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Posts Tagged ‘pixel’


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.


Location, Location, Location

Thursday, January 19th, 2012

If I had to pick one thing that’s definitely better on my old experiment, ATLAS, than on my new experiment, CMS — and especially if I had to pick something I could write publicly without getting into trouble — it would be this: the ATLAS detector is across the street from the rest of CERN. I’m not sure how that was decided, but once you know that, you know where CMS has to be: on the other side of the ring, 5 or 6 miles away. That’s because the detectors have the same goals and need the same beam conditions; two opposite points on the LHC are where a duplicate performance is easiest. The pre-existing caverns from the LEP collider, whose tunnel the LHC now uses, probably also helped determine where the detectors are.

In any case, it used to be that when I wanted to work on my detector, I had only to go across the street. Now I have to drive out of Switzerland and several miles into France. Except, I don’t like driving. So I’ve been working on alternate means of transportation. A few months ago I walked. Last night I had to go to downtown Geneva, so I took the bus. It’s actually pretty good, although the bus stop is a mile away from CMS. There’s also the shift shuttle, which runs from the main CERN site to CMS every 8 hours via a rather roundabout route. And I can bike, once the weather gets better and I get myself a little more road-worthy. To be honest, every option for getting here is much slower than driving, but I enjoy figuring out ways to get places enough that I’m going to keep trying for a while.

I have plenty of chances to try, because I’ll be here in the CMS control room a lot of the time over the next few weeks. Right now, I’m learning and helping with the pixel detector calibration effort. (We’re changing the operating temperature, so all the settings have to be checked.) Soon I’ll be learning to take on-call shifts. So the more I stay here, the more I learn. I got here this morning, and I won’t leave tonight until about 11 pm. I could take the shift shuttle back — or maybe I’ll just get a ride.


How Tracking Works

Tuesday, November 25th, 2008

Author’s note: I didn’t mean for this to end up so complicated that it had equations, figures, and footnotes, but that’s how it turned out. I do apologize for the inconvenience, and if it’s any compensation I can assure you that about half the footnotes are funny.

I’ve written before about how a pixel detector works, but at the time I left as a “topic for another day” the broader question of what a pixel detector is for.  I’m going to answer one part of that question today, and discuss the tracking system, of which a pixel detector is one possible component.1 I’ll have to leave the question of the specific advantages of using pixels, as opposed to other tracking technologies, for another other day.

Regardless of the technology used, the basic idea of a tracker is to put together a bunch of stuff that measures the path a charged particle has taken.   The “stuff” could be silicon, in which electron-hole pairs are separated as the charged particle passes through, and can be used to produce a current, as I explained in my pixel detector entry.  It could also be gas, in which case electron-ion pairs are separated and produce a current in wires; this is the technology used in the ATLAS Transition Radiation Tracker.  If you want to “track” a baseball through the stands, the “stuff” is people: even if you can’t see the baseball in the crowd on other side of the stadium, you can see where it’s gone by who stands up or jumps down and starts grabbing under the seats.  An individual jumping person, or silicon pixel producing a current, is what we call a hit.

Our primary interest actually isn’t in how particles move through the detector, even though that’s what we directly measure.   So let me take a step back now and describe what we are measuring, first and foremost: momentum.

Momentum: What It’s Really All About

The best way I can think of to describe momentum in a few words is to quote Newton and call it the “quantity of motion.”2 It reflects not just the speed and direction (i.e. velocity) of an object, but also the amount of stuff (i.e. mass) that makes up that object.  In ordinary life, if you double the mass then you double the momentum, and if you double the velocity you get double the momentum too; in other words:

  • p = mv

where m is the mass, v is the velocity, and p is the momentum.3 Unfortunately, things get a little more complicated when the particle goes really fast, which they usually do in our detectors; then the equation doesn’t work anymore.  We’ll get to one that does in a minute.

