Undergrad Difference between CMS and ATLAS wrt channels

[josvermeulen]

I was wondering what the differences are between CMS and ATLAS when it comes to their channels. I know they detect for the H →γγ, H → 4l, H → eνμν channels, but I was wondering how they differ in their detection for these channels.

I was also wondering if there were any noticeable differences in the experimental techniques and capabilities. If someone can guide me to a review paper on this I would also appreciate that, but most seem to be filled with technical data and no clear distinction between the both.

Thanks in advance!

 

[fresh_42]

Have you read the homepage of CERN and the many links on the introduction pages?
https://home.cern/about/experiments/atlas

 

[josvermeulen]

Have you read the homepage of CERN and the many links on the introduction pages?
https://home.cern/about/experiments/atlas

I have indeed. The problem is with the specfics itself of the different channels. There seem to be small differences between CMS and ATLAS but I can't seen to really figure it out.

 

[mfb]

Quoting myself from elsewhere, to have it here as well:

They have different detector geometries, but the physics programs and the sensitivities are very similar in most cases.

The main difference is the overall approach with the magnetic field. You want a strong magnetic field in the inner tracking detector, which means you need a large magnet coil.

* If you put that coil around the tracking detector (like ATLAS), the coil is smaller and you have a lot of space for large calorimeters outside. The downside: All particles have to cross the coil, which means you have to make it lightweight, leading to a lower field strength.
* If you put the calorimeters inside the coil (like CMS), you need a huge coil and you have to make the calorimeters very compact to fit in. On the positive side, the coil can be more massive because it doesn't influence the calorimetry any more, therefore it can provide a stronger magnetic field.

The result: ATLAS is large, CMS is heavy. ATLAS can track the directions of photons a bit better, CMS has a slightly better energy resolution for charged tracks, and similar small differences.

On the software side, CMS implemented a particle flow algorithm for jet energies, ATLAS is looking into that as far as I know but they don't have it yet. That improves the energy resolution for jets because you know which track came from where and contributed how much to the energy in the calorimeter.

Analyses are done independently, so naturally the experiments will sometimes use different methods. There is not always a clear best method, sometimes it depends on the detector, and often it is a matter of taste.

The expected sensitivities for the Higgs decay modes are very similar.

I don't know the numbers by heart. ATLAS can determine the origin of photons better, which reduces the background (where the two photons come from different primary vertices) and increases the energy resolution (because you are more likely to find the right point where they come from, important for the invariant mass calculation). CMS has a better intrinsic energy resolution in the calorimeter itself. The background subtraction differs a bit.

Similar for muons, the details differ but the overall strategy is very similar. For the details, you'll have to read the papers.

 

[ChrisVer]

No paper is going to explain the differencies between two separate detectors that only share the same beams...
But nothing is very special about the decay channel searches that allow 1 detector to look at it but halts the other... for example ATLAS could also discover the Higgs boson in the H-->ZZ*-->4l channel with Run-1 data.