by Alessia Saggio

Every year, my university organizes a trip to CERN for Bachelor’s students only, to give them the chance to get acquainted with the world of Particle Physics before starting the Master. This year I was one of the three PhD students who accompanied them, and I thought it would be nice to share my feelings here with you since it was a really nice experience.

In fact, it was my fourth time at CERN, but despite that I had never had the chance to visit the CMS detector, the experiment in which I work – now famous for the discovery of the Higgs boson together with ATLAS.

We actually visited many of the experiments that CERN hosts:

– the AD: the Antiproton Decelerator, a machine that produces low-energy antiprotons for antimatter studies purposes, with its experiments: ALPHA, ASACUSA, ATRAP, BASE;

– COMPASS, a fixed-target experiment located in the North Area that receives muon and hadron beams from the Super Proton Synchrotron (SPS) – one of the pre-accelerators of the LHC – for studies of hadron structure and hadron spectroscopy;

– NA62, also located in the North Area, an experiment to study rare kaon decays in order to check the validity of the Standard Model.

These experiments are interesting under many aspects: their shape and size vary a lot since each of them is built for a specific purpose, they cover many different fields of particle physics and therefore a close look to them was exciting.

To me the most exciting visit we had was the one at the CMS experiment. I found striking to see the whole detector with its different sub-detectors that I knew only on paper and to actually attest how big it is. It is amazing, and not only because of its size: the magnets and the different sub-detectors have different colors, this resulting in a stunning view for your eyes! If you haven’t seen it before, I strongly recommend you to drop by, either if study physics and you know it already, or if you don’t have a clue about it: there are guides that will explain you everything you will want to know about it in a clear and simple way.

In case you are already programming your trip to CERN after these few lines ( 😛 ), here is a simple description of the CMS detector that could help you to better understand it.

CMS is an acronym for Compact Muon Solenoid. It has a cylindrical structure in order to detect the particles produced by the collision between two high-energy proton beams traveling in opposite directions and accelerated by the LHC. A coil of superconducting cables generates a magnetic field of 4 Tesla (~100,000 times the magnetic field of the Earth!).

In most cases, particles are detected by mean of their interaction with matter. Since different particles have different interaction properties, CMS is made up of different layers. From the inner part of the detector (the region close to the collision) to the outer part, one can find:

– Tracker: entirely made of silicon, it tracks the charged particles that curve in the magnetic field thus providing a measurement of the momentum of the particle.

– Electromagnetic calorimeter (ECAL): it measures with high accuracy the energies of electrons and photons. Electromagnetic showers are initiated by the impinging electrons and photons that develop inside the calorimeter producing scintillation light, which is detected, in proportion to the particle’s energy.

– Hadronic Calorimeter (HCAL): it measures the energy of hadrons, i.e. particles made up of quarks and gluons (protons, neutrons, pions and kaons) which penetrate more than electrons and photons due to their different properties of interaction with matter.

– Superconducting solenoid: a huge magnet to generate the 4 T magnetic field;

– Muon chambers and return yoke: muons are charged particles just like electrons and positrons, but 200 times more massive. They can penetrate several meters of iron without interacting, and that’s why they cannot be detected by the ECAL but they need a specific detector. Muon chambers are gas detector that allow the identification of the muons and the measurement of their momenta, interleaved with magnet coils (return yoke) that guide the magnetic field.


Here are some pictures of the CMS detector (first alone in all its glory, then with the UCL students, and with me) taken by my colleagues Martin Delcourt and Sébastien Wertz, which I deeply thank for taking a good camera instead of a miserable smartphone…

P.S. In case you are interested, below you can find some links to book your visit at CMS or at CERN in general: