Nicola Nicassio

# ALICE 3: a next-generation heavy-ion detector for LHC Run 5 and beyond

on behalf of the ALICE Collaboration

###### Abstract

The ALICE Collaboration proposes a completely new apparatus, ALICE 3, for the
LHC Runs 5 and 6. The detector consists of a large pixel-based tracking system
covering eight units of pseudorapidity, complemented by multiple systems for
particle identification, including silicon time-of-flight layers, a ring-
imaging Cherenkov detector, a muon identification system, an electromagnetic
calorimeter and a forward conversion tracker. Track pointing resolution of
better than 10 $\mu$m for $p_{T}$ >200 MeV/$c$ can be achieved by placing the
vertex detector on a retractable structure inside the beam pipe. ALICE 3 will,
on the one hand, enable novel studies of the quark-gluon plasma (QGP) and, on
the other hand, open up important physics opportunities in other areas of QCD
and beyond. The main new studies in the QGP sector focus on low-$p_{T}$ heavy-
flavour production, including beauty hadrons, multi-charm baryons and charm-
charm correlations, as well as on precise multi-differential measurements of
dielectron emission to probe the mechanism of chiral-symmetry restoration and
the time-evolution of the QGP temperature. Besides QGP studies, ALICE 3 can
uniquely contribute to hadronic physics, with femtoscopic studies of the
interaction potentials between charm mesons and searches for nuclei with
charm, and to fundamental physics, with tests of the Low theorem for ultra-
soft photon emission. This paper will cover the detector concept, the status
of novel sensor R&D and the resulting physics performance.

##  1 Introduction

ALICE (A Large Ion Collider Experiment) is a general-purpose experiment at the
CERN Large Hadron Collider (LHC) optimized to study the physics of
ultrarelativistic heavy-ion collisions. The main goal of the ALICE
Collaboration is to study the microscopic dynamics of the strongly interacting
matter produced in such collisions and, in particular, the properties of the
quark-gluon plasma (QGP), a state of matter where quarks and gluons are
deconfined. The LHC Runs 1 and 2 heavy-ion campaigns have already led to
crucial advances in our understanding of the properties of QCD phase diagram
and of the QGP [1]. With the current ALICE 2 and the future ALICE 2.1
upgrades, LHC Runs 3 and 4 will allow further systematic measurements of
fundamental properties for our understanding of the QGP [2]. However, despite
the rich planned physics program, fundamental questions will remain open and
addressing such questions requires substantial improvements in terms of
detector performance and rate capabilities, calling for a next-generation
heavy-ion experiment, the so-called ALICE 3 upgrade planned for LHC Run 5 and
beyond [3].

Figure 1: Left: Schematic view of ALICE 3 state-pf-the-art detector concept.
Right: Improvement in pointing resolution and expected effective statistics in
heavy-ion collisions from ALICE 1 to ALICE 3.

##  2 ALICE 3 physics goals

The ALICE 3 primary physics goal is to achieve a deep understanding of the
underlying mechanisms affecting the different stages of heavy-ion collisions,
from the early-stage medium formation to the heavy-flavour diffusion,
thermalization and hadronization in the QGP. The main new studies in the QGP
sector focus on low transverse momentum ($p_{T}$) heavy-flavour production,
including beauty hadrons, multi-charm baryons and charm-charm correlations, as
well as on measurements of net-quantum number fluctuations to constrain the
susceptibilities of the QGP and on precise measurements of dilepton emission
to probe the time-evolution of the QGP, the early-stage temperature and the
mechanism of chiral-symmetry restoration. Besides QGP studies, ALICE 3 can
uniquely contribute to hadronic physics, with femtoscopic studies of the
interaction potentials between charm mesons and searches for nuclei with
charm, and to fundamental physics, with precision experimntal tests of Low’s
theorem for ultra-soft photon emission and searches for axion-like particles
(ALP) in ultra-peripheral heavy-ion collisions via light-by-light scattering
measurements.

