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Creators/Authors contains: "Nukazuka, G."

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  1. New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarized LiD 6 target. The data were taken in 2022 with the COMPASS spectrometer using the 160 GeV muon beam at CERN, statistically balancing the existing data on transversely polarized proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the u and d quark, as well as the tensor charge in the measured x range are extracted. In particular, the accuracy of the d quark results is significantly improved. Published by the American Physical Society2024 
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    Free, publicly-accessible full text available September 1, 2025
  2. The COMPASS Collaboration performed measurements of the Drell-Yan process in 2015 and 2018 using a 190 GeV / c π beam impinging on a transversely polarized ammonia target. Combining the data of both years, we present final results on the amplitudes of five azimuthal modulations, which correspond to transverse-spin-dependent azimuthal asymmetries (TSAs) in the dimuon production cross section. Three of them probe the nucleon leading-twist Sivers, transversity, and pretzelosity transverse-momentum dependent (TMD) parton distribution functions (PDFs). The other two are induced by subleading effects. These TSAs provide unique new inputs for the study of the nucleon TMD PDFs and their universality properties. In particular, the Sivers TSA observed in this measurement is consistent with the fundamental QCD prediction of a sign change of naive time-reversal-odd TMD PDFs when comparing the Drell-Yan process with deep inelastic scattering. Also, within the context of model predictions, the observed transversity TSA is consistent with the expectation of a sign change for the Boer-Mulders function. Published by the American Physical Society2024 
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    Free, publicly-accessible full text available August 1, 2025
  3. Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive$$\rho ^0$$ ρ 0 meson muoproduction at COMPASS using 160 GeV/cpolarised$$ \mu ^{+}$$ μ + and$$ \mu ^{-}$$ μ - beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$$c^2$$ c 2 $$< W<$$ < W < 17.0 GeV/$$c^2$$ c 2 , 1.0 (GeV/c)$$^2$$ 2 $$< Q^2<$$ < Q 2 < 10.0 (GeV/c)$$^2$$ 2 and 0.01 (GeV/c)$$^2$$ 2 $$< p_{\textrm{T}}^2<$$ < p T 2 < 0.5 (GeV/c)$$^2$$ 2 . Here,Wdenotes the mass of the final hadronic system,$$Q^2$$ Q 2 the virtuality of the exchanged photon, and$$p_{\textrm{T}}$$ p T the transverse momentum of the$$\rho ^0$$ ρ 0 meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ($$\gamma ^*_T \rightarrow V^{ }_L$$ γ T V L ) indicate a violation ofs-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive$$\rho ^0$$ ρ 0 production. 
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