Published: 06 August 2012

Long-term plans of the ALICE Collaboration

The ALICE collaboration would like to contribute to the discussion about the future strategy in particle physics for the coming decade, in the field of high-energy nuclear physics. This initiative comes in time, while we are preparing an upgrade of the ALICE detector, in order to fully exploit the scientific potential of the LHC as a heavy-ion collider. With the planned upgrades, this field of research will move from the present exploratory phase to an era of high-statistics precision measurement. This will allow a detailed characterization of the high-density, high-temperature phase of strongly interacting matter, and detailed tests of the fundamental predictions of the theory. In the following, we outline the basic physics motivation for running the LHC with heavy ions at high-luminosities and summarize the performance gains expected with the upgraded ALICE detector. With the proposed timeline, starting the high-rate operation progressively after 2018 shutdown, the objectives of our upgrade plans should be achieved collecting data until the mid-2020’s. We expect that this programme of research will be considered as a high-priority item by the European Strategy Group.

Physics motivation

At very high temperatures and densities, hadronic matter is expected to undergo a phase transition into a qualitatively different state, where quark and gluon degrees of freedom are liberated. Such conditions were prevailing in the early Universe, a few microseconds after its formation. In heavy-ion collisions at ultra-relativistic energies, nuclear matter is heated and compressed reaching conditions well beyond the phase-transition point, and the same type of medium filling the very early Universe is thus momentarily re-created. At the highest available energies, at the Large Hadron Collider (LHC), far better conditions, than those at the other existing or planned facilities, are achieved to study this new state of matter, called the Quark–Gluon Plasma (QGP). The nearly-vanishing baryon density, the highest available initial temperature and energy density, and the abundance of perturbatively- calculable hard Quantum Chromo-Dynamics (QCD) processes make heavy-ion collisions at the LHC particularly well-suited for precision studies of the QGP properties.

QCD, the well-established theory of strong interactions, predicts (cross-over) phase transitions in strongly interacting matter at high temperatures. A phase transition reflects breaking of a fundamental symmetry in the theory. Above the critical temperature, ordinary hadronic matter, where protons and neutrons are composed of quarks and gluons confined in a colour-neutral state, melts during the deconfinement phase transition. In the deconfined medium, quarks and gluons are not bound into hadrons anymore, however, the effective degrees of freedom of the QGP, formed at temperatures achieved with LHC heavy-ion collisions, are rather complex. The deconfinement phase transition is caused by breaking of Z3-symmetry (symmetry in the limit of pure gauge QCD) at high temperature, and is illustrated in Fig. 1 (left) by a rapid change of the corresponding order parameter, the expectation value of Polyakov loop.


"The experimental demonstration of these phase transitions, the verification of the lattice QCD predictions reflecting the fundamental symmetries of the theory, and a detailed investigation of the properties of strongly interacting matter above the critical temperature are the principal aims of the ALICE scientific programme."

A second phase transition is connected with the generation of hadron masses as a consequence of the presence of a quark–antiquark condensate in the vacuum at low temperature. According to QCD, at high temperatures, the vacuum condensate is reduced, and the masses of quarks drop to their bare values during a chiral phase transition. This can be seen in Fig. 1 (right): the expectation value for the density of vacuum quark–antiquark condensate, the order parameter of chiral transition, exhibits a sudden drop in the vicinity of the critical temperature. The underlying reason for this phase transition is the restoration of the approximate chiral symmetry (exact symmetry in the limit of zero bare quark masses) of the QCD Lagrangian.


Fig1 Recent lattice QCD calculations of the behavior of the order parameters of the deconfinement (Polyakov loop left) and the chiral (quark-antiquark vacuum condensate, right) phase transitions as a function of temperature

The experimental demonstration of these phase transitions, the verification of the lattice QCD predictions reflecting the fundamental symmetries of the theory, and a detailed investigation of the properties of strongly interacting matter above the critical temperature are the principal aims of the ALICE scientific programme. Precise determination of the QGP properties, including critical temperature, degrees of freedom, speed of sound, and, in general, transport coefficients is the ultimate goal in this field. This would go a long way towards a better understanding of QCD as a genuine multi-particle theory. In addition, such precision measurements would shed light on the complex issues of deconfinement and chiral-symmetry restoration.