##  3 ALICE 3 detector concept and R&Ds

All the measurements planned to address ALICE 3 physics goals set strong
requirements on vertexing, tracking, particle identification and rate
capabilities. The combination of these requirements led to the state-of-the-
art detector concept shown in Fig. 1. The key requirements are a retractable
vertex detector with an unprecedented pointing resolution, a compact and
lightweight all-silicon tracker combined with a superconducting magnet system,
extensive identifications of $\gamma$, e±, $\mu^{\pm}$, $\pi^{\pm}$, K± and p±
with different dedicated subsystems, large pseudorapidity acceptance
($|\eta|<4$) and continuous read-out combined with online processing. The
importance of the ALICE 3 upgrade mostly relies on the combined progress in
effective statistics (luminosity $\times$ acceptance) and pointing resolution.
Fig. 1 shows the overall expected improvement by orders of magnitude in the
detector capabilities from ALICE 1 (Runs 1 and 2) to ALICE 2 (Run 3), ALICE
2.1 (Run 4) and ALICE 3 (Runs 5 and 6). In the following, the main features of
the different ALICE 3 subsystems are outlined and some of the corresponding
ongoing R&D activities are presented.

Figure 2: Left: Sketch of the ALICE 3 vertex detector for stable beam (top)
and at beam injection (bottom). Center: Expected pointing resolution for
electrons as a function of $p_{T}$. Right: Expected relative $p_{T}$
resolution for pions as a function of $p_{T}$ assuming B = 0.5T (red) and B =
2T (blue).

###  3.1 Vertexing and tracking

The heart of ALICE 3 is a silicon vertex detector designed to provide a
pointing resolution $\sigma_{DCA}$ better than 10 ${\mu}$m for $p_{T}$ larger
than 200 MeV. The required $\sigma_{DCA}$ can be only achieved by using ultra-
thin silicon sensors, featuring an unprecedented low material budget of 0.1%
of a radiation length, and with the first tracking layer placed at a radius of
5 mm from the beam axis at top energy. However, a wider aperture of
$\approx$15 mm is required at injection energy, demanding for a retractable
detector design. The current baseline consists of wafer-sized, bent Monolithic
Active Pixel Sensors (MAPS) with 10 $\mu$m pixel pitch arranged in 3 barrel
layers and 3 forward disks at each end-cap installed in a secondary vacuum
inside the beampipe and mounted such that they can be retracted during LHC
injection and placed close to the interaction point for data taking, as shown
in Fig. 2. The resulting pointing resolution for electrons as a function of
$p_{T}$ is also shown. The main R&D challenges concern mechanical supports,
cooling and radiation tolerance of the sensors.

The vertex detector is complemented by an outer tracker consisting of 8
cylindrical layers and 9 forward disks at each end-cap equipped with equipped
with MAPS having a pixel pitch of 50 $\mu$m, with a material budget thickness
of $\approx$1% of a radiation length per layer, and installed in a volume of
80 cm radius and $\pm$4 m length around the interaction point. The leading
requirement for the outer tracker is a relative $p_{T}$ resolution better than
1% over a wide $p_{T}$ range. The momentum is reconstructed from the curvature
in a solenoidal magnetic field of B = 2T provided by a superconducting magnet
system featuring an inner radius of 1.5 m. Fig. 2 shows the expected relative
$p_{T}$ resolution for pions at $\eta=0$ as a function of $p_{T}$. The
implementation of additional forward dipoles to keep an excellent relative
$p_{T}$ resolution up to $|\eta|=4$ is under study.

Figure 3: Expected $\eta-p_{T}$ regions in which particles can be separated by
at least $3\sigma$ with the ALICE 3 TOF and RICH systems. Electron/pion,
pion/kaon and kaon/proton separation plots are shown from left to right.

