Hard Scattering Processes in QCD

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Nucleons (protons and neutrons) make up more than 99% of the mass of the visible matter of the Universe. They are composed of quarks and gluons (partons), the fundamental particles of quantum chromodynamics (QCD). The parton structure of the nucleon is encoded in different types of parton distributions (PDFs), which can be extracted from high-energy scattering processes or obtained from first-principles calculations (lattice-QCD). For one crucial PDF, the nucleon transversity, a striking and persistent mismatch exists between results from experiment and lattice-QCD. The PI and his collaborators will revisit and improve the extraction of the transversity from experiment, aiming to obtain state-of-the-art information about the transversity from phenomenology and to find the source of the aforementioned discrepancy. Furthermore, the PI will perform comprehensive calculations in perturbative QCD that are needed to obtain, from lattice-QCD results, two important classes of PDFs that are difficult to address experimentally. The PI will also carefully study the QCD energy momentum tensor, which contains information about fundamental hadron properties like the mass and the spin, as well as pressure and shear distributions. Moreover, the PI will mentor graduate students, conduct outreach activities for high school students and undergraduates, and offer a summer program on hadronic physics for both undergraduate and graduate students.The project deals with three topics in hadronic physics. First, the PI will perform a new fit of the nucleon’s transversity distribution based on data for di-hadron production. After a new leading-order analysis, the transversity will be extracted in a collinear next-to-leading order (NLO) framework. This includes the first-ever NLO calculation, in perturbative QCD, of the relevant di-hadron observables in electron-positron annihilation, lepton-nucleon scattering and proton-proton collisions. The study can shed new light on the longstanding tension between information on the transversity from experimental data and from lattice-QCD. Second, the PI will consider Euclidean parton correlators to enhance our understanding of twist-3 PDFs and generalized parton distributions via lattice-QCD. Specifically, the PI will compute in perturbative QCD the relevant one-loop matching coefficients, which are crucial for relating results from lattice-QCD to the light-cone correlation functions one is interested in. Third, the PI will consider the QCD energy momentum tensor to study the mass decomposition of the pion in QCD, and to explore the pressure as well as the distributions of pressure and shear inside hadrons. A focus will be on properly renormalized operators, an important topic which so far has not received much attention, especially in relation to the pressure and shear distributions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.