Periodic Reporting for period 2 - HIEIC (Heavy ion collisions: collectivity and precision in saturation physics)
Periodo di rendicontazione: 2022-12-01 al 2024-11-30
In 1974 the theory that describes the strong interaction, Quantum Chromodynamics (QCD), was proposed. Its great success is nevertheless restricted to the situations where the coupling constant that determines the interactions between partons, quarks and gluons, is small and perturbative techniques are applicable. Our knowledge on other situations is far less satisfactory. When the coupling is large, partons are confined inside hadrons and the symmetries of QCD are broken. This confinement regime can, at present, only be addressed either by numerical methods or by models. Other situation is when the coupling can be considered small but the number of partons is so large that the field strength is also very large and non-linear phenomena appear. In this region standard perturbative techniques fail. A new regime of QCD where parton densities in hadrons and nuclei are saturated - tamed by the non-linear behaviour of QCD - is expected to dominate the dynamics of high energy collisions involving hadrons and to determine the parton content of hadrons and nuclei when we probe partons that carry a small momentum fraction of the total hadron momentum. This regime shows features that are expected to happen in the confinement region, so its understanding will shed light on the transition between the perturbative and non perturbative regimes of QCD. The present project tries to increase our understanding on this saturation domain by promoting previous calculations to higher precision as required for a meaningful comparison with experimental data, to clearly settle whether this new regime of QCD has been achieved in the existing accelerators or experiments, and for proposing new measurements to be performed in existing or future experiments.
At the Large Hadron Collider (LHC) at CERN, several experiments are analysing proton-proton, proton-nucleus and nucleus-nucleus collisions. All the experiments are either focused on QCD research (e.g. the ALICE experiment) or require knowledge on QCD, QCD backgrounds producing the largestf uncertainty in measurements of characteristics of or deviations from the Standard Model of Particle Physics. The knowledge pursued in these experiments, and therefore in this project, is of fundamental nature. Thus, no immediate application to improve daily life is expected. But large projects as those developed at CERN, and future projects like the Electron Ion Collider (EIC) to be built in the USA for the early 2030’s or others (LHeC, FCC-he) in a more preliminary stage, produce results that become of use for society as it has been in the past: the WWW, accelerator and detector technology for health, new computing techniques,…
The overall objectives of the project aim to increase our precision for understanding the non-linear, high energy regime of QCD: understanding the quantum effects that produce particle correlations observed in data, developing an improved framework for calculations of high order in the saturation domain, a phenomenological description of observables sensitive to saturation in proton and nuclear collisions, and the application of such knowledge to future projects like the EIC, LHeC or FCC-he.
At the Large Hadron Collider (LHC) at CERN, several experiments are analysing proton-proton, proton-nucleus and nucleus-nucleus collisions. All the experiments are either focused on QCD research (e.g. the ALICE experiment) or require knowledge on QCD, QCD backgrounds producing the largestf uncertainty in measurements of characteristics of or deviations from the Standard Model of Particle Physics. The knowledge pursued in these experiments, and therefore in this project, is of fundamental nature. Thus, no immediate application to improve daily life is expected. But large projects as those developed at CERN, and future projects like the Electron Ion Collider (EIC) to be built in the USA for the early 2030’s or others (LHeC, FCC-he) in a more preliminary stage, produce results that become of use for society as it has been in the past: the WWW, accelerator and detector technology for health, new computing techniques,…
The overall objectives of the project aim to increase our precision for understanding the non-linear, high energy regime of QCD: understanding the quantum effects that produce particle correlations observed in data, developing an improved framework for calculations of high order in the saturation domain, a phenomenological description of observables sensitive to saturation in proton and nuclear collisions, and the application of such knowledge to future projects like the EIC, LHeC or FCC-he.
Our studies on the importance of the quantum interference effects in correlated particle production have shown that, in this initial state explanation, two-particle correlations are due to Bose enhancement of the gluons in the projectile wave function and to HBT correlations of the produced gluons. Apart from extending our studies to the correlations of total multiplicity, three and four particle production where we have shown the essential role of quantum interference effects to describe the trend followed by the experimental data (see the image taken from Eur. Phys. J. C 81 (2021) 8, 760), we have also published an invited review article (Eur. Phys. J. A (2020) 8, 215) that discusses the basics of the theory and how different approaches can be used to describe two particle correlations from the initial state perspective.
