Periodic Reporting for period 4 - PartonicNucleus (Understanding the Quark and Gluon Structure of the Nucleus)
Período documentado: 2023-04-01 hasta 2024-09-30
In the past two decades, an important new focus of hadronic physics has been the measurement of the 3D structures of the proton, made possible by theoretical developments on generalized parton distributions (GPD) and transverse momentum dependent parton distribution functions (TMD). While the application of these framework to the study of nuclei has been explored early on, the experimental access was long though to be impracticable, leaving the nuclear 3D structure completely unknown. However, a recent experimental breackthrough has open the possibility to perform these measurements with unique opportunities to explore the partonic structure of the nucleus in completely new ways. Using the GPD framework, we can not only perform a 3D tomography, but also access the non-nucleonic degrees of freedom in the nucleus. The TMD framework on the other hand is sensitive to very different physics in the nuclei, providing a unique access to the nuclear structure in momentum space and, through it, to the gluon saturation scale. Moreover, these experimental developments triggered a renewed interest to measure reactions in which we detect the remnants of the nucleus at the same time as we detect high energy hadrons. Such experiments, on heavy nuclei are one of the best opportunity to understand the origin of the modification of the nuclear quark structure, the so called EMC effect. To perform these experiments, we are building a new detector with an original design named ALERT (A Low Energy Recoil Tracker). This detector will measure the nuclear deeply virtual Compton scattering (DVCS) at a much higher luminosity than in the seminal experiment mentioned above. At the same time, its excellent capability to differentiate nuclear isotopes, will open the opportunity to measure the tagged reactions, in which we detect the remnants of the nucleus. The project will bring about this effort by addressing all the different experimental aspects of the field from the detector R&D to the phenomenology necessary to the interpretation of the data. The goals of the project are (i) to perform independently the quark and the gluon tomography of the helium nucleus, (ii) to measure nuclear TMDs and the gluon saturation scale, (iii) to understand the correlations between the EMC effect and the internal nucleon motions in the nucleus, and (iv) to design and build the ALERT detector necessary to these experiments. These different goals form a coherent program to reset our understanding of the nucleus, from the standard few body system composed of protons and neutrons, to a fully QCD object defined in terms of quarks and gluons. In particular, this project will answer key questions of modern hadronic physics, namely: ”Are quarks and gluons distributions similarly modified in the nucleus?”, ”What is the gluon saturation scale and does it scale in nuclei as A^1/3 ?”, and ”What is the origin of the EMC effect?”
Since the start of the project, the main focus has been the design of the ALERT spectrometer, which is the main technological challenge of the project. In particular, several prototypes have been realized and tested to validate the technological choices and define the design of the drift chamber. A prototype of the final version was tested successfully on beam at the ALTO facility in Orsay (France). We now have completed the wire chamber with a high density of very light aluminum wires and the detector is installed at Jefferson Lab on the beamline of Hall-B. The first data-taking will start in early 2025.
On the transverse momentum dependent structure of the hadrons, from protons to nuclei and on the nuclear DVCS using already existing data from the CLAS collaboration. Also, an effort in the phenomenology has been started to prepare for future experiments with the ALERT detector. Of particular interest in this field, we developped a new Monte-Carlo event generator named TOPEG to simulate the nuclear DVCS process.
On the transverse momentum dependent structure of the hadrons, from protons to nuclei and on the nuclear DVCS using already existing data from the CLAS collaboration. Also, an effort in the phenomenology has been started to prepare for future experiments with the ALERT detector. Of particular interest in this field, we developped a new Monte-Carlo event generator named TOPEG to simulate the nuclear DVCS process.
The ultimate goals of the project are first to build a new generation low energy nuclear recoil spectrometer (ALERT) that opens the way for experiments impossible before, second to perform the first experiment with this detector at the Jefferson Lab to provide completely new information on the nuclear structure in terms of quarks and gluons, third to perform experiments on nuclei based on a modernized framework used only for proton and neutron targets up to now.