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Posts Tagged ‘baseball analogy’

Turning to the Dark Side

Monday, October 3rd, 2011

“So, you’ve turned to the dark side?” I’ve heard it surprisingly often, usually from my new colleagues on CMS. “Yes,” I reply. “My hate makes me powerful.”

We’re just kidding, of course.

I’ve been asked more seriously, on a number of occasions, why I switched from working with ATLAS to working with CMS. There are several ways I can answer that one:

1. Why not? ATLAS and CMS both look for the same exciting things at the LHC: the Higgs boson, supersymmetry, and all sorts of other new physics. They have roughly similar capabilities and, for the most part, conceptually similar designs. So I should be happy to work on either one.

2. It came with the job. Being happy to work on either experiment means I applied to some groups working on ATLAS and some on CMS. The job I ended up with is with Princeton, and they have a CMS group, so…

3. It’s good for our field to exchange techniques and expertise between experiments.

4. It’s good for me to know people from both collaborations and learn different ways of doing things, and good to be forced into doing something completely different than what I did as a graduate student.

So why would switching be a bad idea? Well, mostly, it’s harder. There is more logistics to deal with to get started as a postdoc — on top of the logistics of starting a job — and a lot of time spent learning new software and new organization. And it will take me quite a bit longer to be in a position where I know enough and people have enough confidence in my work to give me significant responsibilities. But all of this, I hope, is transitory.

In the end, neither experiment is the dark side. They do compete with each other — as intended, to keep everyone working hard — but they’re more like opposing sports teams than opposite sides of the Force. You may despise the team across town much of the time, but without them you couldn’t play baseball. And once in a while, players get traded.

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OK, I’ll admit it — instead of writing blog posts or reviewing results that are headed for ICHEP or doing something else productive, I find myself all too easily distracted by information on the current status of the LHC. As the gallant accelerator physicists work to push the machine to higher beam intensities and collision rates, I’m eager to learn about each little bit of progress. It definitely has some meaning to me — the more collisions the LHC produces, the more the experiments can record, and the greater the chance that we will see any particular physics process take place. Especially as we get close to the big ICHEP conference, we are all curious about how much data we might record before then, because that will determine what measurements might possibly be ready. (Of course it’s also determined by how quickly we can push the data through data analyses, how well we can understand detector performance and so forth; let’s not put all of the burden on the LHC.)

It’s not like I can do anything to make the luminosity go up, but I feel better (or at least distracted) by knowing what’s going on at this minute. This is akin to scoreboard watching in baseball, where the outfielders in one game might have their eye on the scoreboard above them to see how the competition is doing. (In fact, back at the Cornell Electron Storage Ring, the display that showed the luminosity numbers for the past 24 hours was called the “scoreboard”, so the analogy fits.)

So, if you want to play along at home, here are a few Web pages you can keep an eye on. Some of these have been mentioned in previous posts on this blog, but it’s been a little while and I’ll give a few more details.

To know what’s happening right now, check out LHC Page 1, which gives the current machine status and the (very) short-term running plan. Here you’ll typically see plots of the amount of beam current and the beam energy in the LHC, and, during periods of collisions for “physics” (i.e. data-taking by the experiment as opposed to studies of collisions done to optimize machine performance) there will be plots of the observed instantaneous luminosity reported by each of the four experiments. (Instantaneous luminosity is a measure of collision rate; its units of inverse centimeter squared per second deserve explanation in a second post.) The experiment reports can also be seen on the LHC Operation page. At other times, it will show the status of preparing to go to collisions, such as “ramp” (increasing beam energy to 3.5 TeV) or “squeeze” (focusing the beams to increase the collision rate). There are also helpful short messages about what’s going on, such as “this fill for physics” or the somewhat unnerving “experts have been called.”

The medium term run plan can be seen on the LHC Coordination screen. Here you can see the goals for the coming week, what administrative limits are currently in place to protect the machine, and the planned activities for the next few shifts.

While the collision rate is interesting, what really counts is the “integrated luminosity”, or the total number of collisions that have taken place. Up-to-date charts can be found here; the data go back to March 30, the start of 3.5 TeV operations. You can see here that the integrated luminosity has been increasing exponentially in time (when the LHC is not in studies periods or technical stops). If the collision rate were the same all the time, the integral would only increase linearly, so this demonstrates just how quickly the LHC physicists are figuring out how to make the machine go.

That’s what I’ve been keeping an eye on. OK, all of you stop looking at Facebook, and distract yourselves with the LHC instead!

