the DELPHI underground exhibition


the LHCb site


Author: Philippe Charpentier

DELPHI is the acronym for DEtector for Lepton, Photon and Hadron Identification. It was an experiment operating at the CERN Large Electron Positron collider (LEP). It was located at pit 8 of what now is the CERN Large Hadron Collider (LHC), 105 metres underground (50 seconds by lift), near the Geneva Airport from which one can see its surface buildings. The journey from CERN (Meyrin site) takes about 10 minutes by bus.

After the stop of LEP in November 2000, part of the DELPHI detector (the so-called endcaps) was dismantled, brought to surface and eventually scrapped. The largest part of it however (the barrel) was left in its cavern to become a visit place. It has been moved from its operation location on the beam line to a corner of the cavern in December 2002. The site is now preparing the installation of the LHCb experiment of the LHC.

NOTE: When you arrive at the LHCb/DELPHI site you will have to park before the entrance (if you don't have access contact the Visits Service).
You will be welcomed at the entrance of the site (personnel lift, PZ, and not machine lift, PM) by the site manager or the guard. He will then lead you through the small square next to the entrance to the glass gallery where the entrance for visitors is.

Each guide should pass through the turnstile with a magnetic card; the site manager/guard will open the SAS for visitors. A book has to be filled by the guide with his/her name and the number of visitors.

The visit starts in the glass gallery with an exhibition showing information on the LEP accelerator, the DELPHI and the LHCb detectors and the DELPHI physics results obtained between 1989 and 2000. The PZ lift will then bring you 105 meters underground to visit the DELPHI underground exhibition.

This document gives you information on:

Safety rules

Visitors should follow all instructions from the guides and not walk out of the indicated path. It is of course forbidden to touch the detectors, but pictures or movies can of course be taken.

There should not be more than 10 visitors per guide, and not more than 2 groups of maximum 10+1 persons should be present at any time in the cavern.All visitors should wear a safety helmet, distributed at the surface by the guides.All children under 12 years of age should be accompanied, under 10 years of age there should be one adult per child. Children under 6 are not admitted in underground areas.

The evacuation should be immediate in case of identified fire or when the emergency alarm is ringing. Visitors should be lead downstairs the platform into the sas giving access to the lift (as it is pressurised). The lift may be used for evacuation if it is operational, as it is a safe place.

The visits to the DELPHI underground exhibition are for the time being not authorised for disabled persons, as it is necessary to climb stairs up. Persons subject to claustrophobia or vertigo might be willing to remain on the surface. There is no restriction in the DELPHI area for people wearing heart stimulators nor hearing aid devices.

Conditions to visit DELPHI

The normal procedure for visiting the DELPHI underground exhibition is to make a request to the CERN visits service. Groups' visits can be arrange by them and they will provide the necessary guides.The LHCb GLIMOS (Group Leader in Matters Of Safety) will notify the visits service of any event that could prevent visits (e.g. lift maintenance). The "gérants de site" will be warned by the visits service for all organised visits, included the number of expected visitors.

For members of the DELPHI or LHCb Collaborations, they are authorised to accompany a group of people under the following conditions:


Restrictions :None
What to see :CERN environment, the Prevessin site, the UA2 site (on the road between Prevessin and DELPHI)
What to say :General subjects :

NOTE: Before leaving the bus, encourage people to leave any large bags and coats behind. In the experimental gallery the temperature is constant: 22°C.
Warn them also that watches could be stopped by the stray field of the LHCb magnet (only when it is on, indicated by signs) so they might wish to leave them behind.
Credit cards are safe but, if visitors are worried they should leave them in the bus.


surface exhibition

Restrictions :None
What to see :Posters, pictures, parts of detectors, physics results.
What to say :

DELPHI in numbers

The underground exhibition

Take the lift with a maximum of 10 visitors and one guide. The visitors reach the lift through the sas that will be open by the "gérant de site" or the guide himself. The guide should use the turnstile and his own valid magnetic card, not use the sas. He should fill in the logbook that he finds in the sas with his/her name, number of visitors, date, entrance time and exit time (when coming back of course).

During the descent, explain to them that downstairs is not only a visit centre but also a working area and they should follow the guide closely, stopping only when authorised.Ask them to mind the step when going into the cavern and turning left (until it is finalised).

When out of the lift, open the cavern door. First sight to the DELPHI barrel from the bottom. Immediately turn left and precede the visitors in the staircase. Stop on the first floor to start explanations on the barrel.

Restrictions :The visitors platform is not reachable to disabled people, as there is a staircase to take from the bottom of the lift..
What to see :The DELPHI barrel
What to say :Describe the parts of the detector that are exhibited

General view
The DELPHI barrel consists in a set of fitting concentric cylindrical detectors. It contains as well a cylindrical solenoidal electromagnet. Explain the role of this set:
The collisions
At the very center of the detector is located the vacuum beam pipe. An ultra high vacuum is present all around the 26.7 km of the ring in order to avoid electrons and positrons to interact with air molecules. From 4 to 8 bunches of particles travel in opposite directions such that they cross at 8 (16) locations around the ring. One of those is at the center of the detector(s). Although 1000 billion particles are present in each bunch, collisions are very rare as electrons are so small (they have a small cross-section). When they collide however, complex mechanisms occur that follow the rules of quantum mechanics. In particular the result is not deterministic (as for car collisions!) and many different phenomena may occur. It is the frequency of such events as well as their properties that physicists analyse and compare with theoretical predictions. What is sure however is that the final state consists in a set of particles escaping the collision region. The detectors are here in order to record as much information as possible on those particles.
Detecting particles
Essentially 3 pieces of information are needed on each emerging particle:
  • its location in space
  • its momentum (almost its energy)
  • its type (electron, muon, pion, kaon, photon, proton…)

The tracking detectors are used to find the location is space of electrically charged particles with a very high precision. These detectors are located inside a magnetic field that curves the trajectories of the charged particles: the higher their momentum is, the larger their radius of curvature is. Measuring the trajectory in space allows to deduce the momentum. Finally a set of detectors are used to stop most particles: the calorimeters. They contain heavy material: lead to stop electromagnetic particles (electrons, photons), iron for most other particles (pions, kaons, protons…) while only muons out of charged particles escape from the iron. The calorimeters are instrumented with sensitive devices in order to measure the energy of the interacting particles. Another set of devices is located outside the iron that will be sensitive to muons. Other more specific sensors may be used for determining the particle type, as in DELPHI with the RICH (see later)

Move to the platform where detectors are shown.

