Visiting

the DELPHI underground exhibition

and

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
• Conditions to visit DELPHI
• The journey from CERN to DELPHI
• The DELPHI surface exhibition
• The DELPHI detector in numbers
• Underground visit of the DELPHI barrel

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:

• The person(s) driving visitors should have the CERN access authorisation, i.e. their magnetic card should grant them access in the underground areas of LHC. If not, they should follow the necessary safety courses (levels 1, 2 and 3).
• The group must be smaller than 10 visitors per DELPHI/LHCb member meeting the above requirement.
• The organiser should make a request to the LHCb GLIMOS (Alasdair Smith) who will check if visits are authorised for the proposed date. He/she should warn the pit 8 "gérants de site" of the visit.They will be in charge of opening the "sas" for the party to access the lift. They will be instructed to refuse access to any group not meeting the previous requirements.
• The responsibility of the DELPHI and LHCb Collaborations cannot be engaged in case visitors do not follow the above instructions.

Journey

• CERN and LEP geographical location (Jura, Geneva, airport).
• CERN history (PS, SPS, LEP, then LHC) and organisation (member states..)
• LEP history (decision, building, 4 experiments...), running from 1989 to 2000
• The new era with the LHC, technological challenge etc...
• CERN across the border : the swiss and french sites, the tunnel
• CERN site of Prevessin (PCR, the control room for SPS, LEP and in future a new one for the LHC)
• UA2 site (where the W and Z° bosons were discovered in 1983, together with UA1)
• General Safety Rules
• Explain the progress of the visit :
  • Exhibition : at least LEP and generalities on DELPHI should be seen before going downstairs
  • Visit of the DELPHI underground exhibition

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.

The DELPHI and LHCb

surface exhibition

• LEP posters :
  • Show the LEP/LHC geographical situation (complement what may have been said during the travel). The LEP/LHC tunnel is not horizontal but has a slope of 1.4% to follow the ground surface and avoid very deep shafts. Circumference 26.7 km.
  • Explain the physics aim of LEP (electron-positron collision, Z° factory, higher energy...). Quote E = mc2 as the fundamental formula to understand the creation of massive particles from the energy stored in the electron and positron beams.
  • An accelerator : beam pipe (ultimate vacuum), acceleration (RF cavities), beam guidance (magnets)
  • CERN and LEP energy consumption : LEP consumed about 60 MW at 45 GeV. CERN consumed in 1995 a total energy of 912 GWh (about 38% of the Geneva canton).
  • LEP costed about 1.3 billion swiss francs (on a period of 10 years). Experiments costed in total 480 millions CHF (140 from CERN, 340 for outside institutes and universities).
  • The electrons and the positrons were grouped into packets (bunches) of more than 250 billions.; 4 bunches circulate in each beam, which provided 8 crossing points. They traveled at a speed extremely close to the speed of light and hence each bunch passed 11,250 times per second in each detector. This allowed 45000 crossings per seconds. The collision rate was however much lower due to the very small probability of collision of electrons with positrons (cross section, i.e. almost their effective size) : 1 collision every 2 s in the best case, when running on the Z peak (LEP 1). For further information about W rate production go to the Frequently Asked Questions.
  • The electrons were produced as for any electron accelerator (e.g. in a TV set) by heating a filament. They were then accelerated by electric fields in a linear accelerator (100 m, up to 600 MeV), then in the PS (630 m in circumference, up to 3.5 GeV), the SPS (6.9 km, up to 21  GeV) and finally in LEP (26.7 km, up to 45 GeV or higher at LEP2). For further information about positron production and injection go to FAQ.
• DELPHI posters :
  • DELPHI cut view : barrel and endcaps
  • Civil engineering and installation pictures
  • The travel of the superconducting magnet. It was built in England at Rutherford Laboratory and traveled by boat and lorry to CERN. This magnet is used to curve the trajectories of particles produced in the collisions (in order to measure their momentum). It is superconducting to save running costs. It is so far the largest superconducting solenoid in the world (6.2 m in diameter, 7.4 m in length, 82 tons, field of 1.2 T for a current of 5000 A).
• Detectors prototypes :
  • TPC : the Time Projection Chamber (barrel tracker)
  • HPC : the High-density Projection Chamber (barrel electromagnetic calorimeter)
  • VD : the silicon microVertex Detector (search for secondary vertices)
  • RICHes : the Ring Imaging Cherenkov counters (particle identification)
  • MUB/MUF : the Barrel and Forward MUon chambers
  • ID : the Inner Detector straw tubes detector
  • STIC : Small-angle TIle Calorimeter (luminometer + forward calorimeter)
• Physics at LEP :
  • The standard model
  • LEP and the early universe
  • The 3 families of neutrinos
• The LHCb experiment
  • It is currently in preparation. Only its magnet has been installed so far, but detector installation will start at autumn 2004
  • Posters show information on the physics of LHCb and details on the LHCb detector
  • More will be added here when the exhibition is compete

