The ALICE experiment at the CERN LHC

TLDR: The Large Ion Collider Experiment (ALICE) as discussed by the authors is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model.

Abstract: ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 161626 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.

Figures

Figure 3.47: Trigger timing diagram for the generation of a high-pt trigger.

Table 5.3: Overview of FMD parameters.

Figure 3.11: Multi-Chip Module (MCM). Left to right: wire bonds connecting the MCM ASICs via the pixel bus to the read-out chips, MCM ASICs, optical package with three optical fibres.

Table 3.14: Overview of TOF parameters.

Table 6.4: Parameters of the ALICE Computing Model wherefrom the computing needs are determined. The proton–proton RAW event size is an average figure. The real pile-up factor will change during the run as a function of the luminosity. The Pb–Pb data rate is an average figure. The actual event size depends on the trigger and the event multiplicity. What is important to retain is that the ‘bandwidth budget’ of 1.25 GB/s will be fully exploited, irrespectively of the chosen trigger.

Figure 8.11: Left: Tracking efficiency for parent tracks as a function of the radial coordinate of the decay vertex. Right: Tracking efficiency for daughter tracks as a function of the radial coordinate of the decay vertex.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "The alice experiment at the cern lhc" ?

The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification ( PID ) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008. 

Q2. Why is the double-track resolution a function of the separation efficiency?

Due to charge diffusion during the drift process, the double-track resolution is a function of the drift time for a given separation efficiency. 

Q3. Why is the readout chambers segmented radially?

Because of the radial dependence of the track density, the readout is segmented radially into two readout chambers with slightly different wire geometry adapted to the varying pad sizes mentioned below. 

Q4. How many low-pt muons are expected to be detected per event?

In central Pb-Pb collisions, about eight low-pt muons from π and K decays are expected to be detected per event in the spectrometer. 

Q5. How many super modules are installed into the ALICE central barrel?

The EMCal is installed into the support structure as twelve super module units, with the lower two super modules being of one-third size. 

Q6. What is the interface layer for the analysis components?

A C interface layer provides access to the analysis components for external applications, which include a PubSub interface wrapper program as well as a simple standalone run environment for the components. 

Q7. Why is the fibre spacing smaller than the radiation length of the absorber?

The fibre spacing is smaller than the radiation length of the absorber, in order to avoid electron absorption in the passive material leading to non-uniformity– 

Q8. How much would the temperature at the half-stave increase?

In normal operation, if a sudden failure of the cooling were to occur, the temperature at the half-stave would increase at a rate of 1 oC/s. 

Q9. How many radiator trays are used to purify C6F14?

A liquid circulation system was implemented to purify C6F14, fill and empty the twenty-one radiator trays at a constant flow, independently, remotely and safely. 

Q10. How much is the contribution of b-decay to the total J/ yield?

At high pt a large fraction of J/ψ’s is produced via b-decay [136]; based on Tevatron measurements [137] the contribution from b-decay to the total J/Ψ yield is ≈ 10% for pt < 3− 4 GeV/c and then it increases linearly to ≈ 40% for pt around 15− 18 GeV/c. 

Q11. What is the material used to support the radiator trays?

The radiator trays are supported by a stiff composite panel, consisting of a 50 mm thick layer of Rohacell sandwiched between two thin 0.5 mm layers of aluminium. 

Q12. What is the DDL's ability to separate the event fragments in memory?

The data formatting introduced by the DDL allows the D-RORC to separate the event fragments in memory, without interrupting the host CPU. 

Q13. What is the need for an advanced access control mechanism?

As the control system controls the often delicate and unique equipment of the sub-detectors the potential danger of serious and irreversible damage imposes a need for an advanced access control mechanism to regulate interactions of the users with the control system components. 

Q14. What is the firmware for the ALICE detector?

The firmware includes a Detector Control System (DCS) card with processor core for handling the Ethernet connection to the ALICE detector control system. 

Q15. How can the SSD be used to operate in a thermal neutral way?

Tests and simulations [29, 30] have shown that the SSD can operate in a thermal neutral way with the water temperature about 5 K below ambient temperature. 

Q16. What was the purpose of the development of the long micro-cable?

The development of the long micro-cable was particularly delicate since it was designed to minimize material while keeping excellent High-Voltage insulation, signal quality and power dissipation. 

Q17. How can the L2 rate of rare events be controlled?

In summary, an effective and robust method to maximise the L2 rates of rare and interesting events can be achieved by controlling the rates of peripheral, semi-peripheral and central events with feedback from the LDC occupancy and with downscaling. 

Q18. How many steps were necessary to have the module ready for final test?

The complexity of the assembly and test procedures (over 20 process steps were necessary) led to an average time of 3 days to have the complete module ready for final test with laser. 

Q19. How will the experiment use the data-taking periods?

The experiment will use the data-taking periods in the most efficient way by acquiring data for several observables concurrently following different scenarios. 

Q20. What is the effect of the mirroring on the distant end of the fibre on the detector?

The mirroring on the distant end of the fibre creates an approximate compensation for the effects of the finite attenuation lengths within the fibre making the longitudinal response of the detector quite uniform.