{"id":52816,"date":"2024-12-18T08:18:21","date_gmt":"2024-12-18T08:18:21","guid":{"rendered":"https:\/\/www.innovationnewsnetwork.com\/?p=52816"},"modified":"2024-12-18T08:17:37","modified_gmt":"2024-12-18T08:17:37","slug":"flavor-asymmetry-of-light-quarks-in-the-nucleon-sea","status":"publish","type":"post","link":"https:\/\/www.innovationnewsnetwork.com\/flavor-asymmetry-of-light-quarks-in-the-nucleon-sea\/52816\/","title":{"rendered":"Flavor asymmetry of light quarks in the nucleon sea"},"content":{"rendered":"
The first direct evidence for point-like constituents in the nucleons came from the observation of scaling phenomenon in Deep-Inelastic Scattering (DIS) experiments. These point-like charged constituents, called partons by Richard Feynman, were found to be spin-1\/2 fermions. These partons were initially identified as the valence quarks in the Constituent Quark Models.<\/p>\n
However, it was soon realised that valence quarks alone could not account for the swarm of low momentum partons observed in DIS. These low-momentum partons, dubbed wee-partons by Feynman, were interpreted as the quark and antiquark sea of the nucleon. DIS experiments, therefore, provided the first evidence for the existence of antiquarks in the nucleon.<\/p>\n
The observation of partons in DIS experiments paved the road to the formulation of Quantum Chromodynamics (QCD) as the theory for strong interactions.<\/p>\n
Nevertheless, the exact form of the parton distribution functions (PDF) cannot be deduced from perturbative QCD. Like many static properties of hadrons, the PDFs belong to the domain of non-perturbative QCD.<\/p>\n
Despite the great progress in Lattice Gauge Theory (LGT) in treating the bound-state properties of hadrons, it remains a challenge to predict PDFs using LGT.<\/p>\n
The early DIS data suggests that the nucleon consists of three quarks in a background of an infinite number of quark-antiquark pairs. The importance of the quark-antiquark pairs in the nucleon is in sharp contrast to the situation for the atomic system, where particle-antiparticle pairs play a relatively minor role.<\/p>\n
In strong interactions, quark-antiquark pairs are readily produced as a result of the relatively large magnitude of the coupling constant \u03b1s, and they form an integral part of the nucleon\u2019s structure.<\/p>\n
There are at least two reasons why it is important to measure the parton distribution functions of the nucleons.<\/p>\n
First, the description of hard processes in high energy interactions requires parton distribution functions as an essential input. Second, many aspects of the parton distributions can be compared with the predictions of theoretical models based on perturbative as well as non-perturbative QCD.<\/p>\n
We present the status of our knowledge of the flavor asymmetry of the light-quark sea in the proton. The opportunities to further explore this subject at existing accelerators and the future Electron-Ion Collider (EIC) will also be discussed.<\/p>\n
The earliest parton models assumed that the proton sea was flavor symmetric, even though the valence quark distributions are clearly flavor asymmetric. Inherent in this assumption is that the content of the sea is independent of the valence quark\u2019s composition. Therefore, the proton and neutron were expected to have identical sea-quark distributions. The assumption of flavor symmetry was not based on any known physics, and it remained to be tested by experiments.<\/p>\n
Experiments with neutrino beams provided strong evidence that the strange-quark content of the nucleon is only about half of the up or down sea quarks. This flavor asymmetry was attributed to the much heavier mass for strange quark compared to the up and down quarks.<\/p>\n
The mass for the up and down quarks being very similar suggests that the nucleon sea should be nearly up-down symmetric. A direct method to check this assumption is to compare the sea in the neutron to that in the proton measured in the DIS experiments.<\/p>\n
Soon after the discovery of partons in electron-proton DIS, electron-deuterium scattering experiments were carried out to extract the electron-neutron DIS. The e\u2212p and the e\u2212n DIS data allowed a comparison of the up and down sea quark distributions in the proton. Despite the large uncertainty of the experimental data, Field and Feynman deduced from these data that the up and down sea quarks are different in the proton.<\/p>\n
They further suggested that the Pauli blocking from the valence quarks would inhibit the down-quark sea more than the up-quark sea, hence creating an asymmetric proton sea.<\/p>\n
However, the large experimental uncertainties prevented a definitive conclusion. A more precise DIS experiment was later carried out by the NMC Collaboration at CERN, showing that the up and down sea quark contents in the proton are different, at a confidence level of 4\u03c3. The Drell-Yan process, described next, would provide a more direct and definitive measurement for this unexpected asymmetry of the proton sea.<\/p>\n
The first high-mass dilepton production experiment was carried out at the AGS in 1969, soon after DIS experiments were performed at SLAC. Drell and Yan interpreted the data within the parton model, in which a quark-antiquark pair annihilate into a virtual photon subsequently decaying into a lepton pair (see Fig. 1).<\/p>\n