  The ALICE Collaboration will undertake a major upgrade of the detector
apparatus during the second LHC Long Shutdown LS2 (2019-2020) in view of the
Runs 3 and 4 (2021-2029). The objective of the upgrade is two-fold: i) an
improvement of the tracking precision and efficiency, in particular in the
low-momentum range; ii) an improvement of the readout capabilities of the
experiment, in order to fully exploit the luminosity for heavy ions envisaged
after LS2. The first goal will be achieved by replacing the Inner Tracking
System with a new tracker, composed of seven layers of silicon pixel detectors.
The new tracker will be made up of about 25000 Monolithic Active Pixel Sensors
with fast readout, resulting in a material thickness reduced to 0.3% (inner
layers) - 1% (outer layers) of the radiation length and a granularity of
$28\times28$ $\mu$m$^2$. The second goal will be achieved, among other
measures, by replacing the readout chambers of the 90 m$^3$ Time Projection
Chamber with Micro Pattern Gaseous Detectors. In particular, the new readout
chambers will consist of stacks of 4 Gas Electron Multiplier foils combining
different hole pitches. The upgraded detector will operate continuously without
the use of a triggered gating grid. It will thus be able to record all Pb--Pb
collisions at the LHC interaction rate of 50 kHz.
