May 23, 2006
I’ve wasted a couple of weeks of my life tracking down a rather odd problem with the CLEO detector, and I feel a certain need to offload. So here follows a rather detailed discussion of the CLEO tracking system — actually, drift chambers in general, so if you’ve ever wondered how exactly we follow all the particles that we create in collisions, here you are.
Say you have a particle whose path you wish to trace. You can’t really look directly at it; you’d need very intense light, which would disrupt the particle’s path, and at any rate isn’t easy to make. We instead put instrumented material in front of the particle. The aim is to use the interactions of the particle with the material to figure out where it was, while having those interactions be so gentle that the particle isn’t greatly affected (usually by using some sort of amplification of a weak signal). This is the principle behind detectors all the way from the cloud and bubble chambers of yesteryear to the silicon (and diamond?) detectors developed today.
CLEO uses a technology called the “drift chamber.” These consist of a volume of gas with lots of wires, running parallel to each other, spread out through the region (the guts of one can be seen in this photo, and the sort of wire arrangement we use is depicted here). The wires are brought to high voltage, with some positive and some negative; this sets up precisely shaped electric fields. As a particle traverses the chamber, it runs into atoms and molecules of gas and knocks electrons off them. The energy required to do this isn’t terribly large, so the particle’s momentum is not affected much (although the energy loss is measurable and important for other reasons).
The liberated electrons start drifting along the electric field lines towards the positively charged (“sense”) wires. Because they keep bumping into more gas, for most of the drift period, their speed is limited by the time between collisions; this “drift velocity,” which depends on the electric field and the type of gas, occupies a lot of some people’s time.
As the electrons approach the sense wires, the electric field increases as the field lines get closer together. At some point near the wire, the electrons accelerate enough between collisions to liberate more electrons when they crash into the gas. These new electrons go on to free more electrons, and so on in what’s called an “avalanche” — this is the main amplification step, where a few tens of primary electrons from the particle are turned into a measurable signal.
So what do we do with this information? The positions of the wires that were hit gives us very rough information on where the particle went, up to a couple of centimeters (a typical separation between wires): if we stop here we have a Multiwire Proportional Chamber. But we can do much better. As mentioned earlier, for most of their drift, the electrons travel at a determined velocity. You can design the field and choose the voltages such that the drift speed is pretty much constant over large regions. This means that the time the electrons take to drift can be translated to a distance measurement — you now know, to a precision more like 100 micrometers, how far away from the wire the particle passed.
Of course, you only know the particle position to within a cylinder of some distance from the wire. At CLEO, the drift chambers are cylindrical, and we know the particles are supposed to come out from the center of the cylinder, so the ambiguity is essentially one of “left” or “right” as the point of closest approach is at practically the same radius as the wire. To break this ambiguity, we stagger the wire layers at different radii, so going “left” or “right” at a certain radius means you hit different wires at the next stage. (Incidentally, the way you do the staggering can cause interesting effects, since positive and negative particles are bent in opposite directions by the experiment’s magnetic field and so produce different stagger patterns.)
One final word: you may have noticed that none of this information tells you where along the length of the wire the particle passed (in the usual experimental setup, this is defined to be the z-axis). We can get this information — at the price of breaking the cylindrical symmetry. Essentially, you give a layer of wires an angle to the z-axis, and tilt another layer in the opposite direction (these are “stereo” layers, as opposed to “axial” layers where the wires are strictly parallel to z). The resulting geometry is a hyperboloid. You can now intersect the two lines you get from the distance measurement, and their intersection gives information on the position along z. This is a really tricky operation, incidentally.
So, on to my obsession for the last week. CLEO uses two drift chambers: an older one called the DR — technically DR3, it’s the third in its lineage — which, with 47 cylindrical layers, provides most of the track information, and a relatively new one, the ZD, which is inside the DR and has six layers. The outermost layers of the DR are stereo, but to get a good measurement of z near the particle production point (and to get a good lever arm to measure angles), the ZD was designed to be all stereo layers.
The timing measurement is all-critical. Unfortunately, there isn’t a little readout telling us when, exactly, a given collision occurred. Collisions are separated by as little as 14 nanoseconds, and our electronics are nowhere near fast enough to react in that time period. We run all events through a “trigger” system to decide whether to take them or not; the trigger makes its choice quite a bit after the actual collision time, and with 42 nanosecond granularity. 42 nanoseconds is over a millimeter of drift distance — much worse than the intrinsic 0.1 millimeter of the detection method.
How do we deal with this? At first, we take the trigger time as the truth. For every event, we then run a program which tries to find little “tracklets” with the DR information. Knowing the timing structure of the collisions from the accelerator, it tries different possible collision times (on the 14 nanosecond clock) near the time the trigger reported, and finds the one where the hits are most consistent with the tracklets; we then take this as the Event Time. In short, the actual time the event occurred, the zero time from which all the drift distances are computed, is obtained entirely from DR information — and this time is used for the ZD as well.
