How a Particle Accelerator and Detector Work

The LHC Experiments

The Large Hadron Collider is the world’s largest particle accelerator, accelerating enormous quantities of protons in opposite directions at energies of 7 TeV, reaching a total energy at collision of 14 TeV.  This 14 times that Fermilab collider, the Tevatron, in Illinois! The particles are “created” and injected into the accelerator which then travel in a circle measured at about 27 kilometers in circumference.  The speed of the beam of particles increases close to that of light, which is approximately 30,000 kilometers per second.  When the protons meet, the collision causes different particles to be created, particles with which we are familiar, and physicists hope, the birth of new undiscovered particles and phenomena.

Accelerators in general are made up of injectors, intermediate accelerators and a main machine, where the very high accelerating occurs.  The detecting usually occurs at multiple detector sites, which are located at intervals along the tunnel, rather the main body of the accelerator.

For example, the picture above shows an underground view of the LHC tunnel and the four main experiments that will be conducted, CMS, ATLAS, ALICE and LHCb, the last one with which Syracuse University is involved!

• CMS (Compact Muon Solenoid) and ATLAS are experiments that will try to answer questions and solve problems about the universe, such as the possible existence of the Higgs mechanism, supersymmetry, dark matter and extra dimensions. 

• ALICE (A Large Ion Collider Experiment) will investigate ultra-high energy proton-proton collisions and lead-lead collisions which may help in understand the beginning of the universe. 

• LHCb (Large Hadron Collider Beauty) will be searching primarily for the decays of B mesons in attempts to investigate CP violation and why there is more matter than antimatter in the universe, as well as testing SUSY theories and the Higgs mechanism. 

• There are two smaller experiments, LHCf and TOTEM, which will investigate more specific phenomena.  LHCf (Large Hadron Collider Forward) is a set of smaller detectors that are located on either side of the ATLAS apparatus, about 450 feet from where the beams will collide in ATLAS, and will be analyzing ultra-high energy cosmic rays via neutral pion creations from ATLAS.  TOTEM (Total Cross Section, Elastic Scattering & Diffraction Dissociation) is a set of silicon detectors located near the CMS apparatus that will be measuring the size of the proton.  

Moments in the Life of a Proton

Before any collisions take place, the protons must go through multiple systems to increase their energy level from their initial energy to the final energy of 7 TeV.  But first, physicists need to have protons to accelerate! How do they do this? Can they “make” protons out of nothing? No way! What actually happens is protons are separated from electrons in hydrogen atoms (which are made up of just that, one electron and one proton) and then sent through a combination of accelerators. 

The picture below shows the routes of each particle, along with other apparatuses that are apart of the CERN accelerator complex.

The first part of the procedure is the linear accelerator, Linac2. This increases energy of the protons and sends them to the Proton Synchrotron Booster (PSB). Protons are then injected at 1.4 GeV into the Proton Synchrotron (PS) at 25 GeV. Next, the Super Proton Synchrotron (SPS) receives the protons from the PS and increases the energy of protons up to 450 GeV.  And finally, the protons are directed to the main LHC tunnel where they circulate, the two separate proton beams circulating in opposite directions, for a certain amount of time. The particles reach speeds close to the speed of light (relativistic) and energies of about 7 TeV, then collide at each experiment site. When the collisions occur head on, a total energy of 14 TeV is obtained!

For ALICE, the Pb ions come from a vaporized lead source and sent through Linac3, another linear accelerator, to Low Energy Ion Ring (LEIR) and then follows the same route as the protons do only colliding at the ALICE apparatus. 

Detecting

Below is a cross section of a typical detector seen at a particle accelerator.

The beam pipe is as long pipe with small cross section in which the particles travel. Depending on the type of experiment being executed, the particles can either enter from both sides of the beam pipe (into the picture and out of the picture), or travel from one side to the other, aiming at a fixed target. 

The tracking chamber records particle tracks to a large system of computers. 

The magnetic coil creates a magnetic field in the tracking chamber such that the charged particle tracks may bend in certain directions enabling physicists to determine the charge of a particle.  This coil also allows for physicists to measure the momenta of the particles passing through. 

These particles deposit on the E-M and Hadron Calorimeters.  The special materials the calorimeters are made of.   Physicists can determine the identity of the particle created after the initial collision by analyzing how far into a layer of the calorimeters a particle travels.  If a particle stops at a layer in the E-M calorimeter then it is known that the particle is electromagnetic, an electron or a photon, for example.  If it continues further into the Hadron calorimeter, the particle then must be a hadron, such as a proton or a neutron. 

If the particles do not stop at either of these layers of the detector and continue through the Magnetized Iron, the particles must then be muons.  Physicists know this because muons can penetrate just about everything.  In fact, muons are passing through us right now, because they are cosmic particles as well (coming from the sky). 

Thus, the Muon Chamber, record the number of particles (or muons) that travel through.

This is a side view of the detector, illustrating the tracks of each type of particle:

(Images: Courtesy of CERN and Particle Adventure)