###  3.2 Particle identification

The tracker is complemented by dedicated particle identification (PID)
systems. With a time resolution of 20 ps, an inner Time-Of-Flight (TOF) layer
(iTOF) at a radius of 20 cm, an outer TOF layer (oTOF) at a radius of 85 cm
and forward TOF disks (fTOF) at $\pm 4.05$ m distance from the interaction
point ensure e/$\pi$, $\pi$/K and K/p separation up to 0.5, 2 and 4 GeV/$c$,
respectively, as shown in Fig. 3. Three sensor technologies have been
identified for dedicated R&Ds: MAPS, Low Gain Avalanche Diodes (LGADs) and
Silicon Photomultipliers (SiPMs). Recent beam test results prove the
feasibility of the 20 ps resolution requirement using LGADs [4] and SiPMs [5].

The TOF system is complemented by a barrel Ring-Imaging Cherenkov (RICH) layer
(bRICH) with an inner radius of 90 cm and forward RICH disks (fRICH) at $\pm
4.05$ m distance from the interaction point based on an aerogel radiator with
a refractive index of 1.03 and a SiPM-based photodetector providing a
Cherenkov angle resolution better than 1 mrad to extend e/$\pi$, $\pi$/K and
K/p separation above 2, 10 and 16 GeV/$c$, respectively, as shown in Fig. 3. A
first small scale prototype was successfully tested on beam, with an
extrapolated angular resolution consistent with the 1 mrad requirement [6].
The main R&D challenges include characterization studies on the radiation
tolerance of the sensors, cooling and the mechanics of the whole RICH system.

For the identification of photons and high energy electrons, up to hundreds of
GeV, an Electromagnetic Calorimeter (ECal) based on Pb-scintillator sampling
technology and including a high energy and spatial resolution segment based on
PbWO4 crystals at $\eta=0$ is foreseen behind the bRICH and the fRICH to cover
the region $-1.5<\eta<4$. The ECal specifications are optimized to enable
precision measurements of radiative decays of $\chi_{c}$ and $D^{*0}$ states
and photon-jet correlations.

The identification of ultra-soft photons with $p_{T}$ down to 1 MeV/$c$, is
only possible by exploiting the Lorentz boost at large $\eta$ through their
conversion to e± pairs. A dedicated Forward Conversion Tracker (FCT)
consisting of 9 disks equipped with ultra-thin MAPS is foreseen to cover the
region $3<\eta<5$. A major R&D challenge is to limit the amount of material in
front of the FCT to less than 10% of a radiation length to suppress the
background from bremsstrahlung processes.

For the identification of muons down to $p_{T}\approx 1.5$ GeV/$c$, a
dedicated Muon Identification system (MID) consisting of a 70 cm thick steel
hadron absorber followed by dedicated muon chambers equipped with scintillator
bars readout using SiPMs covering up to $\eta=1.3$ is planned to be installed
outside of the magnet. Alternative options to scintillators, such as multi-
wire proportional chambers (MWPCs) and resistive plate chambers (RPCs) are
also under study.

##  4 ALICE 3 expected performance

In this section, a selection of projections for the expected ALICE 3 physics
performance are reported highlighting the impact of each subsystem
specifications on the ALICE 3 physics program.

###  4.1 Dielectrons

The hot medium created in heavy-ion collisions emits black-body radiation in
form of real and virtual photons. The latter result in dilepton pairs
production, representing a powerful probe for QGP temperature and the
mechanism of chiral-symmetry restoration studies. Fig. 4 shows the ALICE 3
projection for the expected thermal dielectron invariant mass ($m_{ee}$)
spectrum for central Pb-Pb collisions at a center-of-mass energy per nucleon
pair $\sqrt{s_{NN}}=5.02$ TeV after one month of data taking. In the region
above 1 GeV, the slope of the spectrum provides direct information on the QGP
temperature. The key ALICE 3 ingredients to achieve the desired statistical
and systematic precision are the excellent electron identification down to low
$p_{T}$ achieved combining both TOF and RICH information, the small detector
material budget in order to reduce the combinatorial background from $\gamma$
conversion or $\pi^{0}$ Dalitz decays and the excellent pointing resolution to
efficiently suppress the dominant background from heavy-flavour hadron decays
beyond current limits.

