Comments on: Detectors 101

By: LHC Detector Performs First Test of Fundamental Forces | Cosmic Variance

LHC Detector Performs First Test of Fundamental Forces | Cosmic Variance — Fri, 02 Mar 2007 19:17:08 +0000

[...] A particularly large chunk of the detector â€“ its heaviest piece â€“ was lowered this week. CMS stands for Compact Muon Solenoid and itâ€™s the solenoid itself (preassembled with the central portion of the detector) that made the journey underground Tuesday. The solenoid is a large magnet, generating a 4 Telsa magnetic field (100,000 times stronger than the Earth’s magnetic field) with a total stored energy of 2.66 GigaJoules (equivalent to half a tonne of TNT), and is responsible for our ability to observe tracks and measure the energy of charged particles. Itâ€™s an essential and expensive component of the detector. [...]

By: Lab Lemming

Lab Lemming — Tue, 19 Dec 2006 08:58:12 +0000

Pardon the silly question, but how do you determine how many tracks your big lump of iron actually contains? I hope you don’t have to grind down and etch the whole damn thing…

By: Roberto C

Roberto C — Tue, 19 Dec 2006 08:42:13 +0000

Very concise and informative post. There is something that can be misleading tough, both in the text and in the picture. The hadronic particles can start their shower in the electromagnetic calorimeter, and they usually do. Just, their shower is less compact than an electromagnetic shower, and need a much deeper calorimeter to be fully contained. So the energy of an hadron is the sum of the energy deposited in the ECAL and in the HCAL. A naive reader (if any in this wonderful blog) can be lead to the conclusion that hardons do not interact in the ECAL.
And, I would say that the muon detector is a big chunk of iron instrumented with tracking chambers.

By: The Real World | Cosmic Variance

The Real World | Cosmic Variance — Sun, 17 Dec 2006 17:00:35 +0000

[...] In her post below, JoAnne refers to “the real world” in the literally accurate sense — the physical reality that exists independently of our understanding, in contrast to the tentative frameworks put forward by theorists as hypothetical models of that reality. But there’s a more metaphorical sense in which physicists (and academics more broadly) use the phrase “the real world” — to refer to the socio-economic milieu peopled by those outside the academy. We say things like “she spent a couple of years in the real world before going to grad school,” or “most of the time I hang out with physicists, but I do have some friends in the real world.” [...]

By: Sean

Sean — Sat, 16 Dec 2006 11:39:48 +0000

Thanks, anon., that makes sense. One of those things that just never got explained to me. (I also taught a particle physics class, but we, um, didn’t do a lot about detectors.)

Aaron– A TeV isn’t a lot of energy by macroscopic standards, but you absorb a hundred billion such particles per second and it begins to add up!

By: Jeremy Chapman

Jeremy Chapman — Fri, 15 Dec 2006 22:27:01 +0000

Great post, and very concise. Detector physics is such an interesting area to me because I am always trying to ‘visualise’ physics. I can also attest to the intricacy of the vertex detectors as I am working on a potential upgrade for LHCb’s vertex locator. designing readout electronics that trigger and pick up data from collisions every 25ns is quite a task!

By: Aaron

Aaron — Fri, 15 Dec 2006 17:05:34 +0000

Beautiful post!!! I never realized just how dense those detectors are — from your description, it sounds like they’re made mostly of solid metal! I’m also quite amused by the notion that an off-target beam can actually damage the detector! Even TeV particles have so little energy that it’s hard to see them wrecking anything except each other, but I guess a steady beam can go a long way. It’s certainly nothing I’d like to be sitting in front of!

p.s. “Pion punch” would make a great name for a drink. Preferably one that scintillates! Quinine-containing beverages like tonic water and quinquina often fluoresce nicely under UV light…

By: anon.

anon. — Fri, 15 Dec 2006 16:33:21 +0000

Sean: “it’s because they’re more massive.” In particular, the radiation length scales like mass squared. You should be able to see precisely why this happens if you try to calculate bremsstrahlung, or look up the calculation. You end up with Lorentz factors, which are factors of E/m.

