhouse in the world?

Towards the end of the day my friend Katie showed me the inside of the detector. At the bottom of the detector there’s a crawl space (mind your head on the muon system!) into a series of chambers and tunnels. These give access to some instruments and hardware so that we make some changes or repairs, and through an intricate set of ladders and tunnels you can actually get quite far into the outer parts of the detector. It’s warm down there, and you can see parts of the famous toroid, which is neat. It was around this time that Katie suggested that it would make the best clubhouse in the world. I can see myself enjoying somewhere like that as a 12 year old boy! (Actually, I can see my self enjoying it as an adult, but that’s just how amazing it looks when you’re inside.)

Right now I’m exhausted, and my throat is a little dry, but it’s been an awesome day and I’m glad to have the chance to share some photos of the ATLAS cavern with you!

### Which is the best LHC detector…

Monday, August 8th, 2011

# for B physics?

The title of today’s post is obviously a rhetorical question, because the answer is naturally LHCb. *winks* However I thought I would take some time to prove it with a particular $$B$$ meson decay…

One of the most interesting $$B_s^0$$ meson decays is that into a $$J/\psi$$ and a $$\phi$$ meson, shown below. This is because one of the quantities we can derive from this decay has a very small Standard Model prediction, so any measured excess would be a clear indication of new physics.[*]

This decay mode is so interesting that both ATLAS and CMS as well as LHCb are trying to detect it. Hence giving me the opportunity to directly compare the performance of the detectors. So without further ado, here are the results:So what are we looking at here? These are the invariant mass distributions of the identified $$B_s^0 \rightarrow J/\psi + \phi$$ decays in each detector. In every event, we look for the products of the particular decay we are interested in. In this case, we need to identify two muons from the decay of the $$J/\psi$$ and two kaons from the decay of the $$\phi$$. We then take these four particles and add their four-momenta together, if they did originate from the decay of a $$B_s^0$$ meson, we should see a peak around the $$B_s^0$$ mass of 5366.3 MeV / c$$^2$$. This is represented by the data points in the three plots from each of the experiments. The lines on each of the graphs are fits to the data using a normal distribution for the signal and a straight line for the background. [**]

So what do we look for in these graphs to learn about the performance of each detector? Actually, before we do any comparisons, we need to look the size of the datasets used in each analyses. Luckily for us, the datasets are fairly similar, with LHCb reporting results using 36 pb$$^{-1}$$ of data, CMS using 39 pb$$^{-1}$$ and ATLAS using 40 pb$$^{-1}$$. This means we can basically do a direct comparison of the graphs, though with the caveat that each of the analyses used different selection criteria to select their $$B_s^0$$ candidates. However, we can assume that they have been optimised to select as much signal as possible while rejecting as background as possible.

Okay, now we have established we can compare the graphs, let’s do so. The first thing you might notice is that the graphs look fairly similar. Each experiment has been able to reconstruct a nice $$B_s^0$$ peak from its decay products. Looking closer however, the results have some notable differences, despite each of the experiments looking for the same decay in very similar sized datasets and using the same signal and background distribution shapes.

I’m emphasising the fact that the datasets are similar sizes because you may notice that the number of signal events is fairly different between the three experiments, with 877 events in the signal peak for LHCb, while ATLAS and CMS only see 358 and 377 events respectively. This may not be immediately obvious looking at the height of the signal peaks, but if you notice that each experiment uses different mass binning, it becomes clearer.

So LHCb sees more $$B_s^0 \rightarrow J/\psi + \phi$$ decays than ATLAS and CMS. This is actually expected from the geometry of the detectors. As I mention in my very first post, $$B$$ meson production peaks in the forward region, shown below, where LHCb has coverage while ATLAS and CMS don’t.

Interesting, even though LHCb sees more signal events than ATLAS and CMS, it sees many less background events. This can be seen in the plots above by see how high above 0 the linear background fit is. We can see that LHCb sees less background then ATLAS, which see less background than CMS. The reason for this is that LHCb is much better at identifying kaons and muons at these energies thanks to the RICH subdetectors.

What else can we learn? If we look at the width of the signal fits of the $$B_s^0$$ mass peaks from each experiment, we can see that these are also quite different. The LHCb peak is very narrow at 7 MeV, while the CMS peak is a little wider at 16 MeV and the ATLAS peak is wider again at 27 MeV. These numbers tell us how accurately the momenta of the kaons and muons are measured, and how well the $$B_s^0$$ decay vertices are reconstructed. So we see that LHCb is better at measuring the kaon and muon momenta and reconstructing displaced decay vertices.