Momentum intuitively seems the same as energy of motion, but technically the ideas aren’t exactly the same, and it just so happens that the difference is important to how the LHC detectors work.  One way to think of the energy of a particle is as follows: if you slammed the particle into a big block of metal and then extracted all the ensuing vibrations of the metal’s atoms4 and put them in a usable form, it’s the amount of mechanical work you could do.  In fact, that’s exactly what a detector’s calorimeter does, up to a point.  It’s made of big blocks of metal that absorb the particle’s energy, and then it samples that energy and turns it into an electrical current — not so we can do any kind of work with it, but just so we know how much energy there was in the first place.  So the calorimeter is the piece of ATLAS or CMS that measures the energy of particles and absorbs them; the tracker, by contrast, measures the momentum of particles and lets them pass through.   These two pieces of information are related by the following equation:

  • E2 = p2c2 + m2c4

where p and m are still momentum and mass, E is the energy, and c is the speed of light.  The intuitive understanding of this equation is that the energy of a particle is partially due to its motion and partially due to the intrinsic energy of its mass.  The application to particle detectors is that if you know the mass of a particular particle, or if it’s going so fast that its energy and momentum are both huge so that the mass can be roughly ignored, then knowing the energy tells you the momentum and vice versa — and knowing at least one of the two is critical for analyzing where a particle might have come from and understanding the collision as a whole.  We have both kinds of systems because they have different strengths — for example, some kinds of particles don’t get absorbed by the calorimeter, and some kinds of particles (the uncharged ones) can’t be seen in the tracker — and together, they cover almost everything.

(By the way, the second equation is relativistic; that is, it’s compatible with Einstein’s Theory of Relativity.  That means it always works for any particle at any speed — it might assume that space is reasonably flat or that time really exists, but these are very reasonable assumptions for experimental physicists working on Earth.  For those who haven’t seen the equation before and enjoy algebra problems: what famous equation do you get if you take the special case of a particle that isn’t moving, i.e. with a momentum of zero?)

Particle Motion and Momentum

The next ingredient you need to understand what a tracker does is something I haven’t mentioned yet: the whole thing is enclosed in a huge solenoid magnet, which produces a more-or-less uniform magnetic field pointing along the direction of the LHC beam.  As a charged particle moves through a magnetic field, the force exerted on it by the field works at a right angle to both the direction of motion and the field — I tried to illustrate this in figure 1, where the magnetic field is pointing into your screen if you assume the particle is positively charged.5 This means that as the charged particle flies from the center of the detector, it curves (figure 2).  The amount it curves by is inversely proportional to the momentum, which means that higher-momentum particles curve less.  Along its path, it leaves hits in the detecting material, as I discussed above (red dots, figure 3).  Finally, in a process called track reconstruction, our software “connects the dots” and produces a track — which is just our name for “where we think the particle went” (figure 4).

You’ll notice that figure 2 looks a lot like figure 4, but the conceptual difference is a very important one.  The red line in figure 2 is the actual path followed by the particle, which we don’t see directly, while the black line in figure 4 is our track as determined by detector hits.  If we do our job right, the red line and black line should be almost exactly the same, but that job is complex indeed — literally thousands of person-years have been put into it, including two or three Seth-years6 spent on detector calibration and writing automated tools for making sure the tracking software works properly.

The detector is shown here with only three layers.  Although this would be enough to find a particle’s path in ideal circumstances, we actually have many more: this allows us to still make good measurements even when one layer somehow doesn’t see the particle, and to get a final result for the path that’s more accurate.  And don’t forget that there will actually be many particles passing through the detector at the same time — so we need lots of measurements to be sure that we’re seeing real tracks and not just a bunch of “dots” that happen to “line up”…!

More Than Just Momentum

If you measure the path of a particle, you can do more than just find its momentum; you can also see where it came from, or at least whether it could have come from the same place as another particle.  Pixel detectors excel at making accurate measurements to figure out this kind of thing, but as I said already, to do that subject justice will require another entry.

So there you have it.  In a very broad sense, that’s what I’m working toward when I talk about calibrating the pixel detector.  Tracking provides critical basic information about every charged particle that passes through our detector; combined with data from the calorimeter and the muon systems, this information is what will let ATLAS and CMS measure the properties of the new particles that we hope the LHC will produce.