To increase the precision of the computations of the gluon saturation framework, one needs to include next-to-leading order (NLO) corrections in the strong coupling constant when studying the rapidity evolution equations. In this regard, we obtained a remarkable achievement by providing a novel approach to high-energy evolution which is based on the Born-Oppenheimer approximation. This new approach is expected to be a milestone for the future studies of the evolution equations in QCD.
On the other hand, the precision of the computations can be increased at the level of the observables as well. In Phys. Rev. D 108 (2023) 7, 074003, we have shown that the proper framework to compute cross section for forward inclusive single hadron production in pA collisions beyond leading order is not the collinear factorization, as has been assumed so far, but transverse momentum dependent (TMD) factorization. With these results, we have resolved the long standing problem of unstable NLO corrections in forward pA collisions.
The upcoming EIC in the USA will be the new flagship of the experimental studies of QCD in particle physics, with high luminosity but low energy beams compared to the LHC. The key approximation adopted in the saturation framework is the eikonal approximation which amounts to considering only the leading term in energy and discarding all finite energy corrections. For the energies at the EIC, one should include the finite energy (subeikonal) corrections in the computation of the observables in the saturation framework. We are happy to report that the progresses in subeikonal studies are far more than originally planned and proposed. We studied all possible sources of the subeikonal corrections to parton propagators and to various observables.
On these four aspects, our efforts have resulted in seven, ten (three of them still under review), six and nine publications in peer reviewed journals, respectively (in total 36 publications, 8 proceedings and 3 preprints that are under review), during the course of the project. During the second reporting period, we have 16 publications, 2 proceedings and 3 preprints.
To increase the precision of the computations of the gluon saturation framework, one needs to include next-to-leading order (NLO) corrections in the strong coupling constant when studying the rapidity evolution equations. In this regard, we obtained a remarkable achievement by providing a novel approach to high-energy evolution which is based on the Born-Oppenheimer approximation. This new approach is expected to be a milestone for the future studies of the evolution equations in QCD.
On the other hand, the precision of the computations can be increased at the level of the observables as well. In Phys. Rev. D 108 (2023) 7, 074003, we have shown that the proper framework to compute cross section for forward inclusive single hadron production in pA collisions beyond leading order is not the collinear factorization, as has been assumed so far, but transverse momentum dependent (TMD) factorization. With these results, we have resolved the long standing problem of unstable NLO corrections in forward pA collisions.
The upcoming EIC in the USA will be the new flagship of the experimental studies of QCD in particle physics, with high luminosity but low energy beams compared to the LHC. The key approximation adopted in the saturation framework is the eikonal approximation which amounts to considering only the leading term in energy and discarding all finite energy corrections. For the energies at the EIC, one should include the finite energy (subeikonal) corrections in the computation of the observables in the saturation framework. We are happy to report that the progresses in subeikonal studies are far more than originally planned and proposed. We studied all possible sources of the subeikonal corrections to parton propagators and to various observables.
On these four aspects, our efforts have resulted in seven, ten (three of them still under review), six and nine publications in peer reviewed journals, respectively (in total 36 publications, 8 proceedings and 3 preprints that are under review), during the course of the project. During the second reporting period, we have 16 publications, 2 proceedings and 3 preprints.
We have developed new techniques beyond existing ones. More specifically, (i) a novel approach to high-energy evolution equations in QCD based on Born-Oppenheimer resummation; (ii) TMD factorized framework for the computation of single inclusive hadron production cross section in forward pA collisions beyond leading order; (iii) Sudakov double logarithms for the single inclusive hadron production in Deep Inelastic Scattering (DIS) in the saturation regime; (iv) a computation framework that systematically includes all possible subeikonal corrections to parton propagators and their applications to various different observables, have been provided over the course of this project.
It can be confidently asserted that these outstanding achievements are anticipated to significantly contribute to future research on saturation phenomena, particularly in the context of EIC phenomenology.
It can be confidently asserted that these outstanding achievements are anticipated to significantly contribute to future research on saturation phenomena, particularly in the context of EIC phenomenology.