CERN BROCHURES :

INFORMATIONS BOOKLET FOR TEACHERS

WG LEADER: ANTONELLA DEL ROSSO

HOW TO DETECT A PARTICLE

When a particle passes through a detector it leaves a track like an animal leaves tracks on the sand. 

When a charged particle moves through a medium it interacts with the atoms in the medium. The result of this interaction is the loss of energy. To detect a particle means to convert the deposited energy in some manner that can be recorded. The information recorded can reveal details of particle nature such as its mass and its electric charge as the prints left on the sand could give information on the animal.

Even if detectors come in a variety of types and sizes they follow the same principle. They don’t see directly the particles but they reveal the effects of the particles.

According to Einstein’s law in a collision between two elementary particles (i.e. electron and positron), if there is enough energy in the initial state, it is possible to create new particles. The particles created in the high-energy collision are as small as 10^-16 cm and live for only few hundredths of millionths of a second. Recording these tiny and ephemeral pieces of matter is the job of the detectors.

Imaging has always played and important role in particle physics. In earlier days much of the data was recorded in photographic form. Many of these images look like abstract art painting. The quality of particle imagery and the range of information it provides have both improved over the years. Whereas bubble-chamber photograph shows the track of all the charged particles directly, computers can select the important tracks.

The essential clue to understanding the images of particle physics is that they show the track of the particles and not the particles themselves. How a pion looks like is a mystery but its passage through a medium, solid, liquid or gas, can be recorded. Particle physicists have became as expert at interpreting the types of tracks left by different particles as a zoologist interprets the track of different animals.

For instants, many detectors are based around a magnet, because charged particles bend in a magnetic field. A charged track is the signature of a charged particle and if you know the direction of the magnetic field, the way the track curves tells us if the particle is positive or negative (Fleming’s left-hand rule). The radius of curvature is also important and depends on particle’s velocity and mass, so the momentum of particles can be calculated. The paths of particles with greater momentum bend less than those of lesser momentum. A particle with greater momentum will spend less time in the magnetic field and has greater inertia than the particle with lesser momentum, so it's path will bend less than that of the lesser-momentum particle.

Most of the subatomic zoo particles live less than billionth of a second. This time is long enough to leave measurable tracks. A relatively long life particle leaves a long track passing through a detector. A short life particle usually decays giving birth to new more particles. This decay is often identified as a single track turns into several tracks.

Neutral particles leave no track in a detector so their presence can be deduced only from their interactions or their decay products.

There are three possible ways in which high-energy particles may be detected when they interact with matter

• If the high-energy particles ionize atoms in matter as they pass through it, the ions and the electrons produced may be detected as tiny electrical signals using suitable positioned electrodes.
• A second method of detection is possible if the high-energy particles create ions producing visible tracks.
• A third way of detecting high energy particles can be used if the particles produce excited atoms which lose energy by emitting quanta of light. The light is then converted into an electrical signal by electronics devices.

An ideal detector would reveal the presence of all types of particles. It would show their trajectories and collisions in three-dimensional space and allows us to calculate charge, momentum, and energy. It would resolve positions to within a fraction of a millimeter and react quickly enough to detect millions of events per second and output data on them directly to a computer for processing.

A typical detector, at a modern particle physics laboratory, consists of many different sub-detectors to provide a range of information about the events being studied.

The many sub-detectors are sandwiched together and wrapped around the beam pipe at the annihilation point. The result is a huge multi layer detector.

During a colliding beam experiment the particles radiates at all directions, so the detector is spherical or, more commonly, cylindrical.

The reason that detectors are divided into many components is that each component tests for a special set of particle properties. These components are stacked so that all particles will go through the different layers sequentially. A particle will not be evident until it either interacts with the detector in a measurable fashion, or decays into detectable particles.

 To measure the directions, momenta, and signs of charge for charged particles produced in each collision, we use very finely subdivided sensors ("tracking detectors"). In order to know the locations of the particle tracks precisely, we must know the locations of each of these sensors very precisely. These sensors emit a signal when struck by a charged particle. From these signals and the known detector locations, charged particle trajectories can be reconstructed.

A magnetic field directed parallel to the beam axis and produced by a cylindrical coil of superconducting wire surrounding the tracking detectors, causes the trajectories to bend in circular paths: the radius of each circle determines the momentum, and the location of its center the sign of charge.

The next layer is the calorimeter. It measure the energy carried by particles traversing it. This device is finely subdivided into segments that absorb all the energies of incident particles and emits signals proportional to those energies.

The calorimeter consists of layers of absorber (high-atomic-number material) separated by sensing devices. When high-energy particles traverse the absorber they produce shower. What happens is that their initial energies are transformed into the rest masses of numerous low energy (but still fast-moving) particles. The number of such particles is proportional to the incident energy, and their presence is detected by a sensing system between the absorber plates. The sensing system produces an electric signal so the greater the incident energy, the greater the signal. To determine the precise relation between this electric signal and the corresponding energy one must calibrate the calorimeter.

Usually the calorimeter is makes up two parts depending of the absorber medium. The inner part is the electromagnetic calorimeter, it measures the energy carried by electrons and photons. The outer part is the hadronic calorimeter that measures the energy carried by hadrons (protons, neutrons, pions and other mesons).

Muons are the only charged particle that can travel through all of the calorimeter material and reach the outer layer. The Muon System determines the signs and momenta of muons with better precision than the inner tracking system does. 

The highly penetrating, weakly interacting neutrinos will not be stopped in the detector but their energy can be calculated from the difference between known energy of the annihilation and the total energy measured in the calorimeters.

QUESTIONS

1)      Large detectors are layered detector.

a)      What is the point in having many layers?

b)      What is the function of the tracking system?

c)      What is measured by an electromagnetic calorimeter?

d)      How it is possible to calculate the energy of the particle?

2)      Neutral particles are difficult to detect.

a)      Why?

b)      Why are neutrinos much more difficult to detect than neutrons?

c)      Why are photons easier to detect than either neutrons or neutrino?

UNITS OF ENERGY USED BY PARTICLES PHYSICISTS

The particles in the accelerator are moving extremely close to the speed of light. They obey the rule of Einstein’s theory of special relativity in which moving bodies gain mass as they gain energy, and in which times and distances depend critically upon the reference frame in which they are measured. Particle physicists usually talk of the mass of the particles in term of equivalent amounts of energy.

The mass of a proton, for example, is about 1,67*10^-27 Kg, a small quantity. But, according to Einstein, energy is the product of the mass and the square of the velocity of light  (E=mc² ). In the units used by particle physicists the proton mass become 938 MeV/c². 

To make life easier, the particle physicists also choose to work in units where the velocity of light c is equal to 1, so the proton’s mass becomes simply 938 MeV.

The electron’s mass is significantly smaller than the proton’s at 0,51 MeV.

On this scale the average human mass would be 4*10³¹ MeV.

The mass of a proton at rest is 938 MeV, but when it is accelerated close to the speed of light, its mass increases as it gains energy.

LHC accelerator can accelerate protons to a mass of  7 TeV, this means that its mass can be  effectively  increased 7000 times.

Accelerated to the same extent, the average human would weight as much as a juggernaut.