1 Both ATLAS and CMS have one, but many other detectors at colliders do not, because the technology is complex, relatively new, and expensive.
2 See Corollary III here for what he says about it, if you like your science extra-opaque.
3 I’m really not sure why we always use p for momentum, although a good guess seems to be that it’s related to impetus or impulse.
4 A friend of mine, who has the mysterious superpower of understanding how bulk matter works rather than just mucking about with individual particles, looked at a draft of this and was very concerned that I’m implying that all the energy from such a happening would end up as atomic vibrations. So let the record show that this probably isn’t true. And now, if you’d be so kind, can we pretend it is true? It will make illustrating my point very much easier. Thanks!
5 The particle is definitely not actual size, and don’t ask me why it’s green.
6 A Seth-year doesn’t make nearly as big a contribution as a year of work by any of our real experts, but they do happen to be of particular interest to me.

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Why are you still doing night shifts?

Thursday, November 13th, 2008

This is a question I’ve received recently from a couple of my friends in the theory community.  Theoretical particle physicists are pretty smart people, and they do know a little something about particle detectors — so if they’re wondering, then I’m sure some of you will be curious too!  This is also a chance to see a snapshot of my psychological state at the end of a night shift: I wrote all of this to explain what I was doing between 6:20 and 6:45 in the morning a couple weeks ago.  My only edits are two places where I wrote something incorrect and replaced it with a new explanation in brackets.

To summarize: I’m busy this week and getting an easy entry out of cutting and pasting from my gChat log.

Again, the question was (more or less), “Why are you still doing night shifts when the accelerator, and large parts of the ATLAS detector, are off?”  Here’s my answer:

06:22 calibrate the detector
the pixel detector has 80 million channels (i.e. pixels, 400 x 50 microns)
06:23 they actually live, physically, on about 1700 modules, which talk to various hierarchically-organized computers
06:24 [to transmit the data the 100 meters to the counting room without high voltage or repeaters] we have optical links for transmitting the data from inside the detector until it gets outside
thus we need lasers to turn digital signals into optical light, and then we also need to convert the light back
the lasers have to be timed and powered correctly, as does whatever reads the information
06:25 at the moment, the ATLAS pixel detector isn’t using some fraction like [3%] of its modules, because they aren’t set correctly. in some cases, they may be impossible to set correctly until we can open the detector and replace components — which may be many years
but in other cases, the automatic-setting didn’t work, and we have to take a closer look.
06:26 some experts were in here today to try to recover a few such modules by taking that closer look; now I’m running scans that tell us if they were succesful or not.
06:27 that’s only one example of the kind of thing we do. there are a lot of things you can set on every module, and we have to get them all set right.
06:38 [My friend asks why we run all night, and if we run all the time]
06:43 me: yes, we have finite time, and lots of work to do
and clearly more people than pixel detectors.
06:44 once the cooling goes off, in a few weeks, we have to turn the modules off. then there’s only a few kinds of calibration scans/studies we can do

It’s worth noting that now, two weeks later, all the optical links are working well, except for a very few that are hard-core unrecoverable — thanks to the work of the experts who looked at the tuning and the very small contribution I made by running scans for them overnight.  Our night shifts continue, with a few nights each from over a dozen people in this month alone.   Although the details of the work at the moment are different, but the overall plan is the same: to have our subdetector, the last one installed, be as ready as the rest of ATLAS when data finally arrives next year!


Take the Helm, Mr. Chekov

Monday, October 20th, 2008

Seth on Pixel Data Aquisition shiftAuthor’s note: This entry is mostly for my mother. If it happens to amuse anyone else, this is purely by coincidence. Also, there is no need to leave comments informing me that I’m an enormous nerd; I have noticed.

It’s true, life here at CERN is pretty much like Star Trek, or at least it looks that way sometimes. After training last month, and some very hectic shifts earlier this month, I’ve finally had a chance to get a picture of myself at the Pixel Detector operation station in the ATLAS control room. I have lots of screens with technical information in front of me, and the front of the room has a full seven projection screens.