DELPHI in numbers

• The barrel has been moved 50m away from where it was operated, on the LEP beam line. Its last 5m were made on air-cushion, using the same devices that will be used for moving the CMS detector.
• The LEP and LHC beam line crosses the cavern perpendicularly. The LEP beam was going through the center of the cylinder. When facing the barrel, the electrons were coming from behind, the positrons from the front.
• The detector was behind the counting rooms (3 independent parts, mobile on rails), that are now on your left, being re-used for LHCb.
• The center of LEP and the Jura are in front of you when entering the cavern. Geneva is behind you, Ferney-Voltaire (F) on your right, Meyrin (CH) on your left.
• History of LEP and DELPHI
  • LEP was approved in 1981, DELPHI in 1983
  • LEP and the caverns were dug in 1985 and 1986, the accelerator was installed in 1987 and 1988. The first beam circulated in LEP on 14th July 1989 and the first collisions occurred on 13th August 1989.
  • DELPHI was installed in 1988. All components were brought down through the PX shaft (not visible from there, but the direction is on your left). The heaviest component is the superconducting magnet (82 tons). Only 10 cm clearance was left in the shaft when some components were brought down (counting rooms, magnet...).
  • The DELPHI detector consisted in a central cylindrical part, the barrel, still visible, and two end-caps that were fitting into the cavity visible in the barrel and were like large covers of the same diameter as the barrel.
  • The barrel was installed in front of the PX shaft, the endcaps in their garage position, close to where the barrel now is. They were rolled on their rails into operational position. The barrel was never moved since 1989 until 2003, since they were not sure it would come back into the same position! Endcaps were moved during each winter shutdown for maintenance and repair, removal of the beam pipe and of the microvertex and inner detectors when needed.
  • The detector was operational from the start of LEP on July 14th 1989 until its final breath on November 2nd 2000. Until 1995, LEP was colliding electrons and positrons at an energy very close to that needed to create the Zº vector boson (discovered at CERN in 1983), 91 GeV. Each experiment (including DELPHI) collected more than 6 million events at this energy, allowing an extensive study of the Zº boson.
  • As from 1995, the LEP machine was continuously upgraded in order to increase its energy from 91 GeV to 207 GeV (energy reached in 2000). This higher energy allowed to produce pairs of the charged boson W as well as pairs of Zº bosons. The study of the W bosons at LEP provide the most accurate measurements as well of the properties of this particle.
  • The ever increasing energy of LEP was meant for trying to produce an elusive particle that theorists have predicted: the Higgs boson. It is the particle responsible for giving the mass to other particles that otherwise would be massless. The higher the energy, the more likely it is to pass the threshold for the Higgs boson production. During 2000 when LEP reached its highest energy, indications of the presence of the Higgs boson were obtained by the 4 LEP experiments. This led to a dilemma on whether to stop the LEP as foreseen or to run it for another year, hoping to confirm these hints. The decision was taken at the end of November 2000 not to run further and to dismantle the experiments and the machine to make place for the LHC.
  • Since then, teams of physicists of the Collaborations are still analysing the large amounts of data collected during these 12 years of data taking, presenting results in physics journals and at conferences, doctoral students presenting their thesis on LEP data.
• The DELPHI detector :
  • The weight of DELPHI was about 3200 tons (2000 for the barrel, 600 for each endcap).
  • The total length of cables put end to end would exceed 1100 km (each signal cable carries 16 electronics signals).
  • The signals were treated by electronics located on 3 levels on the side closer to the lift, and 2 levels on the opposite side of the detector. The total power dissipated by the electronics is about 500 kW and hence needs water cooling.
  • The electronics converted the signals into numbers (digitisation) which were read out and compacted by a set of 75 microprocessors, and finally transmitted to the surface control room where the data were stored, controlled and analysed.
• Environment :
  • The cavern temperature and humidity are constant (about 22°C, 40%). The temperature inside the detector during operation was about 30°C due to the electronics heating (water cooled). It must be kept quite stable to keep the detector calibration constant.
  • The cavern is located under the groundwater in the glacial moraine. One can have a look at the rock through the window drilled in the wall of the cavern just on the right when facing the barrel. Water is dropping there. Water is elevated to the surface by means of a set of pumps. A failure of the pumps in 1993 caused the floor of the cavern to be covered by more than 5 cm of water within a few hours!
  • Since most detectors used mixtures of explosive gases (ethane, methane mixed with argon), the biggest danger (for personnel and for the detector) was fire. There was hence a permanent monitoring of the air inside and around DELPHI, reported to a computer on the surface.
  • There was no danger of radiations whatsoever even during the LEP operations, since the detector and the concrete shielding are sufficient to stop the synchrotron radiation which would be the main cause of radiations. Collisions are negligible in that respect. Visits were thus allowed even during LEP operations. The dose of radiations received in the cavern was much lower than on the surface due to the protection by the earth against cosmic rays (also useful to prevent cosmic rays to mimic interactions in the detector).

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 (4.5º above absolute zero, i.e. -269º 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

DELPHI was organised around executive and decision bodies :

• The DELPHI coordination groups around the DELPHI spokesman a restricted team of 8 physicists nominated by the spokesman who act as ministers in the DELPHI government. The spokesman is elected 2 years before starting his term. During this time, he is one of the members of the coordination.
• The DELPHI Executive Committee (DEC) complements the coordination with 8 members elected for 2 years by the collaboration.
• The DELPHI collaboration board (CB) is the decision body. It is formed by one representative per institute, each having equal vote in the decisions. It endorses proposals made by the DEC and the coordination, as the parliament does in a country.
• The projects : each detector team or computing team is lead by a project leader appointed by the project (and endorsed by DEC and CB) for a period of 2 years. He/she is responsible for the data taking of the detector, the software of reconstruction, the simulation of the apparatus, the analysis of physics results.
• The analysis tasks : they deal with parts of the data analysis which are of common interest for several physics analysis (e.g. electron identification, track fitting...)
• The physics teams : they deal with specific physics analysis. They are in charge of running the analysis and preparing physics publications out of it.