Electromagnetic probes MAMI, Jefferson Lab & MAX-Lab Daniel Watts University of Edinburgh

Scope of talk Outline the present engagement of the Glasgow and Edinburgh groups at these facilities. No attempt to be comprehensive Choose two topics reflecting current research: Neutron skins The nucleon excitation spectrum

Timelines, upgrades & UK investment Facility upgrade 0.25 GeV 10 6  s -1 MeV GeV 10 5  s -1 MeV GeV 10 8  s -1

Timelines, upgrades and investment GeV 10 6  s -1 MeV -1 UK Hardware at Max-lab Edinburgh Ge-6 Array Glasgow active target

Timelines, upgrades and investment Facility upgrade 1.5 GeV 10 5  s -1 MeV -1 UK investment at MAMI Glasgow photon tagger Edinburgh Particle-ID detector Edinburgh nucleon polarimeter Glasgow photon beam profiler

Timelines & Upgrades GeV 10 8  s -1 UK Investment at Jefferson Lab Polarised beam setup (Glasgow) Big bite focal plane array(Glasgow) Part of neutron polarimeter for Gen (Glasgow) Nucleon polarimetry R&D (Edinburgh)

Neutron Skins Our knowledge of the shape of stable nuclei is presently incomplete e.g. 208 Pb: RMS charge radius known to < fm RMS neutron radius only known to ~0.2 fm !! Horowitz PRC (2001) New techniques to attack this fundamental problem are important and timely URCA Cooling n → p + e - + e - + p → n + Relativistic mean field Skyrme HF

Angular distribution of  0 → accurate information about matter distribution d  /d  A 2 (q/k  )P 3 2 |F m (q)| 2 sin 2   Neutron skins Photon probe Interaction well understood  0 meson – produced with ~equal probability on protons AND neutrons. Select reactions which leave nucleus in ground state Reconstruct   from   →2  decay

No neutron skin 0.2fm neutron skin 208 Pb( ,  0 ) E  =185±5 MeV 1 st minima No skin 2 nd Max. No skin Neutron Skins : Preliminary analyses One of over 30 spectra!!

A primary motivation of the new EM beam facilities → better establish the nucleon excitation spectrum Use meson photoproduction reactions  + N → N* →   small → resonances are broad (  E  ~ ħ)  Excitation spectrum of nucleon Cross section E  (GeV)   P 33 N(1440)P 11 N(1520)D 13 N(1535)S 11  (1600)P 33

The way forward – double polarisation Polarisation of  target recoil Observable

Edinburgh Recoil Polarimeter: C x, O X, T, P Graphite scatterer Hydrogen target cell n() =n o (){1+A()[P y cos()–P x sin()]

Edinburgh Recoil Polarimeter: C x, O X, T, P First proof of principle for a 4  nucleon polarimeter !! 1000 hours of approved → pion photoproduction Future  multipion etc. SAID PWA MAID PWA p( ,p)  0

Worldwide activity Double polarisation at Jefferson Laboratory Frozen spin polarised target development – beam-target experiments start 2008 (Glasgow, Edinburgh) Strange meson photoproduction e.g.  + p → K + +  0 Self-analysing (weak) decay → recoil polarimetry !! Beam-recoil experiments under analysis (Glasgow) Polarised target will enable complete measurement C.Gordon, K. Livingston et. al.

Worldwide activity Summary New generation of EM beam facilities offer unique opportunities to address fundamental questions in nuclear and hadron physics Other EM beam research programmes with UK leadership Magnetic moments of nucleon resonances Nuclear three-body forces Nuclear short-range correlations Tests of chiral perturbation theory in strange quark sector Search for multiquark states Exotic hybrid mesons and glueballs Nucleon form factors