Periodic Reporting for period 1 - InitialConditions (Initial Conditions for Quark and Gluon Matter Formation at the LHC)
Période du rapport: 2023-09-01 au 2026-02-28
The project Initial Conditions for Quark and Gluon Matter Formation at the LHC (InitialConditions) addresses a central open question in nuclear and particle physics: how the structure and shape of atomic nuclei determine the initial conditions that lead to the formation of quark–gluon plasma (QGP) in ultrarelativistic heavy-ion collisions. Understanding these conditions is essential for interpreting the collective behavior of strongly interacting matter and for linking the fundamental theory of quantum chromodynamics (QCD) to emergent macroscopic phenomena in nuclear systems.
At the Large Hadron Collider (LHC), collisions between heavy nuclei such as lead or xenon recreate, for a fleeting moment, the extreme environment that existed microseconds after the Big Bang. The properties of the resulting QGP depend sensitively on the geometry, density, and fluctuations of the colliding nuclei at the instant of impact. Yet, despite decades of progress, the precise mapping between nuclear structure and the observed collective flow remains poorly constrained. Traditional techniques struggle to disentangle the initial geometry from the subsequent dynamical evolution, limiting our ability to extract quantitative information on nuclear shape and deformation.
This ERC Starting Grant project was conceived to overcome these limitations by developing a new generation of analysis tools capable of probing the initial conditions of heavy-ion collisions with unprecedented precision. Its core objective is to establish robust, model-independent observables that directly connect measurable multi-particle correlations—such as cumulants of transverse momentum and flow coefficients—to the fluctuations in the initial energy density and geometry of the collision system.
The project aims to:
1. Develop and validate a cumulant-based methodology for multi-particle transverse momentum and flow correlations that isolates genuine collective effects from non-flow backgrounds.
2. Apply these tools to high-statistics ALICE data to extract, for the first time, nuclear deformation and shape transitions at ultrarelativistic energies.
3. Explore light-ion collisions (O–O and Ne–Ne) as a new testing ground for QGP formation, establishing a unified framework for small and large nuclear systems.
The new methodologies have already enabled the discovery of a nuclear shape-phase transition in xenon collisions at the LHC—the first direct experimental evidence that nuclear geometry leaves measurable imprints on QGP flow. By integrating advanced statistical, Monte Carlo, and hydrodynamic techniques, the project bridges particle physics, nuclear structure, and computational science. Ultimately, InitialConditions strengthens Europe’s leadership in precision QCD studies and sets new standards for imaging nuclear geometry and the collective behavior of primordial matter.
At the Large Hadron Collider (LHC), collisions between heavy nuclei such as lead or xenon recreate, for a fleeting moment, the extreme environment that existed microseconds after the Big Bang. The properties of the resulting QGP depend sensitively on the geometry, density, and fluctuations of the colliding nuclei at the instant of impact. Yet, despite decades of progress, the precise mapping between nuclear structure and the observed collective flow remains poorly constrained. Traditional techniques struggle to disentangle the initial geometry from the subsequent dynamical evolution, limiting our ability to extract quantitative information on nuclear shape and deformation.
This ERC Starting Grant project was conceived to overcome these limitations by developing a new generation of analysis tools capable of probing the initial conditions of heavy-ion collisions with unprecedented precision. Its core objective is to establish robust, model-independent observables that directly connect measurable multi-particle correlations—such as cumulants of transverse momentum and flow coefficients—to the fluctuations in the initial energy density and geometry of the collision system.
The project aims to:
1. Develop and validate a cumulant-based methodology for multi-particle transverse momentum and flow correlations that isolates genuine collective effects from non-flow backgrounds.
2. Apply these tools to high-statistics ALICE data to extract, for the first time, nuclear deformation and shape transitions at ultrarelativistic energies.
3. Explore light-ion collisions (O–O and Ne–Ne) as a new testing ground for QGP formation, establishing a unified framework for small and large nuclear systems.
The new methodologies have already enabled the discovery of a nuclear shape-phase transition in xenon collisions at the LHC—the first direct experimental evidence that nuclear geometry leaves measurable imprints on QGP flow. By integrating advanced statistical, Monte Carlo, and hydrodynamic techniques, the project bridges particle physics, nuclear structure, and computational science. Ultimately, InitialConditions strengthens Europe’s leadership in precision QCD studies and sets new standards for imaging nuclear geometry and the collective behavior of primordial matter.
During the reporting period, the InitialConditions project achieved its main scientific and technical objectives, advancing heavy-ion and nuclear-structure physics. Work progressed through three work packages (WP1–WP3), focused on methodology development, data analysis, and applications to new collision systems.
WP1 – Cumulant-based methodology
A key achievement is a general algorithm for multi-particle cumulants of transverse momentum (pₜ) and correlations between flow coefficients (vₙ) and pₜ. Extending cumulants from azimuthal angles to momentum enables sensitivity to event-by-event fluctuations in initial energy density and geometry. The algorithm, with realistic detector corrections via analytical and Monte Carlo (AMPT + GEANT3) methods, was integrated into the ALICE framework, enabling high-order correlation analyses with controlled uncertainties.
