Jochen Klein and Marco van Leeuwen describe the physics motivation, detector concept and projected performance of a next-generation heavy-ion programme for LHC Runs 5 and 6.
The ALICE experiment at the LHC was conceived to study the properties of the quark–gluon plasma (QGP), the state of matter prevailing a few microseconds after the Big Bang. Collisions between large nuclei in the LHC produce matter at temperatures of about 3 × 1012 K, sufficiently high to liberate quarks and gluons, and thus to study the deconfined QGP state in the laboratory. The heavy-ion programme at LHC Runs 1 and 2 has already enabled the ALICE collaboration to study the formation of the QGP, its collective expansion and its properties, using for example the interactions of heavy quarks and high-energy partons with the QGP. ALICE 3 builds on these discoveries to reach the next level of understanding.
One of the most striking discoveries at the LHC is that J/ψ mesons not only “melt” in the QGP but can also be regenerated from charm quarks produced in independent hard scatterings. The LHC programme has also shown that the energy loss of partons propagating through the plasma depends on their mass. Furthermore, collective behaviour and enhanced strange-baryon production have been observed in selected proton–proton collisions in which large numbers of particles are produced, signalling that high densities may be reached in such collisions.
During Long Shutdown 2, a major upgrade of the ALICE detector (ALICE 2) was completed on budget and in time for the start of Run 3 in 2022. Together with improvements in the LHC itself, the experiment will profit from a factor-50 higher Pb–Pb collision rate and also provide a better pointing resolution. This will bring qualitative improvements for the entire physics programme, in particular for the detection of heavy-flavour hadrons and thermal di-electron radiation. However, several important questions – for example concerning the mechanisms leading to thermal equilibrium and the formation of hadrons in the QGP – will remain open even after Runs 3 and 4. To address these, the collaboration is pursuing next-generation technologies to build a new detector with a significantly larger rapidity coverage and excellent pointing resolution and particle identification (see “Brand new” figure). A letter of intent for ALICE 3, to be installed in 2033/2034 (Long Shutdown 4) and operated during Runs 5 and 6 (starting in 2035), was submitted to the LHC experiments committee in 2021 and led to a positive evaluation by the extended review panel in March 2022.
Behind the curtain of hadronisation
In heavy-ion collisions at the LHC, a large amount of energy is deposited in a small volume, forming a QGP. The plasma immediately starts expanding and cooling down, eventually reaching a temperature at which hadrons are formed. Although hadrons formed at the boundary of this phase transition carry information about the expansion of the plasma, they do not inform us directly about the temperature and other properties of the hot plasma phase of the collision before hadronisation takes place. Photons and di-lepton pairs, which are produced as thermal radiation in electromagnetic processes and do not participate in the strong interaction, allow us to look behind the curtain of hadronisation. However, measurements of photon and dilepton emission are challenging due to the large background from electromagnetic decays of light hadrons and weak decays of heavy-flavour hadrons.
One of the goals of the current ALICE 2 upgrades is to enable the first measurements of the thermal emission of electron–positron pairs (from virtual photons), and thus to determine the average temperature of the system before the formation of hadrons, during Runs 3 and 4. To further understand the evolution of temperature with time, larger data samples and excellent background rejection are needed. The early-stage temperature is determined from the exponential slope of the mass distribution above the ρ resonance, i.e. pair masses larger than 1.2 GeV/c2 (see “Taking the temperature” figure, upper panel). ALICE 3 would be able to explore the time dependence of the temperature before hadronisation using more differential measurements, e.g. of the azimuthal asymmetry of di-electron emission and of the slope of the mass spectrum as a function of transverse momentum.
The di-electron mass spectrum also carries unique information about the mechanism of chiral symmetry breaking – a fundamental quantum-chromodynamics (QCD) effect that generates most of the hadron mass. At the phase transition to the QGP, chiral symmetry is restored and quarks and gluons are deconfined. One of the predicted signals of this transition is mixing between the ρ and a1 vector-meson states, which gives the di-electron invariant mass spectrum a characteristic exponential shape in the mass range above the ρ meson peak (0.8–1.1 GeV/c2). Only the excellent electron identification and rejection of electrons from heavy-flavour decays possible with ALICE 3 can give physicists experimental access to this effect (see “Taking the temperature” figure, lower panel).
Another important goal of the ALICE physics programme is to understand how energetic quarks and gluons interact with the QGP and eventually thermalise and form a plasma that behaves as a fluid with very low internal friction. The thermalisation process and the properties of the QGP are governed by low-momentum interactions between quarks and gluons, which cannot be calculated using perturbative techniques. Experimental input is therefore important to understand these phenomena and to link them to fundamental QCD.
The heavy charm and beauty quarks are of particular interest because their interactions with the plasma can be calculated using lattice-QCD techniques with good theoretical control. Heavy quarks and antiquarks are mostly produced as back-to-back pairs in hard scatterings in the early phase of the collision. Subsequent interactions between the quarks and the plasma change the angle between the quark and antiquark. In addition, the “drag” from the plasma leads to an asymmetry in the overall azimuthal distributions of heavy quarks (elliptic flow) with respect to the reaction plane. The size of these effects is a measure of the strength of the interactions with the plasma. Since quark flavour is conserved in interactions in the plasma, measurements of hadrons containing heavy quarks, such as the D meson and Λc baryon, are directly sensitive to the interactions between heavy quarks and the plasma. While the increase in statistics and the improved spatial resolution of ALICE 2 will already allow us to measure the production of charm baryons, measurements of azimuthal correlations of charm–hadron pairs are needed to directly address how they interact with the plasma. These will only become possible with the precision, statistics and acceptance of ALICE 3.