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Aggravating the gods

Wednesday, October 14th, 2009

Sigh…
I was reading the New York Times today (ok so I’m a couple of days behind because I was reading it from October 12th) and I came across this article:

The Collider, the Particle and a Theory About Fate

To save you some time, the article references an arXiv paper posted in 2007 about backward causation and time travel. (for those who don’t know arXiv is not a publication, articles are posted, and not necessarily reviewed – although it has to be approved). They argue that any collider searching for the Higgs is destined to fail because god/nature/or whatever doesn’t like little Higgsies. They then point to the failure of the SSC, and the recent failures at CERN as proof.

Instead of trying to refute this, because I think it’s silly, I’d like to discuss correlation. I’ve been watching a lot of baseball recently because the play-offs are going on (poor Rockies just got defeated by the Phillies in a nail biter on the 9th). Anyway, baseball is littered with uncorrelated statistics. Batter X hits a 0.280 on Thursdays against right handed pitchers as opposed to Fridays when he bats 0.320. Does the day that the batter is at bat really change how well he hits? Maybe he’s working for the weekend, but it could be a coincidence or other factors that we’re not taking into account which are correlated and this result is just part of the picture.

I’m sure those who are versed with pastafarians are familiar with the argument that the decreasing pirate population is causing increased global temperature. Sure, the number of pirates has decreased over the past 150 years, while the global temperature has increased (seen below), but does the presence of pirates inherently cause a decrease in the global temperature. I think I’m going to go with probably not. There are clearly other factors to take into account.

Pirate number vs. golbal temperature

Pirate number vs. golbal temperature

Anyway, why do I bring this up? People like to find correlation between things. Our inherent nature as humans forces us to like to try to understand relationships between events. That’s why I want to be a scientist when I grow up. That being said just because as one thing is happening, another happens too, doesn’t mean that they have anything to do with each other. To say that there is evidence to show that god/nature/or anything else is spiting scientists for searching for the Higgs is not only destructive, but unprovable and not science. Fermilab is currently running just fine and searching for the Higgs (a couple people in Stony Brook’s D0 group are doing just that, in fact.) But it also takes away from what the engineers and scientists are doing to make machines like this work. The LHC is a brand new machine pushing the limits of engineering. Of course it’s expensive and things don’t work as we expect. (we have no examples to base our expectations on). And as for the SSC, I think that’s more an example of how scientists need to better explain to Congress why science funding shouldn’t be cut than someone out to stop us from finding the Higgs.

But I guess that’s what I get for reading essays in the NY Times. I should stick to the food section :-)

Until next time,

-Regina

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Rivalries

Tuesday, February 3rd, 2009

People often wonder how we at ATLAS feel about those bozos our good friends at CMS, and vice versa.  The two experiments are trying to discovery exactly the same things, and as Monica discussed a while back, trying to keep from getting scooped by the other experiment will be nerve-wracking.  Personally, I like to think of CMS as the baseball team on the other side of town.  Yes, we plan to beat them at everything, every time — but deep down, we know that if they weren’t there, we couldn’t play baseball.

Obviously we don’t literally need there to be a competing detector in order to record events at the ATLAS detector or look for new physics in them, but having two detectors is actually critical to the LHC’s overall goals.  At many places — especially where new things are being tried — the detectors use different technologies.  For example, the T in ATLAS and S in CMS represent the very different magnet configurations used for the two detectors’ muon systems.  The “worst-case” reason for this is that one detector might incorporate something that never works — and while that would be terrible for the people on that experiment, it’s a lot better to have a backup that can still do the job.  But, in fact, ATLAS and CMS both work just fine so far, and we expect them to continue doing so, which brings me to the second reason that they’re complimentary: we need somebody to check our results.  Of course, we at ATLAS want to (and, needless to say, will) discover everything first — but if CMS never sees the same thing, a discovery will be pretty hard to believe.  The particle physics community really needs two detectors with different designs and different teams and different analysis strategies to get the same answer before we can be sure of what we’ve found.  (Ideally, we’d have different accelerators too, but that’s a little out of our price range nowdays.)

I’d even say it’s a friendly rivalry, more like the San Francisco Bay Area than New York or Chicago.  Although it’s hard to say — I’ve seen hats that say A’s on one side and Giants on the other, but I have yet to see any ATLAS/CMS merchandise.

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Curveballs are Fun

Friday, December 19th, 2008

We’re not big fans of rigid hierarchy in academia, not even on big experiments like ATLAS with multifarious coordinators and project leaders.  On the one hand, this means that nobody ever gives me orders — but on the other hand, it does mean that there are a lot of people who can give me “strong suggestions.”  And sometimes one of those people decides to throw me a curveball…

Friday was a day of two work days.  First I worked a pretty normal eight hours debugging code, then spent the evening at a few holiday parties before heading to the ATLAS Control Room at 11 PM for an eight hour shift.  After I arrived, while waiting for the expert running things to let me do my shift so he could go home and get some sleep, I found an email in my inbox which had been sent only that evening.  It asked me to give a talk at the ATLAS Inner Detector-wide meeting about the activities of the Pixel group over the previous week.  All of the work to be discussed had done by others rather than me, and some of it I hadn’t even been aware of — and the talk was on Monday.

