Periodic Reporting for period 3 - YoctoLHC (Yoctosecond imaging of QCD collectivity using jet observables)
Periodo di rendicontazione: 2022-10-01 al 2024-03-31
Light takes 3 yoctoseconds to travel across a proton. This tiny amount is the time unit in which the processes that YoctoLHC is studying happen. During this time, in some special collisions at the Large Hadron Collider (LHC), the quarks and gluons that are usually confined inside protons and neutrons, interact among themselves created the most perfect liquid in Nature. This new material permeated the whole Universe some millionths of seconds after the Big Bang.
The YoctoLHC main conjecture is that these short-lasting processes can be directly accessed by studying the modification of QCD jets, sprays of collimated quarks and gluons that are also, sometimes, produced in the collision together with, but decoupled from, the bulk of the medium. These jets appear even in much shorter time-scales, in fundamental processes that are very well understood using standard techniques in QCD, Quantum ChromoDynamics, the theory of the Strong Interaction. The fact that jets are produced very early, that the dynamics in the absence of a medium is well-known and that they also interact with the surrounding matter, make them excellent probes to pin-down the structure of the evolving state.
Understanding the origin of complexity, i.e. understanding how collective properties of matter appear from their fundamental constituents and interactions, is one of the most fascinating and challenging open problems in Physics, with far-reaching consequences in all our understanding of Nature, from the evolution of the Universe to the structure of materials.
The YoctoLHC main conjecture is that these short-lasting processes can be directly accessed by studying the modification of QCD jets, sprays of collimated quarks and gluons that are also, sometimes, produced in the collision together with, but decoupled from, the bulk of the medium. These jets appear even in much shorter time-scales, in fundamental processes that are very well understood using standard techniques in QCD, Quantum ChromoDynamics, the theory of the Strong Interaction. The fact that jets are produced very early, that the dynamics in the absence of a medium is well-known and that they also interact with the surrounding matter, make them excellent probes to pin-down the structure of the evolving state.
Understanding the origin of complexity, i.e. understanding how collective properties of matter appear from their fundamental constituents and interactions, is one of the most fascinating and challenging open problems in Physics, with far-reaching consequences in all our understanding of Nature, from the evolution of the Universe to the structure of materials.
During the first reporting period of the project, despite the relevant impact that the COVID19 had in a normal development of the activities, we have made important progress in several items of the proposed research
- Modelling of the interaction of the jet with the QCD medium. One of the long-standing open problems in jet quenching phenomenology was to pin-down, at the same time, the relevance of the multiple scatterings that the quarks and the gluons suffer with the medium and the complete treatment of the corresponding cross sections, including the perturbative tails, responsible for large angle scatterings. YoctoLHC has produced important progress in this subject with: i) the first phenomenological analysis of experimental data with the complete resummation, showing that the inconsistencies previously found between data and theory disappear with this resummation; ii) a new method to compute the spectra of radiated gluons including this resummation with efficient numerical implementation; iii) progress on a new improved opacity expansion that is a good approximation to the whole problem for dense enough media and with the advantage of a better analytical control.
- Effect of the hydrodynamical flow on the jet. The medium produced in the high-energy heavy ion collisions is not static and the velocity fields modify the gluon radiation properties in ways that could be experimentally measured. Should this effect be large enough, a new powerful tool to directly sccess the velocity fields in the medium would be available. In this first months, a formalism was developed to rigorously include these velocity fields in the medium-induced gluon radiation, the building-block of jet quenching
- The evolution equations for jet quenching. Once the building-block for radiation is known, the next step is to understand how this building block has to be iterated, this is known as an evolution equation, or in jet quenching jargon, also a rate equation. YoctoLHC has also studied the effect of different hydrodynamical scenarios in the rate equations, finding that the final result is sensitive to these scenarios and that de differences cannot be accommodated by widely-employed scaling between them. Also, the correction to this equations due to color coherence was computed.
- Color correlations in the initial state, the 3-D structure of the proton and nuclei. The three-dimentional structure of the protons and nuclei is one of the hot topics in QCD at present and extremely relevant for the YoctoLHC physics as it provides the initial conditions that we propose to measure with jet observables.
- Identifying the presence of jets and characterize them in collisions at the LHC require developing sophisticated search algorithms. Different variables and techniques for the clustering of the particles into jets maybe more sensitive to the physics under scrutiny than other. So, devising the right observables is one of the main goals of the project. During this first reporting period, time-like variables have been explored. These time-like variables promise to be more efficient in catching the different snapshots in the evolution of the system at the initial times.
- Modelling of the interaction of the jet with the QCD medium. One of the long-standing open problems in jet quenching phenomenology was to pin-down, at the same time, the relevance of the multiple scatterings that the quarks and the gluons suffer with the medium and the complete treatment of the corresponding cross sections, including the perturbative tails, responsible for large angle scatterings. YoctoLHC has produced important progress in this subject with: i) the first phenomenological analysis of experimental data with the complete resummation, showing that the inconsistencies previously found between data and theory disappear with this resummation; ii) a new method to compute the spectra of radiated gluons including this resummation with efficient numerical implementation; iii) progress on a new improved opacity expansion that is a good approximation to the whole problem for dense enough media and with the advantage of a better analytical control.
- Effect of the hydrodynamical flow on the jet. The medium produced in the high-energy heavy ion collisions is not static and the velocity fields modify the gluon radiation properties in ways that could be experimentally measured. Should this effect be large enough, a new powerful tool to directly sccess the velocity fields in the medium would be available. In this first months, a formalism was developed to rigorously include these velocity fields in the medium-induced gluon radiation, the building-block of jet quenching
- The evolution equations for jet quenching. Once the building-block for radiation is known, the next step is to understand how this building block has to be iterated, this is known as an evolution equation, or in jet quenching jargon, also a rate equation. YoctoLHC has also studied the effect of different hydrodynamical scenarios in the rate equations, finding that the final result is sensitive to these scenarios and that de differences cannot be accommodated by widely-employed scaling between them. Also, the correction to this equations due to color coherence was computed.
- Color correlations in the initial state, the 3-D structure of the proton and nuclei. The three-dimentional structure of the protons and nuclei is one of the hot topics in QCD at present and extremely relevant for the YoctoLHC physics as it provides the initial conditions that we propose to measure with jet observables.
- Identifying the presence of jets and characterize them in collisions at the LHC require developing sophisticated search algorithms. Different variables and techniques for the clustering of the particles into jets maybe more sensitive to the physics under scrutiny than other. So, devising the right observables is one of the main goals of the project. During this first reporting period, time-like variables have been explored. These time-like variables promise to be more efficient in catching the different snapshots in the evolution of the system at the initial times.
The final goal of measuring the transient, very short yoctosecond-lasting, state of matter right after a nuclear collision at the LHC with jet observables demands edge research in several areas, from the theoretical determination of the expected structure to the subsequent hydrodynamical evolution and important improvements in the jet quenching theory and new observables. In this first reporting period, despite important difficulties related to the Covid19 pandemics, YoctoLHC has made significant advances on most of these topics as reported above. The coupling of the jet to the medium has been considerable improved, setting the basis for future progress in the phenomenological search for the yoctosecond structure of the collision; we have proved the relevance of the right resummation of scattering centers to determine the properties of the hot medium; new tools to understand the 3D structure of the proton and nuclei has been developed; and, finally, new observables defined with time-like jet variables have been proposed