Figure 4: Left: Simulated invariant mass spectrum of thermal dielectrons
fitted with an exponential function in the $m_{ee}$ range 1.1-1.8 GeV/$c^{2}$
to extract the early-stage QGP temperature in central (0-10%) Pb–Pb collisions
at $\sqrt{s_{NN}}=5.02$ TeV. Center: Simulated $D^{0}\bar{D^{0}}$ azimuthal
correlation distribution in minimum-bias Pb–Pb collisions with ALICE 3 at
$\sqrt{s_{NN}}=5.52$ TeV. Right: Simulated $\Xi_{cc}^{++}$ significance in
central (0-10%) Pb–Pb collisions at $\sqrt{s_{NN}}=5.52$ TeV as a function of
$p_{T}$.

###  4.2 Heavy-quark correlations

The angular correlation of heavy-flavour hadron pairs is a powerful probe of
QGP scattering, since it is sensitive to the energy loss mechanisms and the
degree of heavy-quark thermalization in the QGP, especially at low $p_{T}$.
Fig. 4 shows the ALICE 3 projection for $D^{0}\bar{D^{0}}$ correlation for
minimum-bias Pb-Pb collisions at $\sqrt{s_{NN}}=5.52$ TeV after six years of
data taking. This measurement will be out of reach in LHC Runs 3 and 4, since
it requires very high $D^{0}$ purity and reconstruction efficiency, as well as
a large $\eta$ coverage, and it will be only possible with ALICE 3.

###  4.3 Multi-charm baryons

Multi-charm baryon yields are powerful probes of the hadron formation
mechanisms since they are expected to be produced by combination of
uncorrelated charm quarks produced in heavy-ion collisions. The ALICE 3
specifications will enable systematic measurements of multi-charm states
currencly inaccessible, such as $\Xi_{cc}^{++}\rightarrow\Xi_{c}^{+}\pi^{+}$
with the subsequent decay $\Xi_{c}^{+}\rightarrow\Xi^{-}\pi^{+}\pi^{+}$. With
the first tracking layer at a distance of 5 mm from the beam axis, strange
baryons like $\Xi^{-}$ can be directly tracked before they decay, thus
improving the capability to identify weak decays from primary and secondary
sources. The resulting expected significance as a function of $p_{T}$ is shown
in Fig. 4.

##  5 Conclusions

ALICE 3 is a next-generation detector planned to replace the present ALICE
apparatus for LHC Run 5 and beyond. Its main goal is to unravel the
microscopic dynamics of the QGP produced in heavy-ion collisions by exploiting
the full potential of the LHC as a heavy-ion collider. Apart from QGP physics,
ALICE 3 also addresses fundamental open questions in QCD physics and beyond.
An innovative detector concept is needed to to ensure high-precision
measurements of heavy-flavour hadrons and electromagnetic probes inaccessible
in Runs 3 and 4, demanding for several novel R&Ds with broad impact also on
future high-energy and nuclear physics experiments.

## References

  * [1] ALICE Collaboration, “The ALICE experiment - A journey through QCD”, CERN-EP-2022-227, 2022, https://cds.cern.ch/record/2838475.
  * [2] Z. Citron et al., “Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams”, CERN-LPCC-2018-07, 2018, https://cds.cern.ch/record/2650176.
  * [3] ALICE Collaboration, “Letter of intent for ALICE 3: A next-generation heavy-ion experiment at the LHC”, CERN-LHCC-2022-009, 2022, https://cds.cern.ch/record/2803563.
  * [4] F. Carnesecchi et al., “Beam test results of 25 and 35 $\mu$m thick FBK ultra-fast silicon detectors”, Eur. Phys. J. Plus 138, 2023, https://doi.org/10.1140/epjp/s13360-022-03619-1.
  * [5] F. Carnesecchi et al., “Measurements of the Cherenkov effect in direct detection of charged particles with SiPMs”, Eur. Phys. J. Plus 138, 2023, https://doi.org/10.1140/epjp/s13360-023-04397-0.
  * [6] N. Nicassio et al., “A combined SiPM-based TOF+RICH detector for future high-energy physics experiments”, IWASI 2023, 2023, https://doi.org/10.1109/IWASI58316.2023.10164558.