Transverse single-spin asymmetries in pion-induced Drell-Yan and J/Psi production at COMPASS

A three-dimensional picture of the nucleon is gradually developing through experimental studies related to generalized parton distributions (GPDs) and transverse momentum dependent (TMD) parton distribution functions (PDFs). There are eight TMD PDFs which describe the nucleon at leading order. Four in particular will be discussed in this thesis. The Sivers and Boer-Mulders functions are predicted to be process-dependent and change sign when measured in semi-inclusive deep inelastic scattering (SIDIS) compared to Drell-Yan scattering (DY). In contrast, the pretzelosity and transversity functions are predicted to be truly universal. TMD PDFs are accessed experimentally through the extraction of transverse spin-(in)dependent azimuthal asymmetries. COMPASS is a fixed target experiment located in the North Area of CERN with a broad physics program. It is uniquely capable of taking SIDIS and DY data under almost identical conditions, and therefore well suited for testing the (non-)universality of the TMD PDFs. It is also one of few experiments capable of probing TMD PDFs with the DY process. In 2015 and 2018 COMPASS collected Drell-Yan data using a 190 GeV/$c$ negative pion beam and a transversely polarized proton target. This thesis focuses on extracting transverse spin-dependent azimuthal asymmetries (TSAs) from dimuon events resulting from DY. The first TSA results from the full COMPASS DY 2015+2018 data sample are presented. The new TSAs favor the predicted sign change for the Sivers function and the universality of transversity and pretzelosity. During the 2015 and 2018 COMPASS runs, dimuons were also produced by the decay of $J/\psi$ mesons. In pion-proton collisions, $J/\psi$ mesons can be produced via quark-antiquark annihilation or via gluon-gluon fusion. TSAs extracted from these $J/\psi$ events can be compared to different theoretical predictions to help determine which production mechanism is dominant at COMPASS kinematics. The TSAs can then be used to learn more about quark or gluon TMD PDFs depending on the production mechanism. The first $J/\psi$ TSA results from pion-proton collisions at COMPASS are presented in this thesis. The small size of the Sivers, pretzelosity and transversity TSAs suggest that gluon-gluon fusion is the dominant production mechanism, and that the gluon TMDs are small or possibly zero. New information about the gluon TMDs is extremely valuable as they are so far not very well studied.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Townsend, April

Contributors dc:contributor

• Riedl, Caroline K
• Perdekamp, Matthias G
• Peng, Jen-Chieh
• Noronha-Hostler, Jaki
• Song, Jun

• Nuclear Physics
• Medium Energy Physics
• Proton Structure
• Proton Spin Structure
• Transverse Single-Spin Asymmetries

Statement dc:rights

• Copyright 2022 April Townsend

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/116203

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F116203