Recoil Polarimetry in Meson Photoproduction at MAMI

Mark Sikora, Derek Glazier, Dan Watts School of Physics, University of Edinburgh, UK

IntroductionThe photon is an ideal probe of the nucleon as its interaction is well understood (QED) and it can be polarized. A new generation of experiments using intense polarized photon beams to produce mesons from the nucleon is aiming to establish the excitation spectrum of the nucleon with sufficient accuracy to distinguish between various theoretical models of nucleon structure (lattice QCD, constituent quark models, holographic dual etc).

These models differ in their predictions of the spectrum and predict excited states which have not yet been observed. These “missing” states may reflect insensitivities of previous measurements, or the states might not exist in nature, highlighting inappropriate degrees of freedom in the models. More comprehensive measurements are crucial to provide constraints on the missing states. Theoretical models predict most of the excited states decay via pion emission back to the nucleon ground state. This motivates the study of the reaction γN→Nπ to access the spectrum. Measuring the recoil polarization of the final state nucleon along with polarized photon beams and nucleon targets is a pre-requisite to achieve the first model independent partial wave analysis to obtain accurate information on the excitation spectrum from these reactions.

TAPS Crystal Ball Beam line

MAMI Facility

The Edinburgh Recoil PolarimeterThe nucleon recoiling from the pion production reaction is spin analysed in a nucleon polarimeter.

The spin-orbit interaction in the nucleon-graphite scattering modulates the resulting angular distribution

n(φsc) = n0{1+AeffPTsin(φsc+φ0)} where Aeff is the analyzing power of graphite and P

T is the transverse polarization of the recoiling

nucleon.

By periodically switching the spin orientation of the photon beam, the asymmetry in the resulting proton yields is fit to a sine function to determine the double polarization observable Cx

n+(φsc) – n-(φsc) n+(φsc) + n-(φsc)

OutlookFuture developments include upgrading the polarimeter for use with a deuterium target to enable measurements of meson photoproduction from neutron targets and polarization transfer in deuterium photodisintegration to assess the onset of perturbative QCD.

The first (preliminary!) measurements of polarization transfer in meson photoproduction were extracted and compared to a partial wave analysis (MAID), which uses the present determination of the excitation spectrum, and previous measurements from Jefferson Lab.

The MAinz MIcrotron in Germany provides an electron beam up to 1.5 GeV, which is converted via bremsstrahlung to an energy tagged photon beam with either circular or linear polarisation. The reaction products are detected with the Crystal Ball, a large acceptance (94% of 4π) calorimeter with high angular resolution. The TAPS detector, composed of 510 BaF

2 crystals,

covers the forward angular region

Lattice QCD and constituent quark model view of the nucleon

Comparison of Cx to MAID partial wave analysis

γ2

Scattered proton detected in Crystal Ball, RCB constructed from center of target

γ1

Reconstruct π0 from γ’s

2.5 cm thick graphite cylinder

Beam

Scattered proton

Rsc = RCB - rRecon

Initial ResultsIn 2008, 200 hours of data was collected using both linearly and circularly polarized photon beams.

The scattering angles for a proton scattering off a carbon nucleus.

= CxPγcircAeff sinφsc

φSample asymmetry. The observable Cx is extracted from the amplitude.

φsc

The new measurements are consistent with the existing data. The theoretical prediction fits the data best at more backward center of mass angles.