Polarisation observables from strangeness photoproduction on a frozen spin target with CLAS at Jefferson Lab

[摘要] This thesis presents the first, preliminary, measurements of the $\Sigma$ and G polarisation observables from strangeness photoproduction on a frozen spin polarised target, for the reactions $\gamma p \rightarrow K^+ \Lambda$ and $\gamma p \rightarrow K^+ \Sigma^0$.The data were collected at the Thomas Jefferson National Accelerator Facility using the CLAS detector in Experimental Hall B in conjunction with the FROST longitudinally polarised frozen spin target and a linearly polarised photon beam in the energy range W = 1.66 to 2.32 GeV.The work forms part of a wider experimental program seeking to obtain experimental data on the excitation spectrum of the nucleon in order to better understand its structure via the measurement of polarisation observables, whose sensitivity to resonances makes them a desirable quantity to measure.By studying strangeness reactions, it may be possible to find "missing" baryon resonances, predicted by symmetric quark models but not observed in previous experiments, whose results are consistent with the di-quark model.It is thought these "missing" resonances remain undiscovered because they have different coupling strengths for different reaction channels, such as the strangeness reactions, whereas the current data is dominated by studies of $\pi N$ reactions.Measurements of the photon asymmetry, $\Sigma$, have been made which agree with previous CLAS measurements, validating the use of the FROST polarised target for the measurement of other polarisation observables, such as the G beam-target double observable.The G observable was measured on the FROST target via two techniques; from beam asymmetry measurements for the two available states of target polarisation, and a novel double asymmetry method intended to combine all the available polarised data.Some inconsistencies between the G measurements for each state of target polarisation are observed, particularly at forward angles, and are much greater once dilution effects from the target material are accounted for.As well as these dilution effects, the double asymmetry technique is also limited by insufficient constraint of the associated parameters from the limited data available on the reactions studied.For $K^{+} \Lambda$, the results are compared to the Kaon-MAID isobaric model calculations, both with and without the inclusion of the missing $D_{13}$ resonance.Both calculations are inconclusive when compared with the data, although at higher energies the calculation without the $D_{13}$ state better matches the trend of the results.For $K^{+} \Sigma$, the model prediction, which does not include any missing states, shows agreement with the trend of the data for some of the energy bins.In light of this new data, refitting of the models should be undertaken, as the next step in the theoretical interpretation of these results.These measurements provide new information to the world dataset of polarisation observables, and with further analysis of the associated systematics of beam polarisation and dilution, the data will provide new insights into the process of strangeness photoproduction.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] meson production, baryon resonances, photoproduction reactions, polarisation in interactions and scattering