Measurement of the Collins and Sivers asymmetries on transversely polarised protons

Abstract

The Collins and Sivers asymmetries for charged hadrons produced in deeply inelastic scattering on transversely polarised protons have been extracted from the data collected in 2007 with the CERN SPS muon beam tuned at 160 GeV/c. At large values of the Bjorken x variable non-zero Collins asymmetries are observed both for positive and negative hadrons while the Sivers asymmetry for positive hadrons is slightly positive over almost all the measured x range. These results nicely support the present theoretical interpretation of these asymmetries, in terms of leading-twist quark distribution and fragmentation functions.

Figures (5)

• Figure 1: Collins asymmetry as a function of $x$, $z$, and $p_T^h$, for positive (closed points) and negative (open points) hadrons. The bars show the statistical errors. The point to point systematic uncertainties have been estimated to be 0.5 $\sigma_{stat}$ for positive and 0.6 $\sigma_{stat}$ for negative hadrons and are given by the bands.
• Figure 2: Sivers asymmetry as a function of $x$, $z$, and $p_T^h$, for positive (closed points) and negative (open points) hadrons. The bars show the statistical errors. The point to point systematic uncertainties have been estimated to be 0.8 $\sigma_{stat}$ for positive and 0.4 $\sigma_{stat}$ for negative hadrons and are given by the bands. For positive hadrons only, an absolute scale uncertainty of $\pm 0.01$ has also to be taken into account.
• Figure 3: Mean values of some kinematic variables in the final data sample. From left to right: mean values of $p_T^h$, $z$ and $Q^2$ as functions of $x$; mean values of $p_T^h$, $x$ and $Q^2$ as functions of $z$; mean values of $x$, $z$ and $Q^2$ as functions of $p_T^h$.
• Figure 4: Collins (upper row) and Sivers (lower row) asymmetry as a function of $W$, for positive (left) and negative (right) hadrons. The closed and open points give the values for the "large $x$" and the "small $x$" samples respectively. The errors are statistical only.
• Figure 5: Mean values of $Q^2$ (left) and $x$ (right) as functions of $W$. The closed and open points give the values for the "large $x$" and the "small $x$" samples respectively.