WP2 – Nuclear shape-phase transition
Applying this to Xe–Xe and Pb–Pb collisions led to a landmark discovery of a nuclear shape-phase transition at ultrarelativistic energies. A Bayesian analysis within a hybrid hydrodynamic model yielded xenon deformation parameters (β2 ≈ 0.21 γ ≈ 30°), marking the first experimental extraction of nuclear shape from collider data. The result, approved by ALICE for a forthcoming Nature publication, links nuclear structure to QGP flow.
WP3 – Light-ion collisions
The project produced the first precision measurements of anisotropic flow in symmetric light-ion systems (O–O and Ne–Ne), published in arXiv:2509.06428. Larger elliptic flow in Ne–Ne confirms neon’s prolate deformation, showing nuclear shape effects persist in the smallest QGP droplets.
Outcomes and Impact
The project delivered:
• A validated algorithm for multi-particle pₜ and vₙ–pₜ cumulants
• First nuclear shape-phase transition observed at TeV energies
• Evidence of shape-driven collectivity in light-ion collisions
• Transferable frameworks now used across ALICE
• An active international team supporting Europe’s leadership in QGP studies
These achievements advance understanding of quark–gluon matter’s initial conditions and nuclear geometry at the LHC.
WP1 – Cumulant-based methodology
A key achievement is a general algorithm for multi-particle cumulants of transverse momentum (pₜ) and correlations between flow coefficients (vₙ) and pₜ. Extending cumulants from azimuthal angles to momentum enables sensitivity to event-by-event fluctuations in initial energy density and geometry. The algorithm, with realistic detector corrections via analytical and Monte Carlo (AMPT + GEANT3) methods, was integrated into the ALICE framework, enabling high-order correlation analyses with controlled uncertainties.
WP2 – Nuclear shape-phase transition
Applying this to Xe–Xe and Pb–Pb collisions led to a landmark discovery of a nuclear shape-phase transition at ultrarelativistic energies. A Bayesian analysis within a hybrid hydrodynamic model yielded xenon deformation parameters (β2 ≈ 0.21 γ ≈ 30°), marking the first experimental extraction of nuclear shape from collider data. The result, approved by ALICE for a forthcoming Nature publication, links nuclear structure to QGP flow.
WP3 – Light-ion collisions
The project produced the first precision measurements of anisotropic flow in symmetric light-ion systems (O–O and Ne–Ne), published in arXiv:2509.06428. Larger elliptic flow in Ne–Ne confirms neon’s prolate deformation, showing nuclear shape effects persist in the smallest QGP droplets.
Outcomes and Impact
The project delivered:
• A validated algorithm for multi-particle pₜ and vₙ–pₜ cumulants
• First nuclear shape-phase transition observed at TeV energies
• Evidence of shape-driven collectivity in light-ion collisions
• Transferable frameworks now used across ALICE
• An active international team supporting Europe’s leadership in QGP studies
These achievements advance understanding of quark–gluon matter’s initial conditions and nuclear geometry at the LHC.
The InitialConditions project has delivered groundbreaking results that redefine the study of nuclear structure and collective behavior in high-energy collisions. For the first time, it established a quantitative link between the geometry of atomic nuclei and quark–gluon plasma (QGP) formation at the LHC.
A newly developed multi-particle cumulant methodology provides unprecedented sensitivity to the initial energy density, size fluctuations, and shape of the colliding system. Extending this framework to transverse momentum correlations and integrating it into the ALICE analysis software sets a new technical standard for precision correlation studies.
Experimentally, the project achieved two major breakthroughs:
1. The first observation of a nuclear shape-phase transition at ultrarelativistic energies, demonstrated in Xe–Xe collisions through nuclear deformation parameters extracted from QGP flow patterns.
2. The first measurements of geometry-driven collectivity in symmetric light-ion collisions (O–O, Ne–Ne), confirming that intrinsic nuclear shapes leave measurable imprints on QGP observables.
These results establish high-energy collisions as a femtoscopic probe of nuclear structure, bridging low-energy nuclear physics and high-energy QCD. The project’s methodologies and open-access software are now widely used across ALICE and transferable to future facilities (e.g. FAIR), ensuring long-term impact.
Future progress will require enhanced computational resources and global theoretical coordination to extend Bayesian modeling and fully exploit predictive power. The outcomes lay the foundation for precision imaging of nuclear geometry at the femtometer scale, transforming our understanding of strongly interacting matter.
A newly developed multi-particle cumulant methodology provides unprecedented sensitivity to the initial energy density, size fluctuations, and shape of the colliding system. Extending this framework to transverse momentum correlations and integrating it into the ALICE analysis software sets a new technical standard for precision correlation studies.
Experimentally, the project achieved two major breakthroughs:
1. The first observation of a nuclear shape-phase transition at ultrarelativistic energies, demonstrated in Xe–Xe collisions through nuclear deformation parameters extracted from QGP flow patterns.
2. The first measurements of geometry-driven collectivity in symmetric light-ion collisions (O–O, Ne–Ne), confirming that intrinsic nuclear shapes leave measurable imprints on QGP observables.
These results establish high-energy collisions as a femtoscopic probe of nuclear structure, bridging low-energy nuclear physics and high-energy QCD. The project’s methodologies and open-access software are now widely used across ALICE and transferable to future facilities (e.g. FAIR), ensuring long-term impact.
Future progress will require enhanced computational resources and global theoretical coordination to extend Bayesian modeling and fully exploit predictive power. The outcomes lay the foundation for precision imaging of nuclear geometry at the femtometer scale, transforming our understanding of strongly interacting matter.