In summary, LHCb sees more signal, less background and better at measuring the particles involved in $$B_s^0 \rightarrow J/\psi + \phi$$ decays compared to CMS and ATLAS. It is therefore clearly the best detector to use for these types of decays. An obvious conclusion, since these decays are what the detector was designed and built to measure, but it is nontheless reassuring to see that the results confirm our hypothesis.

[*] I know that this really isn’t a satisfactory explanation of why this particular decay is interesting, but I didn’t want to get too sidetracked here. I’ll save the details for a future post. This one is long enough already!

[**] I have obviously simplified the selection and analysis process immensely. If you do want to find out more information about each of the analyses, and where I got the graphs and numbers, details can be found here for LHCb, here for ATLAS and here for CMS.

### Don’t Stop Me Now…

Friday, July 29th, 2011

Today I’m going to describe the last, but definitely not least LHCb subdetector, the muon subsystem, which unsurprisingly from the name, is designed to detect muons. Just in case you’ve all forgotten what the LHCb detector looks like, I’ve included a schematic below. The muon subsystem is the rightmost one, with alternating layers of light and dark green.

So why is a completely separate subsystem required to detect muons on top of the previously described vertex location, tracking, particle identification and calorimeter subsystems?

It all comes down to how muons interact with matter. In my last post, I said that the goal of the LHCb calorimeter subsystem is to stop particles in the detector and measure how much energy is produced through interactions with the detector material. However, I left out the important fact that different particles interact differently with detector material. In particular, muons pass through the calorimeters almost without any energy loss. Flip has a very nice explanation about why in this post, where he compares electron interactions to muon interactions… which he hopefully won’t mind if I borrow…

Electrons are light, so let’s imagine that they’re ping pong balls. On the other hand, muons are heavy, so let’s imagine them as bowling balls. As you probably know, the LHC detectors are big and full of stuff… by that I mean atoms, which in turn are made up of a nucleus and a cloud of electrons. We can thus imagine a sea of ping pong balls (kind of like an IKEA ball pit). When electrons hit this ball pit, they end up distributing all of their energy into the other balls. Muons on the other hand, are so massive that they just barrel straight through the ball pit to reach the other side.

Why go to all this effort just to detect muons?

Apart from muons being the only particle you can make farm jokes about, the fact that muons are the only known particles which the calorimeters don’t stop is quite useful. It means that if any signals are seen in a detector that is located behind the calorimeters, they must originate from a muon. This makes searching for decays involving muons much simpler than searching for decays involving other particles, such as electrons. An example of such a decay is the rare $$B_s \rightarrow\mu\mu$$ decay which may reveal new physics, as discussed previous by both Ken and Flip.

So how does LHCb detect muons?

The muon subsystem comprises five rectangular ‘stations’, gradually increasing in size and covering a combined area of 435 square metres. Each station contains chambers filled with a combination of three gases – carbon dioxide, argon, and tetrafluoromethane. The passing muons react with this mixture, and wire electrodes detect the results. In total, the muon subsystem contains around 1,400 chambers and some 2.5 million wires.

Here is a nice photo taken between two of the stations…

So now you know all about the LHCb detector, you should be able to understand the following event display of a $$B_s \rightarrow\mu\mu$$ event. If not, don’t fear, because there’s a very good explanation here.

And that ends my series of posts describing the LHCb detector… I hope you all enjoyed reading them as much as I enjoyed writing them.

### Stop right there, particle!

Tuesday, July 26th, 2011

Looking back over my previous posts, I noticed that I forgot to describe the calorimeter and muon systems before jumping straight to the trigger. The subject of today’s post will thus be the calorimeters and my next post will probably be about the muon system.

So what is a calorimeter? I vaguely remember that in high school chemistry, we performed a calorimetry experiment to measure the energy change in a chemical reaction by measuring the heat released (for those who are enjoy their etymology, calorimeter derives from the Latin word, calor, which means heat).

It is slightly different in particle physics, where the main function of the calorimeter detector subsystems is to measure the energy of produced particles. The materials and techniques vary, however the basic principle of all calorimeter systems is the same: to stop particles in the detector and measure how much energy is produced through interactions with the detector material.

On my very first post, I mentioned that LHCb contains two calorimeters; the electromagnetic calorimeter is responsible for measuring the energy of electrons and photons, while the hadron calorimeter samples the energy of protons, neutrons and other particles containing quarks. The calorimeters provide the main way of identifying particles that possess no electrical charge, such as photons and neutrons.

