The ALICE experiment
ALICE (A Large Ion Collider Experiment) is a detector dedicated to heavy-ion physics at the Large Hadron Collider (LHC). It is designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms.
The ALICE detector weighs 10 000 tonnes, is 26 m long, 16 m high, and 16 m wide. It sits in a vast cavern 56 m below ground close to the village of St Genis-Pouilly in France, receiving beams from the LHC.
During the Long Shutdown 2 (LS2), the Collaboration has completed major upgrades of the detector with two main objectives: to increase the data taking rate by about two orders of magnitude, and to enhance its track reconstruction efficiency and precision for the detection of short-lived particles containing heavy-flavour quarks. With this, ALICE expects to continue its scientific journey at the LHC for many years to come.
The collaboration includes almost 2000 scientists from 174 physics institutes in 40 countries.
1. Time projection chamber (TPC) upgrade
Weighing an enormous 15 tonnes, measuring 5.1 metres in length and 5.6 metres in diameter, the ALICE time projection chamber (TPC) is a 88-cubic-metre cylinder filled with gas and read-out detectors that follows particles’ trajectories in 3D. The readout detectors in the two endplates were previously multi-wire proportional chambers, 72 in total, which have now been replaced by detectors based on gas electron multipliers (GEM), a micro-pattern structure developed at CERN.
These new devices, together with new readout electronics that feature a continuous readout mode, will allow ALICE to record the information of all tracks produced in lead–lead collisions at rates of 50 kHz, increasing the detector’s data acquisition speed by a factor of 100.
2. New inner tracking system (ITS)
Sandwiched between the beam pipe and the time projection chamber (TPC) lies a brand new inner tracking system. This system now improves the ALICE detector’s capacity to pinpoint and reconstruct the short-lived particle trajectories.
With its seven layers of 12.5 billion monolithic active silicon pixel sensors over 10m2 surface area, the new inner tracking system is the largest pixel detector ever built.
The current upgrade relied on new pixel sensors called ALPIDE, which also make up the new Muon Forward Tracker (see below). Each ALPIDE chip contains more than half a million pixels in an area of 15 × 30 mm2 and features an impressive resolution of about 5 μm in both directions – the secret to the subdetector’s improved performances.
3. New muon forward tracker (MFT)
The muon forward tracker (MFT) is a ALICE’s new high resolution silicon tracking detector. Installed in front of the Muon Spectrometer, the MFT extends the precision measurements of the quark–gluon plasma properties towards the forward rapidity region. It is a 0.5 m2 pixel detector, comprising more than 1000 silicon sensors. Like the new inner tracker system (see above), the new MFT uses the same ALPIDE pixel sensor technology. Its high pixel density (each chip houses half a million pixels in an active area of 4.5 cm2) translates into enhanced resolution for high-precision measurements of particle tracks.
The ALICE detector is now equipped to tackle the rich physics opportunities offered by the increased luminosity of the future LHC runs.
4. New fast interaction trigger (FIT)
The new fast interaction trigger (FIT) is a faster trigger for ALICE, as well as being an online luminometer, an initial indicator of the vertex position and a forward multiplicity counter. In offline mode, FIT provides the precise collision time for time-of-flight particle identification. It also yields the collision centrality and interaction plane, and measures cross sections of the diffractive processes.
FIT relies on three state-of-the-art detector technologies underpinning components grouped into five arrays surrounding the LHC beamline, at -1, +3, +17, and -19 metres from the interaction point.
5. New beampipe with a smaller diameter
As a consequence of replacing the inner tracking system, a beam tube with a smaller diameter needed to be installed to replace the existing chamber.
This central vacuum chamber is made of beryllium, a metal that is very light, very resistant and mostly transparent to particles. The new beampipe is about one metre long, 36.4 mm in diameter compared to 50 before, and with a thickness of 0.8 mm, which is at the limit of what can be achieved with current technology.
This change will improve the measurement of the position of particle interactions and help to detect particles with a shorter lifetime, which decay closer to the interaction point.
6. New readout system
The newly upgraded ALICE detector has close to 13 billion electronic sensor elements that are read out continuously, creating a data stream of more than 3.4 terabytes per second. To cope with such a large throughput, a new Online-Offline computing system, called O2, has been deployed.
After first-level data processing, there remains a stream of up to 600 gigabytes per second. These data are analysed online on a high-performance computer farm, consisting of 250 nodes, each equipped with eight graphics processing units (GPUs) and two 32-core central processing units (CPUs). Most of the software that assembles individual particle detector signals into particle trajectories (event reconstruction) has been adapted to work on GPUs.