On the horizon for ALICE
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 series of upgrades during the long shutdowns of the accelerator in the coming years. The new ALICE will have enhanced read-out capabilities and improved efficiency when tracking particles and identifying the vertex of the interactions.
Corrado Gargiulo, ALICE's Project Engineer with ITS prototype. The new ITS will consist of 7 layers of silicon sensors supported by a ultra-light carbon fibre structure.
The LHC has been operated with lead ions for only about two months, but this has been sufficient for ALICE and other LHC experiments to produce results that previous accelerators took several years of operation to produce. “Prior to the start-up of the LHC heavy-ion programme, the nature of the QGP as an almost-perfect liquid had already emerged from the experimental investigations at CERN's SPS and at BNL's RHIC,” says Paolo Giubellino, ALICE Spokesperson. “The LHC experiments have confirmed and extended this basic picture, 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.”
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 luminosity that the experiment will be able to deal with. “Our 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,” explains Paolo Giubellino. “The new ALICE detector will allow the readout of all interactions and will be able to record 1011 lead-lead interactions at a rate of 50 kHz – about two orders of magnitude higher than the current rate capability.”
Besides the partial redesign of the readout electronics, the planned upgrades also include a new beampipe with a smaller diameter; a new Inner Tracking System (ITS); the upgrade of the Time Projection Chamber, where GEM detectors will replace the wire chambers; the upgrade of the forward trigger detectors; and the upgrade of the tools for the online and offline reconstruction of the events. “We are currently testing various solutions for the different upgrades,” explains Luciano Musa, ITS Project Leader. “In particular, we are studying two design options for the ITS: pixel detectors only, or pixel and strip detectors combined. We have also built a prototype of the carbon mechanical structure and have carried out stability and thermal tests.”
The new silicon tracker will allow ALICE to measure charm and beauty production in lead-lead collisions with unprecedented statistical and systematic accuracy, providing crucial information for the understanding of the dynamics (transport, thermalization, hadronization) of heavy quarks in the QGP state. The new ITS will also play a key role in assessing the initial temperature and degrees of freedom of the QGP, as well as the modalities of the phase transition.
The ALICE upgrade will be carried out in phase with the long LHC shutdowns, in particular the second one scheduled for 2017-2018. A major R&D effort has been launched in view of the upgrade, involving technologies ranging from advanced mechanics and electronics to innovative detection systems and a new approach to data treatment. The challenges are, as usual, exciting and these activities, now in full swing, will continue until 2014.
by Antonella Del Rosso