ALICE, the LHC experiment specifically designed to study the physics of the Quark Gluon Plasma (QGP) and, more generally, of strongly interacting matter at extreme energy densities is planning a major upgrade during the Long Shutdown 2 foreseen to start in 2018. The new ALICE will have significantly enhanced read-out capabilities and improved accuracy and efficiency in tracking particles and identifying the interaction vertex.
The LHC operated with lead-lead collisions for about two months, but this was sufficient for ALICE and the other LHC experiments to produce new physics results. that previous accelerators took several years of operation to achieve. ALICE continues the exploration that started in 2000 with the discovery of a new state of primordial matter, the Quark Gluon Plasma at the CERN SPS and was followed by dedicated studies at the RHIC collider at BNL.
The QGP was discovered at the CERN SPS and studied at the dedicated RHIC collider at BNL. The LHC experiments have opened new avenues in our understanding of the properties of this state of hot and dense strongly interacting matter, observing the creation of hot hadronic matter at unprecedented temperatures, densities and volumes and exceeding the precision of all relevant measurements performed over the past decade. The proton-lead run in 2013, that was meant to be a ‘control experiment’ for the understanding of heavy ion collisions, produced a wealth of results due to the observation of unexpected possible collective effects.
To build on this excellent performance, the ALICE collaboration is now seeking to upgrade the detector and enhance its physics capabilities through a significant increase of the luminosity that the experiment will be able to deal with. The upgrade strategy is formulated under the assumption that, after the second long LHC shutdown in 2018, the luminosity with lead beams will gradually increase to an interaction rate of about 50 kHz.
ALICE is focusing on signatures that do not exhibit topologies which can easily be triggered on and therefore plans to read all events into the online system at data rates exceeding 1TB/s. The new ALICE detector will be able to record a total of 1011 lead-lead interactions at a rate of 50 kHz – about two orders of magnitude higher than the current readout rate capability.
Figure 1: Schematic of the ALICE detector.
Besides the partial redesign of the readout electronics to cope with the increased readout rate, the planned upgrades include a new beam pipe with a smaller diameter, a new Inner Tracking System (ITS), a vertex tracker for forward muons, the upgrade of the Time Projection Chamber with Micropattern detectors, the upgrade of the forward trigger detectors and the upgrade of the online and offline system.
The upgraded ITS will improve the track position resolution at the primary vertex by a factor of 3 or even larger with respect to the present detector. It features a standalone tracking efficiency comparable to what can be presently achieved by combining the information of the ITS and the TPC. Monolithic pixel sensors and special carbon fiber support structures will realize the ITS upgrade as an ultra-light 7 layer/10GigaPixel detector that will allow a significant boost of the tracking performance.
Figure 2. Layout of the new ALICE ITS, a 7 layer - ultra low mass monolithic pixel tracker.
The Muon Forward Tracker, a vertex tracker consisting of several discs in the forward region and covering the ALICE muon arm, will also be realized with monolithic silicon pixel sensors, providing the capability to identify secondary vertices and improving mass resolution.
The high collision rate does not allow the TPC to use the traditional gating grid technique for the blocking of ions. Micropattern detectors that have intrinsic ion blocking capability will therefore be employed for the TPC, which allows a continuous readout of the detector.
The massive amount of data arriving from the detector and the need for compression and partial reconstruction requires a very powerful data processing system, called O2, that combines functionalities that are presently split into ‘Online’, ‘Offline’ and the High Level Trigger. This system will then compress and partially reconstruct the events in order to allow efficient storage of all the events.
This ALICE upgrade programme has been approved by the LHCC and is now being detailed in a series of Technical Design Reports. The first one, for the new Inner Tracking System, has already received the final approval at the last meeting of the Research Board and the others are following.
The ALICE upgrade will significantly increase its rate capability and realize a monolithic pixel tracker and a continuously sensitive TPC, two particle detector concepts that have long been dreamed about and that will boost ALICE experimental capabilities for its exciting physics scope. With such an instrument at hand, ALICE will investigate heavy ion collisions after LS2 and beyond LS3 . in the typical pattern of one month per year during the LHC operation.