First Measurement of Transverse-Spin-Dependent Azimuthal Asymmetries in the Drell-Yan Process

Area of Science:

• High-energy particle physics
• Quantum Chromodynamics (QCD)
• Parton physics

Background:

• Transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are crucial for understanding nucleon structure.
• Previous studies on TMD PDFs primarily utilized semi-inclusive deep-inelastic scattering (SIDIS) data.
• Drell-Yan (DY) processes offer a complementary channel to probe TMD PDFs.

Purpose of the Study:

• To measure transverse-spin-dependent azimuthal asymmetries in pion-induced Drell-Yan scattering for the first time.
• To extract different TMD PDFs, including Sivers, Boer-Mulders, transversity, and pretzelosity.
• To test fundamental predictions of QCD regarding the sign of Sivers TMD PDFs in DY versus SIDIS.

Main Methods:

• Utilized a 190 GeV/c pion beam from the CERN SPS.
• Employed a transversely polarized ammonia target.
• Analyzed dimuon events within an invariant mass range of 4.3–8.5 GeV/c² to extract azimuthal asymmetries.

Main Results:

• The first measurement of three azimuthal asymmetries in pion-induced Drell-Yan scattering was performed.
• The observed sign of the Sivers asymmetry is consistent with the QCD prediction of being opposite to that from SIDIS data.
• Two additional asymmetries involving pion Boer-Mulders TMD PDFs convoluted with nucleon transversity or pretzelosity TMD PDFs were presented.

Conclusions:

• The results support the fundamental QCD prediction concerning the sign of the Sivers asymmetry in Drell-Yan processes.
• The findings open possibilities for testing QCD universality predictions by comparing with recent COMPASS SIDIS measurements at comparable energy scales.
• This work provides new insights into the spin structure of hadrons through the Drell-Yan process.