# What is LHC:ATLAS?

Accelerating particles close to the speed of light and colliding them hoping something interesting will happen.

## LHC

The Large Hadron Collider (LHC) is the result of a multinational collaboration with over 10,000 scientists and 100 countries.[1]

By colliding charged particles, mostly protons, at speeds close to that of the speed of light, the LHC has made discoveries ranging from the confirmation of the existence and properties of the Higgs boson[2], to the likely non-existence of supersymmetrical particles [3] [4], and, recently, the discovery of two baryons composed of one bottom quark and two up quarks, and one bottom quark and two down quarks. [5]

## ATLAS

A picture of the caliometers of the ATLAS detector.

ATLAS is one of the four main detectors at LHC, and measures the decay products from the collisions. When protons hit head on[6] in inelastic relativistic collisions[7], they create new particles which spread around in the detector. The atlas detector is constructed as a cylinder with multiple layers. The different layers can measure different properties of the decay products.

### ATLAS Structure

The ATLAS detector is constructed as a cylinder through which the particle beams shoot. Upon a head-on collision,[8] the protons decay to other particles which scatter in all directions. This is possible while conserving momentum precisely because the protons collided head on.

Since we cannot predict where the decay go, the detectors are constructed as approximate onion-like shells around the beam direction as illustrated below.[9][10]

Schematic of ATLAS from Kjende@CERN

In the innermost shell, we have tracking devices such as the Xenon gas tubes, which can track as charged particles go through them, and the PIXEL/SCT trackers. Which is explained well in the following video:

Knowing the trajectory of the particles along with strong magnets which curve the trajectory allows us, with some difficulty when dealing with multiple particles, to calculate the relativistic momentum of the charged particles.

In the next shell, the particles pass through calorimeters which tries to measure the energy of the particles by absorbing them passively through high density materials such as lead, and actively through liquid argon for example.

Electromagnetic and Hadronic calorimeters can stop most particles, but neutrinos and muons.

The next shells detects Cherenkov and transition radiation, and at the outermost shell, we have the the muon spectrometers. [11][12]

ATLAS detectors

On the picture above, we see the inner shells where the trackers can see the charged particles such as the proton, electron and muon. Notice, how the magnets bend the paths of the particles.

In the calorimeters, we see most particles being absorbed and decay to other particles. By observing the decay particles, and considering quantum symmetries, we can figure out the types of decay particles as has been colored in on the image.

Notice further that we cannot directly detect neutrinos in the detector. Instead, we have to rely on inference from looking at the missing total energy in the system because we know there's conservation of energy even if some of it goes to creating new particles.

### Listen to collisions detected by ATLAS

You can listen to the audio of detected collisions from ATLAS courtesy of Ewan Hill below.

The audio is generated by associating the values from different detectors of a subset of the collisions with MIDI tones and normalizing the frequencies to be within the audible range. 30 seconds is approximately equivalent to one event. [13]

1. Most collisions only strays resulting in an elastic collision where the particles get a slight deviation in $\eta$: The pseudorapidity. The fraction of head on collisions versus straying collisions is well known, so the number of straying collisions are measured, so we can normalize the data to get the number of head-on collisions. ↩︎

2. Since we collide the protons with speeds approaching the speed of light, we have to account for the significant relativistic effects. ↩︎

3. It's slightly misleading to talk about single collisions as at every event, which only lasts a few i, about $\mu \approx 50$ collisions happen. ↩︎

4. This is not entirely accurate portrayal as there are places where the detector is less accurate (for example the muon detectors) depending on where in the detector the particles collide. Source: Conversations with Troels Petersen (NBI). ↩︎