Measurement of the strange quark contribution to the proton spin using neutral kaons at HERMES

The HERMES experiment at DESY, Germany was designed to carry out precision measurements of the proton spin structure using polarised deep-inelastic leptonnucleon scattering. The experiment utilises the 27.5 GeV/c electron or positron beam of the HERA accelerator which is longitudinally polarised at HERMES, in combination with a longitudinal polarised internal gas target. The key to understanding the origins of the spin of the nucleon is widely believed to lie in a successful description of the interactions of the quarks and gluons from which it is formed. The internal structure of the nucleon is embodied in parton distributions of the constituent quarks and gluons. The spin-averaged parton distribution functions q(x) of quarks and antiquarks of flavours q = (u, d, s) describe the parton momentum distributions. The differences, or helicity distributions, q(x) = q''(x) − q'#(x), describe the flavour dependent contributions of the constituent partons to the spin of the nucleon. The helicity distribution of the strange quarks sea, S(x), is an essential feature of the spin structure of the nucleon. Its properties reflect the reaction mechanism that is responsible for the formation of the quark sea. Much of the information on nucleon spin structure, in particular, the observation that an unexpected small fraction of the spin of the proton comes from the intrinsic quark spins is based on an analysis of inclusive deeply inelastic scattering and hyperon decay under the assumption of SU(3) symmetry among the structures of the octet baryons. In these experiments (EMC and SMC), the helicity distribution for strange quarks were directly accessible to measurement. The strange quark sea was observed to have a substantial negative polarisation. The violation of the Ellis-Jaffe sum rule observed in that analysis was v vi Abstract attributed to this negative polarisation. To the extent that the virtual sea quarks are generated by gluon splitting, a non-zero strange quark polarisation can be attributed to a substantial polarisation of the gluons. Indeed, it has been speculated that the observed value of the strange sea helicity distribution results from a large positive gluon polarisation. In contrast to the earlier inclusive measurements, recent data from semi-inclusive DIS at HERMES suggest that the strange sea is unpolarised. A full 5 parameter flavour decomposition using data from proton and deuteron targets, although not sensitive to the anti-strange quark spin ¯s, yielded s = 0.028 ± 0.033 ± 0.009. A separate extraction of s+¯s from DIS data on the deuteron alone gave s+¯s = 0.129 ± 0.042 ± 0.129 where the large systematic error reflects the lack of knowledge of the kaon fragmentation functions. This thesis reports a new ”isoscalar” measurement of s + ¯s in which these limitations are avoided. Because strange quarks carry no isospin, the strange seas in the proton and neutron are identical. In the deuteron, an isoscalar target, the fragmentation process in DIS can be described without any assumptions regarding isospin dependent fragmentation. In the isoscalar extraction of s + ¯s only the spin asymmetry for K0 s AK0 s 1,d (x,Q2, z) and the inclusive asymmetry A1,d(x,Q2) are used. An accurate measurement of the total non-strange quark polarisation Q = u + ¯u + d + ¯ d comes directly from A1,d(x,Q2). The fragmentation functions needed for a leading order (LO) extraction of S = s + ¯s are measured directly at HERMES kinematics using the same data. As a result of this analysis, the helicity densities for the strange quarks are consistent with zero with the experimental uncertainty over the measured x kinematic range