By: Sean

Sean — Fri, 15 Dec 2006 11:22:07 +0000

Could someone tell a poor cosmologist why muons don’t get stopped by the electromagnetic calorimeter like electrons do? And don’t tell me “it’s because they’re more massive,” although I suspect that has something to do with the answer. But aren’t they all relativistic in any event, so why does the mass matter? Greater momentum transfer is needed to make them non-relativistic?

(zq– we use SpamKarma, which is an excellent spam filter. In fact we get hundreds of spam comments every day, very few of which get through. Sometimes it’s overzealous.)

By: JoAnne

JoAnne — Fri, 15 Dec 2006 07:25:22 +0000

Alejandro, that was a typo in the text - should have been hermeticity! Hermeticity is a term used in calorimeters to describe maximal coverage for particles under all emission angles, combined with minimal leakage. I’ve fixed the typo in the text now… Hermiticity, on the other hand, is a mathematical property of matrices and operators. The Hermitian conjugate of a matrix is equal to the complex conjugate of the transpose of the matrix. A unitary matrix is a matrix whose inverse is equal to its Hermitian conjugate.

I suppose the theorist in me forced my fingers to type hermiticity instead of hermeticity!

By: Michael Saelim

Michael Saelim — Fri, 15 Dec 2006 02:20:13 +0000

Alejandro, I believe you’re thinking of Hermitian operators. The hermeticity/hermiticity in this post is more related to the idea of a hermetic seal or hermetic calorimeter.

By: Off-Lattice Links « Life on the Lattice

Off-Lattice Links « Life on the Lattice — Fri, 15 Dec 2006 00:07:13 +0000

[...] JoAnne at Cosmic Variance has a nice introductory post about particle detectors. More about the subject, with a description of the CLEO-c detector, can be found at Superweak. [...]

By: Jennifer Ouellette

Jennifer Ouellette — Thu, 14 Dec 2006 23:57:00 +0000

I have clipped and filed this particular post for future reference. It’s one of the best breakdowns I’ve encountered of how, exactly, physicists can “see” something so tiny that lasts a fraction of a second… something that non-scientists often have a very difficult time grasping/appreciating.

By: Arun M

Arun M — Thu, 14 Dec 2006 23:31:33 +0000

Thanks JoAnne, for that down to earth introduction to detectors. The article by Tao Han also made very good reading. As a particle theory student it is nice to understand how the cross sections/lifetimes etc are actually measured by the experimentalists. I look forward to more posts of this nature.

By: On particle detectors « Entertaining Research

On particle detectors « Entertaining Research — Thu, 14 Dec 2006 21:57:24 +0000

[...] Even if you aren’t a HEP-guy, this might be a nice post to read–just to know the amount of engineering that goes into some of the particle physics experiments! [...]

By: zq

zq — Thu, 14 Dec 2006 20:55:16 +0000

I am just curious that if this blog allow anonymous reply, why doesn’t it get lotsa spams.

By: Life on the Lattice

Life on the Lattice — Thu, 14 Dec 2006 19:47:49 +0000

Off-Lattice Links

JoAnne at Cosmic Variance has a nice introductory post about particle detectors. More about the subject, with a description of the CLEO-c detector, can be found at Superweak. [...]

By: Alejandro Rivero

Alejandro Rivero — Thu, 14 Dec 2006 18:27:55 +0000

“and the degree of snugness is called hermiticity. A detector whose components do not have a snug fit is not hermetic”

hermiticity is related to unitarity, is it?

By: Joseph Smidt

Joseph Smidt — Thu, 14 Dec 2006 18:00:52 +0000

Thanks again also for referencing hep-ph/0508097. I’ve been reading the paper and it is great. I learn so much from this blog.

By: graviton383

graviton383 — Thu, 14 Dec 2006 17:33:52 +0000

Excellent job on a very necessary post! Seeing these detectors is a very memorable experience. Steven Hawking visited both ATLAS and CMS last month at CERN (before they begin to observe TeV-scale black holes when the LHC turns on!)

EDT: The ATLAS and CMS detectors weigh on the order of 10,000 Tons and are as large as a building. What you saw on TV was the lowering of a single part of the detector down to the pit where ATLAS is being assembled. An interesting tidbit from an experimeter friend is that the DENSITY of ATLAS is such that it would float on water whereas CMS would sink!