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Archive for November, 2008

Happy Thanksgiving!

Friday, November 28th, 2008

Happy Thanksgiving from CERN! As you can see from the picture, many of us got together yesterday to celebrate (people from the CMS and ATLAS experiments even ate together in the spirit of Thanksgiving).
Holidays here always feel a little different, mainly because US holidays usually aren’t holidays here.
This one wasn’t exactly your typical Thanksgiving, mainly because most of us worked during the day, and we had the dinner at night. There wasn’t much evidence that it was Thanksgiving at CERN, but our celebration felt pretty authentic. We had just about all the necessary foods (no cranberry sauce), incuding not one but two turkeys. And we streamed NFL football live!
The biggest negative about working abroad is being away from family and friends, and that is especially true on Thanksgiving. But yesterday we did a pretty good job of re-creating the family atmosphere, and a phenomenal job of re-creating the feeling of having eaten way too much. Waaaay too much.


Look, ma…

Friday, November 28th, 2008

I occasionally try to convince my decidedly non-academic family that scientists are in fact a subset of humanity, and not a pocket universe unto ourselves. Hey, we even have “vertical integration” meetings!


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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Meet some Atom Smashers

Monday, November 24th, 2008

This week on television, there is a movie that should delight US LHC blog fans. It’s about the particle physics community in the US, at Fermilab, and the work people are doing there. Also, a lot of friends of mine are in it! 🙂

From what I’ve read on the website and seen in the preview of the film, the film focuses on the people doing research at Fermilab and the circumstances they find themselves in: the kinds of questions they want to answer, the position of Fermilab researchers as the LHC starts up, the worries people have about funding and the future of the field.

It looks like the movie has some similar goals to this blog — showing not just the science, but the people behind the science. From the movie’s blog, one of the filmmakers describes his interactions with audiences at the screenings this way:

There is a consistency in the questions we’re asked, whether in Chicago, Vancouver, or Norway. One of the first to come up is “where did you find this topic?” Often the way the question is asked implies “where in the world did you find this topic?” Or even “what on earth were you thinking?”

It seems to be a predictable pattern: the general public is astonished to find that a) scientists are people not that different from everyone else, and b) that their lives involve exciting stories. It reveals the extent of the disconnect many people seem to have regarding science…

“The Atom Smashers” starts tomorrow, Tuesday, in most cities, so set your VCR/TiVo/tune in if you can! You can find out when the film is airing on PBS stations near you by checking this website.


The next next thing

Friday, November 21st, 2008

This week I took a brief trip to Fermilab to attend a little bit of a workshop on upgrades to the CMS detector.  Now yes, this is nutty on its face — what are we doing talking about upgrading a detector that has yet to see its first collisions?  How can you know what needs upgrading?  Unfortunately, the time scales for completing such projects are very long, and we need to start planning now.  By 2013 there are supposed to be LHC upgrades to improve the beam focusing at the collision points, and CMS will want to make changes to make the most of this.  That’s only five years away, and given how long it can take to do the R&D work and then actual construction of the detectors, you have to start now.

It definitely would be better to have some real data, and certainly what we learn from collisions as soon as we have them will inform what we do in the upgrade.  However, we already have plenty of information to chew on. We learned a lot in the course of the construction of the detectors, and know that there are specific problems we would like to fix.  The current detector was already designed some years ago, and there have been technical advances that we would like to take advantage of.  (I was struck by one talk about electronics in which it was stated that there is an increased focus on making components with low power consumption; this is an issue on the computing side too.)  And perhaps in the course of the upgrade studies, we’ll learn something that will allow us to operate the current detector and analyze its data in a more clever way.

Due to my own schedule constraints, I could only stay for a day of the workshop, and thus I won’t claim that I did any real work!  However, it sounds like some progress was made.  We expect that we’ll have to rearrange our tracking system, and as part of it we want to be able to have an online track trigger, something we don’t have right now.  That is a very difficult technical problem, but people agreed on a strawman layout for the tracker that people could at least use for studies, and a general strategy for how the triggering could work.

I am slightly embarrassed to admit that this was my first visit to Fermilab in about six months!  The new baby here at home has kept me from traveling around as much as I ought to.  So there were lots of people to see and say hello to (including the guy in the badge office; my ID had expired and got confiscated at the gate!).  There never is enough time in the day at the lab to talk to everyone I want to see.  I’m looking forward to getting back again soon(ish).