Prior to the start-up of the LHC heavy-ion programme, the nature of the QGP as an almost-perfect, inviscid liquid emerged from the experimental investigation. With the first LHC data, the ALICE collaboration confirmed this basic picture, observing the creation of deconfined matter at unprecedented values of temperatures, densities and volumes. The very low ratio of shear-viscosity to entropy-density of the QGP evidences very short mean free path inside this medium composed of strongly-coupled quasi-particle modes, whose origin is the subject of further experimental study. The progress towards the characterization of this strongly interacting state of matter will now focus on rare probes, and the study of their coupling with the medium and hadronization modes. These include heavy-flavour particles, quarkonia states, real and virtual photons, jets and their correlations with other probes. The cross sections of all these processes are significantly larger at LHC than at previous accelerators. In addition, the interaction with the medium of heavy-flavour probes is better controlled theoretically than the propagation of light partons.

Even for hadrons containing heavy quarks, this investigation involves soft momentum scales, and thus benefits from the ALICE detector strengths: excellent tracking performance in high multiplicity environment and particle identification over a large momentum range. Major highlights of the proposed programme focus on the following physics questions:

•Study of the thermalization of partons in the QGP, with focus on the massive charm and beauty quarks. Heavy-quark elliptic flow is especially sensitive to the partonic equation of state. Ultimately, heavy quarks might fully equilibrate and become part of the strongly-coupled medium.

•Study of the low-momentum quarkonium dissociation and, possibly, regeneration pattern, as a probe of deconfinement and of the medium temperature.

•Study of the production of thermal photons and low-mass di-leptons emitted by the QGP. This should allow to assess the initial temperature and degrees of freedom of the system, as well as to shed light on the chiral nature of the phase transition.

•Study of the in-medium parton energy-loss mechanism that provides both a testing ground for the multi-particle aspects of QCD and a probe of the QGP density. The relevant observables are: jet structure, jet–jet and photon–jet correlations, jets and correlations with high- momentum identified hadrons and heavy-flavour particle production. In particular, it is crucial to characterize the dependencies of energy loss on the parton colour charge, mass, and energy, as well as on the density of the medium.

•Search for heavy-nuclear states such as light multi-? hyper-nuclei 5H, bound states of (??) or the H di-baryon, (?n) bound state, as well as bound states involving multi-strange baryons. A systematic study of light nuclei and anti-nuclei production.

ALICE upgrade

To achieve these goals high statistics and high precision measurements are required, which will give access to the rare physics channels needed to understand the dynamics of this condensed phase of QCD. Many of these measurements will involve complex probes at low transverse momentum, where traditional methods for triggering will not be applicable. Therefore, the ALICE collaboration is planning to upgrade the current detector by enhancing its vertexing and low-momentum tracking capability, and allowing data taking at substantially higher rates.


"To achieve these goals high statistics and high precision measurements are required, which will give access to the rare physics channels needed to understand the dynamics of this condensed phase of QCD. Many of these measurements will involve complex probes at low transverse momentum, where traditional methods for triggering will not be applicable."

The upgrade strategy is formulated under the assumption that, after the second long shutdown in 2018, the LHC will progressively increase its luminosity with Pb beams eventually reaching an interaction rate of about 50 kHz, i.e. instantaneous luminosities of L = 6×1027 cm-2s-1. In the proposed plan the ALICE detector is modified such that most of this increased interaction rate can be inspected. ALICE will then be in a position to accumulate 10 nb-1 of Pb–Pb collisions inspecting O(1010) interactions, which is the minimum needed to address the proposed physics programme, with focus on rare probes both at low and high transverse momenta as well as on the multi-dimensional analysis of such probes with respect to centrality, event plane, multi-particle correlations, etc.

High statistics Pb–Pb measurements will have to be accompanied by precision measurements with p–p and p–Pb collisions to provide a quantitative base for comparison with results from Pb–Pb collisions. Those are furthermore crucial to understand initial state modifications in nuclei, such as shadowing or non-linear QCD evolution, possibly leading to gluon saturation. While the physics of gluon saturation approaches a very different, new regime of QCD, and is therefore interesting in its own right, it is also important for the correct interpretation of final state effects due to the QGP and the influence of initial conditions on observables like elliptic flow. In order to collect a sample of p–p reference data with the statistical significance comparable to that of the Pb–Pb data, the required integrated luminosity for p–p collisions is estimated to about 6 pb-1, taking into account smaller combinatorial background in most of the cases. This results in a p–p data taking at a rate of 500 kHz over a month of running, which would allow to fulfill this programme with one special run at the p–p centre-of-mass energy equal to that of Pb–Pb per nucleon pair. Another option under discussion is the utilization of lighter-ion collisions, which the LHC could produce at higher luminosities; for Ar–Ar collisions an increase of order of magnitude could potentially be reached.