Driving the Pixel Detector is not exactly like driving the USS Enterprise, of course. Where they have a navigator and a helmsman helmsperson, we have a shifter who does Detector Control and one who does Data Acquisition. (I do the latter, although I plan eventually to qualify for both so I can operate the whole thing when everything is very stable.) While they do things like “pivot at warp 2” or “reroute auxilliary power through the main deflector dish to produce a tachyon pulse,” we are more likely to “disable a Read Out Driver to re-enter ATLAS combined running” or “consult the data quality shifter about low statistics in the ID cosmic data stream.” The Pixel Detector has a Shift Leader, who’s sort of like the captain, but they’re only around some of the time if nothing exciting is happening. And of course the Pixel shifters are part of a much larger shift crew, which dwarfs the number of people it apparently takes to operate a starship.

Ok, it’s not that much like Star Trek after all, but — dare I say it? — it’s actually cooler, because it’s real.


Currently I am sitting at Geneva airport waiting for my plane to finally leave for Amsterdam. Looking east I see something the average cernoise is always happy to see: First snow on the Jura.


What Now?

Monday, September 22nd, 2008

Good morning! I’m back at work here at CERN, and I can assure you that there is no pall of doom over the laboratory. Yes, it’s a bummer that collisions won’t happen for a while, but everyone I know still has plenty of work to do to get ready — heck, the only reason I even have time to blog is that I’m waiting for code to compile!

There are plenty of sources for what exactly went wrong, and how long it will officially take to repair; you can see some links in the updates of my last entry. The bottom line is that the needed repair is not a huge one, but it will be very time consuming because of the necessity of warming up the magnets to do it. Why do we need to warm the magnets up? Well, because they’re filled with liquid helium, and you can’t do much work on the magnets while the helium inside. And, as someone asked in a comment, why does it take so long anyway? Didn’t the magnets warm up by a hundred degrees rather quickly during Friday’s malfunction? Yes, they did, but they did it by venting a large amount of helium into the tunnel — and, although helium isn’t dangerous unless there’s so much of it that it crowds out the air, it sure is expensive. The accelerator experts need to slowly warm up, remove, and store the helium; this will save it for future use and prevent damage to the magnets.

So what are we going to do with the next few months? Well, no high-level decisions have been made, and obviously graduate students don’t get to vote on them anyway, but I doubt that there will be collisions in 2008. The old schedule was to slowly get the machine working, and hopefully achieve 5 TeV on 5 TeV collissions sometime in October. If everything went well, this would have allowed maybe a month of physics running before the winter shutdown. (The winter shutdown is CERN’s typical time to do maintenance because electricity is more expensive due to everyone using it for heating; accelerators in places with a lot of air conditioning often shut down in the summer for similar reasons.) After that, the plan was to have a long shutdown during which the machine would be prepared for full energy 7 TeV on 7 TeV collisions, after which it would come online again in Spring 2009. It doesn’t make any sense to shut down the accelerator for repairs, run it for a short while, and then shut it down again for upgrades — so I expect the planned work for the shutdown will begin in parallel with the repairs. Perhaps that means that the LHC will come online at full energy even a bit sooner than it would have otherwise, but bear in mind that that’s speculation based more on my hopes and guesses than on my (non-existent) accelerator-commissioning expertise.

For me and my colleagues working on the ATLAS pixel detector, there is a lot of work still to be done. Our sub-detector is now taking data, but we have a long list of things still to be achieved before it’s operating at its best. We have been doing our utmost to get things ready, but realistically, if the first full energy LHC collisions had been in October, there would have been more work to do: there would still have been a few pieces of our detector shut down because of electronics problems, and the accuracy of our measurements would have been reduced because we didn’t yet know the alignment between different parts of the detector very well. Obviously we would have welcomed that collision data, and used it to continue our improvements, but there was plenty more calibration and commissioning work to do over the winter shutdown. Now we’ll just do that work before we see first collisions instead of after, and hopefully we’ll be in great shape by the time the accelerator is back.

