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Posts Tagged ‘detector’

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…)



Friday, October 5th, 2012

Seth talking at the VERTEX2012 conferenceNever 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.

VERTEX 2012 Conference attendees at Sunrise Peak, JejuThe 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.


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.

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.

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 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.)

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!

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!"

"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?

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!


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!


What made those tracks?

Sunday, May 8th, 2011

Last post I discussed how we reconstruct tracks in LHCb. The next logical step is to talk about how we identify what sort of particle left which track. Continuing with my analogy about animal tracking, animals leave very distinctive tracks related to their paw prints and how they move. You can basically tell what animal left a track by examining it carefully…

The above image was taken from this webpage.

Unfortunately is is not possible with particle tracks. Only given the parameters of a reconstructed track, there is no way to determine what type of particle left that track. More information is required and that is where the RICH1 and RICH2 detectors come in.

The identity of a particle can be determined from its mass. The mass of a particle can be determined from its momentum and speed. The momentum of a charged particle is measured by its deflection in a magnetic field. The purpose of the RICH detectors is to match this information with a measurement of the particle’s speed.

RICH detectors work by measuring emissions of Cherenkov radiation. A charged particle traveling faster than the local speed of light in a medium emits Cherenkov radiation in the form of light, in a cone at an angle which depends on the speed of the particle. The RICH detectors focus the cone of Cherenkov light into a ring using mirrors onto an array of detectors. The radius of this ring provides information about the particle’s speed. Here are a few of the rings seen in RICH2 from an early LHC event.

The system of RICH detectors consists of an upstream detector (RICH1) which uses silica aerogel and \(C_{4}F_{10}\) gas as Cherenkov media located just behind the VELO, and a downstream detector (RICH2) using \(CF_{4}\) positioned after the magnet and tracking system. The use of silica aerogel allows the detector to identify low momentum particles (order of a few GeV), the use of \(C_{4}F_{10}\) allows the identification of higher momentum particles (between 10 GeV to around 65 GeV), while the use of \(CF_{4}\) allows the identification of even higher momentum particles (between 15 GeV to around 100 GeV).

Here is a schematic of the RICH1. Particles will enter the detector from the VELO on the left, then travel through the Cherenkov media, producing Cherenkov light which are reflected by the mirrors into the photon detectors. RICH2 is fairly similar.

The two RICH detectors are responsible for identifying a range of different particles that result from the decay of B mesons. Particle identification is crucial to reduce background in selected final states. For example, in the plots below, we are searching for the decay of a \(B_s\) meson into two \(K\) mesons. On the left, you can see that without the RICH it would be very hard to separate the signal, shown in red, from the backgrounds, since we would have no way of accurately differentiating \(K\) mesons from \(\phi\) mesons and \(\rho\) mesons. We would also have problems differentiating between \(B_s\) mesons and \(B_d\) mesons. On the right, using the RICH detectors, you can see that the signal is much much cleaner. They are very nice, useful detectors!


Are those tracks?

Sunday, May 1st, 2011

Today’s post is a continuation of my description of the LHCb detector. From my other post on identifying vertices using the VELO, we naturally progress to tracking charged particles. As I mentioned in my first post, the VELO along with the TT, T1, T2, and T3 stations are used to reconstruct particle tracks inside LHCb.

Particle tracking is somewhat akin to animal tracking. The first thing you need is some material where particle tracks will leave a trace. It is very hard to find animal tracks on concrete, but very easy on sand or snow…

This where the VELO along with the TT, T1, T2 and T3 come in. When charged particles pass though these detector components, they leave hits. Two different types of technology are used to measure particle interactions. The VELO, TT and the inner sections of the T stations are made of layers of silicon strips while the outer sections of the T stations consist of straw tubes filled with a mixture of argon and carbon dioxide gas. The layout of the TT and T stations is shown below. The silicon sections are coloured purple, while the drift tube sections are coloured blue.

Depending on which detector components register hits, tracks can be classified into four different groups:

  1. Long tracks which pass through all parts of the tracking system, from the VELO, through the TT to the outer T stations;
  2. Upstream tracks which only pass through the VELO and TT stations;
  3. Downstream tracks which only pass though the TT and T stations;
  4. VELO tracks which only pass through the VELO; and
  5. T tracks which only pass through the T stations.

Each of these track types is shown in the image below:

All of these types of tracks are useful for reconstructing B meson events. An example of a reconstructed event is displayed below. The average number of successfully reconstructed tracks in fully simulated B meson events is about 72, which are distributed among the track types as follows: 26 long tracks, 11 upstream tracks, 4 downstream tracks, 26 VELO tracks and 5 T tracks.

You may notice that in the images above, the tracks are curved. This is due to the LHCb dipole magnet. The experiment contains what essentially is a very large horseshoe magnet, which produces a field of 4 Tesla between its two large coils. 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.

So that’s how we measure particle tracks in LHCb and the types of tracks we record. Stay tuned for how we figure out what type of particle left which track…


VELO: it moves!

Sunday, April 17th, 2011

I’m not ashamed to admit that I spend a fair bit of time monitoring what the LHC is doing. Luckily there is a very good set of web pages that allow me to do this, the first of which Brian discusses in this post.

Below I’ve got stills of Page 1 and Page 3 when the beam is in Adjust and Stable modes.

LHC Page 1 - Adjust Mode
Adjust Mode
LHC Page 1 - Stable Beams ModeLHC Page 3 - Stable Beams Mode
Stable Mode

It’s a little hard to see, but I’ve highlighted particular sections of each page. First on Page 1 there’s a small entry titled Movable Devices Allowed In, while on Page 3 there’s a special entry for the LHCb VELO Position.

These two small entries in the LHC status pages are probably all that people outside LHCb know about the VELO, and even then the first entry isn’t an obvious reference.

So what is the VELO? And why does it move?

The VErtex LOcator is the part of the LHCb detector closest to the collisions at the LHC. As I mentioned in my previous post, its job is to measure particle tracks to precisely separate primary and secondary vertices and that this is important to identify B mesons and their decay products.

This is possible through the use of 42 silicon modules which during physics data taking are positioned as close as 5mm from the interaction point.

While it is important for the sensors to be close to the beam during physics data taking for optimal precision; at these small distances, there is a high probability that the modules will sustain damage due to radiation from the high energy proton beam.

To prevent damage, the silicon modules are mounted on custom made bellows, which allow the VELO to retract to the safer distance of 30mm from the beam during periods of beam instability.

Below is a schematic of the silicon modules from the side and from the front, in both open and closed positions, followed by photo of the actual modules during the assembly process.
VELO Schematic

Module assembly

The VELO is a very impressive feat of physics and engineering. It is not an easy task to design a set of silicon chips and associated services to operate so close to a high energy proton beam and its associated electromagnetic effects. And it works! Beautifully, as can be seen from this view of one of the first reconstructed B mesons…

I like to think it is because of Syracuse’s involvement in the subdetector, but Syracuse is just a spoke in the collaboration…

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