The DELPHI detector at CERN's LEP collider


DELPHI (DEtector with Lepton, Photon and Hadron Identification), is a detector for e+e- physics, with special emphasis on powerful particle identification , three-dimensional information, high granularity and precise vertex determination. It is installed at LEP (Large Electron and Positron collider) at CERN where it has operated since 1989.

The figure (click on it to get a clearer version) shows a cut-away view of the DELPHI detector, which consists of a central cylindrical (or ``barrel'') section and two end-caps (or ``forward'' sections), one of which is shown; the overall length and diameter are over 10 m and the total weight is 3500 tons.


Bunches of electrons and positrons travel in opposite directions through vacuum inside the pipe (shown as the black tube through the center) and meet in the middle of the detector. Occasionally (middling several times everyday) an electron and a positron pass close enough to each other (within less than ten-thousand-million-billionths of a metre at current energies!) to collide and annihilate each other. The products of the annihilation fly radially outwards (e.g. figure near).


A huge superconducting solenoid, the largest so far built (shown as the big purple cylinder), produces a powerful magnetic field (1.23 Tesla) parallel to the beam pipe. This bends the trajectory of each charged particle into a spiral whose radius is proportional to the momentum of the particle.

Tracking Detectors

In the barrel part of the detector - tracking detectors: the Vertex Detector (VD), the Inner Detector (ID), the Time Projection Chamber (TPC), the Outer Detector (OD) and the Barrel Muon Chambers (MUB) - and also in the forward part - tracking chambers: the Forward Chambers A and B (FCA and FCB), the Very Forward Tracker (VFT), the Forward Muon Chambers (MUF) and the Surround Muon Chambers (SMC) - are devoted to precise measurement of these trajectories, and hence to the precise determination of the directions and momenta of the charged particles.

The Vertex Detector ( VD ) - nearest the collision point - is an advanced silicon detector providing very precise tracking, principally in order to detect very short lived particles by extrapolating the tracks back towards the interaction point.
The old VD has been made longer by 24 cm and is now the barrel part of the Silicon Tracker. It consists of tree coaxial cylindrical layers of AC coupled silicon strip detectors at average radii of 6.3, 9.0 and 10.9 cm. At present the polar angle coverage extends down to 25 degrees.

The Inner Detector ( ID ) - shown as the smallest green cylinder - is located between the Vertex Detector and the Time Projection Chamber and provides intermediate precision position and trigger information. It consists of two parts: the JET chamber and the Trigger Layers (TL). The JET chamber provides points per track between radii of 12 and 23 cm and the polar angle coverage extends down to 15 degrees.

The Time Projection Chamber ( TPC ) - shown as the big blue cylinder - is the principal tracking device of DELPHI. As well it helps in charged particle identification by measuring the dE/dX. It is a cylinder of 2x130 cm situated between the radii 29 cm and 122 cm. The detector provides points per particle trajectory at radii from 40 to 110 cm between polar angles from 39 to 141 degrees. At least three pad rows are crossed down to polar angles from 20 to 160 degrees.

The Outer Detector ( OD ) - shown as the narrow navy-blue cylinder - consists of five layers of drift tubes located between radii of 197 and 206 cm. The active length of the detector corresponds to polar angles from 42 to 138 degrees. It provides a final precise and direction measurement after the Barrel Ring Imaging Cherenkov detector.

The Forward Chamber A ( FCA ) - shown as the smaller blue component in the forward part - is distant from the interaction point of about 160 cm in z. The chamber covers polar angles from 11 to 32 degrees and from 148 to 169 degrees.

The Forward Chamber B ( FCB ) - shown as the bigger blue component in the forward part - is a drift chamber at an average distance of | z | = 275 cm from the interaction point. The sensitive area of the chamber corresponds to polar angles from 11 to 36 degrees and from 144 to 169 degrees.

The Very Forward Tracker ( VFT ) is located on both sides vertex detectors. It is covers the polar angle from 10 to 25 degrees and from 155 to 170 degrees. The VFT is the forward part of the Silicon Tracker.

The Muon Chambers (MUC = MUB + MUF + SMC) are farthest the collision point, since muons are the only charged particles that can traverse the lead and iron of both calorimeters essentially unaffected. Most muons of momenta above 2 GeV/c are expected to penetrate to The Muon Chambers ,whereas other charged particles are stopped by this material.
Muon identification is achieved by comparing the extrapolations of the reconstructed tracks with the hits in the Barrel ( MUB that covers polar angles from 53.0 to 88.5 degrees and from 91.5 to 127.0 degrees ) and Forward ( MUF that covers polar angles from 20 to 42 degrees and from 138 to 160 degrees ) muon drift chambers. In 1994 a layer of Surrounding Muon Chambers ( SMC ) based on limited streamer tubes was installed outside the endcaps to fill the gap between the barrel and forward regions.

In the barrel part of the detector precise measurement of these trajectories varies from 5-10 micrometers in the Vertex Detector, to a fraction of a millimeter in the Time Projection Chamber and to 1-3 mm in the Barrel Muon Chambers after traversing 5 m of the detector.

Electromagnetic calorimeters and Scintillator counters

Electron and photon identification is provided primarily by the electromagnetic calorimetry system . It is composed of a barrel calorimeter (HPC), a forward calorimeter (FEMC) and two very forward calorimeters, the Small angle TIle Calorimeter (STIC), which replaced the Small Angle Tagger (SAT) in April 1994, and the Very Small Angle Tagger (VSAT). The latter two are used mainly for luminosity measurement. lead-glass.

