Abstract

COMPASS is a fixed target experiment at the CERN SPS M2 beam line, taking data since year 2002. Part of the physics programme is dedicated to the study of the transverse spin and transverse momentum structure of the nucleon, by means of semi-inclusive deeply inelastic lepton scattering (SIDIS). For these measurements, data have been collected using transversely polarised proton (NH 3 ) and deuteron ( 6 LiD) targets. A selection of the results on the azimuthal asymmetries measured is presented, with particular focus on the most recent measurements from the data collected in 2007 and 2010 with the proton target. These results exhibit clear signals for the Collins asymmetry, interpreted as a convolution of a non-zero transversity PDF and Collins fragmentation function, and for the Sivers asymmetry which is related to the Sivers function, the most famous and discussed of the TMD PDFs. The data collected with the 6 LiD target, have also been recently analysed to search for the azimuthal modulations in the production of hadrons which are expected to be present in the unpolarised part of the SIDIS cross-section. The azimuthal hadron asymmetries, which are related to the Boer-Mulders TMD PDF, show strong and somewhat puzzling kinematical dependencies.

Highlights

  • The Collins asymmetries for charged pions and kaons from all the collected proton data are shown in fig. 1, as a function of the Bjorken variable x, the fractional energy of the hadron z, and the hadron transverse momentum phT

  • The Sivers asymmetries measured by COMPASS for pions and kaons on a proton target are shown in fig

  • All the asymmetries measured on deuterium have been found compatible with zero, interpreted as a cancellation of the u and d quark contributions in the isocalar deuterium

Read more

Summary

Introduction

EPJ Web of Conferences in the SIDIS cross section, which can be separately explored and extracted from the same data. The modulations are the combination of two angles, φS and φh, the azimuthal angle of the initial nucleon spin and of the produced hadron momentum. The modulation amplitudes are different structure functions, proportional to convolutions of the TMD PDFs and fragmentation functions (FFs). An important part of the experimental programme of the COMPASS experiment consists in the study of the nucleon structure. In the following a summary of the main measurements related to the transverse structure of the nucleon are presented

The COMPASS experiment
Transverse azimuthal spin asymmetries
Collins asymmetries
Sivers asymmetries
Other transverse spin asymmetries
Unpolarised azimuthal asymmetries
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call