This is fine unless the ZD gets, for some reason, a different time from the electronics than the DR does. As configured when the ZD was installed, the two detectors get their “record your times now” signals from two different circuit boards, so the possibility for a problem existed: if one of the boards broke subtly, the two might not always be in sync. If the two were always out of sync by a constant amount, we would calibrate it away and never see it, so we are only sensitive to one jittering with respect to the other, an even more subtle effect…
And that’s precisely what we saw (and fixed this morning). A few percent of events from the last data run seemed to have much worse tracking in the ZD than they should have. After days of tracing the problem back (made worse because I had just taken over the procedure that raised the red flag, and so I thought I just didn’t understand what I was doing), we decided the timing system was probably to blame. Physical investigation revealed a broken board which output timing commands which jittered, in a random few percent of events, 42 nanoseconds away from where they should have been, in exactly the manner to produce the observed problem. We replaced the board, and the new one seems to be doing OK, but I think I’m going to be eternally paranoid about this.
In the short term, I’m trying to repair the data that we’ve already taken to remove this effect, which seems quite doable. We’re also going to put monitoring in place to catch this quickly if it happens again. Hopefully I can go back to extracting physics results soon, though.
Many thanks to D.P. and K.E. for teaching me what I know about drift chambers…
May 11, 2006
I miss Suck. The back issues give me nostalgia for when the world (or at least I) was young (-er, the back half of the 90s). The dot-com bubble was inflating (with many e-foldings left to go), Netscape 3.01 Gold was the in thing, people believed that you could make webpage communities, OS/2 was still a viable operating system choice, and the grand spirit of Clintonian optimism roamed the land. Computer geeks were going to inherit the earth, and I expected to do CS when I got to college. Good times.
May 6, 2006
Since everyone’s doing it, I’m going to make an attempt at a “generally-accessible” post about a physics topic of interest to me. The particle I’m going to talk about is very obscure, even in the particle physics world, so we’ll build up to it …
CESR, the accelerator that feeds the CLEO experiment, collides electrons and positrons at precisely calibrated energies, and from time to time they will annihilate and produce a quark-antiquark pair. The quark produced has to be lighter than half the center of mass energy, since you have to make a pair. At the energies we are running at now, we can make the up, down, strange, and charm quarks. The first three aren’t all so interesting for us (other experiments study them better), so we concentrate on charm-anticharm production.
One tends to have this image of antimatter as being on a single-minded mission to seek out the nearest bit of matter and annihilate with it, but that’s not really true. The quark and antiquark will only annihilate each other if, in a sense, they wind up at the same place at the same time. The way we produce them, they essentially go into orbit around each other, and this tends to keep them apart. Quantum mechanics means the orbits are fuzzy, and in any amount of time there’s some probability that they’ll meet up (or else the pairing would be stable!), but that behavior is “suppressed.” When this happens for charm-anticharm, the orbiting system’s lifetime, if it can only decay through annihilation, is on the order of 10-20 seconds — an eternity, particle-wise.
States of this kind, where matter and antimatter orbit each other, get the suffix “-onium”: an electron-positron system is positronium, a bottom-antibottom system is bottomonium, and our case, charm-anticharm, is charmonium. (-Onia is the accepted plural…) Quantum mechanics dictates that certain orbits, and only certain orbits, are possible, and we refer to each one as a separate kind of particle (we can do this, even though they are composite systems, the same way we can call an atom a particle). The most famous particle of this family is called the J/ψ, for politico-historical reasons. It is the second lightest charmonium particle, and the lightest that can be easily made at our kind of accelerator. Other charmonium particles get names involving ψ, to remind us that they’re in this family.
When the orbiting pair have enough energy, the system can fall apart another way. The strong interaction, which binds the quarks, doesn’t itself care what type of quark is involved, and is perfectly happy to bind a charm quark to a lighter antiquark, a combination called (for historical reasons) a D meson. When the charm-anticharm system is heavy enough, what usually happens is a light quark pair (down-antidown, or whatever) will be created out of the stew of gluons keeping the charm and anticharm in orbit. Since this can happen anywhere, it is much more likely to happen than the two original quarks meeting up and annihilating. The charm and anticharm then pair up with the lighter quarks, and the charmonium state basically falls apart. When this kind of decay can happen, it is by far preferred over the annihilation, and it leads to the heavy states living much shorter lives (as we pass the threshold, the lifetimes get shorter by a factor of a thousand).
There’s one extra decay mechanism that has to be mentioned. A heavy charmonium state can change to a lighter one by changing the charm-anticharm orbit and dumping the energy somewhere. Depending on the details, it can emit light (exactly like the way light is produced in a neon light, for example), or it can put the energy into gluons, which after some rearranging and quark-popping show up as bound states of light quarks; in particular decays involving two pions are very popular. The particle ψ(3686) (the number indicates the mass in megaelectronvolts) is too light to decay by producing D mesons, but is heavier than the J/ψ, and its primary decay process produces a J/ψ and two pions. Its lifetime is roughly a third that of the J/ψ — the extra decay possibility doesn’t change things much in this case. Charmonium states that can decay to D mesons can decay this way too, but again they prefer not to because the D channel is faster, and so the probability is very low.
