Λ and Σ photoproduction on the neutron

1. Λ and Σ photoproduction on the neutron Pawel Nadel-Turonski The George Washington University for the CLAS Collaboration

2. Outline • Exclusive Measurement of • γn(p) → K0Λ(p) →π+π-π-p(p) • γn(p) → K0Σ0(p) → K0Λγ(p) • Final State Interactions • K-N, Y-N, and Y*-N scattering • The Future Is Polarized 44% 100% NSTAR 2005, Tallahassee, Florida

3. Calculations by C. Bennhold et al. isobaric: H. Yamamura et al., Phys. Rev. C 61, 014001 (2000) coupled-channels: A. Waluyo, private communication (2005) PRELIMINARY NSTAR 2005, Tallahassee, Florida

4. The g10 Experiment at Jefferson Lab • Performed in Hall B • Tagged photon beam of 1 – 3.6 GeV (unpolarized) • Liquid deuterium target • Charged particles detected in CLAS (CEBAF Large Acceptance Spectrometer) • Two track (sector) trigger • 4.71 · 109 events taken with low magnetic field (giving larger acceptance for negative particles) NSTAR 2005, Tallahassee, Florida

5. K0 and Λ Invariant Masses • Only particle ID cuts have been applied here. • Missing mass cuts significantly reduce the background. • All the following plots will have 3σ cuts on the K0 and Λ invariant masses. 3σ cut 3σ cut NSTAR 2005, Tallahassee, Florida

6. Λ and Ks Lifetimes • PDG values: λΛ = 1/τΛ = 3.8 ns-1 λK = 1/τK = 11.2 ns-1 • As expected from the CLAS resolution, there is a slight deficit of events with short path lengths. • The measured Ks lifetime agrees with the PDG value, and the Λ is within 3%. • Vertex reconstruction and event selection are satisfactory. NSTAR 2005, Tallahassee, Florida

7. Missing Masses: Λ – Σ0 separation Σ0 • mm(K0Λ): mass of the ‘spectator’ proton. • mm(K0): mass of the hyperon. Fermi motion is ignored as in an inclusive measurement. • The Λ and Σ0 are well separated in 2D. • The projections show simple double Gaussian fits to regions of interest. • Fitting gives a lower uncertainty in the yields than 2D cuts. • Different limits on mm(K0) can be used to check the fitting procedure. • A missing momentum cut has been applied. Λ PRELIMINARY NSTAR 2005, Tallahassee, Florida

8. Missing Momentum: spectator selection Inclusive Λ Exclusive Σ0 • The ‘missing momentum’ is the momentum of the ‘spectator’ proton if there were no additional particles. • The undetected γfrom theΣ0 is boosted, making the spectators seem forward peaked. • True spectators can be selected by a cut on the missing momentum (magenta colored distribution on lower left). • The spectator protons have an isotropic angular distribution. • The detection threshold for protons in CLAS is just below 0.3 GeV/c. Inclusive Λ extra π spectator cut 2p detected Λ Λ Σ0 PRELIMINARY Exclusive Λ Inclusive Λ FSI ? Exclusive Σ0 Exclusive Λ spectators NSTAR 2005, Tallahassee, Florida

9. γ K+ π - n p p ? p π - Σ-(1385) rescattering on the proton • pmin: minimum momentum of the ‘spectator’ proton before the rescattering, assuming that it propagated on shell. • -t > 0.5, W > 2.0, 0.4 < pmiss < 0.6 • Peak at zero predicted J.-M. Laget, nucl-th/0507035 • See NSTAR 2005 poster by Rebecca Davis for more details. Λ Σ -* deuteron PRELIMINARY NSTAR 2005, Tallahassee, Florida

10. Yields 2p detected Inclusive Λ • Very Preliminary! • Not corrected for photon flux or the CLAS acceptance! • Most high energy photons produce additional particles. • When calculated from the final state, W is not sensitive to Fermi motion. • Will be more interesting when corrected and appropriately binned. Exclusive Σ0 Exclusive Λ PRELIMINARY 2p detected Inclusive Λ Exclusive Σ0 Exclusive Λ NSTAR 2005, Tallahassee, Florida

11. Polarization • Unpolarized photon beam and target • Cross section and recoil polarization (P) • Analysis in progress • Polarized beam and unpolarized LD2 target • Circularly polarized beam + recoil polarization: Cx, Cy • Circular beam polarization available in smaller g2 data set • Linearly polarized beam + recoil polarization: Ox, Oy, T • g8b recently achieved very high linear photon polarization • No CLAS deuterium data with linearly polarized beam! • Polarized beam and polarized target • Frozen spin target (FROST) to be ready in summer of 2006 NSTAR 2005, Tallahassee, Florida

12. Λ Recoil Polarization • Θp is the angle in the rest frame of the Λ between the momentum of the decay proton and n = pγpK. • N± is number of events with the sign of the cosine given by its index. • Not corrected for photon flux or acceptance. • Will be binned in ΘK. PRELIMINARY N- N+ NSTAR 2005, Tallahassee, Florida

13. Polarized FSI • Recent calculations by Miyagawa, Mart, Bennhold, and Glöckle for circularly polarized photons show Cz to be most sensitive to final state interactions. • Calculations for K0 channels using a linearly polarized photon beam are under way. Kaon and hyperon angles – the latter is defined with respect to the momentum of the Y-N system. NSTAR 2005, Tallahassee, Florida

14. Summary • Analysis of the cross sections and recoil polarization for the γn → K0Λand γn → K0Σ0channels is in progress. • Studies of Y-N scattering bring new fundamental understanding and insight into the final state interactions. • We intend to propose a new experiment, using a linearly polarized photon beam and a LD2 target, to measure the polarization transfer. • In the future, the frozen spin target will give access to more polarization degrees of freedom. NSTAR 2005, Tallahassee, Florida