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

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 2 Outline Exclusive Measurement of  γn(p) → K 0 Λ(p) → π + π - π - p(p)  γn(p) → K 0 Σ 0 (p) → K 0 Λγ(p) Final State Interactions  K-N, Y-N, and Y*-N scattering The Future Is Polarized 100% 44%

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 3 Calculations by C. Bennhold et al. PRELIMINARY isobaric: H. Yamamura et al., Phys. Rev. C 61, (2000)coupled-channels: A. Waluyo, private communication (2005)

12-15 October 2005 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 · 10 9 events taken with low magnetic field (giving larger acceptance for negative particles)

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 5 K 0 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 K 0 and Λ invariant masses. 3σ cut

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 6 Λ and K s Lifetimes PDG values: λ Λ = 1/τ Λ = 3.8 ns -1 λ K = 1/τ K = 11.2 ns -1 As expected from the CLAS resolution, there is a slight deficit of events with short path lengths. The measured K s lifetime agrees with the PDG value, and the Λ is within 3%. Vertex reconstruction and event selection are satisfactory.

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 7 Missing Masses: Λ – Σ 0 separation mm(K 0 Λ): mass of the ‘spectator’ proton. mm(K 0 ): mass of the hyperon. Fermi motion is ignored as in an inclusive measurement. The Λ and Σ 0 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(K 0 ) can be used to check the fitting procedure. A missing momentum cut has been applied. PRELIMINARY Σ0Σ0 Λ

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 8 Missing Momentum: spectator selection The ‘missing momentum’ is the momentum of the ‘spectator’ proton if there were no additional particles. The undetected γ from the Σ 0 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. PRELIMINARY Inclusive Λ Σ0Σ0 Λ extra π Λ 2p detected Inclusive Λ spectators spectator cut Exclusive Σ 0 Exclusive Λ FSI ? Exclusive Λ Inclusive Λ

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 9 Σ - (1385) rescattering on the proton p min : minimum momentum of the ‘spectator’ proton before the rescattering, assuming that it propagated on shell. -t > 0.5, W > 2.0, 0.4 < p miss < 0.6 Peak at zero predicted J.-M. Laget, nucl-th/ See NSTAR 2005 poster by Rebecca Davis for more details. deuteron Σ -* Λ ? pπ - p K+K+ n p γ PRELIMINARY

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 10 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. Yields PRELIMINARY 2p detected Inclusive Λ Exclusive Σ 0 Exclusive Λ 2p detected Inclusive Λ Exclusive Σ 0 Exclusive Λ

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 11 Polarization Unpolarized photon beam and target  Cross section and recoil polarization (P)  Analysis in progress Polarized beam and unpolarized LD 2 target  Circularly polarized beam + recoil polarization: C x, C y  Circular beam polarization available in smaller g2 data set  Linearly polarized beam + recoil polarization: O x, O y, 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

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 12 Λ Recoil Polarization Θ p is the angle in the rest frame of the Λ between the momentum of the decay proton and n = p γ  p K. 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 Θ K. N+N+ N-N- PRELIMINARY

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 13 Polarized FSI Recent calculations by Miyagawa, Mart, Bennhold, and Glöckle for circularly polarized photons show C z to be most sensitive to final state interactions. Calculations for K 0 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.

12-15 October 2005 NSTAR 2005, Tallahassee, Florida 14 Summary Analysis of the cross sections and recoil polarization for the γn → K 0 Λ and γn → K 0 Σ 0 channels 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 LD 2 target, to measure the polarization transfer. In the future, the frozen spin target will give access to more polarization degrees of freedom.