I usually write a lot about the CERN LHC, as those who follow me know. I however rarely discuss the machine itself. In a recent post, I detailed the different steps of the LHC success story (see [**here**](https://steemit.com/science/@lemouth/the-large-hadron-collider-the-story-of-the-largest-man-made-experimental-apparatus)).

<div class="pull-left">

<center> ![](https://s26.postimg.org/ys5szhlex/lhc_mag.png)
<sub> [image credits: [CERN](https://cds.cern.ch/record/1741036)]</sub> </center> 
</div>

I have nevertheless not yet described **what is the LHC itself**, and as you can guess, it is the topic of this post.

**I will focus on the machine, the goals behind the building of such a huge accelerator** (I recall this is the largest man-made experiment in the world) **and how it actually works**.

I will also try to motivate the gain for the particle physics community in having such a device operating today.

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## THE LARGE HADRON COLLIDER IN A FEW WORDS ##

The LHC is a machine built at CERN and located at the border between France and Switzerland. It is a ring long of 27 kilometers, buried at a depth varying from 45 meters to 170 meters. 

**Two beams of particles are accelerated in the ring, and then collided at four interaction points** (see the picture below on the right).

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<center> ![](https://s26.postimg.org/sfv93e7qh/lhc_map.png)
<sub> [image credits: [LHC@home](http://lhcathome.web.cern.ch/about)]</sub> </center> 
</div>

The machine has been designed so that it can handle **beams either of accelerated protons or of accelerated lead ions**, so that three types of collisions can occur (proton-proton, proton-lead and lead-lead). 

The size of the ring is such that the particles can be accelerated to energies that have never been reached so far. As a result, **a new energy regime can be probed** (in the aim to discover something). 

Just to recall some special relativity, **reaching high energy means in fact that we get options to produce massive new particles** (Einstein told us that mass is energy).

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## MOTIVATIONS FOR THE LHC ##

As said above, **one of the main motivation of the LHC is to have an experiment allowing us to reach high energies and discover new particles**. But why are we confident some new phenomena can be found there? Actually, we are not. 

The Standard Model of particle physics (the theory describing the world of the elementary particles) is known to contain a set of conceptual issues and limitations. In order to cure these ‘problems’, **physicists have started to design more general theories**. 

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<center> ![](https://s26.postimg.org/8cg853549/cms.jpg)
<sub> [image credits: [CMS](http://cms.web.cern.ch/news/cms-reaches-100-publications)]</sub> </center> 
</div>

Most of these theories predict the existence of new particles that are more massive than the Standard Model ones**. And those new guys have still to be discovered**, in particular at the LHC. 

This machine is currently the best place for conducting the searches.

New particle discoveries are however not the single goal behind the building of the LHC. With the energy available, **the LHC is actually a factory producing tons of particles of the Standard Model**. 

In this way, the properties of those particles (including the Higgs boson) will be scrutinized very deeply, and physicists will be able to look for deviations from the Standard Model.

That is a very good alternative fashion to search for new phenomena (and I recall that all options must be investigated, just to be sure not to miss anything).


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## HANDLING SO MANY COLLISIONS? ##

I said above that mass is roughly speaking energy, so that colliding particles at high energy means that we have a lot of energy to produce new massive particles. The mass-energy equivalence is however also true for the protons that we collide.

Therefore, in order to make them very energetic, there is only a single way: we need to accelerate them a lot. The proton mass being fixed, we need to get kinetic energy. In the LHC tunnel, **protons are accelerated at 99.99999% of the speed of light** before being smashed.

One single beam of protons contains about 2800 well-separated bunches of protons traveling into the LHC ring. The ring being 27 kilometers-long, this means that the speed of a bunch is of about **11000 turns per second**.

**This is equivalent to say that that at each interaction point, 40 millions of collisions occur at every second**. The electronics of the detectors is then such that it is possible to sort out very quickly what is going on in a given collision, and record it if interesting.

<div class="pull-right">

<center> ![](https://s26.postimg.org/ckflrkvxl/cern-data.jpg)
<sub> [image credits: [CERN](https://home.cern/about/updates/2013/02/cern-data-centre-passes-100-petabytes)]</sub> </center> 
</div>

The reason behind the fact that we cannot record everything is that we have a finite amount of disk space and time is needed to treat data. **Out of the 40 millions of collisions happening every second, roughly 1000 of them can indeed be recorded**. 

We must thus rely on the electronics to decide what to record and what to ignore. And we know how to distinguish a boring collision from an interesting one. Most of the stuff ongoing is actually related to the production of low-energetic strongly-interacting particles, for which there is nothing new to learn. 

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## IN PRACTICE ##

Particle colliders simply rely on **electromagnetism**. **An electric field can be used to accelerate particles, and a magnetic field can be used to control their trajectories**.

<div class="pull-left">

<center> ![](https://s26.postimg.org/bpiz070t5/EMfield.jpg)
<sub> [image credits: [NASA](https://www.nasa.gov/mission_pages/sunearth/news/gallery/Earths-magneticfieldlines-dipole.html)]</sub> </center> 
</div>

In the LHC framework, we are dealing with thousands of magnets that allow to keep the beam under control. **The LHC magnets generate a magnetic field of 8.3 Tesla, that is more than 100.000 times the magnetic field of Earth**. 

This huge field allows one to keep the trajectory of the beams under a total control, both before and after the collisions.

Generating such a huge magnetic field is actually a big deal, and one needs to rely on huge electric currents to achieve this. The LHC magnets are fed with a current of **12000 Amperes**, that is about 1000 times what we have in our homes. 

This is why **cryogenics is necessary**: the LHC is cooled down to 1.9 Kelvin, that is cooler than what is found in space, and **superconductivity** can then be used.

## SUMMARY AND REFERENCES ##

In this article, I tried to summarize the main motivations of high energy physicists in building the LHC. I have provided a few details on how the machine and its detectors work, that I hope are useful. These two respectively rely simply on electromagnetism, and on a powerful electronic system.

For more information, I recommend to have a look to on the [**CERN LHC website**](https://lhc-machine-outreach.web.cern.ch/).
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