The ALICE ITS upgrade: Pixels for quarks
Panos Charitos, 28/11/2019

Layers 4, 3 and 5 (from right to left) that together with Layer 6 will form the half Outer Barrel of the new ITS. (Image: CERN)

ALICE, the experiment dedicated to heavy ions at the LHC, will soon have enhanced physics capabilities with a major upgrade of the detectors, data-taking and data-processing systems. They will improve the precision of the extracted characteristics of the high-density, high-temperature phase of strongly interacting matter, the quark-gluon plasma (QGP), together with the exploration of new phenomena in quantum chromodynamics (QCD).

To perform precision measurements of strongly interacting matter, ALICE will focus on rare probes – such as heavy flavour particles, quarkonium (heavy quark-antiquark pairs) states, real and virtual photons, and low-mass di-leptons. Observing rare phenomena requires very large data samples, which is why ALICE is looking forward to the increased luminosity provided by the LHC in the coming years. The interaction rate of lead ions during the LHC Run 3 is foreseen to reach around 50 kHz, corresponding to an instantaneous luminosity of 6×10²⁷ cm^–2s^–1.

This will enable ALICE to accumulate ten times more integrated luminosity (more than 10 nb^-1) and a hundred times larger data sample (about 10¹¹ collisions) than what has been obtained so far. In addition, the upgraded detector system will have a better efficiency for the detection of short-lived particles containing heavy flavour quarks, thanks to improved precision of the inner tracking system (ITS), a silicon tracker based on monolithic pixel sensors.

During the Long Shutdown 2, several major upgrades are being installed in ALICE: a new inner tracking system (ITS) with a new high resolution; a low-material-budget silicon tracker, which extends to the forward rapidities with the new muon forward tracker; an upgraded time projection chamber with gas electron multiplier detectors, along with completely new faster readout; new readout electronics for the muon spectrometer; a new fast interaction trigger detector; and an integrated online-offline computing system to process and store the large data volume.

Thanks to a well-structured R&D programme that attracted international collaborations and an intense period of testing and commissioning, the ALICE ITS is taking shape in CERN’s Building 167. The new detector is part of the upgraded ALICE experiment which during the next runs of the LHC will provide a new set of more precise measurements to meet the long-term goals of the ALICE collaboration in the study of QCD and the characterization of the Quark-Gluon Plasma.

The upgrade of the ALICE ITS will not only boost the physics capabilities of the ALICE experiment but moreover it has advanced a number of technologies beyond the state of the art. The results of this R&D effort currently find applications both in high-energy physics (i.e. the sPHENIX experiment at RHIC, BNL and the Inner Tracking System of the MPD experiment at NICA, JINR) and also in other fields. Perhaps one of the most topical examples is tracking with the development of Proton Computed Tomography (pCT) detectors for hadron therapy. The fast monolithic pixels developed for the ITS upgrade can improve significantly the accuracy of the image with regard to the present state of the art while reducing the exposure time for patients.

An enormous amount of effort has gone into the new ITS over the past years, strengthening collaboration between CERN and institutes from around the world. The fruits of those labours can now be seen while the final installation of the detector in summer 2020 will pave the way for new discoveries that could open up a whole new field of knowledge.

The Upgraded ITS

The new ITS is an all-pixel silicon detector based on CMOS monolithic active pixel sensor (MAPS) covering the a region (when close to the axis of a beam of particles). In the MAPS technology both the sensor for charge collection and the readout circuit for digitization are hosted in the same piece of silicon instead of being bump-bonded together. The chip developed by ALICE is called ALPIDE and uses a 180-nm CMOS process provided by Tower Semiconductor. With this chip, the silicon material budget per layer is reduced by a factor of seven compared to the previous ITS. The ALPIDE chip is 15x30 mm² in size containing more than half a million pixels organised in 1024 columns and 512 rows. Its low power consumption (<40 mW/cm²) and excellent spatial resolution (~5 mm) are perfect for the inner tracker of ALICE.

A longer version of this article first appeared in the EP newsletter: https://ep-news.web.cern.ch/content/alice-its-upgrade-pixels-quarks