The calorimeters: HPC (High Density projection Chamber) and HCAL (Hadronic CALorimeter)
The thick iron plates all around the detector contain a series of sensitive elements that observe either the particles contained in the showers generated by interacting particles or muons passing through and eventually reaching the muon detectors: thick aluminium plates inside and outside (look top right) the iron. Just inside the iron calorimeter is the cryostat of the solenoid (see later) while the copper modules contain the electromagnetic calorimeter (HPC). It is based on sheets of lead located inside a special gas in which the particles generated in the shower drift towards the end of the module where a sensitive device (wire chamber, similar to that invented by Georges Charpak, Nobel Price 1992) is measuring their location using the principle of the Time Projection Chamber (see later TPC). Light diodes can be seen inside the lead of the prepared module that indicate the mechanism.
The superconducting solenoid
In order not to consume the large amount of energy that such a large magnet would need if being operated at normal temperature (around 20 MW), the solenoid is cooled down with liquid helium to a ultra-low temperature (4.5 above absolute zero, i.e. -269 Celsius). At this temperature, the material it is made of (niobium) lets the electric current (5000 amperes) go through without resistance, hence no loss. The magnetic field inside the whole volume was 1.2 tesla.
The tracking detectors
Going from the outside to the inside of the detector are located 5 tracking detectors:
The OD (Outer Detector)
Immediately inside the electromagnetic calorimeter, its outside connectors are visible, but not the sensitive device itself.
The TPC (Time Projection Chamber)
Its outside plate has been cut on the left side in order to show its inside and some of its components. The principle of detection is based on the fact that when a charged particle traverses a gas, it ionises it on its path, producing electrons. These electrons are moved by an electric field parallel to the axis of the cylinder on a length of up to 1.3 m (the ALEPH TPC was even larger). They drift at a very well known velocity such that measuring the time they take to reach the end of the drift volume allows to know with a precision of 250 micrometers where they were generated and hence where the particle was. Hence the name "Time Projection Chamber". The drifted electrons reach a multi-wire proportional chamber (MWPC). The position and the time of arrival of the electrons are digitised and recorded, allowing a 3-dimensional location of the particle.
The ID straws (Inner Detector straws)
Normally located inside the TPC, it is exposed here as if just extracted from its position. It consists of 5 layers of staggered "straws". Each straw is made of aluminium/mylar and contains in its center a thin wire. The difference of potential between the wire and the straw envelope generates an avalanche of electrons when a particle crosses the straws. The time of this avalanche is measured and allows to determine the distance of the particle to the wire.
The ID jet chamber
It is normally inside the straws detector, but is exposed here slightly extracted. It consists of 24 sectors made of thin wires that establish an electric field, allowing ionisation electrons to drift towards detection wires. Again measuring the drift time gives the distance of the particle to the wire.
The VD (Vertex Detector)
This is the innermost and most precise tracking detector. It is located just outside the beam pipe (inside the ID jet). It consists of 3 layers of silicon microstrip detectors in its barrel part, 2 crowns of silicon pixel detectors and 2 crowns of ministrip detectors on either end. The microstrips reach a precision of about 7 micrometers. This extreme precision allows to determine when particles do not originate from the interaction, but from the decay in flight of a short-lived particle (for example a particle with a beauty quark). This technology is now widely used in the LHC detectors, but the LEP detectors were pioneers in this matter.
The particle identification
The identification of particles is performed by four main techniques:
Muon identification
The only charged particles that are able to traverse the lead of the HPC and the iron of the HCAL are energetic muons. Hence sensitive detectors (muon chambers) are located at the outside of the iron calorimeter in order to track those particles.
Electron and photon identification
Those particles interact rapidly in the lead of the HPC, depositing all their energy. Hence an energy deposit in the HPC with almost no energy in the HCAL behind it signs the presence of one of them. If the trackers have localised a charged particle in from the HPC energy deposit, it was an electron (or a positron of course!), otherwise it was a photon. In the electron case one can in addition require that the measured energy corresponds to that measured for the charged particle in the magnetic field.
Hadron identification
All charged particles that are neither muons nor electrons and that cross the full detectors belong to the family of hadrons: protons, pions, kaons. It is not possible to disentangle them from their interaction properties as for muons and electrons. The technique used by DELPHI is unique at LEP. It uses a property of very energetic particles traversing a medium (called the Cherenkov effect): if the particle traverses a medium at a speed that is higher than the speed of light in this medium (this is possible as the speed of light is c/n where c is the speed of light in vacuum and n the refraction index of the medium, always larger than 1. e.g. 1.5 in glass), it emits light with an angle with respect to its own trajectory that depends directly on its speed. Measuring the angle of emission of the light gives thus access to the speed of the particle. As its momentum is measured by the magnetic field, a simple formula (from relativity) allows to determine its mass, hence its nature, as those particles have different masses. In DELPHI, this angle is measured by focusing the light with a set of mirrors in such a way that it forms rings whose diameter is proportional to the angle. This type of detector is called a RICH (Ring Imaging CHerenkov)
Missing energy
When all particles have been measured and identified using the above techniques, there is still one type of particles that has not been seen: the neutrinos. As they are very small, they escape detection and can traverse the whole earth, intergalactic space and even stars. One used a property of physics that says that the energy is conserved in all collisions to get a hint of the presence of neutrinos in the products of the collision: if the total energy visible in the detector is not that of the initial electron-positron pair, this is a sign that a neutrino escaped. Of course this is the most difficult identification, as other particles that would escape the detector would simulate this missing energy. This is why detectors have to be compact and hermetic

The DELPHI Collaboration

The DELPHI collaboration consisted in around 500 physicists from 52 institutions (mostly european, with one american university). During the construction phase (1982 to 1989), one can estimate that close to 1000 technicians and engineers have been involved in DELPHI. Each detector has been built by a team grouping usually a few institutions, the overall coordination being ensured by a technical coordinator at CERN.

DELPHI was organised around executive and decision bodies :

Last Update, July 2004
Copyright Visits Service -CERN 1996 - European Laboratory for Particle Physics