Current Status of the ALICE LS2 upgrades

T. Gunji

Center for Nuclear Study, Graduate School of Science, The University of Tokyo, Japan

1. Introduction

ALICE is one of the experiments at the LargeHadron Collider (LHC) in CERN [1]. ALICE is ded-icated to the studies of strongly interacting matter atextreme energy densities, where a new phase of mat-ter composed of quarks and gluons, called quark-gluonplasma, forms. The ALICE detector is optimized tothe heavy-ion collisions at the ultra-relativistic ener-gies and is designed to measure as many observablesas possible for a wide coverage of transverse momen-tum and pseudo-rapidity [1]. Central barrel detectoris an ensemble of cylindrical detectors, that includesof 6 layers of silicon detectors (ITS), time projectionchamber (TPC), transition radiation detector (TRD),and time-of-flight detector (TOF). There is the muonspectrometer in ALICE at forward rapidity. It con-sists of a front absorber, five tracking stations with thethird one located inside a warm dipole, an iron wall andtriggering chambers. In Run2 LHC operation (2015-2019), ALICE accumulated 0-10% central events, 30-50% mid-central events, and minimum bias events ofPb-Pb collisions. Average collision rate of Pb-Pb col-lisions was 5-8 kHz. Compared to Run1, even thoughthe available highest energies were different, ALICEsuccessfully accumulated x4 larger luminosity in Run2.

2. ALICE upgrade in LS2

After the second Long Shutdown (LS2, 2019-2021),the LHC will deliver Pb beams colliding at an interac-tion rate of about 50 kHz. To fully exploit the LHCpotential in Runs 3 and 4 (2021-2029) for both lowmomentum and high momentum probes, ALICE willrecord all Minimum Bias (MB) events delivered by theLHC. ALICE aims at integrating a luminosity of 13nb−1 Pb–Pb collisions, which corresponds to a mini-mum bias data sample larger by x50-100 with respectto Run 2. In order to benefit from the increasing lu-minosity, ALICE is going to upgrade several detectorsystems during LS2.

The first major upgrade will replace ITS with a newtracker, composed of seven layers of new silicon pixeldetectors. The new tracker will be made up of about25000 Monolithic Active Pixel Sensors with fast read-out and with reduced material thickness reduced downto 0.3% (inner layers) – 1% (outer layers) of the radi-ation length and a granularity of 28×28 µm2 [2]. Thisnew ITS system enables to read out Pb-Pb and pp col-lisions at maximum 100 kHz and 1 MHz, respectively.From the end of 2016 until the end of 2019, a totalof 72000 sensors were produced and tested. During

the second half of 2018 until fall 2019, all seven de-tector layers have been assembled and installed in thehalf-barrels in an assembly clean room at CERN asshown in Fig. 1. Afterwards, full commissioning of thedetector on surface started with all services includingcooling system, power distributions, readout electron-ics and computing system installed. Figure 2 showsfake-hit rates for 54 chips in half-layer 0 as a functionof masked channels. Below 10−10 fake hits per pixeland event is demonstrated when masking only 50 outof 28 × 106 pixels.

Figure 1. New inner half-layers of the upgraded ITS

Figure 2. fake-hit rate of 54 chips as a function of masked

channels

The second major upgrade is to replace the read-out chambers of the TPC with Micro Pattern GaseousDetectors [4]. The upgraded new readout chambersconsists of stacks of 4 Gas Electron Multiplier (GEM)foils with different hole pitches. With this 4 GEM layerconfiguration, ion backflow can be kept less than 1%,which enables the TPC to operate continuously with-out a gating grid. All readout chambers arrived atCERN in early 2019 and were tested using X-ray irradi-ation or particles in the ALICE cavern. The TPC was

moved to surface in March 2019 and the GEM read-out chambers and readout electronics were installed in2019 as shown in Fig. 3. In December 2019, the pre-commissioning on surface started. Figure 4 shows thelaser tracks in the TPC. Further commissioning is con-tinued.

Figure 3. Picture of the TPC equipped with GEM readout

chambers and electronics

Figure 4. laser tracks seen in the TPC

The third major upgrade is to employ new readoutand DAQ systems in order to cope with 3 TB/s rawdata rate under 50 kHz Pb-Pb collisions [5]. The datafrom continuously readout detectors are composed ofseveral constant data streams. Figure 5 shows the ar-chitecture of new DAQ systems in Run3. The data pro-duced by the Front-End Cards (FEC) are transferredto the Common Read-out Units (CRU) which are theFPGA-based backend module installed in a first farmof computers called the First-Level Processors (FLP).In FLP, an initial data volume reduction is performed.The data merging and the final data volume reduc-tion is performed by a second farm of computers calledthe Event Processing Nodes (EPN). 8 GPUs will beequipped in one EPN in order to accelerate data pro-

cessing speed. ALICE is developing a new Softwareframework, called O2, in collaboration with the FAIRat GSI. Figure 6 shows the bandwidth of data tak-ing from FEC of MCH detector to EOS (storage afterEPN farms), where O2 was used for the data trans-portation. It is shown that data taking was smoothand expected bandwidth was achieved. Further devel-opment and commissioning are in progress.

Figure 5. The data processing chains of ALICE in Run3

Figure 6. Full chain readout tests using MCH detector

3. Summary and Outlook

The preparation and commissioning of the upgradesof TPC and ITS during LS2 is on-going. Further com-missioning such as long term stability tests will be per-formed in 2020 and both will be installed in the cav-ern in 2020. New DAQ software framework is underdevelopment and further tests with detectors will beperformed in 2020.

References

[1] ALICE Collaboration, Int. J. Mod. Phys. A 29(2014) 1430044

[2] ALICE Collaboration, CERN-LHCC-2012-013,CERN-LHCC-2013-024

[3] ALICE Collaboration, CERN-LHCC-2015-001[4] ALICE Collaboration, CERN-LHCC-2013-020[5] ALICE Collaboration, CERN-LHCC-2013-019,

CERN-LHCC-2015-006