For me personally, the news is not a big setback. I had already decided (by coincidence, last week) that it would be better to stay at CERN and help with the pixel comissioning work in the winter and early spring, even if it meant forgoing the chance to use 2008 data to write my thesis. The downside of this decision was that it committed me to probably being in graduate school until 2011, for a total of seven years — but the upside was that I would learn more about the detector, and be able to do a more thorough job on my thesis as well. Because of the incident last Friday, it turns out that I didn’t really have a choice after all; but since I had already made the decision, it doesn’t feel like much of a loss.

But certainly this is bad news for a lot of people. Many graduate students and postdocs were counting on 2008 data, and they will now be spending quite a bit longer in their present positions than they had hoped, or making other difficult decisions. And everyone working in particle physics, or interested in particle physics, will now have to wait a few months longer to see what the LHC has in store.


First ATLAS Pixel Tracks!

Sunday, September 14th, 2008

I’m on my 18th hour on training shift since Saturday morning, getting in as much time in the control room as I can, and it’s been a very exciting time. One of my colleagues has just discovered that, last night, we recorded the first cosmic ray tracks in the ATLAS pixel detector!

First ATLAS Pixel Detector Track!

This is very exciting news for us; we’re working right up to the wire to make sure our pixel detector is able to run stably along with the rest of the detector. Collisions are coming soon soon soon!

Update (Sept 15): In response to two excellent questions in the comments, I wrote in a little more detail what you’re looking at in the picture. I figure the explanations might as well go in the entry:

1. What’s the perspective? Where’s the LHC?

You’re looking at the inner part of the ATLAS detector, which is wrapped around one of the collision points of the LHC. The large image in the upper left is a cross-section of the detector; the white dot in the very center is where the LHC beam pipe is. The image along the bottom shows the same tracks from the side; the LHC beam pipe isn’t shown, but it would run horizontally (along the Y’ = 0 cm line).

2. What do the dot colors mean? What’s the line?

All the dots are the actual points at which we have a signal from our detector. The red dots represent the signal that we think was left by a charged particle when it passed through, and the red line is the path we think that particle took (i.e. the “track”). The green dots are also signals in the detector, but we think they’re random firings in our electronics, because we can’t make any tracks out of them.

It may look like a lot of electronic noise, because there are more hits from random firings than from the track. But remember that there were only one or two tracks to be found, whereas we have over eighty million pixels in our detector. Thus the fraction of noisy pixels was actually quite small, and certainly didn’t interfere with finding the track. We also have a list of especially noisy pixels that we can “mask” (i.e. ignore), which will bring down the noise by quite a lot but which we haven’t begun to use yet.


Training Shift Liveblog

Thursday, September 4th, 2008

It may be bedtime back in the United States, but here in Geneva it’s six in the morning, and I’ve just dragged myself out of bed.  That’s because I have a “day” shift, which for some bizarre reason begins at 7AM; thus I’ll have to leave my apartment in downtown Geneva in complete darkness.  This is actually only a training shift, but I’m still very excited; I’ve spent a long time writing various analysis software, and it will be exciting to really get my hands on the detector!

I was actually in the control room for a few hours yesterday evening, watching one of the first times our pixel detector has been integrated with the whole “combined run,” and hoping to see a track.  It was very crowded then; we’ll see how things look at 7 AM.



Getting Ready

Wednesday, September 3rd, 2008

I’m usually fairly reserved about my enthusiasm, but I have to admit that now even I am getting excited about first beam.

The ATLAS pixel detector is up and running in the pit, and I’ve been working hard this week on looking at the data from calibration scans. Since I wrote a lot of the tools for looking at large quantities of pixel calibration data in a systematic way, I’m the most up-to-speed on using them; and since we have to be calibrated and ready to run very soon, there’s a lot of demand for those skills. Being useful, and having a lot to do, makes me happy. I get up early in the morning ready to come to work, and leave only reluctantly in the evening when I’m too tired to get anything done.

I’ve also been trying hard to get all the training I need to run pixel detector shifts, and it looks like my efforts have borne fruit. I have “training shifts” on Friday and Monday, and hopefully after that I’ll be able to do things on my own. The only downside is that the day shifts now start at 7 AM—it’s a good thing I’ve been getting up early ready to come to work!