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

The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS $190\mathrm{GeV}/c$ ${\pi }^{-}$ beam and a transversely polarized ammonia target. Three azimuthal asymmetries giving access to different transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are extracted using dimuon events with invariant mass between $4.3\mathrm{GeV}/c^2$ and $8.5\mathrm{GeV}/c^2$. Within the experimental uncertainties, the observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD PDFs extracted from DY have a sign opposite to the one extracted from semi-inclusive deep-inelastic scattering (SIDIS) data. We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs. A recent COMPASS SIDIS measurement was obtained at a hard scale comparable to that of these DY results. This opens the way for possible tests of fundamental QCD universality predictions.

Figure 1

Top: target rest frame.

Figure 2

The dimuon invariant mass distribution.

Figure 3

The two-dimensional ($x_{\pi }$,$x_N$) distribution of the selected high mass dimuons. The distribution is normalized to have a maximum value equal to one.

Figure 4

The $x_F$ distribution (left) and $q_T$ distribution (right) of the selected high mass dimuons.

Figure 5

Extracted Drell-Yan TSAs related to Sivers, transversity, and pretzelosity TMD PDFs (top to bottom). Inner (outer) error bars represent statistical (total experimental) uncertainties. The normalization uncertainties due to target polarization (5%) and dilution factor (8%) are not included in the error bars.

Figure 6

The measured mean Sivers asymmetry and the theoretical predictions for different $Q^2$ evolution schemes from Refs. [19] (DGLAP), [20] (TMD1), and [21] (TMD2). The dark-shaded (light-shaded) predictions are evaluated with (without) the sign-change hypothesis. Uncertainties are as described in Fig. 5.