Heavier beauty quarks are expected to take longer to thermalise and therefore lose less information through their interactions with the QGP. Therefore, systematic measurements of transverse-momentum distributions and azimuthal asymmetries of beauty mesons and baryons in heavy-ion collisions are essential to map out the interactions of heavy-flavour quarks with the QGP and to understand the mechanisms that drive the system towards thermal equilibrium.
To understand how hadrons emerge from the QGP, those containing multiple heavy quarks are of particular interest because they can only be formed from quarks that were produced in separate hard-scattering processes. If full thermal equilibrium is reached in Pb–Pb collisions, the production rates of such states are expected to be enhanced by up to three orders of magnitude with respect to pp collisions. This implies enormous sensitivity to the probability for combining independently produced quarks during hadronisation and to the degree of thermalisation. At ALICE 3, the precision with which multi-charm baryon yields can be measured is enhanced (see “Multi-charm production” figure).
In addition to precision measurements of di-electrons and heavy-flavour hadrons, ALICE 3 will allow us to investigate many more aspects of the QGP. These include fluctuations of conserved quantum numbers, such as flavour and baryon number, which are sensitive to the nature of the deconfinement phase transition of QCD. ALICE 3 will also aim to answer questions in hadron physics, for example by searching for the existence of nuclei containing charm baryons (analogous to strange baryons in hypernuclei) and by studying the interaction potentials between unstable hadrons, which may elucidate the structure of exotic hadronic states that have recently been discovered in electron–positron collisions and in hadronic collisions at the LHC. In addition, ALICE 3 will use ultra-peripheral collisions to study the structure of resonances such as the ρ′ and to look for new fundamental particles, such as axion-like particles and dark photons. A dedicated detector system is foreseen to study very low-energy photon production, which can be used to test “soft theorems” that link the production of very soft photons in a collision to the hadronic final state.
Pushing the experimental limits
To pursue this ambitious physics programme, ALICE 3 is designed to be a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates. The main tracking information is provided by an all-silicon tracker in a magnetic field provided by a superconducting magnet system, complemented by a dedicated vertex detector that will have to be retractable to provide the required aperture for the LHC at injection energy. To achieve the ultimate pointing resolution, the first hits must be detected as close as possible to the interaction point (5 mm at the highest energy) and the amount of material in front of it be kept to a minimum. The inner tracking layers will also enable so-called strangeness tracking – the direct detection of strange baryons before they decay – to improve the pointing resolution and suppress combinatorial background, for example in the measurement of multi-charm baryon decays.
ALICE 3 is a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates
First feasibility studies of the mechanical design and the integration with the LHC for the vertex tracker have been conducted and engineering models have been produced to demonstrate the concept and explore production techniques for the components (see “Close encounters” image). The detection layers are to be constructed from bent, wafer-scale pixel sensors. The development of the next generation of CMOS pixel sensors in 65 nm technology with higher radiation tolerance and improved spatial resolution has already started in the context of the ITS 3 project in ALICE, which will be an important milestone on the way to ALICE 3 (see “Next-gen tracking” image). The outer tracker, which has to cover the cylindrical volume to a radius of 80 cm over a total length of ±4 m, will also use CMOS pixel sensors. These will be integrated into larger modules for an effective instrumentation of about 60 m2 while minimising the material used for mechanical support and services. The foreseen material budget for the tracker is 1% of a radiation length per layer for the outer tracker, and only 0.05% per layer for the vertex tracker.
For particle identification, five different detector systems are foreseen: a silicon-based time-of-flight system and a ring-imaging Cherenkov (RICH) detector that provide hadron and electron identification over a broad momentum range, a muon identifier starting from a transverse momentum of about 1.5 GeV/c, an electromagnetic calorimeter for photon detection and identification, and a forward tracker to reconstruct photons at very low momentum from their conversions to electron–positron pairs. For the time-of-flight system, the main R&D line aims at the integration of a gain layer in monolithic CMOS sensors to achieve the required time resolution of at least 20 ps (alternatively, low-gain avalanche diodes with external readout circuitry can be used). The calorimeter is based on a combination of lead-sampling and lead-tungstate segments, both of which would be read out by commercially available silicon photomultipliers (SiPMs). For the detection layers of the muon identifier, both resistive plate chambers and scintillating bars are being considered. Finally, for the RICH design, the R&D goal is to integrate the digital readout circuitry in SiPMs to enable efficient detection of photons in the visible range.
ALICE 3 provides a roadmap for an exciting heavy-ion physics programme, along with the other three large LHC experiments, in Runs 5 and 6. An R&D programme for the coming years is being set up to establish the technologies and enable the preparation of technical design reports in 2026/2027. These developments not only constitute an important contribution to the full physics exploitation of the LHC, but are of strategic interest for future particle detectors and will benefit the particle and nuclear physics community at large.