Modus Operandi

Thursday, November 20th, 2008


Schedule Disappointments

Wednesday, November 19th, 2008

As Adam noted yesterday, the date now being cited by the CERN Press Office for the restart of the LHC is early summer.  Unfortunately, there are two reasons why I’m personally not inclined to take this new estimate too seriously.

First, CERN has an increasingly long history of being over-optimistic on LHC start-up times.  At one time it was scheduled to run in 2005. Even into 2007, the official schedule said there would be a low-energy run that year; but in the end, it didn’t start running until almost a year later.  After the accident on September 19, CERN initially announced that the incident would lead to minor delays, then that it would take several months because of the winter shutdown, and now we have the revision that the repairs will push into the summer. Obviously some of these delays were due to unforeseen circumstances, for example the recent accident itself.  But even if all the schedule changes are due to equally-unforeseeable (if less dramatic) issues, the sheer number of revisions seems to suggest that CERN ought to take a step back and consider how it does contingency planning and the certainty with which it expresses its scheduling announcements.

Second, this new announcement is not accompanied by a new detailed schedule.  What would be useful for the experiments is more information about the damage and a full discussion of how the repairs will proceed, along with a range of possible start times depending on how well still-unknown factors turn out; this would let us do better contingency planning for our own maintenance work, not to mention our careers.  It’s very possible that CERN doesn’t yet have all the information about what repairs will be necessary, but then why the new announcement?  What use can it have beyond publicity, and what meaning can anyone possibly extract from it?

I should be clear here what I mean when I talk about “CERN” making announcements.  Obviously I’m not talking about the technicians, engineers, and physicists who work on the LHC; I’m sure they’re doing a great job, and of course they don’t write the press releases or talk to the media.  I’m also not referring to anyone in particular in the CERN Press Office or Management; the Press Office does a lot of good work on outreach, including putting forth an extraordinary effort for First Circulation Day, and the folks who write the press releases aren’t necessarily the ones who decide what they say.  The truth is that I simply don’t know how decisions about these announcements are made, or who makes them.  But somehow the official system for disseminating information is falling short of providing what the physicists working here need or what the public deserves.


PANIC Can Be Good

Tuesday, November 18th, 2008

Especially when PANIC is PANIC 2008, Particles and Nuclei in Collision, which was held in Eilat, Israel this year.  I had the honor of giving a plenary talk on the role of “soft physics”: both for helping understand the physics at RHIC, and how the LHC will contribute to our evolving understanding.

I’ve written about “soft” physics before, both as “day 1” contributions to the LHC physics program, and something fascinating in its own right (but heck, I’m biased).  The idea is that the low-energy particles, which are generally seen as “gunk” to be cut away by analyses looking for new high-mass particles, have very simple features if one compares different energies and different systems (e.g. collisions of protons or heavy ions).  RHIC has been interpreting the heavy ion data in terms of a hot, dense thermalized medium, but treating the very similar p+p data as “reference”.

One interesting thing I learned this week was from a talk by my colleague Mike Lisa from OSU that one can systematize the differences between the transverse momentum distributions in p+p and Au+Au simply by accounting for the basic fact that momentum and energy have to be conserved in detail in every event.  Doing this one finds that both systems have the same parent distribution, and the observed differences are merely from the imposition of conservation laws.  This has two immediate reactions based on people I spoke to: 1) the “trivial” interpretation that all systems are “nothing but phase space”, and 2) the “deep” interpretation that heavy ions both show indications (nearly identical ones!) that the system is similarly hot and dense and “flows” like a fluid.  Unfortunately for those who buy in to #1, #2 has much more experimental evidence supporting it, as Mike and his student point out in their papers.  Interesting stuff and very much worth a look.

And unsurprisingly, I’ve posted photos.



Tuesday, November 18th, 2008

The price tag for the LHC repairs was in the newspaper. And the official estimate for beam back in the LHC is now “summer” instead of “spring”. I remember the day the accident happened and I heard a rumor that the LHC would be down for 3 months.  It seemed too bad to be true.  Things have only gotten worse since then as more has been learned about the extent of the damage.  It is disappointing, but hopefully at this time next year we will be drowning in data.

In the mean time, people on the detector side are still quite busy. There are plenty of repairs to do, analysis of cosmic ray data, physics studies and software improvements, and getting better prepared for next year’s data.  But everyone wishes we had just a little collision data to enjoy during the long winter nights.


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!