The High-density Projection Chamber ( HPC ) - shown as the big green cylinder - is the barrel eletromagnetic calorimeter and is installed as a cylindical layer outside the Outer Detector. It is mounted on the inside the solenoid. The HPC is a cylinder of 2x254 cm situated between the radii 208 cm and 260 cm and consists mainly of lead. The polar angle coverage is from 43 to 137 degrees.

The Forward ElectroMagnetic Calorimeter ( FEMC ) - shown as the green disk component in the forward part - is the forward electromagnetic calorimeter and consists of two 5 m diameter disks (made of lead-glass). The front faces are placed at | z | = 284 cm, covering the polar angels from 8 to 35 degrees and from 145 to 172 degrees.

In order to achieve complete hermeticity for high energy photon detection, additional scintillators have been installed in the cable duct regions (between the barrel and each endcap, and between the HPC modules) the HERmeticity detectors ( HER ). In addition the DELPHI has in the barrel part the Time Of Flight ( TOF ) and in the forward part the HOrizontal Flight tagger ( HOF ). The scintillator counters are also fast trigger for beam events and cosmic radiation.

Hadron calorimeter

The hadron calorimeter ( HCAL ) - shown as the red component of the DELPHI detector - is a sampling gas detector incorporated in the magnet yoke (it consists mainly of iron), the barrel part covering polar angles from 42.6 to 137.4 degrees, and two end-caps from 11.2 to 48.5 degrees and from 131.5 to 168.8 degrees.
The hadron calorimeter provides calorimetric energy measurements of charged and neutral hadrons (strongly interacting particles). In addition is implemented a system to read out the HAC tubes as well as the pads, in order to give a more detailed picture of the hadronic showers and thus better distinction between showers caused by neutral and charged hadrons and better muon identification.

Charged hadron identification

The identification of charged hadrons in DELPHI relies on the specific ionization energy loss per unit length (dE/dX) in the TPC , on the RICH detectors. The RICH technique is based on the detection of Cherenkov light emitted by the particle. The DELPHI RICH contains two radiators of different refractive indices. The liquid radiator is used for particle identification in the momentum range from 0.7 to 9 GeV/c. The gas radiator is used from 2.5 to 25 GeV/c.
The full solid angle coverage is provided by two independent detectors, one in the endcap regions (Forward RICH), and one in the the barrel regions (Barrel RICH).

The Barrel RICH Detector ( Barrel-RICH ) - shown in yellow - is located between the Time Projection Chamber and the Outer Detector. It is a 350 cm long cylinder with inner radius 123 cm and outer 197 cm, divided into two halves by a central support wall, 6.4 cm thick. It covers polar angles between 40 and 140 degrees.

The Forward RICH Detector ( Forward-RICH ) - shown as the yellow disk component in the forward part - is located between 1.7 m < | z | < 2.7 m and covers polar angles between 15 and 35 degrees.

Luminosity measurement

At e+e- colliders, luminosity is measured by counting the number of events of a process with a clear experimental signature, with high statistics and with a cross section which can be calculated theoretically with high precision. The process chosen is Bhabha scattering at small angles, which proceeds almost entirely through the exchange of a photon in the t-channel.
In DELPHI the absolute luminosity is measured using the Small angle TIle Calorimeter (STIC)
and the Very Small Angle Tagger (VSAT).

The Small angle TIle Calorimeter ( STIC ) is a sampling lead-scintillator calorimeter formed by two cylindrical detectors placed on either side of the DELPHI interaction region at a distance of 220 cm, and covers a wider angular region between 29 and 185 mrad in polar angle (from 6.5 to 42 cm in radius).

The Very Small Angle Tagger ( VSAT ) consists of 4 calorimeter modules. Each such module is composed of 12 silicon diodes. It detects electrons and positrons coming from Bhabha scattering between 5 and 7 mrad.

Trigger

In order to cope with high luminosities and large background rates, the trigger system is composed of four successive levels (T1, T2, T3 and T4) of increasing selectivity. The first two trigger levels (T1 and T2) are synchronous with respect to the Beam Cross Over signal (BCO). Each subdetector contributes to the trigger decision with data generated by the respective subtrigger processors.
The global trigger efficiency for electron and muon pairs is consistent with 1 at the level 0.0001 for polar angles between 20 and 160 degrees. Even for single tracks, provided their momentum transverse to the beam exceeds 1 GeV/c, the efficiencies in the barrel (from 42 to 138 degrees) and forward (from 10 to 32 degrees and from 148 to 170 degrees) regions exceed 95%.


The power and signal cables and cooling tubes etc go through the gap between the forward and barrel regions. Electron and photon detection in this gap is provided by the HERmeticity detectors and muon detection is provided by the Surround Muon Chambers.

Most of the elements of DELPHI provide information directly in 3 dimensional form, which is read out via some 200,000 electronic channels by 72 dedicated microprocessors. The data are then merged to form an "event" (event building) and shipped to a central set of computers where they are logged onto mass storage for subsequent analysis. A typical event contains 1 million bits of information.

Design and construction of the DELPHI detector took 7 years. Data have been taken every year for the last 9 years, typically throughout the summer and autumn. During the data-taking periods, LEP and the detectors are operated around the clock. Winter and spring (when electrical power is much more expensive) are reserved for a long shutdown, used for maintenance and modifications (including upgrading of LEP as well as of the detectors!).

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