Abstract
At present, the Boer-Mulders (BM) functions are extracted from asymmetry data using the simplifying assumption of their proportionality to the Sivers functions for each quark flavour. Here we present two independent tests for this assumption. We subject COMPASS data on semi-inclusive deep inelastic scattering on the 〈cos ϕh〉, 〈cos 2ϕh〉 and Sivers asymmetries to these tests. Our analysis shows that the tests are satisfied with the available data if the proportionality constant is the same for all quark flavours, which does not correspond to the flavour dependence used in existing analyses. This suggests that the published information on the BM functions may be unreliable.The 〈cos ϕh〉, 〈cos 2ϕh〉 asymmetries receive contributions also from the, in principle, calculable Cahn effect. We succeed in extracting the Cahn contributions from experiment (we believe for the first time) and compare with their calculated values, with interesting implications.
Highlights
At present it is already recognized that the collinear picture of the parton model, according to which quark momenta are parallel to proton momentum, is a rather rough approximation for the nucleon structure – quarks have transverse momentum
As the cos φh and cos 2φh azimuthal asymmetries receive contributions from both the BM and Cahn effects, we are able to extract information on the Cahn effect from experiment - as far as we know for the first time
The difference asymmetries We consider the production of charged hadrons h± in semi-inclusive deep inelastic scattering (SIDIS) of charged leptons on an unpolarized and a transversely polarized deuteron target: l + d → l′ + h± + X, l + d↑ → l′ + h± + X
Summary
At present it is already recognized that the collinear picture of the parton model, according to which quark momenta are parallel to proton momentum, is a rather rough approximation for the nucleon structure – quarks have transverse momentum. We suggest two independent tests for the above assumption using only measurable quantities – relations between the cos φh , cos 2φh and Sivers asymmetries.
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