Both calorimeters have a sandwich-like structure, with alternating layers of metal and plastic plates. The metal plates are to stop particles, while the plastic plates are to measure the energy released. More technically, when particles hit the metal plates, they produce showers of secondary particles. These, in turn, excite molecules within the plastic plates, emitting ultraviolet light, which is then guided to photomultiplier detectors. The amount of light produced is proportional to the energy of the particles entering the calorimeter.

Above is a photo of the two calorimeters, the one labeled with LHCb ECAL is unsurprisingly the electromagnetic one, while the hadronic one is behind it. It is a little hard to get a sense of scale from the photo, but the electromagnetic calorimeter wall is approximately 6.3 metres by 7.8 metres and 0.5 metres thick, while the hadronic calorimeter wall is around 8.4 metres by 6.8 metres and 1.7 metres thick.

I think that’s all I have to say about the LHCb calorimeters, except to leave you all with this random fact. The specific design of the electromagnetic calorimeter, its alternate layers of scintillator and lead, readout by plastic fibres which run parallel to the plates, is called shashlik, which is also a type of shish kebab… mmm…

### Magnets magnets everywhere…

Tuesday, May 17th, 2011

In my previous posts, I’ve mentioned that LHCb contains a dipole magnet to help measure particle momenta. Particles normally travel in straight lines, but in a magnetic field the paths of charged particles curve, with positive and negative particles moving in opposite directions.
By examining the curvature of the path, it is possible to calculate the momentum of a particle. Particles with greater momentum bend less than those lesser momentum. This is because a particle with greater momentum will spend less time in the magnetic field and thus be affected less.Note that both the above images are from The Particle Adventure which is a fantastic website to learn the basics of particle physics.

Here is an image of the LHCb dipole magnet with some people for size comparison. The magnet is essentially a very large horseshoe magnet with the upper coil being one polarity (N for example) and the lower coil the opposite (S) where the magnetic field is the strongest between them.

Of the major LHC experiments (ALICE, ATLAS, CMS and LHCb), the only other that uses a dipole magnet to bend charged tracks is ALICE. Actually, ALICE has quite an interesting magnet configuration, which you can see from the following schematic. There is a large solenoid magnet (coloured red which was used in the L3 experiment at LEP) in the central region, and a dipole magnet on the single arm forward muon detector.

What’s a solenoid magnet? Here is a schematic of one from physics animations. This type of coil configuration generates a nearly uniform field inside the windings and a comparably weak and divergent field outside. It is the preferred type of magnet within cylindrically symmetric detectors. In fact both ATLAS and CMS use solenoid magnets.

Actually, solenoid is part of the CMS acronym, Compact Muon Solenoid. At 12.9 meters long with an inner radius of 2.95 metres, an outer radius of 3.8 metres, and weighting 12,000 tonnes, the experiment contains the world’s largest superconducting solenoid magnet.

ATLAS on the other hand is named after its other magnet system, A Toroidal LHC ApparatuS. One of the key design features of the ATLAS detector is the unique hybrid superconducting magnet system. It is an arrangement of a central solenoid surrounded by a system of three large air-core toroids, measuring 26 m in length and 20 m in diameter.

What’s a toroidal magnet field? Again, thanks to physics animations, here is a schematic. As you can see, coils are oriented so that the magnetic field goes around in a doughnut type shape. (Toroidal is just a fancy mathematical description for a doughnut shaped object.)

Of course, the LHC experiments aren’t the only uses of magnets at the LHC. The accelerator system is full of them. Dipole magnets are used to steer the protons around the ring, quadrupole magnets are used to focus the beam, sextupole magnets are used to correct chromaticity, octupole magnets are used to correct field errors. What are all these magnets? Here is a nice diagram from some lecture slides I found. As you can see, they are named after the number of magnetic poles they contain (2, 4, 6 and 8, the n in the diagram counts the number of pole pairs). The arrows on the diagram shows the direction of the magnetic fields that each of the magnets produces.

In terms of beam acceleration, dipoles and quadruples are the most important. In fact, if you look carefully at the Fermilab logo below, you might notice the superposition of a quadrupole and a dipole magnet…

I could continue, but this post is getting very long. I hope this very brief introduction illustrates how important magnets are in experimental particle physics. Personally, I wish I had known this when I was taking electromagnetism and electrodynamics in my undergraduate studies, which were actually my worse physics subjects in terms of marks. I really should have paid more attention!