Polarisation transfer in hyperon photoproduction near threshold Tom Jude D I Glazier, D P Watts The University of Edinburgh

Talk Overview Hyperon Photoproduction Polarisation observables Identifying strange decay channels Kinematic analysis Preliminary asymmetry measurements and extraction of C X and C Z T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Hyperon Photoproduction  p  K + +  p +  - (~ 64%) n +  0 (~ 36%)   p  K + +    Important for investigations of nucleon resonances A crucial test of QCD based chiral perturbation theories in the strange quark sector T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Polarisation observables Spin observables – a new constraint for reaction models and baryon resonance structure Circularly polarised photons - polarisation transferred to the recoiling hyperon Characterised by the observables C X and C Z : P X = -P  C X, P y = P, P Z = -P  C z P i = polarisation transferred along axis i, P  = circular polarisation of photon beam  decay is a weak decay (parity violating) Hyperon polarisation - measured by the distribution of it’s decay products T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Linear or circular polarisation of the photon beam Angle between photon polarisation and reaction plane Polarisations transferred with a circularly pol. beam Polarisations transferred with a linearly pol. beam Sixteen different observables in total - beam-target, beam-recoil, target-recoil (all double polarisation), & single polarisation observables Polarisation observables T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

R.A. Adelseck and B. Saghai. Phys Rev C., 42:108, 1990 Centre of mass frame:  rest frame: Beam-recoil measurements Axis defined as:k = Photon beam momentum, q = momentum of the recoiling K + in CM frame T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Beam-recoil measurements 1 R.A. Adelseck and B. Saghai. Phys Rev C., 42:108, 1990 N +/- = number of events detected for a +/- beam helicity P  = extent of polarisation of the photon beam  = weak decay parameter (0.65 +/- 0.04) = a “dilution factor” in the K +  0 channel Flipping the polarisation direction cancels acceptance factors in the detector Measure the polar angle,  i, from axis i in the  rest frame Gives an asymmetry, A, of decay products 1 : T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Previous measurements Previous measurements of C x and C z at the CLAS collaboration at JLab 1 : 1 R. Bradford et al. (The CLAS Collaboration). arXiV:nucl-ex/ v1, 2006  K cm = recoiling K + angle in the CM frame E   = Photon energy W = C of M energy T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Identifying strange mesons K +     (~ 63%) Mean lifetime of K + ~ 12 ns     (~ 21%) First cluster from K + < 3ns Secondary cluster from K +      decay > 10ns A new method of tagging a strange meson reaction channel Identify the K + decay within the crystals of the Crystal Ball T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Mean life ~ 11.6 ns Energy of the secondary cluster Time of the secondary cluster Beam energy for events with a secondary cluster Energy Vs time of secondary clusters Threshold energy for strangeness production T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

 E-E plot for all data  E-E plot for events with a secondary cluster Graphical cut for K + ID T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Kinematic considerations Correcting for the vertex position of the decay:   n +  0: Before Correction After Correction Invariant mass of two photons in coincidence   mass (~ 135 MeV) T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Kinematic considerations Simulated data:Experimental data: Neutron detection from  decay: Calculate neutron momentum from the  and  0 momentum. Angle difference between neutron and detected neutral particle:  p  K + +   p  K + +   T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

 z asymmetry measurements 0.9 – 1.0 GeV1.0 – 1.1 GeV 1.1 – 1.2 GeV1.2 – 1.3 GeV 1.3 – 1.4 GeV1.4 GeV + T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

 x asymmetry measurements 0.9 – 1.0 GeV1.0 – 1.1 GeV 1.1 – 1.2 GeV1.2 – 1.3 GeV 1.3 – 1.4 GeV1.4 GeV + T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Preliminary asymmetry measurements C Z at different energy intervals To do: Suppress K +  0 channel – identify the gamma decay of the  K + energy loss correction Distribution of the decay:  ’p  - Measure the absolute cross sections 1 Measure P - unpolarised beam and target C X at different energy intervals 1 Recent discrepancies in cross sections between CLAS and SAPHIR data: T Mart. arXiv: v1 [nucl-th], 5 th March 2008 T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Secondary cluster energy in TAPS /MeVTiming of secondary cluster in TAPS /ns Simulation of K + detection in TAPS Theta angle of K + detection 100 MeV energy sum in the CB 330 MeV energy sum in the CB T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

K + detection in TAPS with a deuteron target Secondary cluster energy in TAPS /MeVTiming of secondary cluster in TAPS /ns Theta angle of K + detection T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008

Summary New technique of identifying K + mesons (~ 1000 K + identified per hour) (~ 5% efficiency for K +  from simulation, K+ detection efficiency ~20% over all CB angles) The first detection of strange meson photoproduction at MAMI-C First preliminary measurements of C X and C Z at threshold energies More data needed, lower energy sum in the Crystal Ball Opportunity to measure more polarisation observables with polarised targets and/or beam? T Jude Polarisation transfer in hyperon photoproduction near threshold A2 collaboration meeting, Dubrovnik, April 2008