So the overall picture here is charmonia heavy enough (above 3730 MeV, give or take) like to decay to D mesons, if possible, and have short lives; charmonia lighter than that decay by annihilation or emission of photons or pions, and last for a long while. Until last year, all the data we had agreed with this prediction.
In July 2005, our colleagues at the BaBar experiment reported finding a new particle, christened the Y(4260) (4260 again for the mass, Y for “we don’t know what this is”), decaying to J/ψ and two pions. This transition would normally be taken as prima facie evidence that this was in the charmonium family. Now, the Y(4260) is heavier than DD threshold, which means it should really want to decay to D pairs; normally, if we could see the J/ψ π π transition, we would expect to see D pairs at rate orders of magnitude larger. Strangely enough, though, the Y(4260) sits in a dip in the production rate for D pairs; for that reason nobody had ever suspected there might be a charmonium state there.
So the Y(4260) decays very strangely. But it doesn’t stop there. We understand the force binding the charm-anticharm system pretty well (we think), at least for conventional charmonium states, and we can predict what masses the different particles should have. We thought we knew what data objects corresponded to what theoretical objects, and the Y(4260) was an unwanted interloper (it is rather a “who ordered that?” situation). Even after rethinking the assignments from the theory, the Y(4260) just doesn’t fit in nicely, and although attempts have been made to explain it as a conventional (albeit very weird) charmonium state, these aren’t terribly well regarded; the prevailing attitude is that the Y(4260) is not your everyday particle. There’s the exciting possibility that it is a “hybrid,” a long-predicted but not-yet-seen type of particle, where a gluon is actually a permanent part of the makeup of the particle, instead of just an ephemeral messenger keeping the quarks bound together.
So the Y(4260) is exciting. How is CLEO involved? BaBar’s result implied that the accelerator could actually tune to right around 4260 MeV and produce the Y(4260) directly. Since there was only the one BaBar paper, the result needed confirmation, and we were in a good position to provide it. So we ran there, and in a paper published a few days ago in Physical Review Letters confirmed the BaBar discovery and added some new information of our own. We searched for decays that BaBar did not have enough data to go after; we definitely saw one and had evidence for another, and in fact what we did see has already ruled out at least one model of what the Y(4260) could be. The hybrid explanation is still alive…
And the investigations continue!
May 3, 2006
The April meeting was a blur while I was there, and it’s still a blur a week after. Since the talks not given by Special People (e.g. Sean Carroll) are ten minutes long, you get through a lot of them. The number of sessions that run concurrently means that you’re always missing something. There’s also this grad student solidarity curse: you want to make sure that people you know have an audience, even if you’ve heard their talk before, which means you fail to go to sessions where you might actually learn something new.
Very little of shocking significance was revealed at the meeting; you would have heard about it otherwise. So I’m not going to talk about the physics. However, I’ll mention the off-beat talk that I found most memorable: Yuri Orlov’s Sakharov Prize talk. While in the USSR, he became active in human rights monitoring issues, which got him sent to Siberia in the 70s. He was brought to the West as part of a spy exchange deal, and has since been with Cornell’s accelerator physics group.
He’s kept on working on human rights since then, and his talk was about issues of contention in that community — where people who all share the same abstract goal differ on how one pushes for it. Is it ok for campaigners to take sides in a conflict and ignore the behavior of their preferred group (Chechnya, Palestine, Darfur)? Should oppressors and the oppressed be held to the same standards? What is the interplay of human rights, political freedoms, and economic improvement, and should one take precedence to lay the groundwork for the others? Should human rights activists also push specific political programs? (He pointed out that Sakharov, for his part, always remained a commited Marxist-Leninist.)
To paraphrase a comment from his talk: “We know that there are many apparently simple problems in physics that are impossible to solve in closed form. Why do we then expect human interactions, which are so much more difficult to understand, to be optimizable by just sitting down and thinking really hard about it?”
And now, some other highlights (lowlights) of Dallas:
- S.S. and I rode all the way around DFW on the new SkyLink, then wound up being the last people to board our flight home. I saw a Lockheed L1011.
- In other train-related activity, I rode on both the Dallas commuter rail and light rail systems. They were average. The Chicago Metra would never tolerate the commuter rail delays.
- It was too late for us to ride the M-line streetcar, but we stood on the tracks anyway.
- Ever wanted to sit in a steam locomotive engineer’s seat? You can, in Dallas. (It wasn’t even my idea to go to that museum, honest.)
- In one of the sessions, the two non-CLEO speakers pulled out, and the number of non-CLEOns in the audience fluctuated between zero and one. The session was promptly dubbed CLEO West.
- We did not, after all, go to the tower dome next to the hotel. The night we wanted to go it was “closed for a private function.”
- Should a session on professionalism in a physics career really run two hours over?
- My teams are now two for two in winning trivia contests sponsored by the Forum on Graduate Student Affairs. Unfortunately this year I wound up being given a youth T-shirt as a prize by mistake, and let me tell you, it does not fit.
- It is impossible to leave the Hyatt Regency to go anywhere in downtown and not pass within sight of the JFK Memorial and the Book Depository, to say nothing of the Grassy Knoll. It is slightly unnerving, especially as those sites appear to constitute the entire Dallas tourist industry.
And that’s that.