ALICE Collaboration 35 countries, 120 institutes, over 1300 members

To address the topics outlined above and making headway in our quantitative understanding of the properties of the deconfined QGP the ALICE collaboration propose a strategy for upgrading the ALICE detector to be able to make full use of a high-luminosity LHC (L = 6×1027 cm-2s-1 for Pb–Pb). The planned upgrades preserve the excellent tracking and particle identification capabilities, and include: • A new, high-resolution, low-material-thickness Inner Tracking System (ITS). The details of the upgrade are outlined in the Conceptual Design Report (CERN–LHCC–2012–05). With this new detector the resolution of the distance-of-closest approach between a track and the primary vertex will be improved by a factor of about 3, and the standalone ITS tracking performance will be significantly enhanced. • Upgrade of the Time-Projection Chamber (TPC) with replacement of the readout multi-wire chambers with GEM (Gas Electron Multiplier) detectors. This will minimize the feedback of ions from the amplification region and allow running the TPC with continuously-open gate. • Upgrade of the readout electronics of: the TPC, Transition-Radiation Detector (TRD), Time- Of-Flight (TOF) detector, ElectroMagnetic Calorimeter (EMCal), and Muon spectrometer. This will allow for high-rate data taking with these detectors. • Upgrade of the online systems: High-Level Trigger (HLT), data acquisition (DAQ), and trigger system, to adapt for high rates and to increase the data throughput to mass storage to about 20 GB/s. A completely new trigger system will have to be developed, including significant capabilities for topological triggers.

Moreover, a completion of the EMCal to nearly 2? coverage is under consideration. In the course of this, the current ALICE minimum-bias trigger detectors T0 and V0 will be upgraded as appropriate.

In addition to significantly improved performance in vertexing and low-momentum tracking with the upgraded detector, with the high-rate capability ALICE will collect hundred times more statistics for most of the relevant probes, those where an inspection of all collisions is necessary, compared to the approved running scenario, which assumes 1 nb-1 of integrated luminosity. The approved integrated luminosity corresponds to O(109) interactions of which ALICE without the high-rate upgrade could inspect only O(108) events. For the rare probes which utilize a selective trigger, such as high-p jets or di-muons, the statistics will increase by one order of magnitude. Further upgrade proposals for an installation on a similar time scale are currently under discussion within the ALICE collaboration: •The Muon Forward Tracker (MFT) could add secondary-vertex reconstruction capabilities to the muon measurement and enhance the forward heavy-flavour programme. •The Very High Momentum Particle ID (VHMPID), a new RICH detector in the central barrel, could allow additional identification of high-momentum hadrons, which would in particular be useful for advanced studies of hadronization. •The Forward Calorimeter (FoCal) with a highly granular electromagnetic part could allow additional jet and direct-photon measurements at forward rapidities, enhancing the overall photon–jet physics of ALICE and, in particular, enabling studies of gluon saturation.

Conclusion

The main long-term goal of ALICE is to provide a precise characterization of the Quark–Gluon Plasma, the state of deconfined matter produced in high-energy heavy-ion collisions. The timely upgrade of the ALICE detector, and the LHC heavy-ion running till mid-2020’s to accumulate integrated luminosity with Pb–Pb collisions above 10 nb-1, are the necessary conditions to fully exploit the LHC scientific potential in the field of high-temperature QCD. The CERN collider, delivering the highest energy heavy-ion collisions, is a unique instrument, creating this state of high-density strongly interacting matter at the highest temperature, and in clean conditions of practically zero baryon density. At the same time, hard probes, such as heavy-flavour quarks, energetic jets, and electroweak particles, indispensable for precision exploration of the strongly-coupled medium, are produced with large cross sections.

The ALICE upgrade, featuring a new, high-resolution, Inner Tracking System, continuous readout of the Time-Projection Chamber, and the high-rate upgrade of the readout electronics of the other detectors and the online systems, will allow for a qualitative step forward in our understanding of the new state of matter created in ultra-relativistic heavy-ion collisions. This knowledge will help answer fundamental open questions about the basic symmetries and hadron-mass generation, and potentially shed light on the development of the early Universe. The approach pursued by ALICE, accenting the detection of low and medium momentum probes and taking advantage of particle-identification capabilities, will be complemented by the high-momentum and jet capabilities of ATLAS and CMS.

The ALICE collaboration represents already 20 years of the effort in developing, building, and recently exploiting the largest experiment in this field at the forefront of science and technology. Today 1300 scientists and engineers from 35 countries worldwide are involved in this venture; setting new and challenging goals for the coming decade to further advance our understanding of nature. For the ALICE members the continuation of the LHC heavy-ion programme with improved detection facilities is the highest priority in high-energy nuclear physics.