Detector overview

December 14, 2006

JoAnne Hewett has a nice general description of modern collider detectors over at Cosmic Variance. To provide some value-added, let me relate it to CLEO-c:

  • No detector can be perfectly hermetic (cover all of solid angle) because the colliding particles need to get in somewhere and there is accelerator machinery associated with focusing the beams. In CLEO’s case, the tracking system covers 93% of 4π solid angle. The LHC general-purpose detectors do somewhat better, because the products of their collisions will be closer to the beamline than ours are, making “forward” tracking and calorimetry more important.
  • We have a “vertex detector” (the ZD) but it serves a different role: instead of finding the displaced vertices of long-lived particles, it tracks particles near the primary vertex (where the electron-positron collision occured). At our energies, the particles that would normally have displaced vertices don’t have enough relativistic oomph, so their lifetimes aren’t stretched out long enough for us to see their flight distance — the exceptions being the short-lived neutral kaon and the Λ baryon. The ZD is not made out of silicon, since we don’t need that kind of resolution; it is instead a drift chamber.
  • Our main tracking chamber is also a drift chamber. Contrary to the original article, very few detectors rely primarily on silicon tracking for most of the volume (the only one I can think of being CMS); usually there is a gas detector component.
  • Our electromagnetic calorimeter is made out of crystals of cesium iodide (doped with a pinch of thallium to increase the light output). Electrons and photons hitting it create flashes of light whose intensity increases with the incident particle’s energy. There are a whole bunch of different technologies that are used — although crystals are great, they’re also very expensive! The only recent experiment I can think of that uses lead crystals, though, is CMS.
  • We have no hadronic calorimeter. This doesn’t hurt us all that much; we can individually track charged hadrons through our detector (so we really miss only neutrons and long-lived neutral kaons), while at higher energies the tracks merge together into a blob and the only way to measure their (sum) energy reliably is with a calorimeter.
  • Our muon chambers work, but muons need a high momentum to get out there, and they tend not to have it. (These things were designed back in the higher-energy days.
  • Unlike the “energy frontier” experiments, we have an additional system for distinguishing types of particles – the Ring Imaging Cherenkov detector, which uses the Cherenkov light from charged particles travelling quickly through lithium fluoride to measure their velocities. Since we measure their momenta in the tracking system, we can deduce the masses of the particles, and thus whether they are pions or kaons. This kind of additional particle identification system is common at lower-energy experiments.

Particles in China

December 7, 2006

The New York Times discusses China and particle physics, talking (as one might expect) about BES, its recent upgrade, and Chinese work on the ILC (though, oddly, no mention of the Daya Bay neutrino experiment).

It’s a pretty good article with fun anecdotes, though I think 140 of my closest friends might object to the characterization of BES/BEPC as “dominating” and having “hegemony” over the charm energy range… (Yeah, they will after BEPC-II starts, but that’s over a year away!)

No axions for you!

December 7, 2006

If you were to read this U Buffalo press release or drop by Slashdot, you might be left with the impression that the axion has been found. This is not the case, unfortunately. (We’d love for it to be found, really.)

The axion is a particle postulated to solve the strong CP problem, one of the fundamental open questions in particle physics. One of the general principles that guides particle theory is that coincidences are unlikely: if a parameter is generally allowed to be large by the theory, and in the real world it turns out to be small, then you have to explain why it should be small. The parameter in this case is an angle called θ, which a priori could be anything (and has no obvious reason to be zero), but which is constrained by experiment to be less than 10-9 or so. The famous Peccei-Quinn mechanism uses an extra particle to drive θ back to zero dynamically.

I’ve heard varying reports on the accessibility of the actual paper that’s supposed to claim particle discovery (Journal of Physics G34, 129, which I can get using Cornell’s subscriptions). It’s not on arxiv, but there may be preprints floating about. You may need to do a bit of work to get it if you actually want to read it.

The experiment in question appears to be a heavy ion-on-emulsion affair, with only about a thousand actual nuclear interactions from which all conclusions are drawn. The claimed detection channel is electron-positron pairs with displaced vertices, corresponding to a lifetime within a couple of orders of magnitude of that of the D0. Only a thousand or so pairs are used in drawing conclusions; the authors claim to see two excesses of events near 7 and 19 MeV which they attribute to exotic particles, “more than three standard deviations” above background (their italics).

To this physicist, the bumps in question look like perfectly normal statistical fluctuations in a small sample: if you don’t know beforehand where to look for a peak, you take a penalty factor in how significant you can claim your result to be because you get to search in many places, and some of them are going to fluctuate. (The whole blind analysis craze has apparently passed the authors by.) Even if you take the paper’s claims of the statistical significance at face value, though, this does not count as a “discovery”; it is at best “evidence.” Discoveries require five standard deviations (more or less) and reliable, repeatable independent confirmation; that last part is critical (witness the pentaquark fiasco, as belligerently reported and retracted by the New Scientist).

The authors claim that their detection method makes them uniquely sensitive to axions decaying in this mode. This is nonsense. If they can actually pull a signal of an incredibly long-lived (and thus narrow) particle out of a thousand nuclear interactions, this should have been seen all over the place, whether or not the detectors are “electronic.”

This is a lot of text to spend on a non-discovery, but the fast pickup, extending as far as Wikipedia, makes me very annoyed. There is clearly a popular audience which is interested in particle physics results, but which isn’t able to judge the plausibility of fringe claims (as any visit to a Slashdot “science” comment section will show), and who will probably remember and repeat “axion found!” even if (ha, ha) a retraction were posted. I don’t know how one might go about amelioration, though.

As is to be expected, the PDG will tell you everything you ever wanted to know about axion searches, and then some.

Optics and You

December 5, 2006

I just got a new pair of glasses (the last pair met with an unfortunate incident while going through a major intersection on a bike). Still getting used to them. Unlike the previous pair, these lenses are polycarbonate, which is more shatter resistant but, I found to my dismay, much more prone to chromatic aberration than the old plastic ones. Things on-axis are in perfect focus, but as you look to the side, the red is displaced outward and the blue inward, and objects become fuzzy. (Of course, if what you’re looking at is already red or blue, they look fine off-axis.)

If you’re in the market for new glasses, Wikipedia (which, of course, knows all), can tell you about the different materials you can choose for your corrective lenses. If you really want to be old-fashioned, you can apparently still get lenses in crown glass.

For a few minutes of amusement waving your finger in front of your face, you might take a look at this 1952 note in the American Journal of Physics titled “Chromatic Aberration in the Eye“.