ALICE - A Large Ion Collider Experiment

Posted by admin
November 16, 2008

ALICE (A Large Ion Collider Experiment) is a dedicated heavy ion experiment at the LHC. The goal of the experiment is to study strongly interacting matter at extreme energy densities (QCD thermodynamics). Statistical QCD predicts that, at sufficiently high densities, there will be a transition from hadronic matter to a plasma of deconfined quarks and gluons - a transition which in the early universe took place about 100 micro-seconds after the Big Bang. The study of nuclear collisions at high energies utilizes methods and concepts from both nuclear and high energy physics constituting a new and interdisciplinary approach in investigating matter and its interactions.
The ALICE collaboration proposes to build a dedicated, general-purpose detector that will utilize the full potential of the LHC programme including both nucleus-nucleus and proton-proton collisions. Its design is based on the experiencies gained with the existing programs at CERN and BNL and it will address a majority of known sensitive observables like hadrons, di-leptons and photons. The ALICE detector will be the only heavy ion experiment at LHC and its design therefore has to be conservative and robust to be able to observe most of the signals that look promosing today for the QGP formation.
The figure above shows the ALICE experiment. The detector is contained in a big magnet of about 6 meters radius. The detector must have the capability of detecting the produced particles with very high precision. This requires different systems of specially designed detectors based on very advanced technologies.

TPC Overview

The Time Projection Chamber (TPC) is the main tracking detector in the central barrel of theALICE experiment at LHC. Its function is to provide track finding (efficiency larger than 90%), charged particle momentum measurement (resolution better than 2.5% for electrons with momentum of about 4 GeV/c), particle identification (dE/dx resolution better than 10%), and two-track separation (resolution in relative momentum below 5 MeV/c) in the region pt<10 GeV/c and pseudo-rapidity |h|<0.9.
The TPC is cylindrical in shape with an active gas volume that ranges from about 85 cm to 250 cm, in the radial direction, and has a length of 500 cm. along the beam direction. A high voltage (HV) electrode is located at its axial center, which will be aligned to the interaction point, dividing the gas volume in two symmetric drift regions of 250 cm length. The HV electrode, which consists of an aluminized stretched Mylar foil, and two opposite axial potential degraders create a highly uniform electrostatic field in the two drift regions. The potential of the drift region is defined by Mylar strips wound around 18 inner and outer support rods.

Charged particles traversing the gas will leave behind the memory of their passage in the form of a long trace of ionized gas. Depending on the electrical charge and momentum of the particle the trace will be bent weaker or stronger in either direction. In addition the density of the ionization depends on the momentum and identity of the particle. The ionization trace moves at constant velocity to either of the two end plates. The end plates are equipped with wire planes and 560,000 electronics channels, detecting the fundamental properties of the ionization trace (3D image and ionization density). The requirements of good momentum resolution and high rate capability call for a drift gas with low diffusion, low Z and large ion mobility. Extensive investigation of different gas mixtures led originally to the choice of the mixture 90% Ne-10% CO2. More recently it was proposed to add 5% N 2 to the mixture, which turned out to provide, higher gas gain stability and a better control of the fraction of N2 and its influence on the drift velocity. Both gas mixtures, however, require a high drift field (400 V/cm) to secure an acceptable drift time (88ms and 92ms respectively). The field cage is surrounded by double-shelled containment vessels with CO2 as insulator. Composite materials based on carbon fibber were chosen for high mechanical stability and low material budget (only 3.5% of a radiation length for tracks with normal incidence).
The TPC end-plates are each segmented into 18 trapezoidal sectors and equipped with multi-wire proportional chambers with cathode pad readout covering an overall active area of 32.5 m2. The sectors are segmented radially in two chambers with varying pad sizes, optimized for the radial dependence of the track density. There are about 560 000 pads with 3 different sizes: 4 for the inner readout chambers (IROC), 6 x 10 and 6 x 15 mm2 for the outer chambers (OROC), with 159 pad rows radially. For a position resolution not limited by the signal-to-noise ratio, the gas gain has to be sufficiently large. For the chosen pad sizes and rather low dE/dx of the drift gas, a rather large gas gain of 2 104 is necessary at the design noise figure of 1000 e (rms) for the electronics.

The charge collected on the TPC pads is amplified and integrated by a low input impedance charge sensitive amplifier followed by a semi-Gaussian pulse shaper. These analogue functions are realised by a custom integrated circuit (PASA), implemented in a 0.35 mm CMOS technology, which incorporate 16 channels. The circuit has a conversion gain of 12mV/fC, an impulse response function with a peaking time of 150ns and an equivalent noise charge of about 600 e- . Immediately after the PASA, a 10-bit pipelined ADC (one per channel) samples the signal at a rate in the range 5 to 12 MHz. The digitised signal is then processed by a set of circuits that perform the baseline subtraction, tail cancellation, zero-suppression, formatting and multi-event acquisition. The ADC and the digital circuits for 16 channels are contained in a single custom chip, named ALTRO, implemented in a 0.25 m m CMOS technology. The complete readout chain, which has a power consumption of about 40 mW / channel, is contained in Front End Cards (FEC), with 128 channels each, connected to the detector via kapton cables. A readout partition consists of a number of FECs (up to 25), which are controlled by a Readout Control Unit (RCU). The RCU interfaces a readout partition to the Data Acquisition (DAQ), the Trigger, and the Detector Control System (DCS). Every TPC sector (15488 channels) is readout by 6 partitions (121 FECs).
The ALICE Experiment is going in search of answers to fundamental questions, using the extraordinary tools provided by the LHC:

This website aims both at introducing non-initiates to the field of physics covered by ALICE and at providing regular information on the evolution of the experiment, with detailed reports of its results and analysis. It also offers an insight into the scientific community gathered around this project and highlights its contributions to the advancement of our understanding of the universe. So, no matter what your involvement with physics, you are invited to tumble down the rabbit hole into the wonderland of ALICE.

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