The activities for the upgrade of the ALICE detector and instrumentation proceed on schedule. Validated the prototypes, now the components have to be produced, assembled and tested in order to be ready for installation during the next 2-year long shut down of LHC (2019-2020).
Scheme of the upgrade of the ALICE detector. [From Zhongbao Yin's talk at the LHCP2017 Conference]
While the extended end-of-year shutdown has concluded and the LHC has been switched on again, the activities for the upgrade of the ALICE detector have entered a new phase. The prototypes of the various new components have been tested and validated, so that now production can start.
This major upgrade will increase the performance of the detector in order to fully exploit the higher interaction rate of about 50 kHz that is expected in Run 3 of LHC, after the two-year long shut down (LS2) that will start at the end of 2018.
The upgraded ALICE detector will be able to cope with the increased readout rate and will provide better vertex resolution and tracking efficiency at low pT. At the same time, it will preserve its excellent particle identification properties.
The upgrade programme foresees the replacement of the Inner Traking System (ITS) associated to a new beam pipe with a smaller diameter, the introduction of a Muon Forward Tracker (MFT), the upgrade of the Time Projection Chamber (TPC), and the substitution of the V0 and T0 detectors with a new Fast Interaction Trigger (FIT) system. The readout electronics has also been partially redesigned, together with the Central Trigger Processor (CTP) and the DAQ and Offline Data systems.
The new ITS will be a 7-layer barrel structure made of carbon fiber and equipped with dedicated silicon pixel sensors (ALPIDE), replacing the previous 6-layer detector that used strip, drift and pixel sensors. Being smaller (approximately 30 um x 30 um), thinner (50 um on the inner barrel and 100 um on the outer) and monolithic (sensor and readout chip are integrated in the same silicon structure), the ALPIDE pixel chips will guarantee higher granularity and reduced material budget. As a result, the track position resolution at the primary vertex will be improved by a factor of 3 with respect to the present detector.
The ALPIDE chip is employed as well in the Muon Forward Tracker (MFT), which is a new vertex detector for muons; combined with the existing Muon Spectrometer, it will allow precise identification of secondary vertices and better mass resolution. Composed of 5 disks of silicon pixel detectors, it will be placed between the central barrel detector and the hadron absorber of the Muon Spectrometer.
The upgrade of the TPC involves replacing the multi-wire chambers, which limit the event readout rate to 3.5 kHz, with quadruple-GEM chambers designed to minimize ion back-flow and to allow continuous, untriggered readout. New front-end electronics will be also needed. The new TPC will be able to operate at 50 kHz preserving its current performance in terms of tracking, momentum resolution and particle identification.
The FIT will be dedicated to forward trigger and to the measurement of a number of parameters, including luminosity, collision time, as well as multiplicity and centrality of heavy ion collisions. This new detector, which will replace the existing V0 and T0, will consist of two arrays of Cherenkov radiators, equipped with micro-channel plate detectors and photomultipliers, and of a single, large-size scintillator ring. The FIT will provide larger acceptance and finer segmentation than the present two, without compromising the time resolution.
As a consequence of the increased luminosity and interaction rate of LHC, a significantly larger amount of data will have to be processed and selected. Thus, a new Central Trigger Processor and a powerful data processing system integrating some online and offline functionalities have been designed as well.
With this upgraded detector and instrumentation, ALICE will be able to further investigate the properties of Quark-Gluon Plasma in pp, p-Pb and Pb-Pb collisions. In particular, the goal for next runs is to perform high-precision measurements that will shed light on thermodynamics, evolution and flow of the QGP, as well as on parton interactions with the medium.