What's the main difference between ATLAS and CMS detectors in LHC?

petergreat

And can the two run simultaneously?

fss

They are constructed differently and yes, they can run at the same time.

Bill_K

CMS and ATLAS do basically the same thing, but were designed by independent teams making different compromises. As a result the two detectors differ slightly in their capabilities. A comparison gets rather technical.

Similarities: Both detectors consist of four concentric subsystems. From inside to out:

1) Inner tracker, composed of semiconductor chips, contained within a magnetic field. This reconstructs curved tracks for all charged particles.
2) Electromagnetic calorimeter (ECAL). Stops photons and electrons, measuring their energy.
3) Hadronic calorimeter (HCAL). Stops hadrons (pions, etc), measuring their energy.
4) Muon detector. Likewise for muons.

Differences: ATLAS uses a 2T magnetic field surrounding (1). CMS uses a 4T magnetic field surrounding (1), (2) and (3). 4T means the CMS tracker has better momentum resolution but imposes restrictions on the design of the other parts. ATLAS uses Liquid Argon for the ECAL, while CMS uses crystal PbWO₄. For each of the calorimeters, the resolution in space and energy are somewhat different. ATLAS wins on the HCAL, CMS wins on the ECAL.

petergreat

I don't understand how they can run at the same time. If the proton beam is consumed at ATLAS, how can the beam continue to travel through the tunnel to collide at CMS? Or is it the case that the beams are divided between CMS and ATLAS, lowering the luminosity by a half when the two experiments run together?

fzero

The beams are not consumed. They are focused together inside the detectors (see http://lhc-machine-outreach.web.cern...collisions.htm) but most protons do not interact (20 collisions per 10¹¹ protons per bunch) and continue back into the ring.

Vanadium 50

Indeed, most of the beam that's injected is never used in collisions. What happens is that the beam focus degrades over time and after some number of hours it is sufficiently diffuse that it's better to dump and refill.

ZapperZ

Where do you get such numbers?

This is no different than at the Tevatron with DZero and CDF collecting data at the same time. Furthermore, at the LHC, there's also ALICE! Don't forget her!

Zz.

Vanadium 50

And LHCb.

Gokul43201

Or if you like your answers in cheesy rhyme, this comes close:

petergreat

Though Alice is a heavy ion experiment so surely can't run at the same time.

fzero

Alice is designed to measure both pp and ion-ion events: http://aliceinfo.cern.ch/Public/en/C...Chap2_DAQ.html

ZapperZ

Again, as I've questioned in my previous post, where exactly do you get your information from? I believe your statement has been sufficiently addressed in the fzero's response. Or, you can simply look at results out of ALICE so far.

Zz.

Vanadium 50

Indeed, ALICE has more pp papers at the moment than PbPb papers.

jjo

This talk pdf file has technical details that compare the detectors.
https://twiki.cern.ch/twiki/bin/view...s_overview.pdf

This talk has more details but says that Bill_K said in his post in more words and pictures. One of the key parts of the design of the detectors is where to put the magnet. That has an impact on the rest of the detector ... As Bill_K said both "were designed by independent teams making different compromises".

This talk says it's a summary of this paper that compares the detectors - 80 pages.
Annu. Rev. Nucl. Part. Sci. 2006. 56:375–440:
http://vsharma.ucsd.edu/lhc/annurev....103.181209.pdf

Best regards,
JJO