I had never received a request like that before, but believe it or not, I’m not complaining.  Yes, it was rather short notice, but it wasn’t even a strong suggestion, really — I was allowed to opt out if I didn’t have time.  But more importantly, after I thought about it, I decided that giving the talk was entirely a good thing for me.  There are a couple of reasons I can think of to give an inexperienced person the responsibility of summarizing the work of the whole Pixel Collaboration.  One is to give everyone who’s done work on the Pixel Detector a turn to make their participation visible to the wider Inner Detector community, even if their work contributed only indirectly to the material being presented.  (In my case, the contributions were taking shifts and writing tools for analyzing calibration scans.)  Another is to give the person giving the talk the opportunity to learn more about the broader work on the detector.

In my case, it was an opportunity I had to take quickly, so I sprung into action: I checked the agenda for Monday, found that the meeting wasn’t until 3 PM, and decided I could delay the writing of the talk itself until Monday morning.  I did look at the list of topics to cover during my shift, and asked a few questions; then I printed out all the supporting material on Sunday night.  But otherwise I continued with my weekend as scheduled.  This required Monday to be a very productive day: I got up at 6:30 AM to start reading everything I had printed out, then got intto work by 8:30 am and started writing.

Most of the slides were summarized from elsewhere, or even provided for me.  The most important part of what I had to do was to understand what was on them, so that I could provide context for the work and avoid sounding like an idiot if I had to go “off script.”  The way I think about it was that the people who had done the studies had given me intermediate-level information to present, and nobody would expect me to answer really hard stuff during a summary talk, but that I absolutely had to have a command of the basic way in which the material I was presenting fit into the broader picture.  I needed some help with that, and got plenty of it, from the experts who did the original work as well as from the person who asked me to give the talk.

By 3PM, I was ready, but also nervous about talking in a new venue and in front of new people.  I hadn’t given myself time to be nervous up until that point, but I had plenty of it while watching the other four talks ahead of mine.  My strategy during the talk itself was to try to sound confident that I understood everything, unless I actually didn’t know something and had to punt questions to the other pixel people in the room — which it turned out I never did.  In the end, in fact, I was told the talk was clear and went well.   So I suppose I managed to hit the curveball, and it definitely made for a more exciting Monday than usual!

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I’ve been meaning to write a quick note thanking people for their comments on last week’s post about tracking.  When I spend a lot of time on making sure a post really explains something well, it means a lot to me to know that my effort succeeded.  (A note to readers who happen to be my advisor: I didn’t spend too long on it, I swear.  And anyway I was waiting for my code to compile.)  So, thanks!  While I’m here, I figure I might as well share an observation that occured to me while reading the comments, and then answer a question that was asked.

First the observation.  In my experience, if you go to a baseball game and point out that the people on the other side of the stadium “look like” a particle tracker for the ball, your friends stare at you as if you’re crazy.  And yet, if you write about particle physics and manage to compare it to baseball, then it goes over rather well as a feat of science explication.  I conclude from this that the trick to being a tremendous nerd while still being cool is to manage expectations; get your audience to expect you to be an even bigger nerd than you actually are, and they’ll be impressed.

Second, the question: Didi Mouse asked who gets to name any new particles we find.  The answer is that we don’t actually know yet, but it depends on what’s out there.  Many particles — for example, the Higgs boson — have been named already; if we make a discovery that looks more or less like a Higgs boson, we’ll call it a Higgs boson.  There are also theories that predict lots of new particles; often those particles are all named, but according to some regular rule.  For example, Supersymmetry predicts a new particle for every known fundamental particle.  The superpartners have the same name as the original, but with an “s” in front for some spins, and an “ino” at the end for others; electron becomes selectron, quark becomes squark, photon becomes photino, gluon becomes gluino, and (my favorite) W becomes Wino.  If we were sure we’d found Supersymmetry, we’d probably keep those names, but we won’t be sure at first what new theory the particles we’ve found fit into — so what will we do?  I expect the decision will be made as part of the experimental collaborations’ processes for writing and approving papers, because the name for a new particle usually comes from the paper that announces the discovery.  As far as I know, nobody has specific plans for how to handle the naming, but it is a problem we will be delighted to have.

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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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